EXPERIMENTAL STUDIES OF BOILING HEAT TRANSFER OF FOOD SOLUTIONS

Abstract

Vacuum evaporation is widely used in the food technologies. The equipment for this process is well known and methods of calculation and design of vacuum evaporators are described in literature as well. However, in some cases the accuracy of existing methods is not enough. The problem of designing the new, more efficient ap-paratuses those work in regimes, which are not usual, needs to clarify some dependencies. It concerns the problem of boiling heat transfer coefficient determination for such solutes as food products because of the high or sometimes extremely high viscosity of many food staffs is. To take into account properties of products many authors use the Prandtl number. However, determination of these properties exact values is not always possible especially it con-cerns the viscosity of Non-Newtonian fluids, which are the most of food staff. For experimental verification of heat transfer coefficient values, an apple juice was chosen. At first, the theoretical value was obtained with Tolubinskiy dependency using. The vapor bulbs grows rate and surface tension coefficient were solved as for water. The de-pendencies between the heat transfer coefficient and heat flux for apple juice with concentration from 15 to 50 Brix were obtained as result. There are several different equations to calculate the properties of apple juice depending on temperature and concentration and several resulting dependencies that differ from each other were obtained. The comparison with the experimental data that was obtained by authors made possible to choose the right equa-tions for apple juice viscosity determination when the heat transfer calculation error did not exceed 20% that is standard error for used dependency and for many others. To reduce this error on the base of obtained experimental data the correction coefficient was calculated. Therefore, the equation to calculate the heat transfer coefficient for boiling apple juice where maximum error did not exceed 5% was obtained. The experimental research was conduct-ed under atmospheric pressure. To obtain the value of heat transfer coefficient in vacuum condition the Tolubinskiy dependency can be used or addition experiments should be conducted. It depends of required accuracy level.