Penn State College of Engineering
TABLE OF CONTENTS
EXECUTIVE SUMMARY………………………………….. Page 2
ARCHITECTURE AND BACKGROUND…………………. Page 3
MECHANICAL……………………………………………… Page 8
ELECTRICAL……………………………………………….. Page 24
CONSTRUCTION MANAGEMENT ………………………. Page 28
SUMMARY AND CONCLUSIONS………………………… Page 33
APENDICES………………………………………………….. Page 36
EXECUTIVE SUMMARY
This thesis is a yearlong case analysis of the building systems of an existing laboratory located in Fort Worth, Texas. The 220,560 sq. ft. facility contains the fourth chiller plant on the corporation’s grounds. The plant was designed with the capability of future integration with the existing 44°F chilled water campus loop. Currently, the chiller plant services the chilled water demand of the building with the standard constant-flow primary/secondary variable speed pumping arrangement. This report summarizes the impact of converting the plant from constant primary/variable secondary flow to a variable primary flow chilled water configuration.
Currently, the system has four chillers. Three of the chillers operate at 2400 GPM and the other chiller was designed for off peak cooling at 1480 GPM. Since in a variable primary flow system, the flow rate can be reduced to 40% of the design flow or 960 GPM, the fourth 1480 GPM chiller was eliminated from the system.
Constant primary flow/variable secondary flow configuration consumes more pump energy than variable primary flow by maintaining a constant flow through the chiller. Variable primary flow pumping systems save pump energy by operating at lower system head and varying the flow rates through the chillers. At a flat electricity rate of $0.055 per kWh, the total energy savings for a variable flow primary system compared to a primary-secondary system converts into an annual operating cost savings of $18,112.
Converting the chilled water system from a traditional constant primary/variable secondary flow arrangement to a variable primary flow arrangement affects the electrical and mechanical first costs. The lower first cost for variable primary flow pumping is a result of reducing the plant size, equipment costs, labor costs, and material costs. The total cost savings including the cost of the chiller for a field fabrication application $241,637 and for a prefabrication system is $245,757.
BACKGROUND
Primary Project Team
❖ Owner
• Texas Laboratories, Inc is the owner expanding their current laboratory areas.
❖ Design
• Ewing Cole Cherry Brott completed the mechanical, electrical, structural, plumbing, fire protection, and lighting design.
❖ Construction Management
• Austin Commercial, Inc.
❖ Site Work
• Rone Engineers preformed all the site evaluation studies such as soil testing and surveying.
Building Statistics
❖ Dates of Construction
• Construction began August 2002 and is scheduled to be completed by October 2003.
❖ Cost Information
The guaranteed maximum price for the building is approximately 45 million dollars.
❖ Building Function
• The building functions as a laboratory that performs pharmaceutical and medical device research. Research includes the development of eye drops and contact lens solutions.
❖ Location and Site
• The building is located on the North side of the existing Texas Laboratories, Inc. facility in Fort Worth, Texas.
• The site was previously a parking lot that serviced the employees working in an adjacent building.
• The building area was excavated to the elevation which involved a cut greater than ten feet in some areas.
❖ Architecture
• The façade is a combination of brick and glazed curtain wall with varying sizes of square tinted windows.
• The conference rooms have full skylight ceilings that extend into the corridors.
• Since the laboratory replaced an existing parking lot, a corridor was incorporated into the design to link the new parking lot to the adjacent building. The hallway was added to the design to shield employees walking to work from the hot climate.
• The planned building elevation will match the elevation of the existing buildings immediately to the west.
❖ Project Delivery System
• The project is being completed under a traditional design-build method.
[pic]
Building Systems
❖ Mechanical
• The building is divided into six zones to meet the varying air requirements. See Figure 1 for zone division.
• There are 10 air handling units total. Six of the five air handling units service the laboratory areas. Since these units use exhaust 100% of the air, heat pipes are used to recover energy from the exhaust air to the supply outside air. The other four air handling units condition the cage wash area, chiller room, mechanical mezzanine, and office areas.
• The building contains it own chiller plant with three 1300 ton water cooled centrifugal chillers and one 800 ton chiller. Condenser water is cooled through four induced flow cooling towers.
• In the laboratory area, negative air pressures are maintained in the lab and positive air pressures are maintained in the hallways.
❖ Structural
• The structural system is steel frame with vertical cross bracing.
• The floor system is concrete slab on metal deck construction.
• Lateral moment connections account for wind and seismic factors
❖ Lighting
• Lab areas use 8” diameter aperture incandescent downlights and 2x2 recessed fluorescents with 0.125” acrylic lens.
• Offices are lighted with 4” linear (2) lamp fluorescent indirect pendants.
• Animal holding rooms have dimming control systems
❖ Fire Protection
• The building is equipped with a five zone sprinkler system.
• Steel beams are encased with 1 ½” fiber board for two hour fireproofing protection
• The building is split into zones which are divided by two hour fire walls.
❖ Transportation
• The building has one 13’x9’ freight elevator located at the northwest section of the building.
• The second floor is primarily the mechanical mezzanine therefore; there are only two stairways on opposite sides of the building connecting the two floors.
❖ Telecommunications
• The data communications room is located in the mechanical mezzanine.
• All the fire protection, security, control systems, and telecommunications are wired to the data communications room.
MECHANICAL
Design Criteria
❖ Plant Integration
• The building is a part of a series of facilities on the Texas Laboratories, Inc. Campus. Building E contains the fourth chiller plant on the grounds. The plant was designed with the capability of future integration with the existing 44°F chilled water campus loop.
❖ Redundancy
• To avoid the loss of costly research due to the malfunction of the mechanical components, system redundancy was a priority to the owner
• The plant is designed to handle the peak cooling demand in the facility without the energy recovery of the heat pipes or the post cooling. During the design of the six laboratory air handling units, post-cooling coils were added to the design at the request of the owner. The post cooling coils are serviced by a separate chilled water loop than the pre-cooling coils. Four air cooled glycol chillers located outside to the west of the main plant service the post-cooling coils.
❖ Future Expansion
• In the future, the plant must have the capacity to provide chilled water to the adjacent building and another small expansion.
❖ System
• Building requires multiple systems to satisfy the varying air requirements of each zone. Refer to Table 2, to review each space requirement and Figure 1.to view zone spaces.
• Design Outdoor Air Temperatures are from ASHRAE Fundamentals. See Table1 for requirements.
|ASHRAE FUNDAMENTALS OUTDOOR AIR CONDITIONS |
|Winter |Summer |
|Design |Design Dry Bulb and |Mean |Design |
|Dry Bulb |Wet Bulb |Daily |Wet Bulb |
|99% |97.50% |1% |2.50% |5% |Range |
|ZONE |Function |Maximum |Air |Design Temp. |Design Temp. |
| | |CFM |CFM |°F |°F |
|1 |Laboratory |175,000 |175,000 |66 |66 |
|3 |Cage Wash |20,000 |20,000 |72 |72 |
|4 |Offices |35,000 |7000 |75 |75 |
|5 |Chiller Plant |7000 |4500 |80 |65 |
|6 |Mech. Mezz. |30,000 |3000 |80 |65 |
Table 2. Texas Laboratories, Inc. zone design air flow rates and temperature requirements
[pic]
Figure 1. Texas Laboratories, Inc. zone space requirements
Existing Conditions
Constant Primary/Variable Speed Secondary Chilled Water Plant Design
❖ Chillers
• (3) 1400 ton water cooled centrifugal chillers
o (1) for standby
o (2) for peak load cooling
• (1) 800 ton water cooled centrifugal chiller
o Part-load cooling
❖ Primary Constant Speed Pumps
• (3) 2400 GPM, (1) 1480 GPM Horizontal Splitcase
o 50 HP motor
o 55 ft. Head
❖ Secondary Variable Speed Pumps
• (3) 4800 GPM Horizontal Splitcase
o 200 HP motor
o 115 ft. Head
❖ Decoupler
• 14” diameter pipe
o Connects return and supply chilled water piping
o Installed Flow meter signals chillers on and off
❖ Sequence of Operation
Texas Laboratories supplies chilled water to the load with a constant-flow primary/secondary variable speed pumping arrangement, show in Figure 2. The system operates by delivering variable flow chilled water to the cooling coils while maintaining a constant flow rate across the chillers. To maintain a constant flow rate across the chillers, the unused chilled water from the chillers is bypassed. An installed flow meter in the decoupler senses direction of the flow controls the sequencing of the chillers. When the flow through the decoupler is reversed and the return system water mixes with supply chilled water, the system signals another chiller to energize. As the cooling load decreases, a chiller will be shutdown when the flow rate through the chiller is equivalent to the flow rate sensed in the bypass. The secondary chilled water pump adjusts its speed to maintain a set point determined by the differential pressure at the end of the loop.
[pic]
Figure 2. Constant Primary Flow/Variable Secondary Flow Pumping Arrangement
When a chiller is energized, its associated primary water pump is energized and the valve will open. Then the DDC system energizes a condenser water pump with the respective condenser water valve. A sensor in the condenser water temperature to the chiller modulates the tower bypass valve to fully open to the tower. As the temperature rises the valve will modulate to provide flow to the tower. A signal is sent to the tower fan to maintain the set point temperature of 85°F after the valve is open. See Figure 3 to view a schematic of the condenser water loop.
[pic]
Figure 3. Condenser Water Flow Diagra
Since the animal holding areas are more critical than the office space, the chilled water valves to the air handling units that serve the office and cage wash spaces will be closed in the case of very high outdoor air temperatures or system complications.
Alternative Design
Variable Primary Flow Chilled Water Plant
❖ Chillers
• (3) 1400 ton water cooled centrifugal chillers
o (1) for standby
❖ Primary Variable Speed Pumps
• (3) 2400 GPM Horizontal Split case
o 150 HP motor
o 170 ft. Head
❖ Bypass
• 12” diameter pipe
o Connects return and supply chilled water piping
o Installed high performance butterfly valve modulates the flow to provide the minimum flow rate through the chiller
❖ Sequence of Operation
Chilled water is supplied to the load with a variable primary flow pumping arrangement, show in Figure 4. Unlike a constant flow primary/variable flow secondary system configuration which maintains a constant flow rate through the chiller, a variable primary flow arrangement varies the flow according to the load demand. The primary variable speed chilled water pumps modulate their flow to maintain a preset differential pressure at the end of the load loop between the supply and return valves. The differential pressure set point should be set equal or slightly greater than the sum of the pressure drops of the control valve, coil, pipe fittings, and piping friction of the branch circuit between the supply and return mains. As the supply air temperature rise or falls below the design set point, the cooling coil valve is signaled to open or close until the water flow rate through the coils is adequate to meet the load requirements. When the load demands a flow rate below the rated capacity of the chillers, chilled water is bypassed from the supply to the return piping. If the flow rate through the chillers falls below the recommended minimum, there is a risk of freezing water and damaging the evaporator.
[pic]
Figure 4. Variable-flow primary pumping arrangement
The condenser and cooling towers operate similarly to the constant flow primary/variable flow secondary system.
❖ Elimination of the Part-Load Chiller
For Variable Primary Flow, it is not desirable to operate varying sizes of chillers in parallel. Inconsistent chillers complex the bypass valve controls that maintain the minimum flow rate through the chiller. The chiller energy consumption is proportional to the flow rate through the evaporator. The small chiller was eliminated in the variable primary flow design because it takes less pumping energy to operate the 1400 ton chiller with a minimum flow rate of 960 GPM at part loads than operating an 800 ton chiller at low loads with a constant flow rate of 1480 GPM.
The data in Figure 5. illustrates that the chiller energy consumption is almost identical for both variable primary flow and constant primary flow/variable secondary flow. This model was generated by entering the two chiller plant systems into Carrier’s Energy Analysis Program.
[pic]
Figure 5. Monthly Total Chiller Energy Consumption
System Analysis
Design Cooling Loads
Hourly cooling loads were calculated using Carrier’s Hourly Analysis Program (HAP). The program outputs the building load in MBH for each hour of the year. The hourly GPM for chilled water is determined by using the equation:
[pic]
In the equation, the load GPM was solved for by setting the chilled water ΔT to 13°F. The evaporator was designed to produce 44°F supply chilled water from a return water temperature of 57°F. The complete chiller schedule and a summary of the hourly analysis outputs are located in Appendix A.
The building model was based on a 220,559 square foot research laboratory located in Fort Worth, Texas. The facility is divided into 6 zones. The largest and most critical area of the building is Zone 1 which is animal holding rooms that are maintained at a constant year round temperature of 66 °F/55 RH with a RSHR of 0.77. The area is supplied with a 100% outdoor air system served by six air handling units with energy recovery pipes. The surgery zone is supplied by air from the air handling units from Zone 1 and humidified. Two of the six zones house the mechanical equipment and are conditioned to ensure the equipment operates properly. The office zone is a variable air volume system with a dedicated air handling unit. The cage wash area is a control volume system and is supplied 100% outdoor air. With the exception of the animal holding area which has six air handling units, each zone is served by its own air handling unit.
The air handling units for Zone 1 are identical and supply air to a common header before it reached the spaces. The zone cooling requirements were divided by 6 to size one of the six air handling units. Six of these systems were entered into HAP to make up the animal holding area. The loads of the five other zones were able to be evaluated by entering the room requirements and specified ventilation rates from the design schedules.
System Models
To calculate and compare the pump energy savings between a variable flow primary flow and constant flow primary/variable flow secondary configuration, systems were simulated with equation solving software. Pumps were selected by entering the design pump head and flow rate into the Bell & Gossett, Inc. web based equipment selection tool. The constant speed primary, variable secondary speed, and variable speed primary pumps were individually modeled using manufacture’s data. As shown in Figure 6, the pumps models were HCS3 Horizontal Splitcase pumps. Equations for pump efficiency and head were derived from performing linear regressions of manufacture’s data. For the linear regression, the flow rate through the pump was set as the independent variable while the head and efficiency values were set as the dependent variables. The head and efficiency equations were modified using the pump affinity laws. Similar to the procedure used by Bahnfleth1, equations for motor and drive efficiency curves were formed using generic curves found in ASHRAE Handbook2. A piecewise continuous function for motor efficiency as a function of nameplate load was developed by interpolating points off a chart. An equation for drive efficiency as a percentage of design speed was found by interpolating points from data for efficient motors. See Appendix A to view equations and complete system model.
Constant flow primary/variable speed secondary flow model involves one secondary pump and two primary pumps. The existing system has three secondary pumps designed to handle 4800 GPM and 55 ft. of head. See Appendix A for complete pump schedule. Since only one pump is used during the current peak load, a parallel pump model for the operation of the pumps was not necessary. For solving for the energy consumption of the secondary pump, the flow rate is set equal to the flow required by the building load. For the constant primary arrangement, the changeover point is when the system needs two chillers instead of one. According to the building’s specifications, the constant primary pumps only operate when their designated chillers are energized. The system operates on the small chiller until the load demands a greater flow than 1480 GPM. For loads exceeding 1480 GPM, the flow rate is maintained at 2400 GPM at 1780 RPM and the input power is double for when the load flow rate exceeds 2400 GPM. For the variable flow primary model, the program operates the two primary variable speed pumps in parallel. Their changeover point from one to two pumps occurs at the maximum efficiency. To prevent the flow through the chiller from falling below the chiller’s tolerance, the minimum flow rate is set to 40% design or 960 GPM. See Appendix A to view a sample of calculation results.
Simulation Results
Energy Consumption based on Chilled Water Flow Rate
According to the hourly system load results generated by HAP, the current maximum flow rate required for Texas Laboratories is 3100 GPM. As shown in figure 7. the energy savings decrease as the maximum flow rate increases. When the facility is expanded and the peak load demand increases, the energy savings at high loads will decrease as the chilled water flow approaches the plant’s maximum capacity of 4800 GPM. Since at maximum flow rate both systems are operating at full system head, the energy output is almost equal. The small difference in energy output at maximum flow is a result of the greater efficiency of the variable primary flow pumps.
[pic]
Figure 7. Energy Consumption vs. Flow Rate
Energy Analysis Results
Based on this case study, the variable speed primary flow system saves
329,309 kWh per year compared to constant flow primary/variable speed secondary configuration. As shown in Figure 8, the pump energy consumed for a constant flow primary/variable speed secondary system is more than double that for a variable primary flow configuration. An energy savings of more than 50% is attributed to the elimination of the constant primary flow and the greater efficiency of the variable flow primary pumps.
[pic]
Figure 8. Energy Comparison
Annual Cost Savings
At a flat electricity rate of $0.055 per kWh, the total energy savings for a variable flow primary system compared to a primary-secondary system converts in an annual operating cost savings of $18,112.
Plant Operability
Staging Chillers
According to Schwedler3, it is necessary to temporally unload the operating chillers before starting another one to minimize transient flows. Unloading the operating chiller can be accomplished by imposing a demand limit of 50 to 60 percent on the operating chillers. According to the manufacture, the centrifugal chillers are rated to 15% decrease in capacity per minute. At this reduction rate it would take should take 3.5 minutes for the chiller to stabilize when a new chiller is brought online. The isolation valve should open at the same rate the chiller controller can handle changes in flow rate.
The staging of the chillers might produce a brief drop in chilled water temperature. Since the animal holding area is the critical space, the research area will have precedence over the other zones for the available chilled water during staging.
Bypass
The variable primary flow arrangement is compatible with the currently installed 1400 ton centrifugal chillers. According to the manufacture, the 1400 ton chillers have a 40% minimum flow rate. The minimum flow requirement through the bypass will be able to be maintained by a modulating valve. Since the modulating valve is located near the chiller plant, it must be able to withstand high operating pressures. For this system, I chose a high performance butterfly valve. It maintains a linear relationship between the valve position and the flow rate which prevents too much water flowing through the valve when it begins to open.
ELECTRICAL
Electrical Redesign
❖ Existing vs. Redesign Components
Variable primary flow involves less electrical runs than constant primary/variable secondary pumping since there is a reduction in the amount of motors in the system. See appendix for the complete list of components for each system. As shown in Table 9, the existing equipment is replaced with three variable speed primary pumps with 150 HP motors and variable frequency drives. Shown in Figure 10 is the typical electrical feed setup for the constant primary pumps. For the variable speed pumps, the variable frequency drive replaces the function of the starter. See Figure 11, to view the electrical feed configuration for a variable speed pump.
|System |
|System |Wire |Conduit |Circuit Breaker |Switch |Starter |VFD |
|Existing |2808.192 |5611.44 |9194.64 |24660.67 |4523.76 |62578.68 |
|Redesign |1535.112 |1023.408 |5388.96 |18243.36 |0 |43507.2 |
|Cost Savings |1273.08 |4588.032 |3805.68 |6417.312 |4523.76 |19071.48 |
|Total Cost Savings |$39,679 | | | | | |
Table 13. Electrical First Cost Analysis
❖ Mechanical First Cost Savings
Switching from a constant flow primary/variable flow secondary system to a variable primary flow chilled water pumping arrangement has a first cost savings of 40.6%. The total mechanical first cost savings is $231,932. The first cost savings for the valves, flow meters, and strainers were preformed using the same process outlined above for the electrical equipment. The elimination of the 800 ton chiller resulted in a first cost saving of $160,000. Variable primary flow saves in the first cost of the pumps by eliminating the secondary loop pumps. Each 4800 GPM secondary pumps cost $10,250, the constant speed primary pumps each cost $8,989, and the variable primary speed pumps each cost $12,030. Since the variable speed primary pumps are sized to handle a larger head, the variable speed pumps are more expensive than the constant primary speed pumps.
The chiller price and pump material price was obtained from the manufacture. See Appendix B to view the manufacture’s price quote for the pumps. The labor costs for the installation of the pumps and chiller were taken from standard data with the material costs replaced with the actual costs from the quotes. Table 13 summarizes the cost savings for the variable primary flow. The redesign yields a lower cost for all the components with the exception of replacing the flow meter with butterfly valve between the supply and return pumping. See Appendix B for detailed cost breakdown.
|Mechanical First Cost Savings |
| | |pressure gauge | |check |butterfly |Flow meter |
|System |Pumps |and valve |strainer |valve |valve |or butterfly |
|Existing |52997.20 |6737.02 |23158.52 |44743.2 |19842.74 |545.52 |
|Redesign |31468.56 |3031.66 |9925.08 |6748.56 |6177.52 |3943 |
|Difference |21528.64 |3705.36 |13233.44 |37994.64 |13665.21 |-3397.47 |
|Savings |$86,730 | Without Chiller Reduction |
|Total Savings |$231,932 | With Chiller Reduction |
Table 13. Mechanical First Cost Analysis
Prefabricated Verses Field Fabricated Construction Analysis
❖ Overall
Prefabricated pumping systems eliminate the electrical contractor, the controls contractor, and the mechanical contractor from the pump installation process. According to the manufacture, case studies preformed by contractors found that replacing field fabricated components with a prefabricate system saves 5-7% in cost. Additional benefits include quick installation, lower labor costs, less floor space, and easy maintenance.
❖ Analysis
The estimated total price for a prefabricated system for this case study is $120,000. See Appendix B for manufacture’s data. The package system includes all the pumps, valves, controls, motors, variable frequency drives, and electrical feeds. Using R.S. Means to estimate the labor and material costs, the total cost for field fabrication is $125,000. As shown in table 14, using a prefabricated system instead of field fabrication saves 4% of the initial cost in this case.
|Component |Field Fabricated |Prefabricated |
| |Material Costs |Labor Costs |Complete Cost |
|Pumps |36,090 |2,100 |- |
|pressure gauge and valve |3,510 |169 |- |
|strainer |10,350 |1,695 |- |
|check valve |6,900 |1,290 |- |
|butterfly valve |5,760 |2,187 |- |
|VFD |47,400 |4,425 |- |
|Electronic Controls |2,400 |726 |- |
|Total |$125,000 |$120,000 |
Table 14. Field Fabricated and Prefabricated Costs
|Pump |Total Cost |Total Cost |Savings |Savings |
|Construction |VP |CP/VS |w/out chiller |w/chiller |
|Field Fabrication |$288,972 |$411,550 |$96,388 |$241,637 |
|Prefabricated |$284,852 |$396,960 |$100,508 |$245,757 |
Table 15. Final Cost Savings Table
CONCLUSIONS
Recommendations
Variable primary flow chilled water configuration would decrease operating cost and the first cost of the system. Variable primary flow pumping systems save pump energy by operating at lower system head and varying the flow rates through the chillers. At a flat electricity rate of $0.055 per kWh, the total energy savings for a variable flow primary system compared to a primary-secondary system converts into an annual operating cost savings of $18,112.
Converting the chilled water system from a traditional constant primary/variable secondary flow arrangement to a variable primary flow arrangement affects the electrical and mechanical first costs. The lower first cost for variable primary flow pumping is a result of reducing the plant size, equipment costs, labor costs, and material costs. The total cost savings including the cost of the chiller for a field fabrication application is $241,637 and for a prefabrication system is $245,757.
A prefabricated system in this study would save 4% in installation and material costs. Other benefits of prefabricated systems include quick installation, lower labor
costs, less floor space, and easy maintenance.
The efficiency of a variable primary flow system depends on the quality of its components. Chillers should be selected based on their minimum flow rate tolerances to optimize the energy savings during lower cooling loads. To avoid chiller complications during staging, chillers should be selected on their ability to unload quickly. Variable primary flow systems can operate efficiently, provide lower operating costs and first costs with planning, coordination, and quality equipment.
References
1) Bahnfleth, W., and E. Peyer. April 2001. “Comparative Analysis of Variable and Constant Primary-Flow Chilled Water Performance.” HPAC Engineering
2) ASHRAE. 1996. Handbook-HVAC systems and equipment. American Society of Heating, Refrigerating, and Air-Conditioning Engineers. Atlanta Georgia.
3) Schwedler, M. March 2003. “Variable Primary Flow in Chilled Water Systems.” HPAC Engineering.
4) R.S. Means 2002. Mechanical and Electrical Cost Data, R.S. Means Company Co. Inc., Kingston, MA.
APPENDICES
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Austin Commercial, Inc.
Contractor
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AE Design Firm
Texas Laboratories, Inc.
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