Reach-In Freezer With a Partial Frost Pattern
A reach-in commercial freezer using R-404A and a capillary tube has been holding at -4°C although the setpoint is -18°C. The evaporator shows frost only on the first quarter of the coil; the remainder is bare. The suction line is warm to the touch. With the box at -4°C and ambient at 21°C, suction pressure is 8 psig and discharge pressure is 165 psig. Subcooling at the condenser outlet is 0 K and superheat at the evaporator outlet is 25 K. Compressor nameplate RLA is 3.4 A; measured current is 2.1 A. The condenser is clean and the fan is operating. The filter drier is not cold and shows no temperature drop. The owner says another technician added 200 g of R-404A last month, which helped for about a week.
1. Interpret the readings. What do they rule in and rule out?
Read the worked answer
The readings are not normal for a freezer at -4°C. A suction pressure of 8 psig with R-404A corresponds to a very low evaporator temperature, and the 25 K superheat shows the coil is starved. The low current (2.1 A versus 3.4 A RLA) and 0 K subcooling are consistent with a low charge or a restriction. Frost only on the first pass suggests refrigerant is boiling off early, which is typical of a cap-tube restriction or a low charge. The fact that 200 g helped for a week points to a leak, but it does not rule out a restriction; added charge can temporarily raise suction pressure even with a restricted cap tube. The clean condenser and running fan make a condenser airflow problem unlikely.
2. What tests would you perform to distinguish a leak from a restriction, and what would you not do?
Read the worked answer
Recover and weigh the charge. If the recovered charge is significantly below nameplate, leak-test the system; do not just recharge. If the charge is close to nameplate, the problem is a restriction. Check for a temperature drop across the filter drier and at the cap-tube inlet; a cold spot or frost at the cap tube supports a restriction. Compare the frost pattern after a known correct charge: a full frost pattern and normal superheat point away from restriction; a persistent first-pass frost with high superheat points to a restricted cap tube. A compressor valve test is lower priority because the low current and low suction fit starvation, not a failing compressor. Avoid adding refrigerant to see whether it improves, because that masks the cause and can overcharge the system.
3. Outline the repair and verification plan.
Read the worked answer
Repair the leak or replace the restricted cap tube, install a new filter drier, pressure-test with nitrogen, evacuate, and charge by weight to the nameplate. Do not add refrigerant without finding the cause. After restart, verify the evaporator frosts evenly, superheat is within the manufacturer's range, subcooling is appropriate, compressor current is near RLA, and the box pulls down to -18°C. Record pressures, temperatures, charge weight, and leak-test results. If the repair is not economical, explain the options to the owner rather than returning a partially charged system to service.