# HVAC Formulas

## Download Our Free 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

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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 H_{2}O 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} FThe 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 | |
---|---|

T_{f} | 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+2.04901523T+10.14333127_{f}RH-0.22475541 T_{f}RH-6.83783×10^{-3}T^{2}_{f}-5.481717×10 ^{-2}RH+1.22874×10^{2}^{-3}T^{2}_{f}RH+8.5282×10 ^{-4}T_{f}RH^{2}-1.99×10^{-6}T^{2}_{f}RH^{2} |

## 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) Where V = Air Velocity (FPM) |

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) |