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How does an Air Conditioning system works?

Introduction

An air conditioning system which enables the conditions inside to be maintained can be explained as a series of cycles:-

1. The Refrigeration Cycle.
2. The Air cycle.
3. The cooling water cycle.

The Refrigeration Cycle

For all compression type mechanical refrigeration systems, the refrigeration cycle consists of four processes i.e.

1. Heat gain by the refrigerant in the evaporator
2. Pressure rise in the compressor
3. Heat loss in the condenser
4. Pressure loss in the expansion valve.

Compression process, that is the pressure rise is accomplished at the expense of energy added to the compressor in the form of shaft work. The expansion, i.e. pressure loss is allowed to occur adiabatically (with no loss or gain, of heat) in the expansion valve where the energy released by the expanding refrigerant is not utilized for doing any external work but causes evaporation of a portion of liquid. Basically in a refrigeration cycle the heat transfer process does not involve any work and the pressure change processes do not involve any heat transfer.

The four processes of refrigeration cycle can be explained in the four steps -Thermodynamic processes.

1. Evaporation

Liquid refrigerant at low pressure absorbs heat from the space or room to be cooled and evaporates in the evaporator or cooling coil.

2. Compression

The refrigerant vapor drawn from the evaporator is compressed in a compressor consisting of a piston and cylinder. The saturated refrigerant vapor is super heated and its boiling point is raised due to increased pressure.

3. Condensation

The super heated high-pressure vapor rejects its super heat in the condensing medium (water) and condenses into liquid.

4. Expansion

The high pressure liquid refrigerant from the condenser in passing through the expansion valve is reduced to low pressure and temperature. The expansion valve is a combined pressure reducing and monitoring device.

Air Cycle (Ref. Fig.)

The air handling unit comprises of a chamber for mixing the fresh outside air drawn through electronic air Filters with the return air from conditioned room. This chamber is also referred to as weather-maker room. The mixture of fresh air and return air is then passed through the mechanical air filters for eliminating the large size dust particles if present in the return air. The pre-heaters also called as strip heaters are generally switched on it winter season. Spray humidifiers are provided for humidifying the air as per the instructions from sensing device and by pass damper actuated by the damper motor as per the instructions from sensing device Thermostat.

The air further is passed over the cooling coil (evaporator which is cooling-cum-dehumidifying equipment) where transfer of heat takes place. Spray eliminators are provided to arrest the water drops. The re-heaters a set of electrical or warm water heaters are provided for monsoon heating. Air stat is located near the reheated and senses the temperature of reheated and helps in avoiding the fire risks.

The air circulating fan (Blower) is located in blower room driven by the fan motor. The blower draws the air over the evaporator or cooling coils and pushes it to the Air conditioned switch room through supply ducts.

The air further comes back to weather maker room and air cycle is repeated.

Humidistat and thermostat are located in the weather maker room.

 

Water Cycle

Water is used as cooling medium in the condenser in preference to air, because at a given temperature, the heat capacity of water is about 3,500 times more than air. Only in very small systems, air cooling is resorted to in condensers.

Water can be cooled in many ways:-

1. a) by bringing air and water together indirectly as is done in automobile radiator;
2. b) by evaporative cooling, as in a cooling pond, spray pond or cooling tower.

In evaporative cooling, the water can be cooled below the atmospheric dry bulb temperature, whereas in radiator cooling, the water cannot be cooled below the dry bulb temperature. Since the cooling effect is more with evaporative cooling, a smaller amount of circulating water is sufficient. Of all cooling arrangements, the cooling tower is the one mostly used in air-conditioning & refrigeration plants.

In a typical cooling tower system, the water from the sump flows to the centrifugal pump by gravity. The pump forces the water through the condenser, where it picks up heat from the condensing refrigerant. Warm water from the condenser goes to the spray nozzles in the tower. As the sprays fall down, the water comes in contact with the air flowing across. Part of the water evaporates and causes cooling.

As there is continuous evaporation of water, the loss is made good by means of a make-up connection controlled by a float valve, from the city water supply or an overhead tank. A quick fill connection is also given, so that it can be used for filling the cooling tower tank quickly whenever required. An overflow (bleeder) connection is provided so that a small quantity of water is continuously drained out to keep the water fresh and clean and to reduce formation of scales in the water piping. Drain connections is given for draining the water for the purpose of cleaning or shutting down the system for a long time. A strainer is provided at the entrance of the suction pipe in the cooling tower, so that extraneous matter will not get into the pipe and interfere with its working.

Under certain conditions, it becomes necessary that a small portion of the cooled water is not allowed to flow through the condenser but goes directly back to the cooling tower. For this purpose, a valve is provided between the inlet and outlet connections of the condenser to enable a portion of the cooled water to bypass the condenser.

It is seen that water is circulated in the condenser, so that the refrigerant may be condensed to a liquid at high pressure. Where an enormous quantity of cold water is readily available, there is no need to recover the water and reuse it. But in towns and cities, water is too valuable to be used in such an extravagant manner. The warm water leaving the condenser will have to be cooled and re-used.

Chilled water (indirect) system

In this plant chilled water cycle is additional to that of the three cycles provided in direct A/C plants.

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.

AIR CONDITIONING

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.

2. 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.

3. 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.

Example

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.

Example:

The volume of the room, say – 18000cu.ft.(510 cu.mtr.)

Per hour half of this volume is to be replaced by fresh air fan i.e. 9000 cu.Ft.  (205 cu. Mtr).

So, per minute it is to remove =

and capacity of the fan should be = 150 x 2 = 300 cfm.  (8.5 cu.mtr per  minute).

To provide Air movement :

The air conditioners for telephone exchange are different from other buildings. Here heat is continuously generated in the telecom racks (suite). The quick dissipation of heat from the components is the basic need of air conditioning of exchange building. So sufficient air velocity must be available to the racks for quick transfer of heat from the heat generating components. So A.C system provided in the exchange must be capable of supplying enough air with sufficient velocity to each rack.

To remove bad odour :

Positive pressure is maintained in the air conditioned area especially in the switch room the air conditioned area is kept closed to avoid leakage of cold air in case of positive pressure or infiltration of hot air in case of negative pressure in the air conditioned space. Abnormally positive pressure is maintained as continuous leakage exists. Since the air conditioned area remains normally closed bad odour in the air may occur. The continuous leakage and supply of fresh air helps to reduce the bad odour of stagnated air of the air conditioned space.