Research On Modeling And Thermal Characteristics Of Plate Heat Exchanger In Heating System

The plate heat exchanger is an efficient heat exchange device,which is widely used in a centralized heating system because of its compact structure,high heat transfer efficiency,flexible combination of heat transfer surfaces,and convenient assembly and disassembly cleaning.As a part of the thermal energy conversion in the heating system,its operating state is often in non-steady conditions.Therefore,the dynamic characteristics of the plate heat exchanger are studied and analyzed,and the thermal characteristics of the plate heat exchanger under changing conditions are accurately known.The law has very important significance for the operation control of the heat exchanger and even the operation regulation of the heating network.In this paper,based on the heat transfer mechanism of the plate heat exchanger,according to the principle of energy conservation,a dynamic mathematical model for the distribution parameters of the heat exchanger is established for the phase-changeless dynamic heat process of the plate heat exchanger.The main feature of the mathematical model is to consider the thermal diffusion of the heat transfer fluid in the flow channel,ignoring the axial thermal diffusion of the metal plate.Secondly,flow is introduced to distribute the non-uniform parameter?2 in the flow channel,taking into account the influence of uneven flow distribution from the port to the flow channel on the dynamic performance of the heat exchanger.At the same time,for the U-type and Z-type two common flow plate heat exchangers,the phase lags of the inflow and outflow of each fluid are calculated.Using the finite difference method,the nonlinear dynamic differential equations are transformed into algebraic equations through an implicit difference scheme for programming.Under the disturbance of inlet flow,inlet temperature and the combination of the two,the thermal response curves of two types of plate heat exchangers were analyzed in detail.The steady-state calculations verify the temperature value after the dynamic process reaches the steady state.Through comparison with other dynamic models of distributed parameters,the reliability of dynamic process values is theoretically proved.In addition,two kinds of lumped parameter dynamic models for plate heat exchangers have been established for the practical application of engineering and for serving the simulation of the entire central heating network.These models are the total heat capacity parameter model of two mass points and three mass points,and three mass points.The model considers the metal heat capacity of the heat exchanger plate based on the two-mass model.The thermal response curves of lumped parameter model and distributed parameter model under the same disturbance are analyzed and compared,and the error magnitudes of the two lumped parameter models in U-shaped and Z-shaped calculation results are calculated,which is a lumped parameter model.The application provides the theoretical basis.The calculation results show that the three-point lumped parameter model is better than the two-point lumped parameter model model in the cold start-up process,and the lumped parameter model error is within 4% in the steady-state multi-mode disturbance.The lumped parameter model can describe the dynamic process of the plate heat exchanger well.