313A System Troubleshooting & Safety 1 — Questions and Answers
Question 1: A refrigeration system shows high discharge pressure, high suction pressure, and low superheat. What is the most likely cause?
- Refrigerant undercharge
- Refrigerant overcharge (Correct answer)
- Restricted liquid line filter-drier
- Faulty compressor discharge valves
Correct answer: Refrigerant overcharge
High discharge pressure, high suction pressure, and low superheat together indicate refrigerant overcharge — excess refrigerant floods the evaporator (low superheat) and overwhelms the condenser (high discharge pressure).
Question 2: A system has low suction pressure, low discharge pressure, normal subcooling, and high superheat. What is the most likely diagnosis?
- Refrigerant overcharge
- Faulty compressor discharge valves
- Refrigerant undercharge (Correct answer)
- Dirty condenser coil
Correct answer: Refrigerant undercharge
Low suction pressure combined with high superheat and low discharge pressure indicates insufficient refrigerant in the system — the evaporator is starved of liquid, causing high superheat, and the compressor is moving less vapour mass, lowering both pressures.
Question 3: What instrument is used to measure refrigerant pressure at service valves on a refrigeration system?
- A clamp-on ammeter
- A digital manifold gauge set (Correct answer)
- A megohmmeter
- A psychrometric chart
Correct answer: A digital manifold gauge set
A manifold gauge set (or digital manifold) connects to the high- and low-side service valves to measure suction and discharge pressures, allowing the technician to diagnose system performance and charge level.
Question 4: What does a bubbling sight glass indicate when the system is at steady-state operation?
- Normal operation with a full liquid line
- Refrigerant shortage, moisture contamination, or a restriction causing partial vaporization of the liquid upstream of the sight glass (Correct answer)
- The TXV is functioning correctly and modulating flow
- The condenser fan is operating at reduced speed
Correct answer: Refrigerant shortage, moisture contamination, or a restriction causing partial vaporization of the liquid upstream of the sight glass
A bubbling or foamy sight glass at steady state indicates the liquid line contains vapour bubbles, which can be caused by low refrigerant charge, excessive pressure drop, or a restriction in the liquid line producing flash gas.
Question 5: A compressor is running but suction pressure is very high and the system is not cooling. The discharge line is not hot. What is the most likely cause?
- Refrigerant overcharge causing high suction pressure
- Failed compressor discharge or suction valves, causing the compressor to pump poorly (Correct answer)
- Dirty condenser coil blocking heat rejection
- A failed TXV stuck in the open position
Correct answer: Failed compressor discharge or suction valves, causing the compressor to pump poorly
If the compressor is running but the discharge line is not hot and suction pressure is very high, the compressor valves have likely failed — the compressor is spinning but not effectively compressing the refrigerant, so it is not moving refrigerant through the system.
Question 6: What personal protective equipment (PPE) is mandatory when handling refrigerants under Canadian safety regulations?
- Hard hat and steel-toed boots only
- Safety glasses or goggles and insulated gloves as a minimum; a face shield, chemical-resistant apron, and respiratory protection depending on the refrigerant and quantity (Correct answer)
- Flame-resistant clothing only, required for all refrigerants
- Hearing protection and fall arrest harness only
Correct answer: Safety glasses or goggles and insulated gloves as a minimum; a face shield, chemical-resistant apron, and respiratory protection depending on the refrigerant and quantity
Canadian OHS regulations and manufacturer SDSs require at minimum safety glasses/goggles and chemical-resistant gloves when handling refrigerants; additional PPE such as face shields and respirators are required for larger quantities or toxic refrigerants like ammonia.
313A System Troubleshooting & Safety 1
A refrigeration system shows high discharge pressure, high suction pressure, and low superheat.
What is the most likely cause?