Walk-in Cooler Starved Evaporator with R-448A
A 10 ft × 12 ft walk-in cooler holding produce at 38°F is served by a remote air-cooled condensing unit and a TXV with a 25°F superheat setting. The box has drifted to 46°F over two days. At the evaporator, suction pressure is 38 psig (about 20°F saturated), and superheat measured 6 in. from the outlet is 28°F. Frost appears only on the first two rows of the coil; the rest is wet. Liquid line sight glass is clear, and liquid-line temperature is 95°F at 90°F ambient. Condenser subcooling is 12°F. The TXV bulb is clamped on the horizontal suction line and insulated. The technician finds the evaporator fan operating and the coil clean. No recent refrigerant addition.
1. What is the most likely fault, and what measurements support it?
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The evaporator is starved. Low suction (38 psig ≈ 20°F saturation), high outlet superheat (28°F), and a frost line only on the first passes mean liquid refrigerant is not reaching most of the coil. The clear sight glass and 12°F subcooling show liquid is present at the condenser outlet and the charge is not low. A restricted TXV inlet screen or a power head that has lost its charge would both close the valve and produce these readings. Because the suction line is insulated and the bulb is clamped correctly, a bulb problem is less likely. Next, pump down and inspect the TXV inlet screen; then warm the bulb while watching superheat. If superheat falls toward normal, the power head is probably good and the screen was restricted. If superheat does not respond, replace the power head or valve.
2. How would you distinguish a restricted TXV inlet screen from a failed power head using pressure and temperature readings?
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With the system running, measure the temperature drop across the TXV body and inlet screen. A restricted screen usually shows a noticeable temperature drop upstream of the valve seat because refrigerant flashes across the restriction; the valve body may be cold or frosted. A failed power head typically leaves the valve closed with no upstream flashing and no temperature drop across the screen. Then warm the remote bulb with your hand. A good power head should open the valve and reduce superheat; a failed one will not. Also compare the valve's superheat setting to actual: a power head that has lost charge will often hold the valve closed even when the bulb is warmed. If the screen is restricted, cleaning it restores flow and superheat drops.