313A Refrigeration Cycle — Questions and Answers
Question 1: What are the four main components of a basic vapour-compression refrigeration cycle?
- Compressor, condenser, expansion valve, evaporator (Correct answer)
- Compressor, boiler, turbine, condenser
- Pump, heater, cooler, filter
- Fan, coil, thermostat, ductwork
Correct answer: Compressor, condenser, expansion valve, evaporator
The four essential components of a vapour-compression refrigeration cycle are: compressor (raises pressure and temperature), condenser (rejects heat), expansion device/metering device (reduces pressure), and evaporator (absorbs heat). Refrigerant flows through these components in a continuous cycle.
Question 2: In the refrigeration cycle, what state change does the refrigerant undergo in the evaporator?
- Gas to liquid (condensation)
- Liquid to gas (evaporation/boiling) (Correct answer)
- Solid to liquid (melting)
- Gas to solid (deposition)
Correct answer: Liquid to gas (evaporation/boiling)
In the evaporator, low-pressure liquid refrigerant absorbs heat from the space being cooled and changes state from liquid to gas (evaporation). This phase change absorbs a large amount of latent heat, which is what produces the cooling effect.
Question 3: What is superheat in the refrigeration cycle and why is it important?
- The temperature above the boiling point of the refrigerant vapour leaving the evaporator; it ensures no liquid enters the compressor (Correct answer)
- The maximum temperature the system can reach
- The temperature difference between indoor and outdoor air
- The heat generated by the compressor motor
Correct answer: The temperature above the boiling point of the refrigerant vapour leaving the evaporator; it ensures no liquid enters the compressor
Superheat is the temperature of the refrigerant vapour above its saturation (boiling) temperature at a given pressure. Measuring superheat at the evaporator outlet ensures all liquid has evaporated and only vapour enters the compressor, preventing liquid slugging that can damage the compressor.
Question 4: What is subcooling in the refrigeration cycle?
- Cooling the evaporator below 0°C
- The temperature of the liquid refrigerant below its condensing (saturation) temperature at condenser pressure (Correct answer)
- The temperature drop across the compressor
- Reducing the refrigerant charge below recommended levels
Correct answer: The temperature of the liquid refrigerant below its condensing (saturation) temperature at condenser pressure
Subcooling is the temperature difference between the actual liquid refrigerant temperature and its saturation temperature at condenser pressure. Adequate subcooling (typically 5°C to 10°C) ensures solid liquid reaches the metering device, improving system efficiency and indicating proper refrigerant charge.
Question 5: What is the relationship between pressure and boiling point of a refrigerant?
- They are unrelated
- As pressure increases, the boiling point increases; as pressure decreases, the boiling point decreases (Correct answer)
- As pressure increases, the boiling point decreases
- The boiling point is always constant regardless of pressure
Correct answer: As pressure increases, the boiling point increases; as pressure decreases, the boiling point decreases
The boiling point of a refrigerant is directly related to its pressure. This principle is fundamental to refrigeration: the low-pressure side allows the refrigerant to boil at a low temperature (absorbing heat), while the high-pressure side forces it to condense at a higher temperature (rejecting heat).
Question 6: What is the coefficient of performance (COP) in a refrigeration system?
- The speed at which the compressor operates
- The ratio of cooling capacity (heat removed) to the energy input (work done by compressor) (Correct answer)
- The number of refrigeration cycles per hour
- The percentage of refrigerant that evaporates
Correct answer: The ratio of cooling capacity (heat removed) to the energy input (work done by compressor)
COP is the ratio of useful cooling output to the work input required to achieve it. A COP of 3 means the system removes 3 units of heat for every 1 unit of electrical energy consumed. Higher COP indicates greater efficiency. COP varies with operating conditions, particularly evaporator and condenser temperatures.
313A Refrigeration Cycle
What are the four main components of a basic vapour-compression refrigeration cycle?