The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 39
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The key to such flexibility isthe wide range of materials of construction, forming and joining methods, and design features that canbe built into these exchangers (see Schlünder, Vol. 4, 1983; Saunders, 1988; and Yokell, 1990). Most© 1999 by CRC Press LLC4-166Section 4shell-and-tube heat exchangers are manufactured in conformity with TEMA Standards (1988) and theASME Boiler and Pressure Vessel Code (latest edition), but other codes and standards may apply.Construction FeaturesIn the design process, it is important to consider the mechanical integrity under varying operationalconditions and the maintainability (especially cleaning) of the exchanger as equally important with thethermal-hydraulic design.Tubes.
Tubes used in shell-and-tube exchangers range from 6.35 mm (1/4 in.) to 50.8 mm (2 in.) andabove in outside diameter, with the wall thickness usually being specified by the Birmingham wire gauge(BWG). Tubes are generally available in any desired length up to 30 m (100 ft) or more for plain tubes.While plain tubes are widely used, a variety of internally and/or externally enhanced tubes is availableto provide special heat transfer characteristics when economically justified (see subsection on enhancement in Section 4.8). Low fin tubes having circumferential fins typically 0.8 to 1.6 mm (0.032 to 0.062in.) high, spaced 630 to 1260 fins/m (16 to 32 fins/in.) are often employed, especially when the shellside heat transfer coefficient is substantially smaller than the tube-side coefficient.
The outside heattransfer area of a low fin tube is three to six times the inside area, resulting in a smaller heat exchangershell for the same service, which may offset the higher cost of the tube per unit length.The tubes are inserted into slightly oversized holes drilled (or, occasionally, punched) through thetubesheets (items 5 and 13, Figure 4.5.18). The tubes are secured by several means, depending upon themechanical severity of the application and the need to avoid leakage between the streams.
In some lowseverity applications, the tubes are roller-expanded into smooth holes in the tubesheet. For a strongerjoint, two shallow circumferential grooves are cut into the wall of the hole in the tubesheet and the tuberoller-expanded into the grooves; to eliminate the possibility of leakage, a seal weld can be run betweenthe outer end of the tube and the tubesheet.
Alternatively, the tubes may be strength-welded into thetubesheet.Tube Supports. It is essential to provide periodic support along the length of the tubes to prevent saggingand destructive vibration caused by the fluid flowing across the tube bank. A secondary role played bythe tube supports is to guide the flow back and forth across the tube bank, increasing the velocity andimproving the heat transfer on the shell side (but also increasing the pressure drop).
The tube supportis usually in the form of single segmental baffles (item 18 in Figure 4.5.18) — circular plates with holesdrilled to accommodate the tubes and with a segment sheared off to form a “window” or “turnaround”to allow the shell-side fluid to pass from one cross-flow section to the next. The baffles must overlap atleast one full row of tubes to give the bundle the necessary rigidity against vibration. When minimizingshell-side pressure drop is not a priority, a baffle cut of 15 to 25% of the shell inside diameter is customary.Baffle spacing is determined first by the necessity to avoid vibration and secondarily to approximatelymatch the free cross-flow area between adjacent baffles to the flow area in the window; i.e., small bafflecuts correspond to closer baffle spacing.In situations such as low-pressure gas flows on the shell side where pressure drop is severely limited,double segmental and strip baffle arrays can be used.
More recently, a helical baffle arrangement hasbeen introduced (Kral et al., 1996) which causes the shell-side fluid to spiral through the exchangergiving improved heat transfer vs. pressure drop characteristics. Where vibration prevention and/orminimum pressure drop are the main concerns, grids of rods or strips can be used (Gentry et al., 1982).Shells. The shell is the cylinder which confines the shell-side fluid (item 7 in Figure 4.5.18), fitted withnozzles for fluid inlet and exit.
Diameters range from less than 50 mm (2 in.) to 3.05 m (10 ft) commonly,and at least twice that value for special applications. In diameters up to 610 mm (24 in.), shells areusually made from standard pipe or tubular goods by cutting to the desired length; in larger sizes, metalplates are rolled to the desired diameter and welded.© 1999 by CRC Press LLCHeat and Mass Transfer4-167A variety of nozzle arrangements are used for special purposes, and TEMA has a standard code toidentify the major types, as well as the various front and rear head configurations on the tube side. Figure4.5.19 shows these configurations with the corresponding code letters.FIGURE 4.5.19 TEMA nomenclature for shell and tube configurations.
(From TEMA, Standards, 7th ed., TubularExchanger Manufacturers Association, Tarrytown, NY, 1988. With permission.)© 1999 by CRC Press LLC4-168Section 4The E shell (center column, top) has the nozzles on opposite ends of the shell and is the most commonconfiguration. It is used for any of the thermal services (single-phase heating or cooling, vaporization,and condensation). The nozzles may be on opposite sides of the shell as shown, or on the same side;the choice is largely determined by plumbing convenience. The E shell allows countercurrent flow (seebelow) of the two streams if there is one tube-side pass (i.e., the tube-side fluid flows through all of thetubes in parallel).The F shell has both nozzles at one end of the shell and uses a longitudinal baffle on the shell side(shown dashed in the drawing) to force the shell-side fluid to flow to the far end of the heat exchangerand then back to the exit nozzle on the other side of the longitudinal baffle.
Ideally, this allowscountercurrent flow of the two streams if there are two tube-side passes (i.e., the tube-side fluid flowsthrough half of the tubes in one direction, is turned around in the rear head, and returns through theother half of the tubes — see discussion of head types below). However, the longitudinal baffle mustbe carefully sealed to the shell to prevent leakage of the shell-side fluid across it; this is done by weldingthe longitudinal baffle to the shell and front tubesheet (which limits some design options) or by usingmechanical seals. The F shell is mainly used for sensible heat transfer services.The G shell has both nozzles at the center of the shell, with a centrally located longitudinal baffle toforce the fluid to the ends of the shell before returning. While the G shell is used for all services, itsmain application is as a shellside vaporizer with either forced or natural (thermosiphon) convection ofthe boiling fluid; in the latter service, limited leakage across the baffle generally does not greatly degradethe thermal performance and the longitudinal baffle does not need to be perfectly sealed against the shell.The H shell is effectively a double G shell and is employed in the same services.
It is consideredwhen the calculated shell-side pressure drop for a G arrangement is too high and threatens to limit thecirculation rate.The J shell, with one nozzle on top of the shell and two on the bottom, or vice versa, is commonlyused in vacuum-condensing applications because of its low pressure drop.
Two J shells (one inverted)may be mated in series for long-condensing-range mixtures. The nozzles are usually different diameters,with the large diameter accommodating the inlet vapor. The baffles are vertically cut.The K shell (or kettle reboiler or flooded chiller) is exclusively intended for vaporization of liquid onthe shell side, with a condensing vapor (usually steam) or a hot liquid on the tube side as the heatingmedium. The tubesheet diameter is large enough to accommodate the tube bundle, but the shell transitionsto a larger diameter to allow the vapor to disengage from the liquid pool and exit from the top nozzle.A weir or other level control is used to maintain the liquid level, usually just above the top tubes in thebundle.The X shell is intended to provide a well-distributed cross flow of the shell-side fluid, the fluid usuallyentering at the top and exiting at the bottom but occasionally used for upflow or horizontal cross flow.To obtain good distribution, multiple nozzles from a properly designed manifold may be required.Alternatively, the upper tubes in the bundle may be omitted to allow internal redistribution, or a largeplenum chamber may be welded to the top of the shell (“vapor dome” or “bathtub nozzle”), or a divergingtransition section may be placed between the inlet piping and the top of the shell.
The tube supportsmay be complete circles since there is little or no longitudinal shell-side flow. The X shell gives thelowest shell-side pressure drop of any configuration and is often used for low-pressure vapor condensers.Front Head. TEMA recognizes several front head designs as shown in the first column of Figure 4.5.19.Any of these designs will get the tube-side fluid into the tubes, but each has special features whichrecommend it to meet special needs. In Figure 4.5.19 the dashed lines indicate optional features dependingupon need.The A head, a channel with removable cover, bolts directly to the shell flange as shown in Figure4.5.18, the tubesheet in that case being held between them and sealed with gaskets.
Alternatively, thetubesheet may be integral with the shell (see the L rear head in Figure 4.5.19). A removable channelcover permits inspection, cleaning, removal, and replacement of tubes without disturbing the piping.The dashed lines at the center and the lower nozzle indicate that a pass partition plate may be welded© 1999 by CRC Press LLCHeat and Mass Transfer4-169in the channel (and gasketed against the tubesheet and channel cover) to provide for two tube-side passes(as shown in Figure 4.5.18 and required by the F shell design).
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