In the tunnel cold air is forced to flow around fruit trough the vented side of the container. Generally the containers are placed in the cooler so that the air must pass through the
containers before returning to the evaporator surface (Fig.22). Precooling time is reduced increasing the air speed but an air speed over 3.5 m/s could injury the berries and break the paper use for packing, dramatically improving the energy consumption and plant cost (Fig.23 and 24). Using an air speed over 1 m/s is possibile to cool much more grape per day than in room cooling. Due to the short cooling time, despite the high air speed, the water loss rate from the fruit is negligible.
In tunnel cooling heat is carried away primarily by flow of air trough the bunches inside the containers rather than by flow past the outside of the containers as in room cooling. Using high air speed value and container adequately vented, more than a single row of pallet can be cooled at the same time. In this case air movement is always from colder fruit upstream to warmer fruit downstream, which avoids the sweating, but this flow cause a gradient of temperature among the parallel rows of pallets.
The use of precooling tunnel reduces cooling time, removing very fast the field heat. Potential for bunches deterioration is reduced and soon after the treatment, grapes can be stored or shipped in the best condition.
Compare to room cooling, the plant and energy cost are higher but, considering that they are specialized plant, the space required for a specific volume of product and the refrigeration losses are much more lower than in room cooling. Anyway, the use of this method requires higher cost for product movement if it is assigned to storage. Adeguate package design is required in relation to the fixed air speed and cooling time.