RSES Troubleshooting 2 — Questions and Answers
Question 1: A refrigeration system has high discharge pressure and high suction pressure. What is the most likely cause?
- Overcharge of refrigerant (Correct answer)
- Restricted liquid line filter-drier
- Faulty expansion valve stuck closed
- Low ambient temperature
Correct answer: Overcharge of refrigerant
An overcharge of refrigerant raises both suction and discharge pressures above normal operating levels.
Question 2: A technician finds oil accumulation in the evaporator of a low-temperature system. What is the most probable cause?
- Excessive refrigerant charge
- Insufficient refrigerant velocity to return oil (Correct answer)
- Overheating of the compressor
- High condenser subcooling
Correct answer: Insufficient refrigerant velocity to return oil
At low temperatures, refrigerant vapor velocity may be too low to carry oil back to the compressor, causing oil to accumulate in the evaporator.
Question 3: During operation, a system's compressor cycles on and off rapidly (short-cycles). Which condition is the LEAST likely cause?
- Low refrigerant charge
- Faulty low-pressure control set too high
- Oversized compressor for the load
- Dirty condenser coil (Correct answer)
Correct answer: Dirty condenser coil
A dirty condenser coil raises discharge pressure but does not typically cause rapid short-cycling on the low-pressure control.
Question 4: A walk-in cooler compressor runs continuously but the box temperature never reaches setpoint. Suction pressure is low and discharge pressure is normal. What should be checked first?
- Condenser fan motor
- Refrigerant charge (undercharge) (Correct answer)
- Compressor valve efficiency
- Expansion valve superheat setting
Correct answer: Refrigerant charge (undercharge)
Low suction pressure combined with continuous compressor operation and warm box temperature strongly suggests a refrigerant undercharge.
Question 5: What does a frost pattern that stops midway through an evaporator coil typically indicate?
- Excess refrigerant charge
- Partially blocked or malfunctioning expansion device (Correct answer)
- Condenser coil fouling
- High ambient humidity
Correct answer: Partially blocked or malfunctioning expansion device
A frost pattern ending midway indicates refrigerant is fully evaporating before the end of the coil, suggesting a restricted or undersized expansion device.
Question 6: A technician measures 15°F of subcooling at the liquid line service valve. Normal subcooling for this system is 10°F. What does this indicate?
- Refrigerant undercharge
- Refrigerant overcharge or liquid line restriction upstream of measurement point (Correct answer)
- Faulty condenser fan
- Compressor valve leak
Correct answer: Refrigerant overcharge or liquid line restriction upstream of measurement point
Higher-than-normal subcooling suggests an overcharge or a restriction upstream that is backing up liquid refrigerant.
Question 7: A system exhibits liquid slugging at compressor startup. Which corrective action is most appropriate?
- Increase condenser fan speed
- Install or verify operation of a crankcase heater (Correct answer)
- Add refrigerant to the system
- Replace the high-pressure switch
Correct answer: Install or verify operation of a crankcase heater
A crankcase heater prevents refrigerant migration into the oil sump during off-cycles, eliminating liquid slugging at startup.
RSES Troubleshooting 2
A refrigeration system has high discharge pressure and high suction pressure.
What is the most likely cause?