Heat pipes high temperature

Figure XIII-4 illustrates the basic concept of the present KAMADO design. The heat generated by a fuel rod is not removed directly with cooling water but via graphite blocks of a fuel element, i.e., cooling water is heated by high temperature graphite of the fuel element. In case of a loss of coolant or flow, such as a pipe break, turbine trip, etc., the decay heat is removed by passive heat transfer from surfaces of the fuel elements to the reactor water pool operating at atmospheric pressure and low temperature.

Heat pipes are used to perform several important heat-transfer roles ia the chemical and closely aUied iadustries. Examples iaclude heat recovery, the isothermaliziag of processes, and spot cooling ia the mol ding of plastics. In its simplest form the heat pipe possesses the property of extremely high thermal conductance, often several hundred times that of metals. As a result, the heat pipe can produce nearly isothermal conditions making an almost ideal heat-transfer element. In another form the heat pipe can provide positive, rapid, and precise control of temperature under conditions that vary with respect to time. 

Heat/Solvent Recovery. The primary appHcation of heat pipes in the chemical industry is for combustion air preheat on various types of process furnaces which simultaneously increases furnace efficiency and throughput and conserves fuel. Advantages include modular design, isothermal tube temperature eliminating cold corner corrosion, high thermal effectiveness, high reHabiHty and options for removable tubes, alternative materials and arrangements, and replacement or add-on sections for increased performance (see Furnaces, fuel-FIREd). 

The regenerative nature of the Wulff operation permits the recovery of most of the sensible heat in the cracked gas. The gases leave the furnace at temperatures below 425°C, thus obviating the need for special high temperature alloys in the switch valve and piping system. 

J. E. Kemme, J. E. DeveraH, E. S. Keddy, J. R. Phillips, and W. A. Rankin, Temperature Control with High Temperature Gravity A Fst Heat Pipes Los Alamos Scientific Laboratory, 1975, available from National Technical Information Service, Accession No. CONE-750812-10. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

Their main applications have been in heat-resistant structural laminates, electrical laminates resistant to solder baths, chemical-resistant filament-wound pipe and high-temperature adhesives. 

Petroleum products may be treated with various solvents for the removal by selective solubility of undesirable constituents or for the recovery of by-products. The solvent and solute must be separated to yield the desired product and to recover the solvent for reuse. The solvents normally boil at a lower temperature than the products from which they are to be removed and so are generally distilled off as overhead products. The pipe stills used for this service may be single-stage or multi-stage units, depending on the service involved. Some solvents can be removed by the use of steam heated stills. In other cases, the high temperature required necessitates the use of fired heaters and vacuum towers. 

In two stage units, it is often economical to distill more gas oil in the vacuum stage and less in the atmospheric stage than the maximum attainable. Gas formed in the atmospheric tower bottoms piping at high temperatures tends to overload the vacuum system and thereby to reduce the capacity of the vacuum tower. The volume of crude vaporized at the flash zone is approximately equal to the total volume of distillate products. Of course, the vapor at this point contains some undesirable heavy material and the liquid still contains some valuable distillate products. The concentration of heavy ends in the vapor is reduced by contact with liquid on the trays as the vapor passes up the tower. This liquid reflux is induced by removing heat farther up in the tower. 

This type of cast iron is made by high-temperature heat treatment of white iron castings. The mechanical properties of malleable cast irons are given in Table 3.1 usually they are applied to the fabrication of conveyor chain links, pipe fittings and gears. 

For a high-temperature system, a separate subheader may be run up to the point where the temperature drops down to the allowable limit of a less expensive material. It may then be connected to the main flare header (either low pressure or high pressure).To properly evaluate this a heat loss calculation is needed. As a rule of thumb a heat loss of 10 BTU/hr/ft may be assumed for a quick estimate for bare pipe. Consideration should also be given to the need for expansion joints. Main flare headers may be as large as 36 to 42 inches in diameter for a large-capacity plant. Expansion joints of such magnitudes may be so expensive as to call for a separate small header for the hot flare system. 

Merrigan, M., Dunwoody, W., and Lundberg, L., Heat Pipe Development for High-Temperature Recuperator Application, ,/ Heat Recovery Sy St., 2(2) 125-135 (1982)... 

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