The Heat Balance Behind a Cooling Cycle
The time required to cool a batch of product in a vacuum cooler depends on the total heat that must be removed and the cooling capacity of the system. The basic formula is: Q = m × Cp × ΔT + Lv × Δm, where Q is total heat (kJ), m is product mass (kg), Cp is specific heat capacity (kJ/kg·K), ΔT is temperature difference (K), Lv is latent heat of vaporization (2,257 kJ/kg) and Δm is mass of evaporated water (kg).
The formula has two parts and they behave differently. The first term, m × Cp × ΔT, is sensible heat: it depends only on how much product there is and how far its temperature must fall. The second, Lv × Δm, is the heat carried away by water leaving the product as vapour. Because the latent heat of vaporization is large, this term is what makes vacuum cooling work — the product cools itself by evaporating part of its own moisture.
Reference Cycle Times
| Product | Initial Temperature | Target Temperature | Time |
|---|---|---|---|
| Leafy Vegetables | 28°C | 2°C | 25 minutes |
| Mushrooms | 25°C | 4°C | 20 minutes |
| Broccoli | 30°C | 2°C | 30 minutes |
| Cooked Food | 90°C | 10°C | 8 minutes |
The produce rows start from ambient temperature, in the 25–30°C band typical of freshly harvested vegetables. The cooked food row starts from a hot product at 80–90°C, which carries more sensible heat per kilogram but also evaporates faster.
What Shortens or Lengthens a Cycle
An estimate is a starting point, because several variables move it in either direction:
- Temperature span. A larger ΔT adds to the sensible heat term and lengthens the cycle.
- Product structure. Leafy and porous products release moisture freely; dense or sugar-rich products restrict the vapour path and take longer.
- Load and packaging. Closely stacked trays, sealed films or heavy cartons block vapour escape; the same mass loaded loosely cools faster.
- Equipment condition. Pump displacement and condensing capacity set the achievable rate; a worn door seal or a fouled condenser lowers it.
Using the Estimate
Treat the calculated time as a planning figure and confirm it with a trial on the actual product and load, since load density, batch weight and product variety all shift the result. A practical figure should therefore be stated per product and per batch: where the material and the batch weight differ, the cycle time differs with them.