HVAC Compressors and Condensing Units 2 — Questions and Answers
Question 1: What is 'compressor superheat' and why is it important?
- Heat added to liquid refrigerant before expansion
- The temperature rise of refrigerant vapor above its saturation point before entering the compressor (Correct answer)
- The discharge temperature of refrigerant leaving the compressor
- Heat removed from refrigerant in the condenser
Correct answer: The temperature rise of refrigerant vapor above its saturation point before entering the compressor
Compressor superheat is the temperature of refrigerant vapor above its saturation point at the compressor inlet, ensuring only dry vapor enters to protect the compressor.
Question 2: What is a hermetic compressor?
- A compressor with an external belt-drive motor
- A compressor where the motor and pump are sealed in a single welded housing (Correct answer)
- A compressor designed only for commercial applications
- A compressor that uses magnetic refrigerant
Correct answer: A compressor where the motor and pump are sealed in a single welded housing
A hermetic compressor encloses the motor and compression mechanism in a welded, gas-tight shell so the motor windings are cooled by refrigerant vapor.
Question 3: What symptom indicates a compressor is short-cycling?
- The compressor runs continuously without shutting off
- The compressor starts and stops frequently in short intervals (Correct answer)
- The compressor produces a loud grinding noise during operation
- The condenser fan runs but the compressor does not start
Correct answer: The compressor starts and stops frequently in short intervals
Short-cycling is when the compressor repeatedly starts and stops within short intervals, which can be caused by refrigerant issues, dirty coils, or oversized equipment.
Question 4: What is the purpose of a crankcase heater on a compressor?
- Increase compressor efficiency during summer operation
- Prevent refrigerant migration into the oil during the off cycle (Correct answer)
- Heat the refrigerant before it enters the evaporator
- Protect the compressor from low ambient temperatures during operation
Correct answer: Prevent refrigerant migration into the oil during the off cycle
A crankcase heater keeps the oil warm during the off cycle to prevent refrigerant from migrating into and mixing with the compressor oil, which could cause damage on startup.
Question 5: Which compressor type is most commonly used in large centrifugal chiller applications?
- Reciprocating compressor
- Scroll compressor
- Rotary compressor
- Centrifugal compressor (Correct answer)
Correct answer: Centrifugal compressor
Centrifugal compressors are used in large tonnage chillers because they efficiently handle very large volumes of low-pressure refrigerant vapor.
Question 6: What causes a compressor to 'trip' on its internal overload protector?
- Low suction pressure from a clean filter
- Overheating of the motor windings due to excessive current or poor cooling (Correct answer)
- Excessive refrigerant subcooling at the condenser outlet
- Low discharge pressure in the system
Correct answer: Overheating of the motor windings due to excessive current or poor cooling
The internal overload protector opens when motor windings overheat from excessive amperage draw or insufficient refrigerant cooling of the motor.
Question 7: What is 'discharge superheat' in a refrigeration system?
- The temperature difference between outdoor air and condensing temperature
- The temperature of refrigerant vapor at the compressor outlet above its condensing saturation temperature (Correct answer)
- The amount of subcooling at the condenser outlet
- The difference in suction and discharge pressures
Correct answer: The temperature of refrigerant vapor at the compressor outlet above its condensing saturation temperature
Discharge superheat is measured at the compressor outlet and indicates the temperature of discharge gas above its saturation point at the discharge pressure.
HVAC Compressors and Condensing Units 2
What is 'compressor superheat' and why is it important?