How long an ice pack lasts is a race between the cold the pack has stored and the heat entering the container from outside. The stored cold depends on the pack type, the number of packs and whether they are fully frozen; the heat coming in depends on the container's insulation, the outside temperature and lid openings. A figure in a catalogue was measured for one combination of these. GEBHARDT also states that transport time depends on the type and number of plates and calculates it per application; you can request that calculation from Aris Makina with your route and product details.
| Factor | Effect on the time |
|---|---|
| Container insulation | Makes the largest difference; heat enters wherever insulation is weak |
| Outside temperature | The bigger the inside-outside gap, the faster heat enters |
| Pack type and number | Sets the total stored cold |
| Lid openings | Each opening lets cold air out |
| Starting load temperature | A warm load uses up the first hours of the pack |
Container and pack are one system. The pack melts to answer the heat entering the container, so the cheapest way to extend the time is often a well-seated lid and a thicker wall rather than more packs. Each pack added to a thin-walled bag answers the leaking heat for a while longer but does not stop the leak.
The product stores cold too. The WHO guide on shipping containers notes that the load with the lowest thermal mass is the most sensitive to temperature change; a half-empty container warms faster than one filled with the same product. Product should enter the container at its target temperature, because every hour spent cooling a warm load is taken off the journey.
A pack that is not fully frozen never took on part of the cold it could store and runs out early. For GEBHARDT plates the freezer must run at least 5 °C below the melting point, and the recommended freezing time is 24 hours. A plate that leaves before meeting this condition is one of the common reasons the time falls short.
Measure your own duration
- prepare the container with a normal shipment load and your usual number of packs
- place one logger at the most sensitive point of the load, in many containers a top corner, and one outside the container
- set the logger to record at least every 30 minutes, preferably every 5-10 minutes
- for freeze-sensitive goods, also monitor the point closest to the packs; dropping below the lower limit is an excursion too
- open the lid as often as in the field and note the time of each opening
- record the hour at which the target is first exceeded
- repeat the trial three times in the hottest and coldest season, with the smallest and largest load
If the first excursion came at hour 9, plan the route shorter than 9 hours with a margin. If the time falls short, work through this order: gasket and lid seating first, then pre-cooling of the product, then the number of lid openings, and pack quantity last. Do not count a change as an improvement until the same trial has been repeated.
