Design a cold room to be small, to have a thick door with a tight seal and a latch that pulls it shut, to insulate the floor as well as the walls, to carry a vapour barrier on the warm side, and to have an inner door or airlock if it is opened often. Warm food loaded into the room can add more heat than the air let in at the door, so load it a little at a time.
Part of Refrigeration
The layout, door, floor, shelving, vapour barrier and loading habits of a small cold room built around an evaporator.
Size, Door And Floor Decide How Much Cold Leaks Out
The corpus carrier gives the design points. Keep the room small, because every cubic metre of air cooled costs energy; it suggests 1 × 1.5 × 2 m as enough for a small community. Make the door thick, insulated and tightly sealed with rubber strips or greased leather, with a latch that pulls it tight to the frame, since the door is the single biggest source of cold loss. Insulate the floor, because cold sinks and an uninsulated floor bleeds into the ground. Fit a vapour barrier on the inside of the insulation. Use slatted shelves, metal or sealed wood, so air moves around the stock. Walls and insulation are covered in Insulation & Construction; this page concerns the room around them.
Air that leaks through gaps is a second route, and the corpus quantifies it for ordinary rooms in Air Sealing with Primitive Materials. A cold room’s door seal and frame are the same problem with colder air on the other side.
What You Need
- A door at least as well insulated as the walls, with a gasket and a latch that closes it against the frame.
- Sealing material for the frame: rubber strip or greased leather, as the carrier lists.
- A vapour barrier on the warm side of the insulation, from waxed cloth, fat-treated skins or pitch-coated fabric.
- Slatted shelves and a thermometer positioned away from the coil and the door.
- A drain for defrost water and a way for the room’s air to leave safely.
- An inside release on the door. A person must be able to open the door from inside.
Method: Lay Out And Load The Room
Step 1: Fix the evaporator at the top or back, away from the door. Cold air falls, so the coil’s cold sinks through the stock.
Step 2: Put the airlock between the room and the outside if it is opened often. Open the outer door, step in, close it, then open the inner door, as the carrier describes. The vestibule means the cold room never meets outside air directly.
Step 3: Load in small batches. When it is right, the room’s thermometer recovers within the hour. When it is going wrong, it climbs and stays up after loading.
Step 4: Leave space round every stack. Air must be able to reach every item; a solid block of crates cools from the outside only.
Step 5: Check the seal in daylight. A thin line of light round the door is a leak. Mend it.
Worked Example: Warm Food Costs More Than The Air
The room inside is 1 × 1.5 × 2 m, which is 3 m³.
- Air in the room: 3 m³ × 1.2 kg/m³ = 3.6 kg (air density of 1.2 kg/m³ is a recalled figure, flagged).
- Cooling that air by 26 K from 30 °C to 4 °C, with a specific heat of 1.0 kJ/kg·K (also recalled, flagged): 3.6 × 1.0 × 26 = 94 kJ. That is the sensible heat of one complete change of the room’s air.
- Load 10 kg of food 20 K warmer than the room. Treating food as mostly water, with the corpus value of 4.186 kJ/kg·K: 10 × 4.186 × 20 = 837 kJ.
- Ratio: 837 ÷ 94 = 8.9.
So 10 kg of warm food brings in as much heat as nearly nine complete changes of the room’s air. The numbers show where the load is: in what you put in, not the draught from a quick door opening. Moisture in the air adds latent heat that this sensible-heat figure leaves out, and that moisture freezes onto the coil as frost, which is another reason to keep the door shut. The 94 kJ is not what a door opening costs: only a fraction of the room’s air changes each time.
Common Mistakes
- A big room for a small load. Every extra cubic metre is more wall and more air to cool.
- A door that sags. It leaves a gap along the bottom that nobody notices.
- Loading hot food all at once. The room warms for hours.
- A drain that freezes. Defrost water backs up on the floor and freezes.
- No inside release. A person shut in a cold room can die of cold or, with an ammonia coil, of vapour.
Safety
A cold room is a trap if the door cannot be opened from inside
Fit an inside release before the room is used. If the evaporator holds ammonia, a leak fills a closed room with irritant vapour in the place people go to fetch food, and a warning smell is no use to someone who cannot get out. Ventilate the room from outside and keep the evaporator joints few and welded; see Ammonia Toxicity.
What’s Next
- Learn next: Food Cold Chain — what to keep in the room and how to handle it
- Combine with: Evaporation — the coil that cools it