Hvac Flashcards
What are flaws?
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
What can flow under layers of paint, flux, rust, slag, and pipe insulation?
Vapor
Vapor can flow under layers of paint, flux, rust, slag, and pipe insulation
What is important to do at the leak site?
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
Besides loose paint, slag, flux, or rust, what else must be removed at the leak site?
Pipe insulation, oil, and grease
Must also remove pipe insulation, oil, and grease to avoid contaminating the detection tips
Standing leaks
Detected while the unit is at rest or off
The most common of all leaks
Pressure dependent leaks
Detected as the system pressure increases
Pressure dependent leaks
Temperature-dependent leaks
Associated with the heat of expansion
Temperature-dependent leaks
What is the first step in performing a standing pressure test?
Pressurize the system with dry nitrogen to a pressure no higher than the lowest system test pressure
Example: Pressurize the system to 100 psi
What is the second step in performing a standing pressure test?
Allow the system to rest for ten minutes
No additional information
What is the third step in performing a standing pressure test?
Mark the needle positions on the gauge manifold
No additional information
What is the final step in performing a standing pressure test?
Monitor gauge needle position
No additional information
Vibration-dependent leaks
Occur during unit operation
Vibration-dependent leaks occur during unit operation.
Combination-dependent leaks
Require two or more conditions to leak
Combination-dependent leaks require two or more conditions to leak.
Cumulative microleaks
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.
What can vapor flow under at the leak site?
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.
Why is it important to clean the leak site?
So system is more efficient
What else must be removed from the leak site?
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.
What does air contain?
Oxygen, nitrogen, and water vapor
Air contains oxygen, nitrogen, and water vapor
What is nitrogen?
A non-condensable gas
Nitrogen is a non-condensable gas
What do non-condensables cause?
A rise in the system’s operating head pressure
Non-condensables will cause a rise in the system’s operating head pressure
What do oxygen and water vapor cause in the system?
Chemical reactions
Oxygen and water vapor cause chemical reactions in the system
What do oxygen and water vapor produce in the system?
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
Is repairing leaks usually as economical as adding refrigerant?
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.
What is the best field practice when it comes to system leaks?
The best field practice is to repair system leaks whenever possible.
Additional information: Repairing leaks helps maintain system efficiency and prevents environmental harm.
Who establishes requirements regarding when leaks must be repaired?
The EPA (Environmental Protection Agency) establishes requirements regarding when leaks must be repaired.
No additional information provided.
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.
No additional information provided.
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
Schrader valve caps should be put back on the valve after service
What do chemical combinations create?
Hydrofluoric or hydrochloric acids
Example sentence: Chemical combinations can create hydrofluoric or hydrochloric acids.
What does evacuation consist of?
Degassing + dehydration
None
What does moisture + acid + oil result in?
Sludge
None
What can sludge cause in system components?
Plugged components
None
What can proper evacuation eliminate?
Formation of acid and sludge
None
TEV or TXV (Thermostatic Expansion Valve)
Measuring and adjusting superheat
Mounting the sensing bulb
Use on multi-circuit evaporators
Identification and application
AEV (Automatic Expansion Valve)
EEV (Electronic Expansion Valve)
Fixed Metering Devices
Pistons and capillary tubes
Primary function of a TEV
Maintain superheat
TEV stands for Thermostatic Expansion Valve
Opening force/liquid line temp
Bulb pressure
Opening force is generated by the pressure in the sensing bulb
Closing forces
Evaporator pressure
One of the closing forces is generated by the evaporator pressure
The TEV closes when the evaporator pressure is too low
Closing forces
Adjustable spring
Another closing force is provided by an adjustable spring
Notes for the next slide
Assume an R22 system, and
There is R22 in the sensing bulb
These notes are important for understanding the operation of the TEV
Measuring Superheat
• Superheat =
Suction line temperature – Evaporator temperature
•
High superheat:
–
Evaporator is “Starving”
• Because all liquid has boiled off too soon
•
Low Superheat:
–
Evaporator is “Flooding”
• Because not all liquid has boiled off
Decrease spring pressure
– Allows more refrigerant into evaporator
•
Decrease spring pressure
– Allows more refrigerant into evaporator
•
Increase spring pressure
–
Decrease refrigerant to evaporator
Distributors create pressure drop
• An external equalized TEV is required
– It senses pressure at the evaporator outlet
• Internal equalized valves will not work
Expansion Valve connections
Flange, swage, flared
Rule of Thumb
Warmer Vapor on top
Cold oil on bottom
Bulb mounted on side works well too
Using a bulb
Make sure it is on a smooth surface of pipe
• Tip: Use stainless steel hose clamps
Best location:
– On horizontal suction line
• If necessary:
– On vertical suction line
For bulb placement
Best location:
– On horizontal suction line
• If necessary:
– On vertical suction line
For bulb placement
High pressure drop
–
Increases valve capacity
•
High liquid temperature
– Decreases valve capacity
•
Almost offset each other
– Within 12% of original capacity
Select valve by
Refrigerant
– Tonnage
– Application:
• “C” is Medium temperature
• “Z” is Low temperature
• “ZP40” Low temp, outlet pressure limited to 40 psig
Freezer nominal ratings
A freezer requires 36,000 Btuh
– Is a 3 ton TEV correct for a -20°evaporator
To know
Capacity is a letter, not a number
• Tonnage is a range of capacity
Balanced Port TEV
In commercial refrigeration
When inlet pressures vary
Large swings in ambient temp
More consistent TEV operation
AEV
Maintain evaporator pressure, doesn’t get too cold
Prevents pressure from going too low
Water cooler…slush machines
EEV
A computer board controls the valve
A electric motor opens and closes the valve
Good accuracy
Controller
Receives input from sensors
– Provides output power to EEV motor
Sensors
Temperature thermistors
– Pressure transducers
Capillary tube
Regulates flow by pressure drop
The longer the tube, the more the pressure drop
The smaller and bigger the tubing inside diameter
Temperature difference
Space temp minus Evaporator temp
All evaporators
Dehumidify… remove moisture
Temperature difference
Air temperature entering the evaporator minus (–)
refrigerant temperature inside the evaporator
Delta T
Air temperature entering the evaporator minus (–)
air temperature leaving the evaporator
Delta T used in
A/c
TD is used in
Commercial refrigeration application
TD is used in
Commercial refrigeration application
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
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
Superheat
Measurable heat above saturation temperature
– Verifies full evaporation
• Liquid flooding can cause compressor damage
Evaporator Superheat
Heat measured at evaporator outlet
Compressor or Total Superheat
Heat measured near compressor inlet
Air conditioning is considered to be
High temp refrigeration
Heat transfers to coil
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
Refrigerant boiling temp
Evaporator …coil operating temp
Refrigerant condensing temp
Condensor
Refrigerant condensing temp
Condenser
Heat Exchange Characteristics of
the Evaporator part 1
Rapid heat transfer rate between two liquids
• Slower heat transfer rate between two vapors
Types of evaporators
Finned tube is most common, multiple circuit and stamped plate
Types of evaporators
Finned tube is most common, multiple circuit and stamped plate
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
Heat transfer
Air side—between fins and air to be cooled
• Heat conduction—between fins and tubes
• Refrigerant side—between tubes and evaporating
refrigerant
Heat transfer
• Heat conduction—between fins and tubes
• Refrigerant side—between tubes and evaporating
refrigerant
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
Dry-Type Evaporator Performance
evaporators are considered to be “dry” when all of
the liquid boils before leaving the coil
Hot Pulldown (Excessively
Loaded Evaporator)
When refrigerated space has warmed up
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
Evaporator and Defrost Efficiency
Controller: Frost Removal
• Frost removal by sublimation
The control system is based around a refrigerant
pump-down cycle
Defrost methods
Hots gas and electric heaters
Evaporators for Low-
Temperature Application
Made for frost buildup
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
Tev
Meters the refrigerant to the evaporator by using a
thermal sensing element
The Liquid Charge Bulb
• Charged with a fluid characteristic of the refrigerant
in the system
•
The Cross Liquid Charge Bulb
different refrigerant than the system
• Does not follow the pressure/temperature
relationship of the system
•
Maintains a constant evaporator superheat
• If the evaporator superheat is high,
the valve will
open
• Superheat ensures that no liquid refrigerant
leaves
the evaporator
•
Low superheat increases
the net refrigerant effect
Liquid Charge Bulb: May reach high temperatures
during defrost
Diaphragm and bulb are not
actually full of liquid;
•
Pressures inside bulb cause
excessive pressures
over the diaphragm
•
Valve opens wide,
overfeeding the evaporator
Closes the valve faster
on a rise in evaporator
pressure
•
Valve closes during
the compressor off cycle
•
Helps prevent liquid refrigerant from flooding over
into the compressor at startup
