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Direct cool is one of the two major types of techniques used in domestic refrigerators, the other being the "frost-free" type. Direct-cool refrigerators produce the cooling effect by a natural convection process. However, as opposed to frost-free units, direct-cool units require manual defrosting of the interior. Direct cool is less expensive in production and in operation, as it consumes less energy.
Direct cool refrigerators generally have the cooling compartment inside the fridge. The water vapour from the fruits and vegetables stored exert pressure on the walls of the cooling unit and freeze. Hence, the frost is formed.
Video Direct cool
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Source of the article : Wikipedia