1.refrigeration

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1.refrigeration

  1. 1. Refrigeration and Air conditioning
  2. 2. Lesson Plan • Vapour compression refrigeration cycle • Components of a refrigeration system • Pressure enthalpy chart • superheating & sub-cooling • Heat exchanger • Coefficient of performance ( COP ) • System Capacity • Direct and Indirect Expansion System • Back Pressure Regulating valve • Types of compressor • Oil Separator • Filter/Drier • Throttling device • Capacity control Method
  3. 3. Basic refrigeration cycle: • Heat energy flows from a hot region to a cooler region. • Vapour Compression Refrigeration System uses a circulating refrigerant as a medium which 1) absorbs & removes heat from space to be cooled 2) rejects the heat elsewhere (cooler) Heat energy Refrigerant flow cooler Cooling water Cold room
  4. 4. Vapour Compression System Low Pressure Side High Pressure Side Liquid receiver Expansion valve Compressor Evaporator Liquid Condenser Heat in Heat out Hot Gas Gas 4 numbers principle components : (1) Evaporator (2) Compressor (3) Condenser (4) Expansion Valve
  5. 5. Vapour Compression System Low Pressure Side High Pressure Side Liquid receiver Expansion valve Compressor Evaporator Liquid Condenser Heat in Heat out Hot Gas Gas EVAPORATOR: 1) The evaporator coils are located in the compartment to be cooled. 2) The low pressure liquid refrigerant ,after passing through the expansion valve, expands. 3) Takes in heat from the surrounding and evaporates. 4) The gas is then sucked up by the compressor.
  6. 6. Vapour Compression System Low Pressure Side High Pressure Side Liquid receiver Expansion valve Compressor Evaporator Liquid Condenser Heat in Heat out Hot Gas Gas COMPRESSOR : 1) Compresses the refrigerant (gaseous state). 2) Raising its Temperature & Pressure. 3) Discharges refrigerant to Condenser.
  7. 7. Vapour Compression System Low Pressure Side High Pressure Side Liquid receiver Expansion valve Compressor Evaporator Liquid Condenser Heat in Heat out Hot Gas Gas LIQUEFACTION: 1) Hot refrigerant gas cooled in the condenser. 2) Condensed liquid refrigerant flows into a receiver. 3) Then liquid refrigerant flows to the expansion valve.
  8. 8. Vapour Compression System Low Pressure Side High Pressure Side Liquid receiver Expansion valve Compressor Evaporator Liquid Condenser Heat in Heat out Hot Gas Gas EXPANSION: 1) The expansion valve acting as a regulating valve, limits the amount of refrigerant flowing through. 2) Resulting in reduction of pressure of the liquid and expansion takes place.
  9. 9. P-H chart ( Pressure – Enthalpy chart ) Sub cooled liquid • Pressure – Absolute pressure Unit : bar , psi • Enthalpy – Total amount of energy per unit weight of substance. Unit : BTU / Lb or kJ / kg • The lines ,saturated liquid & vapour respectively are plots of pressure vs enthalpy for the saturated state of a given refrigerant. • This chart is used to understand the property changes that takes place in each phase of the cycle. Saturated liquid line Saturated vapour line Superheated region Sub-cooled region Liquid – vapour mixture
  10. 10. • Enthalpy – Total amount of energy per unit weight of substance. Unit : BTU / Lb or kJ / kg • Entropy – Measure of heat dispersion in a system divided by temperature. Unit : BTU / Lb / deg change or kJ / kg / deg change for a substance. Refrigeration Cycle : Pressure-Enthalpy graph
  11. 11. Ideal Refrigeration Cycle : Pressure-Enthalpy chart Superheated vapour Sub cooled liquid Liquid vapour mixture Ideal Refrigeration Cycle : Pressure – Enthalpy chart Enthalpy ( BTU / lbs or KJ / kg ) Pressure(absolute) P1 P2 1 2 34
  12. 12. Refrigeration Cycle : Pressure-Enthalpy chart Non ideal Refrigeration Cycle : Pressure – Enthalpy chart , showing superheating & sub cooling Sub cooled liquid Superheated vapour Liquid vapour mixture Enthalpy ( BTU / lbs or KJ / kg ) Pressure(absolute) 1 4 2 Liquid to Vapour Transformation in EVAPORATOR Throttling at expansion valve Vapour to Liquid transformation in CONDENSER Work done in the compressor Liquid vapour mixture superheated Superheated subcooling 3
  13. 13. Non ideal Refrigeration Cycle : Pressure – Enthalpy chart , showing superheating & sub cooling Sub cooled liquid Superheated vapour Liquid vapour mixture Enthalpy ( BTU / lbs or KJ / kg ) Pressure(absolute) 1 34 2Liquid to Vapour Transformation in EVAPORATOR Throttling at expansion valve Vapour to Liquid transformation in CONDENSER Work done in the compressor H1 H2 H3 The amount of heat that the refrigerant absorb must equal the cooling load. (1) Refrigerant cooling load ( F ) = cooling load / ( H2 – H1) (2) Work done by compressor = F x ( H3 – H2 ) (3) Heat rejected by condenser = F x ( H3 – H1 ) (4) Heat absorbed by evaporator = F x ( H2 – H1 ) Coefficient of Performance (COP) = heat absorbed by refrigerant / Energy required driving compressor = ( H2 – H1) / ( H3 – H2)
  14. 14. Pressure-Enthalpy chart Pressure ( bar ) Enthalpy ( KJ / kg of refrigerant ) Liquid to Vapour Transformation in EVAPORATOR at -13 deg C Vapour to Liquid transformation in CONDENSER at 42 deg C Work done in the compressor Throttling at expansion valve 1 4 1-2 : EVAPORATOR – extraction of heat from room 2-3 : COMPRESSOR – compression work 3-4 : CONDENSER – energy thrown to sea 4-1 : EXPANSION VALVE – throttling at the expansion valve 3 2 16 3.2 150 304 365 For each kg of refrigerant flow , Energy extracted from meat room : 304 - 150 = 154 KJ / kg Work spent on compressor = 365 – 304 = 61 KJ / Kg Coefficient of Performance ( COP ) = Energy extracted from room / Energy spent = 154 / 61 or 2.52
  15. 15. • Pressure – Enthalpy chart , of a practical cycle (refer to page 8) • Effects of pressure loss resulting from friction.
  16. 16. Superheating & Sub-cooling compressor condenser receiver refrigerant control (expansion valve) evaporator Heat exchanger Saturated liquid Superheated suction vapour Saturated suction vapour Sub cooled liquid Improvement in cycle efficiency with a heat exchanger – as compared to another cycle where vapour is superheated without producing any useful cooling Page 7
  17. 17. Refrigeration system capacity • Rate at which system removes heat from. • Rate depends : (1) mass of refrigerant circulated per unit time (2) refrigerating effect per unit mass circulated (undercooling increases the refrigerating effect)
  18. 18. Two systems employed: • Direct Expansion System • Indirect expansion system aka Brine System
  19. 19. Direct Expansion System : Provisional Refrigeration System Condenser Cooling water in / out Fan/blower expansion valve Solenoid stop valve Thermostat Temperature sensor MEAT ROOM LP pressure switch HP pressure switch Refrigerant compressor Sight glass Drier Evaporator Capillarytube : Refrigerant flow From FISH ROOM From VEGETABLE ROOM To FISH ROOM To VEGETABLE ROOM Oil separator Oil return to compressor sump Bulb T1 T2 receiver Oil pressure switch Purging line LP pressure gauge Oil pressure gauge HP pressure gauge Back pressure regulating valve
  20. 20. Indirect Expansion (Brine System) Condenser / Receiver Cooling water in / out expansion valve Solenoid stop valve Thermostat Temperature sensor LP pressure switch HP pressure switch Refrigerant compressor Sight glass Drier Evaporator Capillarytube : Refrigerant flow Oil separator Oil return to compressor sump Bulb T1 T2 pump Brine header tank Secondary refrigerant to various compartment Oil pressure switch
  21. 21. Back pressure regulating valve • Normally fitted to higher temperature rooms, ie the vegetable room not for the fish room or meat room. Purpose : • Act as system balancing diverters – a) When all solenoid valves are opened, the valve restrict liquid flowing into the vegetable room & therefore deliver the bulk to the colder rooms. b) Limits the pressure drops across the expansion valve by giving a set minimum pressure in the evaporator coil. Prevents cold air blowing directly onto delicate vegetables.
  22. 22. Refrigerant Compressor types: • Reciprocating • Rotary • Centrifugal • Screw
  23. 23. Oil Separator Gas from compressor Float Oil to compressor crankcase Oil Gas to condenser Internal baffles Page 12
  24. 24. Liquid-line Filter / Drier Desiccant (dehydrating material) Refrigerant in Clean,dry refrigerant Fine filter to remove small particles Course filter to remove large particles Felt pad Drying agent : silica gel or activated alumina Page 13
  25. 25. Condenser: • Air cooled type – up to 5 hp • Large capacity – shell & tube type , SW cool • Tubes – aluminium brass (option ext. fins) • Water velocity < 2.5 m/s minimise erosion • Anodes – avoid corrosion non ferrous metals
  26. 26. Throttling device: • Metering of refrigerant – rate suitable to maintain designed operating pressures at different load. • Maintain pressure differential between HP & LP side. The pressure of the refrigerant is reduced as it passes through the small orifice of the throttling device. With the reduction in pressure, the corresponding boiling point of the liquid is reduced. Types of throttling devices: • Hand expansion valves • Automatic constant pressure expansion valve • Thermostatic expansion valve • Externally equalized expansion valve • Pressure balancing expansion valve • Expansion valves with centrifugal type distributors • Flow control device for flooded evaporators
  27. 27. Expansion valve • automatic expansion valve • thermostatic expansion valve • externally equalised thermostatic expansion va
  28. 28. Automatic expansion valve Page 18
  29. 29. Thermostatic expansion valve (TEV) Page 19
  30. 30. External equalised thermostatic expansion valve Page 20
  31. 31. Capacity control methods • Manual start/stop • Speed variation • Cylinder unloading reciprocating compressor • Suction side throttling centrifugal compressor • Inlet guide vane centrifugal compressor • Hot gas bypass • Compressor in parallel • Slide valve Screw compressor - control effective working length of rotor. To maintain constant temperature, a constant pressure must be present in the EVAPORATOR. Ideally, the compressor should remove from the EVAPORATOR exactly the volume of refrigerant that boils off in it. Change in loading : change in quantity of boiling off the refrigerant.
  32. 32. Unloading device
  33. 33. Screw compressor LOBES DRIVE SHAFT Min BYPASS GAS OUTLET DISCHARGE PORT INLET UNLOADING PISTON Max SLIDE VALVECYLINDER NORMAL LOADING Page 12
  34. 34. Screw compressor LOBES DRIVE SHAFT Min BYPASS GAS OUTLET DISCHARGE PORT INLET UNLOADING PISTON Max SLIDE VALVECYLINDER REDUCE LOADING

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