Heat enters a container through its outer surface, while cold is stored in its internal volume. As a container grows, its volume rises faster than its surface: double the edges of a box and the surface goes up four times, the volume eight times. With the same wall thickness and the same eutectic plate, the leak surface of a small container is larger relative to the cold it stores, so it loses that cold sooner.
The ratio is the outer surface area divided by the volume worked out from the inner dimensions. For two wheeled models of the GEBHARDT Isotec TBX it comes out like this.
| Model | Outer size (mm) | Outer surface | Inner volume | Surface / volume |
|---|---|---|---|---|
| TBX 410 | 580 x 803 x 1799 | 5.9 m2 | 0.38 m3 | 15.4 |
| TBX 795 | 757 x 954 x 1929 | 8.0 m2 | 0.77 m3 | 10.5 |
A small container is not a poor one. For the same hold time it needs more cold source per unit of volume, and in delivery work it is refreshed often with new plates. Wall thickness shifts the balance too: a thick wall slows the heat but eats into inner volume, and in a small container that loss weighs more. Which size in the range from the TBX 410 up to 1500 litres suits a job is set by the length of the route and the volume of the load; once you send us those, we choose the container size and the number of plates together.
