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 (heating or cooling capacity) can be expressed as

hs = 1.08 q dt         (1)

where

hs = 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:

hl = 0.68 q dwgr         (2)

or

hl = 4,840 q dwlb         (3)

where

hl= latent heat (Btu/hr)

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

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

dwlb = 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:

ht = 4.5 q dh         (4)

where

ht= 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:

ht = hs + hl

    = 1.08 q dt + 0.68 q dwgr         (5)

Example - Heating Air

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

hs = 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 and required air volume

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 and required air volume

SHR - Sensible Heat Ratio

The Sensible Heat Ratio can be expressed as

SHR = hs / ht     (6)

where 

SHR = Sensible Heat Ratio

hs = sensible heat

ht = total heat (senible and latent)

Related Topics

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

Related Documents

Search the Engineering ToolBox

- "the most efficient way to navigate the Engineering ToolBox!"

Engineering ToolBox - SketchUp Edition - Online 3D modeling!

3D Engineering ToolBox - draw and model technical applications

Engineering ToolBox - SketchUp Edition - add standard and customized parametric components - like flange beams, lumbers, piping and more - to your SketchUp model - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro. Add from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!
About the Engineering ToolBox!

close