U-Bend Tubes

For U-bend tubes the following modifications are required Frictional pressure drop, APf  

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Modifications include the use of U-bend tubes to facilitate bundle rentoval for inspection and cleaning. 

Examination Both tubes showed cracks extending circumferentially above 180°C on the tension side of the U-bend. Tube 1 showed a relatively... 

As with other removable-bundle exchangers, the inside of the shell, and to the outside of the tubes. However, unlike the straight-tube exchanger, whose tube internals can be mechanically cleaned, there is no way to physically access the U-bend region inside each tube, so chemical methods are required for tubeside maintenance. 

The recommended minimum radius of bend for various tubes is given in Table 10-5. These measurements are for 180° U-bends and represent minimum values. 

TEMA, Par. RGB 2.31 recommends the minimum wall thinning of tubes for U-Bends by the minimum wall thickness in the bent portion before bending, p. 

Figures 10-25A-E are for U-bundle tubes and require a wide blank space across the center of the tubesheet to recognize the U-bend requirement at the far end of the tube bundle, see Figures 10-lE, 10-lAitem U, and 10-lK Figures 10-25F-Kare for fixed tubesheet layouts.

Elimination of one tubesheet reduces initial cost. Tube bundle is removable for inspection and cleaning. Full tube bundle minimizes shell-side bypassing. U-bends permit each tube to expand and contract individually. Tube bundle expansion is independent of shell no expansion diaphragm is required. 

The tube bundle is an assembly of tubes rolled into tubesheets and assembled into headers. See Figures 10-175, 10-176, 10-178, 10-179 and 10-180. The usual headers are plug and cover plate but can accommodate U-bend types if the design so dictates. 

The dissipated energy due to flow resistance and turbulence F consists of four parts the energy consumption in the riser, that in the separator tubes, X that in the down-comer, and that due to the flow direction changes of the U-bending channel at the top and bottom of the reactor,/,. Therefore, F can be written as ... 

If U-tubes are used the number of tubes will be slightly less than that given by equation 12.3a, as the spacing between the two centre rows will be determined by the minimum allowable radius for the U-bend. The minimum bend radius will depend on the tube diameter and wall thickness. It will range from 1.5 to 3.0 times the tube outside diameter. The tighter bend radius will lead to some thinning of the tube wall. 

Not all shell and tube exchangers have a floating head. Many exchangers have individual U bends for each tube. Then, each of the U bends functions like a mini-floating head for each tube. 

Typically, the tubing in the convection zone is placed in a hairpin fashion or series of connected U bends and is suspended horizontally. The steam that will be used as a diluent is also heated in the convection zone in other tubes. This superheated steam and the gaseous hydrocarbons are... 

Tubular reactors normally take one of three geometries. The most common consists of tubing of the same diameter connected by U bends. The internal diameters are often 4 in. The total length of one reactor coil may be 300 ft. Four to eight coils are commonly placed within a single furnace. 

Id. For U-bends, heat treat the entire 18Cr-8Ni SS tube at 1.850 F (1,000 0) minimum after bending. Use type 321 or type 347 SS only if the U-bends are heat treated. 

FIQ. 8.13. (Left) Spiral coils, "(a) Shows tlie small U-bend ready to hook into the metal loop on tlie forming tool, (b) Tlie elevation of tlie tube at tlie beginning, (c) Tlie elevation of tlie tube when winding is under way. 

Series flow between passes of the absorber was achieved through U-bends instead of headers, allowing viewing of the solution flow in the axial direction of the tubes. The coolant entered an inlet manifold, split into two rows of tubes in parallel, flowed through... 

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