A frozen storage room keeps frozen product frozen; a blast freezer freezes fresh product. The difference is large: bringing 1 tonne of meat from +4 °C to −18 °C takes far more cooling than keeping the same meat frozen for a month, because a large latent heat is removed during freezing. That is why a blast freezer is fitted with an evaporator and condensing unit 5–10 times more powerful for its volume.
Speed is the key to quality: in slowly frozen product, large ice crystals rupture the cell walls, and on thawing there is water loss and texture damage. With −35 °C air and an air velocity of 3–5 m/s the product core reaches −18 °C in 4–12 hours (depending on thickness). The evaporator has wide fin spacing (8–12 mm), high air flow and hot gas or powerful electric defrost; the compressor is a low-temperature series, usually two-stage or with an economiser; the panels are 150–200 mm.
A blast freezer is usually built together with a storage room: the product is blast frozen, then moved to the −20 °C store. In tunnel-type blast freezing the product passes through on trolleys or a conveyor; IQF systems freeze small pieces individually. The "blast freezer" preset in the cooling load calculator gives the capacity for the daily quantity of product to be frozen and the freezing time.