A refrigerated cabinet defrosts by switching the cooling off and letting itself warm up to clear the ice, and the temperature log shows that as a rise four times a day. Cabinet 4 falls by two degrees instead, and the log is a legal record.
I am a store engineer for a supermarket group and I cover about twenty stores.
How a defrost works, because the chart shape is the whole of this.
A chiller runs below freezing at the evaporator, so moisture out of the air freezes on the coil. Ice on a coil insulates it, so every cabinet defrosts on a timer: typically four times a day, for twenty minutes or so.
In a standard off cycle defrost, the compressor simply stops. The fans may run, the coil warms up to above freezing, the ice melts and runs away to the drain, and then the cooling comes back on.
During that the cabinet warms. It has to. You have switched the cooling off and you are deliberately melting ice in it, and the product rides it out because a pack of food has thermal mass and twenty minutes is not long enough to matter.
On the chart it is a hill. Air temperature climbs from two degrees to six or seven, then drops back. Four hills a day, every day, in every cabinet in every store in this country.
Cabinet 4 in one of my stores goes down to zero.
Every defrost. Four times a day. The air temperature falls by about two degrees for the duration of the cycle and comes back up to two when the cooling comes back on.
I have been doing this for nineteen years and when I first saw that chart I assumed the defrost was not happening.
A cabinet that never defrosts ices up, and an iced coil loses performance, and within a week you have a cabinet at eight degrees and a lot of binned sandwiches.
Cabinet 4 does not ice up. I have had the rear panel off it on eleven visits and the coil is clean.
What has been checked, in the order an engineer checks it.
The controller and its probes. Swapped, then the whole controller replaced with a different model in 2022, and a data logger put in alongside with its own independent probe.
The logger agrees. Two independent instruments watch the air in that cabinet fall when the cooling stops.
The defrost is actually happening. I have put a clamp meter on the compressor circuit and watched it: at the defrost time the compressor stops, the current goes to the fans only, and twenty two minutes later it starts again. So the cooling really is off.
The pipework. It is on a pack system with five other cabinets on the same suction line, and a liquid line that does not shut properly would keep feeding the evaporator and keep it cold. That is the one plausible mechanical answer and I chased it for a year.
The solenoid was replaced in 2022. I have isolated the liquid line by hand during a defrost, with a gauge on it, so no refrigerant can reach that evaporator at all.
It still falls by two degrees.
And the evaporator temperature is the thing that finishes it. I put a surface probe on the coil in 2023. During the defrost, with the compressor off and the line isolated, the coil goes from minus six to minus nine.
Which is the part I have stopped telling other engineers.
A coil with no refrigerant flow and no compressor is a lump of aluminium in air above its own temperature. It warms. That is not refrigeration, that is a cup of tea going cold.
Ours gets colder, by three degrees, for twenty two minutes, four times a day, and then comes back to where it was when the cooling comes on.
I have written that in a service report once, in 2023, in the plainest language I could manage, with the logger data attached. The report went to the technical manager for chilled, who rang me up and was perfectly decent about it and asked whether the probe could have been loose.
What happens about it.
Nothing needs to. That is the whole reason it has run like this for four years.
A cabinet that holds its temperature better than specification during a defrost is a cabinet that passes every check there is. The temperature log is the legal record and it is the best log in that store. Product life is better in cabinet 4 than in the others and the store manager puts the high value lines in it.
Environmental health have looked at those charts twice. The officer in 2022 commented that our defrost management was the best she had seen, and I said thank you.
What I do.
I service it on the schedule, I download the logger, and I file the chart.
The one thing I have changed is that I do not work on it during a defrost.
You can, and it is convenient, because the cooling is off and you are not fighting a cold coil.
In February 2023 I had the panel off cabinet 4 in the middle of a defrost with the liquid line isolated by my own hand and a probe taped to the coil reading minus eight, and I had my arm in behind the fan deck, and a coil at minus eight with no flow in it feels like a coil at minus eight, which is to say it takes the feeling out of your fingers in about a minute.
What I could not account for at the time, and have not been able to since, is that the air coming off the back of that coil with the fans running and nothing cooling it was moving downwards, out of the bottom of the cabinet, across the floor, which air that cold does do, and there was a great deal more of it than a cabinet that size contains.