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    A dynamic modelling methodology for the simulation of industrial refrigeration systems : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biotechnology and Bioprocess Engineering at Massey University
    (Massey University, 1992) Lovatt, Simon James
    A dynamic modelling methodology has been developed for the computer simulation of industrial refrigeration systems. A computer program, RefSim, has been developed which embodies the new methodology. RefSim contains a total of 33 separate models - 11 derived from existing models, six which are substantially enhanced and 16 which are new. In general, these models were derived from thermal considerations and ignored the effect of hydrodynamic processes in the refrigeration circuit. Models can be dynamically linked together as specified by the input data in order to simulate a complete plant. The program includes a set of simulation utilities which reduce the amount of work required to develop models. The object-oriented features of inheritance, encapsulation and polymorphism are used extensively. Substantial model development was carried out to achieve accurate predictions of the heat release profile during chilling and freezing of food product as product cooling makes the greatest contribution to both mean and peak heat loads in many industrial refrigeration plants. The new ordinary differential equation (ODE) model was tested against finite difference (FD) calculations for a range of product shapes and Biot numbers. The ODE model predicted to within ±10% of the FD calculation during almost all of the cooling process under the test conditions. The ODE model required several orders of magnitude less computation than FD while being capable of extension to shapes that could not be handled by FD. To test the new ODE model against experimental data, a differential air temperature method to measure the cooling food product heat load profile was developed. Both the FD and ODE methods predicted the heat load profile of freezing meat cartons to within the experimental margin for error (±10%). The ODE model also predicted the heat load profile of freezing lamb carcasses to a similar level of accuracy. Three refrigeration plants (a laboratory water chiller, a 18500 lamb per day meat processing plant, and a 6000 lamb/1000 beef per day meat processing plant) were surveyed to obtain data for testing the whole simulation environment. RefSim was found to predict the measured data satisfactorily in most cases. The results were superior to those from a commercial refrigeration simulation environment and comparable to an enhanced version of that environment which included the new ODE product heat load model. Differences between the measured values and those predicted by Refsim were probably more attributable to uncertainties in the simulation input data than to model deficiencies. RefSim was found to be a flexible environment which was general enough to simulate both simple and complex refrigeration systems. Unusual components could be simulated by combining existing models rather than implementing custom models. Nevertheless, the simulation results have indicated a number of areas for further model improvement. The effects of air mixing and the thermal buffering of structural materials were shown to be modelled poorly for some refrigerated rooms. There is some scope for improving the chilling stage of the ODE product heat load model.
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    High temperature superconducting flux pump : a thesis in the partial fulfilment of the requirements for the degree of Masters of Engineering in Mechatronics at Massey University, Turitea Campus, Palmerston North, New Zealand
    (Massey University, 2011) Walsh, Rowan Martin
    Electromagnets play an important role in everyday life from motors and generators to such devices as Magnetic Resonance Imaging (MRI). The larger a magnetic field needed in such a device the more demand there is in power consumption and space. The development of superconductors (SC) and their ability to dissipate negligible power has made it possible to create larger fields in a more cost effective and space efficient manner. The disadvantage of the SC is that it needs to be operated at cryogenic temperatures (typically below 110 K). Cryogenics is a significant cost factor so the less power needed for cooling a SC magnet the more cost effective the system can become. The main heat load comes from the ohmic dissipation of the current leads necessary to energise the magnet. The current leads provide the electrical connection from the power supply at room temperature to the coils in the cryogenic environment. To circumnavigate this heat load a superconducting device known as a flux pump (FP) can be embedded in the cryogenic environment. The flux pump operation can be generalised as a DC generator, which minimises ohmic heating and makes the power supply needed to energise the magnet superfluous. HTS 110 is a magnet manufacturer who has assigned the task of implementing a flux pump into a commercial magnet application and developing a complete system capable of controlling a homogenous current level in their magnets. Such a flux pump module was developed and successfully incorporated into an existing SC magnet. This thesis details the mechanical design, control hardware, required software used to create this technology and the benefits it presents. Compared to the current lead technology, the developed flux pump module reduced the heat load on the cryo-cooler by almost a factor of 5. In addition a produced magnetic field of 750 mT was achieved, which corresponds to a current of 68 A in the magnet coils. Such a high field could not have been obtained in the present setup with conventional technology. The large heat load caused by current leads would increase the overall temperature of the coils and in turn decrease the current capability of the circuit to approximately 20 A, resulting in a field that is more than three times smaller. A full control system using field strength as feedback to control the rotation of the FP was implemented. This included an iterative hardware design and fabrication process followed by software implementation, that led to a high level system encompassing a PID type control algorithm. The control system achieved a field stability of 30 parts per million (PPM) which puts the system in reach for the needed NMR stability criterion of 1 PPM.