Refrigeration and air-conditioning has become necessary in every field of human utility in houses, schools, cinema houses, restaurants, cars, buses, railways. It is also used in industries for various processes. Basically it is employed for the comfort of human beings in domestic appliances and industrial equipment. In the industry it is employed to maintain the temperature of factories. working place so that workers can do their duties efficiently. It is also used for producing ice, air conditioning of buildings, coaches and milk vanes.
REFRIGERATION
Some Commonly Used Terms :
1. Refrigeration: It is the process of removing heat from substances under controlled conditions. It also includes the process of reducing and maintaining the temperature of the body below the general temperature of its surroundings.
2. Refrigerator: This is a machine by which the temperature of substances is lowered by storing them in this machine.
3. Refrigerator system: The mechanism used for producing low temperature in a body whose temperature is already below the surroundings temperature is called a refrigeration system.
4. Air Conditioning: It involves the control of the temperature about 25°C and 50% humidity and motion of air in an enclosure. Air conditioner consists of filters for removing lint and dust and also having equipment for removing odours etc.
5. Refrigerant: It is a substance (generally a gas) which is circulated in a refrigeration system. It absorbs heat from a cold body and delivers it to a hot body. Freon, ammonia carbon dioxide, sulpher dioxide are the commonly used refrigerants.
Refrigerating Effect
The heat carrying ability of the refrigerant is called the refrigerating effect. The refrigerating fect may be found by determining the amount of heat that one kg of the refrigerant is capable of absorbing as it passes through the evaporator.
Properties of Refrigerants
The refrigerants used should possess the following properties:
1. It should be non-corrosive.
2. It should be non-inflammable,
3. It should be non-toxic.
4. It should be free from objectionable odour.
5. It should get liquified at a reasonable low pressure.
6. Economical in cost in the quantity to be used and easy availability.
7. No action with lubricating oil.
Refrigeration Cycle
In order to lower the temperature of a body below that of its surroundings and then maintain it, a continuous extraction of the heat from the body is required. This is done by a mechanism called the refrigeration system. The total amount of heat being rejected to the outside body consists of two parts i.e. heat extracted from the body to be cooled and the heat equivalent to the mechanical work required for extracting it.
The refrigeration cycle starts from the evaporator inlet (1) in which low pressure liquid undergoes expansion, absorbs hent and evaporator changing to a low pressure vapour at the outlet of the evaporation (2). During the process of expansion, about 20-25% of the refrigerant changes to vapour.
The compressor (3) sucks this vapour from the evaporator and increases in pressure. It is then discharged to the condenser (4) where the heat is removed from the vapour and condensed into liquid. Now the refrigerant moves into the drier, which prevents the plugging of the flow control device by dirt and moisture etc., and then refrigerant flow into the evaporator continues and this process continues till a temperature control device i.e., thermostat stops the compressor. When the temperature again increases, the compressor starts once again and the cycle continued.
Refrigeration System
There are three refrigeration systems:
(i) Vapour compression refrigeration system.
(ii) Vapour absorption refrigeration system.
(iii) Thermo-electric refrigeration system.
Vapour Compression Refrigeration System
This is the most popular system of refrigeration employed in domestic and industrial refrigeration and air conditioning plants. It works on the principle that when liquid evaporates, it absorbs heat. Once the refrigerant is charged in the system, the same refrigerant is used again and again.
The vapour compression cycle is chiefly composed of four stages i.e., compression, condensation, expansion and vaporisation. The purpose of the refrigeration cycle is to remove heat from refrigerant by again putting it into liquid form so that it may be used again and again.
The four main stages of vapour compression cycle are discussed as below:
(i) Vapourisation: From the expansion valve, the low pressure refrigerant at low temperature enters the evaporator which is placed near or around the material to be cooled down. Before entering the evaporator, the refrigerant is in the liquid state and when it absorbs heat from the material to be refrigerated it is transformed from a liquid to a vapour state. The process is called evaporation.
(ii) Compression: The compressor draws the vapours from the evaporator through the return piping and then compresses these vapours until their temperature is raised above that of the condensing medium. The compressor is driven by external mechanical energy which may be supplied by an engine or electric motor. Generally, electric motors are employed for this purpose. This process is called compression.
(iii) Condensation: When the temperature of the vapour is raised above that of the condensing medium by the compressor, then the heat of vaporization will flow from the vapour to the condensing medium to condense the refrigerant to a high pressure liquid. This high pressure liquid then flows to the receiver where it is stored until it is supplied to the cooling unit through the expansion valve. This process is called condensation.
(iv) Expansion: As the compressor withdraws the refrigerant vapour from the evaporator. The cooling unit must be supplied with a low temperature, low pressure refrigerant capable of absorbing heat. This is done by a liquid control valve known as an expansion valve. This valve reduces the pressure of the high pressure liquid from the receiver to a low pressure liquid capable of absorbing heat. This also maintains a constant supply of liquid in the evaporator and acts as a dividing point between high and low pressure sides of the system. The process is called expansion.
Vapour Absorption Refrigeration System
It is one of the oldest methods of producing refrigerating effects. The refrigerant, commonly used in this system is ammonia.
In the vapour absorption refrigeration system, the compressor is replaced by an absorber, a pump, a generator and a pressure reducing valve. In addition to condenser, receiver, expansion valve and evaporator as in the vapour compression refrigeration system. In this system the vapour refrigerant from the evaporator is drawn into an absorber where it is absorbed by the weak solution of the refrigerant forming a strong solution.
This strong solution is pumped to the generator where it is heated by some external source. During the heating process, the vapour refrigerant is driven off by the solution and enters into the condenser where it is liquified. The liquified refrigerant then flow into the evaporator and thus the cycle is completed.
AIR CONDITIONER
Air conditioning may be defined as the production of an artificial atmosphere specially adapted to particular requirements or air-conditioning means control of temperature, humidity, purity and movement of air. The working conditions and comfort of human beings is improved by air conditioning. The humidity is maintained at 60% being most suitable for human beings. By air conditioning we heat the rooms in winter and cool them in summer. The air is also made dust free. The air conditioner is based on the principle of vapour compression refrigeration system.
The various parts of a room air conditioner are:
1. Compressor unit
2. Condenser
3. Receiver
4. Evaporator
5. Capillary tube
6. Impeller fan
7. Centrifugal fan
8. Partition
9. Air filter
Working
In the case of a window type air-conditioner, the evaporator faces towards the inside of the room whereas the condenser faces towards the outside of the room.
The centrifugal fan sucks air from the room and passes it over the evaporator coils. As the room air passes over the evaporator coils (the surface temperature of the coils is very low) it is cooled and this cooled air is supplied to the room. An impeller fan sucks outside air through the sides and throws it over the condenser and cools down the refrigerant passing through the condenser tubes.
The evaporator side and the condenser side are completely separated from each other by means of a partition.
Electrical Circuit of Air Conditioner
The following electrical components are fitted with the air conditioner.
1. Motor compressor with overload protection.
2. Starting relay
3. Starting and running capacitor
4. Fan motor with capacitor
5. Thermostat
6. On/Off switch with speed control
When the supply is 'ON", the high current flows in the motor starting winding, it starts and also the relay contacts close and connect the starting capacitor to the starting winding. As the motor gradually reaches the normal speed, the current in the running winding decreases, making the relay open, thus disconnecting the supply to the starting winding of the motors.
The overload protector protects the motor against excessive overload. This protection is provided inside the compressor and its function is to open the compressor motor circuit. Basically it is a bimetallic dise with contacts. When excessive current flows in the motor, the temperature increases thus opening the circuit automatically to stop the motor. The electrical circuit comprises the following components:
1. Fan Motor: 1-phi split phase motor is used for fan motors. The fan motor is provided with an ris overload protection placed in winding so that in the event of overheating it is protected.
2. Starting Capacitor: The starting capacitor (C) is of large capacity. The rating of any capacitor is given in terms of voltage and KVAR, whereas its capacitance is in microfarad. It remains in the circuit for a short time, till the motor starts.
3. Running Capacitor: The running capacitor (C) is of smaller capacitance but heavy duty and is oil filled. It remains in the circuit continuously and improves the starting torque and power factor.
4. Thermostat: The function of the thermostat is to monitor the room temperature and start or stop the motor. When the room temperature increases, the contacts of the thermostat close and start the compressor and when the temperature decreases the contacts open up and the compressor of the air conditioner stops.
Advantages of Room Air Conditioners
1. Saving in installation.
2. Only those rooms which need cooling will have their units running.
3. Low initial cost.
4. Flexibility of operation.
5. Duct work is eliminated.
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