Floating Tube Bundle

This design will take care of the expansion of tubes when temperature variation is greater for the fluids on tube side than on the shell side. 

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Kettle-type reboilers, evaporators, etc., are often U-tube exchangers with enlarged shell sec tions for vapor-liquid separation. The U-tube bundle replaces the floating-heat bundle of Fig. 11-36. ... 

The tube bundle is removable, and the floating tube sheet moves (or floats) to accommodate differential expansion between shell and tubes. The outer tube hmit approaches the inside diameter of the gasket at the floating tube sheet. Clearances (between shell and OTL) are 29 mm (IV in) for pipe shells and 37 mm (I ifi in) for moderate-diameter plate shells. 

With an even number of tube-side passes the floating-head cover serves as return cover for the tube-side fluid. With an odd number of passes a nozzle pipe must extend from the floating-head cover through the shell cover. Provision for both differential expansion and tube-bundle removal must be made. 

Tube-Side Passes Most exchangers have an even number of tube-side passes. The fixed-tube-sheet exchanger (which has no shell cover) usually has a return cover without any flow nozzles as shown in Fig. 11-35M Types L and N are also used. All removable-bundle designs (except for the U tube) have a floating-head cover directing the flow of tube-side fluid at the floating tube eet. 

Many of the rear end types allow the head to "float" as the tube bundle expands and contracts. 

From Table 3-4 it is then possible to pick a shell diameter that can accommodate the number of tubes required. Please note that Equation 3-2 calculates the total number of tubes required and not the number of tubes per pass. Similarly, Table 3-4 lists the total number of tubes and not the number per pass. There are fewer total tubes in the same diameter exchanger for more passes of the tube fluid because of the need for partition plates. There are fewer tubes for floating head than fixai head designs because the heads and seals restrict the use of space. U-tubes have the lowest number of tubes because of the space required for the tightest radius bend in the U-tube bundle. 

Kettle Reboiler U-Bundle or Floating Tube Sheet Single Shell Poss... 

Sheli Cover Bolted to Shell. Shell Cover Bolted to Shell. Floating Head Caver Baited ta Floating Head Cover Boited Tube Sheet or its Backing Ring. Expansion of Tube Bundle to Tube Sheet or its Provided by External Packing Gland or by Internal Bellows... 

In order to allow for the removal of the tube bundle and for considerable expansion of the tubes, a floating head exchanger is used, as shown in Figure 9.63. In this arrangement one tube plate is fixed as before, but the second is bolted to a floating head cover so that the tube bundle can move relative to the shell. This floating tube sheet is clamped... 

The detailed design of the tube bundle must take into account both shell-side and tube-side pressures since these will both affect any potential leakage between the tube bundle and the shell which cannot be tolerated where high purity or uncontaminated materials are required. In general, tube bundles make use of a fixed tubesheet, a floating-head or U-tubes which are shown in Figures 9.62, 9,63 and 9.64 respectively. It may be noted here that the thickness of the fixed tubesheet may be obtained from a relationship of the form ... 

Adequate space must be left around all equipment so that it can be easily serviced and operated. For instance, a floating head heat exchanger must have enough clear space so that the tube bundle can be removed from the shell and taken elsewhere for repairs. One company had problems with a heat exchanger during startup. They tried to remove the tube bundle, but found that they had not allowed adequate space and had to knock an opening in a brick wall. They replaced the wall with a door so that they would not need to remove any more walls when they needed to service it again. 

It is quite true that the floating head permits differential rates of thermal expansion between the shell-and-tube bundle of an exchanger. However, the floating head cannot permit differential rates of thermal expansion between individual tubes. 

Unless specified, all water is untreated, brackish, bay or sea. Notes H = horizontal, fixed or floating tube sheet, U = U—tube horizontal bundle, K = kettle type, V = vertical, R = reboiler, T=thermosiphon, v = variable, HC = hydrocarbon, C) = cooling range At, (Co) = condensing range At. 

A floating head-type shell-and-tube heat exchanger is recommended for this application because of the need to provide capacity for thermal expansion of the tube bundle. The floating head also enables easy withdrawal of the tube bundle for cleaning purposes. This factor may be very advantageous, not because the streams are subjected to fouling, but because of the possibility that platinum carryover from the reactor will be deposited on the walls of the tubes. Platinum recovery is improved by providing easy access. 

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