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Hvac Flashcards

(109 cards)

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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
Who establishes requirements regarding when leaks must be repaired?
The EPA (Environmental Protection Agency) establishes requirements regarding when leaks must be repaired. ## Footnote No additional information provided.
26
What factors may determine when leaks need to be repaired?
The factors that may determine when leaks need to be repaired include the type of system, refrigerant used, and/or total system charge. ## Footnote No additional information provided.
27
Systems with Schrader Valves take longer to evacuate than systems with service valves
Field service valves are used to replace Schrader valve stems while the system is under pressure ## Footnote Schrader valve caps should be put back on the valve after service
28
What do chemical combinations create?
Hydrofluoric or hydrochloric acids ## Footnote Example sentence: Chemical combinations can create hydrofluoric or hydrochloric acids.
29
What does evacuation consist of?
Degassing + dehydration ## Footnote None
30
What does moisture + acid + oil result in?
Sludge ## Footnote None
31
What can sludge cause in system components?
Plugged components ## Footnote None
32
What can proper evacuation eliminate?
Formation of acid and sludge ## Footnote None
33
TEV or TXV (Thermostatic Expansion Valve)
Measuring and adjusting superheat Mounting the sensing bulb Use on multi-circuit evaporators Identification and application
34
AEV (Automatic Expansion Valve)
35
EEV (Electronic Expansion Valve)
36
Fixed Metering Devices
Pistons and capillary tubes
37
Primary function of a TEV
Maintain superheat ## Footnote TEV stands for Thermostatic Expansion Valve
38
Opening force/liquid line temp
Bulb pressure ## Footnote Opening force is generated by the pressure in the sensing bulb
39
Closing forces
Evaporator pressure ## Footnote One of the closing forces is generated by the evaporator pressure ## Example The TEV closes when the evaporator pressure is too low
40
Closing forces
Adjustable spring ## Footnote Another closing force is provided by an adjustable spring
41
Notes for the next slide
Assume an R22 system, and There is R22 in the sensing bulb ## Footnote These notes are important for understanding the operation of the TEV
42
Measuring Superheat • Superheat =
Suction line temperature – Evaporator temperature •
43
High superheat: –
Evaporator is “Starving” • Because all liquid has boiled off too soon •
44
Low Superheat: –
Evaporator is “Flooding” • Because not all liquid has boiled off
45
Decrease spring pressure
– Allows more refrigerant into evaporator •
46
Decrease spring pressure
– Allows more refrigerant into evaporator •
47
Increase spring pressure –
Decrease refrigerant to evaporator
48
Distributors create pressure drop
• An external equalized TEV is required – It senses pressure at the evaporator outlet • Internal equalized valves will not work
49
Expansion Valve connections
Flange, swage, flared
50
Rule of Thumb
Warmer Vapor on top Cold oil on bottom Bulb mounted on side works well too
51
Using a bulb
Make sure it is on a smooth surface of pipe • Tip: Use stainless steel hose clamps
52
Best location: – On horizontal suction line • If necessary: – On vertical suction line
For bulb placement
53
Best location: – On horizontal suction line • If necessary: – On vertical suction line
For bulb placement
54
High pressure drop –
Increases valve capacity •
55
High liquid temperature
– Decreases valve capacity •
56
Almost offset each other
– Within 12% of original capacity
57
Select valve by
Refrigerant – Tonnage – Application: • “C” is Medium temperature • “Z” is Low temperature • “ZP40” Low temp, outlet pressure limited to 40 psig
58
Freezer nominal ratings
A freezer requires 36,000 Btuh – Is a 3 ton TEV correct for a -20°evaporator
59
To know
Capacity is a letter, not a number • Tonnage is a range of capacity
60
Balanced Port TEV
In commercial refrigeration When inlet pressures vary Large swings in ambient temp More consistent TEV operation
61
AEV
Maintain evaporator pressure, doesn’t get too cold Prevents pressure from going too low Water cooler…slush machines
62
EEV
A computer board controls the valve A electric motor opens and closes the valve Good accuracy
63
Controller
Receives input from sensors – Provides output power to EEV motor
64
Sensors
Temperature thermistors – Pressure transducers
65
Capillary tube
Regulates flow by pressure drop The longer the tube, the more the pressure drop The smaller and bigger the tubing inside diameter
66
Temperature difference
Space temp minus Evaporator temp
67
All evaporators
Dehumidify… remove moisture
68
Temperature difference
Air temperature entering the evaporator minus (–) refrigerant temperature inside the evaporator
69
Delta T
Air temperature entering the evaporator minus (–) air temperature leaving the evaporator
70
Delta T used in
A/c
71
TD is used in
Commercial refrigeration application
72
TD is used in
Commercial refrigeration application
73
Why measure superheat
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
Some other reasons
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
Superheat
Measurable heat above saturation temperature – Verifies full evaporation • Liquid flooding can cause compressor damage
76
Evaporator Superheat
Heat measured at evaporator outlet
77
Compressor or Total Superheat
Heat measured near compressor inlet
78
Air conditioning is considered to be
High temp refrigeration Heat transfers to coil
79
Evaporator
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
Refrigerant boiling temp
Evaporator …coil operating temp
81
Refrigerant condensing temp
Condensor
82
Refrigerant condensing temp
Condenser
83
Heat Exchange Characteristics of the Evaporator part 1
Rapid heat transfer rate between two liquids • Slower heat transfer rate between two vapors
84
Types of evaporators
Finned tube is most common, multiple circuit and stamped plate
85
Types of evaporators
Finned tube is most common, multiple circuit and stamped plate
86
Heat Exchange Characteristics of the Evaporator (2 of 2)
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
Heat transfer
Air side—between fins and air to be cooled • Heat conduction—between fins and tubes • Refrigerant side—between tubes and evaporating refrigerant
88
Heat transfer
• Heat conduction—between fins and tubes • Refrigerant side—between tubes and evaporating refrigerant
89
The Flooded Evaporator
• 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
Dry-Type Evaporator Performance
evaporators are considered to be “dry” when all of the liquid boils before leaving the coil
91
Hot Pulldown (Excessively Loaded Evaporator)
When refrigerated space has warmed up
92
evaporator controller monitors evaporator coil efficiency
to initiate defrost • Minimizes the effects of defrost on space temperature • Maintaining a lower temperature difference (TD) will reduce frost formation
93
Evaporator and Defrost Efficiency Controller: Frost Removal
• Frost removal by sublimation The control system is based around a refrigerant pump-down cycle
94
Defrost methods
Hots gas and electric heaters
95
Evaporators for Low- Temperature Application
Made for frost buildup
96
Liquid Cooling Evaporators (Chillers)
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
Tev
Meters the refrigerant to the evaporator by using a thermal sensing element
98
The Liquid Charge Bulb
• Charged with a fluid characteristic of the refrigerant in the system •
99
The Cross Liquid Charge Bulb
different refrigerant than the system • Does not follow the pressure/temperature relationship of the system •
100
Maintains a constant evaporator superheat • If the evaporator superheat is high,
the valve will open
101
• Superheat ensures that no liquid refrigerant
leaves the evaporator •
102
Low superheat increases
the net refrigerant effect
103
Liquid Charge Bulb: May reach high temperatures
during defrost
104
Diaphragm and bulb are not
actually full of liquid; •
105
Pressures inside bulb cause
excessive pressures over the diaphragm •
106
Valve opens wide,
overfeeding the evaporator
107
Closes the valve faster
on a rise in evaporator pressure •
108
Valve closes during
the compressor off cycle •
109
Helps prevent liquid refrigerant from flooding over
into the compressor at startup
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