Hot-Gas Defrost Overpressure on a Cold Morning
A two-stage ammonia plant serves four freezer rooms at -30 C evaporating and a high-side receiver that normally sits at 35 C condensing on an evaporative condenser. On a cold morning the condenser fans are cycling and the receiver pressure falls to 8.5 barg. During hot-gas defrost of one freezer, the defrost timer is set for 45 minutes. At 30 minutes the defrost termination sensor reads -5 C but the room air temperature is climbing, and the high-side pressure rises from 8.5 to 13.5 barg. The defrost relief valve on that evaporator begins to weep. The operator wants to extend the defrost timer to 60 minutes to 'get it fully clear'.
1. Explain why high-side pressure rose during defrost and why extending the defrost time is the wrong move.
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Hot-gas defrost dumps high-pressure discharge gas into the evaporator, so the evaporator becomes a condenser and its pressure is tied to the high side. With the condenser fans cycling on a cold morning, the high side has little capacity to absorb that extra heat, so receiver pressure climbs from 8.5 to 13.5 barg. The weeping relief is telling you the evaporator circuit is overpressured.
Extending defrost to 60 minutes adds more hot gas and more heat to a circuit that is already overpressured and already above termination temperature. The correct action is to stop defrost on the termination sensor, verify the termination sensor is reading correctly, confirm the condenser is rejecting heat (fans running, water flow, no air lock), and check that the defrost relief and its setpoint are correct. If defrost is slow because of ice buildup, fix drainage and door seals, not the timer.
2. The termination sensor reads -5 C while room air is rising. What does that discrepancy suggest, and how would you confirm it?
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A termination sensor reading colder than the room air it is supposed to be clearing suggests the sensor is either not representative (buried in ice, mounted where hot gas passes, or in a pocket) or failed. If the coil is genuinely clear, the sensor should be approaching termination setpoint as the coil warms; a stuck -5 C reading while the room warms points to a sensor problem or a sensor that is not seeing the coil.
Confirm by comparing the termination sensor against a second reading (an adjacent coil sensor or a handheld probe on the coil), checking the sensor is clear of ice and correctly located, and watching whether the reading changes when hot gas is isolated. Do not override termination based on a single suspect sensor; a false termination or a missed termination both cause problems, and on this system the relief valve is already indicating overpressure.