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 VariablesSystem VariablesOutput Variables
RHRelative Humidity %eAmbient vapor pressure in kPaTdDewpoint temperature in degrees C
TDrybulb temperature in degrees CGAMMAConstant based upon ambient barometric pressureTwWetbulb temperature
DELTAConstant
Equations
e(RH / 100) * 0.611*EXP(17.27*T/(T+237.3))
Td[116.9 + 237.3 ln(e)] / [16.78 – ln(e)]
GAMMA0.00066*P (Use P = 98.642 kPa. This is equal to 29.15 “Hg… about the pressure we normally experience.)
DELTA4098*(e / Td + 237.3)^2
Wetbulb Temperature in Degrees F Equals:
Tw1.8 * [[(GAMMA*T) + (DELTA*Td)] / (GAMMA + DELTA)] + 32
Dewpoint Temperature in Degrees F Equals:
Td1.8 * [[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 VariablesOutput Variables
T1Entering air temperature of the coil in degrees FTONNSDewpoint temperature in degrees F
T2Leaving air temperature of the coil in degrees F
CFMVolume of air passing through the coil
Equation
TONNS1.08*(T1 – T2)*CFM

## Chiller Tonnage Output

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

Input VariablesOutput Variables
T1Chilled water return temperature in degrees FTONNSEnergy output of the chiller
T2Chilled water supply temperature in degrees F
GPMVolume of water passing through the chiller
Equation
TONNSGPM*(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
T1Chilled water return temperature in degrees F
T2Chilled water supply temperature in degrees F
GPMVolume of water passing through the chiller
KWKilowatts
Output Variables
COPEnergy output of the chiller
Equation
COP(T1 – T2) * GPM * 0.0417 / (0.28433 * KW)

## VAV Box Air Flow Rate (CFM)

Input Variables
PvPressure in inches of H2O from PV3
Output Variables
VVelocity of the air
CFMCubic feet of air per minute
Equation
QAV
0.0763 is the density of dry air at 60o F
The duct diameter units are in ft.
CFM1096Π(Duct Diameter/2)2((Pv/.0763))

## Heat Index Calculation

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

Input Variables
TfOutside air temperature in degrees F
RHOutside air relative humidity % (enter 50 for 50%, etc.)
Output Variables
HIHeat 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
VOutside air velocity in Miles per Hour
TOutside air temperature in degrees F
Output Variables
WCWind chill temperature
Equation
WC0.0817(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)