CARO Evaporators and Condensers 2 — Questions and Answers
Question 1: What is the main purpose of maintaining superheat at the outlet of a direct-expansion (DX) evaporator?
- To subcool the liquid line
- To increase the condensing pressure
- To speed up oil return to the receiver
- To ensure only vapor returns to the compressor (Correct answer)
Correct answer: To ensure only vapor returns to the compressor
Superheat confirms all liquid has boiled off, protecting the compressor from liquid slugging.
Question 2: Why does a flooded evaporator generally transfer heat better than a DX evaporator of the same size?
- Its refrigerant is always superheated
- It uses no surge drum
- Nearly all of its internal surface is wetted with liquid refrigerant (Correct answer)
- It runs at a higher condensing pressure
Correct answer: Nearly all of its internal surface is wetted with liquid refrigerant
Liquid-wetted surfaces have far higher heat transfer coefficients than surfaces contacting superheated vapor.
Question 3: Heavy frost accumulation on an air-cooling evaporator coil typically causes which condition?
- Rising room humidity and rising suction pressure
- Increased evaporator capacity
- Higher condenser water temperature
- Reduced airflow and a falling suction pressure (Correct answer)
Correct answer: Reduced airflow and a falling suction pressure
Frost insulates the coil and blocks airflow, so less heat is absorbed and suction pressure drops.
Question 4: In a hot gas defrost system, what heat source melts the frost from the evaporator coil?
- Electric heaters mounted on the drain pan only
- Warm condenser cooling water sprayed on the coil
- Compressor discharge vapor routed into the coil (Correct answer)
- Outdoor air drawn through the coil
Correct answer: Compressor discharge vapor routed into the coil
Hot gas defrost sends high-pressure discharge vapor into the coil, where it condenses and releases heat.
Question 5: An evaporative condenser rejects most of its heat by which mechanism?
- Conduction through the casing to ambient air
- Radiation from the fan motor
- Sensible heating of the sump water only
- Evaporation of spray water over the coil (Correct answer)
Correct answer: Evaporation of spray water over the coil
Evaporating water absorbs large amounts of latent heat from the refrigerant inside the coil.
Question 6: Non-condensable gases trapped in a condenser most directly cause which problem?
- Low discharge temperature
- Reduced oil in the crankcase
- Excess liquid in the evaporator
- Abnormally high head pressure (Correct answer)
Correct answer: Abnormally high head pressure
Non-condensables occupy condenser surface and add their partial pressure, raising head pressure.
Question 7: What is the main operating penalty of running a refrigeration system at a higher-than-necessary condensing pressure?
- Higher evaporator capacity
- Lower discharge temperature
- Increased compressor power per ton of refrigeration (Correct answer)
- Reduced risk of oil carryover
Correct answer: Increased compressor power per ton of refrigeration
A higher compression ratio requires more horsepower per ton and lowers efficiency.
CARO Evaporators and Condensers 2
What is the main purpose of maintaining superheat at the outlet of a direct-expansion (DX) evaporator?