An energy-based representation of a counter flow single phase heat exchanger
Smuts, Stefan Benjamin
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Energy is a fundamental part of human civilisation and the expected rise in global energy demand is approximately 1.7% per annum until 2030. With limited fossil fuels available for energy generation, other ways of energy production and conservation must be investigated. One way to achieve this objective is to evaluate the energy used in an industrial process and to determine if this energy evaluation can be used to increase efficiency of the said industrial process. In order to address the problem of evaluating the energy representation of a heat exchanger, an analytical model must be derived, verified and validated. The sensitivity of the energy representation for several fault conditions is evaluated and possible applications of the energy representation are identified. The analytical model is derived by applying the staggered grid approach and the laws of conservation of mass, momentum and energy to the heat exchanger. Verification was done by comparing the analytical model results to the results of a Flownex® simulation. Flownex® is validated thermodynamic and hydraulic simulation software that excels at simulations where a fluid is a driving factor. Validation was done using a supercritical CO2 test bench that consists of a compressor, gas cooler, expansion valve, and an evaporator. The gas cooler can be approximated as a heat exchanger, as it is where hot CO2 is cooled with water. The gas cooler was therefore used for validation. Bejan  created entropy interaction–energy interaction graphs, using the first two laws of thermodynamics and visualises the changes in energy and entropy of a system. Faults induced include fluid leakage, heat leakage, and fouling. The purpose of faults in the heat exchanger system is to measure the sensitivity of the energy representation to changes in the heat exchanger operation. An emerging property of the graphing technique is that entropy generated is also shown. The entropy generation number is an indication of the efficiency of the system. Because the energy representation is sensitive to changes in the operating conditions of the heat exchanger and the efficiency of the heat exchanger can be seen, possible applications include fault detection and isolation (FDI) and optimisation. Future research includes a more accurate model encompassing more detail regarding the real world system and improved manners to detect faults as not all faults could be identified.
- Engineering