Удосконалення вакуум-випарного апарата для концентрування плодово-ягідної сировини

This study considers the process of heat and mass transfer during the boiling of fruit and berry pastes in a vacuum evaporator with a mixer that has heating blades. The vacuum evaporator has been improved by modernizing the heat supply system with a flexible film-like electric heater of the radiant type. The heating element is evenly placed on the outer surface of the working container; the unified mixer has its own heating surface with an area of 0.7 m2. This solution not only provides a stable thermal field throughout the volume of the apparatus but also reduces the time for the system to enter the operating mode. In addition, this solution makes it possible to reduce the inertia of the heating process, improve resource efficiency, and avoid local overheating of the product, which is especially important when boiling thermolabile fruit and berry masses. The process of boiling a semi-finished product from apples, jujubes, and blueberries was tested. It was found that at a temperature of 25°C, a paste with a mass fraction of dry matter of 30% has a dynamic viscosity coefficient of 428 Pa∙s, which is 1.5 times higher than that of a puree with 15% dry matter (290 Pa∙s). It was determined that under boiling conditions at temperatures of 52…55°C and a residual pressure of 13…16 kPa, the effective viscosity of a product with a dry matter content of 15 to 30% is within 12…28 Pa∙s (shear rate of 1 s⁻1). The transient characteristic during heating in the improved apparatus is 30% less than that in the basic one. The metal consumption indicators of the improved structure are reduced by 45%, and the boiling time of fruit and berry puree (from 15% to 30% dry matter) is reduced by 16%, which is explained by the increase in the heating surface to a value of 4.4 m2. The specific heat consumption for heating the system decreased from 134.5 to 119 kJ/kg, which confirms the positive effect of the structural solution proposed in this work