CFC Compressor & Component Diagnostics 5 — Questions and Answers
Question 1: A technician diagnoses a compressor with liquid slugging damage. Which internal component is most commonly damaged first by this condition?
- Motor stator windings
- Suction and discharge valve reeds (Correct answer)
- Crankshaft main bearings
- Piston pin retaining clips
Correct answer: Suction and discharge valve reeds
Valve reeds are thin, flexible metal components that fracture almost immediately when subjected to the hydraulic shock of liquid refrigerant or oil slugging.
Question 2: A system equipped with a crankcase pressure regulator (CPR) valve at the compressor suction uses it primarily to:
- Prevent refrigerant migration during the off-cycle
- Limit the suction pressure to protect the compressor motor from overloading at startup (Correct answer)
- Reduce suction superheat during peak load conditions
- Control evaporator pressure in a multi-evaporator system
Correct answer: Limit the suction pressure to protect the compressor motor from overloading at startup
A CPR valve limits suction pressure entering the compressor to prevent motor overload during pull-down when suction pressure is initially very high.
Question 3: Which condition would cause a compressor discharge temperature to exceed 300°F (149°C)?
- Low compression ratio with adequate suction superheat
- High compression ratio combined with high suction superheat (Correct answer)
- Low ambient temperature reducing head pressure
- Normal compression ratio with 10°F suction superheat
Correct answer: High compression ratio combined with high suction superheat
High compression ratio (high head pressure) combined with high suction superheat produces extremely high discharge temperatures because both factors increase the temperature of the compressed gas.
Question 4: A technician performs a pump-down test and the system reaches the low-pressure cutout in an unusually short time. This indicates:
- The system is operating normally with correct refrigerant charge
- The system is undercharged with refrigerant (Correct answer)
- The metering device is overfeeding the evaporator
- The low-pressure control setpoint is set too high
Correct answer: The system is undercharged with refrigerant
An undercharged system has insufficient refrigerant mass to maintain evaporator pressure, causing rapid pump-down to the low-pressure cutout.
Question 5: When diagnosing a compressor with a suspected internal leak across the discharge valve, which field measurement best confirms this?
- High discharge superheat measurement at the condenser inlet
- Compressor off-cycle pressure equalization occurring faster than normal (Correct answer)
- Low amperage reading compared to nameplate FLA
- High refrigerant subcooling at the condenser outlet
Correct answer: Compressor off-cycle pressure equalization occurring faster than normal
A leaking discharge valve allows high-side pressure to bleed back to the low side during the off-cycle much faster than the normal equalization time, confirming internal bypass.
Question 6: A compressor on a CFC-11 centrifugal chiller experiences surge repeatedly at part-load conditions. What is the primary cause of compressor surge?
- Refrigerant overcharge reducing evaporator superheat
- The pressure differential exceeding the compressor's ability to produce flow at reduced speed (Correct answer)
- High suction pressure caused by a fouled evaporator
- Excessive oil viscosity at low ambient temperatures
Correct answer: The pressure differential exceeding the compressor's ability to produce flow at reduced speed
Surge occurs when the pressure differential across a centrifugal compressor exceeds its ability to maintain stable flow at reduced capacity, causing cyclical flow reversal.
Question 7: After recovering CFC refrigerant from a system with a burned-out compressor motor, what additional step is required before recharging with virgin or reclaimed refrigerant?
- The recovered refrigerant can be immediately recharged after oil separation
- The system must be dehydrated, acid-flushed, and the recovered refrigerant sent for reclamation to ARI-700 standards (Correct answer)
- The system only needs a new filter-drier before recharging
- The refrigerant must be recharged within 24 hours to prevent refrigerant degradation
Correct answer: The system must be dehydrated, acid-flushed, and the recovered refrigerant sent for reclamation to ARI-700 standards
A burnout contaminates refrigerant with acid and carbon, requiring full system decontamination and sending recovered refrigerant to a certified reclaimer to meet ARI-700 purity standards before reuse.
CFC Compressor & Component Diagnostics 5
A technician diagnoses a compressor with liquid slugging damage.
Which internal component is most commonly damaged first by this condition?