Milk arrives twice a day and in a short time; it has to be cooled quickly from 35 °C to 4 °C. A condensing unit that met this instantaneous load directly would be very large and would stand idle for the rest of the day. In an ice bank a small unit runs for hours and builds up ice on the tubes in the tank; when the load comes, the tank water is pumped to a plate heat exchanger, the water returning warm melts the ice and falls back to 0–1 °C.
The power of ice lies in its heat of fusion: as 1 kg of ice melts it absorbs 334 kJ, that is, 93 Wh of heat; 1 tonne of ice means about 93 kWh of cooling. To store the same energy in water with a 5 K temperature difference, 16 tonnes of water are needed. Example: cooling 10,000 litres of milk from 35 °C to 4 °C takes about 350 kWh; this load is met with around 4 tonnes of ice, or with less than half of that if mains or well water is used for pre-cooling.
The ice thickness on the tubes is usually limited to 25–40 mm; as it thickens the ice behaves like an insulator, the evaporating temperature falls and efficiency decreases. An ice thickness sensor stops the compressor. The evaporating temperature is between −5 and −10 °C. Air blown in from the bottom of the tank, or an agitator, keeps the water moving and ensures that the ice melts evenly and quickly; in still water the outlet temperature rises. The gap between the tubes is left wide enough for the water to circulate even when the ice has reached its full thickness.
Apart from dairies, the main fields of use are dough water in bakeries, immersion chilling in poultry slaughterhouses, beverage production and the pre-cooling of fruit and vegetables with ice water. Building ice on the night tariff lowers the electricity cost and reduces the connected power. Points to watch: that the ice does not bridge between the tubes and block the tank, that the water is kept clean and that the tank is well insulated. The capacity of the condensing unit is chosen according to how many hours the ice is to be built in. In an uninsulated tank part of the ice melts to no purpose.