In this article, I am going to discuss the domestic refrigerator working principle and construction. You will find this article interesting and informative. So let us start.
The common type of domestic refrigerator has a cabinet shaped with compressor, the condenser and receiver fitted in their basement. The expansion valve and evaporator coils are exposed in the storage cabinet with the pipings, carrying liquid refrigerant passing through the body. Generally, methylene chloride, Freon-12, and Freon-11 are used as the refrigerants.
Refrigeration is not only provided with double-walled cabinet packed with materials having high thermal insulation such as fiberglass, cork or expanded rubber but also all around the inside of door flap soft rubber seal is used which makes the cabinet airtight.
Also the door is provided with automatic closing mechanism—door hinges are provided in such a way that door flap when left in open position automatically comes to closing position due to gravity and as it approaches closing position it is attracted by the magnetic strip fitted behind the sealing rubber ring and thus the door is closed with snap action.
All this is done to prevent leakage of atmospheric heat inside the refrigerator. The main precaution to be taken is that very hot things should not be put in the refrigerator if it is done it will quickly evaporate the refrigerant in evaporator coils producing a large vapor pressure increasing the duty of the compressor. It may damage the motor, which is short-time rated.
Domestic Refrigerator Working Principle
The heat of the items, to be cooled is carried to the evaporator coils by means of air trapped in the cabinet. The working fluid, known as a refrigerant, used in refrigerator readily evaporates and condenses or changes alternately between the vapor and liquid phases without leaving the refrigerator. The refrigerant keeps circulating from evaporator coil to condenser till compressor motor is connected to the supply.
During evaporation, it absorbs heat from items placed in the refrigerator and in condensing or cooling or liquefying it rejects heat outside the refrigerator. The heat absorbed from items placed in the refrigerator during evaporation is used as its latent heat for converting it from liquid to vapor. Thus a cooling effect is created in the working fluid. And this decreases the temperature inside the refrigerator.
When a predetermined value of the temperature is achieved inside the refrigerator, thermostat switch operates and disconnects the compressor motor from the electric supply. Further circulation of refrigerant and its cooling effect stops.
After some time, when the temperature increases and reaches up to a predetermined value, thermostat operates again and connects the compressor motor to the supply. And the cooling process starts again. This cycle is repeated continuously to maintain the temperature in a predetermined temperature range.
In this way, the refrigerant is circulated through the coils of the refrigerator to maintain the temperature in the required temperature range.
Electrical Circuit of a Refrigerator
The electrical circuit of a refrigerator is shown in Figure. The refrigerator is provided with a door push switch, which closes on the opening of the refrigerator and puts the lamp on. Split-phase single phase induction motor is used in the sealed compressor unit to run the compressor. Thermal overload release is provided to protect the motor from damage against the flow of over-current.
Thermostat switch is provided to control the temperature inside the refrigerator. The temperature inside the refrigerator can be adjusted using the temperature control screw. Greater the distance between contacts, the greater will be the temperature inside the refrigerator and vice versa. To protect the motor against under-voltage use of automatic voltage regulator is essential since, in case of fall in applied voltage, the motor will draw heavy current to develop the required torque and will become hot.
Vapor Compression System Parts
The refrigerant in this unit is circulated through the various components of the system with the help of a motor installed in the compressor unit where it undergoes a number of changes in its state or condition. Each cycle of operation consists of the four fundamental changes in the state of the refrigerant:
(i) expansion (ii) vaporization (iii) compression and (iv) condensation.
The vapor compression system of a domestic refrigerator consists of the following five essential parts:
Compressor: The low pressure and temperature vapor refrigerant from the evaporator is drawn into the compressor through the inlet or suction valve, where it is compressed to high pressure and temperature. The high pressure and temperature vapor refrigerant is discharged into the condenser through the delivery or discharge valve.
Condenser: The condenser or cooler consists of coils of pipe in which the high pressure and temperature vapor refrigerant is cooled and condensed. The refrigerant, while passing through the condenser, rejects its latent heat to the external surrounding air. Thus hot refrigerant vapor received from the compressor is converted into liquid form in the condenser.
Receiver: The condensed liquid refrigerant from the condenser is stored in a vessel, known as a receiver, from where it is supplied to the expansion valve or refrigerant control valve.
Expansion Valve or Throttle Valve: The function of this valve is to allow the liquid refrigerant under high pressure and temperature to pass at a controlled rate after reducing its pressure and temperature. Some of the liquid refrigerant evaporates as it passes through the expansion valve, but the greater portion is vaporized in the evaporator at the low pressure and temperature.
Evaporator: An evaporator consists of coils of pipe in which the liquid-vapor refrigerant at low pressure and temperature is evaporated and changed into vapor refrigerant at low pressure and temperature. During the evaporation process, the liquid-vapor refrigerant absorbs its latent heat of vaporization from the medium which is to be cooled (i.e. items placed in the refrigerator).
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1. A one tonne refrigerating machine means that
(a) the total weight of the machine is one tonne.
(b) the quantity of the refrigerant used is one tonne.
(c) one tonne of water can be converted into ice.
(d) one tonne of ice when melts from and at 0°C in 24 hours, the refrigerating effect is equivalent to 14,000 kJ per hour.
2. The capacity of a refrigerating machine is expressed as
(a) inside volume of the cabinet.
(b) lowest temperature attained.
(c) gross weight of machine in tonnes.
(d) rate of abstraction of heat from the space being cooled.
3. Which of the following properties of a refrigerant is undesirable ?
(a) High critical temperature.
(b) Low specific heat of liquid.
(c) High boiling point.
(d) Low specific volume of vapor.
(e) High latent heat of vaporization.
4. Various desirable properties which should be possessed by refrigerants are
(a) non-corrosive and non-inflammability.
(b) no-toxic and freedom from objectionable odor with low working pressure.
(c) both (a) and (b).
(d) none of the above.
5. In a refrigeration cycle, the flow of refrigerant is controlled by
(b) expansion valve.
6. Which part of the vapor compression refrigeration cycle, produces the refrigeration effect ?
(c) Expansion valve.
7. ……is used as refrigerant in a vapor absorption refrigerator.
(a) Acqua ammonia
(d) Sulfur dioxide
8. The refrigerant Freon 12 is a compound consisting of
(a) carbon, fluorine and chlorine.
(b) carbon, chlorine and hydrogen.
(c) carbon, fluorine and hydrogen.
(d) chlorine, bromine and iodine.
9. Ammonia is preferred as a refrigerant in large commercial installations because
(a) it is non-tonic.
(b) it has a low working pressure.
(c) it is relatively cheap.
(d) it has low latent heat.
10. In a vapor compression system, which of the following units is adversely affected by the presence of moisture ?
(b) Expansion valve.
11. In a vapor absorption system lithium bromide is used as
(d) cooling substance.
12. Vapor absorption refrigeration systems have the advantage(s) of
(a) absence of moving parts.
(b) universal power source.
(c) automatic defrostration.
(d) both (a) and (b).
13. Thermoelectric refrigeration system has the advantage(s) of
(a) small initial cost.
(b) absence of moving parts.
(c) ease of automatic control by variation of magnitude of current.
(d) both (b) and (c).
14. In a domestic refrigerator, compressor and motor are assembled in a single unit known as ……….. sealed unit.
(d) none of these
15. Hermetically sealed units have the advantage(s) of
(a) relatively silent operation.
(b) minimum gas leakage problem because no moving part extends through the sealing housing.
(c) minimum maintenance requirement.
(d) all of the above.
16. Which of the following motors is used in household refrigerators ?
(a) DC shunt motor.
(b) Reluctance motor.
(c) Single phase induction motor.
(d) Synchronous motor.
17. The compressor of a domestic refrigerator has been noted to run for comparatively large durations, it indicates that
(a) thermostat has become defective.
(b) the machine is overloaded.
(c) capillary tube is choked.
(d) none of the above.
18. In a refrigerator, defrosting may be done by
(a) opening the door of the refrigerator.
(b) stopping the compressor for a while.
(c) placing trays of warm waters in the refrigerator.
(d) any of the above.
19. Types of water coolers are
(a) instantaneous or pressure type.
(b) storage type.
(c) exhaust fan type.
(d) both (a) and (b).
20. Air-conditioning is the simultaneous control of……….in a confined space.
(c) air movement
(d) temperature, humidity, purity and movement of air
21. ……….is a process which is generally used for summer air-conditioning.
(a) Cooling with dehumidification
(b) Chemical dehumidification
(c) Adiabatic cooling
(d) Cooling with humidification
22. ………..is a process that is generally used in winter air-conditioning.
(c) Heating and humidification
(d) Heating and dehumidification
23. Air is dehumidified by
(b) chemical absorption.
(d) both (a) and (b).
24. Compared to individual systems, the central air-conditioning system has ………… overall efficiency.
25. The main disadvantage(s) of central type air-conditioning s/are that
(a) dusting is very costly.
(b) absence of any individual room temperature adjustments.
(c) cigarette smoke and bacteria present in the return air from infected rooms redistribute to healthy rooms.
(d) all of the above.
26. Which of the following unitary-central systems are in common use ?
(a) Fan coil units.
(b) Induction units.
(c) All air high velocity systems.
(d) All of the above.
27. The conditions conductive to comfort depends upon
(c) air movement.
(d) air purity.
(e) all of the above factors.
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