Refrigeration and Air conditioning


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Refrigeration and Air conditioning

  1. 1.  BY Mr. ADE S.L.
  2. 2. 5.1 Introduction  The mechanism used for lowering or producing low temp. in a body or a space, whose temp. is already below the temp. of its surrounding, is called the refrigeration system.  Here the heat is being generally pumped from low level to the higher one & is rejected at high temp. 
  3. 3. Objectives  • • • • Basic operation of refrigeration and AC systems Principle components of refrigeration and AC systems Thermodynamic principles of refrigeration cycle Safety considerations
  4. 4. Generic Refrigeration Cycle 
  5. 5. Refrigeration  The term refrigeration may be defined as the process of removing heat from a substance under controlled conditions. It also includes the process of reducing heat & maintaining the temp. of a body below the general temp. of its surroundings.
  6. 6. Contd….  In other words the refrigeration means a continued extraction of heat from a body whose temp is already below the temp. of its surroundings.
  7. 7. Coefficient of performance(COP)   C.O.P is a measure of efficiency of a refrigeration cycle/ system.  It is the ratio of refrigerating effect to the energy spend.  Refrigerating effect is the amount of heat removal/ absorbed from the substance to be cooled.  The energy spend may e in the form of work in VCR or heat in VAR.
  8. 8. REFRIGERATORS AND HEAT PUMPS The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators. Another device that transfers heat from a low-temperature medium to a hightemperature one is the heat pump. Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only. The objective of a refrigerator is to remove heat (QL) from the cold medium; the objective of a heat pump is to supply heat (QH) to a warm medium. for fixed values of QL and QH
  9. 9. Refrigerator & Refrigerant  A refrigerator is a reversed heat engine or a heat pump which takes out heat from a cold body & delivers it to a hot body. The refrigerant is a heat carrying medium which during their cycle in a refrigeration system absorbs heat from a low temp. system & delivers it to a higher temp. system.
  10. 10. Unit of refrigeration   The capacity of refrigeration unit is generally expressed in “Tons” of refrigeration.  The rate of removal of heat in cooling operation was expressed in terms of kilograms or tons of ice required per unit time usually or a day.
  11. 11. One ton of refrigeration   The quantity of heat required to remove from one ton ice within 24 hours when initial condition of water is zero degree centigrade, because the same cooling effect will be given by melting the same ice.
  12. 12. Uses of Systems • •  and cargo Cooling of food stores Cooling of electronic spaces and equipment • • • • • CIC (computers and consoles) Radio (communications gear) Radars Sonar Air conditioning for crew comfort
  13. 13. 5.2 Typical Refrigeration Cycle 
  14. 14. Refrigeration Cycle  In refrigeration system the heat is being generally pumped from low level to higher one & rejected at that temp. This rejection of heat from low level to higher level of temp. can only be performed with the help of external work according to second law of thermodynamics.
  15. 15. Contd….  The total amount of heat being rejected to the outside body consist of two parts:- the heat extracted from the body to be cooled . - the heat equivalent to the mechanical work required for extracting it.
  16. 16. Vapour compression cycle
  17. 17. Vapour compression cycle  3 Condenser High Pressure Side 4 Expansion Device Compressor 2 1 Evaporator Low Pressure Side
  18. 18. Vapour compression cycle  3 Condenser High Pressure Side 4 Expansion Device Compressor 2 1 Evaporator Low Pressure Side
  19. 19. Vapour compression cycle The superheated vapour enters the compressor where its pressure is raised  3 Condenser High Pressure Side 4 Expansion Device Compressor 2 1 Evaporator Low Pressure Side
  20. 20. Vapour compression cycle Low pressure liquid refrigerant in evaporator absorbs heat and changes to a gas  3 Condenser High Pressure Side 4 Expansion Device Compressor 2 1 Evaporator Low Pressure Side 20
  21. 21. Vapour compression cycle The high pressure superheated gas is cooled in several stages in the condenser  3 Condenser High Pressure Side 4 Expansion Device Compressor 2 1 Evaporator Low Pressure Side
  22. 22. Vapour compression cycle 
  23. 23. Components • • • • • •  Refrigerant Evaporator/Chiller Compressor Condenser Receiver Thermostatic expansion valve (TXV)
  24. 24. Evaporator/Chiller  • • • Located in space to be refrigerated Cooling coil acts as an indirect heat exchanger Absorbs heat from surroundings and vaporizes • • Latent Heat of Vaporization Sensible Heat of surroundings
  25. 25.
  26. 26. Compressor •  Superheated Vapour: • • • • • Enters as low press, low temp vapour Exits as high press, high temp vapour Temp: creates differential (DT) promotes heat transfer Press: Tsat allows for condensation at warmer temps Increase in energy provides the driving force to circulate refrigerant through the system
  27. 27. Condenser  • • • Refrigerant rejects latent heat to cooling medium Latent heat of condensation (LHC) Indirect heat exchanger: seawater absorbs the heat and discharges it overboard
  28. 28. Receiver  • • Temporary storage space & surge volume for the sub-cooled refrigerant Serves as a vapor seal to prevent vapor from entering the expansion valve
  29. 29. Expansion Device  • • • Thermostatic Expansion Valve (TXV) Liquid Freon enters the expansion valve at high pressure and leaves as a low pressure wet vapor (vapor forms as refrigerant enters saturation region) Controls: • • Pressure reduction Amount of refrigerant entering evaporator controls capacity
  30. 30.
  31. 31. Vapour absorption refrigeration system 
  32. 32. Vapour absorption refrigeration system 
  33. 33. Vapour absorption refrigeration system   In the absorption refrigeration system, refrigeration effect is produced mainly by the use of energy as heat. In such a system, the refrigerant is usually dissolved in a liquid. A concentrated solution of ammonia is boiled in a vapour generator producing ammonia vapour at high pressure. The high pressure ammonia vapour is fed to a condenser where it is condensed to liquid ammonia by rejecting energy as heat to the surroundings. Then, the liquid ammonia is throttled through a valve to a low pressure. During throttling, ammonia is partially vapourized and its temperature decreases.
  34. 34. Vapour absorption refrigeration system  is fed to an evaporator  This low temperature ammonia where it is vaporized removing energy from the evaporator. Then this low-pressure ammonia Vapour is absorbed in the weak solution of ammonia. The resulting strong ammonia solution is pumped back to the Vapour generator and the cycle is completed. The COP of the absorption system can be evaluated by considering it as a combination of a heat pump and a heat engine
  35. 35. Domestic refrigerator  Domestic refrigerator
  36. 36. ICE plant 
  37. 37. ICE plant   The simple VCR cycle is used in primary circuit using ammonia as a refrigerant and brine is a secondary circuit.  This is indirect method of cooling used for ice production.  The ice can contain water which reject heat to the brine which is circulated in secondary circuit.  In evaporator the heat of brine is transfer to the refrigerant in primary circuit and brine is cooled.
  38. 38. ICE plant   The vapour refrigerant form in evaporator is suck by compressor then it compressed to a high pressure and this is condensed in a condenser with the help of cooling water.  The high pressure liquid ammonia is collected in the receiver and then it is passed through expansion valve.  The throttle liquid ammonia at low pressure and temperature enter in an evaporator, so brine in cooled and ammonia absorb heat and form vapour.
  39. 39. 5.3 Psychrometry and Air Processes  1. Atmospheric air    Atmospheric air is not completely dry but a mixture of dry air and water vapor. In atmospheric air, the content water vapor varies from 0 to 3% by mass. The processes of air-conditioning and food refrigeration often involve removing water from the air (dehumidifying), and adding water to the air (humidifying).
  40. 40. 2. The thermal parameters of moist air t (1) Dry bulb temperature    Dry bulb temperature is the temperature of the air, as measured by an ordinary thermometer. The temperature of water vapor is the same as that of the dry air in moist air. Such a thermometer is called a dry-bulb thermometer in psychrometry, because its bulb is dry. (2) Wet bulb temperature tWB:  Wet bulb temperature is thermodynamic adiabatic temperature in an adiabatic saturation process, and measured by a wet bulb thermometer.
  41. 41. (3) Dew point temperature tDP:  When the unsaturated moist air is cooled at constant vapor pressure or at constant humidity ratio, to a temperature, the moist air becomes saturated and the condensation of moisture starts, this temperature is called dew point temperature of the moist air. (4) Relative humidity Ф:    Relative humidity is defined as the ratio of the mole fraction of the water vapor in a given moist air to the mole fraction of water vapor in a saturated moist air at the same temperature and the same atmospheric pressure. Relative humidity is usually expressed in percentage (%). From the ideal gas relations, relative humidity can be expressed as xw Pw  xw, sat  Pw, sat
  42. 42. (5) Degree of Saturation μ:  Degree of saturation is defined as the ratio of the humidity ratio of moist air w to the humidity ratio of saturated moist air wsat at the same temperature and atmosheric pressure. (6) Humidity ratio (Moisture Content) w:    The humidity ratio is the mass kg of water vapor interspersed in each kg of dry air. It should be noted that the mass of water refers only to the moisture in actual vapor state, and not to any moisture in the liquid state, such as dew, frost, fog or rain. The humididy ratio, like other several properties to be studied- enthalpy and specific volume-is based on 1kg of dry air.
  43. 43. (7) Specific Volume/Moist Volume v:  Specific volume of moist air v , m3/kgdry is defined as the total volume of the moist air (dry air and water vapor mixture) per kg of dry air. (8) Specific Enthalpy:    Specific enthalpy of moist air h (kJ/kgdry) is defined as the total enthalpy of the dry air and water vapor mixture per kg of dry air. Enthalpy values are always based on some datum plane. Usually the zero value of the dry air is chosen as air at 0℃, and the zero value of the water vapor is the saturated liquid water at 0℃.
  44. 44. Psychrometric chart 
  45. 45. Psychrometric chart    A psychrometric chart graphically represents the thermodynamic properties of moist air. It is very useful in presenting the air conditioning processes. The psychrometric chart is bounded by two perpendicular axes and a curved line:    1) The horizontal ordinate axis represents the dry bulb temperature line t , in℃ ; 2) The vertical ordinate axis represents the humidity ratio line w , in kgw/kgdry.air 3) The curved line shows the saturated air, it is corresponding to the relative humidity Ф=100% .
  46. 46.   The psychrometric chart incorporates seven parameters and properties. They are dry bulb temperature t , relative humidity Ф , wet bulb temperature tWB, dew point temperature tDP , specific volume v, humidity ratio w and enthalpy h. ①Dry-bulb temperature t is shown along the bottom axis of the psychrometric chart. The vertical lines extending upward from this axis are constant-temperature lines. ②Relative humidity lines Ф are shown on the chart as curved lines that move upward to the left in 10% increments. The line representing saturated air ( Ф= 100% ) is the uppermost curved line on the chart. And the line of Ф = 0% is a horizontal ordinate axis itself.
  47. 47. 2. Main air handing processes and their variations in properties (1) Sensible cooling along a cooling coil, or sensible heating along a heating coil  tDp rel ati ve 2a 1a 1b 2b  The sensible cooling can only take place under the condition when the temperature of the cooling coil is not below the dew point temperature of the air being processed. tW 3 dew-point temp B dry-bulb temp wet -bu lb t em p w humidity ratio During this process, the relative humidity of the air will increase. φ  hu mi di ty The sensible cooling happens when the air is cooled without altering the specific humidity. φ =1 00 % 
  48. 48. Cont.…. tDp re la tiv e It should be noted that there should be no water within the heating system because the evaporation of the water will increase the specific humidity of the air. φ  2a 1a 1b 2b tW 3 dew-point temp B dry-bulb temp wet -bu lb t emp w humidity ratio The sensible heating is similar to sensible cooling, but with the dry bulb temperature increasing. φ =1 00 %  hu mi di ty 
  49. 49. (2) Adiabatic humidification and dehumidification using a humidifier or chemical dehumidifier tDp re lat iv e 2a 1a 1b 2b tW 3   During the adiabatic humidification process along the constant wet bulb temperature line, the specific humidity of air will increase. Reduction in dry bulb temperature will happen as the evaporated water will absorb heat . dew-point temp B dry-bulb temp wet -bu lb t em p w humidity ratio A humidifier performs this function by supplying the water vapor. φ  hu mi di ty The adiabatic humidification occurs when water vapor, of which temperature is near the wet bulb temperature of the moist air, is added to the air . φ =1 00 % 
  50. 50. 5.4 Air Conditioning  • • • Purpose: maintain the atmosphere of an enclosed space at a required temp, humidity and purity Refrigeration system is at heart of AC system Types Used: • • • Self-contained Refrigerant circulating Chill water circulating
  51. 51. Air conditioning   Air conditioning is the science which deals with the supply and maintaining desirable internal atmospheric condition irrespective of external condition.  Air conditioning may be defined as simultaneous control of temperature, humidity, motion of air and purity of air within the enclosed space.
  52. 52. Classification of air conditioning   a) According to purpose. i) comfort air conditioning system ii) Industrial air conditioning  b) According season of year. i) Winter air conditioning ii) Summer air conditioning iii) Year round (All weather air conditioning)
  53. 53. Classification of air conditioning   c) According to equipment arrangement i) central air conditioning ii) unitary air conditioning d) According to working substance used. i) All air system ii) chilled water system iii) Air water system
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