When insulation and cold sources are calculated, the discussion is usually about heat entering through the walls. The load carries heat of its own, and the size of it often comes as a surprise. Turkey's Ministry of National Education module on selecting refrigerated vehicles collects the thermal data of foods in a table given in kcal.
The two columns in the table. Specific heat is the heat needed to warm or cool one kilogram of product by one degree, given separately for before and after freezing. The heat of freezing is what the water inside the product needs to change phase, and it is taken without any change in temperature. The second number is far larger than the first.
| Product | Specific heat, above freezing | Below freezing | Heat of freezing |
|---|---|---|---|
| Milk | 0.90 kcal/kg°C | 0.46 | 70.0 kcal/kg |
| Beef | 0.75 | 0.40 | 55 |
| Chicken | 0.70 | 0.42 | 71 |
| Shrimp | 0.81 | 0.43 | 60 |
| Cheese | 0.44 | 0.31 | 30.0 |
| Butter | - | 0.25 | 13 |
| Egg | 0.73 | 0.40 | 52 |
| Lettuce | 0.96 | 0.48 | 76 |
Working it through one trip
Say 600 kg of milk has to come down from +8 °C to +2 °C. Milk does not freeze here, so the above-freezing figure applies.
600 × 0.90 × 6 = 3,240 kcal. Converted to kilojoules, 3,240 × 4.1868 ≈ 13,565 kJ, roughly 3.8 kWh.
Freezing the same milk would be a different sum: 600 × 70 = 42,000 kcal ≈ 175,800 kJ, roughly 48.8 kWh. The ratio is thirteen to one.
Cooling by six degrees is one job. Changing phase is another job entirely, and transport equipment does the first.
Once those two lines are calculated, the question of when a refrigerated vehicle is necessary stops being guesswork.
Why that is a practical result
Freezing does not happen on the road. A vehicle or a container is built to hold a load at its target temperature; its heat-removal capacity is small next to a blast tunnel at the plant. Any round that plans to load warm goods and cool them in transit is asking equipment to do work it cannot do. That is why pre-cooling has to be finished before loading rather than during it.
On a frozen load a small thaw costs a lot
If 500 kg of frozen chicken rises from -18 °C to -15 °C, the below-freezing figure gives 500 × 0.42 × 3 = 630 kcal. A small number. But if 10 kilograms of that load crosses its freezing point and begins to thaw, the heat of freezing in the same table gives 10 × 71 = 710 kcal. A two per cent thaw is a larger movement of heat than warming the entire load by three degrees.
That is what makes the -18 line so decisive on a temperature chart. A swing below the line changes the product's heat; a swing across the line changes its structure.
The limits of the table
The values in the module are guide values. Water content in the same product varies with season, breed and processing, and the heat of freezing depends directly on water content. A calculation destined for a contract uses the product's own analysis. For choosing equipment, the order of magnitude is enough.
