Ceiling Ice and a Frozen Drain Pan in a -22 °C Cold Store
A 12 m × 8 m × 3 m cold store holds palletised frozen food at -22 °C. Two 6 kW electric-defrost air coolers run four defrosts per day, 45 minutes each, terminated at 12 °C coil temperature with a 10 °C fan delay. The store door is 2.4 m × 2.5 m and opens about 20 times per hour during a six-hour dispatch window. Outside the door the ambient is 18 °C and 65% RH. After three weeks, staff report ice on the ceiling within 2 m of the door and a frozen block in the drain pan; the evaporator fins themselves are clear. Product temperature is still acceptable, but defrost water is dripping onto the floor and refreezing. The defrost termination sensor is clipped to a return bend at the bottom of the coil, and the drain-pan heater is wired through the same contactor as the defrost elements. The site wants to increase defrost frequency to six per day.
1. Why would increasing defrost frequency from four to six per day probably worsen the symptoms, and what does the ice pattern near the door tell you?
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More defrosts add heat and meltwater to a store that is already struggling to clear water. If the drain-pan heater is energised only while the elements are on, water remains in the pan after the elements switch off and refreezes; six cycles per day means six chances to build that block, not fewer. The ice on the ceiling within 2 m of the door is not a coil-defrost pattern. It points to moist outside air entering through the doorway, contacting the cold ceiling and freezing there. That moisture load will not be removed by more defrosting. Increasing frequency may also raise product temperature and add unnecessary heat. The correct first response is to treat door infiltration and drain clearing as separate faults, not to shorten the defrost interval.
2. Give a diagnostic sequence that separates door infiltration from a defrost-termination or drain-heater fault.
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Start with a full defrost observation: log coil temperature, termination time and drain-pan heater voltage from the start of defrost until the fans restart. Confirm whether the pan heater is live after the elements drop out; if it is not, the wiring is a direct cause. Check the termination sensor position against the manufacturer’s location; a bottom return bend can read cold while the top of the coil is still iced. Inspect the drain pan slope, trap, drain line and insulation, and confirm water flows freely. In parallel, check door seals, closers, strip curtain or air curtain, and door heater. Use a smoke pencil at the door with the store at temperature to show infiltration. Run one logged defrost with no changes and compare the ice pattern before and after; ceiling ice near the door with a clear coil confirms infiltration, while ice across the coil and a frozen pan confirms defrost/drain faults.
3. Specify the corrective actions and the checks you would make after repair.
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Relocate the termination sensor to the manufacturer’s specified position, normally where the coil is last to clear, and secure it so it senses coil temperature rather than air. Rewire the drain-pan heater so it operates during and for a timed period after defrost, independent of the element contactor, and repair the drain slope, trap and insulation. Restore the original four defrosts and address infiltration with seals, closers, strip curtain and discipline on door openings. After repair, run at least two complete defrosts with logging: termination must occur before the 45-minute maximum, drain water must clear, no ice should form in the pan, and the fans must restart only after the coil is clear. Check ceiling and floor near the door after 24 hours and confirm product temperature remains within specification.