313A Compressors & Condensers 2 — Questions and Answers
Question 1: What is the function of the condenser in a refrigeration system?
- To expand the refrigerant from high to low pressure
- To reject heat from the refrigerant to the ambient medium, condensing the refrigerant from vapour to liquid (Correct answer)
- To separate oil from refrigerant vapour
- To filter moisture and debris from the refrigerant
Correct answer: To reject heat from the refrigerant to the ambient medium, condensing the refrigerant from vapour to liquid
The condenser rejects the heat absorbed in the evaporator plus the heat of compression to the outdoor air or cooling water, condensing the refrigerant vapour back into a liquid.
Question 2: What is the approach temperature of an air-cooled condenser?
- The difference between entering air temperature and leaving air temperature
- The difference between condensing temperature and entering air temperature (Correct answer)
- The difference between discharge temperature and condensing temperature
- The temperature of the air at the condenser fan inlet
Correct answer: The difference between condensing temperature and entering air temperature
Approach temperature is the difference between the condensing temperature (saturation temperature at condensing pressure) and the entering air dry-bulb temperature, and a low approach temperature indicates good heat transfer.
Question 3: What effect does dirty condenser coils have on a refrigeration system?
- Lower discharge pressure and lower condenser temperature
- Higher discharge pressure, higher condensing temperature, and reduced system efficiency (Correct answer)
- Reduced suction pressure and higher superheat
- Increased refrigerant flow rate through the expansion device
Correct answer: Higher discharge pressure, higher condensing temperature, and reduced system efficiency
Dirty condenser coils reduce airflow and heat transfer, causing the condensing temperature and discharge pressure to rise, which increases compressor work and reduces system efficiency and capacity.
Question 4: What type of condenser uses water as the heat rejection medium and is commonly used in large commercial systems?
- Air-cooled condenser
- Evaporative condenser
- Water-cooled condenser (shell-and-tube or plate heat exchanger) (Correct answer)
- Desuperheating condenser
Correct answer: Water-cooled condenser (shell-and-tube or plate heat exchanger)
Water-cooled condensers, such as shell-and-tube or brazed plate heat exchangers, use water from a cooling tower or municipal supply to reject heat from the refrigerant, offering high efficiency in large commercial applications.
Question 5: What is the purpose of non-condensable gas purging in a refrigeration system?
- To remove excess refrigerant charge from the high side
- To eliminate air or nitrogen trapped in the system that raises condensing pressure and reduces efficiency (Correct answer)
- To blow out debris from the condenser coil fins
- To purge oil accumulation from the condenser
Correct answer: To eliminate air or nitrogen trapped in the system that raises condensing pressure and reduces efficiency
Non-condensable gases such as air or nitrogen do not condense and accumulate at the top of the condenser, artificially raising discharge pressure and reducing heat transfer efficiency.
Question 6: What is the 'head pressure control' strategy used on air-cooled condensers in cold Canadian winters?
- Reducing refrigerant charge to lower head pressure
- Cycling condenser fans off or modulating fan speed to maintain minimum condensing pressure during cold weather (Correct answer)
- Opening the bypass valve to route refrigerant around the condenser
- Increasing compressor speed to compensate for low ambient temperature
Correct answer: Cycling condenser fans off or modulating fan speed to maintain minimum condensing pressure during cold weather
In cold weather, air-cooled condensers can overcool the refrigerant and reduce condensing pressure below the minimum needed for proper expansion device operation, so fan cycling or variable-speed fans maintain minimum head pressure.
313A Compressors & Condensers 2
What is the function of the condenser in a refrigeration system?