Tham khảo tài liệu 'developments in heat transfer part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 310 Developments in Heat Transfer Fins are generally not used in high temperature units because of suspended dirt particles that will foul and low available pressure drops. The advantage of fins is negligible. Ceramic and high-temperature alloys such as Alloy 800H 617 230 are used for construction materials. The thickness and mechanical design of selected materials are mainly governed by thermal stress but the extent of the materials oxidation thermal shock bearing capability and erosion from suspended dirt particles fouling and corrosion because of metallic salts sulfates etc. also need to be considered. Differential expansion is an important factor in high temperature units and should be accounted for by using either expansion bellows or by using bayonet-type units. Floating tube sheets cannot generally be used because sealing gaskets or packing materials do not work effectively at such high temperatures. Heat losses from the outside surface to the environment have to be considered in the mechanical design of the unit and design of the foundation. Gases air liquid metals or molten salts are preferred over steam for high temperature heat transfer because the latter require a very thick shell and tubes to contain its high pressure. Therefore the thermal stress during startup shutdown and load fluctuations can be significant for high temperature heat exchangers. The heat exchanger must be designed accordingly for reliability and long life. The thermal capacitance should therefore be reduced for high temperature heat exchangers for shorter startup time. High temperature heat exchangers also require costly materials contributing to the high cost of balance of power plant. Heat exchanger costs increase significantly for temperatures above 675 C. Heat exchanger types and classifications A variety of heat exchanger types with various features are used in industry. This subsection generally explains how to classify and categorize them. According to Kakac and Liu .
