Arsenyev P. Investigation of heat transfer processes in welded multi pass plate heat exchangers for chemical industry

The dissertation is dedicated to the important scientific and practical problem of the improvement of the energy efficiency of production facilities by the enhancement of the efficiency of the industrial processes of gases heat recuperation by the use of the multi pass welded plate heat exchangers. An analytical survey of the scientific and technical information was carried out on the improvement of the energy efficiency of the chemical industry by the heat recovery in the technological processes. It is shown that the improvement of the effect of integration of thermal processes by the improvement of the level of heat recovery is possible with the use of compact heat exchangers with intensive heat transfer, such as plate heat exchanger. Analysed are the possibilities for improving the process of the ammonia production by the recovery of heat energy in the columns for synthesis. The main requirements are formulated for heat exchange equipment operating at the high pressure and temperature conditions of the units for the ammonia synthesis. Works with methods of calculation of plate heat exchangers with the intensified process of heat transfer are analysed. The analysis of turbulent heat transfer in the channels of complex geometric shape of plate heat exchangers with cross movement of gaseous heat carriers in the channels is performed. The analysis was performed using the analogy of heat transfer and momentum. Using a three-layer model of turbulent flow, the equation for calculating heat transfer based on the data on the hydraulic resistance of the corrugated field of the channels of the plate heat exchanger is obtained. It is proved that the exponent at the Prandtl number as a multiplier in the correlation equations for the calculation of heat transfer in gas flows in the channels of plate heat exchangers must be greater than that for liquid flows. For the range of Prandtl numbers from 0,5 to 7, the recommended value of this exponent is c = 0.5. The description of the experimental stand and model of the welded plate heat exchanger for research of process of heat transfer and pressure losses at cross movement of heat carriers in channels of mesh-flow type formed by round corrugated plates is presented. The stand and its equipment with control and measuring devices allows to carry out researches in a sufficient range of change of the basic parameters of process and measurement of flow rates, temperatures and pressure of heat carriers with sufficient accuracy. Experimental study of heat transfer and pressure drop in the model of a welded plate heat exchanger confirmed for the case of transverse motion of heat carriers in the apparatus the validity of the expressions proposed for the channels of plate heat exchangers mesh-flow type of different geometry. The dependence of heat transfer efficiency (ε) on the number of heat transfer units (NTU) in one pass of a plate heat exchanger with cross movement of heat carriers is also estimated. The proposed equation can be used in the calculation of plate heat exchangers with total counter and cross motion of coolants within individual courses. An equation for calculating the pressure loss in the channel of a welded plate heat exchanger taking into account the pressure loss in the main corrugated field and the local hydraulic resistance at the inlet and outlet of the channel is proposed. The adequacy of the proposed equations for the calculation of heat transfer efficiency and pressure losses in the channels of a welded plate heat exchanger with round plates and the possibility of their use in engineering calculations of plate heat exchangers are confirmed. A generalized mathematical model of the heat transfer process between single-phase heat carriers in a multi-pass welded plate heat exchanger with cross-counter current motion of heat carriers has been developed. The analysis of test results of a sample of a welded plate heat exchanger installed in an industrial ammonia synthesis column at an ammonia production plant is performed. The description of the column and the experimental sample of the heat exchanger is given. The results of industrial tests confirmed the suitability of the welded plate heat exchanger for operation at pressures up to 32 MPa and temperatures up to 520 ° C of the ammonia synthesis column. In industrial conditions, the adequacy of the developed mathematical model and the equations obtained in laboratory exams, which are the basis for the model development are confirmed. A 15% increase in the productivity of the ammonia column due to the increase of the catalyst load in the column is shown and the advantages of using a plate heat exchanger in comparison with a tubular heat exchanger when operating under the same conditions are confirmed.