CARO Evaporators and Condensers 5 — Questions and Answers
Question 1: Why must an evaporator be pumped out before hot gas is introduced for defrost?
- To avoid hydraulic shock from hot gas hitting liquid refrigerant (Correct answer)
- To allow the fans to keep running
- To drain oil into the receiver
- To raise the room temperature
Correct answer: To avoid hydraulic shock from hot gas hitting liquid refrigerant
Hot gas entering a coil full of cold liquid can collapse vapor and cause dangerous liquid hammer.
Question 2: If liquid backs up in a condenser because the drain to the receiver is restricted, what happens?
- Suction pressure rises sharply
- The receiver overfills
- Subcooling disappears completely
- Head pressure rises because effective condensing surface is lost (Correct answer)
Correct answer: Head pressure rises because effective condensing surface is lost
Liquid flooding the condenser tubes reduces the surface available for condensing, raising head pressure.
Question 3: What health hazard makes water treatment and cleaning of evaporative condensers especially important?
- Ammonia dissolving into the water
- Ozone generation
- Legionella bacteria growth (Correct answer)
- Carbon monoxide buildup
Correct answer: Legionella bacteria growth
Warm recirculating water can breed Legionella, which spreads through drift and causes Legionnaires' disease.
Question 4: In a gravity-flooded evaporator, where is the surge drum normally located?
- Next to the condenser
- Above the evaporator coil (Correct answer)
- Below the evaporator coil
- Inside the high-pressure receiver
Correct answer: Above the evaporator coil
The drum sits above the coil so liquid can flow by gravity into the coil and vapor can rise back to the drum.
Question 5: Why do low-temperature freezer evaporator coils typically use wider fin spacing than cooler coils?
- To increase refrigerant velocity
- To reduce the refrigerant charge in the receiver
- To allow frost to build up without quickly blocking airflow (Correct answer)
- To lower the condensing temperature
Correct answer: To allow frost to build up without quickly blocking airflow
Wider fin spacing lets more frost accumulate between defrosts before airflow is choked.
Question 6: What is subcooling in a condenser?
- Lowering the evaporator pressure below atmospheric
- Heating vapor above its saturation temperature
- Cooling vapor below its dew point
- Cooling liquid refrigerant below its saturation temperature (Correct answer)
Correct answer: Cooling liquid refrigerant below its saturation temperature
Subcooling is the temperature of liquid below the saturation temperature at its pressure.
Question 7: A room requiring a warmer evaporator temperature than the common suction main is controlled by which device at the evaporator outlet?
- Thermal expansion valve
- Relief valve
- Back-pressure (evaporator pressure) regulator (Correct answer)
- High-pressure float
Correct answer: Back-pressure (evaporator pressure) regulator
A back-pressure regulator holds the evaporator at a higher pressure, and therefore temperature, than the suction main.
CARO Evaporators and Condensers 5
Why must an evaporator be pumped out before hot gas is introduced for defrost?