Where the product shape is irregular, the only way to extract its heat will be by using a cold fluid surrounding it. Air is the obvious choice – it is economical,hygienic and relatively non-corrosive to equipment. The air temperature will be of the order of –40°C and the air speed over the product will be high, to get good heat transfer. Circulation of air around stacked product in a cold room is used for batch processes. This requires good air distribution and an optimum value exists between the decrease in freezing time and the increasing power required to drive the fans to produce higher air speeds