U-bend exchangers

Generally, U-bend exchangers should not be installed in a vertical position. Vertical U-bends are difficult to vent or drain on the tubeside because connections cannot be provided at the U-bend end. 

Multipass exchangers with straight tubes preferrably should not be installed vertically unless tubeside velocities are relatively high and provisions are made to ensure that the tubes operated completely flooded. 

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Material Type 316 stainless steel U-bend exchanger tube. Temperature of tube side. Service life is 5 years at 21°C. 

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. 

Water can be trapped behind heat exchanger baffles and then suddenly vaporized by circulation of hot oil. It can also be trapped in dead-ends and U-bends in pipework (see Section 9.1.1). Such U-bends can form when one end of a horizontal pipe is raised by thermal expansion. The trays in a distillation column were damaged during startup when hot gas met water, from previous steaming, dripping down the column [3J. Section 17.12 describes an incident somewhat similar to a foamover. 

Figure 10-25A. Tubesheet layout for U-tube exchanger. Tube passes two or four. Tube sizes and pitch % in. on in.A. Radius of bend 2Vj X tube diameter.

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. 

The U-tube exchanger (Fig. 4) is the best solution to the thermal stress problem, because each tube is free to expand or contract independently of the others or of the shell. However, there are disadvantages to the U-tube exchanger the tubes cannot be mechanically cleaned around the bend, the inner tubes cannot be individually replaced, the long-radius U-tubes are particularly subject to vibration, and single-pass tube-side flow is not possible. 

When the required length of spiral has been formed it should be allowed to cool for a few seconds. The former is now cautiously rotated in the opposite direction and the spiral held lightly and firmly. This should free the small U-bend from the metal loop and the helical coil can be easily slid off. No annealing is necessary and as soon as the coil is cool enough to handle the ends can be prepared for further work in making a condenser or heat exchanger. 

The cathode area is sometimes the noncold-worked area in a component that has been partially cold worked, for example, the straight-run tubing in a U-bend heat exchanger bundle. 

Stress relief also reduces stress cracking in materials that have been cold worked. Conunon applications are for spun or pressed heads and for the U-bends in heat exchanger bundles. 

FIG. 11-4 Continued) LMTD correction factors for heat exchangers. In all charts, R = Ti- T2)/ t2-ti) and S = t2-ti)/ Ti - ti). (i) Cross-flow (drip type), two horizontal passes with U-bend connections (trombone type), (j) Cross-flow (drip type), helical coils with two turns. 

Simonson et al (1994) constructed a VXD based on the same principle, except that they exchanged the position of the permanent magnet and the drive/pickiip wires. In this VXD the permanent magnet was mounted on the vibrating tube close to the U-bend, while two wire-coil electromagnets were fixed to the block. Blencoe et al (1996) developed a VXD for determine the densities of fluids in the range 10-200 MPa and 150-500 °C. 

For double pipe exchangers, assume an 8" schedule 20 OD and a 6" schedule 40 ID pipe with a length of 12 ft including u-bend. 

Stress corrosion cracking of a U-bend waste fuel heat exchanger tube. 

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. 

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