A frozen storage room is where blast-frozen product waits. The −18 °C limit is not arbitrary: at this temperature micro-organisms do not multiply and chemical deterioration slows greatly. Food legislation requires −18 °C or below at every point of quick-frozen food; during transport and distribution it allows a brief rise of at most 3 °C. In practice rooms are set to −20/−22 °C to leave a margin; ice cream is kept at −25 °C, and oily fish and seafood to be stored for a long time at −25/−30 °C.
The room is noticeably more heavily equipped than a positive room: 120–150 mm panels, an insulated floor heated from below, a frozen room door with a heated frame, a pressure equalising valve, and an evaporator with 7–10 mm fin spacing and electric or hot gas defrost. The evaporating temperature is between −28 and −32 °C; under these conditions the compressor delivers less than half the cooling it would in a positive room for the same electricity. That is why every loss, such as door leakage, inadequate insulation and unnecessary defrost, costs twice as much in a frozen room.
The most common mistake is trying to freeze product in a frozen storage room. The room is calculated on the assumption that the daily intake will arrive at −12/−15 °C at the warmest; if fresh product is put in, it will not freeze for days, it warms the frozen product next to it and it smothers the evaporator in ice. Freezing is done in the blast freezer, and the product is transferred from there to the store. The second enemy is temperature fluctuation: with every warming and cooling the water in the product moves to the surface, snow builds up inside the packaging and the surface dries out (freezer burn). A steady −20 °C preserves better than a fluctuating −24 °C.
Points to watch in operation: product is stacked at least 10 cm clear of the walls and floor and so that it does not block the evaporator’s air throw; a strip curtain is fitted at the door and, if possible, an anteroom at 0/+5 °C is built; a temperature recorder and alarm are used. For anyone trapped inside, a door handle that opens from the inside and an alarm button are indispensable safety equipment. The cooling load calculator gives a preliminary capacity from the room dimensions, panel thickness and daily intake; you can find the storage times of products at −18 °C in the food storage conditions guide.