Brochure

Calculating Air Changes per Hour
ACH = Q x 60 / Room Volume ft
3
Q = ft
3
/minute (CFM)
Calculating Air Velocity (Standard, 70 °F @ 29.92 in. Hg)
V = 4005 x √VP
VP = (V/4005)
2
V = Velocity, fpm
VP = Velocity pressure, in. wc
Calculating Air Velocity with density correction (Actual)
V = 1096.7 x √VP/D
V = Velocity, ft/minute (fpm)
VP = Velocity pressure, in. wc
D = Density, lbs/ft
3
, 0.075 x (530/460+T
act
) x (P
act
/ 29.92)
T
act
= measured dry bulb temperature of the actual airstream, °F
P
act
= absolute pressure of the actual airstream, in. Hg
Calculating Air Flow (Standard, 70 °F @ 29.92 in. Hg)
Q = A x V
Q = ft
3
/minute (CFM)
A = duct area (ft
2
)
V = Velocity, ft/minute (fpm)
Calculating Air Flow with density correction
Correcting for standard cfm
SCFM = ACFM(530/(460+T
act
))(P
act
/29.92)
SCFM = standard flow rate
ACFM = actual flow rate= measured flow rate
T
act
= measured dry bulb temperature of the actual airstream, °F
P
act
= absolute pressure of the actual airstream, in. Hg
Correcting for actual cfm
ACFM = SCFM((460+T
act
)/530)(29.92/P
act
)
SCFM = standard flow rate
ACFM = actual flow rate = measured flow rate
T
act
= measured dry bulb temperature of the actual airstream, °F
P
act
= absolute pressure of the actual airstream, in. Hg
Calculating % of Outside Air (%OA)
% OA = (RAT - SAT) / (RAT - OAT) x 100
SAT = (%OA x OAT) + (%RA x RAT) / 100
OAT = (SAT x 100) - (%RA x RAT) / %OA
RAT = (SAT x 100) - (%OA x OAT) / %RA
RAT = Return Air Temperature
SAT = Supply Air Temperature (or mixed air temperature)
OAT = Outside Air Temperature
%RA = Percentage Return Air
Calculating Duct Pressure
VP = TP-SP
TP = VP+SP
TP = total pressure, in. wc
SP = static pressure, in. wc
VP = velocity pressure, in. wc
Using Fan Laws to Assess Performance Changes
CFM
2
= CFM
1
x (RPM
2
/ RPM
1
)
SP
2
= SP
1
x (CFM
2
/CFM
1
)
2
BHP
2
= BHP
1
x (CFM
2
/ CFM
1
)
3
CFM
1
= Cubic feet minute (Existing)
CFM
2
= Cubic feet minute (New)
SP
1
= Static pressure (Existing)
SP
2
= Static pressure (New)
BHP
1
= Power consumed by propeller (Existing)
BHP
2
= Power consumed by propeller (New)
Airflow quick
reference guide