RSES Troubleshooting 4 — Questions and Answers
Question 1: A commercial refrigeration system's discharge temperature is excessively high. Which of the following is NOT a likely cause?
- High suction superheat
- High compression ratio
- Refrigerant overcharge causing high head pressure
- Low ambient temperature at the condenser (Correct answer)
Correct answer: Low ambient temperature at the condenser
Low ambient temperature at the condenser reduces head pressure and discharge temperature, making it an unlikely cause of excessive discharge temperature.
Question 2: What symptom would a technician observe if a TXV is hunting (oscillating)?
- Steady suction pressure with normal superheat
- Fluctuating suction pressure and superheat swinging above and below setpoint (Correct answer)
- Constant low suction pressure
- High head pressure with low suction pressure
Correct answer: Fluctuating suction pressure and superheat swinging above and below setpoint
A hunting TXV repeatedly over- and under-feeds refrigerant, causing suction pressure and superheat to oscillate continuously.
Question 3: A technician checks a system and finds oil in the sight glass of the liquid line. What does this indicate?
- Normal condition — some oil always circulates
- Refrigerant overcharge causing oil logging
- Low refrigerant charge with oil becoming visible (Correct answer)
- Moisture contamination in the refrigerant
Correct answer: Low refrigerant charge with oil becoming visible
When refrigerant charge is low, the remaining refrigerant carries proportionally more oil, making oil visible in the sight glass.
Question 4: During a leak check using electronic leak detection, false positives are being triggered near a drain pan. What could cause this?
- Refrigerant accumulation in the drain pan from a real leak
- Cleaning solvents or certain drain pan treatments off-gassing halogen compounds (Correct answer)
- Excessive superheat in the evaporator
- Non-condensables in the system
Correct answer: Cleaning solvents or certain drain pan treatments off-gassing halogen compounds
Some cleaning agents and drain pan treatments contain halogenated compounds that can trigger halide-based electronic leak detectors.
Question 5: A technician measures 0°F superheat at the suction service valve of a running system. What immediate risk does this present?
- Compressor motor overheating from low voltage
- Liquid refrigerant entering the compressor, risking mechanical damage (Correct answer)
- Expansion valve failure from overpressure
- Condenser coil freeze-up
Correct answer: Liquid refrigerant entering the compressor, risking mechanical damage
Zero superheat means liquid refrigerant is present in the suction line, and liquid entering the compressor can cause valve and piston damage.
Question 6: Which of the following best describes 'flash gas' in the liquid line and why it is problematic?
- Vapor formed when liquid refrigerant pressure drops below saturation, reducing expansion valve capacity (Correct answer)
- Gas released from oil at high temperatures, contaminating refrigerant
- Nitrogen introduced during charging that displaces refrigerant vapor
- Flash evaporation in the condenser that reduces subcooling
Correct answer: Vapor formed when liquid refrigerant pressure drops below saturation, reducing expansion valve capacity
Flash gas occurs when liquid refrigerant pressure drops below its saturation point before the expansion device, reducing the valve's effective capacity.
Question 7: A refrigeration technician finds that a compressor's amp draw is well below nameplate rating. What could this indicate?
- Refrigerant overcharge
- Worn compressor valves reducing pumping efficiency (Correct answer)
- Clogged condenser reducing heat rejection
- High suction superheat
Correct answer: Worn compressor valves reducing pumping efficiency
Worn or broken compressor valves reduce the work done per cycle, resulting in lower-than-normal current draw.
RSES Troubleshooting 4
A commercial refrigeration system's discharge temperature is excessively high.
Which of the following is NOT a likely cause?