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HVAC > Hvac > Flashcards

Hvac Flashcards

1
Q

What are flaws?

A

Exist at every joint fitting, seam, or weld

Flaws may be too small to detect but given time, vibration, temperature, and environmental stress, these flaws become larger detectable leaks

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2
Q

What can flow under layers of paint, flux, rust, slag, and pipe insulation?

A

Vapor

Vapor can flow under layers of paint, flux, rust, slag, and pipe insulation

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3
Q

What is important to do at the leak site?

A

Clean the leak site by removing loose paint, slag, flux, or rust

It is important to clean the leak site by removing loose paint, slag, flux, or rust

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4
Q

Besides loose paint, slag, flux, or rust, what else must be removed at the leak site?

A

Pipe insulation, oil, and grease

Must also remove pipe insulation, oil, and grease to avoid contaminating the detection tips

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5
Q

Standing leaks

A

Detected while the unit is at rest or off

The most common of all leaks

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6
Q

Pressure dependent leaks

A

Detected as the system pressure increases

Pressure dependent leaks

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7
Q

Temperature-dependent leaks

A

Associated with the heat of expansion

Temperature-dependent leaks

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8
Q

What is the first step in performing a standing pressure test?

A

Pressurize the system with dry nitrogen to a pressure no higher than the lowest system test pressure

Example: Pressurize the system to 100 psi

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9
Q

What is the second step in performing a standing pressure test?

A

Allow the system to rest for ten minutes

No additional information

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10
Q

What is the third step in performing a standing pressure test?

A

Mark the needle positions on the gauge manifold

No additional information

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11
Q

What is the final step in performing a standing pressure test?

A

Monitor gauge needle position

No additional information

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12
Q

Vibration-dependent leaks

A

Occur during unit operation

Vibration-dependent leaks occur during unit operation.

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13
Q

Combination-dependent leaks

A

Require two or more conditions to leak

Combination-dependent leaks require two or more conditions to leak.

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14
Q

Cumulative microleaks

A

All the individual leaks that are too small to detect with standard tools

Cumulative microleaks refer to all the individual leaks that are too small to detect with standard tools.

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15
Q

What can vapor flow under at the leak site?

A

Vapor can flow under layers of paint, flux, rust, slag, and pipe insulation

Example sentence: Vapor can flow under layers of paint, making it important to properly clean the leak site.

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16
Q

Why is it important to clean the leak site?

A

So system is more efficient

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17
Q

What else must be removed from the leak site?

A

Must also remove pipe insulation, oil, and grease to avoid contaminating the detection tip

Additional information: Removing pipe insulation, oil, and grease helps maintain the integrity of the detection process.

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18
Q

What does air contain?

A

Oxygen, nitrogen, and water vapor

Air contains oxygen, nitrogen, and water vapor

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19
Q

What is nitrogen?

A

A non-condensable gas

Nitrogen is a non-condensable gas

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20
Q

What do non-condensables cause?

A

A rise in the system’s operating head pressure

Non-condensables will cause a rise in the system’s operating head pressure

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21
Q

What do oxygen and water vapor cause in the system?

A

Chemical reactions

Oxygen and water vapor cause chemical reactions in the system

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22
Q

What do oxygen and water vapor produce in the system?

A

Acids that deteriorate system components, electroplating of the running gear, and the breakdown of motor insulation

Produce acids that deteriorate system components, electroplating of the running gear, and the breakdown of motor insulation

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23
Q

Is repairing leaks usually as economical as adding refrigerant?

A

No, repairing leaks is usually not as economical as adding refrigerant.

Example sentence: It is often more cost-effective to simply add refrigerant instead of repairing leaks.

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24
Q

What is the best field practice when it comes to system leaks?

A

The best field practice is to repair system leaks whenever possible.

Additional information: Repairing leaks helps maintain system efficiency and prevents environmental harm.

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25
Q

Who establishes requirements regarding when leaks must be repaired?

A

The EPA (Environmental Protection Agency) establishes requirements regarding when leaks must be repaired.

No additional information provided.

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26
Q

What factors may determine when leaks need to be repaired?

A

The factors that may determine when leaks need to be repaired include the type of system, refrigerant used, and/or total system charge.

No additional information provided.

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27
Q

Systems with Schrader Valves take longer to evacuate than systems with service valves

A

Field service valves are used to replace Schrader valve stems while the system is under pressure

Schrader valve caps should be put back on the valve after service

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28
Q

What do chemical combinations create?

A

Hydrofluoric or hydrochloric acids

Example sentence: Chemical combinations can create hydrofluoric or hydrochloric acids.

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29
Q

What does evacuation consist of?

A

Degassing + dehydration

None

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30
Q

What does moisture + acid + oil result in?

A

Sludge

None

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31
Q

What can sludge cause in system components?

A

Plugged components

None

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32
Q

What can proper evacuation eliminate?

A

Formation of acid and sludge

None

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33
Q

TEV or TXV (Thermostatic Expansion Valve)

A

Measuring and adjusting superheat
Mounting the sensing bulb
Use on multi-circuit evaporators
Identification and application

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34
Q

AEV (Automatic Expansion Valve)

A
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35
Q

EEV (Electronic Expansion Valve)

A
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36
Q

Fixed Metering Devices

A

Pistons and capillary tubes

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37
Q

Primary function of a TEV

A

Maintain superheat

TEV stands for Thermostatic Expansion Valve

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38
Q

Opening force/liquid line temp

A

Bulb pressure

Opening force is generated by the pressure in the sensing bulb

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39
Q

Closing forces

A

Evaporator pressure

One of the closing forces is generated by the evaporator pressure

The TEV closes when the evaporator pressure is too low

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40
Q

Closing forces

A

Adjustable spring

Another closing force is provided by an adjustable spring

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41
Q

Notes for the next slide

A

Assume an R22 system, and
There is R22 in the sensing bulb

These notes are important for understanding the operation of the TEV

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42
Q

Measuring Superheat
• Superheat =

A

Suction line temperature – Evaporator temperature
•

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43
Q

High superheat:
–

A

Evaporator is “Starving”
• Because all liquid has boiled off too soon
•

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44
Q

Low Superheat:
–

A

Evaporator is “Flooding”
• Because not all liquid has boiled off

45
Q

Decrease spring pressure

A

– Allows more refrigerant into evaporator
•

46
Q

Decrease spring pressure

A

– Allows more refrigerant into evaporator
•

47
Q

Increase spring pressure
–

A

Decrease refrigerant to evaporator

48
Q

Distributors create pressure drop

A

• An external equalized TEV is required
– It senses pressure at the evaporator outlet
• Internal equalized valves will not work

49
Q

Expansion Valve connections

A

Flange, swage, flared

50
Q

Rule of Thumb

A

Warmer Vapor on top

Cold oil on bottom

Bulb mounted on side works well too

51
Q

Using a bulb

A

Make sure it is on a smooth surface of pipe
• Tip: Use stainless steel hose clamps

52
Q

Best location:
– On horizontal suction line
• If necessary:
– On vertical suction line

A

For bulb placement

53
Q

Best location:
– On horizontal suction line
• If necessary:
– On vertical suction line

A

For bulb placement

54
Q

High pressure drop
–

A

Increases valve capacity
•

55
Q

High liquid temperature

A

– Decreases valve capacity
•

56
Q

Almost offset each other

A

– Within 12% of original capacity

57
Q

Select valve by

A

Refrigerant
– Tonnage
– Application:
• “C” is Medium temperature
• “Z” is Low temperature
• “ZP40” Low temp, outlet pressure limited to 40 psig

58
Q

Freezer nominal ratings

A

A freezer requires 36,000 Btuh
– Is a 3 ton TEV correct for a -20°evaporator

59
Q

To know

A

Capacity is a letter, not a number
• Tonnage is a range of capacity

60
Q

Balanced Port TEV

A

In commercial refrigeration
When inlet pressures vary
Large swings in ambient temp
More consistent TEV operation

61
Q

AEV

A

Maintain evaporator pressure, doesn’t get too cold
Prevents pressure from going too low

Water cooler…slush machines

62
Q

EEV

A

A computer board controls the valve

A electric motor opens and closes the valve

Good accuracy

63
Q

Controller

A

Receives input from sensors
– Provides output power to EEV motor

64
Q

Sensors

A

Temperature thermistors
– Pressure transducers

65
Q

Capillary tube

A

Regulates flow by pressure drop

The longer the tube, the more the pressure drop

The smaller and bigger the tubing inside diameter

66
Q

Temperature difference

A

Space temp minus Evaporator temp

67
Q

All evaporators

A

Dehumidify… remove moisture

68
Q

Temperature difference

A

Air temperature entering the evaporator minus (–)
refrigerant temperature inside the evaporator

69
Q

Delta T

A

Air temperature entering the evaporator minus (–)
air temperature leaving the evaporator

70
Q

Delta T used in

A

A/c

71
Q

TD is used in

A

Commercial refrigeration application

72
Q

TD is used in

A

Commercial refrigeration application

73
Q

Why measure superheat

A

helps determine evaporator efficiency
– Latent heat is absorbed only as liquid boils
– Sensible heat is absorbed only when it is all vapor
• Evaporator superheat “Rule of Thumb”:
– Average 10°; Acceptable range 5°to 20

74
Q

Some other reasons

A

Superheat too HIGH (above 20°):
– “Starving” evaporator
• Refrigerant boiling off too soon
• Superheat too LOW (below 5°):
– “Flooding” evaporator
• Not all refrigerant is vaporized

75
Q

Superheat

A

Measurable heat above saturation temperature
– Verifies full evaporation
• Liquid flooding can cause compressor damage

76
Q

Evaporator Superheat

A

Heat measured at evaporator outlet

77
Q

Compressor or Total Superheat

A

Heat measured near compressor inlet

78
Q

Air conditioning is considered to be

A

High temp refrigeration
Heat transfers to coil

79
Q

Evaporator

A

Maintained at a temperature that is lower than the
medium being cooled
• Removes both latent and sensible heat from the air
in the refrigerated box
• Latent heat is in the form of moisture
• Sensible heat reduces air and product temperature

80
Q

Refrigerant boiling temp

A

Evaporator …coil operating temp

81
Q

Refrigerant condensing temp

A

Condensor

82
Q

Refrigerant condensing temp

A

Condenser

83
Q

Heat Exchange Characteristics of
the Evaporator part 1

A

Rapid heat transfer rate between two liquids
• Slower heat transfer rate between two vapors

84
Q

Types of evaporators

A

Finned tube is most common, multiple circuit and stamped plate

85
Q

Types of evaporators

A

Finned tube is most common, multiple circuit and stamped plate

86
Q

Heat Exchange Characteristics of
the Evaporator (2 of 2)

A

Relationship between the medium giving up heat
and the heat exchange surface
• Related to the velocity of medium over the coil
• If velocity is low, the film acts as an insulator

87
Q

Heat transfer

A

Air side—between fins and air to be cooled
• Heat conduction—between fins and tubes
• Refrigerant side—between tubes and evaporating
refrigerant

88
Q

Heat transfer

A

• Heat conduction—between fins and tubes
• Refrigerant side—between tubes and evaporating
refrigerant

89
Q

The Flooded Evaporator

A

• Designed to operate full of liquid
• Coil efficiency is maximized
• Other devices must be used to prevent liquid from
entering the compressor
• Normally use a float-type metering device to keep
the liquid level in the coil high

90
Q

Dry-Type Evaporator Performance

A

evaporators are considered to be “dry” when all of
the liquid boils before leaving the coil

91
Q

Hot Pulldown (Excessively
Loaded Evaporator)

A

When refrigerated space has warmed up

92
Q

evaporator controller monitors evaporator coil
efficiency

A

to initiate defrost
• Minimizes the effects of defrost on space
temperature
• Maintaining a lower temperature difference (TD) will
reduce frost formation

93
Q

Evaporator and Defrost Efficiency
Controller: Frost Removal

A

• Frost removal by sublimation

The control system is based around a refrigerant
pump-down cycle

94
Q

Defrost methods

A

Hots gas and electric heaters

95
Q

Evaporators for Low-
Temperature Application

A

Made for frost buildup

96
Q

Liquid Cooling Evaporators (Chillers)

A

Different type of evaporator is used
• Dry-type expansion evaporator in small systems
• Flooded type for larger-tonnage chillers
• Usually have more than one refrigerant circuit to
prevent pressure drop

97
Q

Tev

A

Meters the refrigerant to the evaporator by using a
thermal sensing element

98
Q

The Liquid Charge Bulb

A

• Charged with a fluid characteristic of the refrigerant
in the system
•

99
Q

The Cross Liquid Charge Bulb

A

different refrigerant than the system
• Does not follow the pressure/temperature
relationship of the system
•

100
Q

Maintains a constant evaporator superheat
• If the evaporator superheat is high,

A

the valve will
open

101
Q

• Superheat ensures that no liquid refrigerant

A

leaves
the evaporator
•

102
Q

Low superheat increases

A

the net refrigerant effect

103
Q

Liquid Charge Bulb: May reach high temperatures

A

during defrost

104
Q

Diaphragm and bulb are not

A

actually full of liquid;
•

105
Q

Pressures inside bulb cause

A

excessive pressures
over the diaphragm
•

106
Q

Valve opens wide,

A

overfeeding the evaporator

107
Q

Closes the valve faster

A

on a rise in evaporator
pressure
•

108
Q

Valve closes during

A

the compressor off cycle
•

109
Q

Helps prevent liquid refrigerant from flooding over

A

into the compressor at startup

HVAC (16 decks)

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