ATP Cooling Cycle & Heat Transfer Principles 2 — Questions and Answers
Question 1: In a vapor-compression refrigeration cycle, what happens to refrigerant temperature during the throttling process through an expansion valve?
- Temperature increases due to pressure drop
- Temperature decreases as enthalpy remains nearly constant (Correct answer)
- Temperature remains unchanged while pressure drops
- Temperature increases due to flash evaporation
Correct answer: Temperature decreases as enthalpy remains nearly constant
The expansion valve is an isenthalpic device, so enthalpy stays approximately constant while pressure and temperature both drop.
Question 2: Which heat transfer mode is primarily responsible for heat exchange inside a fin-and-tube evaporator coil?
- Radiation from refrigerant to tube wall
- Forced convection on the air side and conduction through the tube wall (Correct answer)
- Natural convection only
- Conduction through stagnant air gaps
Correct answer: Forced convection on the air side and conduction through the tube wall
Forced convection moves air over the fins while conduction transfers heat through the tube wall to the refrigerant.
Question 3: What does the coefficient of performance (COP) of a refrigeration system represent?
- The ratio of compressor work to heat rejected at condenser
- The ratio of cooling effect to the net work input (Correct answer)
- The ratio of condenser heat to evaporator heat
- The ratio of mass flow rate to compressor power
Correct answer: The ratio of cooling effect to the net work input
COP = Q_evaporator / W_compressor, expressing how much useful cooling is obtained per unit of work consumed.
Question 4: A technician notices that the condenser outlet temperature is 10°F above the ambient temperature. This condition is known as:
- Subcooling
- Superheat
- Flash gas
- Approach temperature (Correct answer)
Correct answer: Approach temperature
Approach temperature is the difference between condenser leaving liquid temperature and the entering ambient (or cooling medium) temperature.
Question 5: Which law governs the rate of conductive heat transfer through a flat wall?
- Newton's Law of Cooling
- Stefan-Boltzmann Law
- Fourier's Law of Heat Conduction (Correct answer)
- Kirchhoff's Law
Correct answer: Fourier's Law of Heat Conduction
Fourier's Law states Q = -kA(dT/dx), relating heat flux to thermal conductivity, area, and temperature gradient.
Question 6: In a refrigeration system, flash gas in the liquid line before the expansion valve causes:
- Increased cooling capacity due to pre-evaporation
- Reduced capacity and possible expansion valve hunting (Correct answer)
- Lower compressor discharge temperatures
- Improved subcooling at the evaporator inlet
Correct answer: Reduced capacity and possible expansion valve hunting
Flash gas reduces the density of refrigerant entering the expansion valve, starving the evaporator and causing erratic valve operation.
Question 7: The latent heat of vaporization of a refrigerant is best described as:
- Heat added to raise refrigerant temperature by 1°F
- Heat required to change liquid refrigerant to vapor at constant pressure (Correct answer)
- Heat rejected during compression
- Sensible heat stored in the refrigerant vapor
Correct answer: Heat required to change liquid refrigerant to vapor at constant pressure
Latent heat of vaporization is absorbed during the phase change from liquid to vapor at constant temperature and pressure.
ATP Cooling Cycle & Heat Transfer Principles 2
In a vapor-compression refrigeration cycle, what happens to refrigerant temperature during the throttling process through an expansion valve?