The CRC Handbook of Mechanical Engineering. Chapter 4. Heat and Mass Transfer (776127), страница 40
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Additional pass partitions may be providedto allow four, six, or any even number of tube-side passes. This permits the designer to utilize theavailable tube-side pressure drop to increase velocity, improve the heat transfer coefficient, and possiblyreduce fouling. A second nozzle is required on the channel for multipass designs.The B, or bonnet, front head reduces the number of gasketed joints and thus the opportunity forleakage, but it does not permit inspection of the tubes without breaking the piping connection.
It isgenerally less expensive than the A head.C and N heads retain the removable cover feature of the A head but, respectively, replace the channelto-tubesheet and the tubesheet-to-shell gasketed connections with welds to minimize leakage possibilities. The N head is particularly used in nuclear services.The D head is mainly used in feed-water heater applications where tube-side pressures are in the 100to 400 bar range.
The internal partition (optional) need only withstand the 1 to 2 bar pressure dropthrough the tubes so it can be of lightweight construction. The high-pressure closure against the atmosphere uses a shear key ring to lock the main closure in place.Rear Head. A variety of rear head designs are used in shell-and-tube exchangers, primarily because ofthe need to accommodate thermally induced stresses.
During operation, the tubes and the shell havedifferent temperatures and therefore will expand (or try to) different amounts, even if there were noresidual stresses in the exchanger before start-up and even if the entire exchanger is made out of thesame material. The thermal stress problem is exacerbated if there are residual stresses, or if the exchangeris made of different materials, or during transient operation (including start-up and shutdown). If thetemperature differences are small, the structure may be able to accommodate the thermal stresses safely;usually, however, it is necessary to make specific provision to allow the shell and the tubes to expandor contract independently. Failure to do so can result in buckling, bending, or even rupture of the shellor the tubes, or destruction of the tube-to-tubesheet joint.A simple solution is to incorporate an expansion joint or a bellows into the shell (or in certain specialapplications, into the tube-side piping internal to the shell cover).
However, this solution cannot coverthe entire range of pressures and temperature differences encountered in practice. Further, it is usuallypossible to incorporate other desirable features, such as removable bundles, with thermal stress relief inthe variety of rear head designs available. These are shown in the last column of Figure 4.5.19.The L and M rear heads correspond to the A and B front heads previously described. As shown, theyrequire a fixed tubesheet design; that is, the tubesheets are rigidly fastened to the shell, and thermalstress relief, if necessary, must be provided by a shell-side expansion joint or bellows.
The tube bundlecannot be removed for inspection or mechanical cleaning on the shell side. However, the outer tube limit(OTL) — the diameter of the tube field circumscribing the outermost tubes in the bundle — can be aslittle as 0.4 in. (10 mm) less than the inside diameter of a pipe shell and 0.5 in. (12.7 mm) for a rolledshell. Therefore, the tube field can be very full, giving more tubes and minimizing bypass flow. Similarcomments apply to the N rear head, except that more clearance must be left between the outermost tubesand the shell.The type P head uses packing between the skirt on the rear tubesheet and the shell extension to sealthe shell-side fluid against leakage.
The compression on the packing has to be adjusted to preventexcessive leakage on the one hand and to allow limited movement of the tube-side head on the other,so the shell-side fluid must be benign and cheap (not surprisingly, it is often cooling water). On theother hand, leakage between the two fluids can occur only through tube hole leaks. Because of thetubesheet skirt, clearance between the outermost tubes and the shell must increase compared with typesL or M; accordingly, fewer tubes are possible in a given shell, and sealing strips to partially block thebundle-to-shell bypass stream are recommended.
When the floating head cover and packing gland areremoved, the tube bundle can be pulled out of the shell for inspection and cleaning.The TEMA S split-ring floating head design uses a split backing ring to hold the floating head coverand its gasket to the tubesheet. The split backing ring is bolted to the cover with a bolt circle outside© 1999 by CRC Press LLC4-170Section 4the diameter of the tubesheet.
Therefore, when the split ring is removed, the entire tube bundle may bepulled out of the shell. Tube count is similar to type P design and sealing strips are recommended.Usually, the split-ring floating head is used with an even number of tube passes so that a plain bonnettype shell cover can be used. However, as shown by the dashed lines in Figure 4.5.19, single tube-sidepass design (and countercurrent flow) can be achieved by use of a packing gland on the exit pipingthrough the bonnet; alternatively, a deep bonnet can be used together with an expansion joint or bellowson the tube-side exit piping.The pull-through floating head, type T, uses a floating head cover that flanges directly to the tubesheet,reducing the possibility of internal leakage compared with type S, but also eliminating more tubes aroundthe periphery.
Sealing strips are a virtual necessity. Single tube-side pass design is similar to type S, butis rarely used.TEMA type U uses a bundle of U tubes and hence requires no rear head at all. The U-tube bundleeffectively eliminates the thermal stress problem between shell and tubes, because each tube is free toexpand or contract independently. The U bundle is also the cheapest construction because the cost of asecond tubesheet is avoided.
However, there are a number of drawbacks: designs must have an evennumber of tube-side passes, mechanical cleaning of the smaller bend radius tubes in the U bend isimpossible, individual tubes cannot be replaced except in the outer row, some tube count is lost becauseof minimum bend limits, and the U bend must be carefully supported against vibration or kept out ofthe cross-flow stream by placing the shell nozzle upstream of the bend. The tube side in the U bend issusceptible to erosion, especially with two-phase or particulate-containing fluids.Type W uses two sets of packing, often with a lantern ring in between.
This construction is generallylimited to benign fluids and low to very moderate pressures and temperatures.Other Features. Numerous other components are necessary or optional to construction of shell-and-tubeexchangers. Probably the most complete discussion is given by Yokell (1990).Principles of DesignDesign Logic. The design of a shell-and-tube exchanger involves the following steps:1. Selection of a set of design features which are required for mechanical integrity and ease ofmaintenance, and which will likely lead to satisfying the thermal requirements within the allowablepressure drops, and at lowest cost.2.
Selection of a set of dimensions for the actual exchanger.3. For the dimensions selected in (2), calculation of the thermal performance of the heat exchangerand both tube-side and shell-side pressure drops, using available rating procedures.4. Comparison of the thermal performance calculated in (3) with that required and examination ofthe pressure drops calculated in (3) to ensure that the allowed pressure drops are reasonably usedbut not exceeded.5.
Adjustment of the dimensions selected in (2) and repetition of steps (3) and (4) until the criteriaare satisfied.6. Completion of the mechanical design to satisfy code requirements.7. Cost estimation.Basic Design Equations. The basic design equation for a shell-and-tube exchanger in steady-state service isA* =òqT0dqU * (Th - Tc )(4.5.87)where A* is the heat transfer area required in the heat exchanger, m2 (ft2); qT is the heat transfer rate ofthe heat exchanger, W (Btu/hr); U* is the local overall heat transfer coefficient referenced to area A*,W/m2 K (Btu/hr ft2 °F); and Th and Tc are the local hot and cold stream temperatures, K (°F).
The *© 1999 by CRC Press LLC4-171Heat and Mass Transfersuperscript on A* and U* only means that a consistent reference area must be used in defining theseterms. For example, for an exchanger with plain tubes, it is customary to use the total outside heattransfer area of all of the tubes in the exchanger, Ao, as the reference area, and then Uo is the overallheat transfer coefficient referenced to Ao. If the exchanger has low-finned tubes, A* may refer either tothe total outside area including fins or to the inside tube heat transfer area; the choice is optional, butmust be spelled out. Since Th and Tc generally vary with the amount of heat transferred (following thefirst law of thermodynamics, and excepting isobaric phase transition of a pure component) and U* mayvary with local heat transfer conditions, in principle Equation 4.5.87 must be numerically integratedwith Th , Tc, and U* calculated along the path of integration, and this process is performed by the mostadvanced computer-based design methods.For many applications, certain reasonable assumptions can be made allowing the analytical integrationof Equation 4.5.87 to give (Schlünder, Vol.
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