Conclusiones y prospectiva

10.1.1 Descriptions of refrigerated CTUs can be found in informative material 3, section 1.3.

10.1.2 For land transport, the refrigerated semi-trailer is the most popular form of vehicle although for local deliveries and short haul operations rigid vehicles are also used. The external dimensions of European semi-trailers can be as large as 13.6m (long) x 2.6m (wide) x 2.7m (high) although in other countries they may be larger.

10.1.3 For marine use the most common type of container is the 40ft high cube integral refrigerated container, which has an inbuilt refrigeration unit similar to the refrigerated semi-trailer. The smaller 20ft version is available but only constitutes 7% of the world’s refrigerated fleet. 10.1.4 As with all types of transport equipment, there are mass restrictions which may limit the

volume of the more dense produce which can be carried. This is more often found with frozen cargo.

10.2 How does a mechanically refrigerated vehicle work?

The refrigeration unit fans cause temperature controlled air to circulate around the inside of the vehicle floor, walls, doors and roof to remove heat which is conducted from the outside. Some of the air should also flow through and between the cargo, particularly when carrying fruit and vegetables, where heat of respiration may be a significant proportion of the heat load. The various components of the heat load of a refrigerated CTU are given in Figure 9.11.

Figure 8.11 Heat load of a refrigerated CTU 10.3 Top air delivery systems

Top air delivery is used predominately on refrigerated semi-trailers. Air is ducted from the refrigeration unit to the end of the vehicle or passes through and around the load returning via the floor or space under pallets. For chilled cargoes horizontal channels are required between rows of cartons to allow good return airflow through the load, whereas block stows are recommended for hard frozen cargoes that have been fully precooled. Some trailers are fitted with a false bulkhead wall with metal grill or holes in the lower part for return air passage. The cargo may be stacked against this bulkhead. Where return air bulkheads are not used it is a common practice to set wooden pallets on end between the front wall and the front of the load thus creating a return air channel.

Figure 8.12 Top air delivery reefer

Refrigeration unit Latent heat of condensation of water Air cooler Fan heat Heat flowing into CTU from outside Temperature

inside Temperature outside

Temp

Time

Floor

10.4 Bottom air delivery systems

10.4.1 Bottom air delivery is generally used in marine containers. Air is blown through the evaporator into a plenum chamber, which distributes the flow evenly across the width of the floor. Depending on the stowage pattern the air passes along the floor to be circulated up through and around the stow returning via the roof space. With respiring cargoes, the most even temperature distribution is attained if the load completely covers the floor and the packaging or dunnage has been designed to allow a high proportion of the air to circulate through the load as well as around it. Where precooled frozen cargoes are concerned, a block stow is acceptable as only the heat from the container fabric has to be removed.

Figure 8.13 Bottom air delivery reefer

10.4.2 The heat, gained by the air as it circulates around the vehicle, is removed in the evaporator section. The air also picks up moisture from the produce and also from air from the refreshing vents when in use in ambient conditions with high humidity. This is deposited on the evaporator as water or ice, depending on the coil temperature. When ice is formed the air flow through the evaporator becomes restricted and defrosting becomes necessary when the flow falls to 75% of the frost free rate.

10.4.3 The rate of air circulation within the vehicle is equivalent to 60 to 90 air changes per hour of the empty volume. Some container operators are increasing the rate to 120 for chilled cargoes. Under maximum summer temperatures of 30°C and 0°C set point, for example, the range of air temperatures would be about 1.5°C at full speed and 2.5°C at half speed on 40ft semi-trailers. Tighter tolerances are achieved on marine containers where a 1°C spread would not generally be exceeded.

10.5 Floor designs

10.5.1 There are generally four alternatives available, a T-bar section floor, a castellated section floor, a perforated floor or the pallet.

10.5.2 T-bar section floors - cause minimum obstruction to air flow, but can be damaged by fork lift trucks and are difficult to keep clean.

10.5.3 Castellated floors - some obstruction to flow of air and increased pressure drop, very strong and easy to clean.

10.5.4 Perforated floors - used traditionally in refrigerated ships and have been modified for use in containers. Give less obstruction to air flow and better distribution in the container than castellated. Difficult to clean unless removable.

10.5.5 Pallets - may be used with flat floors which are easily cleaned.

10.5.6 Road vehicles generally use flat checker plate or G.R.P. floors and marine containers are fitted with T-bar section floors.