Transient boiler heat exchanger thermal behaviour analysis
Master Thesis
2014
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University of Cape Town
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Abstract
Coal fired power plants that were built in the past four decades are aging. The main aging mechanisms are creep and thermal fatigue. Creep results from the high temperatures at which the components operate. Thermal fatigue is due to thermal stresses and these stresses result from temperature gradients within the material. Cycling of these thermal stresses accelerate the creep in a process called creep- fatigue aging. The boiler and its final heat exchangers and headers are the main components that are affected by these mechanisms. The aging of these components results in high maintenance costs, reduction of the plant reliability and availability, and contribute to increased safety risks for the plant and personnel. Therefore, there is a need to understand the steady state and dynamic behaviour of the components of these plants in order to predict the stresses that the material experience. This report discusses an investigation to the possibility of modelling the thermal dynamic behaviour of typical boiler heat exchanger components which have to withstand the highest temperature of a Pulverised Fuel Rankine cycle power plant. Thus, illuminating the issues that need to be addressed in modelling such heat exchangers. Modelling approaches of heat exchangers are systematically presented, starting with the use of exact analytical solutions. This is followed by the application of finite volume numerical method. Finishing off with the use of the Flownex software. The exact analytical solutions are used to characterise the transient temperature distribution in solid materials with simplified heat transfer, highlighting the dependence of the solutions on the Fourier number and Biot number. These solutions are further used to calculate thermal stresses generated in the material, illustrating the relationship between thermal stresses and temperature gradients. Furthermore, a finite volume solution is applied to modelling an infinitely long tube. I t is illustrated that for transient conduction heat transfer problems, the solution depends on both physical space discretisation and time- wise discretisation. The numerical solution is verified against the exact analytical solution. Finally, the Flownex software is used to illustrate the issues that need to be addressed when modelling the transient behaviour of a heat exchanger . For this purpose only the average area discretisation scheme is used since it allows for any generic solid structure to be modelled, provided that the appropriate level of discretization is applied. The Flownex modelling starts by modelling transient conduction heat transfer within an infinitely long tube. The Flownex solution is verified against the finite volume numerical solution. The Flownex solution depends on thickness discretisation, especially for thick cylindrical components. Finite tubes are also modelled on Flownex including axial discretisation and layout simplification of the tubes. Flownex is also used to model a heat exchanger bundle using two methods; a tube by tube method and a method that involves the combination of all ii the tubes into one tube. The product of the thermal resistance and the capacitance of the system governs the transient simulations for both methods.
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Reference:
Gwebu, E. 2014. Transient boiler heat exchanger thermal behaviour analysis. University of Cape Town.