313A Refrigeration Fundamentals & Thermodynamics 2 — Questions and Answers
Question 1: On a pressure-enthalpy (P-h) diagram, what does the area under the saturation dome represent?
- The superheated vapour region
- The two-phase (liquid-vapour) mixture region (Correct answer)
- The subcooled liquid region
- The critical point only
Correct answer: The two-phase (liquid-vapour) mixture region
The area under the saturation dome on a P-h diagram represents the two-phase region where both liquid and vapour coexist at saturation conditions.
Question 2: What is the coefficient of performance (COP) of a refrigeration system?
- The ratio of heat rejected at the condenser to work input
- The ratio of refrigeration effect to the work input to the compressor (Correct answer)
- The ratio of compressor discharge pressure to suction pressure
- The ratio of evaporator temperature to condenser temperature in Kelvin
Correct answer: The ratio of refrigeration effect to the work input to the compressor
COP is defined as the useful refrigeration effect (heat absorbed at the evaporator) divided by the compressor work input, indicating system efficiency.
Question 3: What happens to the boiling point of a refrigerant as pressure increases?
- It decreases
- It remains constant
- It increases (Correct answer)
- It becomes unpredictable
Correct answer: It increases
As pressure increases, the boiling (saturation) temperature of a refrigerant also increases, which is why high-side pressure corresponds to higher condensing temperatures.
Question 4: In a standard vapour-compression cycle, which process occurs in the compressor?
- Isobaric heat rejection
- Isothermal expansion
- Isentropic compression of refrigerant vapour (Correct answer)
- Throttling of liquid refrigerant
Correct answer: Isentropic compression of refrigerant vapour
The ideal compressor process is isentropic (adiabatic and reversible) compression, raising both the pressure and temperature of the refrigerant vapour.
Question 5: What is sensible heat?
- Heat that cannot be detected by instruments
- Heat that changes the temperature of a substance without changing its phase (Correct answer)
- Heat stored in the refrigerant's pressure
- Heat transferred only by radiation
Correct answer: Heat that changes the temperature of a substance without changing its phase
Sensible heat is heat energy that causes a measurable temperature change in a substance without changing its physical state.
Question 6: What is the purpose of calculating the refrigeration effect in system design?
- To size the electrical disconnect
- To determine how much heat the evaporator can absorb per kilogram of refrigerant (Correct answer)
- To calculate pipe wall thickness
- To determine the colour code for refrigerant cylinders
Correct answer: To determine how much heat the evaporator can absorb per kilogram of refrigerant
The refrigeration effect (kJ/kg) tells the designer how much heat each kilogram of refrigerant absorbs as it passes through the evaporator, which is used to size the system for the required cooling load.
313A Refrigeration Fundamentals & Thermodynamics 2
On a pressure-enthalpy (P-h) diagram, what does the area under the saturation dome represent?