How does an Air Conditioning system works?
An air conditioning system which enables the conditions inside to be maintained can be explained as a series of cycles:-
- The Refrigeration Cycle.
- The Air cycle.
- The cooling water cycle.
The Refrigeration Cycle
For all compression type mechanical refrigeration systems, the refrigeration cycle consists of four processes i.e.
- Heat gain by the refrigerant in the evaporator
- Pressure rise in the compressor
- Heat loss in the condenser
- 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.
Liquid refrigerant at low pressure absorbs heat from the space or room to be cooled and evaporates in the evaporator or cooling coil.
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.
The super heated high-pressure vapor rejects its super heat in the condensing medium (water) and condenses into liquid.
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 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:-
- a) by bringing air and water together indirectly as is done in automobile radiator;
- 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.