Determination of the heat transfer coefficient of a rotary film evaporator with a heating film-forming element

A model of a rotary film evaporator with a film-forming element with a reflective heated surface has been developed. This will allow stabilizing the hydraulic movement of the cut wave flow due to the reflective surface of the geometric shape for the forced direction of the cut raw material to the heating surface. Autonomous heating of the reflective surface additionally provides a temperature effect in the conditions of movement of particles of raw materials after cutting. The analysis of the experimental and theoretical parameters of heat transfer made it possible to substantiate the criterion equation for determining the heat transfer coefficient of an evaporator with the proposed film-forming element and a reflective heated surface for calculating the coefficient from the working surface to the raw material. The resulting equation takes into account the influence of the vertical component of the motion of the raw material film, centrifugal movement during the rotation of the film-forming element, mixing of the boiling film of the raw material with steam bubbles, and the geometric characteristics of the film-forming blade on the hydrodynamic flow of the raw material. The calculation of the rotary-film evaporator was carried out using the criterion equation and the obtained useful heat exchange surface – 0.75 m2. The specific metal consumption in a rotary film evaporator with a film-forming element having a reflective surface is 57 kg/m2, compared to the vacuum evaporator traditionally used in canning industries (410 kg/m2), which is 7.1 times less. The duration of the temperature effect on the raw material is also reduced: a rotary film evaporator – 200 s and 3600 s in a traditional apparatus. The data obtained will be useful for the design of rotary-film devices of different geometric parameters using articulated blades with a reflective plate.

Improved rotary film evaporator for concentrating organic fruit and berry puree

This paper reports the improved rotor-film evaporator with the lower arrangement of the separating space, the auger-type discharge of concentrated organic fruit and berry paste, and preheating the puree with secondary steam. The working surface of the evaporator is heated by a flexible film resistive electric heater of the radiating type with an insulating outer surface. Peltier elements installed in the device make it possible to provide low-voltage power for exhaust fans from the thermal secondary steam. The puree fed for processing is preheated by 8...10 °С by the heat from the concentrated product and secondary steam. For the experiment, fruit and berry blended puree from apples, quince, and black currants was used. The structural and mechanical properties of blended puree have been determined when the temperature changes within 55...75 °С, in particular, the effective viscosity varies in the range of 22...6 Pa∙s, the maximum shear stress ‒ 29...8 Pa. Effective regions in the fruit and berry puree concentration process have been established: Kmin=Vpaste/Vpuree=0.190; Kmax=Vpaste/Vpuree=0.725 When concentrating fruit and berry pastes with an initial solids content of 9...15 % to the resulting content (29...31 %), it is advisable to apply a surface load of 0.048...0.121 kg/m2s. By calculation, the reduction of the specific energy consumption for heating the volume of the product unit has been confirmed: a rotor-film evaporator – 547 kJ/kg over a period of 75 s, compared to the basic vacuum evaporator – 1,090 kJ/kg, respectively, over 1.08 hours. The results could be useful when designing evaporating equipment for rotor-film-type devices in order to concentrate various blends of fruit and berry raw materials under conditions of using the energy of secondary steam.

Determination of heat transfer coefficient in advanced rotary film evaporator

The object of research is the process of concentrating fruit and vegetable purees in an improved rotary film evaporator. The existing hardware design of traditional processes for processing fruits and vegetables, as a rule, is not unified enough, inconvenient in operation and is designed for high productivity. Concentration of fruit and vegetable purees occurs mainly in vacuum evaporators of periodic and continuous operation at a temperature of 60–80 °C under vacuum, which allows them to significantly preserve their nutritional value. But the duration of the process remains very significant (in devices of periodic action up to 75–90 minutes). One of the most problematic areas in the concentration of fruit and vegetable raw materials is significant losses of biologically active substances. At the same time, an important indicator of the quality of the process of concentrating pasty fruit and vegetable pastes is the value of the heat transfer coefficient, which characterizes the efficiency of the heat transfer method and the design features of the mixing device, taking into account the thermophysical characteristics of the product. To create conditions for conducting research to determine the heat transfer coefficient, it is necessary to use instrumentation with precise regulation of the necessary technological parameters. To study the heat transfer coefficient when concentrating fruit and vegetable purees, an automatic installation of an improved rotary evaporator was designed. The improvement of the rotary film evaporator (RFE) is carried out due to the lower location of the separating space by installing a screw discharge of the paste and preheating the output puree with secondary steam. The experimental dependences of the heat transfer coefficient on the product flow rate make it possible to determine the rational values of the flow rate of the RFE feedstock at various values of the rotor shaft speed. It is found that the heat transfer coefficient is influenced to a large extent by the product consumption, and the rotor speed acts to a lesser extent, only the relative speed of fluid passage around the developed hinged blade changes. It is found that when the frequency changes from 0.3 to 1.7 s–1, an increase in the heat transfer coefficient by 1.45 times is observed, which is explained by a more intensive degree of mixing of the product by the blades.