Requirement of Air-Conditioning Systems

Essential requirement for an air-conditioning system are as follows:

1. Filters
2. Heating
3. Cooling
4. Humidification
5. Dehumidification
6. Air circulation

Figure 28.5 shows a simple air-conditioning plant.

Figure 28.5 Typical air-conditioning plant

1. Filters

These are used to have effective cleaning of the air so as to remove dust or other suit particles from it. Filters used for this purpose are either dry or viscous type. Dry filters are commonly used. These filters are made of close-grained felt, cloth or paper through which air is forced. When they become dirty, they may be cleaned by vacuum-cleaning methods. Viscous filters are made from a coarse-grained material coated with an adhesive or a viscous oil of the non-drying type. Thus they form sticky surfaces which attract and remove the dust particles. With use they become dirty, which are washed, recoiled and used again.

When exceptionally dust-free air is required, electric precipitators are used. They consist of a high-intensity ionising field with a secondary field where the dust is precipitated. These are expensive.

2. Heating

During winter season, the air-conditioner has to provide slightly heated air to the inside of the building. This arrangement further compensates the heat loss from the room. The pre-heating incoming air may be carried out by warm-air furnaces or by coils around wherein the hot water or steam is circulating. Before entering the room the incoming air is allowed to pass over warm-air furnaces. As a matter of fact, the circulation of hot water or steam around the coils makes them hot and where the incoming air comes into contact with heated coils, it also gets heated.

3. Cooling

During the summer season, the air-conditioner has to provide cooled air inside the room, for which the incoming air is to be cooled before its entry into the room. It is further to state that the cooling is necessary to remove excessive heat from the air. The cooling can be done by surface cooling, spray cooling or evaporative cooling.

4. Humidification

In winter the outside air may have high relative humidity. The relative humidity becomes low when the air is heated. If too low air becomes too dry giving a parched feeling. However, if air is too much humidified, it also results in feeling of discomfort. For human comfort proper control of humidity is therefore necessary. Hence, humidification is an important part of the air-conditioning system and so it is also called as the heart of air-conditioning system.

Humidification may be achieved by passing air over pans of water with large surfaces or by passing air through spray nozzles which break the water into tiny drops. The sprayed water may be reused. Eliminator plates are used to remove the droplets from the air (Fig. 28.5). They are arranged in such a way that the air is made to follow a tortuous path between them.

5. Dehumidification

This is obtained by first lowering the temperature of the air below the dew point. This is followed by condensing out the necessary amount of moisture and then re-heating with dry heat to the desired temperature. Cooling of the air is done then. Two common methods of cooling, viz., mechanical refrigeration or cooling by water spray may be adopted. Mechanical method is similar to household refrigerator. The second method is one wherein warm air is passed through a water spray. By the action of evaporation the air gets cooled. This method is good for dry climates. In humid climates, mechanical refrigeration would be preferred. Re-heating is done to ensure no heaviness due to high humidity.

6. Air Circulation

Air circulation or air distribution is the act of circulation or distribution of conditioned air inside a room or enclosure. This is one of the most essential requirement of the air-conditioned system. The minimum fresh air requirement is based on the number of users and whether they are smokers or non-smokers. The conditioned air should be spread in the room at such height and location so as to mix thoroughly with the air present in the room. The air circulation is performed using one of the apparatus, viz., air pumps, air delivery system (consisting of supply and return ducts) and air distribution system (consisting of inlets and outlets).