32
Air Handling Unit
Installation, Operation, and Maintenance Manual
April 2021
14.0 Useful Equations
14.1 General Equations
Total Pressure
= Velocity Pr Static Pressure
Velocity Pressure
= (Velocity / 4005)
2
CFM
2
= CFM
1
* (RPM
2
/ RPM
1
)
Static Pressure
2
= Static Pressure
1
* (Velocity
2
/Velocity
1
)
2
Hp
2
= Hp
1
* (RPM
2
/ RPM
1
)
3
= Hp
1
* (CFM
2
/ CFM
1
)
3
Evap Cooler Efficiency
= (Edb-Ldb) / (Edb-Ewb)
Evap Cooler gal/hr
= [CFM * (Edb-Ldb)] / 10,000
AHU Leak Class
= [(% Leakage) * (Supply CFM) * 100] / [(Area sq. ft) * (Test Pressure)
0.65
]
WPD
2
= WPD
1
* (GPM
2
/ GPM
1
)
2
Water Velocity ft/sec
= (GPM * 0.41) / [(tube diameter)
2
]
Pump hp
= (GPM * ft. head) / (3960 * efficiency)
Steam lb/hr
= CFM * (T
2
-T
1
) / 1000
Total Heat
= 4.5 * cfm * (h
2
-h
1
)
Total heat
= 500 * GPM * (T
2
-T
1
)
Sensible Heat
= 1.085 * cfm * (T
2
-T
1
)
Total Heat
= Mass flow * Cp * (T
2
-T
1
)
Motor Heat
= (bhp * 2524) / Efficiency
Fan Heat
(Deg F) = (0.37 * Static Pressure) / (Motor Eff * Fan Eff) ≈ 0.58 * Static Pressure
fn (natural frequency)
= 188 * (1/ Spring Deflection)
^(1/2)
Weight of Water in a Coil (in pounds)
≈ π * (½ tube diameter)
2
* tube length * number of tubes * 0.00433 * 8.3
14.2 Fan Law Equations
New CFM
= (New RPM * Existing CFM) / Existing RPM
New sp
= Existing sp * (New RPM / Existing RPM)
2
New bhp
= Existing bhp * (New RPM / Existing RPM)
3
Static Efficiency
= (CFM * static pressure) / (6356 * bhp)
Total Efficiency
= (CFM * total pressure) / (6356 * bhp)
