Free CARO Evaporators and Condensers Questions and Answers — Questions and Answers
Question 1: An operator observes a steady increase in the condenser approach temperature. Which of the following is the MOST likely cause?
- Fouling or scale buildup on the condenser's heat transfer surfaces. (Correct answer)
- A significant drop in the ambient wet-bulb temperature.
- Excessive subcooling of the liquid refrigerant leaving the condenser.
- The system has a low refrigerant charge.
Correct answer: Fouling or scale buildup on the condenser's heat transfer surfaces.
The condenser approach temperature is the difference between the refrigerant's condensing temperature and the temperature of the cooling medium (water or air) leaving the condenser. Fouling, such as mineral scale or algae, acts as an insulator on the heat transfer surfaces, reducing the condenser's efficiency. To reject the same amount of heat, the condensing temperature must rise, which increases the approach temperature. [11, 29]
Question 2: In an industrial ammonia refrigeration system, what is the primary purpose of an automatic purger connected to the high side of the system?
- To remove refrigerant that has broken down over time.
- To drain excess lubricating oil from the condenser.
- To remove non-condensable gases like air and hydrogen. (Correct answer)
- To vent off high-pressure liquid refrigerant during an over-pressure event.
Correct answer: To remove non-condensable gases like air and hydrogen.
Non-condensable gases, such as air that may have entered the system during maintenance or hydrogen from the breakdown of ammonia, accumulate in the condenser. These gases do not condense at the same temperature and pressure as the refrigerant, which increases the overall head pressure, reduces system efficiency, and increases energy consumption. An automatic purger is designed to safely and efficiently remove these non-condensable gases. [3, 15, 18]
Question 3: During a hot gas defrost cycle for a freezer evaporator, which of the following actions occurs?
- The compressor shuts down and liquid refrigerant is drained from the evaporator.
- The evaporator fans continue to run to circulate warm air from the freezer space.
- Cold liquid refrigerant is circulated through the coil to melt the frost.
- Hot, high-pressure vapor from the compressor discharge is diverted into the evaporator. (Correct answer)
Correct answer: Hot, high-pressure vapor from the compressor discharge is diverted into the evaporator.
In a hot gas defrost system, hot discharge gas from the compressor is redirected from the condenser to the evaporator inlet. This hot gas flows through the evaporator coil, warming it from the inside out and melting any accumulated frost or ice. The evaporator essentially functions as a condenser temporarily during this cycle. [23, 27]
Question 4: A flooded-type shell-and-tube evaporator is being used to chill brine. Where are the refrigerant and the brine typically located within this component?
- The refrigerant is in the tubes, and the brine is in the shell.
- Both the refrigerant and the brine are mixed together in the shell.
- The brine is in the tubes, and the refrigerant is in the shell. (Correct answer)
- The refrigerant and brine flow through alternating plates.
Correct answer: The brine is in the tubes, and the refrigerant is in the shell.
In a typical flooded shell-and-tube evaporator used for liquid chilling, the refrigerant fills the shell and boils on the outside surface of the tubes. The liquid being cooled (the brine) flows through the inside of the tubes, transferring its heat to the boiling refrigerant. [37]
Question 5: Which of the following conditions would most likely cause an evaporator to become 'starved' of refrigerant?
- An overcharged system.
- A dirty or clogged filter-drier upstream of the expansion device. (Correct answer)
- The superheat setting on the expansion valve is set too low.
- The evaporator fans are shut off.
Correct answer: A dirty or clogged filter-drier upstream of the expansion device.
A starved evaporator is not receiving enough liquid refrigerant to utilize its entire heat transfer surface effectively, resulting in low suction pressure and high superheat. A restriction, such as a clogged filter-drier, will cause a significant pressure drop before the expansion valve, reducing the flow of liquid refrigerant into the evaporator and causing it to be starved. [19, 26]
Question 6: An operator at a plant with an evaporative condenser notices that the discharge (head) pressure is significantly higher than normal on a mild day. The water spray nozzles are working correctly and the sump water level is normal. What is the MOST appropriate next check?
- Check for a low refrigerant charge in the system.
- Verify that the condenser fans are operating correctly and airflow is not obstructed. (Correct answer)
- Check the superheat at the evaporator outlet.
- Ensure the compressor's oil level is correct.
Correct answer: Verify that the condenser fans are operating correctly and airflow is not obstructed.
An evaporative condenser uses both air and the evaporation of water to reject heat. If the water side appears to be functioning, the next logical step is to check the air side. If the fans are not running, running in reverse, or if airflow is blocked by debris (like leaves or dirt), the condenser's ability to reject heat will be severely limited, causing a rise in discharge pressure. [7, 8, 14]
Free CARO Evaporators and Condensers Questions and Answers
An operator observes a steady increase in the condenser approach temperature.
Which of the following is the MOST likely cause?