Making ice on demand means removing the heat of cooling the water to 0 °C and then the heat of freezing it: about 84 kJ plus 334 kJ, or roughly 418 kJ, for each kilogram starting at 20 °C. Put shallow metal trays of clean water against the coldest part of the evaporator and expect hours, not minutes; the carrier gives 4 to 8 hours depending on the system.
Part of Refrigeration
Producing ice with a working refrigeration system: the energy it takes, how to set up trays, and how to keep the ice safe to use.
The Heat Of Freezing Dominates, And Making Ice Is A Long Job
The corpus gives the latent heat of melting ice as 334 kJ/kg in Sensible vs Latent Heat, which is as much as warming water by 80 K. Making ice reverses that: the machine must first cool the water to 0 °C and then pull out the 334 kJ/kg, all at the evaporator. The refrigeration carrier’s method is simple: shallow metal trays of clean water in the coldest part of the evaporator, directly against the coils if possible, with ice forming in 4 to 8 hours depending on system capacity.
Ice extends the corpus cold chain: pack it around medicines or perishables for transport, as Cold Chain describes, and keep a stock for emergencies.
What You Need
- Shallow metal trays, steel or tinned, so the water has a large cold surface and a short distance to freeze through.
- Clean water, fit to drink. Freezing does not purify it, and the ice is used where people drink and handle food.
- A place against the coldest part of the evaporator.
- A way to release the blocks: a brief run of warm water on the outside of the tray.
Method: Making A Batch
Step 1: Fill the trays with clean water, not to the brim. Water expands when it freezes, so leave room.
Step 2: Put the trays on or against the evaporator. Good contact matters; a gap of air between tray and coil slows the freezing.
Step 3: Leave them closed in for the hours it takes. Check by looking, not by opening the box repeatedly.
Step 4: Release and store the ice at once. Move the blocks to an insulated store, as in Ice Houses, so the work of making them is not lost.
Worked Example: What A Batch Costs In Cooling Power
Inputs: 1 kg of water at 20 °C, the corpus water heat capacity of 4,186 J/kg·K and the corpus latent heat of 334 kJ/kg.
- Cooling to 0 °C: 4,186 × 20 = 83,720 J = 83.7 kJ, about 84 kJ.
- Freezing: 334 kJ.
- Total per kilogram: 83.7 + 334 = 417.7 kJ, about 418 kJ.
- For a 2 kg batch: 2 × 417.7 = 835 kJ (2 kg is an assumed tray load).
At the carrier’s 4 to 8 hours the average cooling needed is:
- Over 4 hours: 835,000 J ÷ 14,400 s = 58 W.
- Over 8 hours: 835,000 J ÷ 28,800 s = 29 W.
So the carrier’s timing is consistent with an evaporator delivering a few tens of watts of cooling to the trays. This is the ideal figure: it leaves out cooling the tray metal, heat leaking into the box and the cooling of the ice below 0 °C, so the real average is higher or the time longer. The carrier does not give a tray load, so the 2 kg is an assumption.
Common Mistakes
- Trays too deep. Thick water freezes from the outside in, slowly, and may split the tray.
- Dirty water. The ice carries it into drinks and wounds.
- Leaving the box open to check. Warm air and moisture undo the work and ice up the coil.
- Ice stored in the machine. It melts when the machine stops.
Safety
Ammonia dissolves in water, so a leak at the coil can poison the ice
If the evaporator holds ammonia, a pinhole leak at the coil can pass into trays pressed against it, and the ice then carries a toxic, irritant solution without looking any different. Do not make ice for drinking or for food contact on a coil that carries ammonia unless the trays are separated from it. Metal sticks to a very cold coil and can tear skin, so handle with dry gloves. See Ammonia Toxicity.
What’s Next
- Learn next: Medical Cold Chain — where the ice buffer matters most
- Combine with: Ice Houses — storing it