CRST Installation & Maintenance Practices 5 — Questions and Answers
Question 1: What type of oil is required in systems using R-410A refrigerant?
- Mineral oil
- Alkylbenzene oil
- Polyolester (POE) oil (Correct answer)
- Naphthenic oil
Correct answer: Polyolester (POE) oil
R-410A and other HFC refrigerants require POE oil because it is miscible with HFCs, ensuring adequate lubrication and oil return throughout the system.
Question 2: A refrigeration technician installs a new evaporator and notices the coil temperature is uneven across its face. What is the MOST likely cause?
- Overcharged refrigerant
- Unequal refrigerant distribution due to a faulty distributor or plugged distributor nozzle (Correct answer)
- Excessive superheat at the TXV bulb
- High ambient temperature at the condenser
Correct answer: Unequal refrigerant distribution due to a faulty distributor or plugged distributor nozzle
A faulty or plugged refrigerant distributor causes uneven flow to evaporator circuits, resulting in uneven coil temperatures and reduced capacity.
Question 3: During a maintenance inspection, a technician finds that the compressor contactor contacts are pitted and burned. What is the correct action?
- File the contacts smooth and return to service
- Apply contact cleaner and re-energize
- Replace the contactor (Correct answer)
- Increase the supply voltage to reduce arcing
Correct answer: Replace the contactor
Pitted and burned contactor contacts indicate excessive arcing and wear; the contactor must be replaced to prevent premature failure and compressor damage.
Question 4: What is the purpose of a hot gas defrost system in a commercial refrigeration installation?
- Pre-heat refrigerant before it enters the compressor
- Use high-pressure discharge gas to melt frost from the evaporator coil without electric heaters (Correct answer)
- Raise suction pressure during high-load periods
- Superheat the liquid line to prevent flash gas
Correct answer: Use high-pressure discharge gas to melt frost from the evaporator coil without electric heaters
Hot gas defrost diverts high-temperature discharge gas through the evaporator coil, efficiently melting frost accumulation without electric resistance heaters.
Question 5: When installing multiple evaporators on a single condensing unit, which component ensures proper refrigerant distribution and independent temperature control for each case?
- A common TXV sized for the total load
- Individual EPRs (evaporator pressure regulators) or individual TXVs per evaporator (Correct answer)
- A single solenoid valve at the condensing unit
- A common liquid line filter-drier only
Correct answer: Individual EPRs (evaporator pressure regulators) or individual TXVs per evaporator
Individual EPRs or TXVs for each evaporator allow independent temperature control and proper refrigerant metering regardless of load differences between cases.
Question 6: A technician measures 20°F of subcooling in the liquid line of a properly charged system. What does this indicate?
- The system is overcharged with refrigerant
- The liquid refrigerant has been cooled 20°F below its condensing saturation temperature, which is normal (Correct answer)
- There is moisture in the system
- The TXV is malfunctioning
Correct answer: The liquid refrigerant has been cooled 20°F below its condensing saturation temperature, which is normal
Subcooling of 10–20°F is normal and indicates the liquid refrigerant has cooled below its saturation point, reducing flash gas at the metering device.
Question 7: What safety precaution must be taken before performing brazing work on a refrigeration system that contains or previously contained refrigerant?
- Pressurize the system to check for leaks first
- Ensure the system is fully recovered and purged with nitrogen before applying heat (Correct answer)
- Perform brazing while refrigerant is still present to prevent air entry
- Only braze in areas with a fire extinguisher nearby, no purge required
Correct answer: Ensure the system is fully recovered and purged with nitrogen before applying heat
Refrigerant must be fully recovered and the system purged with dry nitrogen before brazing, as heat applied to refrigerant can produce toxic decomposition products.
CRST Installation & Maintenance Practices 5
What type of oil is required in systems using R-410A refrigerant?