Dynamic simulation and control strategy of an absorption heat transformer with a falling film evaporator

Abstract

The performance of an absorption heat transformer (AHT) with falling film heat exchangers is determined by the heat sources, the wetting of the heat transfer tubes, and by the thickness of the falling film. Additionally, an incorrect management of the heat sources may result in wasted energy. A control strategy implemented in an AHT can be a tool to address these issues. Therefore, in this work, the implementation of two PID controllers in an AHT model is presented. The control was implemented in an AHT model developed in gPROMS® using its control libraries. The energy balances, correlations for heat transfer coefficients, and the wetting efficiency of the unit are considered. The first PID controller is proposed to guarantee the complete falling film evaporation by controlling the evaporator heat source outlet temperature. The Second PID controller helps to reach an optimal heat transfer rate by controlling the evaporator inlet mass flow rate to obtain complete wetting efficiency and a minimum film thickness. The temperature and mass flow rate in the heat sources of the generator and evaporator are manipulated. Therefore, the control system aids in optimizing energy, avoiding a useless increment of the heat source temperature and mass flow rate. First, a sensitivity analysis is carried out to establish the effect of the manipulated variables on the inlet and the evaporator vapor mass flow rate. The results of the first PID controller show that the evaporator temperature can be decreased by controlling the system when the amount of fluid to evaporate decreases. For the second PID controller, to maintain a constant inlet mass flow rate, the generator temperature needs to be increased by approximately 2 °C due to the increment in the condenser temperature caused by environmental conditions. When the mass flow rate is manipulated, the process can be controlled up to a limit because the process has a limit of vapor production at a high mass flow rate. The fair-flow configuration of the heat sources with the implementation of PID controllers was adequate because the inlet mass flow rate evaporated completely.