Section 3.13 High Temperature Heat Transfer Fluids

In this chapter in Section 3.1 we consider mechanical drives. In Sections 3.2 and 3.3 furnaces and exchangers, condensers and reboUers are considered followed by fluidized bed with coil in the bed, Section 3.4 and static mixers. Section 3.5. Direct contact systems are considered next liquid-liquid. Section 3.6 gas-liquid cooling towers. Section 3.7 gas-liquid quenchers. Section 3.8 gas-liquid condensers. Section 3.9, and gas-gas thermal wheels. Section 3.10. Heat loss to the atmosphere is described in Section 3.11. Refrigeration, steam generation and high temperature heat transfer fluids are presented in Sections 3.12 to 3.14, respectively. Tempered heat exchange systems are considered in Section 3.15. 

Shiralkar and Griffith (1970) determined both theoretically (for supercritical water) and experimentally (for supercritical carbon dioxide) the limits for safe operation, in terms of the maximum heat flux for a particular mass flux. Their experiments with a twisted tape inserted inside a test section showed that heat transfer was enhanced by this method. Also, they found that at high heat fluxes, DHT occurred when the bulk fluid temperature was below and the wall temperature was above the pseudocritical temperature. 

Heat transfer to the tubes on the furnace walls is predominantly by radiation. In modern designs this radiant section is surmounted by a smaller section in which the combustion gases flow over banks of tubes and transfer heat by convection. Extended surface tubes, with fins or pins, are used in the convection section to improve the heat transfer from the combustion gases. Plain tubes known as shock tubes are used in the bottom rows of the convection section to act as a heat shield from the hot gases in the radiant section. Heat transfer in the shield section will be by both radiation and convection. The tube sizes used will normally be between 75 and 150 mm diameter. The tube size and number of passes used depend on the application and the process-fluid flow rate. Typical tube velocities will be from 1 to 2 m/s for heaters, with lower rates used for reactors. Carbon steel is used for low temperature duties stainless steel and special alloy steels, for elevated temperatures. For high temperatures, a material that resists creep must be used. 

Prediction of the heat-transfer coefficient for this problem is complicated by the fact that the local coefficient is very high at the start of heat transfer and decreases as the conduction into or out of the fluid builds up an adverse temperature gradient. A number of analytical and numerical solutions for this and other constant cross-sectional geometries are given by Shah and London [11], but most... 

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