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source; ö is the waste heat lost to the environment; η is the Carnot efficiency; TH is the temperature of the waste heat source; and TO is the ambient temperature, 77°F [25°C]. 2.2.3 Temperature and Material Selection The temperature of the waste heat source also has important ramifications for material selection in heat exchangers and recovery systems. Corrosion and oxidation reactions, like all chemical reactions, are accelerated dramatically by temperature increases. If the waste heat source contains corrosive substances, the heat recovery surfaces can quickly become damaged. In addition, carbon steel at temperatures above 800°F [425oC] and stainless steel above 1,200°F [650oC] begins to oxidize. Therefore, advanced alloys or composite materials must be used at higher temperatures. Metallic materials are usually not used at temperatures above 1,600°F [871oC]. Alternatives include either bleeding dilution air into the exhaust gases to lower the exhaust temperature, or using ceramic materials that can better withstand the high temperature. In the case of air bleeding, the quantity of heat contained in the exhaust stream remains constant, but the quality is reduced due to the temperature drop. 2.3 Waste Stream Composition Although chemical compositions do not directly influence the quality or quantity of the available heat (unless it has some fuel value), the composition of the stream affects the recovery process and material selection. The composition and phase of waste heat streams will determine factors such as thermal conductivity and heat capacity, which will impact heat exchanger effectiveness. Meanwhile, the process specific chemical makeup of offgases will have an important impact on heat exchanger designs, material constraints, and costs. Heat transfer rates in heat exchangers are dependent on the composition and phase of waste heat streams, as well as influenced by the deposition of any fouling substances on the heat exchanger. Denser fluids have higher heat transfer coefficients, which enables higher heat transfer rates per unit area for a given temperature difference (Table 5). Table 5 General Range of Heat Transfer Coefficients for Sensible Heat Transfer in Tubular Exchangers10 Fluid Conditions Water, liquid Light organics, liquid Gas (P = 1,000 kPa) Gas (P = 100200 kPa) 5 x 103 to 1.5 x 103 to 2.5 x 102 to 8 x 10 to 1 x 104 2 x 103 4 x 102 1.2 x 102 Another key consideration is the interaction between chemicals in the exhaust stream and heat exchanger materials. Fouling is a common problem in heat exchange, and can substantially reduce heat exchanger effectiveness or cause system failure. Figure 3 displays an abandoned recuperator previously used in an aluminummelting furnace. Deposition of substances on the heat exchanger surface can reduce heat transfer rates as well as inhibit fluid flow in the exchanger. In other cases, it will degrade the heat exchanger such that it can no longer be used. Methods for addressing fouling are numerous and include filtering contaminated streams, constructing the exchanger with advanced materials, increasing heat exchanger surface areas, and designing the heat exchanger for easy access and cleaning. Nevertheless, the problem of fouling remains a significant challenge in thermal science. A 1992 study by Pacific Northwest National Laboratories examined 231 10 Heat Transfer Coefficient (W/(m2 • °K)PDF Image | Waste Heat to Energy Tech Opportunities in US Industry
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