RSES Evaporator Systems 3 — Questions and Answers
Question 1: What is the primary reason a hot-gas defrost system heats evaporator coils faster than electric defrost?
- Hot gas condenses inside the coil, releasing latent heat directly to the frost (Correct answer)
- Hot gas is at a higher temperature than electric heater surfaces
- Hot gas defrost uses more total energy than electric defrost
- Hot gas bypasses the expansion device entirely
Correct answer: Hot gas condenses inside the coil, releasing latent heat directly to the frost
When hot discharge gas condenses in the evaporator, it releases its latent heat of condensation directly to the coil, providing high heat transfer rates.
Question 2: A plate evaporator used in a beverage cooler develops oil logging. What is the most effective corrective action?
- Increase suction gas velocity by reducing evaporator circuits or raising suction pressure slightly (Correct answer)
- Add more refrigerant to flush the oil out
- Install a larger TXV to increase flow
- Lower the evaporating temperature to vaporize the oil
Correct answer: Increase suction gas velocity by reducing evaporator circuits or raising suction pressure slightly
Oil logging in evaporators is corrected by increasing suction gas velocity, which carries trapped oil back to the compressor through the suction line.
Question 3: During off-cycle defrost, what limits how completely frost is removed from a low-temperature evaporator?
- The ambient or box temperature may not be warm enough to melt all frost before refrigeration restarts (Correct answer)
- The compressor continues to run, re-freezing the coil
- Off-cycle defrost is only effective at evaporating temperatures above 0°F
- The expansion valve starves the evaporator during defrost
Correct answer: The ambient or box temperature may not be warm enough to melt all frost before refrigeration restarts
Off-cycle defrost relies solely on warm air from the refrigerated space, which may not carry enough heat to fully defrost deeply frosted coils in sub-zero applications.
Question 4: What does a 'starved evaporator' mean in refrigeration terminology?
- The evaporator receives insufficient liquid refrigerant, resulting in high superheat (Correct answer)
- The evaporator is flooded with liquid, causing low superheat
- The evaporator fan is undersized for the coil capacity
- The evaporator coil is oversized for the system
Correct answer: The evaporator receives insufficient liquid refrigerant, resulting in high superheat
A starved evaporator receives too little refrigerant, so the refrigerant evaporates early and the remaining coil surface superheats vapor without contributing to cooling.
Question 5: In a flooded evaporator system, refrigerant level is maintained by a:
- Float valve that admits liquid to maintain a set liquid level (Correct answer)
- Thermostatic expansion valve sensing bulb temperature
- Pressure-regulating valve on the suction line
- Hand expansion valve set by the technician
Correct answer: Float valve that admits liquid to maintain a set liquid level
Flooded evaporators use a float valve (low-side or high-side float) to maintain the correct liquid refrigerant level in the evaporator vessel.
Question 6: Why is it important to pitch evaporator drain pans toward the drain outlet?
- To prevent water from accumulating and refreezing on the pan, causing blockages (Correct answer)
- To increase airflow velocity across the evaporator coil
- To improve refrigerant distribution through the evaporator circuits
- To reduce the weight load on the evaporator mounting brackets
Correct answer: To prevent water from accumulating and refreezing on the pan, causing blockages
Standing water in an improperly pitched drain pan will refreeze, eventually blocking the drain and causing water to overflow into the refrigerated space.
Question 7: A secondary refrigerant (brine) evaporator system offers which advantage over a direct-expansion system in large industrial plants?
- Reduced refrigerant charge in the system and easier leak detection (Correct answer)
- Higher evaporating temperatures for the same load
- Faster system response to load changes
- Lower pumping energy than compressor work
Correct answer: Reduced refrigerant charge in the system and easier leak detection
Secondary refrigerant systems contain refrigerant only in the central plant equipment, dramatically reducing the total system charge and limiting refrigerant leak exposure.
RSES Evaporator Systems 3
What is the primary reason a hot-gas defrost system heats evaporator coils faster than electric defrost?