Refrigeration and Air Conditioning-Unit-1.pptx

R&AC

1. Refrigeration and Air conditioning

2. Introduction Refrigeration: Refrigeration is the science of producing and maintaining temperatures below that of the surrounding atmosphere . The aim of the refrigeration is to cool some product or space to the required temperature. Air Conditioning refers to the treatment of air so as to simultaneously control its temperature, moisture content, cleanliness, odour and circulation, as required by occupants, a process, or products in the space.

3. Applications of Refrigeration Storage of raw fruits and vegetables Icing of fish Meat and poultry Dairy products Beverages Separation and condensation of gases Medical Desalination of water Ice manufacture

4. Applications of Air conditioning Industrial air conditioning Laboratories Printing Manufacture of precision products Textile, phrama , computer rooms, photographic Comfort air conditioning The residences, offices, shopping centers, stores, large buildings, theatres, hospital, auditorium etc.

5. Basic methods of producing low temperature Melting of solid Evaporation of solid Evaporation of liquid

6. History of Refrigeration NATURAL REFRIGERATION Art of Ice making by Nocturnal Cooling: Evaporative Cooling: Cooling by Salt Solutions ARTIFICIAL REFRIGERATION Vapour Compression Refrigeration Systems Vapour Absorption Refrigeration Systems Non Conventional refrigeration

7. Natural Refrigeration In olden days refrigeration was achieved by natural means such as the use of ice or evaporative cooling. In earlier times, ice was either: 1. Transported from colder regions, 2. Harvested in winter and stored in ice houses for summer use or, 3. Made during night by cooling of water by radiation to stratosphere

8. PERFORMANCE OF REFRIGERATION SYSTEM   The performance of the refrigeration system is expressed in terms of coefficient of performance which is defined as the ratio of heat absorbed by the refrigerant while passing through the evaporator to the work input required for compressor. COP = Refrigeration effect (R N ) / Work required (W) Relative COP is the ratio of actual COP to theoretical COP Relative COP = Actual COP / Theoretical COP

9. STANDARD RATING OF A REFRIGERATING MACHINE Rating of a refrigerating machine is given by a unit of refrigeration known as standard ton of refrigeration. Ton of refrigeration is defined as the refrigeration effect produced by the melting of one ton of ice from and at 0 C in 24 hours. Latent heat of fusion of ice is 336 kJ / kg. So 1 ton of refrigeration is given by 1 TR = 336 x 1000 / 24 = 14000 kJ / h. TR is basically American unit of refrigeration which is generally in BTU. After conversion it is rounded to 210 kJ / min or 3.5 kJ / sec.

10. Heat pump and Refrigerator

11. Coefficient of performance COP is defined as the ratio of refrigeration effect to work input. COP = Q/W Q= heat supplied to or removed from the system (COP) HP = 1+ (COP) R

12. AIR REFRIGERATION Introduction   Air cycle refrigeration systems belong to the general class of gas cycle refrigeration systems uses air as the working fluid. The air does not undergo any phase change during the cycle. Here all the internal heat transfer processes are sensible heat transfer processes. Air cycle refrigeration systems find applications in air craft cabin cooling and also in the liquefaction of various gases.

13. Air refrigeration system is classified into two categories Closed system Open system

14. Advantages of closed system over open system In closed system the suction to the compressor is at high pressure. Sizes of compressor and expander can be kept within reasonable limits by using dense air. In closed system no need of driers as in open system to remove the moisture present in air. Coefficient of performance of closed system is higher than that of open system. In open system air can expand up to the atmospheric pressure only. Where as in closed system there is no such restriction.

15. Air Standard Cycle analysis   Air cycle refrigeration system analysis is considerably simplified if one makes the following assumptions: The working fluid is a fixed mass of air that behaves as an ideal gas The cycle is assumed to be a closed cycle with all inlet and exhaust processes of open cycles being replaced by heat transfer processes to or from the environment

16. Air Refrigeration System using Reversed Carnot Cycle Reversed Carnot cycle is an ideal refrigeration cycle for constant temperature external heat source and heat sinks. Figure shows the schematic of a reversed Carnot refrigeration system using a gas as the working fluid along with the cycle diagram on T-s and P-v coordinates.

17. Reversed Carnot cycle

18. Process 1-2: Reversible, adiabatic compression in a compressor Process 2-3: Reversible, isothermal heat rejection in a compressor Process 3-4: Reversible, adiabatic expansion in a turbine Process 4-1: Reversible, isothermal heat absorption in a turbine The cycle consists of the following four processes

19. Limitations of Carnot cycle:   Carnot cycle is an idealization and it suffers from several practical limitations. It is very difficult to achieve isothermal heat transfer during processes 2-3 and 4-1. The volumetric refrigeration capacity of the Carnot system is very small leading to large compressor displacement, which gives rise to large frictional effects. All actual processes are irreversible; hence completely reversible cycles are idealizations only.

20. Reverse Brayton cycle (or) Bellcoleman Cycle

21. From the above expression for COP, the following observations can be made: For fixed heat rejection temperature (T 3 ) and fixed refrigeration temperature (T 1 ), the COP of Bell Coleman cycle is always lower than the COP of reverse Carnot cycle COP of Bell Coleman cycle approaches COP of Carnot cycle as T 1 approaches T 4 (thin cycle), however, the specific refrigeration effect [c p (T 1 -T 4 )] also reduces simultaneously. COP of reverse Bell Coleman cycle decreases as the pressure ratio r p increases

22. Aircraft Refrigeration systems Simple Air refrigeration system Boot strap air cooling system Regenerative air cooling system Reduced ambient air cooling system

23. Merits and Demerits of Air refrigeration Merits: The air is readily available and no cost of refrigerant The air is non toxic and non inflammable The weight per ton of refrigeration is less The pressure in the system is low, therefore piping and ducting are simple design Demerits: it has low co efficient of performance The rate of air circulation is relatively large.

24. Simple Air Refrigeration system

25. T-S diagram of simple air refrigeration system

26. Air refrigeration with evaporative cooling

27. T-S diagram of simple evaporative cooling system

28. Boot strap Air Cooling system

29. T-S diagram for Boot strap cooling system

30. Boot strap Air Evaporative cooling system

31. T-S diagram for Boot strap air evaporative cooling system

32. Regenerative air cooling system

33. T-S diagram for regenerative air cooling system

34. Reduced ambient air cooling system

35. T-S diagram for reduced ambient air cooling system

36. Comparison of Air refrigeration systems Simple air cooling system gives maximum cooling effect on the ground surface and decreases as the speed of aircraft increases. Bootstrap system requires the airplane to be in flight so that ram air can be used for cooling in the heat exchangers. For high speed aircrafts , the boot strap evaporative or regenerative systems are used because they give lower turbine discharge temperature.

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