Mechanical Sciences Refrigeration

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Author:
ereim
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28845
Filename:
Mechanical Sciences Refrigeration
Updated:
2010-08-09 06:51:05
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Refrigeration
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Chap 7
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  1. Metering device used to reduce the high liquid refrigerant pressure down to a low
    pressure vapor-mist in the evaporator
    Automatic expansion valve (AXV)
  2. The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit
    British thermal unit (Btu)
  3. The refrigeration effect that can be achieved by the system. It is equal to the rate at which heat can be removed from the medium to be cooled.
    Capacity
  4. A length of tubing with a small inside diameter.
    It acts as a constant throttle on the refrigerant
    Capillary tube
  5. Mechanical device used to receive or remove the refrigerant vapor from the evaporator, increasing the pressure and temperature of the refrigerant vapor
    Compressor
  6. Heat exchanger designed to completely evaporate the refrigerant in the coil itself, with only a vapor leaving the coil outlet.
    Dry-type evaporators
  7. Heat exchanger in the refrigeration system where heat is removed from the product: refrigerant in the evaporator absorbs heat from the material being cooled.
    Evaporator
  8. Heat exchanger completely filled with liquid refrigerant, designed so that the refrigerant level is maintained. Part of the liquid refrigerant evaporates in the coil, and this vapor goes to the accumulator
    Flooded-type evaporators
  9. A condition of liquid refrigerant returning to the compressor through the suction line before it vaporizes. It can occur when the evaporator is overfed or there is too much refrigerant in the system.
    Floodback
  10. Heat added to or removed from a substance to bring about a change in phase, with no change in temperature
    Latent heat
  11. Indicates combined operating efficiency of motor and compressor











    Performance Factor (Btu/watt-hr)


    =


    Capacity (Btu/hr)
    Power Input (watt)





    Performance factor
  12. Reduces the high pressure in the liquid line to the low pressure in the evaporator
    Refrigerant-flow control
  13. Heat added to or removed from a substance, which results in a temperature change to that substance, and no change in phase.
    Sensible heat
  14. Indirect measuring device used to monitor compressor suction line temperature and control the flow of refrigerant into the evaporator based on this temperature
    Sensing bulb
  15. The temperature of a liquid, below saturation (boiling/condensing) temperature
    Subcooling
  16. Catches liquid that leaves the evaporator through the suction line before it can reach the compressor
    Suction accumulator
  17. The temperature of a vapor, above saturation (boiling/condensing) temperature.
    Superheat
  18. Use of thermistors, exposed to the refrigerant in the suction line to control the expansion needle valve opening
    Thermal-electric expansion valve
  19. Metering device controlled by the temperature of refrigerant at the evaporator outlet.
    Thermostatic expansion valve (TXV)
  20. The temperature of a vapor, above saturation (boiling) temperature
    Superheat
  21. The temperature of a liquid, below saturation (condensing) temperature.
    Subcooling

    • The heat added to a liquid to bring about a change in phase, with no change in temperature. Heat added to a liquid at its boiling temperature will vaporize the liquid, without changing its temperature.



    Latent heat
  22. The heat added to a substance, which results in a temperature change to that substance, and no change in phase.
    Sensible heat
  23. DESCRIBE the basic mechanical refrigeration cycle
    The refrigeration cycle is divided into two pressure sections: the high pressure side and the low pressure side

    • The compressor raises the pressure of the fluid. The high pressure side starts when gasses
    • leave the compressor, and continues through to the metering device. The metering device reduces system
    • pressure. The low pressure side continues from the metering device through to the suction of the compressor.
  24. Describe the two pressure sections of a refrigeration cycle.
    Low pressure, superheated, refrigerant vapor is supplied to the suction of the compressor. As the compressor pistons compress the gas, the gas is heated.

    The hot, high pressure gas flows from the compressor discharge through the condenser. The cooling medium (water or air) of the condenser absorbs heat from the hot, high pressure refrigerant gas.

    • Receiving or removing the refrigerant vapor from the evaporator, so that desired pressure and temperature can be maintained.
      Increasing the pressure of the refrigerant vapor through the process of compression and simultaneously increasing the temperature of the vapor so that it will transfer its heat to the condenser cooling medium.



    Compressor
  25. Transfers the heat from the refrigerant to a medium that can absorb it and move it to a final disposal point
    Condensor
  26. Installed to collect the high pressure, low temperature liquid refrigerant as it leaves the condenser
    Receivers
  27. Sometimes call the cooling coil, it is the part of the refrigeration system where heat is removed from the product: air, water, or whatever is to be cooled.
    Evaporator
  28. A simple device that traps liquids leaving the evaporator
    Accumulator
  29. This is the most common type of compressor used in domestic, smaller commercial, and industrial condensing unit applications.
    Reciprocating Compressors
  30. In this type compressor, vapor, as it is moved rapidly in a circular path, moves outward.
    Centrifugal Compressor
  31. The low pressure refrigerant vapor from the suction line is drawn into the opening and fills the space behind the blade as it revolves.
    Rotary Compressor
  32. The rotors trap and compress vapor as they revolve inside an accurately machined compressor cylinder.
    Screw-Type Compressor
  33. This compressor uses two offset spiral disks to compress the refrigerant vapor
    Scroll compressor
  34. EXPLAIN the factors that affect compressor performance.
    • Capacity
    • Performance Factor
    • Balancing
  35. Explain the operation of the following metering devices:
    a. Automatic expansion valve
    b. Thermostatic expansion valve
    c. Thermal-electric (solid-state) expansion valve
    d. Capillary tube
    Automatic expansion valve (AXV) is used to reduce the high pressure of the liquid refrigerant down to a low pressure vapor-mist in the evaporator.

    • A thermostatic expansion valve (TXV), controls the refrigerant flow in the
    • evaporator by the temperature of refrigerant at the evaporator outlet.

    • The thermal-electric expansion valve depends upon the use of thermistors, directly
    • exposed to the refrigerant in the suction line, to control the expansion valve needle opening.

    • Capillary Tube is simply a length of tubing with a small inside diameter. It acts as a constant
    • throttle on the refrigerant.
  36. DESCRIBE failure mechanisms and symptoms associated with a refrigeration system
      • Insufficient cooling
      • Improper refrigerant charge
      • High head pressure
      • Low suction pressure
      • High suction pressure
  37. DESCRIBE causes for the following refrigeration system conditions:
    a. High head pressure
    b. Low suction pressure
    c. High suction pressure
    High head pressure is caused by either restriction in the compressor discharge line or by poor heat transfer in the condenser, Also by Overcharge of refrigerant

    Low Suction pressure Either a restriction in the suction line or poor heat transfer in the evaporator causes low suction pressure. Undercharge of refrigerant

    High suction pressure Either heavy load conditions or improper metering of the refrigerant to the evaporator causes high suction pressure
  38. Caused by either restriction in the compressor discharge line, poor heat transfer in the condenser or Overcharge of refrigerant.
    High Head Pressure
  39. Caused by a restriction in the suction line, poor heat transfer in the evaporator or Undercharge of regrigerant
    Low Suction Pressure
  40. Caused by Heavy Load conditions or improper metering of the refrigerant
    High Suction Pressure

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