Baffles tube-support

The four tube support baffles shown in this exchanger serve a dual function ... 

Find the distance between the adjacent tube support baffles, in inches. 

Count the number of tubes at the edge of the tube support baffle. As shown in Fig. 19.3, this would be seven. 

The tube support baffles must have a diameter somewhat smaller than the ID of the shell. 

Shell-side seal strips. See if you cannot find an old tube bundle lying around your plant. Many such bundles have pairs of metal strips set around the edge of the tube bundle. These metal strips are typically V4 in thick and 4 in wide. They extend down the length of the tubes. As seen in Fig. 19.4, they are inserted in grooves cut in the tube support baffles. These seal strips often increase heat-transfer efficiency by 5 to 10 percent. 

Please note that the tube support baffles are normally installed with the baffle cut in a vertical position. I have shown the baffle cut in Fig. 19.1 in a horizontal position for clarity. Baffles are installed vertically in most exchangers to reduce buildup of sludges in the bottom of the shell and avoid trapping vapors in the top of the shell. 

The shell-side cross-flow velocity may be altered in much smaller increments, by changing the tube support baffle spacing. This is one advantage of placing the fluid with the poorer heat-transfer properties on the shell side. But there is another, far more critical, advantage in placing the fluid with the poorer heat transfer properties on the shell side. 

The rapid changes in direction, due to the tube support baffles... 

Figure 26.1 Shell-and-tube heat exchanger. Tube support baffles 90 from true orientation.

From a theoretical heat-exchanger perspective, the triangular tube pitch shown in Fig. 26.7 is best. The term pitch refers to the geometry and distance between the holes drilled into the tube-support baffles. 

Shell-side flow should be mostly at right angles or perpendicular to the tubes. Unavoidably, as the fluid flows from the inlet nozzle to the outlet nozzle, there is some component of flow parallel to the tubes. The bigger the tube support baffle spacing, the greater the component of the flow parallel to the tubes. A larger baffle cut also increases the component of the flow parallel to the tubes. Thus, to promote perpendicular flow to the tubes or cross-flow, velocity baffle spacing should be about 20 to 30 percent of the bundle diameter. The baffle cut should be about 25 to 35 percent of the bundle diameter. By baffle cut, I mean the cutout section of the round baffle. 

Caution To prevent galvanic corrosion of the tube bundle, the entire tube bundle should be constructed of the same metallurgy this would include the tube support baffles, tube sheets, and tie rods. 

The manufacturers also claim that because of all these points of contact they are able to avoid the use of baffles for tube support and therefore eliminate problems of vibration. (Tube vibration in traditionally designed shell-and-tube exchangers particularly at the point where the tubes pass through the tube support baffles can cause the tubes to fail and leak.)... 

Although the traditional shell-and-tube heat exchangers have segmental tube support baffles at 90° to the line of the tubes, the helically baffled tube bundle has tube support baffles sloped so as to induce a helical flow pattern on the shell side (see Fig. 27.6). 

Figure 27.6 Schematic arrangement showing helical tube support baffles in relation to tube alignment.

For an equivalent shell-side AP, the helical baffle will be less fouling and will provide less film resistance, and less fouling resistance than will a conventional vertical tube support baffle. 

Number of tubes at edge of the tube support baffle = 20... 

The second reason that shell-side flow is not subject to laminar flow is that the liquid only flows the diameter (several feet) of the shell, before its direction is reversed by the tube support baffles. That s not far enough to establish laminar flow. In the ordinary range of operations, there is no such thing as laminar flow on the shell side. 

Don t make the mistake of using c.s. tube support baffles and 316 s.s. tubes. A galvanic cell will be created that will destroy the c.s. tube support baffles. I ve seen the result of this error on an HDS unit reactor effluent. 

To dam off this bypass area, a pair of seal strips is needed. These are typically M-inch thick, as long as the tubes, and perhaps 3 or 4 inches wide. They are set in grooves cut in the tube support baffles on either side of the impingement plate (Fig. 28.1). 

However, when I wrote that the seal strips needed to be as long as the tubes, that is not quite right. One of the pairs of seal strips must have one strip that stops at the first tube support baffle. If not, the shell-side feed would be trapped above the impingement plate. 

If we have vertically cut the tube support baffles (which is normal, at least in the United States), then the seal strip that is aligned with the edge of the first shell-side tube support baffle should extend to the tubesheet. 

Normal design practice is to end both seal strips at the first tube support baffle. This is wrong design practice. It allows the shell-side flow to bypass the exchanger area between the tubesheet and the first tube support baffle. To avoid this problem, the seal strip at the edge of the first tube support baffle must be extended to the tubesheet (see Fig. 28.1). 

The only one of the above that is in widespread use, and used with success in the correct application, is the helical baffle exchanger (see Figure 27.6). The tube support baffles are set at an angle to induce a helical flow pattern on the shell side. The idea is to avoid the dead zones that occur in a conventional vertical tube support baffle. Thus, AP and fouling are both reduced on the shell side. 

Tube support baffles Used to support tubes in a shell-and-tube heat... 

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