Airflow Formulas
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Transcript of Airflow Formulas
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Air Flow formulas
CFM = Duct area sq ft x Velocity Standard Air=70F @ 29.92 HG(Mercury)
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: newis the same as 1 and oldis the same as 2
Fan Law #1
=
old
new
old
new
RPM
RPM
CFM
CFM
Formulas for problem solving
CFM new =CFMold X
old
new
RPM
RPM RPM new = RPMold X
old
new
CFM
CFM
Fan Law #22
old
new
CFM
CFM=
old
new
SP
SPOr
old
new
CFM
CFM=
old
new
SP
SP
Formulas for problem solving
CFM new =CFMold xold
new
SP
SP SP new =
2
old
new
CFM
CFM
Fan Law #3
3
old
new
CFM
CFM=
old
new
BHP
BHPor
old
new
CFM
CFM= 3
old
new
BHP
BHP
Formulas for problem solving
CFM new = CFM old X 3old
new
BHP
BHP BHP new = BHP old x
3
old
new
CFM
CFM
Volume calculations:
Air Changes Per Hour =Cubicfeet
CFMx60 CFM = Room Volume x
60
perhourAirChanges
Calculating Outside Air quantities:
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OAT =( ) ( )
OA
RAxRATMATx
%
%100 O=Outside A=Air R=Return M=Mixed
T=Temperature
%OA=( )
( )OATRAT
MATRAT
x 100
MAT =( ) ( )
100
%% RAxRATOAxOAT +
RAT =( ) ( )
RA
OAxOATMATx
%
%100
Velocity and Velocity with density correction:
For Standard Air (70F @ 29.92"): For Other Than Standard Air:
V VP= 4005 VP
V=
4005
2
V
VP
Den= 1096 7.
DenT
BAR=
+0075
530
460 29 92.
.
Airflow stations flow:
oHinchesPCCFM V 2)(=
3
2)( oHinchesP
CFMCv
=
2
2)(
=
Cv
CFMoHinchesP
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 hour4.5 x 0.24 = 1.08
BTUH M sp ht T = . .
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M = Mass Flow Rate in Lbs. per Hour. sp.ht. = Specific Heat in BTU/Lb. per DegreeF.
CFM BTUH SensibleT DryBulb= ( )
. ( )108 BTUH CFM T Den= 108
0075.
.
TBTUH
CFM=
108.
BTUH Total CFM hDen
( ) ..
= 4 50075 h = Enthalpy in BTU per Lb.
MBHBTUH
=1000
Fan calculations:
Static fan efficiency:FanBHP
CFM SP
SE=
6356
FanTipSpeedFPM RPMCirc in
= .( .)
12 Circ in
Ts FPM
RPM.( .)
( )=
12
RPMTs FPM
Circ in=
( )
.( .)
12
( )( )( )
BL C D d D d
C= + + +
2 157
4
2
.
FanRPM MotorRPMd
D=
MotorRPM FanRPMD
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. =
PstimatedBHExsistingE
BHPMax
aanSheaveDiExsistingF
.3
.
Max. Motor sheave = Existing Motor sheave Dia. XPstimatedBHExsistingE
BHPMax.3
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New Motor Sheave size Diameter new = Dia.old x
old
new
RPM
RPM
New Fan Sheave size Diameter new = Dia.old x
new
old
RPM
RPM
2005, National Energy Management Institute Committee. All rights reserved.
