CARO Refrigeration Controls and Instrumentation 2 — Questions and Answers
Question 1: What is superheat in a refrigeration system, and where is it measured?
- The temperature difference between condensing and ambient, measured at the condenser
- The temperature of refrigerant vapor above its saturation temperature, measured at the evaporator outlet (Correct answer)
- The heat added by the compressor, measured at discharge
- The subcooling temperature, measured at the liquid line
Correct answer: The temperature of refrigerant vapor above its saturation temperature, measured at the evaporator outlet
Superheat is the degrees of temperature a refrigerant vapor has above its saturation point, and it is measured at the evaporator outlet to ensure no liquid enters the compressor.
Question 2: A low-pressure controller on an ammonia system is set to cut out at a specific suction pressure. What does a cut-in pressure setting represent?
- The pressure at which the compressor shuts down
- The pressure at which the compressor restarts after a low-pressure shutdown (Correct answer)
- The discharge pressure limit
- The oil pressure safety setpoint
Correct answer: The pressure at which the compressor restarts after a low-pressure shutdown
The cut-in pressure is the suction pressure at which the low-pressure controller closes and allows the compressor to restart after conditions have returned to acceptable levels.
Question 3: What is subcooling and why is it beneficial in a refrigeration system?
- Cooling vapor below dew point to improve suction line efficiency
- Cooling liquid refrigerant below its saturation temperature to increase system capacity and prevent flash gas (Correct answer)
- Lowering discharge temperature to reduce compressor wear
- Reducing suction superheat for better heat transfer
Correct answer: Cooling liquid refrigerant below its saturation temperature to increase system capacity and prevent flash gas
Subcooling cools liquid refrigerant below its condensing temperature, increasing the system's net refrigerating effect and preventing flash gas in the liquid line.
Question 4: Which control device is typically used to maintain a constant evaporating temperature by regulating suction pressure?
- Thermostatic expansion valve
- Evaporator pressure regulator (EPR) (Correct answer)
- Discharge check valve
- Head pressure controller
Correct answer: Evaporator pressure regulator (EPR)
An evaporator pressure regulator (EPR) maintains a minimum evaporating pressure, which controls the evaporating temperature in multi-temperature or product-protection applications.
Question 5: What is the purpose of a head pressure control valve in cold weather operation?
- To reduce suction pressure during low ambient conditions
- To maintain sufficient condensing pressure so the expansion device operates correctly during cold weather (Correct answer)
- To increase compressor discharge temperature
- To bypass refrigerant around the evaporator
Correct answer: To maintain sufficient condensing pressure so the expansion device operates correctly during cold weather
Head pressure control valves ensure condensing pressure does not drop too low in cold ambient conditions, which would starve the expansion valve of adequate liquid pressure.
Question 6: An ammonia system's suction pressure gauge reads 0 psig. What approximate evaporating temperature does this correspond to for ammonia (R-717)?
- -28°F (Correct answer)
- +32°F
- -28°C
- 0°F
Correct answer: -28°F
For ammonia (R-717), 0 psig (atmospheric pressure) corresponds to a saturation temperature of approximately -28°F, which is its normal boiling point at atmospheric pressure.
CARO Refrigeration Controls and Instrumentation 2
What is superheat in a refrigeration system, and where is it measured?