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HVAC Formulas

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

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
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)