RSES Evaporator Systems 5 — Questions and Answers
Question 1: Which defrost termination method is considered the safest in preventing over-defrosting and unnecessary heat gain in a refrigerated space?
- Temperature-terminated defrost using a sensor on the coil (Correct answer)
- Time-terminated defrost with a fixed clock timer
- Pressure-terminated defrost sensing suction pressure
- Manual defrost initiated by an operator
Correct answer: Temperature-terminated defrost using a sensor on the coil
Temperature termination ends defrost as soon as the coil reaches a set temperature (typically 50–58°F), preventing excess heat from being added to the refrigerated space.
Question 2: What is the effect of non-condensable gases (such as air) trapped in the evaporator of an absorption refrigeration system?
- They reduce the partial pressure of refrigerant vapor, raising the evaporating temperature and reducing capacity (Correct answer)
- They increase evaporating pressure, improving capacity
- They have no effect because the evaporator only handles liquid refrigerant
- They mix with the absorbent, improving absorption efficiency
Correct answer: They reduce the partial pressure of refrigerant vapor, raising the evaporating temperature and reducing capacity
Non-condensable gases displace refrigerant vapor in the evaporator, lowering its partial pressure and raising the effective evaporating temperature, which reduces cooling capacity.
Question 3: A technician notices the suction line is sweating all the way back to the compressor. What does this indicate about evaporator performance?
- The evaporator is flooding or overfeeding refrigerant, with liquid returning in the suction line (Correct answer)
- The evaporator superheat is too high, indicating a starved coil
- The system refrigerant charge is low
- The evaporator fan has failed, reducing heat transfer
Correct answer: The evaporator is flooding or overfeeding refrigerant, with liquid returning in the suction line
Sweating along the entire suction line indicates liquid refrigerant is present in the suction gas, a sign of evaporator flooding or a failed TXV.
Question 4: In a parallel evaporator system serving multiple refrigerated cases, why is a solenoid valve on each case suction line important?
- It isolates individual cases during off-cycle defrost or shutdown without affecting other cases (Correct answer)
- It regulates refrigerant superheat for each individual case
- It equalizes pressure between cases operating at different temperatures
- It prevents refrigerant migration to the lowest pressure case
Correct answer: It isolates individual cases during off-cycle defrost or shutdown without affecting other cases
Individual suction solenoid valves allow each case to be isolated during defrost or maintenance cycles so that the remaining cases can continue operating normally.
Question 5: What is the function of an evaporator pressure regulator (EPR) valve installed in a multi-temperature suction system?
- To maintain a minimum evaporating pressure in a higher-temperature evaporator sharing suction with a lower-temperature one (Correct answer)
- To increase refrigerant flow to the evaporator under heavy load conditions
- To prevent suction pressure from rising above the compressor's design limit
- To regulate superheat at the evaporator outlet independently of the TXV
Correct answer: To maintain a minimum evaporating pressure in a higher-temperature evaporator sharing suction with a lower-temperature one
An EPR valve holds the evaporating pressure (and thus temperature) in a warmer evaporator at its setpoint, preventing it from pulling down to the lower evaporator's pressure.
Question 6: When sizing an evaporator for a walk-in freezer, which heat load component is often the largest contributor in a busy retail application?
- Infiltration heat gain from door openings (Correct answer)
- Conduction through insulated walls
- Heat from evaporator fan motors
- Respiration heat from stored product
Correct answer: Infiltration heat gain from door openings
In a busy retail walk-in freezer, frequent door openings introduce large amounts of warm, humid air, making infiltration the dominant heat load component.
Question 7: What condition is indicated when an evaporator coil is completely blocked with ice, the box temperature is rising, yet the suction pressure is lower than normal?
- The ice blockage is restricting airflow, reducing heat absorption so the refrigerant cools excessively and suction pressure drops (Correct answer)
- The TXV is overfeeding the evaporator causing flooding
- The refrigerant charge is low, starving the evaporator
- The condenser is fouled, raising discharge pressure and lowering suction pressure
Correct answer: The ice blockage is restricting airflow, reducing heat absorption so the refrigerant cools excessively and suction pressure drops
An ice-blocked coil starves the refrigerant of heat input because air cannot reach the coil surface, causing the refrigerant to over-cool and pull suction pressure below normal.
RSES Evaporator Systems 5
Which defrost termination method is considered the safest in preventing over-defrosting and unnecessary heat gain in a refrigerated space?