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```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
⎛ RPMnew ⎞
CFM new = CFM old X ⎜
⎟
⎝ RPMold ⎠
⎛ CFMnew ⎞
RPM new = RPM old X ⎜
⎟
⎝ CFMold ⎠
Fan Law #2
⎛ CFMnew ⎞
⎟⎟
⎜⎜
⎝ CFMold ⎠
2
=
SPnew
⎛ CFMnew ⎞
Or ⎜
⎟
SPold
⎝ CFMold ⎠
=
SPnew
SPold
Formulas for problem solving
CFM new = CFM old x
SPnew
⎛ CFMnew ⎞
SP new = ⎜
⎟
SPold
⎝ CFMold ⎠
2
Fan Law #3
3
BHPnew
CFMnew
⎛ CFMnew ⎞
or
=
⎟ =
⎜
BHPold
CFMold
⎝ CFMold ⎠
3
BHPnew
BHPold
Formulas for problem solving
CFM new = CFM old X
3
BHPnew
BHPold
⎛ CFMnew ⎞
BHP new = BHP old x ⎜
⎟
⎝ CFMold ⎠
3
Volume calculations:
Air Changes Per Hour =
CFMx60
Cubicfeet
CFM = Room Volume x
Calculating Outside Air quantities:
AirChangesperhour
60
(MATx100) − (% RAxRAT )
OAT =
%OA
O=Outside A=Air R=Return M=Mixed
T=Temperature
(RAT − MAT )
(RAT − OAT )
%OA =
MAT =
x 100
(%OAxOAT ) + (% RAxRAT )
RAT =
100
(MATx100) − (%OAxOAT )
% RA
Velocity and Velocity with density correction:
For Standard Air (70F @ 29.92"):
⎛ V ⎞
⎟
VP = ⎜
⎝ 4005 ⎠
V = 4005 × VP
530
BAR
×
Den = 0.075 ×
460 + T 29.92
For Other Than Standard Air:
2
VP
Den
V = 1096.7 ×
Airflow stations flow:
CFM = CV × ∆P(inches) H 2o
Cv = 2
CFM
∆P(inches) H 2o
2
⎛ CFM⎞
∆P(inches)H 2o = ⎜
⎟
⎝ 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.
F.
BTUH ( Sensible)
108
. × ∆T ( DryBulb)
BTUH
∆T =
108
. × CFM
CFM =
sp.ht. = Specific Heat in BTU/Lb. per Degree
BTUH = CFM × 108
. × ∆T ×
BTUH (Total ) = CFM × ∆h × 4.5 ×
Den
0.075
Den
0.075
h = Enthalpy in BTU per Lb.
BTUH
= MBH
1000
Fan calculations:
FanBHP =
Static fan efficiency:
FanTipSpeedFPM = RPM ×
RPM =
Ts( FPM ) × 12
Circ.(in.)
BL = 2C + ( 157
. × ( D + d )) +
Circ.(in.)
12
CFM × SP
6356 × SE
Circ.(in.) =
( D − d)2
Ts( FPM ) × 12
RPM
FanRPM = MotorRPM ×
4C
d
D
D
MotorRPM = FanRPM ×
d
BL = Belt Length (in.) C = Distance between shaft centers (in.)
(in.)
d = Motor Sheave dia. (in.)
D = Fan sheave dia.
ExsistingFanSheaveDia.
Max. (min.) Fan sheave Dia. =
3
Max.BHP
ExsistingEstimatedBHP
Max. Motor sheave = Existing Motor sheave Dia. X 3
Max.BHP
ExsistingEstimatedBHP
⎛ RPMnew ⎞
New Motor Sheave size Diameter new = Dia.old x ⎜
⎟
⎝ RPMold ⎠
⎛ RPMold ⎞
New Fan Sheave size Diameter new = Dia.old x ⎜
⎟
⎝ RPMnew ⎠