Air Flow formulas - NEMI

Air Flow formulas

CFM = Duct area sq ft x Velocity (Mercury)

Standard Air= 70F @ 29.92" HG

1 cubic foot of standard air = 0.075 pounds 13.3 cubic feet of standard air = 1 pound

FAN LAWS:

Remember RPM is interchangeable for CFM

Note: new is the same as 1 and old is the same as 2

Fan Law #1

CFMnew = RPMnew CFMold RPMold

Formulas for problem solving

CFM new = CFM old X RPMnew RPMold

RPM new = RPM old X CFMnew CFMold

Fan Law #2

CFMnew CFMold

2

=

SPnew SPold

Or

CFMnew CFMold

=

SPnew SPold

Formulas for problem solving

CFM new = CFM old x

SPnew SP new = CFMnew 2

SPold

CFMold

Fan Law #3

CFMnew 3 = BHPnew or CFMnew = 3 BHPnew

CFMold BHPold CFMold

BHPold

Formulas for problem solving

CFM new = CFM old X 3 BHPnew BHPold

BHP new = BHP old x CFMnew 3 CFMold

Volume calculations:

CFMx60

Air Changes Per Hour =

Cubicfeet

CFM = Room Volume x AirChangesperhour 60

Calculating Outside Air quantities:

OAT = (MATx100) - (%RAxRAT ) O=Outside A=Air R=Return M=Mixed

%OA T=Temperature

%OA

=

(RAT (RAT

- MAT ) - OAT )

x

100

MAT = (%OAxOAT ) + (%RAxRAT )

100

RAT = (MATx100) - (%OAxOAT )

%RA

Velocity and Velocity with density correction:

For Standard Air (70F @ 29.92"):

For Other Than Standard Air:

V = 4005? VP

VP

=

V 2 4005

Den

=

0.075 ?

530 460 +

T

?

BAR 29.92

VP V = 1096.7 ?

Den

Airflow stations flow:

CFM = CV ?

P(inches)H 2o

Cv = 2

CFM

P(inches)H 2o

P(inches)H2o = CFM2 Cv

Air Thermal calculations:

sp.ht = 0.24 13.3 cu.ft. = 1 lb. 0.075 lbs. = 1 cu.ft. .075 lbs. x 60 min. = 4.5 lbs per hour 4.5 x 0.24 = 1.08

BTUH = M ? sp.ht.?T

M = Mass Flow Rate in Lbs. per Hour. sp.ht. = Specific Heat in BTU/Lb. per Degree F.

BTUH(Sensible) CFM = 1.08 ? T(DryBulb)

T

=

BTUH 1.08 ? CFM

BTUH(Total) = CFM ? h ? 4.5? Den 0.075

BTUH = MBH 1000

Den BTUH = CFM ? 1.08 ? T ?

0.075

h = Enthalpy in BTU per Lb.

Static fan efficiency:

Fan calculations:

CFM ? SP FanBHP = 6356 ? SE

FanTipSpeedFPM = RPM ? Circ.(in.) 12

RPM = Ts(FPM ) ? 12 Circ.(in.)

Circ.(in.) = Ts(FPM ) ? 12 RPM

BL = 2C + (1.57 ? ( D + d )) + ( D - d ) 2

4C D MotorRPM = FanRPM ? d

FanRPM = MotorRPM ? d D

BL = Belt Length (in.) C = Distance between shaft centers (in.) D = Fan sheave dia. (in.) d = Motor Sheave dia. (in.)

Max. (min.) Fan sheave Dia. = ExsistingFanSheaveDia.

3

Max.BHP

ExsistingEstimatedBHP

Max. Motor sheave = Existing Motor sheave Dia. X 3

Max.BHP

ExsistingEstimatedBHP

New Motor Sheave size Diameter new = Dia.old x RPMnew RPMold

New Fan Sheave size Diameter new = Dia.old x RPMold RPMnew

? 2005, National Energy Management Institute Committee. All rights reserved.

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