Industrial Waste Heat Recovery, Using a Loop Thermosyphon Heat Exchanger

For many years, the industrial sector has been a major consumer of energy. As fossil fuels become scarcer and more expensive, many companies are re-assessing their essential energy needs. Saving energy by waste heat recovery is important, not only for cost reasons but also for reducing primary energy consumption and thus reducing carbon dioxide production. In industries such as food and pharmaceuticals, where mixing of hot exhaust gases with the heated fluid is prohibited due to health reasons, practical waste heat recovery presents a major technical problem. In this situation a simple shell and tube type heat exchanger cannot be used because a small leak in any of the tubes may lead to mixing of the fluids, which is totally unacceptable. Loop thermosyphon heat exchangers not only solve this problem but in comparison with shell - and - tube type heat exchangers they are compact and allow efficient heat transfer from heat source to heat sink without using any duct work. Avoiding ductwork is a great advantage since laying duct is expensive, requires space and no obstacles, which is rare in an existing industry. This paper describes a successful application of waste heat recovery from flue gases in a bakery, using loop thermosyphon heat exchangers (LTHE) in which the vapor and liquid streams are separated. Four LTHEs with water as working fluid were designed, constructed and tested under medium temperature operating conditions. The face dimensions of the condenser and the evaporator sections of these LTHEs were kept similar. Each condenser and evaporator had two rows and each row consisted of 12 plate-finned tubes. The LTHEs were used to recover heat energy from the flue gas exhaust of the main oven in the bakery. This heat energy recovered from the flue gas was used to heat up the proofing oven. Formerly the proofing oven had been heated by steam supplied from a gas fired steam generator. The design and performance of the loop thermosyphon heat exchangers together with their heat recovery system are explained and discussed. In this study, the evaluation of the thermal performance of the LTHEs was based on the effectiveness-NTU method. The average effectiveness of each LTHE was found to be 51% and the average heat recovery from the flue gases by each LTHE was 19.3 kW.