Walk-In Cooler Floodback After an R-448A Retrofit
A supermarket walk-in cooler measuring 10 ft by 8 ft by 8 ft is maintained at 38°F. The system was retrofitted from R-404A to R-448A six weeks ago. It uses a semi-hermetic compressor, a thermostatic expansion valve with an external equalizer, and an air-cooled condenser. The store reports that product near the evaporator is freezing and the compressor runs continuously. Measured at steady load: box temperature 36°F, suction pressure 45 psig, suction line temperature at the evaporator outlet 24°F, liquid line temperature 82°F, condensing pressure 210 psig, sight glass clear, and the compressor oil sight glass is foamy with oil level at the bottom of the glass. The suction line and compressor body are frosted. The TEV bulb is clamped on the horizontal suction line and insulated. The service port is on top of the suction line.
1. Calculate the approximate superheat and subcooling and explain what the readings indicate about the operating condition.
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At 45 psig, R-448A saturation is approximately 18°F to 20°F, so superheat is roughly 24°F minus 19°F, or about 5°F. At 210 psig, saturation is approximately 102°F, so subcooling is about 102°F minus 82°F, or 20°F. Normal walk-in cooler superheat at the evaporator outlet is about 8°F to 12°F, and normal subcooling is about 10°F to 15°F. The measured 5°F superheat is too low and the 20°F subcooling is too high. Low superheat with a frosted suction line and compressor body indicates liquid refrigerant is reaching the compressor. High subcooling indicates excess liquid is backed up in the condenser, which is consistent with overcharge. The oil sight glass foaming and low oil level indicate refrigerant is diluting the oil. The freezing product near the evaporator is a symptom of a flooded evaporator, not a correct box temperature.
2. Is the clear sight glass conclusive evidence that the charge is correct? Explain the relationship between overcharge and TEV overfeed.
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No. A clear sight glass only means there are no vapor bubbles at that point in the liquid line. It can be clear with a correct charge, an overcharge, or a restricted liquid line that keeps the line full. It can also show bubbles when the charge is adequate but the condenser subcooling is low or the liquid line has a pressure drop. Subcooling is the charge indicator for this system. An overcharge raises subcooling and can force excess liquid through the TEV if the valve cannot close enough. A TEV that is overfeeding, has a loose bulb, has a blocked external equalizer, or is oversized will also produce low superheat and floodback even if the charge is only slightly high. The correct diagnosis separates the two: first correct the charge using subcooling, then adjust or repair the TEV using superheat. Because R-448A has glide, use dew-point saturation for superheat and bubble-point saturation for subcooling.
3. Outline the step-by-step correction and the readings you would monitor afterward.
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First confirm the TEV bulb is clean, tight, insulated, and mounted on the suction line at the correct clock position, and verify the external equalizer line is clear. Then recover refrigerant in small increments while watching subcooling, because the system is already overcharged. Target 10°F to 15°F subcooling at design condensing conditions. Once subcooling is correct, measure superheat at the evaporator outlet and adjust the TEV to 8°F to 12°F. If superheat remains low after charge correction, inspect the TEV seat, power head, and equalizer. Verify evaporator fan operation and defrost termination. After adjustment, monitor compressor oil level, oil appearance, suction line frost line, compressor amperage, box temperature, and product temperature. Confirm the frost line stays at the evaporator and does not migrate to the compressor. Record the final charge, subcooling, superheat, and oil level. R-448A is a blend, so charge as liquid and do not use the sight glass as the final charge criterion.