Every day people buy fruit and vegetables. A problem with these produces is that they are usually not locally grown, but they have often undergone long journeys. Because fruit is perishable, they easily become damaged and rot during a long transport. The most cost effective way to extend the shelf life of fruit is by cooling it. Since it is desirable that fruits are in the same state as if they were freshly harvested, it is important that they are cooled to their optimum temperature as quickly as possible after they have been harvested. The most common pre-cooling method is forced air cooling.

The work has been done in collaboration with Billerud Fresh Services AB. The company's goal is to reduce wastage of fruits and vegetables through improved packaging solutions, with paper as raw material, thereby improving both the economic and environmental efficiency throughout the value chain, from harvesting and cooling until the fruit is on the store shelf.

The purpose of this study is to investigate what effects the use of electricity in forced-air cooling and how it can be reduced. If the energy use during the air-cooling could be reduced, then carbon emissions and economical cost of the cooling process can be reduced. This allows growers to be able to afford a complete cooling process and the losses from fruit that has been damaged due to bad cooling will decrease. This increases the value of the fruit value chain and suppresses the increase of greenhouse gases.

The aim of this study is to describe how the cooling homogeneity and the fan energy power during forced air cooling of fruits and vegetables depends on the box and the California Tunnel design.

By simulations in COMSOL Multiphysics 4.3, the impact of the vent hole ratio on the pressure drop is studied. Simulation in COMSOL has also been made to estimate how the flow varies between boxes in a California Tunnel and how this tunnel can be modified to provide a more homogeneous cooling. Experiments have been conducted to compare the cooling homogeneity in an existing box and a new box designed to create a uniform air flow.

The results of the experiment showed that the cooling homogeneity could be increased by changing the box design, thereby reducing electricity consumption. The simulation of the California tunnel showed that a modified tunnel arrangement could reduce the airflow difference between the boxes from that at a classic arrangement being 60 % to be less than 1 %. This allows the homogeneity to increase and electricity can be saved. The results of the simulations showed that if the vent hole ratio is halved, the fan energy demand will increase fourfold. Therefore it is important that, when designing a box, the vent hole ratio is high for electricity use to become low.