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Preventing leakage
High pressure and leakage problems go together. A wide variety of standard gaskets are available to suit most applications For very high pressures,a Bridgeman-type closure may be employed. This device takes advantage of the high pressure to effect the seal. For joining tubes to tube sheets, seal welding or strength welding may be necessary, besides the common tube roiling. For cover plates or closure heads, seal-welded membranes may be employed. These seals can be removed
by using a cutter and then be rewelded.Constructions for Reducing Size and Cost When one of the fluids has a low heat transfer coefficient for a reasonable pressure drop, the use of extended surface on the heat exchanger is often more economical, as for example, in the aircooled condenser used in the Southwest. This surface takes on a number of different forms. Fins or studs may be welded on or mechanically attached to tubing. Some extended surface is produced by rolling or forming the fins from the base tube metal. In general, the least costly means involve mechanical attachment of extended surface, but the best thermal and mechanical bond required by severe service is obtained by welding, brazing, or rolling on the fins or studs. If the outside diameter of the finned surface is no greater than the outside diameter of expanded tube ends, the
surface is termed "low fin," and the tubing may be used interchangeably with smooth tubing of the same diameter,thus lending itself well to standard constructions. Extended surface may also be use dinside tubes. This can be accomplished in cast tubes and also extruded tubes, and some special types of heat exchangers are available with this construction.A compact and generally economical
arrangement is sometimes achieved by employing plate-type heat exchangers.One such surface arrangement consists of an assembly of flat corrugated plates. Another form is the spiral plate exchanger. More recently, progress has been achieved in the design and fabrication of so-called “packed-surface”heat exchangers wherein extended surface is employed on both sides of the fluid-piding plates. A variety of fabrication techniques has been developed to accomplish seal, manifolding,and general structural integrity of plate-type heat exchangers, although such exchangers are generally restricted to applications under 200 p.s.i.
Provisions for Extreme Corrosion Resistance
Where low volume and relatively low pressures and temperatures are involved,but highly corrosive conditions exist, impervious graphite and glass have been employed successfully as heat
exchanger materials. Graphite can be formed in almost any shape and, thus, typical shell-and-tube heat exchanger designs can be used. The type may depend on whether the fluid on one side or on both sides of the heat exchanger is corrosive. One graphite exchanger arrangement consists of a block with passage sin which the corrosive fluids are directed perpendicularly to one another. Glass is used to coat the inside or outside of metal tubes and to line the inside of shells in some heat exchanger constructions. The structural strength of one material may be combined with the corrosive resistance of another by employing metallic cladding. A thin layer of stainless steel is often rolled or spot-welded to a less costly base material, such as carbon steel. Electrolytic or chemical plating is also employed to lay a corrosion- resistant film on a structural material. Sometimes fluids employed in a process may not be compatible with the same tubing material. In this case, it is possible to obtain a dual-metal tube, with one material, such as a nickel alloy, on one side and aluminum, for example, on the other. Such tubing is often produced by co-drawing in order to obtain a tight bond between the two materials.RECEIVEfDor review
换热器的机械设计在许多配置和类型所必需的各种流体和宽范围的温度和压力，在化学工业中遇到的热交换器使选择设计的经济学一个复杂的问题