Effect Of Parameters Of Supply Air On Pressure Precooling Of Fruits And Vegetables

Pressure precooling makes it possible to drop the temperature of the fruits andvegetables rapidly. One the other side it needs pressure produced by the high speed runningof the fan, while the refrigeration system works under the high load and the energyconsumption is large. Analyzing and changing the operating condition of the refrigerationsystem and the fan, optimizing the pressure precooling device, therefore coming up withthe project of energy saving and controlling which guides the pressure precoolingproduction, could reduce the energy consumption effectively, and has a great significant inthe popularization and promotion of pressure precooling.The air supply parameter of the pressure precooling is controlled by pressure fan andrefrigeration system, which directly affect the effect of the pressure precooling and theenergy consumption of the device. Tomatoes are chosen as experimental objects in thispaper, precooling effect and energy consumption of the device are analyzed under differentair supply condition. The main contents and conclusions are as follows,1. The measurement of the thermal physical property of the tomatoesThe heat conductivity coefficient and specific heat capacity of tomatoes underdifferent temperature are measured, showing that they increase with the temperature. Theheat conductivity coefficient changes linearly with temperature, while the specific heatcapacity does not change significantly with temperature.2. The experimental study of the pressure precooling of tomatoesPressure precooling experimental table is built, the changing characteristic of the coretemperature of tomatoes with the time under different air supply velocity and temperatureare measured, and the tomatoes’ principle of cooling under different precooling conditionsis analyzed. The results show that, the air supply velocity and temperature are the mainfactors which affect the precooling time. Under the experimental condition, the airflow rateincreases from9.324m3/(h·kg) to12.312m3/(h·kg)m/s, with the precooling time shortenedby19.9%. The airflow rate increases from12.312m3/(h·kg) to14.58m3/(h·kg), with theprecooling time shortened by21.6%; the air supply temperature decreases from4℃to2℃,with the precooling time shortened by33.9%. The air supply temperature decreases from2℃to0℃, with the precooling time shortened by46.36%..3. Theoretical analysis about heat transfer of the precooling processThe air supply parameters affect greatly to the pressure precooling effect. The mathmodel of sphere fruits and vegetables’ pressure precooling heat transfer is built, the coolingspeed of tomatoes under different air supply velocity and temperature are analyzed by MATLAB, therefore coming to the conclusion of increasing the precooling rate by risingthe air supply velocity and reducing the air supply temperature.The math and physical model of pressure precooling of tomatoes kept in boxes arebuilt, the distribution of flow field inside boxes and the changing process of the tomatoes’core temperature with time are simulated by FLUENT. The results show that thetemperature field is determined by speed field; the greater the wind speed is, the tomatoes’temperature drops more rapidly. After the wind speed reaches1.7m/s, the increasing ofwind speed affects little on the precooling rate and homogeneity. It is suggested that theprecooling wind speed of tomatoes is1.7~2.2m/s. The air supply temperature affect greatlyto the precooling time, with every drop of2℃wind temperature, the precooling time isshortened by16.16~22.8%The experiment and simulation results match properly, proving the accuracy of themodel and the reliability of the calculation method, therefore providing the theoretical basisof the tomatoes’ pressure precooling.4. Precooling process optimization researchDue to the temperature dropping characteristics during the precooling process of thetomatoes, the optimization program of variable wind speed and evaporation temperature iscarried. The precooling wind speed remains0.9m/s while the tomatoes’ temperature dropfrom27℃to15℃,and0.57m/s from15℃to5℃, the pressure fan saves energy of44.9%;The air supply temperature remains4℃while the tomatoes’ temperature drop from27℃to10℃, and2℃while10℃to5℃, the refrigeration system saves energy of10.53%.The research result has certain guiding significance to the air supply parameterselecting, and provides reference to the optimization and energy saving of the pressureprecooling devices.