RSES Evaporator Systems 2 — Questions and Answers
Question 1: What condition causes frost to form unevenly on an evaporator coil, with heavy frost near the inlet and little near the outlet?
- Low refrigerant charge (Correct answer)
- Restricted liquid line
- Flooded evaporator
- High superheat
Correct answer: Low refrigerant charge
Low refrigerant charge causes the available refrigerant to evaporate early in the coil, leaving the outlet section with only superheated vapor and no frost formation.
Question 2: A direct-expansion evaporator serving a walk-in cooler shows adequate airflow but poor temperature pull-down. The suction pressure is normal. What is the most likely cause?
- Dirty evaporator coil fins (Correct answer)
- Overcharged system
- Faulty condenser fan
- Low ambient temperature
Correct answer: Dirty evaporator coil fins
Dirty or frosted fins restrict heat transfer between the air and refrigerant even when airflow volume appears adequate, reducing the effective cooling capacity.
Question 3: Which evaporator design is best suited for applications requiring precise humidity control, such as produce storage?
- Large surface area, low TD evaporator (Correct answer)
- Small surface area, high TD evaporator
- Flooded shell-and-tube evaporator
- Plate-type evaporator
Correct answer: Large surface area, low TD evaporator
A large surface area evaporator operating at a low temperature difference (TD) between air and refrigerant minimizes moisture removal from the stored product.
Question 4: In a multi-circuit evaporator, what is the purpose of a distributor assembly?
- To divide liquid refrigerant equally among all circuits (Correct answer)
- To separate liquid from vapor before the evaporator
- To regulate superheat at the evaporator outlet
- To prevent liquid slugging in the compressor
Correct answer: To divide liquid refrigerant equally among all circuits
A distributor ensures each parallel circuit receives an equal mass flow of refrigerant, preventing some circuits from flooding while others starve.
Question 5: What happens to evaporator capacity when the suction pressure is lowered below design conditions?
- Capacity decreases because the refrigerant density and mass flow rate drop (Correct answer)
- Capacity increases because the temperature difference with the load increases
- Capacity stays the same since TD with the air increases proportionally
- Capacity increases because refrigerant boils faster at lower pressure
Correct answer: Capacity decreases because the refrigerant density and mass flow rate drop
Lower suction pressure reduces refrigerant vapor density, which decreases mass flow through the compressor and lowers the actual refrigerating effect.
Question 6: A technician measures 20°F superheat at the evaporator outlet on a system designed for 10°F. What adjustment should be made to the TXV?
- Open the valve (decrease spring tension) to allow more refrigerant flow (Correct answer)
- Close the valve (increase spring tension) to restrict refrigerant flow
- Replace the TXV with a larger capacity valve
- Add refrigerant to raise suction pressure
Correct answer: Open the valve (decrease spring tension) to allow more refrigerant flow
High superheat indicates the TXV is underfeeding the evaporator; reducing spring tension allows the valve to open more and feed more refrigerant.
Question 7: Which refrigerant property most directly determines the evaporating temperature in an evaporator operating at a fixed suction pressure?
- Saturation temperature corresponding to that pressure (Correct answer)
- Latent heat of vaporization
- Liquid density at the inlet
- Specific heat of the refrigerant vapor
Correct answer: Saturation temperature corresponding to that pressure
The evaporating temperature is the saturation temperature of the refrigerant at the prevailing suction pressure, as defined by the pressure-temperature relationship.
RSES Evaporator Systems 2
What condition causes frost to form unevenly on an evaporator coil, with heavy frost near the inlet and little near the outlet?