313A Refrigeration Cycle 2 — Questions and Answers
Question 1: What happens to the refrigerant in the condenser?
- It absorbs heat and evaporates
- It rejects heat and changes from a high-pressure superheated gas to a high-pressure liquid (Correct answer)
- It expands and drops in pressure
- It is compressed to a higher pressure
Correct answer: It rejects heat and changes from a high-pressure superheated gas to a high-pressure liquid
In the condenser, high-pressure superheated refrigerant gas from the compressor rejects heat to the surrounding medium (air or water). As it loses heat, it desuperheats, condenses from gas to liquid, and may subcool further. This heat rejection is what removes the heat absorbed by the evaporator plus the heat of compression.
Question 2: What is the purpose of the compressor in the refrigeration cycle?
- To cool the refrigerant
- To raise the pressure and temperature of the refrigerant vapour so it can reject heat in the condenser (Correct answer)
- To reduce the pressure of the refrigerant
- To filter impurities from the refrigerant
Correct answer: To raise the pressure and temperature of the refrigerant vapour so it can reject heat in the condenser
The compressor raises the pressure and temperature of the refrigerant vapour from the evaporator. By increasing the pressure, the condensing temperature rises above the ambient temperature, allowing the condenser to reject heat to the surroundings. The compressor is the component that drives the entire cycle.
Question 3: What is a pressure-enthalpy (P-h) diagram used for in HVAC?
- To measure duct pressure losses
- To graphically represent the refrigeration cycle, showing the state of the refrigerant at each point and the energy involved in each process (Correct answer)
- To calculate building heat loads
- To size electrical circuits for equipment
Correct answer: To graphically represent the refrigeration cycle, showing the state of the refrigerant at each point and the energy involved in each process
A pressure-enthalpy (P-h) diagram plots refrigerant pressure on the vertical axis and enthalpy (heat content) on the horizontal axis. It shows the saturation curve, the state of the refrigerant at each point in the cycle, and allows calculation of system capacity, COP, and heat transfer at each component.
Question 4: What is the saturation temperature of a refrigerant?
- The maximum operating temperature of the system
- The temperature at which the refrigerant changes state (boils or condenses) at a given pressure (Correct answer)
- The temperature at which the oil separates from the refrigerant
- The ambient air temperature at the outdoor unit
Correct answer: The temperature at which the refrigerant changes state (boils or condenses) at a given pressure
Saturation temperature is the temperature at which a refrigerant changes between liquid and vapour states at a specific pressure. At saturation, liquid and vapour coexist. Every refrigerant has a unique pressure-temperature relationship that technicians use for system diagnosis.
Question 5: In a heat pump system, how does the cycle differ from a standard air conditioning cycle?
- Heat pumps use a different refrigerant
- A reversing valve changes the direction of refrigerant flow, allowing the system to heat or cool by swapping the functions of the indoor and outdoor coils (Correct answer)
- Heat pumps operate at higher pressures
- There is no difference between them
Correct answer: A reversing valve changes the direction of refrigerant flow, allowing the system to heat or cool by swapping the functions of the indoor and outdoor coils
A heat pump uses a reversing valve (four-way valve) to reverse the direction of refrigerant flow. In cooling mode, the outdoor coil is the condenser and the indoor coil is the evaporator. In heating mode, the functions are reversed — the outdoor coil absorbs heat (evaporator) and the indoor coil rejects heat (condenser).
Question 6: What is latent heat and why is it significant in the refrigeration cycle?
- The heat measured by a thermometer
- The heat energy absorbed or released during a phase change (liquid to gas or gas to liquid) without a temperature change (Correct answer)
- The heat generated by friction in the compressor
- The heat stored in the building walls
Correct answer: The heat energy absorbed or released during a phase change (liquid to gas or gas to liquid) without a temperature change
Latent heat is the energy absorbed or released when a substance changes phase without changing temperature. In refrigeration, the large amount of latent heat absorbed during evaporation provides the cooling effect, and the latent heat released during condensation is how the system rejects heat. This is far more effective than sensible heat transfer alone.
313A Refrigeration Cycle 2
What happens to the refrigerant in the condenser?