Reach-In Freezer Starving: Restriction vs. Undercharge vs. Weak Compressor
A reach-in freezer with a 1/2 HP R-404A condensing unit and a TXV is holding 18°F instead of 0°F. Ambient is 75°F and the condenser is clean. At steady state the readings are: suction pressure 6 psig; suction-line temperature at the evaporator outlet 2°F; head pressure 190 psig; liquid-line temperature at the condenser outlet 83°F; liquid-line temperature just before the TXV 74°F; compressor current 4.2 A versus 6.0 A nameplate; sight glass clear. The evaporator frost pattern is heavy on the first passes and light near the outlet. The compressor is not hot to the touch, and the crankcase is not sweating. The unit uses a liquid-line filter-drier and a TXV with an inlet screen. The box is empty and the door gaskets are intact.
1. Calculate superheat and subcooling, and explain what the combination tells you. Why is undercharge unlikely?
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At 6 psig, R-404A saturation is about -30°F; superheat = 2 - (-30) = about 32°F. At 190 psig, saturation is about 93°F; subcooling = 93 - 83 = about 10°F.
The combination is high superheat with normal subcooling. Undercharge is unlikely because subcooling is normal and the sight glass is clear; undercharge would normally lower subcooling and produce bubbles. The low compressor current (4.2 A vs 6.0 A) shows reduced mass flow, not a loaded compressor. The 9°F drop from the condenser outlet (83°F) to the TXV inlet (74°F) is excessive for a clean liquid line and points to a pressure drop/restriction between those points, likely the filter-drier, a line kink, or a solenoid. The evaporator is starved, which matches the frost pattern and high superheat.
2. If the temperature drop across the filter-drier were only 1°F, how would you distinguish a restricted TXV from a weak compressor?
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If the drier drop is only 1°F, check the TXV and compressor separately. A restricted TXV (plugged inlet screen, failed power element, lost bulb charge, bulb not clamped) will keep suction pressure low and superheat high even as the valve is wide open. A weak compressor usually shows low head pressure, higher-than-normal suction pressure, low current, and poor capacity; superheat may be normal or low because the evaporator is not being pumped down. Useful checks: pump down or isolate the evaporator, verify bulb charge and clamp, inspect the TXV screen and distributor, compare suction/head to the compressor performance curve, and check discharge temperature and current. Do not condemn the compressor until the liquid supply and TXV are proven good.
3. Describe the repair and the verification steps after the repair.
Read the worked answer
Replace the restricted filter-drier and any kinked line or screen; brace with nitrogen while brazing, leak-test, and evacuate with a micron gauge. Recharge by weight/subcooling, then verify 8-12°F evaporator superheat, 10°F subcooling, compressor current near nameplate, and a normal frost pattern. Confirm the box pulls down to 0°F with product load.