Refrigeration

1. Refrigeration P R Venkatesh, Mech Dept, RVCE, B'lore

2. Refrigeration • Refrigeration is the removal of heat from a material or space, so that it’s temperature is lower than that of it’s surroundings. • When refrigerant absorbs the unwanted heat, this raises the refrigerant’s temperature (“Saturation Temperature”) so that it changes from a liquid to a gas — it evaporates. The system then uses condensation to release the heat and change the refrigerant back into a liquid. This is called “Latent Heat”. P R Venkatesh, Mech Dept, RVCE, B'lore

3. P R Venkatesh, Mech Dept, RVCE, B'lore

4. Desirable properties of a refrigerant • Boiling point: An ideal refrigerant must have low boiling temperature at atmospheric pressure. • Freezing point: An ideal refrigerant must have a very low freezing point because the refrigerant should not freeze at low evaporator temperatures. • Evaporator and condenser pressure: In order to avoid the leakage of the atmospheric air and also to enable the detection of the leakage of the refrigerant, both the evaporator and condenser pressures should be slightly above the atmospheric pressure. P R Venkatesh, Mech Dept, RVCE, B'lore

5. Desirable properties of a refrigerant • Latent heat of evaporation: the latent heat of evaporation must be very high so that a minimum amount of refrigerant will accomplish the desired result, in other words, it increases the refrigeration effect. • Specific volume: The specific volume of the refrigerant must be very low. The lower specific volume of the refrigerant at the suction of the compressor reduces the size of the compressor. P R Venkatesh, Mech Dept, RVCE, B'lore

6. Desirable properties of a refrigerant • Specific heat of liquid and vapour: A refrigerant must have low specific heat when it is in liquid state and high specific heat when is vaporized. The low specific heat of the refrigerant helps in sub cooling of the liquid and high specific heat of the vapour helps in decreasing the superheating of the vapour. Both these desirable properties increase the refrigerating effect. P R Venkatesh, Mech Dept, RVCE, B'lore

7. Desirable properties of a refrigerant • Viscosity: The viscosity of a refrigerant at both the liquid and vapour states must be very low as it is improves the heat transfer and reduces the pumping pressure. • Toxicity: The refrigerant should be non toxic, because any leakage of the toxic refrigerant increases suffocation and poisons the atmosphere. • Corrosiveness: The refrigerant should be non corrosive to prevent the corrosion of the metallic parts of the refrigerators. • Chemical stability: An ideal refrigerant must not decompose under operating conditions. P R Venkatesh, Mech Dept, RVCE, B'lore

8. Desirable properties of a refrigerant • Coefficient of performance: The coefficient of performance of a refrigerant must be high so that the energy spent in refrigeration will be less. • Odour: The refrigerant must be odourless, otherwise some food stuff such as meat, butter, etc. loses their taste. • Leakage tests: The refrigerant must be such that any leakage can be detected by simple tests. • Action with lubricating oil: The refrigerant must not react with the lubricating oil used in lubricating the parts of the compressor. P R Venkatesh, Mech Dept, RVCE, B'lore

9. List of commonly used refrigerants The most commonly used refrigerants are • Ammonia – Used in vapour absorption refrigerator. • Carbon dioxide – Used in marine refrigerators. • Sulphur dioxide- Used in household refrigerators. • Methyl chloride-Used in small-scale refrigeration and domestic refrigerators. • Freon -12 – Used in domestic vapour compression refrigerators. • Freon-22 – Used in air conditioners. P R Venkatesh, Mech Dept, RVCE, B'lore

10. Principle of refrigeration: • In refrigeration, the heat is to be removed continuously from a system at a lower temperature and transfer it to the surroundings at a higher temperature. • This operation according to the Second Law of Thermodynamics can only be performed by the aid of the external work. • Therefore in a refrigerator, power is to be supplied to remove the heat continuously from the refrigerator cabinet to keep it cool at a temperature less than the atmospheric temperature. P R Venkatesh, Mech Dept, RVCE, B'lore

11. Refrigeration effect and unit of refrigeration: In a refrigeration system, the rate at which the heat is absorbed in a cycle from the space to be cooled is called refrigerating effect. • The capacity of a refrigeration system is expressed in tons of refrigeration which is the unit of refrigeration. • A ton of refrigeration is defined as the quantity of heat absorbed in order to form one ton of ice in 24 hours when the initial temperature of the water is 00 c. P R Venkatesh, Mech Dept, RVCE, B'lore

12. Refrigeration effect and unit of refrigeration: Here it should be noted that one American ton (2000 pounds) is taken as the standard in the refrigeration practice. In S.I System 1 ton of refrigeration = 210 kJ/min = 3.5 kW P R Venkatesh, Mech Dept, RVCE, B'lore

13. Coefficient of performance: • The performance of a refrigeration system is expressed by a factor known as the coefficient of performance (COP).The COP of a refrigeration system is defined as the ratio of heat absorbed in a system to the work supplied. If Q = Heat Absorbed or Removed, kW W = Work supplied, kW COP = Q/W P R Venkatesh, Mech Dept, RVCE, B'lore

14. Parts of a Refrigerator P R Venkatesh, Mech Dept, RVCE, B'lore

15. Parts of Refrigerator • Evaporator: It consists of metal tubing which surrounds the freezing & cooling compartments to absorb heat and to evaporate the refrigerant. • Circulating system (Compressor or pump): It consists of mechanical devices powered by an electric motor which increase the pressure of the refrigerant and helps to circulate the refrigerant. • Condenser: Here, the latent heat is rejected from the refrigerant to atmosphere and it returns to liquid state and re circulated. • Expansion device: It is a device used to reduce the pressure and temperature of the liquid before it passes to the evaporator. P R Venkatesh, Mech Dept, RVCE, B'lore

16. Types of Refrigeration systems • Vapour Compression Refrigeration • Vapour Absorption refrigeration • Vapour compression refrigeration makes use of mechanical energy supplied by the compressor to circulate the refrigerant. • Vapour absorption refrigeration makes use of the ability of a absorbent to absorb large volumes of vapour when cold & give off its vapours when heated. P R Venkatesh, Mech Dept, RVCE, B'lore

17. Vapour Compression Refrigeration P R Venkatesh, Mech Dept, RVCE, B'lore

18. Vapour Compression Refrigeration P R Venkatesh, Mech Dept, RVCE, B'lore

19. Vapour Compression Refrigeration • The refrigerant in the form of vapour at low temperature & pressure enters the compressor and gets compressed to high temperature & pressure. • The compressed vapours then enter the condenser where heat is transferred to a cooling medium such as air or water. The refrigerant is condensed to liquid form. • The refrigerant then enters the expansion valve which reduces the pressure and temperature by throttling process.(to about -100C) • It finally passes on to the evaporator where it absorbs heat from surroundings and becomes low pressure dry vapour, and the cycle is repeated. P R Venkatesh, Mech Dept, RVCE, B'lore

20. Vapour Absorption Refrigeration P R Venkatesh, Mech Dept, RVCE, B'lore

21. Vapour Absorption Refrigeration P R Venkatesh, Mech Dept, RVCE, B'lore

22. Vapour Absorption Refrigeration • The low pressure ammonia vapour leaving the evaporator is readily absorbed in the cold water in the absorber and becomes strong ammonia solution. • The strong ammonia solution is pumped to the heat exchanger and the vapour gets separated due to high pressure & temperature. • The ammonia vapours then pass to the condenser where it gets condensed into liquid after rejecting heat to atmosphere. • The high pressure ammonia liquid is then expanded in the throttle valve to get low pressure & temperature liquid. • The low pressure & temperature ammonia liquid then enters the evaporator where it extracts the heat and gets vapourized. The cycle then repeats itself. P R Venkatesh, Mech Dept, RVCE, B'lore

23. Comparison between Vapour compression & Vapour Absorption Refrigeration P R Venkatesh, Mech Dept, RVCE, B'lore

24. Air Conditioning • Air conditioning is defined as a process of providing cool and congenial indoor atmosphere for human comfort or for industrial purposes. • Air conditioning involves artificial cooling, humidifying or dehumidifying, cleaning and re circulating the surrounding air. • An air conditioner mainly consists of an evaporator, condenser, compressor, two fans & capillary tube. • It is generally mounted on a window such that the evaporator unit is inside the room and the condenser part projecting outside the building. P R Venkatesh, Mech Dept, RVCE, B'lore

25. Air Conditioner P R Venkatesh, Mech Dept, RVCE, B'lore

26. Air Conditioner P R Venkatesh, Mech Dept, RVCE, B'lore

27. Air Conditioning • The high pressure, low temperature liquid refrigerant from the condenser is passed to the evaporator coils through the capillary tubes where it undergoes expansion. • The evaporator fan continuously draws air from the interior space through an air filter by forcing it to pass over the evaporator coils. • The refrigerant absorbs the heat from the air, gets evaporated and then compressed in the compressor. • The high pressure & high temperature refrigerant vapour passes through condenser coils where it gets cooled by air drawn by the condenser fan from the atmosphere. • This condensed refrigerant once again enters the capillary tube & the process repeats itself. P R Venkatesh, Mech Dept, RVCE, B'lore

28. Humidity & its control in Air Conditioning • Humidity is defined as the moisture present in the atmospheric air. • Absolute humidity is the weight of water vapour contained in given volume of air & is expressed as gm/m3 of air. • Specific humidity is the weight of water vapour to the total weight of air volume of air & is expressed as gm/kg of air. • Relative humidity is the ratio of actual vapour content of air to the vapour content of saturated air at the same temperature. P R Venkatesh, Mech Dept, RVCE, B'lore

29. Humidity & its control in Air Conditioning • When a controlled atmosphere is required in air conditioning, the humidity of air is varied. • When dry air is required, it is usually dehumidified by passing the air through adsorptive chemicals such as silica jel. • On the other hand, air is humidified by circulation through water baths or sprays. • The level of discomfort is high when the atmosphere is saturated with water. • Psychrometry is the study of properties of air & water vapour mixture. P R Venkatesh, Mech Dept, RVCE, B'lore

30. Central Air Conditioning • Central air conditioning, is an air conditioning system that uses ducts to distribute cooled and/or dehumidified air to more than one room, or uses pipes to distribute chilled water to heat exchangers in more than one room, and which is not plugged into a standard electrical outlet. • With a typical split system, the condenser and compressor are located in an outdoor unit; the evaporator is mounted in the air handler unit. • With a package system, all components are located in a single outdoor unit that may be located on the ground or roof. P R Venkatesh, Mech Dept, RVCE, B'lore

31. Advantages of Central air conditioning • When the air handling unit turns on, room air is drawn in from various parts of the building through return-air ducts. • This air is pulled through a filter where airborne particles such as dust and lint are removed. • Sophisticated filters may remove microscopic pollutants as well. The filtered air is routed to air supply ductwork that carries it back to rooms. Whenever the air conditioner is running, this cycle repeats continually. • Because the condenser unit (with its fan and the compressor) is located outside the home, it offers a lower level of indoor noise than a free-standing air conditioning unit. P R Venkatesh, Mech Dept, RVCE, B'lore