Refrigeration lets a settlement keep perishable food for days to months, hold vaccines and insulin within their safe range, make ice on demand, and move heat where it is wanted. Start with the smallest cold box holding the most valuable load, usually medicines, because the wall area that must be kept cold grows with the cube root of the volume squared: a box of 0.1 m³ has about a tenth of the wall area of a 3 m³ room.
Part of Refrigeration
What a working refrigerator is used for in a rebuilding community, how to rank the uses, and the less obvious one: moving heat in the other direction.
Four Uses Rank By How Much Cold They Need And How Much Is At Stake
Food. The carrier ranks storage temperatures: 0 to 4 °C for fresh meat, dairy and eggs, giving days to one or two weeks; −5 to 0 °C for fish and soft fruit, one to four weeks; −18 °C for any food, months to a year; 2 to 8 °C for vegetables and cheese, two to six weeks. Cold slows bacterial growth and does not stop it. Detail is in Food Cold Chain.
Medicine. Vaccines and some other drugs lose potency when warm, and a break in cold storage can ruin a batch without showing. Medical Cold Chain sets out the principle, and the corpus’s Cold Chain sets out the vaccine side.
Ice. For fever, for transport of cold goods and for the cold chain itself. Ice Making gives the energy arithmetic.
Process cooling. Cooling condensers, fermentations or other processes. The same cycle serves.
A Small Cold Box Is Far Cheaper Than A Cold Room
The heat that leaks into a cold space is proportional to its surface area. Volume rises with the cube of size and area with its square, so a larger space has less area per unit of volume but needs far more area in total. For a fixed volume the question is how much surface there is to insulate and keep cold.
Worked Example: Wall Area Of A Small Box Against A Room
Compare two cubes with the same insulation and the same temperature difference.
- A 0.1 m³ box has a side of 0.1^(1/3) = 0.464 m and a surface of 6 × 0.464² = 1.29 m².
- A 3 m³ room has a side of 3^(1/3) = 1.442 m and a surface of 6 × 1.442² = 12.5 m².
- Ratio: 12.5 ÷ 1.29 = 9.7.
With the same insulation, the room gains about ten times the heat through its walls. It also holds thirty times the volume, so per cubic metre of storage the room is the more economical, but a settlement that has one machine and one vaccine supply to protect gets there sooner with a small insulated box. The wall-heat arithmetic for a real room is in Insulation & Construction.
The Warm Side Is Useful Heat
A refrigerator rejects, at its condenser, the heat it took from the box plus the work it was given. The Refrigeration Cycle shows the bookkeeping. That warm side can heat water or a room. When the aim is the warm side and not the cold, the same machine is called a heat pump. The condenser water arithmetic in Condensation shows how much water a given load can warm.
Common Mistakes
- Starting with a walk-in room. The cold room is the second project, after a small box that has proved the machine.
- Mixing loads. Vaccines and raw meat belong in separate cold spaces.
- Forgetting that the machine heats its surroundings. A cold room’s machine in a closed shed overheats the shed.
Safety
The things refrigeration protects are only as safe as the temperature record
A cold box with no thermometer gives false assurance, and a break in cold that nobody notices can ruin vaccines or let food reach unsafe temperatures. Fit a thermometer, read it at set times and write it down. The refrigerant circuit itself is a hazard; see Safety.
What’s Next
- Learn next: Food Cold Chain — farm to table
- Combine with: Medical Cold Chain — the highest-value load