Cooling and Heating Equations

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Sensible Heat

The sensible heat in a heating or cooling process of air (heating or cooling capacity) can be expressed as

h_s = 1.08 q dt         (1)

where

h_s = sensible heat (Btu/hr)

q = air volume flow (cfm, cubic feet per minute)

dt = temperature difference (^oF)

Latent Heat

The latent heat due to moisture in the air can be expressed as:

h_l = 0.68 q dw_gr         (2)

or

h_l = 4,840 q dw_lb         (3)

where

h_l= latent heat (Btu/hr)

q = air volume flow (cfm, cubic feet per minute)

dw_gr = humidity ratio difference (grains water/lb dry air)

dw_lb = humidity ratio difference (lb water/lb dry air)

• 1 grain = 0.000143 lb = 0.0648 g

Total Heat - Latent and Sensible Heat

Total heat due to both temperature and moisture can be expressed as:

h_t = 4.5 q dh         (4)

where

h_t= total heat (Btu/hr)

q = air volume flow (cfm, cubic feet per minute)

dh = enthalpy difference (btu/lb dry air)

Total heat can also be expressed as:

h_t = h_s + h_l

    = 1.08 q dt + 0.68 q dw_gr         (5)

Example - Heating Air

An air flow of one cfm is heated from 32 to 52^oF. Using (1) the sensible heat added to the air can be expressed as:

h_s = 1.08 (1 cfm) ((52 ^oF) - (32 ^oF))      

    = 21.6 (Btu/hr)

Sensible Heat Load and Required Air Volume Chart

Sensible heat load and required air volume to keep temperature constant at various temperature differences between entering air and room air are indicated in the chart below:

• Sensible Heat Load and Required Air Volume Chart (pdf)

Latent Heat Load and Required Air Volume Chart

Latent heat load - humidifying and dehumidifying - and required air volume to keep temperature constant at various temperature differences between entering air and room air are indicated in the chart below:

• Latent Heat Load and Required Air Volume Chart (pdf)

SHR - Sensible Heat Ratio

The Sensible Heat Ratio can be expressed as

SHR = h_s / h_t     (6)

where 

SHR = Sensible Heat Ratio

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Related Topics

• Air Conditioning - Air Conditioning systems - heating, cooling and dehumidification of indoor air for thermal comfort

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• Sensible Heat Ratio - SHR - Sensible Heat Ratio - SHR - is defined as the sensible heat or cooling load divided by the total heat or cooling load