RSES Evaporator Systems 4 — Questions and Answers
Question 1: What is the purpose of a suction line accumulator installed between the evaporator and compressor?
- To prevent liquid refrigerant slugs from reaching the compressor (Correct answer)
- To increase suction gas superheat beyond TXV settings
- To store extra refrigerant during low-load conditions
- To filter oil out of the suction gas
Correct answer: To prevent liquid refrigerant slugs from reaching the compressor
A suction line accumulator traps liquid refrigerant and oil, releasing only vapor to the compressor to prevent liquid slugging damage.
Question 2: Which factor most significantly affects the air-side heat transfer performance of a fin-and-tube evaporator?
- Fin spacing and fin surface cleanliness (Correct answer)
- Refrigerant type used in the tubes
- Tube material (copper vs. aluminum)
- Number of refrigerant circuits
Correct answer: Fin spacing and fin surface cleanliness
Fin spacing determines airflow resistance and surface area, while cleanliness ensures that the full fin surface area is available for heat exchange.
Question 3: A technician installs a TXV with a remote sensing bulb. Where should the bulb be clamped for proper superheat sensing?
- On the suction line outside the refrigerated space, after the evaporator outlet (Correct answer)
- On the liquid line before the TXV inlet
- Inside the refrigerated space on the return air duct
- On the discharge line near the condenser
Correct answer: On the suction line outside the refrigerated space, after the evaporator outlet
The TXV sensing bulb must be clamped on the suction line after the evaporator outlet to sense actual refrigerant superheat leaving the evaporator.
Question 4: What effect does an excessively high evaporator fan speed have on evaporator performance in a low-humidity storage application?
- Increased moisture removal from stored product due to greater air-to-coil contact (Correct answer)
- Improved heat transfer with no effect on humidity
- Reduced coil frosting rate due to higher air velocity
- Increased evaporating temperature due to higher airflow
Correct answer: Increased moisture removal from stored product due to greater air-to-coil contact
Higher fan speed increases the rate at which moist air contacts the cold coil, causing more moisture to condense and freeze on the coil and dehumidify the stored product.
Question 5: During hot-gas defrost of a low-temperature evaporator, the system pressure may rise significantly. What component protects the low-side from excessive pressure?
- A defrost pressure-relief or pressure-limiting valve on the evaporator outlet (Correct answer)
- The TXV, which closes under high pressure
- The liquid line solenoid valve
- The compressor suction service valve
Correct answer: A defrost pressure-relief or pressure-limiting valve on the evaporator outlet
A pressure-limiting or defrost relief valve vents excess pressure from the evaporator to the suction line or condenser to prevent low-side pressure from exceeding safe limits.
Question 6: What is 'temperature difference' (TD) as applied to evaporator selection and why does it matter?
- The difference between the air entering the evaporator and the evaporating refrigerant temperature, affecting humidity and capacity (Correct answer)
- The difference between suction and discharge pressures expressed as a temperature
- The superheat measured at the evaporator outlet
- The difference between box setpoint and ambient outdoor temperature
Correct answer: The difference between the air entering the evaporator and the evaporating refrigerant temperature, affecting humidity and capacity
TD determines how aggressively the coil pulls heat and moisture from the air; a higher TD increases capacity but also removes more moisture, affecting product quality.
Question 7: A glycol chiller evaporator shows reduced capacity but the refrigerant side readings are normal. What is the most likely cause on the glycol side?
- High glycol concentration reducing heat transfer and flow rate (Correct answer)
- Low refrigerant superheat causing liquid slugging
- Refrigerant overcharge flooding the evaporator
- Faulty TXV allowing too much refrigerant flow
Correct answer: High glycol concentration reducing heat transfer and flow rate
Excessive glycol concentration increases viscosity and reduces the glycol-side heat transfer coefficient, significantly cutting the chiller's effective capacity.
RSES Evaporator Systems 4
What is the purpose of a suction line accumulator installed between the evaporator and compressor?