Walk-In Cooler Repeat Refrigerant Loss
You are called to a convenience store walk-in cooler using R-404A. The box is 8 ft by 10 ft by 8 ft and should hold 35°F; product temperature is 44°F. Outdoor ambient is 88°F. The remote condensing unit has a clean condenser, and the evaporator fan is running. Measured: suction pressure 22 psig, discharge pressure 210 psig, suction line temperature at the coil 55°F, liquid line temperature at the drier outlet 90°F. The sight glass shows bubbles. The owner says another technician added 3 lb of refrigerant two weeks ago and the problem returned. A filter-drier and sight glass are on the liquid line.
1. Interpret the superheat and subcooling condition and explain why a recharge alone is not an acceptable repair.
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Using R-404A pressure-temperature relationships, 22 psig corresponds to roughly 0°F saturation, so a 55°F suction line is about 55°F superheat: very high. At 210 psig, saturation is roughly 95°F, so a 90°F liquid line is only about 5°F subcooling: low. High superheat plus low subcooling plus bubbles means the TXV is receiving mostly flash gas, not solid liquid.
That pattern can come from a low charge or from a liquid-line restriction. Adding refrigerant without finding out which one simply repeats the previous repair. If there is a leak, the added refrigerant will escape again; if there is a restriction, the added charge may stack in the condenser and make pressures look better briefly without fixing the starved evaporator. The correct next step is to test the liquid line for a pressure/temperature drop and to leak-search the system.
2. List the steps to distinguish a restricted liquid line from a low charge, and state the repair for each.
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- Measure temperature on both sides of the filter-drier and any other liquid-line device. A noticeable drop, frost, or sweating at the drier points to a restriction.
- If no restriction is found, perform a leak search: electronic detector at brazed joints, Schrader ports, flare fittings, and coil returns; follow with a nitrogen pressure test if needed.
- For a restriction: recover refrigerant, replace the filter-drier, confirm the TXV screen is clean, evacuate to a deep vacuum, and weigh in the charge.
- For a leak: repair the leak, pressure-test, replace the drier if the system was opened, evacuate, and weigh in the charge.
A recharge without one of these repairs is not a diagnosis; it is a temporary masking of the failure.
3. What records and regulatory steps should accompany the repair if the system holds more than 5 lb of R-404A?
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The technician must be EPA Section 608 certified, must not vent refrigerant, and must follow the applicable leak-repair and recordkeeping requirements for appliances above the 5 lb threshold. That includes documenting refrigerant added, recovered, and the leak repair, and keeping service records available. If the leak cannot be repaired, the owner must be informed of the options and the rules that apply to continued operation. The invoice should state the measured readings, the leak location or restriction found, the repair performed, and the final weighed charge, not just refrigerant added.