Evaporators and the commercial Refrigeration system chapter 21

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salvatorefricano
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296255
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Evaporators and the commercial Refrigeration system chapter 21
Updated:
2015-03-01 11:46:50
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commercial refrigeration avaporators
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Description:
high, med, low temp refrigeration, identify all types of evaporators,
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  1. A refrigeration designed for the storage of flowers or candy is considered a _______ type
    High temperature
  2. If a low temp evap is forming frost, what would be the problem
    no problem,frost formation is  normal
  3. If an evap has a pressure drop of 10 PSIG and the low side pressure is 15 PSIG, what is the pressure at the inlet of the evap
    25 Psig
  4. If a Low temp evap is forming excess ice, which of the following could be a possible problem
    • The unit is not staying in defrost long enough
    • the evap fan motor is defective
    • the defrost heater is defective
  5. Refrigerant entering the evap is approximently _______ vapor
    25%
  6. Excess evap superheat _______
    will reduce the cooling capacity of the cooling system
  7. if a high temp evap is forming ice, which of the following is a possible problem
    evap fan not running
  8. A typical supper heat for an evap operating under normal condition would be _______
    8 - 12 * F
  9. What type of defrost, and fins would A forced draft high temp evap have_______
    • no need for defrost
    •  it's fins are very close together
  10. The heat required for off- cycle defrost is obtained from blank
    the space temp when the compressor is off
  11. If the application requires a coil design to operate below 32*F, there must be provision for _______
    Defrost
  12. how would a forced draft medium temp evap be defrosted
    by off cycle defrost, when the compressor is off.
  13. A direct expansion-type evap is also called what
    dry-type evaporator
  14. If a R-22 refrigerant system is operating wit a 25.4 psig suction line pressure, what is the evap supper heat if the outlet temp is 35*F
    9*F
  15. The fin spacing on a low temp evap is_______than on a medium-temp evap
    farther apart
  16. During the defrost cycle of a forced draft low temp evap. the evap fan would _____
    turn off
  17. An evap made up of two pieces off sheet-metal that has an impression of refrigerant piping is called a __________evap
    Stamped
  18. The refrigerant in the evap must be cooler than the air passing over it for heat to transfer to the evap 

                            TRUE or FALSE
    True
  19. A stamped plate evap usually does not use a fan to move air across it

                         TRUE or FALSE
    TRUE
  20. When the velocity of the air moving over the evap is slow the film factor acts as an insulates and slows the rate of heat exchange

                             TRUE or FALSE
    True
  21. Boiling temp. is the same as
    evaporator temp.
  22. fresh food is maintained at ________
    35*F
  23. How do you get TD?
    What does TD controls?
    • Evap. tamp. - box tamp. =TD. 
    • It controls humidity
  24. What is air conditioning considered to be and what is the typical coil temp.
    • High temp/ 47*F to 60*F
    • Medium temp 28*F to 40*F
    • low temp -20*F to 0*F
  25. What are the refrig temp ranges?
    • High temp/ 47*F to 60*F
    • Medium temp 28*F to 40*F
    • low temp -20*F to 0*F
  26. Low side gauge converted to a temp =
    Boiling (actual)
  27. Box temp - evap temp=
    Boiling (design)
  28. What is the definition of TD?
    It's the difference between the refrigerant temp inside the coil and the space being cooled.
  29. When is the biggest heat lose in the system?
    when the refrigerant changes state this is called latent heat or hidden heat
  30. Common materials used in the evaporator
    nickel, copper, aluminum, steal, bras, stainless  steal
  31. Rapid heat transfer rates between two ______
    liquids
  32. Slower heat transfer rates between two ________
    vapor
  33. Relationship between the medium giving up heat and the heat exchange surface
    film factor
  34. Film acts as an insulator when ?
    velocity is low
  35. Temperature difference between two mediums
    • large temp difference = high heat transfer rate 20 * TD
    • Small temp difference = ow heat transfer rate 10*F TD
  36. Natural convection (draft) evaporators
    • Made up of fins and tubes
    • physically large, very low velocity (film factor)
    • Located high, near the ceiling of the cooler
  37. Mechanical draft evaps
    • Use fans to move air across the coil
    • It also improves heat transfer rate
    • Physically smaller then natural draft evaps
  38. Stamped plate evaporators are made of what, and are designed to do what
    • Made of metal plate stamped with grooves that provide a path for the refrigerant
  39. What are the functions of the fins in the evap
    to increase surface area
  40. What is considered and excessive pressure drop in the evap
    2 1/2 or more
  41. Multiple coils reduce _________
    Reduces the temp differences drastically and lowers pressure drop.
  42. What is the header
    It is the outlet of each circuit that is tapped into the side of a straight pipe it will become the suction line
  43. More liquid enters the coil then vaper
  44. Latent and heat is more concentrated then sensible heat
  45. The greatest heat transfer takes place during latent and heat transpher
  46. Flooded evap is designed to operate _______
    Full of liquid
  47. What devices used to prevent liquid from entering the compresser
    • Low side float
    • Accumulator
  48. Chiller barrel is a flooded evap
  49. what is a dry evaporator
    when 100% of the  the refrigerant boils before leaving the coil.
  50. evaporator outlet temp- the evaporator saturation temp=
    superheat
  51. what does high superheat indicate
    underfed evaporator or starved evaporator
  52. what does low superheat indicate
    overfed evaporator or flooded evaporator
  53. closing TXV will do what to the superheat
    lower the superheat
  54. opening the TXV will do what to the superheat
    raise the superheat
  55. when does hot pull down occur
    • any time your box goes above C/I of the low pressure switch LPS.
    • ie: excessively loaded box
  56. When should you take superheat readings
    when the system is @ 5*F above the C/O
  57. Pressure drop in the evaporator should be _____
    as low as possible below 2 1/2 #
  58. What is done when the pressure drop in the evap is higher than 2 1/2#
    • a multiple circuit evap is used
  59. liquid type evap are commonly called chiller barres. how much superheat is good
    0-1 superheat
  60. Low-Temp evaporators are designed differently.  fins are spaced farther apart to accommodate frost buildup without sacrificing airflow
  61. low temp evap methods to defrost the coil
    • hot gas defrost sends the compressor discharge gas to the evap to melt the ice
    • electric heaters can also be used.
  62. high temp coil uses what type of defrost
    none. no need because the box temp is not designed to go below 32*F
  63. Medium temp coil uses what type of defrost
    random defrost. anytime the compressor shuts down, it defrosts by the space temp inside the box by blowing the warmer air over the coil.
  64. low temp coil uses what type of defrost
    planned defrost. relies on a timer , and some type of mechanical device to defrost the coil
  65. FPI. Fins Per Inch. how many FPI does ac, med and low temp coils have
    • ac has 15+ fpi
    • med has 7 to 14 fpi usually 10
    • low has 6 fpi
  66. Where is Hot gas defrost piped in the system
    It will be piped from the discharge line to the evaporator after the TXV, but before the distributors
  67. How does Hot gas defrost work.
    • Compressor discharge gas pumped directly to the evaporator coil during the defrost cycle
    • hot gas from the compressor condenses to a liquid  in the evap coil.
    • Liquid from the evap vaporizes in the accumulator before returning to the compressor
  68. How does Electric Defrost work.
    • Electric heating element are located next to the evaporator coil
    • The compressor and the fan motor are not running during defrost
    • At the end of the defrost cycle, the compressor cycles on before the fan, to allow the evap coil to get cold before the fan energized

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