Psychrometric Properties: Dry Bulb Temperature, Wet Bulb Temperature, Dew Point Temperature

Here are some more important properties of air:

The temperature of the air measured by the ordinary thermometer is called as the dry bulb temperature of air, commonly referred as DBT. When ordinary thermometer is exposed to the atmosphere, it indicates the dry bulb temperature, which is nothing but the atmospheric temperature.

The wet bulb temperature of air is also measured by the ordinary thermometer, but the only difference is that the bulb of the thermometer is covered by the wet cloth. Temperature of the ordinary air measured by the thermometer when it is covered by wet cloth or wick is called as the wet bulb temperature, commonly referred to as WBT. When the air comes in contact with the wet cloth it absorbs some moisture and gives up some heat, sue to which the temperature of the air reduces. This reduced temperature measured by the thermometer is called as the wet bulb temperature.

If the moisture content of the air is very low, it will give up more heat to the cloth and the wet bulb temperature of air will also be comparatively lower. On the other hand, if the moisture content of air is high it will loose lesser heat to the air and wet bulb temperature will be higher. The more is the moisture or water vapor content of the air more is the wet bulb temperature. Thus the wet bulb temperature indirectly indicates the moisture content present in the air or we can say that it is affected by the relative humidity of the air.

The wet bulb temperature of the air is always less than the dry bulb temperature of air. The difference between the DBT and WBT is called as wet bulb depression.

The DBT and WBT of the air are measured by a special thermometer called Sling thermometer. It comprises of two ordinary thermometers, but one of them is covered with the wet wick or cloth. The uncovered thermometer measures dry bulb temperature, while the one covered with wet cloth measures wet bulb temeprature. Since both the thermometers are placed side-by-side, the mositure from wet wick may affect the readings of dry bulb thermometer, to avoid this the wet bulb thermometer is placed at the lower level. The wet wick should be clean and free of dirt to avoid the erroneous readings.

Towards one end of this thermometer there is a small handle that enables the rotation of the thermometers. To measure DBT and WBT, the cloth is wetted and the instrument is rotated for about a minute. The process has to be repeated several times so as to ensure that the lowest possible WBT is obtained.

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When the temperature of the air is reduced, the temperature of the water vapor present within the air also reduces. The water vapor within the air is similar to the superheated steam in the vapor condition, but at much lower pressure. When we reduce the temperature of the superheated steam, its sensible heat content starts reducing and at some particular temperature (100 degree C) it starts condensing. In a similar manner, when the temperature of the air is reduced continuously, the point comes when the water vapor within the air starts getting converted into dews. The temperature at which the water vapor within the air at some temperature starts condensing is called as the dew point temperature of the air or DPT. When the dew is formed the air is said to be in saturated condition.

When any object is cooled below the dew point temperature of the air, we can see the formation of sweat on its surface. A glass of water filled with chilled water is can example.

The dew point temperature of air depends upon its mositure content. Higher the mositure content of the air, higher is the dew point temperature. For the saturated air, the dry bulb temperature, the wet bulb temperature and dew point temperature are same.

1) Book: Principles of Refrigeration by Roy J. Dossat

2) Book: Basic Refrigeration and Air Conditioning by P N Ananthanarayan