Air Conditioning system | AC System

Chilled water AC Plant

In this system, the heat inside the air conditioned space is picked up by cold supply air, which gets heated up and this warm return air gives up its heat to the chilled water in the dehumidifier (cooling coil) after passing over the preheater and mechanical filter. In chilled water system, heat picked up by the chilled water is given up to the refrigerant at the chiller. Refrigerant vapor gets compressed in the compressor and gives up the heat at the condenser to the water, which is taken to the cooling tower and given up in the atmosphere.

Thus by a series of heat transfer process, both the heat entering the conditioned space and the heat produced inside it are given back to the outside atmosphere. In chilled water plant, the boiling refrigerant is not used to extract heat directly from the air. It is used for chilling water to a low temperature and this chilled water is circulated through the cooling coil, where it picks up heat from the air passing over its surface and comes back to the chiller to be chilled again. A separate pump set is used for circulating this chilled water. This is known as the chilled water system or indirect system because heat is first removed by chilled water. Where as in direct system the heat is directly picked up from return air and given up at cooling tower.


  1. Introduction: –

With the advancement of science and technology, specially the electronics equipments, the Air Conditioning has become a day-to-day need. Besides the industrial need, the Air conditioning is used for human comforts. Even in the household it serves the people in the form a refrigerator, deep fridger, so, its need is felt from household to the office, work place, industries, shops etc.

Presently every individual has to have some knowledge of Air Conditioning to meet his day-to-day need.

This booklet includes giving the reader the most basic knowledge of Air conditioning and refrigeration.

  1. Objective:

The objective of this booklet is to give the knowledge which will make a person capable to attend the minor defects arising out of day to day working and to carry out the preventive maintenance of the Air-cooled Air Conditioning system or units.

  1. Terminologies:

Air conditioning engineering required some technical terms to be frequently used. These terms are described below.

Air Conditioning

This word is more or less known to everybody. To general masses it appears to be a system of activities, which keep the temperature of a desired place within certain limits. However its utility varies from place to place. For example, whatever is suitable for human comforts may not be suitable for cold storage.

For telephone exchange building the meaning of Air Conditioning is different from others. The Air conditioning is provided for the exchange to cater the following needs.

  1. To maintain the temperature

As per the latest instruction of the B.S.N.L the temperature in the switch room of an electronics exchange is to be kept 230  30C. It means that the switch room temperature should be maintained between 200C to 260C within the range of temperature the telecom equipment would maintain its own properties without any adverse effect. So if the temperature range in the switch room is maintained within the above-specified limits there will be no adverse effect on the functioning. So if the thermostat of the A.C unit is set at following range the specified status of temperature can satisfactorily be maintained.

Setting of the thermostat:
  1. Cutout temperature: 230C
  2. Cut in temperature: 250C

The temperature difference of cut in and cut out settings is called differential. It is adjusted by a screw provided in the thermostat itself.

  1. To maintain humidity:

As per the latest practice in the department the relative humidity in the switch room to be maintain at 45%  15 %. Means the switch room relative humidity range is to be 30% to 60%.

The relative humidity in the switch room is controlled by apparatus dew point temperature. It is well known fact that cooling coil or the evaporator does two functions. One is removal of heat by absorbing heat from the air conditioned area and other function is to remove moisture by condensing it into water droplets. The former one is sensible cooling and the later one is latent cooling. The total cooling of the evaporator remains same. Only the division between these two functions can be adjusted by regulating the apparatus dew point temperature.


Switch room temperature is maintained – 740F

Relative humidity to be maintained – 450F

The apparatus dew point is to be set at – 510F

Dust Level:

The A.C units supply cold air to the specified space required for cooling. This air is available to the evaporator as return air from that space, some times fresh air and leakage in the package room. The mixed air contains a lot of dust particles, which are harmful for the telecom equipments. So it is desired that the air supplied after cooling is free from dust particles. Absolute cleaning of air is not possible. However cleaning is done to reduce the quantum of dust particles in the supply air. Two types of filters are used for cleaning the air to be supplied to the air conditioned area.

The coarse filter is provided in the return air path to filter out the dust particles of larger size. So its efficiency is referred as 90% down to 20 microns. It means the filter will block 90% of the dust particles having the diameter of 20 microns.

The fine filters, filters out the dust particles of 5 microns having the efficiency of 99.9%.

Fresh Air:

Many times the air conditioned space is also utilized for human occupancy. The human being requires oxygen for his breathing. So fresh air is supplied to the air conditioned space to provide sufficient oxygen for human occupancy. The fresh air is taken from outside, from a place which has high oxygen content. The fresh air contains higher quantum of heat, moisture and dust particles. So the fresh air is provided with coarse and fine filters. As per the latest circular of the department the quantum of fresh air to be supplied is half air change per hour. However the capacity of the fresh air fan should be one air change per hour.

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