Refrigeration & Air-Conditioning

Module 7. Psychrometry

Lesson 28
HUMIDITY MEASUREMENT, DEHUMIDIFIERS, HUMIDITY CONTROL

28.1 Introduction

The measurement of dbt and wbt is a basic way to know the psychrometric properties of air by using psychrometric chart. It is necessary to locate the point on sychrometry chart to read values of other parameters such as relative humidity, dew point temperature, specific humidity etc. The dbt and wbt of the air are measured by using following type of psychrometers.

28.2 Laboratory Psychrometric

This is the simplest type of psychrometer used for the measurement of dbt and wbt of the air. It consists of two thermometers, one measure the dbt of the air and second measures wbt as its bulb is covered with wet cloth as shown in Fig. 28.1 (a). This type of psychrometer is commonly used in laboratories.

Fig. 28.1 Laboratory psychrometric

As the air is passed over a thermometer covered with wet wick, the moisture contained in the wick tends to evaporate. The cooling effect of evaporation lowers the temperature measured as compared to dry bulb thermometer. When the temperature measured by the wet bulb thermometer reaches steady state, then the heat absorbed from the thermometer bulb required for the evaporation of water vapour going into air is equal to the heat given by the air by convection to the thermometer. The evaporation rate of water from the wet wick depends upon the humidity of the air passing over it. The air having low humidity gives more evaporation rate and the drop in temperature measured by the wet bulb thermometer will be more and vice versa. This shows that the wbt is a measure of degree of saturation or relative humidity of the air.

28.2.1 Sling psychrometer

This psychrometer consists of two mercury thermometers mounted on a frame provided with a handle as shown in Fig. 26.2 (a). The psychrometer can be rotated with the helps of a handle to produce necessary air motion. The air velocities of 5 m/s to 10 m/s are recommended to get more accurate value of wbt. The rotating motion of sling psychrometer provides necessary air velocity over the thermometer.

Fig. 28.2 Sling psychrometer

28.2.2 Aspirating psychrometer

The aspirating psychrometer is shown in Fig 28.3 which has a small blower at the top for producing rapid motion of air over the thermometer bulbs. A provision is also made in the psychrometer to prevent errors due to radiant heat exchange. This type of psychrometer is used for measuring the dbt and wbt at a particular interval of time mostly for measuring the atmospheric conditions of air.

Fig. 28.3 Aspirating psychrometer

28.2.3 Continuous recording psychrometer/data logger

Continuous recording of psychrometers are available for measurement of dbt and relative humidity of the air using various types of sensors and transducers. The values of the air quality will be recorded and it is possible to get digital output of data.

28.3 Dehumidifiers

Dehumidification of air is required to control the RH of the air for comfort air conditioning, storage, food production, packing etc. The process of dehumidification by cooling has been explained in chapter 27. There are three main methods of dehumidification, viz.

a) Reducing the temperature of air below dew point

b) Absorption of moisture from air

c) Adsorption of moisture from air

Reducing the temperature of air below dew point: This may be carried out by two methods- either by passing the air over a cold surface below DPT, or by passing the air though a spray of cold water below DPT