Cooling and Heating Equations

Latent and sensible cooling and heating equations - in imperial units

Sensible Heat

The sensible heat in a heating or cooling process of air 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)

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: