Abstract
Heat exchangers used in various industries, most often work in conditions of variable flows and temperatures. At the same time, the existing theories of calculation of heat exchanger modes are based on the use of constant dimensionless parameters in any mode of operation. The purpose of this work is to obtain dependencies to determine the effect of coolant temperatures on the variable parameter of the heat exchanger. Using the simulation method, dependencies were found that describe the change in the heat exchanger parameter which made it possible to obtain a general formula for the change in the heat exchanger parameter at varying coolant temperatures. To test the applicability of the existing relations describing the change in the heat exchanger parameter and the formula obtained, a large number of heat exchangers were calculated in variable operating modes. Comparison with the simulation results showed that the ratios of the known theories of heat exchangers do not work in all modes and their application can lead to significant errors. A formula has been obtained allows one to find the effect of coolant temperatures on the variable parameter of the heat exchanger. The formula can be used to predict the modes of large systems.
Highlights
To assess the efficiency of heat transfer processes in variable operating modes, the values used in the theory of heat exchangers are widely used, which include heat exchanger efficiency, log average temperature difference t, heat exchanger surface F and the number of heat transfer units NTU
The convenience of using dimensionless complexes, such as NTU, is in the assumption that the heat transfer coefficient and the efficiency of the heat exchanger are practically independent of the temperatures of the heat exchanging media [13, 14]
The temperature range of the primary coolant was chosen in the range of 70150 С, the secondary one in the range of 30-60 С, which corresponds to the working temperature ranges of heattransfer agents in the second stage heater at the thermal point for evaluating the formulas given in [16]
Summary
To assess the efficiency of heat transfer processes in variable operating modes, the values used in the theory of heat exchangers are widely used, which include heat exchanger efficiency , log average temperature difference t, heat exchanger surface F and the number of heat transfer units NTU. The convenience of using dimensionless complexes, such as NTU, is in the assumption that the heat transfer coefficient and the efficiency of the heat exchanger are practically independent of the temperatures of the heat exchanging media [13, 14] This allows to reduce the number of unknown variables which is especially important when calculating the associated heat exchanger systems, it is often necessary to solve the problem by the method of successive approximations [5, 12, 15]. For explicit heat the following equation is applied [11, 19]
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