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3.3.1 Challenges to Recovering LowTemperature Waste Heat Lowtemperature heat recovery faces at least three challenges: • Corrosion of the heat exchanger surface: As water vapor contained in the exhaust gas cools, some of it will condense and deposit corrosive solids and liquids on the heat exchange surface. The heat exchanger must be designed to withstand exposure to these corrosive deposits. This generally requires using advanced materials, or frequently replacing components of the heat exchanger, which is often uneconomical. • Large heat exchange surfaces required for heat transfer: Heat transfer rates are a function of the thermal conductivity of the heat exchange material, the temperature difference between the two fluid streams, and the surface area of the heat exchanger. Since lowtemperature waste heat will involve a smaller temperature gradient between two fluid streams, larger surface areas are required for heat transfer. This limits the economics of heat exchangers. • Finding a use for lowtemperature heat: Recovering heat in the lowtemperature range will only make sense if the plant has a use for lowtemperature heat. Potential enduses include domestic hot water, space heating, and lowtemperature process heating. Other options include using a heat pump to “upgrade” heat to a higher temperature to serve a load requiring higher temperatures (Section 3.3.2). Additionally, lowtemperature power generation technologies are slowly emerging (Section 3.4.1). Technologies are available that can cool gases below dew point temperatures to recover lowtemperature waste heat. Options include deep economizers, indirect contact condensation recovery, direct contact condensation recovery, and recently developed transport membrane condensers. These technologies are discussed below. Commercialization has been limited due to high costs and because facilities lack an end use for the recovered heat. When facilities lack an enduse for waste heat, some have found other means for recovery, including heat pumps and lowtemperature power generation. These technologies are also frequently limited by economic constraints. 3.3.2 LowTemperature Heat Exchange 3.3.2.1 Deep Economizers Deep economizers are designed to cool exhaust gas to 150160oF [65oC71oC] and to withstand the acidic condensate depositing on its surface. Designs include the following options: • Installing a “throwaway” section on the cold end of the economizer. The tubing in the cold end will degrade over time and will need to be repeatedly replaced. The frequency of replacements will depend on the flue gas composition and the material of construction. • Designing the economizer with stainless steel tubes. Stainless steel can withstand acidic gases better than the mild steel typically used in construction. • Using carbon steel for the majority of the heat exchanger, but using stainless steel tubes in the cold end where acidic deposits will occur. • Using glasstubed heat exchangers (mainly for gasgas applications such as air preheaters). • Using advanced materials such as Teflon.18 3.3.2.2 Indirect Contact Condensation Recovery Indirect contact condensation recovery units cool gases to 100 to 110oF [3843oC]. In this range, the water vapor in gases will condense almost completely. Indirect contact exchangers consist of a shell & tube heat exchangers. They can be designed with stainless steel, glass, Teflon, or other advanced materials. 19 PDF Image | Waste Heat to Energy Tech Opportunities in US Industry
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