CRST Refrigeration Systems & Components 5 — Questions and Answers
Question 1: A refrigeration system has excessively high discharge temperature. Which component failure could cause this if suction pressure is also low?
- Overcharged condenser
- Leaking discharge valve allowing re-expansion into the cylinder (Correct answer)
- Flooded evaporator
- Faulty liquid line solenoid valve stuck open
Correct answer: Leaking discharge valve allowing re-expansion into the cylinder
A leaking discharge valve allows hot high-pressure gas to leak back into the cylinder, which is re-compressed and creates abnormally high discharge temperatures.
Question 2: What is the purpose of an equalizer line on a thermostatic expansion valve (TXV)?
- Balance refrigerant flow between parallel evaporators
- Sense evaporator outlet pressure to compensate for coil pressure drop (Correct answer)
- Supply refrigerant to the valve bulb
- Connect the valve to the receiver
Correct answer: Sense evaporator outlet pressure to compensate for coil pressure drop
An external equalizer line senses actual evaporator outlet pressure, allowing the TXV to accurately control superheat even when there is significant pressure drop across the coil.
Question 3: Which refrigerant property determines the saturation temperature at a given pressure?
- Latent heat of vaporization
- Pressure-temperature relationship unique to each refrigerant (Correct answer)
- Specific volume of the vapor
- Global Warming Potential (GWP)
Correct answer: Pressure-temperature relationship unique to each refrigerant
Each refrigerant has a unique pressure-temperature (P-T) relationship; at any given pressure, the saturation temperature is fixed and can be read from a P-T chart.
Question 4: A technician finds oil logged in an evaporator coil. What is the most likely contributing cause?
- Too much subcooling in the liquid line
- Insufficient refrigerant velocity to carry oil through the coil (Correct answer)
- Overcharged system with excess refrigerant
- Faulty TXV bulb losing charge
Correct answer: Insufficient refrigerant velocity to carry oil through the coil
When refrigerant velocity in the evaporator is too low, oil cannot be carried along and accumulates (logs) in the coil, reducing heat transfer.
Question 5: What is the function of a head pressure control valve on an air-cooled condenser in cold ambient conditions?
- Prevent refrigerant from migrating to the condenser during off cycles
- Maintain minimum head pressure by flooding condenser coil with liquid refrigerant (Correct answer)
- Reduce compressor speed when ambient drops below setpoint
- Open bypass around the condenser when pressure is too high
Correct answer: Maintain minimum head pressure by flooding condenser coil with liquid refrigerant
Head pressure control valves flood condenser coils with liquid refrigerant in cold weather, reducing effective condensing area and maintaining minimum operating pressure.
Question 6: Which factor most directly determines the compression ratio in a refrigeration system?
- Refrigerant type and molecular weight
- Ratio of absolute discharge pressure to absolute suction pressure (Correct answer)
- Compressor displacement volume
- Condenser surface area
Correct answer: Ratio of absolute discharge pressure to absolute suction pressure
Compression ratio is calculated by dividing absolute discharge pressure by absolute suction pressure, and directly affects compressor efficiency and discharge temperature.
Question 7: A refrigeration technician discovers a system running with extremely high compression ratio. What is the most likely consequence?
- Improved coefficient of performance (COP)
- Excessive discharge temperature and reduced volumetric efficiency (Correct answer)
- Lower suction superheat
- Reduced condenser load
Correct answer: Excessive discharge temperature and reduced volumetric efficiency
High compression ratios cause discharge temperatures to rise sharply and reduce volumetric efficiency because re-expanded gas from clearance volume occupies more cylinder space.
CRST Refrigeration Systems & Components 5
A refrigeration system has excessively high discharge temperature.
Which component failure could cause this if suction pressure is also low?