HVAC Thermodynamics 2 — Questions and Answers
Question 1: What term describes the heat absorbed by a refrigerant as it changes from liquid to vapor at constant temperature?
- Sensible heat
- Latent heat of vaporization (Correct answer)
- Specific heat
- Superheat
Correct answer: Latent heat of vaporization
Latent heat of vaporization is the energy absorbed during the phase change from liquid to vapor without a temperature change.
Question 2: Which thermodynamic process occurs when a gas expands without any heat transfer to or from the surroundings?
- Isothermal
- Isobaric
- Adiabatic (Correct answer)
- Isochoric
Correct answer: Adiabatic
An adiabatic process involves no heat exchange with surroundings, so temperature changes are driven entirely by work.
Question 3: In a standard vapor-compression refrigeration cycle, where does the refrigerant absorb heat from the conditioned space?
- Condenser
- Compressor
- Expansion valve
- Evaporator (Correct answer)
Correct answer: Evaporator
The evaporator coil absorbs heat from the indoor air, causing the refrigerant to evaporate and cool the space.
Question 4: What is the Coefficient of Performance (COP) of a refrigeration system if it removes 12,000 BTU/hr using 2,000 BTU/hr of work input?
- 2.0
- 6.0 (Correct answer)
- 0.5
- 4.0
Correct answer: 6.0
COP = heat removed / work input = 12,000 / 2,000 = 6.0.
Question 5: Which law of thermodynamics states that heat will not spontaneously flow from a cold body to a hot body?
- Zeroth Law
- First Law
- Second Law (Correct answer)
- Third Law
Correct answer: Second Law
The Second Law of Thermodynamics establishes the natural direction of heat flow and the concept of entropy.
Question 6: Subcooling in a refrigeration system refers to cooling the refrigerant below its:
- Evaporating temperature
- Condensing temperature (Correct answer)
- Critical temperature
- Dew point temperature
Correct answer: Condensing temperature
Subcooling occurs when liquid refrigerant is cooled below the saturation (condensing) temperature, increasing system efficiency.
Question 7: What happens to the boiling point of a refrigerant when pressure is increased?
- It decreases
- It stays the same
- It increases (Correct answer)
- It becomes unpredictable
Correct answer: It increases
Higher pressure raises the saturation temperature (boiling point) of a refrigerant, a fundamental pressure-temperature relationship.
HVAC Thermodynamics 2
What term describes the heat absorbed by a refrigerant as it changes from liquid to vapor at constant temperature?