Calculating the Enthalpy of Air

Air conditioning engineers must consider how their devices impact the temperature of the air, which is partially determined by the heat content–measured in kilojoules (kJ) per kilogram (kg)–of the air. Heat content, or enthalpy, of air has two components: the kinetic motion of the air molecules that can be measured by a thermometer, and the latent (hidden) heat contained within evaporated water. Since all air contains a least a small percentage of water, both factors must be taken into account when calculating the enthalpy of air.

Step 1

Calculate the enthalpy in the air alone by multiplying the temperature of the air, in degrees Celsius, by 1.007 and subtracting 0.026 from the answer. For example, consider air at a temperature of 30 degrees C.

Air Enthalpy = 1.007 x 30 – 0.026 = 30.184 kJ per kg.

Step 2

Calculate the enthalpy in the water vapor by plugging the water content of the air (in kg per kg) and the air temperature into the following formula:

Water vapor enthalpy = water content of air x (2501 + 1.84 x temperature).

Consider air that has a water content of 0.01 kg per kg of air.

Water vapor enthalpy = 0.01 x (2501 + 1.84 x 30) = 25.01 kJ per kg.

Step 3

Add the air enthalpy to the water vapor enthalpy to determine the total atmospheric enthalpy:

Total enthalpy in air = air enthalpy + water vapor enthalpy = 30.184 + 25.01 = 55.194 kJ per kg.

The air in this example has 55.194 kJ of enthalpy per kg.