Variable air supply velocity of forced-air precooling of iceberg lettuces: Optimal cooling strategies

Abstract

The air supply velocity has a direct impact on the cooling rate and energy consumption for forced air precooling of horticultural produce, which was often investigated at a constant air supply velocity throughout the whole precooling process previously. In this paper, the effects of different constant air supply velocity on cooling time, uniformity, heat load and energy consumption during different precooling stages were analyzed theoretically by numerical simulation. The simulated results are consistent with the experimental data. The analyzed results showed that the air supply velocity had less influence on the half cooling time, but it had significant impact on energy consumption whether before or after half cooling time, especially when air supply velocity exceeds 2 m/s. With the increased air supply velocity, the internal temperature gradient of individual product and cooling uniformity after half cooling time were all enhanced. Furthermore, from half cooling time to 7/8 cooling time, the proportion of fan heat load increased gradually with the precooling going on and became the main heat load (more than 50%), which means the refrigerating capacity is mainly used to remove the heat generated by ventilating fan in the later stage of precooling. Finally, a novel precooling strategy (variable supply air velocity precooling) was proposed and a method to determine optimal precooling condition from four constant and five variable supply-air velocity plans was established.