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 
Download HVAC_Formula_TablesDewpoint_and_Wetbulb_Temp.pdf (121 kb)
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 
Download HVAC_Formula_TablesAir_Handling_Unit_Tonnage_Output.pdf (88 kb)
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 
Download HVAC_Formula_TablesChiller_Tonnage_Output.pdf (87 kb)
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) 
Download HVAC_Formula_TablesChiller_Coefficient_of_Performance.pdf (89 kb)
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 60^{o} F The duct diameter units are in ft. 

CFM  1096Π(Duct Diameter/2)2(√(Pv/.0763)) 
Download HVAC_Formula_TablesVAV_Box_Air_Flow_Rate_CFM.pdf (113 kb)
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 
Download HVAC_Formula_TablesHeat_Index_Calculation.pdf (103 kb)
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 
Download HVAC_Formula_TablesWind_Chill_Temp_Calc.pdf (88 kb)
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) 
Download HVAC_Formula_TablesPressure_Measurment.pdf (111 kb)