234 Meat refrigeration Fig. 11.1 Multi-deck display cabinet for wrapped products The duct provides two functions: (1)to provide cold air through the holes n the rear panel and(2)to provide an air curtain at the front of the cabinet The holes in the rear panel direct chilled air over the food and the air curtain provides a thermal barrier between the chilled display area and the like perfect' cabinet would have its chilled air form a closed cycle, much like a domestic refrigerator when the door is closed. In reality warm mois air from the surrounding store entrains with chilled air from the air curtain causing a loss of chilled air from the cabinet and a gain of warm air and moisture 11.1.3 Air curtain The air curtain differs from a solid door as it provides no physical barrier between customer and product, but is similar to a door in that it does provide a thermal barrier. The air curtain is a jet of chilled air of about l ms that exits the duct at the top of the cabinet and falls down the face of the cabinet to the return grille. Owing to the fact that the temperature of the air from the air curtain is lower than the surrounding air it is denser and therefore is aided by natural convection in its downward motion. The air curtain is very sensitive and its effectiveness has other implica tions. An ineffective air curtain is likely to have the following effects Increased icing up and therefore more defrosts of the refrigeration coilThe duct provides two functions: (1) to provide cold air through the holes in the rear panel and (2) to provide an air curtain at the front of the cabinet. The holes in the rear panel direct chilled air over the food and the air curtain provides a thermal barrier between the chilled display area and the store. A ‘perfect’ cabinet would have its chilled air form a closed cycle, much like a domestic refrigerator when the door is closed. In reality warm moist air from the surrounding store entrains with chilled air from the air curtain causing a loss of chilled air from the cabinet and a gain of warm air and moisture. 11.1.3 Air curtain The air curtain differs from a solid door as it provides no physical barrier between customer and product, but is similar to a door in that it does provide a thermal barrier. The air curtain is a jet of chilled air of about 1ms-1 that exits the duct at the top of the cabinet and falls down the face of the cabinet to the return grille. Owing to the fact that the temperature of the air from the air curtain is lower than the surrounding air, it is denser and therefore is aided by natural convection in its downward motion. The air curtain is very sensitive and its effectiveness has other implica￾tions. An ineffective air curtain is likely to have the following effects: • Increased temperature of product, • Increased icing up and therefore more defrosts of the refrigeration coil, 234 Meat refrigeration Fig. 11.1 Multi-deck display cabinet for wrapped products