Dynamic Modelling and Control of Counter-Flow Heat Exchangers for Heating and Cooling Systems

A heat exchanger is an essential component in district heating and cooling networks as it transfers thermal energy from energy centers to substations or customers. Thermal loads are typically supplied by regulating the temperature output of one stream of the heat exchanger and by modifying the mass flow rate of the other one. The availability of dynamic models suitable for control design can aid this process to be performed in an efficient way. In this paper, thermal dynamic models of counter-flow heat exchangers are developed. The models encapsulate key heat exchanger characteristics and consider the dynamic calculation of heat transfer coefficients-which includes the hydraulic behavior of the streams. Although heat exchangers are non-linear systems, linearized models are obtained to design simple yet effective and robust controllers in the frequency domain. These are validated in an advanced process simulation software. It is shown that a good temperature control performance in heat exchangers can be achieved for a wide range of operating conditions with a simple PI controller.