Walk-In Cooler With Bubbles at the TXV
A walk-in cooler serving a restaurant holds a 38°F setpoint but is running at 42°F. It uses R-404A, a receiver, a thermostatic expansion valve, and a remote condensing unit 25 ft away with a 12 ft vertical rise to the evaporator. The liquid line is 3/8 in. and the suction line is 7/8 in. Outdoor ambient is 90°F. Suction pressure is 35 psig with 55°F suction line temperature. Liquid pressure is 200 psig with 78°F liquid line temperature. The sight glass at the TXV inlet shows bubbles, and the TXV is hunting. The filter drier is warm to the touch and shows no measurable temperature drop. The condenser is clean and the fan is running. The box has product and the door gaskets are good. The last technician added 3 lb of R-404A and the bubbles did not clear. The unit has no liquid-line solenoid; it pumps down on the low-pressure control. The nameplate charge is not legible. You need to determine whether the system is undercharged, restricted, or has a low-load or airflow problem.
1. Calculate superheat and subcooling and interpret the sight glass. List the most likely causes.
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R-404A at 35 psig has a saturation temperature near 9°F; superheat = 55°F - 9°F = 46°F. At 200 psig saturation is about 86°F; subcooling = 86°F - 78°F = 8°F. The combination of high superheat, low subcooling, and bubbles at the TXV inlet points to a liquid-feed problem. Ranked causes: (1) undercharge or a leak; (2) liquid-line restriction upstream of the sight glass; (3) insufficient subcooling to overcome the 12 ft vertical lift, allowing flash gas at the TXV. A low-load or airflow problem would usually give low superheat, not 46°F. The warm filter drier with no measurable temperature drop makes a drier restriction less likely, but it does not rule out a restriction at a king valve, strainer, or undersized liquid line.
2. Why is adding refrigerant until the sight glass clears unsafe? What tests separate undercharge from a restriction?
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Adding until clear can overcharge the system, raise discharge pressure, flood the compressor, and hide a leak. The sight glass can bubble from flash gas caused by vertical lift or a restriction, not just low charge. Tests: measure temperature drop across the filter drier and any strainer (more than about 2°F indicates restriction); check the liquid line at the receiver outlet and at the TXV inlet; isolate the liquid line and observe whether bubbles persist; perform a pump-down and standing-pressure test; and recover and weigh the charge. If the recovered weight is below the nameplate and the system holds pressure, undercharge is confirmed. If the charge is correct but the TXV inlet still flashes, check subcooling and line sizing/lift.
3. Recommend next steps, including how the vertical lift affects the diagnosis.
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First leak-search the system, repair any leak, replace the filter drier, evacuate, and charge by weight. Since the nameplate is illegible, use the condensing unit and line-set charging tables or contact the manufacturer with model, serial, and line length. The 12 ft lift costs roughly 6 psi of liquid pressure; with only 8°F subcooling, the liquid can flash before the TXV. Verify that the 3/8 in. liquid line is approved for the tonnage and lift; if not, upsize or add subcooling per manufacturer. Check the TXV strainer and bulb. Do not leave the system with a bubble in the sight glass if it is caused by flash gas; correct the charge and piping, not just the symptom. Document recovered weight and final superheat/subcooling.