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# HVAC Formulas

Computrols believes that our customers should have the ability to operate their own HVAC system and building automation controls. That is why we do our best to provide them with as many free resources as possible. Here, you will find our HVAC formulas. On larger screens you can either scroll down to view each individually or download the PDF. On smaller screens such as phones you must download the PDF to view the formulas. The PDF includes:

• Dewpoint and Wetbulb Temperature
• Air Handling Unit Tonnage Output
• Chiller Tonnage Output
• Chiller Coefficient of Performance
• VAV Box Air Flow Rate (CFM)
• Heat Index Calculation
• Wind Chill Temperature Calculation
• Pressure Measurement Dewpoint and Wetbulb Temperature

The following equations are used to calculate the wetbulb temperature of air given the drybulb temperature and relative humidity %. The equation assumes that the ambient barometric pressure is constant at a value of 29.15 “Hg since the change in wetbulb temperature is very insignificant with changes in the ambient barometric pressure.

Input Variables System Variables Output Variables
RH Relative Humidity % e Ambient vapor pressure in kPa Td Dewpoint temperature in degrees C
T Drybulb temperature in degrees C GAMMA Constant based upon ambient barometric pressure Tw Wetbulb temperature
DELTA Constant
Equations
e (RH / 100) * 0.611*EXP(17.27*T/(T+237.3))
Td [116.9 + 237.3 ln(e)] / [16.78 – ln(e)]
GAMMA 0.00066*P (Use P = 98.642 kPa. This is equal to 29.15 “Hg… about the pressure we normally experience.)
DELTA 4098*(e / Td + 237.3)^2
Wetbulb Temperature in Degrees F Equals:
Tw 1.8 * [[(GAMMA*T) + (DELTA*Td)] / (GAMMA + DELTA)] + 32
Dewpoint Temperature in Degrees F Equals:
Td 1.8 * [[116.9 + 237.3 ln(e)] / [16.78 – ln(e)]] + 32

## Air Handling Unit Tonnage Output

The following equation calculates the refrigeration output in Tonns of a coil.

Input Variables Output Variables
T1 Entering air temperature of the coil in degrees F TONNS Dewpoint temperature in degrees F
T2 Leaving air temperature of the coil in degrees F
CFM Volume of air passing through the coil
Equation
TONNS 1.08*(T1 – T2)*CFM

## Chiller Tonnage Output

The following equation calculates the refrigeration output in Tonns of a chiller.

Input Variables Output Variables
T1 Chilled water return temperature in degrees F TONNS Energy output of the chiller
T2 Chilled water supply temperature in degrees F
GPM Volume of water passing through the chiller
Equation
TONNS GPM*(T1 – T2) / 24

## Chiller Coefficient of Performance

The following equation calculates the ratio of energy used to the energy output of a chiller.

Input Variables
T1 Chilled water return temperature in degrees F
T2 Chilled water supply temperature in degrees F
GPM Volume of water passing through the chiller
KW Kilowatts
Output Variables
COP Energy output of the chiller
Equation
COP (T1 – T2) * GPM * 0.0417 / (0.28433 * KW)

## VAV Box Air Flow Rate (CFM)

Input Variables
A Duct area in sq. ft
Pv Pressure in inches of H2O from PV3
Output Variables
V Velocity of the air
CFM Cubic feet of air per minute
Equation
Q AV
0.0763 is the density of dry air at 60o F
The duct diameter units are in ft.
CFM 1096Π(Duct Diameter/2)2((Pv/.0763))

## Heat Index Calculation

The following equation calculates the heat index of the outside air.

Input Variables
Tf Outside air temperature in degrees F
RH Outside air relative humidity % (enter 50 for 50%, etc.)
Output Variables
HI Heat index
Equation
HI -42.379+2.04901523Tf+10.14333127RH
-0.22475541TfRH-6.83783×10-3T2f
-5.481717×10-2RH2+1.22874×10-3T2fRH
+8.5282×10-4TfRH2-1.99×10-6T2fRH2

## Wind Chill Temperature Calculation

The following equation calculates the wind chill temperature of the outside air.

Input Variables
V Outside air velocity in Miles per Hour
T Outside air temperature in degrees F
Output Variables
WC Wind chill temperature
Equation
WC 0.0817(3.71(V)^0.5 + 5.81 – 0.25V)(T – 91.4) + 91.4

## Pressure Measurement

Velocity Pressure

Pv = (V/4005)2 or V = 4005 Pv

Where V = Air Velocity (FPM)
Pv = Velocity Pressure (in. w.g.)

Equivalent Measures of Pressure
1lb. per square inch = 144lbs. per sq. ft.
= 2.036in. Mercury at 32°F
= 2.311ft. Water at 70°F
= 27.74in. Water at 70°F
1 inch Water at 70°F = .03609lb. per sq. in.
= .5774oz. per sq. in.
= 5774oz. per sq. in.
= 5.196lbs. per sq. ft.
1 ounce per sq. in. = 1272in. Mercury at 32°F
= 1.733in. Water at 70°F
1ft. Water at 70°F = .433lbs. per sq. in.
= 62.31lbs. sq. ft.
1 Atmosphere = 14.696lbs. per sq. in.
= 2116.3lbs. per sq. ft.
= 33.96ft. Water at 70°F
= 29.92in. Mercury at 32°F
1in. Mercury at 32°F = .491lbs. per sq. in.
= 7.86oz. per sq. in.
= 1.136ft. Water at 70°F
= 13.63in. Water at 70°F
Compression Ratio
Compression Ratio = Absolute Discharge Pressure / Absolute Suction Pressure
Absolute Discharge Pressure = gauge reading + 15psi
Absolute Suction Pressure = gauge reading + 15psi
Refrigerant Mass Flow Rate
Mass Flow Rate
(Pounds/Minute)
= Piston Displacement X Refrigerant Density
= (Cubic Feet/Minute) X (Pounds/Cubic Feet)