Development of MATLAB Based Dynamic Model of the Helium Liquefier / Refrigerator

Abstract

Static simulation of the thermodynamic model of the liquefaction / refrigeration process was reported by a number of researchers using commercially available software as rigorous controls are required for high pressure (HP) and low pressure (LP) stream, turbo expander flow, Joule-Thomson valve opening and Dewar pressure. However these softwares are very expensive. On the other hand dynamic modelling of the components or sub-systems is necessary to control the actual system under realistic conditions. Mathematical modelling is nothing but the representation of any system through a set of mathematical equations which reflects the behaviour of the system under consideration. Performances of the control system can be evaluated by proper dynamic modelling of the liquefaction process and subsequent simulation of the control system in synchronisation with the models is necessary. It is planned to implement the entire control system by four split models as mentioned earlier, which can be later integrated for the entire process. The design methodology includes mathematical modelling of different sub-systems and incorporation of control system in each of them which stabilizes load disturbances and process parameter fluctuations by controlling or modifying some manipulated variables using actuators. A dynamic model of the process is developed using some commonly available mathematical software so that it can be easily interfaced with the SCADA developed in open platform and the Programmable Logic Controller (PLC). It will enable the real time check of the control system in line with the dynamic model. In the current work screw compressors with associated components (compressor and valves), Dewar and turbine is mathematically modelled and in each case, one controller is also designed in parallel with them which will control the process. The behaviour of the control systems are examined and recorded during the load disturbances in cold box (affects the HP and LP of the system), heat load fluctuation in Dewar (affects the boil-off rate and the pressure inside Dewar) and during speed control of the turbine (at the starting). Different control philosophies are designed to get the optimum control technique and suitability of these control systems are checked. The final work is to attach them together in association with already developed models of heat exchanger and J-T valve and link them to SCADA and PLC to establish a control interface between actual system and the operator.