CRST CRST Refrigerants & Thermodynamics 2 — Questions and Answers
Question 1: What is 'superheat' in a refrigeration system?
- The temperature of suction vapor above the saturation temperature at suction pressure (Correct answer)
- The temperature difference between discharge gas and ambient air
- The excess heat removed by the condenser beyond condensation
- The temperature rise of refrigerant through the compressor
Correct answer: The temperature of suction vapor above the saturation temperature at suction pressure
Superheat is the number of degrees that suction vapor exceeds the saturation temperature at the measured suction pressure, ensuring only dry vapor enters the compressor.
Question 2: Per EPA Section 608, what is the maximum allowable leak rate for refrigeration equipment with a charge of 50 lbs or more used in commercial processes before repair is required?
- 30% per year (Correct answer)
- 10% per year
- 50% per year
- 5% per year
Correct answer: 30% per year
EPA Section 608 requires owners of commercial refrigeration equipment with charges of 50 lbs or more to repair leaks when the annual leak rate exceeds 30%.
Question 3: When recharging a system with an HFC blend refrigerant (such as R-410A), how must the refrigerant be added from the cylinder?
- As a liquid (cylinder inverted or using a liquid valve) (Correct answer)
- As a vapor only through the suction service valve
- As a vapor only through the discharge service valve
- It can be added as either liquid or vapor interchangeably
Correct answer: As a liquid (cylinder inverted or using a liquid valve)
HFC blends like R-410A must be charged as liquid to maintain the correct blend ratio, as vapor-charging can cause fractionation and alter the refrigerant composition.
Question 4: What happens to the pressure-temperature relationship of a refrigerant mixture that 'glides'?
- The temperature changes during phase change at constant pressure (Correct answer)
- The pressure and temperature remain fixed during phase change
- The mixture separates into its components at low temperatures
- The boiling point increases as pressure decreases
Correct answer: The temperature changes during phase change at constant pressure
Zeotropic refrigerant blends exhibit temperature glide, meaning the evaporation or condensation temperature shifts (slides) across a range during phase change at constant pressure.
Question 5: What is the primary purpose of a refrigerant receiver in a commercial refrigeration system?
- Store liquid refrigerant to accommodate system demand changes and ensure liquid feed to the expansion device (Correct answer)
- Separate oil from refrigerant on the suction side
- Remove moisture and contaminants from the refrigerant
- Reduce refrigerant velocity before entering the condenser
Correct answer: Store liquid refrigerant to accommodate system demand changes and ensure liquid feed to the expansion device
A liquid receiver stores excess refrigerant charge, provides a stable liquid supply to the expansion device, and accommodates load variations without flooding or starving the system.
Question 6: Which pressure-enthalpy diagram characteristic indicates a refrigeration system is operating with excessive head pressure?
- The condensing process line moves to the right (higher enthalpy/temperature) at elevated pressure (Correct answer)
- The evaporator line moves upward on the pressure axis
- The compression line becomes horizontal
- The expansion process crosses the saturation curve
Correct answer: The condensing process line moves to the right (higher enthalpy/temperature) at elevated pressure
High head pressure shifts the condensing portion of the P-H diagram to the right and upward, indicating the refrigerant must be cooled to a higher temperature before condensing, which reduces system efficiency.
CRST CRST Refrigerants & Thermodynamics 2
What is 'superheat' in a refrigeration system?