On a continuous production line the product arrives without stopping; carrying it to a blast freezer on trolleys interrupts the line. In a spiral freezer the product goes onto the belt, the belt travels round a rotating drum in 20–40 helical tiers, and the product comes out frozen at the other end. A few hundred metres of belt fit on a floor area of a few square metres; this is its advantage over a straight tunnel freezer. The evaporator and fans are inside the enclosure, beside the belt tiers. The infeed and outfeed can be arranged on the same side or opposite each other.
The air temperature is −30 to −40 °C and the air velocity over the product 3–6 m/s. The residence time is set by the belt speed and varies between 15 and 90 minutes according to the thickness of the product: thin meatballs and burgers 20–30 minutes, chicken pieces and börek 40–60 minutes, thick ready-meal trays longer. Capacities range from 200 kg an hour to more than 5 tonnes. The belt width is 400–1,000 mm; it may be stainless steel or modular plastic.
Suitable products are those frozen individually or in trays whose shape has to be preserved: meatballs, nuggets, chicken pieces, fish fillets, mantı, börek, pizza, pastry products, ice cream and ready meals. For small-grained products such as peas and sweetcorn a fluidised bed IQF tunnel is more suitable. With unpackaged product the water loss is around 1–2%; the lower the air temperature and the shorter the time, the smaller the loss. For sticky and soft products a rapid crust freeze on the belt is provided at the infeed.
Because the cooling load is large and the evaporating temperature low, two-stage, economiser or CO₂ systems are used; the evaporator has wide fin spacing. The moisture entering with the product and through the open infeed and outfeed openings forms frost on the evaporator; the running time is usually limited to 16–20 hours, after which come defrost and washing. If the air balance at the infeed and outfeed openings is not set up correctly, warm air is drawn in and frosting accelerates. The capacity is determined by a cooling load calculation according to the entry temperature of the product.