The load carries two separate heat terms: the product's own heat and the heat entering through the container wall. The wall term is calculated with K, surface area and temperature difference, as in the insulation article; the product term follows here.
Two kinds of data. Sensible heat is the energy needed to change the product's temperature, given in kcal/kg°C. Heat of freezing is the energy needed to freeze or thaw the water, given in kcal/kg. According to the ASHRAE Refrigeration Handbook, food does not freeze at a single temperature but over a range, so the calculation uses the product's initial freezing point.
| Product | Freezing point °C | Above freezing, kcal/kg°C | Below freezing, kcal/kg°C | Heat of freezing, kcal/kg |
|---|---|---|---|---|
| Milk | -0.6 | 0.90 | 0.46 | 70.0 |
| Beef | -2 | 0.75 | 0.40 | 55 |
| Chicken | -3 | 0.70 | 0.42 | 71 |
| Cheese | -10/-15 | 0.44 | 0.31 | 30.0 |
600 kg of milk (example)
Inputs: 600 kg of milk, measured start +8 °C, target +2 °C. The milk stays above its freezing point of -0.6 °C, so only sensible heat counts: 600 × 0.90 × (8 - 2) = 3,240 kcal.
If the milk goes from +8 °C to -18 °C, three terms add up:
- Sensible heat down to the freezing point: 600 × 0.90 × (8 - (-0.6)) = 4,644 kcal
- Heat of freezing: 600 × 70.0 = 42,000 kcal
- Sensible heat below freezing: 600 × 0.46 × (-0.6 - (-18)) = 4,802 kcal
The total is 51,446 kcal, and 82% of it is the heat of freezing. That is about sixteen times the 3,240 kcal needed to cool the same milk from +8 °C to +2 °C. Cooling to +2 °C only lowers the temperature; cooling to -18 °C spends most of the energy freezing the water.
Once those two lines are calculated, the question of when a refrigerated vehicle is necessary stops being guesswork.
Frozen load: a small thaw is an expensive term
The MEB table gives the freezing point of chicken as -3 °C. If 500 kg of frozen chicken warms from -18 °C to -15 °C, the load stays frozen and only sensible heat counts: 500 × 0.42 × 3 = 630 kcal.
If 10 kg of that load reaches -3 °C and thaws, that part needs 10 × 71 = 710 kcal. Thawing just 2% of the load therefore takes more heat than warming the whole load from -18 °C to -15 °C. This is why the -3 °C line matters.
The limits of the table
The MEB table gives orders of magnitude. Because water content varies by product, a contract calculation should use the product's own analysis data. As a check, the ASHRAE composition table lists whole milk at 87.69% water and a latent heat of 293 kJ/kg; 0.8769 × 333.6 = 292.5 kJ/kg, or about 69.9 kcal/kg, close to the MEB's 70.0. Its specific heats, 0.93 above and 0.43 below freezing in kcal/kg°C, differ from the MEB's 0.90 and 0.46 by roughly 3% to 7% (1 kcal = 4.1868 kJ, NIST).
