1,200 Packs a Shift: Sizing Freezing Capacity for a Packing House

Twelve hundred 500 g packs a shift is 600 kg of gel that has to go from 20 °C in the filling room to -18 °C at the core before the first truck leaves at 05:00. That is not a volume problem, it is a kW problem, and buying freezer space without doing the kW sum is how packing houses end up with packs that read -3 °C at the centre when the pallet is loaded.

The good news is that the sum is short. The bad news is that most quotes for blast freezers are written in cubic metres, which tells you nothing about whether the unit can pull the heat out in the hours you have.

The heat you have to remove

For 600 kg of a water-based gel in a 10-hour window: sensible cooling from 20 °C to 0 °C is 600 × 20 × 4.2 = 50,400 kJ. Latent freezing is 600 × 334 = 200,400 kJ. Total 250,800 kJ over 10 hours is 25,080 kJ/h, which is 6.97 kW of refrigeration. Add 25-30 percent for door openings, fan heat, frost on the coil and the fact that a blast freezer rarely runs at rated capacity, and you need roughly 9 kW of installed duty.

As a working rule: 1 kW of refrigeration duty freezes about 170 packs of 500 g in a 10-hour overnight pull. That single line has settled more capacity arguments than any datasheet.

Why a bigger box is not a bigger freezer

A 12 m³ blast freezer rated at 8 kW and a 20 m³ unit rated at 8 kW will freeze the same mass at the same rate. The larger unit will take longer to pull down after a door opening, because there is more air and more surface to re-cool. Volume buys you loading convenience and air circulation space; it does not buy duty.

What volume does affect is stacking. Packs stacked more than eight deep in still air insulate each other, and the middle of the stack will lag the outside by 6-8 hours. If your freezer is full to the ceiling, you have effectively derated it by a third.

The failure that gets blamed on the SAP

A plant in Ecuador upgraded to a higher-absorbency SAP, and the packs started coming out soft in the middle. The SAP was fine. What had changed was the fill: the new grade held more water per kilo, so each pack carried 30 g more mass, and 1,200 packs became 636 kg instead of 600 kg. That is a 6 percent increase in heat load, on a freezer that was already running at 95 percent of duty in a 30 °C room.

The fix was not a different polymer. It was an extra 90 minutes of pull time and splitting the load into two batches. Two weeks and one argument with the freezer supplier, all because the fill weight changed and nobody re-ran the kW sum.

The same thing happens when a plant switches from 400 g to 500 g packs without changing the count per tray. The freezer was sized for 3,000 units and is now asked to pull 1,500 kg. Weight is the unit that matters, not pieces, and a production plan written in pieces will understate the load by 25 percent every time.

Practical checks on the shop floor

• Probe the core of three packs from the centre of the stack, not the surface, at the end of every pull
• Log the pull start and end times on the batch sheet; if the pull is stretched, the freezer is short of duty
• Keep the evaporator clear of frost; 3 mm of frost on the coil can cost 10-15 percent of capacity
• Do not load warm packs into a freezer that is already holding product for dispatch
• Leave 100 mm between pallets for air movement, even if it costs you a row

If you are adding a second shift, re-run the sum rather than the schedule. Doubling the packs without doubling the pull window is the same as halving the freezer, and the packs will tell you so at the worst possible hour.

For a new line, size the freezer against the fill weight you intend to run, not the one you are running today — the logistics ice pack guide has a worksheet that ties fill weight to pull time.