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Evap, Cond, MD, Receivers > RACT Unit 21 > Flashcards

RACT Unit 21 Flashcards

1
Q

_________ is the process of removing heat from a place it is not wanted and transferring it to a place where it makes little or no difference

A

refrigeration

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2
Q

Typically, the fresh food compartment temperature is about _____F

A

35F

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3
Q

Heat in refrigerator is transferred to a cold refrigerant coil and pumped by the system compressor to the condenser where it is released into the room much like using a ______ to move water

A

sponge

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4
Q

High temp refrigeration applications will normally involve box temperatures of ____F to ____F. High temp is commercial refrigeration

A

47 to 60F

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5
Q

Medium temp applications range from ___F to ___F. The household refrigerator fresh food compartment is medium temp

A

28 to 40F

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6
Q

Low temperature applications produces temperature below the freezing point of water, 32F. Low temp food storage applications generally start at ____ and go as low as ____F

A

0F and as low as -20F

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7
Q

The ________ in a refrigeration system is responsible for absorbing heat into the system from whatever medium is to be cooled.

A

evaporator

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8
Q

The heat absorbing process is accomplished by maintaining the evaporator coil at a ______ temperature than the medium to be cooled

A

lower

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9
Q

The evaporator operating at these low temperatures removes ______ and ______ heat

A

latent and sensible heat

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10
Q

Two important factors in understanding refrigeration are the refrigerating _______ temperature and _______ temperature

A

boiling temperature and condensing temperature

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11
Q

The _________ temp of the liquid refrigerant determines the coil operating temperature

A

boiling temperature

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12
Q

When the evaporator removes heat from the air and lowers the temperature of the air, _______ heat is removed

A

sensible

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13
Q

When moisture is removed from the air, _______ heat is removed

A

latent

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14
Q

The difference in temperature between the air being cooled and evaporator is directly related to the amount of ________that will be removed

A

moisture (humidity)

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15
Q

The greater the difference in temperature the more ______ is removed

A

moisture

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16
Q

The rate of heat exchange is governed by 3 things;

  1. The evaporator ______ through which heat has to be exchanged
  2. The _____ to which the heat is transferred.
  3. The ____ factor. Relationship between the medium giving up heat and the heat exchange surface.
  4. The _______ _________ between two mediums in which the heat exchange is taking place
A
  1. material
  2. medium
  3. film factor
  4. temperature difference
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17
Q

What type of evaporators were the first for cooling air?

A

natural-convection type

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18
Q

The stamped evaporator was one of the first designs to create a ______ pipe surface that improved efficiency

A

larger

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19
Q

Finned-tube evaporators are used more today than any other type of heat exchanger. It is efficient because the fins are in good contact with the _____ carrying the refrigerant

A

pipe

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20
Q

The longer the evaporator the more ______ drop associated with refrigerant flowing through them

A

pressure drop

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21
Q

The ______ (tonnage) of the heat exchanger can vary with its height and length

A

capacity

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22
Q

An efficient evap design incorporates an aluminum parallel-flow, flat-plate-and-fin configuration with small parallel channels inside the flat plate. The plates are _______, streamlined _____ , each one of which is split into smaller parallel ports

A

flattened , tubes

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23
Q

Name two ways to increase internal surface area;

  1. Increasing the number of ______ channels inside the flat plates
  2. increasing the number of ____ plates (decreasing space between them)
A
  1. parallel

2. flat

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24
Q

The vertical orientation of flat plates allows condensate removal to occur naturally, relieving any water drainage issues from the evaporator. This technology is being used with ______ as well as _________

A

condensers, as well as evaporators

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25
Q

Some of the benefits of this parallel-flow, plate-and-fin heat exchanger technology are as follows;

  1. reduces ______ pressure through the coil
  2. _____ system costs
  3. The all-aluminum coil, header, fins enhance _________ resistance
  4. Reduced internal _______ reduces ref charge
  5. Reduces ____ depth for the evap and cond which leads to easier cleaning
  6. reduced _____ area of cond and evap
  7. smaller _______ for the condensing unit
A
  1. static
  2. lower
  3. corrosion resistance
  4. volume
  5. coil
  6. face
  7. footprint
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26
Q

Field repair of leaking heat exchangers can be accomplished by;

  1. Recovering the ________
  2. Cleaning the leaking area with a ______
  3. Brushing the area with a ____ brush
  4. Pulling a vacuum with a ______ pump
A
  1. refrigerant
  2. solution
  3. wire brush
  4. vacuum pump
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27
Q

Evaporator evaluation can be considered as one of the ______ ____ in organized trouble shooting

A

starting point

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28
Q

The evaporator _____ heat, the compressor _____ it and the condenser _____ it

A

absorbs, pumps, rejects

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29
Q

How an Evaporator functions correctly;

  1. Warm ____ passes through a small
    opening in the expansion device into the low pressure side
  2. Due to pressure drop across the expansion device, ____% liquid and ____% vapour occurs
  3. As sat. mix moves through the evaporator more liquid changes to a ______ . This is called _____ and is a result of heat absorbed into the coil from whatever medium the evaporator is cooling
  4. Finally, near the end all the liquid is boiled to a vapour and the refrigerant is known as _______ vapour
A
  1. liquid
  2. 25% vapour 75% liquid
  3. vapour, boiling
  4. saturated vapour
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30
Q

The latent heat absorbed during the change of state is much more ________ than the sensible heat that is added to the vapour leaving the coil

A

concentrated

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31
Q

To get max eff from the evap heat exchange, some evaporators are operated full of liquid or flooded and are equipped with a device to keep the liquid refrigeration from passing to the _______

A

compressor

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32
Q

The _______ evaporators are specially made and normally use a float type metering device to keep the liquid level as high as possible

A

flooded

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33
Q

If the evaporator is not flooded when the refrigerant starts out as a partial liquid and boils away to a vapour in the heat exchange pipes it is known as a ____\ type or direct expansion evaporator

A

dry

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34
Q

To check performance of a dry type evaporator the technician would first make sure that the refrigerant coil is operating with enough _______ inside the coil

A

liquid

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35
Q

To determine if enough liquid is inside the coil to check evaporator performance , the technician must calculate the evaporator ________ which is done by comparing the ______ temp of the refrigerant inside the coil with the line temp leaving the coil. The difference should be __ to ___F

A

superheat

boiling temp

8 to 12F

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36
Q

The difference in temperature between the boiling refrigerant and the evaporator outlet temp is known as ________ ________

A

evaporator superheat

37
Q

Super heat is the ________ heat added to the vapour refrigerant after the change of state has ocuxted

A

sensible

38
Q

Superheat is the best method of checking to see when a refrigerant coil has a ______ level of refrigerant

A

proper

39
Q

When a metering device is not feeding enough refrigerant to the coil, the coil is said to be ______ and the superheat is greater

A

starved

40
Q

A hot pull down is essentially an excessively ________ evaporator

A

loaded

41
Q

When the refrigerated space has been allowed to warm up considerably, the system must got through a ____ ________

A

hot pulldown

42
Q

When a system is under a hot pull down, the technician should allow the system to _________ before trying to calculate an evaporator superheat reading

A

stabilize

43
Q

When a dry type coil is fed too much refrigerant, not all refrigerant changes to a vapour. This is thought of as a _______ _____

A

flooded coil

44
Q

Multicircuit evaporators are used when the coil would become too _____ for a single circuit

A

long

45
Q

A dry type evaporator has to be as full as possible with refrigerant to be ______

A

efficient

46
Q

Each circuit in a multi circuit evap should be feeding the same amount of refrigerant. If this needs to be checked the service technician can check the _____ ______ tap for the boiling pressure which can then be converted to to temperature. The temp will then have to be checked at the ______ of each circuit to see if any circuits are being _______ or ________

A

common pressure tap

outlet

overfed or starved

47
Q

Some reasons for uneven feeding on a multi-circuit evaporator are;

  1. blocked ________ system
  2. dirty _____
  3. uneven ____ distribution
  4. coil circuits of _______ lengths
A
  1. distribution
  2. coil
  3. air
  4. different
48
Q

With larger evaporators and longer suction lines it is best to measure the refrigerants pressure at the _______ outlet not at the compressor ______ valves -when measuring evaporator superheat

A

evaporator outlet

compressor service valves

49
Q

It is best to measure refrigerant pressure at the same location that the evaporator ______ temp is taken when measuring superheat

A

outlet

50
Q

_____ taps can be used to gain access to evaporator outlet pressure for more accurate readings

A

line

51
Q

A different type of evaporator is required for liquid cooling. It functions much like the one for cooling air and is normally a ___ type expansion evaporator in smaller systems.

A

dry type

52
Q

Liquid cooling evaporators have saturated liquid/vapour refrigerant in the shell and the water to be chilled flows in the _____ bundles

A

tube

53
Q

Liquid cooling evaporators use a ____ side float to meter the refrigerant into the shell of the evaporator to maintain the proper refrigerant level

A

low side float

54
Q

Liquid cooling evaporators have more then ____ refrigerant circuit to prevent pressure drop

A

more than one

55
Q

Liquid evaporators have a normal superheat range similar to air-type (__ to ___F)

A

8 to 12F

56
Q

List two problems that are typical on the liquid side of the evaporator;

  1. mineral _______ build up on the liquid side, acting as an insulator and causing a poor heat exchange
  2. poor _______ of the liquid to be cooled where a circulating pump is installed
A
  1. mineral deposits

2. circulation

57
Q

A pull down can not be _______

A

rushed

58
Q

Low temperature evaporators uses for cooling space or product to below freezing are designed differently because they require the coil to ______ below ________

A

operate below freezing

59
Q

Low temp coils have wider spacing than medium temp coils to prevent ____ flow blockage when freezing occurs

A

air flow blockage

60
Q

Low temp evaporators are normally ____ type. They have one or more ____ to circulate air across the coil

A

dry type

fans

61
Q

The defrosting of a low temp evaporator coil has to be done by _______ the temp of the coil above freezing to melt the ice

A

raising

62
Q

defrosting of low temperature evaporators can be accomplished with heat from inside the system using hot gas from the discharge line of the compressor or using __________ resistance heaters

A

electric

63
Q

Hot gas defrost is accomplished by routing a ___ _____ line from the comp discharge line to the outlet of the expansion valve and installing a solenoid valve to control the flow

A

hot gas

64
Q

When hot gas enters the evaporator to melt the ice it will soon turn to a liquid. This liquid will either go to an _________ or flood the compressor crankcase if there is no _________

A

accumulator

65
Q

flooding compressor can cause a lower oil level in the crankcase and score the bearings in the compressor which is known as ___________ washout

A

bearing

66
Q

Electric defrost heaters are often embedded in the actual _______ _____ and cannot be removed if they burn out

A

evaporator fins

67
Q

If the evaporator fan is not turned off during the defrost, tow things will happen;

  1. The heat from defrost will be transferred directly to the _________ space
  2. The ______, conditioned air will slow down the defrost process
A
  1. conditioned

2. cold

68
Q

Defrost occurs when the thermostat is ________ and the refrigeration system cycles _____

A

satisfied

off

69
Q

Medium temperature refrigeration systems operate with evap temperatures below ___F and box temperatures above 32F

A

32F

70
Q

The evaporators function is to efficiently transfer heat heat from the surrounding area to the phase-changing ________ flowing through the ________

A

refrigerant

evaporator

71
Q

As the evaporators heat load changes, an _______ type _________ device at the entrance of the evaporator will control the flow of refrigerant through the evaporator

A

expansion type metering device

72
Q

The expansion type metering device will ensure that all of the liquid in the evap has fully _________ prior to exiting

A

vaporized

73
Q

Many low temp commercial and industrial evaporator assemblies consist of:

  1. The evaporator _____
  2. F___
  3. An ______ device
  4. _______ heaters
  5. A liquid line ________ valve
A
  1. The evaporator coil
  2. fans
  3. an expansion device
  4. defrost heaters
  5. Liquid like solenoid valve
74
Q

Although frost is unavoidable on an evaporator it is the most common cause of ___________ in the evaporator coils

A

inefficiency

75
Q

The dew point temperature is the temperature where ________ starts to form

A

moisture

76
Q

When the evaporators coil temp drops below the dew point temperature, moisture begins to collect on the evaporators coil surface. Eventually this moisture turns to _____ if temp continues dropping

A

ice

77
Q

The control system is based around a refrigerant __________ cycle

A

refrigerant pump down cycle

78
Q

In a pump down cycle, once the refrigerated space temperature is satisfied, the controller deenergizes (closes) the liquid line _______ valve and turns the evap fans off. The comp continues to run until any liquid refrigerant in the evap and suction line is fully vapourized and pumped to the _______ of the system

A

solenoid valve

high side

79
Q

During the ref pump down, the temperature of the evaporator coil will _____ because of the low pressure created in the coil from the pump down process.

This residual/flywheel effect will drop the _________ below the refrigerated space temp

A

drop

coil temp

80
Q

Once the coil temperature is below the space temperature in a pump down cycle, the controller will then cycle the evaporator fans back on without starting the _______
The space temp is now controlled by only the _____ cycling on and off blowing air over a very cold evaporator coil

A

compressor

fans

81
Q

Frost removal by Sublimation:

During punk down, the evaporator fans are blowing air through a very cold compressor coil with ______ accumulating on its surfaces. The frost will change its phase to vapour through the process of ________.

A

frost , sublimation,

82
Q

_________ is a process where a solid changes directly to a vapour, skipping the liquid phase

A

sublimation

83
Q

The sublimation process reduces the frost on the coil _______

A

naturally

84
Q

As frost changes to a vapour during sublimation, it also returns valuable _______ to the refrigerated space to maintain higher humidity and reduce product shrinkage

A

moisture

85
Q

Evaporator Efficiency Controller and Evaporator Temperature Difference (TD):

The evaporator efficiency controller uses a proprietary defrost control algorithm that eliminates the dependency on _____ for defrost _______

A

time

initiation

86
Q

Evaporator Efficiency Controller and Evaporator Temperature Difference (TD):

Instead of depending on time, evaporator efficiency controllers monitor evaporator coil _______ to initiate defrost and also minimize effects of defrost on space temp

A

efficiency

87
Q

Evaporator Efficiency Controller and Evaporator Temperature Difference (TD):

Maintaining a lower TD between the evap coil and the refrigerated case throughout the run time will reduce rate of frost formation on the ____. This will lead to extended time between defrost and save _____.

A

coil

energy

88
Q

Evaporator Efficiency Controller and Evaporator Temperature Difference (TD):

Important functions of the evaporator efficiency controller;

  1. Monitors evaporator efficiency and ______
  2. Initiates defrost once the evap coil efficiency is less than ___%
  3. creates an initial evap profile from series of __________
  4. identifies the temp relationship between the evaporator coil and refrigerated space temp through two _______ locations
A
  1. Monitors evaporator efficiency and performance
  2. Initiates defrost once the evap coil efficiency is less than 90%
  3. creates an initial evap profile from series of measurements
  4. identifies the temp relationship between the evaporator coil and refrigerated space temp through two sensor locations

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