Transportation 195 Table 9.2 Freezing point and latent heat of fusion of ome eutectic solutions Eutectic solution Freezing Latent heat of oint(C) fusion(kWhm-) Carbonate of soda Potassium bicarbonate Potassium chloride Ammonium chloride Sodium chloride ource: Lenotre. 1988. methods are commonly used for charging up the plates: (1) when the vehicle is in the depot the solutions are frozen by coupling the plates to stationary refrigeration plants via flexible pipes and(2)a condensing unit on the vehicle is driven by an auxiliary drive when the vehicle is in use and an electric motor when stationary To provide the required cooling capacity, the plates should be mounted so that air can circulate freely over both sides and over the product. Most systems rely on gravity circulation but some are equipped with fans, ducts and dampers for temperature control Eutectic systems are chosen for the simplicity, low maintenance and quietness of their operation but can suffer from poor temperature control. 9.3.1.3 Liquid nitrogen A typical liquid nitrogen system consists of an insulated liquid nitrogen storage tank connected to a spray bar that runs along the ceiling of the transport vehicle. Liquid nitrogen is released into the spray bar via a ther mostatically controlled valve and vaporises instantly as it enters the body of the vehicle. The air is then cooled directly utilising the change in the latent and sensible heat of the liquid nitrogen. Once the required air tem- perature has been reached the valve shuts off the flow of liquid nitrogen and the temperature is subsequently controlled by intermittent injections of liquid nitrogen Many advantages are claimed for liquid nitrogen transport systems (Table 9.3). It is also claimed thatthat long hauls can be carried out sin vehicles are available that will maintain a chilled cargo at 3C for 50h after a single charge of liquid nitrogen and that overall costs are comparable with mechanical systems. 9.3.2 Observations of transport Gill and Phillips(1993)found that the deep temperature in beef sides and quarters at the time of their loading into transport vehicles in three USA plants ranged from 6 to 18C(Fig 9. 1). Maximum surface temperaturesmethods are commonly used for charging up the plates: (1) when the vehicle is in the depot the solutions are frozen by coupling the plates to stationary refrigeration plants via flexible pipes and (2) a condensing unit on the vehicle is driven by an auxiliary drive when the vehicle is in use and an electric motor when stationary. To provide the required cooling capacity, the plates should be mounted so that air can circulate freely over both sides and over the product. Most systems rely on gravity circulation but some are equipped with fans, ducts and dampers for temperature control. Eutectic systems are chosen for the simplicity, low maintenance and quietness of their operation but can suffer from poor temperature control. 9.3.1.3 Liquid nitrogen A typical liquid nitrogen system consists of an insulated liquid nitrogen storage tank connected to a spray bar that runs along the ceiling of the transport vehicle. Liquid nitrogen is released into the spray bar via a ther￾mostatically controlled valve and vaporises instantly as it enters the body of the vehicle. The air is then cooled directly utilising the change in the latent and sensible heat of the liquid nitrogen. Once the required air tem￾perature has been reached the valve shuts off the flow of liquid nitrogen and the temperature is subsequently controlled by intermittent injections of liquid nitrogen. Many advantages are claimed for liquid nitrogen transport systems (Table 9.3). It is also claimed that that long hauls can be carried out since vehicles are available that will maintain a chilled cargo at 3 °C for 50h after a single charge of liquid nitrogen and that overall costs are comparable with mechanical systems. 9.3.2 Observations of transport Gill and Phillips (1993) found that the deep temperature in beef sides and quarters at the time of their loading into transport vehicles in three USA plants ranged from 6 to 18 °C (Fig. 9.1). Maximum surface temperatures Transportation 195 Table 9.2 Freezing point and latent heat of fusion of some eutectic solutions Eutectic solution Freezing Latent heat of point (°C) fusion (kW h m-3 ) Carbonate of soda -3 92 Potassium bicarbonate -6 85 Potassium chloride -10 94 Ammonium chloride -15 89 Sodium chloride -21 74 Source: Lenotre, 1988