Packs per Night: Freezer Capacity Math for a 48-Hour Chilled Lane

A pack maker won a contract for 9,000 packs a day and had a 12 cubic metre blast freezer the sales team believed could handle it. In practice the freezer turned out 4,200 packs in 24 hours, and the shortfall was not compressor power: it was the 22 mm thickness of the pack and the way crates were stacked, which left the centre of each crate at minus 6 while the surface read minus 24.

Freezer capacity planning for gel packs is a heat transfer problem disguised as a scheduling problem. The number that matters is not how many packs fit in the room, but how many reach core temperature before the truck leaves. Here is how to work that out and where the usual plans break.

Freezing a Kilo of Gel Takes How Much Energy

Start with the physics. A 1 kg gel pack at 20 degrees must lose sensible heat down to 0 (about 84 kilojoules), then the latent heat of freezing (roughly 320 kilojoules for a 96 percent water gel), then a further 38 kilojoules to reach minus 18. Call it 440 kilojoules, or 0.12 kilowatt-hours per pack, ignoring losses.

A 12 cubic metre freezer with 6 kW of compressor capacity at minus 25 should therefore freeze about 50 packs an hour on paper. Real rooms deliver 30 to 40 percent of that, because heat has to travel through the pack and then out through the air, and air is a poor conductor. The pack surface reaches setpoint in an hour while the core takes many more.

The Number That Governs Throughput: Half-Thickness Squared

Freezing time scales roughly with the square of the half-thickness of the pack divided by the thermal diffusivity of the gel. That is why a 30 mm pack takes more than twice as long as a 15 mm one, and why flat packs are the single biggest capacity lever available to a pack maker. A 1 kg pack at 22 mm in still air at minus 25 needs about 11 hours to reach minus 18 at the core; the same weight at 14 mm needs about 5.

Air speed matters almost as much. At 0.5 m/s the surface film coefficient is low and the pack sits in its own cold boundary layer; at 3 m/s, typical of a blast freezer with good fan placement, freezing time drops by 35 to 45 percent. A cheap way to gain capacity is to rearrange crates so air can move, not to buy another compressor.

Working Out Packs per Night for a Real Room

Take the room’s usable crate volume, not its gross volume. A 12 cubic metre room typically holds 60 percent in crates and clearance, so 7.2 cubic metres. At 1 kg packs in 20-pack crates with 30 percent void, that is roughly 5,000 packs per load. If core freezing takes 11 hours in that room, and you need 3 hours for loading, unloading and the room to recover, you get one full load per 14 hours, or about 8,500 packs a day in two shift-aligned cycles.

That number changes completely if the packs are 14 mm: 5 hours plus 2 hours of handling gives three cycles a day, or about 15,000 packs. Same room, same compressor, nearly double the output, because the product design changed.

Where the Plan Breaks, and What to Ask a Freezer Supplier

Three failure modes. Crates stacked tight against the evaporator coil block airflow to the middle of the room, so half the load is still soft at hour 12. Freezer doors opened every twenty minutes for picks destroy the room temperature and add hours to every cycle. And packs loaded warm from a 30 degree filling room carry 4 percent more heat than packs from a 20 degree room, which adds roughly 40 minutes per cycle and compounds across the day.

The fixes are unglamorous: leave 150 mm between crates, batch picks into two windows per shift, and hold filled packs in a 15 degree staging area before they go into the freezer.

Ask for the pull-down time for a defined load in kilograms of water at a defined starting temperature, not the room’s air temperature. Ask what air speed is measured at the crate face. And ask for the recovery time after a door opening, because that is the number your nightly throughput actually depends on.

The other half of the equation is the pack itself. A formulation that holds its water and freezes into a firm block delivers the rated hold time; a weak gel freezes into a slush with a soft core and gives back hours. Our SAP grades for ice packs are specified with freeze curves at two thicknesses, which makes the capacity planning above a calculation rather than an experiment.