Psychrometric Properties of Air: Saturated Air, Relative Humidity of Air etc

Having seen the composition of air, now let us see some important psychrometric properties of air:

The total pressure exerted by the air is called as atmospheric pressure or barometric pressure, denoted by P. It is the air pressure measured by the ordinary barometer.

The pressure exerted by the dry air in the mixture of air is called as partial pressure of dry air, denoted by P_d.

As we have seen previously, the air contains moisture or water vapor. As per the Dalton’s law of partial pressure, this water vapor exerts certain pressure independent of the other gases in the air. The pressure exerted by the water vapor within the air is called as partial pressure of water vapor (P_w). The more is the content of water vapor within the air, more is the partial pressure of the water vapor within the air. In the saturated air, the partial pressure of water vapor is maximum.

As per the Dalton’s law of partial pressure: P = P_d + P_w

The air consists of the water vapor or moisture at all times no matter what its temperature and pressure is. When we add more moisture to the air in an enclosed space at certain temperature, the air tends to keep on absorbing the moisture. However, after reaching certain stage, the air is no more able to absorb the moisture and all the extra moisture is converted into dew or fog. The air that contains maximum amount of moisture that it can hold at particular temperature is called as saturated air.

The quantity of the moisture that the air can hold depends on its temperature. The more is the temperature of the air, the more quantity of the moisture it can absorb.

Let us suppose that the partial pressure of water vapor when the air is saturated is P_ws, then as per the Dalton’s law of partial pressure:

P = P_d + P_ws

The absolute humidity of the air is defined as the total mass of water vapor present in the unit volume of air at any given condition of temperature.

The term relative humidity is more widely used in the field of HVAC or air conditioning. It is defined as the ratio of the mass of water vapor present in the unit volume of the air at certain temperature to the maximum mass of the water vapor that can be accommodated in the unit volume of air when it is saturated. We have seen that the partial pressure of the water vapor depends on the mass of the water vapor in the air. Hence we can also define relative humidity as the ratio of the partial pressure of the water vapor at a given temperature to the partial pressure of water vapor when the air is saturated at the same temperature. Let us say

Relative Humidity RH = Amount of water vapor in unit volume of unsaturated air/ Amount of water vapor in unit volume of saturated air

RH = Partial Pressure of water vapor in unsaturated air/ Partial pressure of water vapor in saturated air

RH = Pw/Pws *100

Relative humidity is expressed in percentage. It indicates the actual humidity present in the air at the prevalent temperature conditions and widely used by the air conditioning engineers.