Thirty-Eight Degrees in the Trailer, Forty-Eight Hours to Destination: Doing the Heat-Load Sum

A three-pallet consignment of chilled salmon left a packing house in Adelaide at 6 p.m. on a Thursday with eight 1 kg gel packs per 20-litre box, a configuration that had worked all winter. It reached Perth on Saturday with the fish at 7.4 degrees and two boxes rejected. The forecast had read 38 degrees for both days, and nobody had rerun the numbers since March. The packs had not failed; the box had been asked to carry more heat than it was sized for.

Most cold-chain sizing arguments are settled by a single sum that takes ten minutes and a manufacturer’s UA figure. This article walks through that sum for a 48-hour summer lane, shows where the simple answer is optimistic, and sets out how to add margin without doubling your gel spend.

The Only Two Numbers the Box Cares About

Every insulated shipper has a UA value, the overall heat transfer coefficient in watts per kelvin, usually quoted by the manufacturer and typically between 0.5 and 1.1 W/K for a 20-litre expanded polystyrene box and 0.35 to 0.6 for a vacuum panel or thick PUR box. Multiply UA by the temperature difference between outside and inside and you have the heat leak in watts. A 20-litre EPS box at 0.8 W/K, sitting in a 38 degree trailer with a 4 degree payload, sees a 34 kelvin difference and therefore 27 watts of continuous heat ingress.

Over 48 hours that is 27 multiplied by 48, or 1,310 watt-hours, which is 4.7 megajoules. Melting one kilogram of ice absorbs 334 kilojoules, so the box needs about 14 kg of ice-equivalent just to stand still, before you account for pulling the payload down from 6 degrees to 4 and for the packs themselves warming.

Fourteen Kilos of Ice-Equivalent Is Not Fourteen Kilos of Pack

A gel pack is not pure ice. A typical 1 kg pack is roughly 99 percent water plus SAP and salts, so call it 0.95 kg of freezable water, and it also arrives with sensible heat: from minus 18 to 0 degrees a pack absorbs about 38 kilojoules per kilo as it warms, which is worth another 11 percent. That gets you to roughly 12 packs for the standing load. Add 15 percent for the payload pull-down and door openings at the depot, and the practical answer for this lane is 14 packs of 1 kg per box, not eight.

The catch is that 14 packs of 1 kg will not fit in a 20-litre box alongside 10 kg of fish. This is where buyers discover that the box, not the SAP, is the binding constraint: the honest options are a bigger box, a thicker wall, a phase-change pack with a colder plateau, or splitting the consignment into 24-hour legs with a re-ice stop.

Where the Plain Sum Is Too Optimistic

Three effects push real hold time below the calculated figure. Solar loading on a trailer roof can add 20 to 30 percent to the effective ambient for the top layer of cartons, so pallet position matters and the top tier is always the first to fail. Door openings at consolidators dump a slug of warm humid air into the box in seconds, and every opening costs minutes of stored cold. And gel packs at the centre of a tightly packed box give up their cold more slowly than packs against the wall, so the average is not the minimum.

A practical fix is to size at 42 degrees rather than the forecast 38 and to place the last two packs on the top face, where the heat arrives. That costs about 15 percent more gel per box and removes most of the surprise.

What to Write on the Lane Sheet

Record the UA value and its source, the design ambient, the transit hours including dwell, the payload mass and its starting temperature, and the resulting pack count with the margin stated separately. Then validate once with two data loggers, one in the payload centre and one against the lid. If the lid logger peaks more than 2 degrees above the payload, your margin is in the wrong place and the top packs need to move.

For lanes above 30 degrees ambient or beyond 36 hours, the SAP itself also needs a second look, because gel that releases water under stack load loses cold contact with the payload. The super absorbent polymer powder behind a 48-hour pack needs high absorbency under load, not just a high free swell figure, and that is the difference between a pack that still has shape at hour 40 and one that has slumped into a puddle.