HVAC Thermodynamics 5 — Questions and Answers
Question 1: What happens to entropy in a truly reversible process?
- It increases
- It decreases
- It remains constant (Correct answer)
- It becomes undefined
Correct answer: It remains constant
In a perfectly reversible (ideal) process, total entropy change is zero; real processes always increase entropy.
Question 2: The heat rejection rate of a condenser equals the:
- Evaporator heat absorption only
- Compressor work only
- Sum of evaporator heat absorption plus compressor work input (Correct answer)
- Difference of evaporator heat minus compressor work
Correct answer: Sum of evaporator heat absorption plus compressor work input
By energy conservation (First Law), the condenser must reject the heat absorbed in the evaporator plus the work added by the compressor.
Question 3: What is the dew point temperature in the context of refrigerant pressure-temperature relationships?
- Temperature at which refrigerant starts to condense from 100% vapor (Correct answer)
- Temperature at which refrigerant starts to boil from 100% liquid
- The critical temperature of the refrigerant
- The temperature at full subcooling
Correct answer: Temperature at which refrigerant starts to condense from 100% vapor
The dew point is the saturation temperature at which the last bit of vapor begins to condense when cooling a refrigerant mixture.
Question 4: Why is it important to avoid liquid refrigerant entering the compressor suction?
- Liquid is too cold for the compressor bearings
- Liquid is incompressible and can cause mechanical damage (slugging) (Correct answer)
- Liquid reduces the compression ratio
- Liquid increases superheat readings
Correct answer: Liquid is incompressible and can cause mechanical damage (slugging)
Liquid refrigerant cannot be compressed and can cause catastrophic compressor damage known as liquid slugging.
Question 5: In a heat pump operating in heating mode, which component acts as the heat absorber from outdoor air?
- Indoor coil (condenser in cooling mode)
- Reversing valve
- Outdoor coil (acting as evaporator) (Correct answer)
- Accumulator
Correct answer: Outdoor coil (acting as evaporator)
In heating mode, the outdoor coil absorbs heat from outdoor air (even cold air) acting as an evaporator to deliver heat indoors.
Question 6: What does a higher EER (Energy Efficiency Ratio) value indicate about an air conditioning system?
- Higher cooling capacity regardless of efficiency
- Greater energy consumption per BTU
- More cooling output per watt of electricity consumed (Correct answer)
- Lower SEER rating
Correct answer: More cooling output per watt of electricity consumed
EER = BTU/hr of cooling divided by watts of input; a higher EER means more efficient cooling at a single operating condition.
Question 7: Which thermodynamic cycle forms the theoretical basis for the vapor-compression refrigeration system used in most HVAC equipment?
- Carnot cycle
- Rankine cycle
- Reversed Rankine (vapor-compression) cycle (Correct answer)
- Brayton cycle
Correct answer: Reversed Rankine (vapor-compression) cycle
The reversed Rankine cycle, using a working fluid that changes phase, is the thermodynamic model for vapor-compression refrigeration.
HVAC Thermodynamics 5
What happens to entropy in a truly reversible process?