The density of the air at 20 oC is 1.205 kg/m 3. The surface temperature of the cooling coil is 0 oC (32 oF) (C). H we = water evaporation enthalpy (2502 kJ/kg) Example - Cooling and Dehumidifying Airġ m 3/s of air at 30 oC (86 oF) and relative humidity 60% (A) is cooled down to 15 oC (59 oF) (B). The latent heat flow rate can be expressed as The sensible heat flow rate can be expressed asĬ p = 1.01 - specific heat air (kJ/kg oC) The total heat flow rate can be split into sensible and latent heat. At 0 oC and atmospheric pressure the density is 1.293 kg/m 3. Note! The density of air varies with temperature. The total heat flow rate through a cooling coil can be calculated as The relationship between the Contact Factor and the Bypass Factor can be expressed asīPF = 1 - β (3) Heat Flow in a Cooling Coil The Bypass Factor - BPF - (or BF) is also used to express cooling coil efficiency as T = temperature ( oC) Bypass Factor - BPF The efficiency of a cooling coil can be expressed with the Contact Factor - β -as Note! This process decreases the specific humidity and increases the relative humidity. The amount of condensated vapor will be the difference in specific humidity x A - x B. In the real world a limited surface is never 100% effective and the final state of the cooled and dehumidified air will be somewhere on the straight line between point A and C - point B. With a cold surface of unlimited size and a very small amount of air, it would be possible to reach point C. The air cools in the direction of point C, which is the intersection point of the cold surface temperature (the cooling surface dew point temperature or the apparatus dew-point - t ADP) and the saturation line. This process is indicated in the Mollier diagram as below. Latent heat - vapor - is removed from the humid air. If the temperature on a cold surface is lower than the dew point temperature - t DP - of the humid air, vapor in the air condensates on the surface. Note! The specific humidity is constant but the relative humidity will increase.

The process is very similar (opposite direction) to the sensible heating process and the heating formulas can be adapted to calculate change in enthalpy and temperature. The air cools along a constant specific humidity - x - line as indicated in the Mollier diagram below: It is Sensible Heat - the 'temperature heat' - in the air that is removed. If the temperature on a cooling surface - t C - is above or equal to the dew point temperature - t DP - of the surrounding air, the air will be cooled without any change in specific humidity. When cooling air - part of the energy is used to separate the water (latent heat), and the rest is used to lower the air temperature (sensible heat).

Dew point temperature