Novel tubes for optimizing heat exchange in power producing boilers
Lignite utility boilers, as their name suggests, burn lignite for the production of mainly power but also heat and/or cold. For these boilers the heat exchange system is of crucial importance to remove excessive amounts of heat from the system. The tubes used for the heat exchange rapidly degrade due to fouling. In simple terms, fouling is the deposition of ash particles resulting from the burning of lignite on the heat exchanger surfaces. Minimizing the fouling of the tube surfaces was the objective of the current research project. Such minimization has been achieved with immediate and very important consequences. The overall performance of the boilers increases by as much as 90% if the heat transfer is at 25% of the fouling rate and 60% of the pressure drop. These percentages are based upon the standard configuration of the boilers used for power production in Greece Project partners have succeeded in a substantial reduction of the fouling rate using asymmetric tube arrangements and/or non-circular tubes. The tubes bundle design, named DDEFORM, have been numerically and experimentally tested and are being protected by patents. The developed configuration of the tubes is not restricted to utility boilers but is also applicable to heat exchangers used for cooling, as in the case of automobile radiators. In general the proposed configuration may be applied whenever fouling conditions may appear or pressure drop is an important factor for operation. Throughout the project, partners have conducted many numerical and experimental investigations on fouling minimization and have developed new fouling tests. Upstream flow perturbation, asymmetric arrangements and non-circular tubes were investigated for optimizing tube bundle efficiency. An experimental loop was built for use with industrial heat exchangers. The loop was used for measuring particle deposition on cylindrical in-line tube bundle and on an enhanced in-line tube bundle. The methodology developed will allow a thorough testing of heat exchangers and filters under fouling conditions so that manufacturers can use the best configuration of the new shaped-tubes to reduce fouling. Apart from the power generation industry that will greatly benefit from the results of this project there are a number of other possible application sectors. The automotive industry for engine cooling radiators, the chemical industry where heat exchangers often operate in highly fouling conditions and finally the filters manufacturers that may use the knowledge acquired for the reverse problem they are faced with, that of capturing particles.
