194 Meat refrigeration Types of refrigeration system ajority of current road transport vehicles for chilled foods are refrig- using either mechanical, eutectic plates or liquid nitrogen cooling 93.1. Mechanical units Many types of independent engine and/or electric motor driven mechani cal refrigeration units are available for lorries or trailers. One of the most common is a self-contained 'plug unit which mounts in an opening pro- vided in the front wall of the vehicle. The condensing section is on the outside and the evaporator on the inside of the unit, separated by an insu- lated section that fits into the gap in the wall. Units have one or two com- pressors, depending upon their capacity, which can be belt driven from vehicle but are usually driven direct from an auxiliary engine. This engine may use petrol from the vehicle's supply, an independent tank, or liquid petroleum gas. Many are equipped with an additional electric motor for standby use or for quiet running, for example when parked or on a ferry Irrespective of the type of refrigeration equipment used, the product will not be maintained at its desired temperature during transportation unless it is surrounded by air or surfaces at or below that temperature. This is gravity, around the load. Inadequate air distribution is probably the princ usually achieved by a system that circulates moving air, either forced or ple cause of product deterioration and loss of shelf-life during transport Conventional forced air units usually discharge air over the stacked or sus- pended products either directly from the evaporator or through ducts towards the rear cargo doors. Because air takes the path of least resistance it circulates through the channels which have the largest cross-sectional area. These tend to be around rather than through the product. If products have been cooled to the correct temperature before loading and do not gen erate heat, then they only have to be isolated from external heat ingress. Surrounding them with a blanket of cooled air achieves this purpose. Care has to be taken during loading to avoid any product contact with the inner surfaces of the vehicle because this would allow heat ingress during trans- port. Many trucks are now being constructed with an inner skin that forms a return air duct along the side walls and floor, with the refrigerated air being supplied via a ceiling duct 9.3.1.2 Eutectic plates Eutectic plate cooling systems are used in refrigerated vehicles serving local distribution chains. The eutectic plate consists of a coil, through which a primary refrigerant can be passed, mounted inside a thin tank filled with a eutectic solution. Standard eutectic solutions freeze at temperatures between-3 and -50C and Table 9.2 lists some that have been applied in food chilling systems. A number of these plates are mounted on the wall and ceilings or used as shelves or compartment dividers in the vehicles. Two9.3.1 Types of refrigeration system The majority of current road transport vehicles for chilled foods are refrig￾erated using either mechanical, eutectic plates or liquid nitrogen cooling systems. 9.3.1.1 Mechanical units Many types of independent engine and/or electric motor driven mechani￾cal refrigeration units are available for lorries or trailers. One of the most common is a self-contained ‘plug’ unit which mounts in an opening pro￾vided in the front wall of the vehicle. The condensing section is on the outside and the evaporator on the inside of the unit, separated by an insu￾lated section that fits into the gap in the wall. Units have one or two com￾pressors, depending upon their capacity, which can be belt driven from the vehicle but are usually driven direct from an auxiliary engine. This engine may use petrol from the vehicle’s supply, an independent tank, or liquid petroleum gas. Many are equipped with an additional electric motor for standby use or for quiet running, for example when parked or on a ferry. Irrespective of the type of refrigeration equipment used, the product will not be maintained at its desired temperature during transportation unless it is surrounded by air or surfaces at or below that temperature. This is usually achieved by a system that circulates moving air, either forced or by gravity, around the load. Inadequate air distribution is probably the princi￾ple cause of product deterioration and loss of shelf-life during transport. Conventional forced air units usually discharge air over the stacked or sus￾pended products either directly from the evaporator or through ducts towards the rear cargo doors. Because air takes the path of least resistance it circulates through the channels which have the largest cross-sectional area. These tend to be around rather than through the product. If products have been cooled to the correct temperature before loading and do not gen￾erate heat, then they only have to be isolated from external heat ingress. Surrounding them with a blanket of cooled air achieves this purpose. Care has to be taken during loading to avoid any product contact with the inner surfaces of the vehicle because this would allow heat ingress during trans￾port. Many trucks are now being constructed with an inner skin that forms a return air duct along the side walls and floor, with the refrigerated air being supplied via a ceiling duct. 9.3.1.2 Eutectic plates Eutectic plate cooling systems are used in refrigerated vehicles serving local distribution chains. The eutectic plate consists of a coil, through which a primary refrigerant can be passed, mounted inside a thin tank filled with a eutectic solution. Standard eutectic solutions freeze at temperatures between -3 and -50 °C and Table 9.2 lists some that have been applied in food chilling systems. A number of these plates are mounted on the walls and ceilings or used as shelves or compartment dividers in the vehicles.Two 194 Meat refrigeration