What a blast freezer actually has to remove
A blast freezer is not judged by how cold it gets. It is judged by how many kilograms of a specific product it can take from a specific starting temperature to a specific core temperature in a stated time. Those four variables are the whole specification, and any quotation missing one of them is not a specification at all.
The load to be removed has three parts: sensible heat above freezing, the latent heat of crystallisation, and sensible heat down to the final core temperature. On a cooked product the first part is often larger than people expect, because the product enters the cabinet hot.
Taking 500 kg of cooked product from 65 °C to −18 °C core, with reasonable values for specific heat and latent heat, needs on the order of 260 MJ to be removed from the product itself. Add frames, trays and cabinet structure and the real figure sits nearer 285 MJ. That is the number the refrigeration system has to deliver — not the cabinet volume, and not the nameplate.
Why the cheap unit is slower than its badge
Two cabinets can carry the same compressor and still differ by a factor of two in real throughput. The difference lives in the evaporator and the airflow, not the compressor.
- Undersized evaporator. A smaller coil cannot absorb the peak load in the first hour, when the product is hottest and the load is highest. The compressor runs continuously and the product still takes twice as long.
- Air short-circuiting. If supply and return are poorly placed, cold air takes the easy path across the top of the cabinet instead of through the stack. The trays nearest the return freeze; the ones at the back do not.
- Ice build-up. A cabinet that freezes fast also frosts fast. Without an adequate defrost cycle, coil blockage quietly removes capacity over a shift.
- Door discipline. Every opening restarts the pull-down. A cabinet sized with no margin has nothing left to catch up with.
Air-blast, plate and tunnel: picking the right type
| Type | Best suited to | Typical cycle | Where it struggles |
|---|---|---|---|
| Air-blast batch | Mixed products, small batches | 2 - 6 h | Loose or lightweight product |
| Plate contact | Flat, regular blocks | 1 - 3 h | Irregular shapes, thin items |
| Tunnel / continuous | Single high-volume product | 20 - 60 min | Product changeovers |
| Blast chiller | Cook-chill, above 0 °C | 60 - 120 min | Freezing duty |
The commonest buying error is a batch air-blast cabinet chosen for a product that never changes. If one product runs all day at volume, a tunnel will do the same job with far less handling and far less energy per kilogram.
Worked payback on a 500 kg batch
Take two options. A cheaper cabinet with a nominal 12 kW of refrigeration capacity, and a properly sized unit at 25 kW. Illustrative figures, but the structure of the arithmetic is what matters.
| Item | Undersized unit | Correctly sized unit |
|---|---|---|
| Batch handled per cycle | 500 kg | 500 kg |
| Cycle time to −18 °C core | about 8 h | about 4 h |
| Cycles per 24 h shift | 2, with little margin | 4, with margin |
| Daily throughput | 1 000 kg | 2 000 kg |
| Product above −5 °C during freeze | long window | short window |
| Compressor duty during pull-down | continuous, saturated | continuous, within rating |
The undersized cabinet is not half the throughput for half the price. It is half the throughput, with the compressor running saturated for most of the shift, and a longer period during which the product is passing through the temperature range where ice crystal size is decided. In most food businesses the quality difference and the compressor warranty cost land before the purchase-price saving does.
The five questions that produce a comparable quote
- kilograms per cycle, and product type (including fat and moisture content)
- starting temperature and required final core temperature
- maximum time allowed per cycle
- evaporating temperature and compressor model number
- defrost method and expected cycles per day
Sizing with margin, not hope
A blast freezer is a peak-load machine. Averaging its duty over a shift is the fastest way to buy something that never quite performs. We size the evaporator for the first hour of the cycle, when the incoming product is at its hottest and the coil is clean, then confirm that the compressor can hold that duty at the design ambient without exceeding its rating.
If your batch size, product mix or ambient is unusual, send us the four variables above. We will come back with a capacity figure and the curve behind it, so the two options can be compared on throughput rather than on price per box.
Liang leads the refrigeration test bench at Dixue and has spent nineteen years matching compressors to cabinets.
