RSES Refrigeration Fundamentals & System Components 5 — Questions and Answers
Question 1: The refrigerating effect of a system is defined as:
- Total heat rejected at the condenser per pound of refrigerant
- Heat absorbed per pound of refrigerant in the evaporator (Correct answer)
- Work done by the compressor per pound of refrigerant
- Enthalpy change across the expansion device
Correct answer: Heat absorbed per pound of refrigerant in the evaporator
Refrigerating effect (RE) = h_evaporator_out - h_evaporator_in, representing the heat absorbed by the refrigerant per unit mass in the evaporator.
Question 2: Which component in a refrigeration system is responsible for changing high-pressure liquid refrigerant to a low-pressure liquid/vapor mixture?
- Compressor
- Condenser
- Metering device (expansion valve or orifice) (Correct answer)
- Accumulator
Correct answer: Metering device (expansion valve or orifice)
The metering device (TXV, fixed orifice, or capillary tube) creates a pressure drop that flashes some liquid to vapor, lowering refrigerant temperature.
Question 3: When checking system superheat at the evaporator outlet, a technician finds it is much higher than the TXV setpoint. The most likely cause is:
- Refrigerant overcharge
- TXV restricted or underfeeding the evaporator (Correct answer)
- Condenser coil fouled
- Excessive subcooling in the liquid line
Correct answer: TXV restricted or underfeeding the evaporator
A restricted or underfeeding TXV allows less refrigerant into the evaporator, causing the refrigerant to become highly superheated before leaving the coil.
Question 4: In a two-stage refrigeration system, the economizer (intercooler) is used to:
- Subcool the discharge gas from the high-stage compressor
- Cool the discharge gas from the low-stage compressor and subcool liquid before the low-side expansion device (Correct answer)
- Separate oil between the two compression stages
- Heat the suction gas entering the low-stage compressor
Correct answer: Cool the discharge gas from the low-stage compressor and subcool liquid before the low-side expansion device
The intercooler cools the low-stage discharge gas (reducing high-stage work) and subcools liquid refrigerant, improving overall system efficiency.
Question 5: The purpose of a liquid line solenoid valve in a refrigeration system is to:
- Regulate refrigerant flow rate continuously
- Stop refrigerant flow to the evaporator when the system is off or on demand (Correct answer)
- Separate oil from liquid refrigerant
- Control discharge pressure
Correct answer: Stop refrigerant flow to the evaporator when the system is off or on demand
A liquid line solenoid valve is an electrically operated on/off valve that stops refrigerant flow to prevent evaporator flooding or pump-down the system.
Question 6: Which refrigerant blend is classified as 'zeotropic' and exhibits temperature glide during phase change?
- R-134a
- R-410A
- R-407C (Correct answer)
- R-744
Correct answer: R-407C
R-407C is a zeotropic HFC blend whose components boil at different temperatures, resulting in a temperature glide (change in temperature during evaporation or condensation).
Question 7: A high-pressure cut-out (HPCO) safety control in a refrigeration system is set to:
- Shut down the compressor if suction pressure falls below a minimum value
- Shut down the compressor if discharge pressure exceeds a safe maximum (Correct answer)
- Open the liquid line solenoid at high condensing pressure
- Start a condenser fan when head pressure rises
Correct answer: Shut down the compressor if discharge pressure exceeds a safe maximum
The HPCO stops the compressor when discharge pressure exceeds its setpoint to protect the system from dangerously high pressures.
RSES Refrigeration Fundamentals & System Components 5
The refrigerating effect of a system is defined as: