Sensible and Latent Heat of Air

The total heat content of the air or enthalpy of air comprises of the sensible heat and the latent heat. The sensible heat is the heat absorbed or lost during the change in temperature of the air. The latent heat is the heat lost or absorbed during change in phase of the water vapor present in the air. Let us see these heats in more details.

The sensible heat of the air is mainly due to the dry air present in the air. The dry air exists in the superheated condition in the air since it comprises of gases like nitrogen, oxygen, and other inert gases that have very low boiling point or liquefaction point. Since there is no change in the phase of dry air, it has only the sensible heat. When the temperature of the air is changed, it is the temperature of the dry air that changes and it either absorbs or liberates the sensible heat. The water vapor present within the air also absorbs or liberates the sensible heat, but this quantity is very small so it is not taken into account.

The sensible heat of the air is calculated as follows:

SH = m*0.133*DBT

Where: m is the mass of the dry air, 0.133 is the specific heat of air in Kcal/kg and DBT is the dry bulb temperature of the air.

From the above formula is clear that the sensible heat of the air depends on dry bulb temperature of the air and the mass of the dry air.

Since there is no change in the phase of the dry air present in the air mixture, it does not have any latent heat. The water vapor present within the air undergoes changes in the phase when the air is either cooled or heated hence the latent heat within the air is present due to water vapor. The latent heat of the air due to water vapor depends on the amount of moisture present in it. The dew point temperature of the air when it is saturated decides the amount of moisture content present within the air. Hence the latent heat of air depends on the dew point temperature of air. This also implies that as long as the dew point temperature of the air remains constant, its latent heat will also remain constant no matter if the dry bulb temperature of the air increases or decreases.

The latent heat of the air is calculated as follows:

LH = m*w*h_w

Where: m is the mass of dry air, w is the specific humidity of dry air, and h_w is the specific enthalpy of water vapor taken from the steam tables as the enthalpy of water vapor at dew point temperature.

The total heat content of the air is the sum of the sensible heat of the air and the latent heat of the air. Thus,

Total heat of the air = SH + LH

The sensible heat depends on dry bulb temperature of air while latent heat depends on dew point temperature of the air, hence the total quantity of heat in the air depends on the dry bulb and dew point temperature of the air. Further, for any combination of the dry bulb and dew point temperature, there can be only one wet bulb temperature, hence the total quantity of heat in the air also depends on the wet bulb temperature.

It will also be interesting to note here that for various combinations of air having different dry bulb and dew point temperature, but the same wet bulb temperature, the total heat content of the air remains constant. The only change will be the proportion of the sensible heat and the latent heat. On the similar lines, if the dry bulb temperature of air is constant, the total heat content of the air depends on the wet bulb temperature of the air.