SECTION III preliminary tab procedures
SECTION III PRELIMINARY TAB PROCEDURES
A INITIAL PLANNING
Since testing, adjusting and balancing (TAB) of HVAC systems can best be accomplished by following systematic procedures, the entire TAB process should be thoroughly organized and planned. All activities including the organization, procurement of required test instrumentation and the actual system balancing should be scheduled as soon as practical after the contract to the TAB work has been consummated. Since building space loads often vary with each change of season and since space temperature levels are a significant factor in TAB work, the total system, which includes both the air and hydronic portions of the system, must be balanced along with all satellite systems (such as exhaust systems operating independently in the same conditioned space).
Preparatory work includes the planning and scheduling of all NEBB TAB procedures, collecting the necessary data, reviewing the data collected, studying the systems to be balanced, making schematic system layouts, recording the published data on the test report forms, and finally, making preliminary field checks of the HVAC system plans and specifications by the NEBB Certified TAB Firm indicates that the systems may not be able to be balanced properly, the HVAC system designer should be sent a written notification containing suggestions for changes or the addition of balancing equipment (dampers or valves) that would allow corrective action before starting the balancing procedures. The NEBB Certified Firm is not responsible for the design, installation, or function of the HVAC systems or equipment. Occasionally, a system cannot be balanced or made to perform in accordance with the contract documents regardless of the number of balancing dampers or valves that can be installed.
B PRELIMANARY REVIEW
1. PROCUREMENT OF DATA
Obtain all applicable plans and contract documents including change orders, approved shop drawings and equipment submittals, paying particular attention to the items listed below.
A. HVAC Equipment Performance Data
Performance data, including fan and pump curves, should be obtained for all HVAC equipment.
Fan performance data must relate to the actual job requirements and include items such as inlet vanes and altitude and temperature effects. Many times data is general and is not adjusted for these conditions.
Fan performance also can be affected by improper design of ductwork near the fan inlet and/or discharge. This phenomenon is called "system effect", and it cannot be measured in the field. It can be calculated, however, by using tables and charts found in the SMACNA "HVAC Systems--Duct Design" manual or thy AMCA "Fans and Systems Publication 201-90". Performance of lower pressure fans can be substantially reduced by "system effect".
Fan curves or prototype curves can be obtained upon request. I none of these can be obtained, limited curves may be developed from tabulated catalog data. Use caution and determine if the pressures given are internal or external to the equipment and are total pressures or static pressure. Pump curves can be handled in a similar manner.
Take particular note of equipment substitutions that might affect air and water pressure drops of heat exchangers, cooling towers, coils and condensers.
These include changes in coil size, fin spacing, fin configuration, number of rows, cooling coil wet or dry ratings, or number of tubes in the coil face. Data should include air and hydronic pressure differences, the direction of air and water flow (so that proper field installation checks can be made), temperature differentials, capacities, operating temperature and pressures, and limit or safety temperatures and pressures.
B. Electrical Data
Look for changes of horsepower ratings caused by substitutions. Note phase substitutions, especially on packaged equipment, such as ½ hp, single phase on the nameplate instead of the ½ hp, three phase specified. Often these changes can be discovered from the shop drawings or submittals. Motor data not available from shop drawings should be obtained from the field.
Motor starters, sizes, locations, and thermal overload protection ratings should be checked against horsepower, phase and voltage for substitutions. Coordinate this information with the electrical drawings and with the electrical contractor to verify that the specified electrical service is being installed to each piece of HVAC equipment.
Check all sheaves, pitch diameters, belt sizes and number, limits of adjustments, type of belt guards, etc. Insure that all belt guards and other fan accessories do not restrict airflow.
C. Air Distribution Devices
Manufacturer's recommendations on device testing is available in most cases. Effective Area (K factors) usually can be obtained for all air terminal devices for the velocity measuring instrument recommended. Also obtain air patter adjustment and sound data. Manufacturer's Data and Test Procedures also is available on all other rated or adjustable air handling devices, such as variable air volume boxes, constant volume regulators, static pressure control dampers and all other similar equipment.
Air pressure drop data across louvers, filter banks, sound traps, remote coils and other devices in the air distribution system should be obtained. Note whether louvers are provided with screening; whether filter pressure drop data is for clean, partially dirty, or dirty filters; whether sound trap pressure drops are certified and can be confirmed and whether all pressure drops for substituted equipment are within design limitation.
D. Accessory Devices
Air Venting Devices--Determine locations in piping systems and if manual or automatic.
Automatic Temperature Control (ATC) and Energy Management System (EMS) Diagrams--Obtain necessary data required only for TAB work.
Manufacturer's Catalogs--Obtain on all HVAC equipment, including pumps, air moving and air terminal devices in addition to shop drawings and submitted data where possible.
Hydronic Equipment--Obtain location and ratings of expansion tanks, relief valves, pressure reducing valves and flow control devices.
Maintenance--Obtain operating and maintenance instructions for all equipment (if available--for review).
2. SYSTEM REVIEW AND ANALYSIS
After all preliminary data has been collected, a study of each HVAC system may be performed. Two basic reasons for the importance of system review and analysis are (1) to isolate any discrepancies in the data or drawings that may prevent the proper balancing and performance of a system, and (2) to establish the best approach for testing and balancing. Given an agenda, the design engineer, architect, and owner may review what procedures will be employed during the actual testing and balancing of their systems. The agenda also offers an excellent vehicle to present any discrepancies found during the system review and analysis.
A. System Components and Types
Review the plans, specifications and equipment data. Examine each system noting such things as the types and locations of the areas served, types of system, types of components used such as fans, pumps, boilers, chillers, coils, VAV boxes, etc. Note such things as primary and secondary systems, interlocked or interconnected systems, possible tie-ins to existing systems and the location of motor control centers, breakers, etc. Review the equipment schedules, the temperature control drawings and particularly review in detail the operating sequences of each system.
Review most current plans for all applicable notes, sections and details. Look for possible additional fans or other equipment that my not be listed in the schedules.
B. System Schematic Drawings
It is recommended that a schematic layout of each HVAC duct system be prepared either as shown in Figure 3-1 or using a reduced in size set of design drawings. A similar drawing should be made also for extensive piping systems. In large buildings or where there is more than one system, make a separate layout for each floor or for each system. All dampers, regulating devices, terminal units, supply outlets, return and exhaust inlets should be indicated. Also, show the sizes, velocities and flow for main and branch circuits or ducts. Include the air intakes and exhaust air and relief air louvers where applicable.
Indicate any changes in the design of the HVAC system layouts made during construction. For rapid identification and for reporting purposes, all outlets should be numbered similar to that shown in Figure 3-1. The same applies when there are many fan coil or heating units in hydronic systems. Add general notes indicating thermostat locations and other special conditions needed by the TAB team in the field.
C. Study of Systems and Data
After preliminary data has been collected, analyze the data and study all portions of the HVAC systems. The purpose of the study is to find discrepancies in the data or drawings and to establish the best testing and balancing approach. Listed below are items that should be considered.
1) Performed data for all equipment.
2) Total quantity of fluid flow for all terminal units or devices compared to the designated total fan or pump capacities.
3) Location and function of all devices which might impact the system or change the system operation, such as:
a) smoke and fire campers
b) automatic control dampers
c) static pressure dampers
d) automatic control valves
e) flow control devices
f) air vents (hydronic systems) and provisions for make-up water
g) strainers and filters
h) safety devices.
4) Correcting factors if required (such as altitude and temperature).
5) Desired locations to obtain duct and piping flow measurements.
6) Access requirements to take measurements and to adjust volume dampers and balancing valves (ceiling access doors and other accesses must be large enough to do the job).
7) Location of volume dampers and balancing valves required to balance systems.
8) System features which may contribute to an unbalanced condition.
9) Required test report forms.
10) Instrumentation required.
11) ATC and EMS sequence of operation, and ability to index (set) system configuration as required.
12) Sequence of balancing the systems. (Often temporary heat or partial occupancy requirements may require priority. Be careful of dirt after temporary use).
13) Air filtration requirements such as:
a) filter locations
b) special duty filters
c) air filters in place
d) requirements for prefilters
e) system to be balanced with temporary filters or the system to be balanced with an artificial pressure drop across filter bank to simulate special duty filters.
14) Special requirements provided by other contractors that must be completed prior to doing the testing and balancing work, such as ceiling plenums, access doors, light fixtures with troffers, wall openings, architectural louvers, door louvers or door undercutting.
3. THE AGENDA
It is recommended that an agenda be prepared and submitted on jobs, prior to installation of the HVAC systems. The agenda should include a preliminary reporting of any discrepancies that would prevent the proper balancing of the project. It is important to notify the appropriate parties of apparent discrepancies prior to construction or the purchase of any equipment. The agenda should include brief descriptions about the systems and their operation. It should include all proposed balancing procedures and note any items excluded. To be effective, agendas should be submitted early in the process and with adequate time for review by the owner/engineer.
A. Balancing Devices
Review the project drawings, schematics, and details to insure that all necessary balancing devices such as volume dampers and balancing valves are provided to facilitate the balancing procedure. Identify additional dampers or any devices necessary to properly balance the systems. Report an balancing devices that my be inaccessible.
B. System Capacity Review
Check that the total flow requirements of all system terminal units equals the design fan or pump capacities. If a diversity procedure is applicable it should be established and clearly defined in the agenda. Be sure to verify that the system is capable of variable volume operation and control if a diversity balancing procedure is utilized.
Look for obvious static and head pressure discrepancies. Review the scheduled pressure drops of components and compare to the fan and pump capacities for each system. If a fan is undersized, the system designer, purchaser or owner should be notified before the fan is purchased, not after it is installed and operating unsatisfactorily.
C. Locations of Flow Measurements
Determine the desired locations for the measurement of air and water flow quantities, and if access is readily available. Unfortunately, some of these locations (particularly Pitot tube traverse hole locations) will end up being relocated later due to unforeseen or unpredictable conditions. Many times, test plugs may have to be installed before the ducts are externally insulated.
Locate where Pitot tube traverses of duct mains and branches are to be made. Determine the number of readings, calculate velocities and set up the duct traverse test reports.
D. Sequence of Operation
Examine the ATC and EMS control system diagrams to determine how to set the HVAC sytem components (ATC and EMS dampers, terminal boxes, etc.) and whether full heating or cooling is required for testing. Also consider any possible sequences that may result in an unbalanced system operation.
4. TAB INSTURMENT SELECTION
Review section II--"TAB Instruments" to select the instruments that are best suited for the TAB work to be done. Select instruments that will give the measurements required in the least amount of time and provide the accuracies required. Prepare a list of all instruments to be used and include the calibration information. If there is any doubt about the instrument's accuracy, it should be calibrated before the TAB work is scheduled.
5. REPORT FORMS
Select the report forms based upon tests to be conducted and record design data obtained from the system drawings, plans, and submittals. Perform all calculations that are possible to do before the actual TAB work is started. This can save many field labor hours. If terminal airflows are to be determined by velocity readings, calculate the terminal velocities using the manufacturer's "K" factors.
6. SPECIAL INSTRUCTIONS
Prepare instructions for the TAB team. The instructions should include any special job conditions, special TAB procedures, and any anticipated problems. They also can include the name of the job superintendent, important phone numbers, the project engineer's name, and any other useful data.
C PLANNING TAB FIELD PROCEDURES
1. SCHEDULING
Prepare a schedule and plan of attack for the TAB work. Review the job progress schedule know when systems are expected to be ready for balancing. Estimate how many TAB technicians are required.
It is recommended that the amount of time that it will take to perform the TAB work be determined. Notify all contractors involved that there will be a specific period of time needed to perform the work after the job is compete and ready for balancing. Indicate items that must be completed such as all doors, windows, ceilings, thermostats, diffusers and registers, and that all controls are in automatic operation.
If partial occupancy is planned, find out which sections of the building will be needed. Determine if it's possible to perform the TAB work for this area only.
2. FIELD READINESS CHECK
Contact supervisors of the other trades involved to inquire about what work is complete and operable. Also, ask about special conditions.
A checklist (Figure3-3) may be used by installing contractors to verify that the building and the building systems are ready for the TAB work to begin.
D PRELIMINARY FIELD PROCEDURES
After completing initial planning, systematically follow the steps listed below.
Note that the responsibilities will be different if the TAB firm is also the installing contractor. If the TAB contractor is only doing TAB work, the installing contractor has responsibility for assuring satisfactory startup procedures have been achieved.
1. HVAC EQUIPMENT
a) Confirm that all HVAC equipment fans have been checked and verify that:
1) equipment matches the test report data such as model number, make, arrangement, class, etc.
2) test report forms have had data entered that must be obtained form the field.
3) all bearings have been lubricated.
4) fan wheels clear the housings: improper clearance can greatly affect fan performance, especially backward inclined fans.
5) all foreign objects have been removed (such as shipping restraints, and protective covers).
6) motors and bases have been fastened securely.
7) all drives have been correctly aligned.
8) all drive set screws and keyways are tight.
9) belt tensions are correct.
10) fan rotation direction is correct.
11) duct flexible connections are properly aligned.
12) vibration isolators or bases have the correct springs and in the right location, and that the springs are not collapsed. Be sure that the equipment is level and the isolators are not totally compressed. Check for the proper seismic restraints if they are required.
13) static pressure controls are free and operable.
14) equipment drains are piped and trapped properly (no moisture present).
15) all equipment is clean and free of paper, rags, and other foreign objects.
16) belt guards are in place.
b) Check the airflow pattern from the outside air louvers/dampers and the return/exhaust air dampers through to the supply air fan discharge (and mixing dampers if present).
c) Locate all star-stop, disconnect switches, electrical interlocks and motor starters. Motor starters must be equipped with thermal overload protection of the proper size.
d) Check availability of electrical power to all equipment needed for TAB work and verify the compatibility of voltage and phase.
e) Confirm the operation of all unit and related dampers and their sequence of operation (includes fire and smoke dampers). Proper damper position during startup and TAB work is very critical. If dampers are closed, restricting the airflow, serious damage can be done to casings, housing and ductwork. It is generally best to work with an automatic temperature control (ATC) technician during startup to ensure that all ATC dampers are positioned properly. However, it may be necessary to manually secure some dampers into position before starting the fan.
f) All dampers should be in a position to ensure the desired path for air to travel through the correct components of the system and not cause a choked or blocked condition. For startup and balancing, the following damper settings are recommended for the most common systems.
1) Where separate minimum and maximum O.A. damper is used in conjunction with a minimum position controller, the damper should be opened approximately to the percentage of minimum outside airflow. If the system used 100 percent outside air, the damper will have to be fully open. (Note: Don't leave O.A. dampers open when the unit is not in use, especially during cold weather when freezeups may occur.)
2) Return air dampers should be opened.
3) Exhaust air dampers usually are left closed. Some systems may require opening them to a percentage equal to the minimum outside air damper setting. This is where minimum relief or exhaust air is specified.
4) Face and bypass dampers should be set so that the airflow is through the coil. Multizone dampers should be set so the air will flow through the cooling coil. Confirm that the coils are sized for an airflow equal to the fan design. Occasionally coils are sized for less airflow that the fan. In this case, the bypass damper should be left open an amount equal to the excess fan airflow so that the total airflow will not be restricted.
5) Fire and smoke dampers generally must be in the open position. More complex systems use a variety of damper configurations, particularly where multiple fans and sequences are involved. In this case, the sequence of operation must be studied and dampers must be open or closed accordingly.
6) Vortex and other fan limiting dampers are often used with variable air volume (VAV) systems. It is safest to start the systems with these dampers throttled somewhat and then open them up slowly, observing the static pressure and amperage accordingly.
g) Check for any type of temporary blockage over the outside air inlet opening and the exhaust air discharge openings (such as polyethylene, cardboard or plywood), that may have been placed during construction. Look for any other types of debris or blockage in both the outside air and the return air duct systems.
h) Check the coils for cleanliness and check the alignment of the fans. Confirm that the piping connections are correct.
i) Confirm that condensate drains from the cooling coil drain pans have been provided and that they are properly trapped and functioning. Improper drain traps are a common cause of leakage, flooding and moisture carryover. The trap depth on "draw through" units should be 1 inch (25 mm) greater than the total suction static pressure.
j) Check the entire unit and the internal components for proper leak sealing. Leaks will cause whistling, possible moisture carryover and short circuiting of the air. Particularly check around pipes and panel holes. Have the leaks sealed.
k) If the system has spray systems, they should be clean and operating.
l) Confirm that the correct size and type of filters are installed for the TAB work. (Sometimes different filters are used temporarily during startup and construction.)
1) If the permanent filters are to be used, confirm that they are the correct size and type of filter by comparing them with the submitted data and the NEBB Test Report Form.
2) Check the filters for cleanliness. If they aren't clean, have them replaced before starting TAB work.
3) Confirm that the filters and filter frames are properly installed and are airtight. Any leaks must be corrected.
4) If the unit has been running with no filters or very dirty filters, be alerted for possible dirty or clogged coils, etc.
2. DUCT SYSTEM CHECKS
a) Walk each duct system from the supply air or exhaust air to the last terminal Check that:
1) The ductwork is complete and installed correctly and if there are any openings in the duct work or any endcaps missing, and that all access doors are closed and secured tight.
2) All terminals, boxes, reheat coils, etc. are installed.
3) The installation matches the plans.
4) All the necessary architectural items are installed, such as doors, partitions, ceilings and ceiling plenums and windows.
5) The system really is ready for balancing. (This is important because the TAB contractor often is pressured by the owner, general contractors, mechanical contractors, or system designer to start the TAB work before the building and/or the systems are ready.)
b) Confirm that adequate balancing dampers have been provided and that all volume dampers, fire dampers and smoke dampers are installed at the correct locations. Verify that they are wide open and that adequate access has been provided.
c) Confirm that any terminal boxes, such as VAV boxes and mixing boxes are installed and accessible. Confirm that the controls are energized and in operation. Thermostats must be installed and operable. For startup and full air flow testing, most boxes will have to be set in the full cooling position by setting down the thermostat. This will insure maximum airflow and the least amount of system restriction.
Caution: Some boxes are furnished with normally closed dampers. Starting a complete system with closed terminal box dampers will result in excessive system static pressures and possible duct system damage.
d) Verify that all terminal devices are installed and that their dampers are open. Confirm that the terminals are of the same size and type as specified. Frequently, substitutions are used due to space configurations or availability. This will usually change the A k or K factor. (The size and type check can be performed while making the preliminary readings at the terminals.)
e) Inspect the system for leakage. Specifically check access doors and hardware, coils, humidifiers, pipe penetrations, duct connections, flex duct and terminal connections. Confirm that any specified sealing has been done correctly.
f) Where plenum ceilings are utilized, confirm that they are airtight. Pipe penetrations and any other holes should be sealed. Any air barriers must be well sealed. This is extremely important on supply ceiling plenums.
g) Confirm that any required openings between partitions, etc. have been installed and are open to insure proper air passage.
h) Pitot tube test holes must be located in the field to confirm that they are accessible. Confirm that the actual duct installation matches the plans and that adequate straight sections of duct work are available for the tests. Look for obstructions that will hamper swinging the Pitot tube, such as pipes, ceiling supports, lights, etc. Pitot tube test holes must be sealed or capped when not being used.
i) If the duct is insulated, any insulation removed for the test will have to be replaced and resealed. Use caution when removing insulation so that a neat repair can be easily made.
j) If the Pitot tube test holes and caps have already been installed by others, confirm that they are in the correct ducts and at satisfactory locations.
3. HYDRONIC PIPING SYSTEM CHECKS
a) Confirm that the system has been hydrostatically tested and is free of leaks.
b) Trace the system piping from the source (e.g. boiler, chiller, heat exchanger) to all terminal units to determine:
1) Completeness and integrity of the installation.
2) Any variations between actual installation and design.
3) That all required valves (manual and automatic) outlined in the agenda are open.
4) That accessibility is readily available for testing of all balancing devices, flow meters, and terminal units.
c) Confirm that the system has been cleaned, flushed, filled and all air purged. Verify that strainer baskets have been cleaned and construction baskets (if used) have been removed. Maintaining system cleanliness is a full-time job, especially during initial system operation. Monitor the system cleanliness during the TAB work and be aware of any debris build-up that may affect the final system balance.
d) Verify that the system water level and pressures are correct for the height of the highest terminal units. Procedures for open and closed systems are outlined below:
1) Open System--Confirm that the system water level is correct and verify the operation of the make-up water device. Open systems with low static heads require special care on initial start-up to prevent inducing air into the pump. A good practice is to initially start the pump with the discharge valve partially closed so that the pump volume drawn from the sump will not exceed the make-up water capacity. Monitor the sump level and slowly open the discharge valve until the system is in full operation.
2) Closed System--Inspect the pressure reducing valve(s) (PRV) for proper installation and operation. The setting of the PRV should always maintain a minimum 4 psi static pressure at the highest point of the system. Verify compression device(s) for proper pressures and/or levels.
e) Verify the proper installation of piping safety devices! Do not attempt to operate any system without backflow prevention, or a closed system without the proper pressure relief valves.
f) Confirm that all of the systems have been installed in accordance with the contract documents and are ready to be tested, adjusted and balanced. Check list (Figure 3-3) can be used as a guide by the TAB team.
4. PUMPS
a) Confirm that all pumps have been checked and verify that:
1) the equipment matches the test report data such as model number, make, type, rpm, etc.
2) the test report forms have had data entered that must be obtained from the field.
3) all bearings have been lubricated.
4) rotation is free and correct.
5) motors have been aligned and fastened securely.
6) pump bases have been correctly grouted.
7) air has been bled from pump casing where required.
8) all drive set screws and keyways are tight.
9) vibration isolation and flexible pipe connectors are the correct size and type and in the proper position and alignment.
10) all equipment is clean and free of foreign objects.
11) drive guards are in place.
12) access has been provided for pressure and/or temperature readings.
b) Locate all start-stop, disconnect switches, electrical interlocks and motor starters. Motor starters must be equipped with thermal overload protection of the proper size.
c) Check availability of electrical power to all equipment needed for TAB work and verify the compatibility of voltage and phase.
d) Verify that all strainers are clean.
e) Check system temperature and pressure combinations at pump inlets for possible flashing or cavitation problems.
5. BOILERS
a) Verify that boilers have been started and tested for proper and safe operation in accordance with the manufacturer's instruction and that all safety and operating controls have been tested, adjusted and set for the proper operation.
b) Locate and confirm that all combustion air openings and barometric or draft control dampers for the boilers are of proper size for the fuel being used.
c) Confirm proper settings and types of all operational and safety control devices, both for temperature and pressure.
d) Confirm proper operation and lubrication of boiler equipment, motors, pumps, and boiler feed water equipment.
e) Check for proper location and correct piping to air vents, air elimination fittings and compression tanks.
f) Verify that water levels of steam boilers are steady and that the boilers have been properly flushed and/or "blown down".
g) Verify boiler nameplate data and add missing information (such as serial numbers, etc.) to test report forms.
6. HEAT EXCHANGERS
a) Confirm size and physical data.
b) Verify proper piping methods, connections for flow, pipe sizes, venting devices, etc.
c) Verify airflow direction.
d) Inspect face areas for fin damage, air leakage from tube sheets, fluid leakage from tubes or piping, foreign matter, etc.
e) Confirm provisions for pressure and temperature measurements.
f) Confirm operation type and size of automatic valve, expansions vales, and other control equipment. (Temperature control valves usually are set for full flow during TAB procedures.)
7. REFRIGERATION EQUIPMENT
a) Confirm that all crankcase heaters are in operation. DO NOT START REFRIGERATION EQUIPMENT UNLESS CRANKCASE HEATERS HAVE BEEN IN OPERATION AT LEAST 24 HOURS BEFORE START-UP (LARGER SYSTEMS MAY REQUIRE MORE TIME)
b) Verify that the compressor and nameplate corresponds to preliminary test report forms. Record nameplate data not found on the test report forms. Verify thermal overload protection and safety switch locations.
c) Confirm the settings for all high pressure, low pressure and oil failure switches and that proper startup adjustments have been made.
e) Confirm that compressors are filled with lubricating oil.
f) Check coupling alignment and seal leakage on open compressors.
g) Inspect vibration isolation and compressor level.
h) Inspect piping flexible connectors for alignment and restraint.
i) Confirm that filter driers and liquid indicators have been installed and that the systems have the correct refrigerant charge.
j) Confirm that all equipment has been piped correctly and that airflow and fluid flow is in the correct direction.
k) Confirm the location and settings of chilled water flow switches and chiller safety controls.
l) Vent air from chillers and condensers as required.
m) Confirm provisions in piping for pressure, temperature, and flow measurements.
n) Check superheat settings.
8. CONDENSERS/COOLING TOWERS
a) Verify that nameplate data corresponds to the preliminary test data reports. Record missing data. Verify thermal overload protection and safety switch locations.
b) Confirm that condenser piping has provisions for pressure, temperature and flow measurements.
c) Check that condenser piping is correct for the flow necessary for the required heat transfer.
d) Check cooling tower sump water level and make-up water valve.
e) Confirm that provisions have been made for condenser water bleed off and chemical treatment.
f) Check cooling tower fan motors for correct rotation, voltage and phase.
g) Check for obstructions that can cause short circuiting of airflow into or from the tower or condenser.
h) Check spray nozzle pattern and confirm that all are working.
i) Verify that all control dampers or head pressure controls are working properly.
9. COILS/TERMINAL UNITS
a) Verify that the units are piped correctly including required valves, vents, and safety devices.
b) Confirm that provisions are available for making the required temperature and pressure measurements.
c) Confirm the proper installation and operation of automatic temperature control devices.
d) Purge all air from terminal units on hydronic systems.
e) Verify the proper airflow direction and fan rotation.
f) Inspect the coils on both sides for fin damage and blockage.
g) Confirm voltages and phases on electrical equipment of terminal units
10. STEAM PIPING SYSTEMS
a) Confirm that the system is free of leaks and that it has been hydrostatically tested.
b) Confirm that all strainers have been cleaned.
c) Inspect pressure reducing valve operation and settings.
d) Confirm settings and locations of all safety valves.
e) Confirm that all manual and automatic valves are in the required position for TAB work.
f) Inspect and verify that the water level in the boiler(s) is correct.
g) Confirm that provisions have been made to obtain temperature, pressure, and flow measurements.
h) Confirm that steam traps are operating properly on all equipment, at ends of mains, and at all drip points.
SECTION V--GENERAL AIR SYSTEM TAB PROCEDURES
A BASIC AIR SYSTEM TESTING PROCEDURES
Section III -- "Preliminary TAB Procedures" covered the preparation work that must be done prior to the actual testing, adjusting and balancing of thd HVAC systems on the job. Confirm that these preliminary procedures have been completed and check lists prepared. Do not attempt to balance a system before installation has been completed and the system is ready to be balanced.
1. PREPARATION
The following balancing procedures are basic to all types of air systems.
a) Confirm that every item affecting the airflow of a duct system is ready for the TAB work, such as doors and windows being closed, ceiling tiles (return air plenums) in place, etc.
b) Confirm that all automatic control devices ill not affect TAB operations.
c) Establish the conditions for the maximum demand system airflow which generally is a cooling application with "wetted" coils.
2. SYSTEM STARTUP
a) After verifying that all dampers are open or set, have the job electrician start all related systems (return, exhaust, etc.) and the system being balanced with each fan running at the design speed (rpm). Upon starting each fan, immediately check the fan motor and fan drive for malfunctions, and the motor amperage. If the amperage exceeds the nameplate full load amperage, stop the fan to determine the cause or to make the necessary adjustments.
b) Quickly go to each automatic damper that hasn't been blocked or disconnected and confirm that the damper is being controlled automatically and is in the correct position. There will be some effect on the airflow if these dampers are "hunting." This is undesirable while doing air balancing. Therefore, the dampers or their controls should b e blocked out to keep them in the desired position. All dampers should be set for a full flow "cooling" condition.
c) Again confirm that all related system fans serving each area within the space being balanced are operating. If they are not pressure differences, and infiltration or exfiltration may adversely influence the balancing. Preliminary studies will have revealed whether or not the supply air quantity exceeds the exhaust air quantity from each area. Positive and negative pressure zones should be identified at the time.
d) If a supply fan is connected to a return air system and an outside air intake, set all system dampers and controls so that the air returned from the individual rooms or areas supplied by the fan is returned via the related return air system. Normally this will involve opening an outside air damper to the minimum position, opening the return air damper, and closing exhaust air and relief air dampers. (if the supply system is associated with a return air system and/or an independent exhaust system, make sure all systems are operating and all related dampers are set properly for the TAB work.)
FIGURE 5-1 Making a Pitot Tube Traverse
3. FAN TESTING
a) Determine the volume of air being moved by the supply fan at design rpm by one or more of the acceptable methods, such as:
1) Pitot tube traverse of the main duct or the ducts leaving fan discharge if good location available.
2) Fan curves or fan performance charts. In order to determine fan performance using a fan curve or performance rating chart, it is necessary to take amperage and voltage readings. In addition, a static pressure reading across the fan must be recorded. With rpm, brake horsepower and static pressure, the fan manufacturer's data sheets may be used to determine the aiflow predicted by the manufacturer. Fan performance can deviate from the fan curves if "system effect" or other system installation defects are present.
3) Where impossible to take good Pitot tube traverses of duct system use total sum of terminal device air volume readings.
4) Anemometer readings across coils filter, and/or dampers on the intake side of the fan. This is used as an approximation only.
b) If the fan volume is not within plus or minus 10 percent of the design capacity at design rpm, determine the reason by reviewing all system conditions, procedures and recorded data. Check and record the air pressure drop across filters, coils, eliminators, sound traps, etc. to see if excessive loss is occurring. Particularly study duct and casing conditions at the fan inlet and outlet for "system effect."
c) Always recheck the amperage whenever any rpm change or major damper setting change is made.
d) Many fan rooms are also return air or outside air plenums. When taking readings in these rooms, it will be necessary to reference the manometer or anemometer to the atmosphere. This is accomplished by running a length of hose from the opposite port of the instrument to the outside or atmosphere. Take advantage of existing possible openings available for static pressure (SP) readings such as access doors, handles, bolt holes etc. that may be removed to get the SP probe into the airstream. If none are available, new ones will have to be drilled. Be sure to close or cap them when finished.
e) Using the methods outlined above, determine the volume of air being handled by a return air fan is used; and/or if a central exhaust fan system is used, also determine the aiflow being handled by the exhaust fan. If several exhaust fans, such as power roof ventilators, are related to particular supply air system, it generally is not necessary to measure the aiflow of each such exhaust fan until after the supply air system is balanced.
f) If the measured airflow of the supply air fan, central return air fan or central exhaust air fan varies more than 10 percent from design, adjust the drive of each fan discharge static pressure, amperage and air volume measurements. Confirm that the fan motor is not overloaded.
g) After balancing the return air system and the associated supply air system, the return air damper should be closed; the relief air dampers should be 100 percent open and the return air fan, if used, static pressure and system airflow should be checked again. If it is necessary to increase the system static pressure and thereby reduce the fan airflow, adjust the exhaust air damper to a maximum position less than 100 percent open. Recheck the supply fan airflow with the outside air damper in the full open position.
4. SYSTEM AIRFLOW
a) Make a preliminary survey, spot checking air circulation in various rooms. With knowledge of the supply, return or exhaust fan volumes and data from the survey, deter if the return air or exhaust air system should be balanced before the supply air system is balanced. In continuation of this procedural outline, the assumption is made that the supply air system balance is not restrained by the exhaust air system or the return air system. However, if such a restraint exists the exhaust air system or the return air system should be balanced prior to continuing with the supply air system.
b) The most accurate and accepted field test of airflow is by a Pitot tube traverse of the duct being tested. Anemometer traverses across coils and/or filters are a poor substitute and should only be used in an emergency and with considerable reservation. Field tests have shown that they will vary considerably. They usually tend to read higher than the actual system airflow, but no definite pattern has evolved.
c) A total of the terminal readings will be useful to compare with the Pitot traverse readings when system air leakage is suspected. There will be instances when they will be the only field readings available for the system total airflow. Fan curves can be used when other required data can be obtained, such as SP, rpm and Bhp. Experience has shown, however, that often all of the field readings will not fall into place on the fan and system curves. Therefore, it is best to make Pitot tube traverses whenever possible and use them in conjunction with the other test data and fan and system curves to tell what actually is happening.
d) The accuracy of a Pitot tube traverse is determined by the availability of a satisfactory location to perform the traverse. Reasonably uniform airflow through the duct is necessary. Ideally, there should be six to ten diameters of straight duct upstream from the test location. Realistically, this condition will no be found very often in the field, therefore, use the best locations available. Avoid getting close to elbows, offsets, transitions or anything else in the duct that is creating turbulence.
e) If the Pitot tube traverse readings are taken at a good location and the readings are reasonably steady and uniform, these readings are going to give the most accurate field measurement of the system airflow and should be used accordingly. When the readings are not steady and uniform, they should be used in conjunction with the other test data and the fan curves to make a determination. The fan curve and fan speed data, when used with the calculated brake horsepower, will give the most accurate field readings that can be relied on heavily. Static pressures will be the least accurate field readings along with airflow readings, depending on how and where they were taken. But with a combination of these readings. one should be able to make a reasonable determination of the performance of the fan.
f) Don't be surprised when all of this data doesn't fall into place on the correct fan curve. Field readings are not that accurate, and fan curves do not reflect installed conditions (fans are tested in a laboratory under ideal conditions). Accurate HVAC system airflow readings should be taken with a wet cooling coil. If this is not possible, allow for some loss of airflow. When the coil is in use and wet, a 5 to 15 percent difference in airflow and static pressure readings is common.
5. SYSTEM DEFICIENCIES
a) Compare the actual results of the above tests with the specified performance of the fan. If the fan airflow is not within + 10 percent of design, try to find the reason for the difference. Determine if the pressure drops across the duct system components (such as coils, filters, sound attenuators, eliminator blades, etc.) agree with the manufacturer's ratings. Observe the duct system configurations at the inlet and discharge of the fans. Compare these with the contract drawings. Notice if any radical changes were made to the duct system layout during installation. If any corrections are needed, report this to the appropriate persons.
b) If there are no obvious deficiencies, and the airflow is high the fan can be slowed by adjusting the drives or making drive changes. When the airflow is low, the fan speed should be increased. Before doing this, determine if there is adequate motor horsepower available. The new airflow-horsepower relationship can be determined by use of the fan laws found in Section IX. The fan curves are a better reference, if available. If any sizable upward change is made, the fan manufacturer's data should be checked for the maximum allowable rpm for this fan and its bearings. If horsepower and static pressure data is available and the fan speed can be increased, adjust the drives accordingly to obtain the desired airflow.
c) When new systems do not perform as designed, new drives and motors are often required. The financial responsibility for these items does not usually belong to the NEBB TAB Firm, but the submitted readings will have a lot to do with determining responsibility. Be sure to include the necessary data together with explanations about how and where the readings were taken. The above steps also should be used for any return air or exhaust air fan associated with the HVAC system in question.
B BASIC AIR SYSTEM BALANCING PROCEDURES
1. OBJECTIVE OF SYSTEM BALANCING PROCEDURES
Balancing air systems may be accomplished in various ways. Regardless of method, the objectives remain the same and the system will be considered balanced in accordance with NEBB procedural standards when:
a) The value of the air quantity of each inlet or outlet device is measured and found to be within + 10 percent of the design air quantities (unless there are reasons beyond the control of the NEBB TAB Firm -- see Section VI-B, Work Completion); and
b) The terminal in the circuit with the greatest resistance shall be fully open.
2. STEPWISE METHOD
a) Make Pitot tube traverses on all main supply and major branch ducts to determine the air distribution. Investigate any branch that is very low in capacity to make sure that no blockage exists.
b) Adjust the volume damper on each branch that is high on airflow. Monitor the static pressure (SP) at a point downstream of the balancing damper. Slowly close the damper until the SP comes down to the new required SP determined by the equation SP2/SP1=(airflow2/airflow1)2. This should give approximately the correct airflow for this zone. This procedure should be used on each zone with high airflow, usually starting with the highest one first. Then remeasure the SP in all of the zones. There usually will be some interaction between the zones. Some of the adjusted zones may need adjusting again. The zones that were low in airflow should have increased, and now some of these may be high and may themselves need adjusting. After the zones are adjusted to the new calculated SP, proceed to the terminal units.
c) There will be instances where a branch damper will need adjusting but there won't be any satisfactory location for a Pitot tube traverse. In this instance, it will be necessary to take airflow readings at all of the terminals in the zone and total them. Use this total, take a reference SP as detailed earlier, and then proceed to balance the zone. Often this will result in a decrease in accuracy, but one should still be able to get the zone set close enough to proceed.
d) Without adjusting any terminal device, measure and record the airflow at each terminal in the system. In making adjustments, adjust volume dampers instead of extractors (if installed) or the dampers at the air terminals. If the throttling process at the terminal involves closing the damper to a degree that generated noise, evaluate the design airflow capacity of the branch duct.
e) Using the appropriate instruments and procedures as delineated earlier, measure and record a preliminary reading at each terminal unit. This is a good time to confirm that the size and type of terminal device installed is what was specified. If not, the A k factor will surely be different. Unless a direct reading hood for airflow is being used, be absolutely sure that the manufacturer's published A k factor and measurement procedures are being used. Many hours have been wasted searching for a problem when the wrong A k factor was used. Direct reading hoods eliminate the need for A k factors and special procedures, and they also decrease the time needed for balancing.
f) After testing and recording all of the terminal units, total the readings on a zone or branch basis. Compare the totals to the comparable zone duct traverse reading and the required airflow. The total airflow for the terminal units should be close to the traverse reading for the zone or branch. The terminal unit total usually will be a little lower due to some expected leakage found in unsealed or partially sealed supply air ductwork. The accuracy of a good Pitot tube traverse is usually considerably better than most terminal readings. On occasion, there will be a higher airflow total from the terminals than from the Pitot tube traverse readings.
g) If the readings indicate a loss of more than ten percent of air in the duct system, the system will not be able to be balance properly. Investigate duct connections, terminal connections, and the plenums of linear diffusers. Also recheck for open access doors, holes in the ducts, etc. Notify the proper persons to have the leaks corrected.
h) When it is determined that airflow is within + 10 percent of design, proceed with the TAB work. Since system airflow will go first to the points of least resistance, usually the airflow of terminals closest to the fan will be higher and those near the end of the system will be lower. The results of preliminary readings will indicate what the system is doing and where the problems exist.
i) Review the readings and start adjusting the terminals that are highest on airflow. On the first "adjusting pass" through the system, it usually helps to throttle these terminals to about ten percent under design airflow. This will allow for the possible buildup as other terminals are adjusted.
j) After adjusting the high airflow volume terminals, proceed to make another pass through the entire zone or system. Adjust each terminal to the specified airflow, assuming that sufficient air is available. After two adjusting passes, most systems should be in good balance. An additional pass will probably be necessary to "fine-tune" the system. Mark all dampers at the point of final adjustment for ease of resetting in the event of tampering.
k) Verify the fan capacity and operating conditions again and make a final adjustment to the fan drive if necessary.
l) If the supply system was tested with dry coil surfaces and is designed for dehumidification, the total air quantity should be rechecked under wet coil conditions. (If this is not possible, add 5 to 15 percent to the system setting instead.)
m) After the supply, return and exhaust systems are properly balanced, the supply air fan capacity should be checked with 100 percent outside air if this alternative is included in the system design. Appropriate damper adjustments should be made if necessary.
n) Record the "as balanced" state of the system on report forms for all terminals and duct apparatus.
o) Verify the action of all fan control dampers, shut down controls, and airflow safety controls.
p) Prepare the report forms and submit as required. (See Section VIII--NEBB TAB Report Forms).
3. PROPORTIONAL BALANCING (RATIO) METHOD
a) Perform the initial air systems TAB work as described under Subsection A-"Basic Air Systems Testing Procedures".
b) Select the supply air duce branch farthest from the fan. All terminal units or outlets should be numbered on a schematic drawing. (For example, see Figure 5-5, outlets number 4 to 9.)
c) Preferably using a direct reading flow hood, record the measured airflow "Qm" from each of the outlets on the branch duct.
d) Calculate the percentage (X%) of design airflow "Q d" for each outlet (Q m/Q d=X%). For example, 250 cfm (125 l/s) measured airflow divided by 200 cfm (100 l/s) design airflow equals 125%.
e) The outlets are renumbered in their degree of percentage of design from the lowest to thehighest as shown in the following example (based on Figure 5-5):
Outlet Design Meas.
No. Q d Q m % _____ cfm l/s cfm l/s _____
6 200 100 150 75 75
9 200 100 160 80 80
5 200 100 170 85 85
8 200 100 180 90 90
7 200 100 200 100 100
4 200 100 210 105 105
f) The branch damper for Outlet number 6, the lowest percentage of design, is not adjusted. The damper for Outlet number 9, the next lowest percentage of design, is adjusted until the airflow volume decrease to about 155 cfm (77 l/s). Outlet number 6 airflow volume should then come up to about 155 cfm (77 l/s). This should be verified by measurement, and these two outlets (6 and ( should be in balance.
g) Outlet number 5, which is the next lowest percentage of design, is adjusted down to about 160 cfm (80 l/s). Outlets 6 and 9 should then come up to near 160 cfm (80 l/s) and number 9 should be measured to verify this. Outlets numbers 6,9 and 5 should all be basically in balance.
h) This procedure is followed, proportionally balancing the next highest percentage of design outlet to the previous one balanced. This should bring all outlets balanced earlier into balance with it.
i) Using Figure 5-5, the three (3) outlets numbered 1 to 3 would be proportional balanced to each other in the same manner. The two branches then could be proportional balanced using the same procedures used with the terminals.
j) If a branch duct has outlets with varying airflows, the percentage of design is calculated for each and the same procedures are used, balancing to the percentage of design airflow for each.
k) Upon completion of proportional balancing of all outlets and branches, recheck the supply air fan capacity to the final Q m / Q d percentage. If Q m is lower than Q d, the fan airflow volume must be increased to the design airflow (Q d).
l) Continue the TAB work by following steps "k" through "p" in Subsection B-2 "Stepwise Method."
SECTION VI TAB PROCEDURES FOR SPECIFIC AIR SYSTEMS
A INTRODUCTION
The TAB procedures for basic air systems found in Section V - "General Air System TAB Procedures" are the foundation for the testing, adjusting and balancing of any air distribution system. There are, however, certain different or additional procedures that should be used when balancing other than single duct, constant volume air systems.
Even though some of the duct systems addressed in this section are considered "obsolete" by the HVAC industry, TAB firms may encounter them when rebalancing or retrofitting systems in older buildings. Procedures will follow for:
* Variable air volume (VAV) systems
* Multizone systems
* Dual duct systems
* Induction unit systems
* Process exhaust sir systems
B VARIABLE AIR VOLUME (VAV) SYSTEMS
1, There are may types of variable air volume (VAV) systems (see Figure 6-1) being used that can fall into two basic categories: (1) by-pass systems and (2) reduced airflow or turn down systems. They further can be connected to single or dual supply air ducts, have constant or variable airflow on the primary side of the VAV boxes with variable air flow on the secondary or distribution side. The variable airflow rate can be from 100 percent to 0 percent of full flow.
Variable air volume terminal units (VAV boxes) also can be classified as pressure independent and pressure dependent. A pressure independent device has a volume regulator which will maintain the proper airflow regardless of the terminal inlet static pressure. A pressure dependent device will allow the airflow to vary in accordance with the inlet static pressure.
Usually, variable air volume systems are designed with a diversity factor which means that the supply fan airflow (cfm l/s) capacity is less than the sum of the airflows of all the terminal devices. If the diversity factor is not given, it can be approximated by dividing the supply air fan maximum airflow by the sum of the airflows of all VAV terminal units and converting the decimal number to a percentage.
2. PRIMARY AIR VOLUME CONTROL
The supply air fan installation for a VAV system is similar to that for constant volume systems except that the fan air volume must be varied. Inlet or discharge dampers and variable speed drives or motors may be used to control the system airflow. A static pressure sensor usually located about two-thirds of the way from the fan to the end of the duct system, senses the supply air duct static pressure and sends a signal back to the apparatus controlling the fan airflow volume. Verify that the controls are set to maintain a constant static pressure at the sensor location, as the system airflow varies.
Systems with combination return/exhaust air fans require special attention by the NEBB TAB Supervisor. Building pressure will vary if the return air fan volume does not vary closely with the supply air fan volume. Three common methods used are building static control, open loop control and closed loop control.
A. Building Static Control
Building static control senses differential pressures between a typical room and outdoors, and increases the volume of air handled by the return/exhaust air fan as building pressure increases. Difficulty in determining a typical room on large systems and obtaining a stable outdoor reference are disadvantages of this control method.
B. Open Loop Control
Open loop control uses an adjustable span and start point on the supply air and return air fan controls to sequence the return air fan operation with the supply air fan (Figure 6-2). This system requires close attention by the NEBB TAB Supervisor. If the system load varies significantly among major zones the supply air fan serves, resistance in the return air system may not vary in direct proportion to resistance in the supply air system. Open loop control does not sense the effect of resistance variance between the supply air and return air systems, and building pressures may vary when major load variation occurs.
C. Closed Loop Control
The closed loop control senses changes in the volume of air the supply air fan delivers and uses a controller having a second input proportional to the return air fan flow to reset the return air fan (Figure 6-5). Square root factors should be applied where flow is measured as velocity pressure, enabling comparison of linearized signals. Controlling flow eliminates the effects of different fan or vane characteristics. The controller can be set to maintain the difference in the airflow required between th supply and return air fan to maintain building pressurization and accommodate auxiliary exhaust systems. If this difference is also the minimum amount of outside air the system requires, the flow controller on the fans will maintain the minimum ventilation rate regardless of variations in outside air and return air damper characteristics. Multiple point Pitot tubes or flow measuring stations should be used to sense velocity pressure at the fans on VAV systems, since the velocity profile will vary with flow; single-point Pitot tubes will give inaccurate readings of total airflow.
The system designer should locate the static pressure sensor on the drawings, as it depends to some extent on the type of variable air volume terminal unit used. Pressure dependent units without controllers should be near the static pressure midpoint of the duct run to ensure minimum pressure variation in the system. Where pressure independent units are installed, pressure controllers may be at the end of the duct run with the highest static pressure loss to ensure maximum fan horsepower savings while maintaining the minimum require pressure at the last terminal unit. However, as the flow through the various parts of a large system varies, so does the static pressure losses. Some field adjustment or relocation may be required.
3. GENERAL VAV PROCEDURES
Prior to beginning the TAB work, verify that the temperature control contractor's sequence of operation complements the terminal u nit or VAV box manufacturer's factory installed control system. Inspect primary air ducts to ensure adequate entry conditions. VAV systems with diversity factors should be operated at maximum system airflow with all "peak load" terminal units wide open, to check actual fan motor current and voltage before initiating the TAB work on the system.
To check out the supply air fan, the total airflow of the VAV boxes or terminal units should be indexed to equal the design airflow of the fan. If the system has a diversity, meaning that the total airflow of all terminal units is more than the fan design airflow volume, place a selected number of terminal u nits at a maximum set point flow condition until the total airflow of the terminal units equals the design airflow of the fan. This means that some of the terminal units may be in a minimum flow condition. Distribute the reduced airflow terminal units throughout the system so that they are not on one major branch. When the system airflow is equal to the designed fan airflow, the fan can be tested using the methods described in Section V, Subsection A -- "Basic Air System Testing Procedures". VAV box balancing procedures follow.
4. PRESSURE INDEPENDENT VAV SYSTEM BALANCING PROCEDURES
Pressure independent VAV boxes have the ability to maintain a constant maximum and minimum airflow as long as the box inlet static pressure is within the design range of the VAV box. The manufacturer's published data provides the static pressure operating range and the minimum static pressure drop across each terminal u nit for a given airflow. Use this data to verify that adequate pressure is available for the terminal unit to function properly. The objective of balancing pressure independent VAV boxes is the same, regardless of the type of controls used. They must be adjusted to deliver the specified maximum and minimum airflows (the minimum airflow can be zero in man instances).
For simplification, consider each pressure independent VAV box and its associated downstream ductwork to be a separate supply air duct system. If there is adequate static pressure and airflow available at the box inlet, the box and its associated outlets can be balanced.
Two common methods usually are available to verify that the VAV box is delivering the correct airflow. First, most boxes have taps in the lines going to the regulator from the sensor inside the box. Connect a manometer to these taps to read the pressure differential at the sensor. Most manufacturers provide data to convert these readings to the actual airflow. Field testing, however, has proved that this method is not always accurate. Often the inlet duct configuration and the type of sensor will affect the signal sent to the regulator, causing erroneous readings. The second method is to total the terminal unit outlet readings to verify that they are delivering the proper amount of air. This test also will aid in locating duct leakage.
After the related fan systems have been adjusted in accordance with previous procedures, the VAV system should be tested, adjusted and balanced using the following procedures:
a) Determine if the system designer has taken into account a diversity factor when selecting the equipment by comparing the fan design airflow with the total airflow of the terminal units shown on the drawings. If a diversity factor was used in equipment selection, set the number of units meeting this factor in the full cooling mode and the remaining terminals in the fully closed position. Distribute the "closed boxes" throughout the system. If this procedure tends to over cool the building, the system supply air temperature should be raised. Check to see if the low airflow conditions create undesirable problems in air distribution.
b) Set outside air dampers to the minimum position and exhaust and/or return air dampers to a position that simulates full load.
c) Index and verify that the fan volume control is set for maximum flow. Observe the amperage of the fan motor for possible overload.
d) Identify the VAV box or terminal unit that is the most critical to the supply air fan airflow and static pressure. Several spot checks should be made. In most systems this unit is usually the most remote.
e) Measure the static pressure at the most critical unit. The entering static pressure at this VAV box should be no less than the sum of the VAV box manufacturer recommended minimum inlet static pressure plus the static pressure or resistance of the ductwork and the terminals on the discharge side of the VAV box.
f) Measure the total system airflow using Pitot tube traverse readings. Airflow totals must be within + 10 percent of design. If this is not the case, make the necessary adjustments prior to continuing on to the next step.
g) Because of terminal unit pressure independent characteristics, it is possible to balance all of the boxes on a system, even if the system pressure is low. When there is inadequate static pressure, index adjacent boxes into the minimum airflow position to increase the static pressure to simulate design conditions. Adjust the static pressure regulating controller until the static pressure is at the point required in item e) above. Usually boxes and outlets of a system can be completely balanced to their design maximum and minimum regardless of fan capacity. This method of simulating or providing adequate static pressure also applies to balancing systems with diversity.
h) With the VAV box set at the maximum flow rate, measure the total airflow being delivered. If necessary, adjust the controller or regulator to deliver the specified airflow following the manufacturer's recommended procedures. When the total airflow is correct, balance the outlets.
i) After the outlets are balanced, set the VAV box for a minimum airflow. Check the total airflow and adjust the minimum setting on the box, when required, following the manufacturer's recommendations. The individual outlets should be re-tested in the minimum position, but they should stay proportionally balanced. It is not unusual, however, for the terminals to be slightly out of balance in the minimum position. This condition, if found, should be reported, but leave the system balanced in the maximum airflow position. The VAV box and its associated outlets are now in balance and they should stay in balance as long as the inlet static pressure to the box stays within the design static pressure range given by the manufacturer.
j) With the system operating on maximum return air and minimum outside air, determine the total return airflow by using Pitot tube traverse readings (if possible) and/or fan characteristics and temperature readings. Adjust the fan speed for design requirements and balance the return air system. A slight negative static pressure reading may be found in the mixed air plenum at this point.
k) Measure the static pressure at the most critical terminal unit (see item e). If required, adjust the static pressure regulating controller at the main supply air sensing station to ensure that adequate static pressure is maintained at the most critical terminal unit.
It must be approximately the same as previously measured. Measure the actual static pressure at the main supply air sensing station. Slowly set the static pressure regulating controller until the fan speed device or vortex dampers begin to change the supply air duct static pressure.
l) Record the final fan performance information on the test report form.
m) A return air fan (if used) should be adjusted to maintain a slightly positive pressure in the building. This may be accomplished by damper adjustment and/or fan speed adjustment.
5. PRESSURE DEPENDENT VAV SYSTEM BALANCING PROCEDURES
Pressure dependent VAV boxes or terminal units have no automatic volume controller to regulate the airflow as the inlet static pressure to the box changes. The airflow delivered by the box for any given condition will change at any time the inlet static pressure changes. Since the airflow delivered is dependent on the inlet static pressure furnished, the VAV boxes are considered pressure dependent.
These VAV boxes may have an inlet balancing damper in addition to an automatic temperature control (ATC) damper. The ATC damper may have limiters to provide for adjustments of the minimum and maximum position. The NEBB TAB Supervisor must realize that every change in damper setting, either manual or automatic, is going to affect the airflow in the system.
A. Non-diversity Systems
Non-diversity systems are balanced similar to constant volume systems.
a) Put all of the VAV boxes and the supply air fan in a full flow condition.
b) Test and adjust the fan to design airflow using methods previously described.
c) Adjust the inlet dampers to each box to obtain the design airflow and verify the operation of the static pressure regulating controller.
d) Retest and adjust the fan airflow for final maximum readings.
e) Test and record the operating static pressure at the sensor that controls the HVAC unit fan, if provided, and verify the operation of the static pressure controller.
f) If a minimum airflow is specified, put the system into a minimum flow mode. Verify that the fan is maintaining a constant static pressure if control is provided.
g) Adjust each VAV box to deliver the correct minimum airflow.
h) Test and record the values of the downstream terminals with minimum airflow.
i) Adjust the outside air/return air dampers and the return air fan (if used) as outlined in paragraphs (j) to (m) above under subsection 4. "Pressure Independent Vav System Balancing Procedures".
B. Diversity Systems
Diversity systems can be the most difficult VAV systems to balance satisfactorily. Any procedure used will be a compromise, and shortcomings will appear somewhere in the system under certain operating conditions. The NEBB TAB Supervisor can expect that some find tuning will be necessary after the initial TAB work is complete.
To eliminate possible misunderstandings later, and agenda with the proposed balancing procedures should be submitted and approved by the system designer or authorized persons before the TAB work is started. This practice is recommended for all jobs, but it is critical on jobs with these particular systems.
Generally, pressure dependent diversity systems are balanced as follows:
a) Put the system into a mode where it will require approximately the same airflow as the maximum HVAC fan design airflow by placing the required number of VAV boxes in a minimum airflow position. Stagger the boxes set at minimum airflow so they all are not in one location or on one zone.
b) Test and adjust the supply air fan to deliver the design airflow with the variable airflow control device set at "maximum." This device may be an inlet damper, discharge damper, variable speed drive, or variable speed motor.
c) After the fan has been set to deliver the system design airflow, set all of the VAV boxes being tested at this time to full airflow.
d) Starting at the supply air fan end of the system, adjust the inlet damper of each VAV box to provide the correct airflow for that box, and then balance its downstream terminal outlets. Do this to each VAV box that has the correct airflow available with the system in this condition. These VAV boxes and their associated outlets are now balanced. Adjust the minimum airflow requirements at this time.
e) The remaining VAV boxes will still be low on airflow due to the diversity in the system. Starting with the boxes that are closest to the design airflow, systematically place adjacent boxes to minimum airflow until just enough air is available to balance the VAV box being tested. Do not close the inlet damper on this box. Leave it wide open. Balance the downstream terminal outlets and then set the box minimum airflow, if provided. Use these same procedures on each remaining box.
f) There may be some boxes that will not produce the required airflow. In this case, proportionally balance the downstream outlets to as high a percentage of their design airflow as possible. Be sure to leave the box inlet damper wide open. During normal operation of the system, when more static pressure becomes available, the required airflow should be delivered.
g) Test and record the operating static pressure at the sensor (if provided). However, some experimentation will probably be necessary with the final setting. On these systems, the operating pressure usually will have to be increased to assure that all of the VAV boxes will have an adequate airflow available during normal operation.
h) Adjust the outside air/return air dampers and the return air fan (if used) as outlined in paragraphs (j) to (m) above under subsection 4. "Pressure Independent VAV System Balancing Procedures".
6. VAV BOXES/TERMINAL UNITS
A typical VAV box or terminal unit is used to modulate primary airflow to satisfy space temperature and airflow requirements. Design of units will vary based upon application and differences among manufactured products. It is important to consult manufacturers' specifications to obtain information regarding performance and operating characteristics.
A. Single Duct Turndown (Shutoff) Boxes
The basic components consist of a plenum box with a primary air inlet, damper or air valve with actuator, and an air discharge. When the VAV box is pressure independent, a primary air velocity sensor and controller also will be included.
Typical balancing procedures follow:
a) Verify fan and control operation.
b) Index the VAV box to maximum (cooling) position. Some boxes may have to be set to the minimum position first.
c) Test total airflow delivered by the VAV box using one of the following methods:
1) Inlet velocity sensor (where furnished).
2) Total of air being delivered from the outlets.
3) Pitot tube traverse of box inlet or discharge.
d) Adjust total airflow using components provided as follows:
Pressure Independent:
Adjust the controller to achieve design maximum airflow. Index the VAV box to minimum (heating) and adjust the controller to achieve design minimum airflow.
Pressure Dependent:
Adjust maximum airflow by using one of the following:
1) Adjust the stops or limiting devices on the actuator operated damper in the VAV box.
2) Adjust the manual volume damper in the primary air inlet or VAV box discharge.
3) Balance with the dampers at the terminal outlets. (Use this method only if no other means is provided. This method may result in excessive noise).
Index the VAV box to the minimum position. Adjust minimum airflow using stops or position limiting devices on the actuator operated damper in the VAV box.
e) After the total airflow is adjusted, index the VAV box to maximum (cooling) position and balance terminal outlets.
B. Fan Powered VAV Boxes
Fan powered VAV boxes are VAV boxes that contain individual supply air fans. The variations of operating sequences are numerous and it is imperative that the manufacturer's data be reviewed. The actual balancing procedures may be included. Otherwise, TAB procedures must be developed based on a possible combination of procedures previously described. Identify the fan powered VAV box application as either a series or parallel type.
1. Series Type, Balancing Procedures (Pressure Independent)
A constant volume fan-powered induction type VAV box mixes primary air with induced air by using a continuously operating fan located in the VAV box discharge (see Figure 6-6). It provides a relatively constant volume of air to the space and comes either with or without an auxiliary heating coil. As more or less cold primary air flows through the unit, ceiling plenum return air is induced to mix with the primary air, providing enough total airflow for a constant discharge. To avoid short circuiting of primary air through the fan terminal into the return air plenum, a balancing device permits the discharge to be adjusted so that total air delivery equals the maximum primary air load requirement. Series type VAV boxes include a primary air velocity sensor and controller.
Typical balancing procedures are listed below:
a) Verify fan controls operation.
b) Index VAV box to the maximum (cooling) position.
c) Using test ports on primary air velocity sensor, determine the primary airflow and adjust to design. In no ports are provided, a Pitot tube traverse of the primary air duct may be used if installation conditions permit.
e) Make preliminary fan speed and/or discharge damper adjustment by obtaining a neutral pressure condition at the return inlet with primary air in the maximum position.
f) Test airflow at the terminal outlets and reset fan speed and/or discharge damper if needed to obtain design airflow.
g) Verify neutral pressure at the return inlet. If air is spilling or inducing, touch up adjustments may be required.
h) Return to the maximum (cooling) position and balance the terminal outlets.
NOTE:
* On series VAV box systems only, volume dampers may be used to restrict airflow if fan airflow cannot be reduced, provided that a noise problem is not created.
* Fan speeds usually are variable by using either solid state speed controls, multi-position switches or wiring selection. Discharge dampers also may be provided.
2. Parallel Type, Balancing Procedures (Pressure Independent or Dependent)
Parallel Flow VAV box components include a plenum box with a discharge outlet, return air inlet with a fan, and a primary air inlet (see Figure 6-7). Primary air enters through an actuated air valve or damper. Return (secondary) air is induced into the plenum by the fan, which is usually equipped with a backdraft damper. Air discharged from the plenum into the system can be either primary air, secondary air or a mixture of both. The most common sequence cycles the fan off when in the cooling mode so that all air is primary air.
When full cooling is no longer needed, the primary air begins to decrease. At a predetermined setpoint, the fan comes on and return air is mixed with the primary air. On full heating, primary air may be completely shut off. Consult project specifications for the specific sequence specified. Most parallel VAV boxes are pressure independent and include a primary air velocity sensor and controller. Heating coils may be provided at the return inlet or at the VAV box discharge.
Typical balancing procedures follow:
a) Verify fan and controls operation.
b) Index the controls to operate the fan with the primary air valve or damper closed.
c) Measure the fan airflow by making a Pitot tube traverse, totaling the terminal outlet readings or return inlet readings.
e) Adjust the fan airflow to design by adjusting the fan speed or dampers., whichever is provided.
f) Index the VAV box to the minimum and set the minimum primary airflow. Use the pressure taps on the inlet velocity sensor or take a Pitot tube traverse of the inlet duct if possible.
g) Verify that the fan cycles on and off at the proper point in the operating sequence. Verify the proper operation of the heating coil if provided.
h) The VAV box now can be indexed to cooling and the terminal outlets can be balanced.
C. Bypass VAV Boxes
In the bypass variable air volume system, the supply fan supplies a constant volume of air at all load conditions and the terminal device diverts air from the supply outlets to the return plenum, thus reducing or varying the volume of air to the zone. Bypass variable air volume terminal units generally are used on smaller systems.
To balance the bypass variable air volume system, the same techniques as discussed above for "Pressure Independent VAV" and "Pressure Dependent VAV" systems are used except that the supply fan does not have airflow or static pressure controls. On the minimum airflow position of the terminal device, the bypass outlet usually is equipped with a volume damper which is adjusted to simulate the same pressure loss imposed upon the terminal device as the downstream duct system.
D. Induction VAV Boxes
Induction VAV boxes use primary air from a central fan system to create a low pressure area within the box by discharging the primary air at high velocities into a plenum. This low pressure area usually is separated from a ceiling return air plenum by an automatic damper. The induced air from the ceiling is mixed with the primary air, so that the actual airflow being discharged form the box is considerably more than the primary air airflow. Most of these induction boxes are designed for VAV operation, but a few are constant volume boxes.
Study the manufacturer's data before attempting to do the TAB work, because many operating sequences are available. Balancing will consist of setting the primary airflow, both maximum and minimum. The discharge air is a total of the primary air and the induced air. Some boxes have adjustments for the induction damper setting. After the box is set, the downstrem air outlets can be balanced in the conventional manner.
7. VAV BOX VOLUME CONTROLS
A. Non-System Powered
Non-system powered VAV boxes are powered by electricity or pneumatic air from an independent source. Controllers may have screw, thumbnuts, potentiometers or sliding-type adjusters to set the VAV box airflow. Electronic direct digital controls (DDC) have greater versatility and are easily interfaced with central ATC/EMS using a dedicated communication terminal.
B. System Powered
System powered boxes are powered by the HVAC duct system inlet static pressure and/or velocity pressure. System powered boxes usually have a higher required minimum inlet static pressure than non-system powered boxes. Although the higher static pressure may not be needed for the airflow quantity, it will be needed to operate the controls. Since most system powered boxes are normally open, it is possible to have VAV box that is delivering the designed amount of air, but the controls will not operate due to low system static pressure at the VAV box inlet.
With a high diversity factor, it is possible to have the HVAC duct system not develop sufficient pressure to activate the VAV box control systems upon startup, as a lock of static pressure leaves all of the boxes wide open. With a system of wide open boxes, static pressure cannot build up until some of the boxes are either manually or automatically closed to allow the rest of the VAV boxes to start controlling and begin close down.
8. COMBINATION SYSTEMS
System applications may incorporate independent VAV boxes and pressure dependent VAV boxes on the same system, either with or without diversity. Balancing procedures will have to be tailored to each job, but it is recommended that the pressure independent boxes are balanced first, since once they are balanced, they will not be affected by changing static pressures as the rest of the system is being balanced.
If a system has many pressure dependent boxes, they may consume most of the system airflow and static pressure on the initial system start up, since they will be wide open. Either set some of these boxes to a minimum airflow position or partially close the inlet dampers on some boxes to build up the static pressure in the system. After setting all of the pressure independent VAV boxes, use the procedures detailed previously for pressure dependent systems and balance the downstream air outlets.
C MULTIZONE SYSTEMS
A multizone unit (Figure 6-10) uses one fan that can blow air through two paths, usually a cooling coil and a heating coil or a perforated plate, before being discharged from the HVAC unit. After the air passes through each coil into a cold air plenum or a hot air (or neutral) plenum, the air then passes through mixing dampers into two or more zone ducts serving various spaces. Each zone duct, usually close to the unit, has a manual volume damper that is used to balance the airflow to each zone. These balancing dampers definitely are required, since the mixing dampers are not capable of controlling the total airflow quantity.
Some unit will not have a heating coil, but will just bypass the return air/outside air mixture when cooling is not needed. Multizone systems normally are balanced with all the zones in the full cooling position.
There are, however, exceptions. The cooling coil may be designed for less air than the fan delivers and the total system requires, because the building normally will not need full cooling in all zones at the same time. This diversity is caused by the sun load of the spaces changing from east to west during the day. It will be necessary to check the manufacturer's data to determine if the cooling coil is sized for full airflow or if a diversity factor has been used. If there is a diversity, set enough zones into full cooling to equal the design airflow of the coil. The remaining air will then go through the heating coil or by the bypass.
During normal operations, there will be some variation in airflow as each zone satisfies its individual requirements. It is also not uncommon for the system to move less air when it is in the heating mode.
If the cooling coil is sized for the full fan airflow, put all zones into full cooling by setting each zone thermostat to its lowest point. The system is then balanced (similar to any low pressure, constant volume system detailed earlier) as outlined below:
a) Adjust and set the fan; put the system into full flow through the cooling coil with allowance for diversity.
b) Make a Pitot tube traverse of each zone and total the results.
c) Make any required fan adjustments to obtain the design total airflow.
d) Adjust each zone damper to obtain the proper airflow in each zone. This should be done by making a traverse if possible. This type of system usually cannot be balanced satisfactorily without zone balancing dampers. If they are not shown on the construction drawings, the NEBB TAB Supervisor should notify the proper people to have them installed.
e) Once each zone has the correct airflow, the terminals can be balanced by using the previously described methods.
D DUAL DUCT SYSTEMS
Dual duct systems (see Figure 6-11) use both a hot air duct and a cold air duct to supply air to mixing boxes. Mixing boxes may operate in a constant air volume mode or in a variable air volume mode. They are usually pressure independent, but they may be either system powered or have external control systems.
1. CONSTANT VOLUME SYSTEMS
Each mixing box has a thermostatically controlled mixing damper to satisfy the space temperature requirements. A mixture of the hot and cold air is controlled to maintain a constant airflow to the space.
The balancing procedures are as follows:
a) Adjust the supply air fan using procedures for constant volume systems (see Section V, Subsection A -- "Basic Air System Testing Procedures"). It is common practice to set all the mixing boxes to their full cold airflow position for setting the fan volume, but first verify that the cooling coil is designed to handle the same airflow as the HVAC duct system. It may be designed for less airflow creating a diversity that will require some mixing boxes to be set in a heating position for a total system flow test.
b) An initial static pressure check should be made at the end of the longest duct run to ascertain that the required minimum static pressures is available. The minimum static pressure at the inlet to the box would be the manufacturer's published static pressure drop across the box added to the static pressure resistance of the downstream duct and terminal air outlets. If the boxes have been present, a low reading at this location usually will indicate a need to increase from the fan. If adequate static pressure is available, proceed with the system TAB work.
c) Pitot tube readings of the airflow quantities should be taken in both ducts (hot and cold) and totaled, even if all the mixing boxes are set on cold. A low airflow reading combined with low static pressure at the farthest box inlet will indicate a need to increase the airflow furnished by the fan. However, low airflow combined with high static pressure can indicate that the boxes may be set at an airflow rate that is too low which holds back the air in the system and develops a higher static pressure.
Testing and adjusting of the constant volume mixing boxes consists of the following:
a) Verify the hot and cold operation by moving the thermostat adjustment back and forth and observing the temperature and volume change of the air.
b) Verify that there is an adequate inlet static pressure before making volume adjustments. If adequate static pressure is not available in either the hot or cold position, put the box into the opposite position where enough pressure is available. Perform Steps c) and d) while the system is in this condition, which will insure adequate static pressure at the boxes. The boxes and terminals will then be in proper balance and will continue to be whenever adequate system pressure is available.
c) Adjust the mixing box for the correct total airflow. The total airflow from the box may be obtained by making Pitot tube traverse readings, totaling the airflow of the outlets or if provided, by pressure tap readings at the box.
d) The terminals can then be balanced.
e) After the system has been satisfactorily balanced, recheck the static pressure at the end of the duct run. If it is much higher than required, the fan should be slowed down to decrease noise, mechanical wear and operating costs.
2. VARIABLE AIR VOLUME SYSTEMS
Each mixing box is thermostatically controlled to satisfy the space and temperature requirements. As the box airflow changes from cold to hot, the quantity of the hot air discharged is increased as the cold air is reduced or shut off. The available sequences are numerous and it is imperative that the NEBB TAB Supervisor review the operating sequence for the individual box being balanced.
Testing and adjusting is similar to a dual duct constant volume system except that VAV capability is incorporated and will have to be taken into account, such as:
a) The mixing box adjustments incorporate a maximum and minimum adjustment and each needs to be adjusted.
b) After the boxes have been set for the correct airflow, the terminals can then be balanced.
It is not possible to cover all of the various operating sequences here, as each may require a different balancing procedure. Consult the manufacturer's data when available and follow the recommended procedure.
E INDUCTION UNIT SYSTEMS
Induction unit systems (Figure 6-12) use high or medium pressure fans to supply primary air to the induction units. Since they usually are located around the perimeter of the building, it is common practice to run many risers up and down the building and feed the units from a common header duct from the risers. As these systems use high or medium pressure, take extra precautions to avoid building up excessive system pressures and causing damage. Check to see that the induction unit dampers, as well as the system dampers, are wide open before starting the HVAC unit primary air fan.
Airflow readings at the induction units are taken by reading the static pressure at one of the nozzles and comparing it to the manufacturer's published data. The design static pressure and airflow will be shown on the manufacturer's submittal data for the various size units on the job.
a) Adjust the primary air fan using previously described methods for constant volume systems. With a new or wide open system, allow for a 10 percent reduction in airflow while balancing.
b) Using a dry type manometer, hose and a probe (which can be a length of metal tubing of the correct diameter), read the static pressure at the nozzle. By comparing it to the unit charts or data, determine the airflow being delivered. By taking readings of the top and bottom unit on each riser, determine which riser dampers will need to be throttled. Assuming that most of the risers are typical, the riser with the higher pressure should be throttled to build up the lower pressure ones, until all the riser static pressures are close to being equal. If the required airflow of each riser varies considerably, take Pitot tube traverses of the riser and adjust them in this manner.
c) Proceed with the first pass to test and adjust each induction unit working from the supply air fan outward., If this is a new wide open system, adjust the units closest to the fan to about 10 percent to 15 percent under design airflow, anticipating some airflow build-up as the rest of the system is adjusted.
d) On the first pass, find any blockages that may be in the system. If units with extremely low or zero static pressure are found, that usually indicates a duct obstruction. Since so many smaller size risers are used, it is quite common to have debris fall down into them while the building is under construction. The debris will usually stop in a transition (reducer) or elbow. Unfortunately, these risers are not usually very accessible and repairs to the system by the installing contractor can be time consuming.
e) Make two to three "adjusting" passes around the system. The final balancing pass should be just a reading pass of the units to record the results.
f) As with all HVAC duct systems, the fan speed should be set so that the fan is delivering the correct airflow to the induction units located at the end of the longest duct run with their dampers wide open. The system balancing dampers should not be closed to where there is excessive static pressure everywhere in the system.
g) Normally the flow of water in induction unit coils is automatically controlled to adjust room temperature. Some systems use the primary air source to power the controls and move a secondary air damper for adjusting room temperature. In such cases, it is extremely important that the manufacturer's minimum static pressure in the plenum of each unit be maintained.
F PROCESS EXHAUST AIR SYSTEMS
Exhaust air systems with hoods are found in various types of ventilating systems. Hoods in restaurants and institutions are the type most frequently encountered in HVAC work. Most kitchen hoods are designed for a face velocity of about 100 feet per minute (0.5 m/s) at the hood entrance. Capture velocity is necessary to insure entrainment of steam and grease laden vapors from the equipment below. Some municipalities have ordinances setting minimum requirements for face or hood perimeter velocities at kitchen hoods, and further require that an authorized representative be present at the time of the balancing work.
1. KITCHEN EXHAUST/MAKEUP AIR SYSTEMS
Kitchen makeup air systems must be in operation when the balancing takes place. Sometime make-up is achieved by means of relief grilles from adjoining areas. The thermal anemometer is a good instrument for measuring these low face velocities. Some swinging vane anemometers (Valometer)can be used at velocities under 100 fpm (0.5 m/s) using the low flow probe. A Pitot tube used with a micromanometer also can be used. When making a Pitot tube traverse of the duct from the hood, be sure to correct for air density if elevated temperatures are present or predicted.
Most kitchen hood exhaust ducts are made of heavy gauge metal, and are covered with a thick fire resistant insulation. A Pitot tube traverse of the duct is the most accurate way to test, but the test holes will need to be plugged with moisture tight, fire resistant metal plugs or caps; and often, holes are not allowed. Avoid putting holes in the bottom of the duct where moisture or grease can accumulate and/or leak out. If possible, put the test holes in the side of a riser. Never use plastic or rubber test plugs in a kitchen exhaust duct. Also, be aware that even if the correct airflow is obtained by the Pitot tube traverse, the hood face velocity may not be sufficient to satisfy local ordinances. In this case speed up the fan if the system designer approves and the fan/drive components can accommodate the increase.
Velocity readings across grease filters are not usually reliable. Accurate free area correction data is not usually available and it would be influenced by the condition of the filters.
2. FUME HOODS
Fume hoods frequently are found in laboratory buildings, hospitals and cleanrooms. See the NEBB "Procedural Standards for Certified Testing of Cleanrooms" for additional information. As experiments are conducted in confined areas, the hoods are designed to prevent the escape of toxic or noxious fumes. Make up air must be provided from the HVAC system or from a separate system that some hoods have built in to minimize the loss of conditioned air from the laboratory. These systems also must be accurately balanced.
When toxic experiments will be performed by the occupants, when permissible, a smoke candle test should be made to ensure that vapors do not escape. Often the whole room is designed to be under a negative pressure, so the room also should be smoke tested if permissible. Some of the more sophisticated hoods have a built in exhaust fan working in series with the system exhaust fan.
Some fume hoods are designed to exhaust the same amount of air with the work door open or closed. Many fume hoods are variable volume and maintain a constant face velocity at the hood opening as the hood door is raised or lowered. The airflow varies to maintain the same velocity through the opening, no matter how far the door is open. Face velocity testing should be done with the door wide open unless otherwise specified. Readings are best taken with a thermal anemometer and should be taken in equal area rectangles similar to a traverse. Be sure to keep well out of the airstream.
A hand held smoke generator should be used to insure that the entire face of the hood is drawing air into the hood. Be sure that air is not swirling and escaping back out of the hood. A 30-second smoke candle should be set of in the hood also. This will insure that the hood will contain the exhaust fumes without escaping back into the space.
Fume hoods are used mostly in laboratories. When balancing laboratories, carefully study the drawings, noting the airflow and pressure differentials between different areas and rooms. Balancing is critical in these areas and will require precise readings and adjustments to obtain the correct positive and negative pressure relationship between space. Even with airflow readings within acceptable tolerances, some adjustments may have to be made to the airflows to obtain the correct pressures. If there is any question, consult with the system designer or the laboratory operator.
3. INDUSTRIAL EXHAUST HOODS AND EQUIPMENT
Industrial exhaust air systems with hoods fall into two categories. One group, similar in many respects to laboratory fume hoods, is used over vats such as dip tanks and plating tanks. Exhaust hoods are often placed at one end above the tank and make up air hoods are placed at the opposite end. This permits vapors to be swept from the tank surface but still leaves the top open for overhead handling equipment. Often an exhaust duct will be connected direct to a piece of equipment with no external hood. Other times, hoods may be used just to remove heat from equipment. Heat recovery systems also are being used more frequently. Here again, makeup air becomes critical and air density must be corrected in calculations.
The balancing procedure is basically the same as any other exhaust air system. A Pitot tube traverse of the exhaust air duct is the preferred method where possible. The differences are mainly in how to test the various inlet openings. If an inlet opening velocity must be measured, obtain the free area opening by measuring it and then calculate what the velocity should be. Quite often this will not be possible due to irregular shapes and/or obstructions.
A digital or thermal anemometer is a very valuable instrument for this type of work as the probe is small enough to get into obstructed places. But here again, review the equipment manufacturer's data, as the procedures for setting up and testing the equipment often may be available.
A second group of industrial exhaust air systems is used to remove and convey solid materials. Sawdust, wood chips, paper trimmings, etc., are transported at high velocities through these exhaust system. These systems must be balanced so that velocities do not fall below predetermined transport velocities below which the materials would drop out.
Balancing of these systems is done with blast gates which are installed in lieu of dampers and are used to temporarily shut off unused branches. In addition to velocity readings, static pressure readings of the pressure differential between the room and the hood should be recorded in a convenient reference point at each hood or intake device. This will permit easy future checks designed to spot any deviation in exhaust volumes from original volumes.
SECTION VII HYDRONIC SYSTEM TAB PROCEDURES
A HYDRONIC SYSTEM MEASUREMENT METHODS
1. BASIC BALANCING METHODS
The best possible method for flow measurement of hydronic systems cannot be determined without reviewing the systems. There are five basic methods available for measuring the flow quantity in a piping system:
a) with flow meters,
b) with calibrated balancing valves,
c) using the equipment pressure loss,
d) by heat transfer, and
e) using pump curves
It is preferable to balance hydronic systems by direct flow measurement. This balance approach is very accurate because it eliminates compounding errors introduced by the temperature difference procedures. Balance by direct flow measurement allows the pump to be matched to the actual system requirements (pump impeller trim). Proper instrumentation and good preplanning is needed. Water flow instrumentation must be installed during construction of the piping system; they can consist of all or a combination of the following:
a) System components used as flow meters -- Control valves, terminal units, chillers, etc.
b) Flow meters -- Venturi, orifice plate, and pitot tube
c) Pumps
d) Flow limiting devices and balancing devices
System circumstance often dictates a combination of flow and temperature balance. In many cases, it may not be economically sound or even necessary to install flow indicating devices at every terminal. For example, in reheat, induction, and radiation systems, temperature readings can be used to set the flow. Branch piping and risers should still be set with primary flow measuring devices. The water balance is undertaken using all the pressure measuring methods available and verified by total heat transfer using air and water temperature readings. The pressure readings provide the necessary accuracy for a good balance only if verified by a heat balance.
2. USING FLOW METERS
A flow meter usually is deemed to be the most reliable method for measuring the system flow. Flow meters usually are permanently installed in the hydronic piping system and are used for the measurement and adjustment or flow to pumps, to primary heat exchange equipment, at each zone, and at terminal units. Flow meters such as the venturi, orifice, and annular types, require the use of a differential pressure gauge and flow charts provided by the manufacturer to calculate the system flow. Always verify that installation of the flow meters is in accordance with recommended practices given by the manufacturer. There must be adequate amounts of straight section of piping upstream and downstream from the flow meter to prevent erroneous readings affecting final system balance.
Note: Verify that the pressure units of the differential pressure gauge and the pressure units found on the flow charts provided by the manufacturer are identical. If pressure units are not the same, (i.e. psi, in. w.g., ft.w.g., Pa, mm.w.g., m3h) pressure conversions will be required.
3. USING COMBINATION VALVE/FLOW METERS
Two types of these combination valves are being used, self adjusting and field adjusted.
A self-adjusting valve/flow meter utilizes internal mechanisms that constantly change internal orifice openings to compensate for varying system differential pressures while maintaining a pre-set flow rate. No external adjustment is available with this device. Pressure taps, providing measurement of valve differential pressure, allow measurements of the system flow.
Calibrated balancing valve/flow meters are field adjustable devices. Pressure loss of the valve is measured similar to that of a flow meter. A chart or graph, provided by the valve manufacture, indicated actual flow rates at various valve positions and differential pressures. Unlike a flow meter, the flow coefficient of a calibrated balancing valve/flow meter changes with adjustment of the valve. Always be aware of the actual valve position when calculating the system flow.
4. EQUIPMENT PRESSURE LOSS METHOD
Actual system flow rates may be established by using HVAC equipment pressure loss calculations, provided the following two items are available:
a) certified data from the equipment manufacturer indicating rated flow and pressure losses;
b) and an accurate means for determining the actual equipment pressure losses.
When the design criteria of the equipment and the actual pressure loss is known, the flow rate may be calculated by using the equation:
Flow 2 = flow, delta P 2/ delta P 1
5. HEAT TRANSFER METHOD
Approximate flow rates may be established at heating and cooling terminal units by using both air and hydronic measured heat transfer data and the proper equations. Although the equations used are slightly different, both are based upon the First Law of Thermodynamics (heat transfer). Each determines the total heat transfer rate of the terminal unit at the time of testing, and then the flow rate is calculated based upon the fluid heat transfer rate (water temperature difference). See Section IX for the correct equations.
However, at less than full load conditions, temperature difference balancing procedures could provide hydronic flow rates varying from 60 to 175 percent of design under circumstances of an actual 50 percent load when airflow readings are + 10 percent accuracy.
6. USING PUMP CURVES
Flow may be established at circulating pumps through a series of differential pressure testing and pump curve analysis. When circulating pumps are installed in series with any item of HVAC equipment, the equipment flow rate may be assumed to be equal to that of the pump.
B BASIC HYDRONIC SYSTEM PROCEDURES
Section III -- "Preliminary TAB Procedures" outlined the preparation work that must be done prior to the actual testing, adjusting, and balancing of the HVAC systems. Confirm that these preliminary procedures have been completed and check lists prepared. Do no attempt to balance a hydronic system before the installation has been completed and all of the air systems have been balanced.
The following balancing procedures are basic to all types of hydronic distribution systems:
a) Confirm that all necessary electrical systems temperature control systems, all related hydronic piping circuits and all related duct systems are functional and that any necessary compensation for seasonal effects have been made.
b) Verify that all hydronic systems have been cleaned, flushed, refilled and vented as required.
c) Verify that all manual valves are open, or preset as required and all temperature control (automatic) valves are in a normal or desired position.
d) Verify that all automatically controlled devices in the piping or duct systems will not adversely affect the balancing procedures.
e) With the pump(s) off, observe and record system static pressure at the pump(s).
f) Place the systems into operation, check that all air has been vented from the piping systems and allow flow conditions to stabilize. Verify that the system compression tank(s) and automatic water fill valve are operating properly.
g) Record the operating voltage and amperage of the pump(s) and compare these with nameplate ratings and thermal overload heater ratings. Verify the speed of each pump.
h) If flow meters or calibrated balancing valves are installed, which would allow the flow rate of the pump circuit(s) to be measured, perform the necessary work and record the data.
i) With the pump(s) running, slowly close the balancing cock fully in pump discharge piping and record the discharge and suction pressures at the pump gauge connections. Do not fully close any valves in the discharge piping of a positive displacement pump. Severe damage may occur.
Using shut-off head, determine and verify each actual pump operating curve and the size of each impeller. Compare this data with the submittal data curves. If the test point falls on the design curve, proceed to the next step; if not, plot a new curve parallel with other curves on the chart, from zero flow to maximum flow. Make sure the test readings were taken correctly before plotting a new curve. Preferably one gauge should be used to read differential pressure. It is important that gauge readings should be corrected to center line elevation of the pump.
j) Open the discharge balancing cock slowly to the fully open position; record the discharge pressure, suction pressure and total head. Using the total head, read the system water flow from the corrected pump curve established in step i. Verify the data with that from flow meters and/or calibrated balancing valves if used (see step h).
If the total head is higher than the design total head, the water flow will be lower than designed. If the total head is less than design, water flow ill be greater; in which case the pump discharge pressure should be increased by partially closing the balancing cock until the system water flow is approximately 110 percent of design. Record the pressures and the water flow. Check pump motor voltage and amperage and record. This data should still be within the motor nameplate ratings. Start any secondary system pumps and readjust the balancing cock in the primary circuit pump discharge piping if necessary. Again record all readings See Subsection E of this section for the balancing of primary-secondary systems.
k) If orifice plates venturi meters or other flow measuring or control devices have been provided in the piping system branches, an initial recording of the flow distribution throughout the system should be made without making any adjustments. After studying the system adjust the distribution branches or risers to achieve balanced circuits as outlined above. Vent air from low flow circuits. Then proceed with the balancing of terminal units on each branch.
l) Before adjusting any balancing cocks at equipment (i.e. chillers, boilers, hot water exchangers, hot water coils, chilled water coils, etc.) take a complete set of pressure drop readings through all equipment and compare this with submittal data readings. Determine which are high and which are low in water flow. Vent air from low flow circuits or u nits and retake readings.
m) Make a preliminary adjustment to the balancing cocks on all units with high water flow, setting each about 10 percent higher than the design flow rate.
n) Take another complete set of pressure, voltage and ampere readings on all pumps in the system. If system total flow has fallen below design flow, open the balancing cock at each pump discharge to bring the flow at each pump with 105 to 110 percent of the design reading (if pump capacity permits).
o) Make another adjustment to the balancing cocks on all units which have readings more than 10 percent above design flow in order to increase the flow through those units with less than design flow.
p) Repeat this process until the actual fluid flow through each piece of equipment is within plus or minus 10 percent of the design flow.
q) Make a final check of the pressures and the flow of all pump and equipment; of the voltage and amperage of pump motors; and record the data.
r) Where three-way automatic valves are used, set all bypass line balancing cocks to restrict the bypassed water to 90 percent of the maximum demand through coils, heat exchangers and other terminal units.
s) After all TAB work has been completed and the systems are operating within plus or minus 10 percent of design flow, mark or score all balancing cocks, gauges, and thermometers at final set points and/or range of operation.
t) Verify the action of all water flow safety shut-down controls.
u) Prepare all NEBB TAB report forms and submit as required using the guidelines provided in Section IV.
C PIPING SYSTEM BALANCING
Many systems use combinations of the piping applications outlined below. As it is necessary to apply balancing procedures correctly, a procedure (or system) may be required to be broken down into several steps that correspond to the source, outlet and piping.
All of the balancing procedures outlined below have two things in common:
a) Balancing of a forced circulation system starts at the pump. Pump testing and adjustment, as described in Subsection B, must be done prior to any adjustments to system piping or terminal units.
b) System terminal units are maintained in the full flow position (i.e. control valves open to coil and closed to bypass) during the entire balancing procedure.
1. ONE-PIPE SYSTEMS
a. Series Loop
Upon adjustment of the source flow rate, balancing of a series loop is accomplished with the use of one balancing device located in the loop. Balancing valves are normally located at the end (return) of a series loop piping arrangement. Adjustment of more than one device within a loop will directly affect the flow and heat transfer of other terminals in the loop. Total loop flow is the primary concern in the adjustment of series loop systems.
b. One-Pipe (Monoflow)
Upon adjustment of the source flow rate, balancing of a one-pipe (single main) system is initiated with the first terminal unit supplied by the source. Adjustment of individual terminal unit flow rates may be accomplished by any of the acceptable methods utilizing a balancing device normally located in the return piping. Adjustment of the system continues sequentially from the first to the last terminal unit served.
2. TWO-PIPE SYSTEMS
a. Direct-Return
Upon adjustment of the source flow rate, balancing of a two-pipe, direct-return system is initiated with the first terminal unit supplied by the source. Adjustment of terminal unit flow rates may be accomplished by any of the acceptable methods as outlined in the previous subsection. Adjustment of system terminal units continues sequentially from the first to the last terminal unit served. Several passes of the system terminal units may be required to achieve balanced conditions. Therefore, on the first pass it is recommended that flow to the first 1/3 section of the system should be adjusted 10 percent below the design requirements. The reasoning for this is that the system differential pressure will increase as terminal unit flow rates are adjusted. On the second pass of balancing, flow rates for the first section of terminal units usually will have increased, but they should remain within acceptable tolerances.
b. Reverse-Return
Adjustment and balancing of two-pipe reverse-return systems is usually much easier due to the inherent equal system pressure differential resulting from the piping arrangement. For Example, a properly sized reverse-return piping system serving ten terminal units of identical capacity and resistance may require adjustment to the total system flow rate only. Flow will then be equally distributed to the terminals as a result of the piping application. However, many reverse-return systems do not contain identical terminal units. A review of the terminal units with specific attention to unit resistance should be made. Adjust the terminal flow rates starting with the units of lease resistance and work toward the units with the greatest resistance.
3. THREE-PIPE SYSTEMS (SUMMER-WINTER)
Due to the nature of this unusual piping application, the heating and cooling water systems may be viewed as diversity operations. In other words, the required terminal unit flow rate may vary with actual load conditions. Balancing should e performed on a system in a maximum load condition. Therefore, source and terminal unit adjustment should be accomplished with all terminal units (or as many as stipulated by diversity procedure) in the wide-open position.
Although connected to one another, the cooling and heating piping systems should be balanced as independently as possible with the given system conditions. Be aware that incorrect piping of automatic temperature control valves or improper system pressurization may result in extreme difficulties while performing the balancing procedures. "Crossed piping" and/or incorrect valve applications are another common installation problem.
4. FOUR-PIPE SYSTEMS (SUMMER-WINTER)
Typical four-pipe systems are simply two independent two-pipe systems and should be addressed accordingly. Four pipe systems may also be seen as the heart of "summer-winter systems". They are reviewed in the following Subsection E.
D BALANCING SPECIFIC SYSTEMS
The basic steps previously outlined in Subsection B form the foundation for balancing any hydronic distribution system. In this subsection, additional or special balancing procedures are outlined for use in balancing specific types of hydronic distribution systems. All equipment such as boilers, chillers, compressors, etc., shall be started by, and operated under, the supervision of the responsible contractor or the designated authority.
1. CHILLED AND/OR HOT WATER EQUIPMENT
a) Flow through chillers, HVAC unit coils and heat exchangers should be measured by using flow meters or calibrated balancing valves if installed. Otherwise use the equipment manufacturer's certified pressure drop tables and curves or use the pressure drop characteristics of automatic control valves. If three-way control valves are used, measure the pressure difference with full flow both through the coil or unit and the bypass. Set the bypass lien balancing cock to maintain a constant pressure with the control valve in either position.
b) When fan-coil units or induction units are used with a direct return piping system, flow measurements for each unit should be made, either by using calibrated balancing valves, by taking pressure readings across each coil, from pressure readings across each automatic water valve, or (as a last resort) from water or air temperature readings.
When a reverse return piping system is installed, a flow measurement should be made at each set of risers to make sure that all units are getting the correct flow of water to provide a fairly uniform water temperature drop. All automatic water valves must be open and coils must have the rated airflow when measurements are being made.
c) After all of the fan-coil type units have been put into operation with all automatic valves fully open and full flow through the coils, take the entering and leaving water temperatures of all chillers, boilers, heat exchangers and coils. Record and compare with design conditions.
d) When units or systems have multiple coil sections, where possible, balance the water flow by establishing the design water pressure drop across each coil. A less accurate method of balancing multiple coil sections involves reading the water temperatures at each coil section with insertion thermometers or contact pyrometer probes, and adjusting the balancing cocks until uniform temperatures are obtained.
e) Complete the TAB procedures by recording the required data on NEBB TAB report forms for submittal.
2. COOLING TOWER SYSTEMS
a) With the system off, confirm that the water level in the tower basin is at the correct level and that the piping system has been cleaned and flushed. On towers with variable pitch fan blades, verify that the setting of the blades is correct for the test conditions.
b) With pump(s) off, observe and record the system static pressure at the pump(s).
c) Place the system into operation and allow the flow conditions to stabilize. Check the operation of the water makeup valve and blowdown.
d) Record the operating voltage and amperage of all fan and pump motors and compare these with nameplate ratings and thermal overload heater ratings.
e) Record the speed of each pump.
f) With the pump(s) running, slowly close the balancing cock in each pump discharge line and record shutoff discharge and suction pressures at the pump gauge connections. Do not use this method if a positive displacement pump(s) is used. Using the shutoff head, determine (and verify) the actual pump operating curve and the size of each impeller. Compare this data with the submittal data curves. If the test point falls on the design curve, proceed to the next step; if not, plot a new curve parallel with other curves on the chart, from zero flow to maximum flow. Make sure the test readings were taken correctly before plotting a new curve. Preferably a single gauge should be used to read differential pressure. It is important that gauge readings be corrected to center line elevation of the pump.
g) Establish a uniform water distribution within thee tower where possible, and check for clogged outlets or spray nozzles. Check for vortex conditions at the tower condenser water suction connection.
h) Record the inlet and outlet pressures of the condenser(s) and check against the manufacturer's design pressure difference.
i) When a three-way control valve is used in the condenser water piping at the tower, measure the pressure difference with full water flow going both through the tower and/or through the bypass line. Set the bypass line balancing cock to maintain a constant pressure at the pump discharge with the control valve in either position.
j) Start the tower fan(s) and check rotation, gear box, belts and sheave alignment. Measure and record fan motor amperes, voltage, phase and speed.
k) Take the inlet and outlet air dry bulb and wet bulb air temperature readings. Take test readings continually with a minimum of time lapse between readings. Note wind velocity and direction of the time of the test.
l) Take the inlet air temperature readings between 3 and 5 feet (0.9 and 1.5m) from the tower at all inlets. These readings shall be taken halfway between the base and the top of the inlet and then averaged.
m) If the cooling tower has a ducted inlet or outlet, make a Pitot tube traverse of the duct to verify the airflow.
n) If verification of the HVAC refrigeration equipment data is included in the TAB specifications, have the refrigeration system started. Verify the head and suction pressures and compare with design. After operation stabilizes under a normal cooling load, measure and record the condenser water inlet and outlet temperatures. Observe and record the percent of load on the compressor where possible.
o) After setting the three-way control valve (to control head pressure) in the condenser water line (step i), verify and record that it operated to maintain the correct head pressure by varying the flow at the tower. On units that have a fan cycling control, verify that the fan cycles to maintain design condenser water temperature. If fan inlet or outlet damper controls are used, verify that the dampers modulate to maintain the design condenser water temperature leaving the tower.
p) Make another complete set of pressure, voltage and ampere readings at the pump(s). If the pump(s) capacity has fallen below design flow, open the balancing cock(s) at the pump discharge to bring flow within 5 to 10 percent of the design reading, if possible.
q) Make final measurements of all pump, fan and equipment data, and record on the TAB report forms.
r) After all balancing work has been completed and the system is operating within plus or minus 10 percent of design flow, mark or score all balancing cocks, gauges, and thermometers at final set points and/or range of operation.
s) Verify the action of all water flow safety and shutdown controls.
t) Prepare all NEBB TAB report forms and submit as required.
3. BOILERS
a) Verify that the boiler(s) and/or system has been cleaned, flushed, and started; that all safety and operating controls have been tested, adjusted and set; and that the burner(s) is operating properly.
b) With the boiler(s) operating under normal conditions, check the following:
* Boiler feed pump(s) or makeup water system(s) and compression tank operation.
* Boiler, burner and pump nameplate data.
* Boiler control settings (operating pressures and temperatures).
* Water flow rates and inlet and outlet temperatures (hot water boilers).
* Steam boiler water level proper and steady.
c) On initial runs, hot water systems normally require additional air venting. Confirm that automatic air vents are operating and vent air manually as required.
d) Coil and main drip steam traps can be checked for proper operation with a surface contact thermometer.
e) Confirm that all automatic temperature control valves and steam pressure reducing valves in the system have the proper setting or mode of operation for the TAB work.
f) Confirm that all pipe strainers are clean.
g) The distribution of steam systems is set by the piping design and layout; therefore, no field balancing is required.
h) Follow the basic TAB procedures for hot water systems described above.
i) Prepare TAB report forms and submit as required.
E TAB PROCEDURES FOR OTHER APPLICATIONS
1. VARIABLE VOLUME FLOW
A. Heating Systems at Reduced Flow Rates
The typical heating only hydronic system often operates satisfactory at reduced flow because of the water flow/heat transfer relationship, as shown in Figure 7-4.
A decrease in terminal unit flow rate to 50 percent of design requirement still allows about 90 percent of heat transfer capability. The reason for the relative insensitivity to changing flow rates is that the governing coefficient for heat transfer is the air side coefficient. A change in internal or water side coefficient with flow rates does not materially affect the overall heat transfer coefficient. This means that (1) load ability for water-to-air terminals is basically established by the mean air-to-water temperature difference, (2) a high order of design temperature difference exists between the air being heated and mean water temperature in the coil (a substantial change in mean water temperature is necessary before terminal load ability is measurably changed), and (3) a substantial change in the mean water temperature (load ability) requires a very substantial change in water flow rate. A reduction in terminal heating capacity caused by an inadequate flow rate often can be overcome by simply raising the system supply water temperature. Designing near the upper temperature limits [250o F (121o C) for low pressure code] does not allow the temperature increase safety factor to be applied.
The previous comments apply to allowable flow variation for heating terminal units selected for a 20o F (11oC) temperature drop (st) and the general order of 200oF (93o C) supply water temperature. Changes in design supply water temperature and design temperature drop affect permissible flow variation. When 90 percent terminal capacity is acceptable for a system application, the flow variation can be approximated, as shown in Figure 7-5.
Note that heating system tolerance to unbalance decreases with increases in the design st and with decreases in supply water temperature. As a general rule, however, system tolerance to flow rates less than design is important.
B. Cooling Systems at Reduced Flow Rates
Chilled water terminal units are much less tolerant to flow variation. This illustrated in Figure 7-5, which compares chilled and heating terminals for flow reduction that will establish 90 percent of design heat transfer capacity.
Many dual-temperature changeover systems are completed and first started during the heating season. Reasonably adequate heating ability in all terminals may suggest that the system is balanced adequately. As shown in Figure 7-5, 40 percent of design flow through the terminal provides 90 percent terminal design heating with about 140o F (60o C) supply water and a 10o F (5.6o C) st. Increased supply water temperature establishes the same heat transfer at terminal flow rates of less than 40 percent design.
The majority of dual-temperature systems establish a decreased flow during the cooling season because of the introduction of chiller pressure drop against the distribution pump.
The flow reduction can reach 25 percent, meaning that during chiller operation, a terminal that original heated satisfactorily could receive only 30 percent of the originally defined design flow rate.
Under these circumstances, TAB work will become mandatory during spring chilled water startup. Balancing procedures therefore must be related to the least tolerant system application or the cooling season. The procedures follow below under "3--Summer-Winter Systems".
The major reason for lessened tolerance of chilled water terminals to decreased flow is that the air-to water-temperature difference is much less than that with the heating cycle.
The general change for chilled heat transfer with changes in water flow rate is shown in Figure 7-6. The curves shown are based on ARI rating points: 45o F (7.2o C) inlet water at a 10o F (5.6o C) rise with air at 80o F (26.7o C) DB and 67o F (19.4o C) WB.
Table 7-1 Load-Flow Variations
%Design Other Load, Order of %
Flow at
Loan Type 90% Load Sensible Total Latent
Sensible 65 90 84 58
Total 75 95 90 65
Latent 90 98 95 90
Dual temperature systems are designed to chilled flow requirements and often operate on a 10o F (5.6o C) temperature drop as full-load heating.
The basic curve applies to catalog ratings for lower drybulb temperatures, provided a consistent entering air moisture content or vapor pressures is maintained [e.g., 75o F (23.9o C) DB, 65o F (18.3o C) WB]. Deviation from the curves shown is to be expected with changes in inlet water temperature, temperature rise, air velocity, and DB and WB conditions. Figure 7-6 should be considered only as a general representation of variable change, not as a fact that applies to all chilled water applications.
If the chilled water terminal is matched to the load, the load variation to 90 percent design can be interpreted to three flow variations, as shown in Table 7-1. Note that load-flow variation for Figure 7-5 is stated for total load.
Table 7-1 and Figure 7-6 illustrate that the first loss with reduced chilled terminal flow rate is latent capability. Table 7-1 defines that permissible flow variation from design will be related to the following application requirements: (1) when high latent capability is needed, operational terminal flow rate must substantially meet design flow and (2) the application where sensible load control is predominant provides for a much wider terminal flow tolerance.
C) Variable Speed Pump Curves
Figure 7-7 shows pump curves from 50 percent to 100 percent capacity using a variable speed drive. Several efficient plot points A, B, and C are used as "system curve" start points to describe variable speed pump operation curves. The intersection of the percent base speed pump curves with the system "control curve" can be determined as points 1 through 5 in figure 7-7. Each intersection point states a pump, flow, head, and efficiency point. These points, in combination with variable speed drive efficiency, permit an indication of the power draw for the pump drive combination at the flow points illustrated.
D) Balancing Variable Volume Systems
Variable volume systems have the following characteristics:
a) A system diversity is usually present (required terminal flow rate exceeds pump and primary heat exchange unit).
b) Two-way control valves are utilized at terminal units creating a variable system flow rate or pressure differential.
c) Some form of differential pressure control (such as variable speed pumping) normally is used to maintain system differential pressure requirements.
Variable volume systems are tested with simulated full load system conditions. This procedure usually requires the temporary isolation of portions of the system piping and terminal units. When circulating pump and terminal unit capacities are within acceptable tolerances, terminal unit balancing may be performed in accordance with procedures stipulated by the piping configuration of the system. Upon completion of balancing procedures with a portion of the system isolated, the isolated units are then opened and an equal capacity of units closed. Units isolated for the initial balancing procedure are than balanced to design flow rates.
Specific units and procedures involved in the diversity balancing procedure should be delineated in an agenda for approval prior to initiating field testing.
2. PRIMARY-SECONDARY SYSTEMS
Primary-secondary systems (Figure 7-8) may appear to be too complex when first reviewed, but a proper system analysis will result in a relatively simple balancing procedure. First, address the primary loop. The source (primary pump) may supply outlets (primary bridges) in any of the possible piping arrangements described previously. The duty of the primary pump is to supply proper circulation to the primary bridges and return water back to the source. Initial balancing should therefore be restricted to the primary loop and it's components. Note that secondary systems should be in full flow operation during primary loop balancing.
Upon adjustment of primary pump flow rate, primary bridge piping is adjusted using a procedure applicable to the piping arrangement of the loop. When primary loop flow rates have been adjusted to design quantities, testing of the secondary systems may begin. Testing of each secondary system should be accomplished independently with procedures applicable to the piping arrangement of the secondary loop.
3. SUMMER-WINTER SYSTEMS
Characteristics of summer-winter piping applications as stated above in "1-Variable Volume Flow," dictate that initial system testing and balancing be accomplished in the summer mode of operation. Design terminal unit cooling flow rates are usually much greater than that required for heating. As the terminal unit may only be adjusted to satisfy one flow rate, that flow rate must be the greatest required, normally that of the cooling application. The system piping should be analyzed and set to accommodate the requirements of summer operation prior to pump testing. Insure that no bypasses are open and that summer-winter changeover valves (manual or automatic) are functional and open to the cooling mode of operation. Proceed to test and balance the pump(s), test and balance the terminal units of the system in accordance with recommended procedures outlined for the piping application.
Upon completion of system balancing in the cooling made of operation, switch the system operation over to the heating mode. Balance applicable pumps and equipment unique to the hot water piping without disturbing the valve settings accomplished during the summer mode balancing procedure.
If necessary, balancing of summer-winter systems may be accomplished in the winter mode of operation provided system pump and terminal units are set to design chilled water flow rates.
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