Psychrometry for Air Conditioning

Psychrometry for Air Conditioning

Psychrometry is the study of air and water vapour mixtures.

Air is made up of five main gases i.e.

Nitrogen 78.03%, Oxygen 20.99%, Argon 0.94%, Carbon Dioxide 0.03%, and Hydrogen 0.01% by volume.

The Ideal Gas Laws are used to determine psychrometric data for air so that the engineer can carry out calculations.

To make life easier a chart has been compiled with all the relevant psychrometric data indicated.

This is called the Psychrometric Chart.

A typical chart is shown below.

Air at any state point can be plotted on the psychrometric chart.

The information that can be obtained from a Psychrometric Chart is as follows:

1. Dry bulb temperature
2. Wet bulb temperature
3. Moisture content
4. Percentage saturation
5. Specific enthalpy
6. Specific volume.

The following is a brief description of each of the properties of air.
1. Dry bulb temperature This is the air temperature measured by a mercury-in-glass thermometer. 2. Wet bulb temperature

This is the air temperature measured by a mercury-in-glass thermometer which has the mercury bulb wetted by gauze that is kept moist by a reservoir of water.

When exposed to the environment the moisture evaporates from the wetted gauze, which gives a lower reading on the thermometer.

This gives an indication of how ‘dry’ or how ‘moist’ the air is, since in ‘dry’ air the water will evaporate quickly from the gauze, which depresses the thermometer reading.

3. Moisture content

This is the amount of moisture in air given in kg of moisture per kg of dry air e.g. for room air at 21^oC dry bulb and 15^oC wet bulb, the moisture content is about 0.008 kg/kg d.a.

This is a small mass of moisture ( 0.008 kg = 8 grams) per kg of dry air or 9.5 grams per cubic metre of air.

4. Percentage saturation

The Percentage saturation is another indication of the amount of moisture in air.

This is the ratio of the moisture content of moist air to the moisture content of saturated air at the same temperature.

When air is saturated it is at 100% saturation and cannot hold any more moisture.

5. Specific enthalpy

This is the amount of heat energy (kJ) in air per kg.

If heat is added to the air at a heater battery for example, then the amount to be added can be determined from Specific enthalpy change.

6. Specific volume

This is the volume of moist air (dry air + water vapour) per unit mass.

The units of measurement are m³ per kg.

Also specific volume = 1 / density.