Matsegora O. Investigation of local features of the influence of heat transfer surface contaminants on heat transfer in plate heat exchangers

The dissertation is devoted to the decision of an actual scientific and practical problem of increase of efficiency of heat recovery at the industrial enterprises of the chemical industry with use of plate heat exchangers (PHE) of a collapsible design at the expense of decrease in level of pollution on a heat exchange surface and forecasting their steady work. The dissertation considers the issues of intensification of heat transfer processes and development of methods for forecasting heat exchange surface contamination, design of plate heat exchangers and heat exchange systems taking into account local features of heat transfer surface contamination on heat transfer during PHE operation. The application of the threshold model allows to find the critical level of pollution, after which the pollution does not increase, which will create schedules for the operation of PHE with forecasting the period of operation between cleaning (service intervals). Based on the threshold model of pollution formation over time, a mathematical model of a plate heat exchanger prone to pollution was developed, which is represented by a system of ordinary differential equations. The given mathematical model allows to analyze productivity of PHE in the conditions of pollution of a surface of heat transfer and to define parameters of process in time taking into account influence of geometry of corrugation of plates that allows using this approach for increase of heat recovery in the heat exchanger. Attempting to achieve the desired result by simply adding plates with the same angle of corrugation β to the main direction of flow does not lead to the desired effect, as at the same time increases the thermal resistance of the deposit and after some time it can become much more significant than in PHE with fewer plates. The paper develops a physical and mathematical model that takes into account the distribution of heat transfer process parameters along the PHE channel, which allows predicting the development of pollution over time in different places along the length of the channel. The development of the sediment layer is taken into account by the pollution model, which is represented by the equation in dimensionless form. The relative influence of various factors is taken into account by empirical coefficients that can be identified from the monitoring of thermal and hydraulic characteristics of PHE. The model also allows predicting the change in pressure losses in the PHE with the development of the deposition layer and the corresponding decrease in the cross-sectional area of the channels. On the basis of the considered mathematical model tests of productivity of PHE taking into account occurrence of pollution in the conditions of the operating evaporating installation of sugar factory are carried out and data of tests of the M15M heat exchanger with the heat exchange area of 93 m2 are analyzed. The parameters of the proposed mathematical model of pollution formation were determined on the basis of industrial test data. Differences between the obtained test data and the values estimated by the model range from ± 8%. Calculations were made for the use of PHE for hot water supply. Experimental data from the literature for the boiler system of district heating, which is built on an "open" scheme, were taken as a basis. Experimental studies were performed with PHE type M10B. The empirical parameters of the proposed model of pollution formation were determined by the method of least squares according to experimental data for different flow rates. Comparison of data for all experiments with total heat transfer coefficients calculated by the model showed a discrepancy between the calculated and experimental results ± 7%. The dissertation presents the calculations of the experimental installation on the basis of PHE when used in public utilities for heating tap water, which was installed at the site. A heat exchange system was developed and implemented on the basis of modern PHE for water heating for the needs of chemical water purification for industrial enterprise. The created mathematical model is integrated into a package of applications for automation of process of optimum designing of individual thermal points for TW systems. The practical significance of the obtained results of the dissertation for the chemical industry and utilities is a reasonable choice of design and configuration of plate heat exchangers taking into account the contamination of the heat exchange surface. The method of calculating the thermal resistance of contaminants in the heat exchanger along the plate will take into account this factor at the design stage of heat exchange equipment and improve the system of operation of existing equipment. Keywords: plate heat exchanger, heat transfer, hydraulic resistance, contamination of heat exchange surface, mathematical model, dynamics of pollution growth.