Heat transfer characteristics of air heater heating elements.

A project report submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering.This project is a part of a research programme by Eskom to develop power station rotary regenerative air heaters that are more compatible with South African coal properties. Large costs are involved in the replacement of air heater surfaces due to the erosion caused by the abrasive South African coal ash. The performance of an air heater L governed by numerous parameters, some of which (such as erosion) are unpredictable. A laborar: 'v testing facility which can produce a real-situation environment for air heater research is not only impractical but also expensive. Hence it has been decided to generate a computer simulation model of a power station air heater. The various alternatives for the improvement of existing air heaters will be assessed using this computer model. Extensive information regarding the influence of various parameters such as thermal performance, erosion, flow resistance, corrosion, etc. is necessary as input to the simulation model. Various test facilities have been commissioned to obtain this information. This project is an experimental study on the thermal performance of the regenerative air heater heating elements using a thermal test facility situated at the Eskom Technology Research and Investigations centre in Johannesburg. The facility uses the single-blow transient technique to establish the heat transfer coefficients of various air heater heating elements. Ten different types of heating elements were tested and the results were analysed .. These test results and recommendations give useful indications for power station application even before the final simulation model is available. The primary objective of this project was to find the heat transfer coefficient and also to make correlations between Colburn j factor, Fanning friction factor and Reynolds number. Packs were tested for various air tlow rates, ranging from Reynolds number 1200 to 6000. The test results were analysed and the correlations were made. A detailed uncertainty analysis was done and found that the results are showing less than 7 % error which is acceptable. The consistency of the test results was tested by a repeatability test and the results were quite satisfactory. The single blow method used in this project considered the longitudinal conduction within the material and it can accommodate any arbitrary inlet fluid temperature history. A comparative study of the various packs was done and traced certain trends. The high density packs gave better heat transfer and high pressure drop. The packs with alternate corrugated and flat plates had lower heat transfer performance, due to the fact that the tlat plates do not create such turbulence to the flow to reduce the boundary layer thickness to enhance the heat transfer. In short the flat plates only add weight to the pack, making it heavier than other packs. Based on the correlations and trends obtained from the analysis, some recommendations are also made. A further modification of the test facility was recommended to include a wider range of flow from very low Reynolds number to very high. This is necessary to find whether the packs with flat plates really perform better for highly turbulent tlow. The non - adiabatic nature of the side wall has to be considered for better reliability of the results. Some other recommendations are made to make the testing of packs more convenient.Andrew Chakane 201