Take a sample parcel: 300 x 200 x 150 mm, with an outer surface of 0.27 m². The outside temperature is 30 °C, the product must stay at 2 °C and transit takes 6 hours. Neither the depot nor the van may refrigerate, so the ice inside absorbs the heat that passes through the parcel surface. You can work out how much ice is needed.
How much ice melts. Heat gain is Q = U × A × ΔT. U = λ / d, where λ for foam is 0.031 W/mK and d is the wall thickness. ΔT = 30 - 2 = 28 K. The heat of fusion of ice is 333.5 kJ/kg. Divide the total heat gain by this figure to get the kilograms of ice that melt:
| Wall thickness | U, W/m²K | Ice melted in 6 h | Ice melted in 24 h |
|---|---|---|---|
| 20 mm | 1.55 | 0.76 kg | 3.0 kg |
| 31 mm | 1.00 | 0.49 kg | 2.0 kg |
| 62 mm | 0.50 | 0.24 kg | 1.0 kg |
The calculation assumes a single uniform foam wall, with no openings, no leakage and no difference at the base. A real parcel departs from these conditions, but the table gives the order of magnitude. EPP or foam of another density has a different λ; enter the declared value of your own material. For gel packs, run the same sum with the maker's latent heat rather than ice's.
With the outer size held fixed, every extra millimetre of wall comes out of the inner volume. Going from 20 mm to 31 mm cuts the heat gain by about 35%, and from 31 mm to 62 mm by 50%. The table therefore also shows the trade-off between heat and space.
Dry ice needs its own rules. Dry ice (solid carbon dioxide) is very cold and releases gas as it sublimates. In the US, 49 CFR 173.217 requires packaging that lets the gas escape and the net mass to be marked on the outside of the package. IATA Packing Instruction 954 sets the same venting requirement for air transport and a limit of 200 kg per package. A sealed, airtight box therefore fails; inform the carrier before each shipment.
The target temperature sets the packing threshold. The canteen hygiene guide of the Turkish Ministry of Agriculture and Forestry keeps chilled food at 4 °C or below and frozen food at -18 °C or below. Use these two figures as the reference for a courier parcel too. The packing order is as follows:
- Cool the product to 4 °C before packing; warm product directly shortens the time the ice takes to melt.
- Put the coolant above the product and beside it, with a thin separator between them so the product does not freeze.
- Fill the empty space with foam padding; air in a gap carries heat and lets the product shift.
- Place the label under a clear sleeve so that condensation does not make it unreadable.
Send a temperature logger inside the parcel as an ordinary courier shipment and read it on arrival. The duration that comes out is the real capacity of that parcel, on that route, in that season. Set it against the table above and you will know how much ice to add next time.