Tube-side passes

For odd niimher of tube side passes, floating head requires packed joint or expansion joint. 

There is no luTiitation on the number of tube-side passes. Shell-side passes can be one or more, although shells with more than two shell-side passes are rarely used. 

The floating-head cover is usually a circular disk. With an odd number of tube-side passes, an axial nozzle can be installed in such a floating-head cover. If a side nozzle is required, the circular disk is replaced by either a dished head or a channel barrel (similar to Fig. 11-36/) bolted between floating-head cover and floating-tube-sheet sldrt. 

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. 

The layout of the heat exchanger tubesheet determines the number of tubes of a selected size and pitch that will fit into a given diameter of shell. The number of tubes that will fit the shell varies depending upon the number of tube-side passes and even upon whether there is a shell-side pass baffle that divides the shell itself into two or more parts. 

The horizontal natural circulation systems do not use a kettle design exchanger, but rather a 1-2 (1 shell side, 2 tube-side passes) unit, with the vaporized liquid plus liquid not vaporized circulating back to a distillation column bottoms vapor space or, for example, to a separate drum where the vapor separates and flows back to the process system and where liquid recirculates back along with make-up feed to the inlet of the horizontal shell and tube reboiler. See Figures 10-96A-C. 

Remember to keep a standard length if possible and maintain a tube-side pass condition to realize the film conditions established in Step 4. U-tubes are a good selection for this type of service, and a ketde-type shell is usually used. 

For heat transfer performance, horizontal baffles to isolate tube-side passes in horizontal bundles are preferred over vertical baffles that isolate groups of tubes in vertical columns. The expansion of capacity by adding more tube bundles or sections in parallel is easier, and the MTD is better with the horizontal pass plates. The fan drive may be by any of the available means, including ... 

Assume tube-side passes and calculate hj, hj in the usual manner. 

In an exchanger with one shell pass and several tube-side passes, the fluids in the tubes and shell will flow co-currently in some of the passes and countercurrently in the others. For given inlet and outlet temperatures, the mean temperature difference for countercurrent flow is greater than that for co-current or parallel flow, and there is no easy way of... 

With one shell-side pass and two tube-side passes, then from equation 9.213 ... 

Np = number of tube-side passes, ut = tube-side velocity, m/s,... 

We can have any even number of tube-side passes two, four, six, eight, etc. But this certainly limits our flexibility to optimize the tube-side velocity. 

Figure 8.12. Arrangements of cross baffles and tube-side passes, (a) Types of cross baffles, (b) Rod baffles for minimizing tube vibrations each tube is supported by four rods, (c) Tube-side multipass arrangements.

The required heat-transfer area of 19.5 m2 is based on an overall heat-transfer coefficient of 102 W/(m2 K). The best exchanger geometry for this application includes six internal baffles, one shell-side pass and two tube-side passes. The shell is fabricated from standard carbon steel piping of nominal pipe size 30, schedule number 80. The 112 tubes required are each 1.83 m long and 38.1 mm (1.5 in.) o.d. (BWG 12). The tubes must be fabricated from stainless steel type 250 for reasons of temperature tolerance. 

As the shell side fluid is clean, triangular pitch might be suitable and 460 x 25 mm o.d. tubes on 32 mm triangular pitch with 4 tube side passes can be accommodated in a 0.838 m i.d. shell and still allow room for impingement plates. 

The former is closer to a standard shell size and 166 x 19 mm tubes on 25.4 mm square pitch with two tube side passes can be fitted within a 438 mm i.d. shell. In this event, the water velocity would be slightly less than 1 m/s in fact (1 x 146/166) = 0.88 m/s, though this would not affect the overall coefficient to any significant extent. 

For one shell side pass, two tube side passes, Fig. 9.71 applies and F = 0.98. 

Arrangements with 4 and 6 tube side passes require water outlet temperatures in excess of the condensing temperature and are clearly not possible. With 2 tube side passes, T = 327.2 K at which severe scaling would result and hence the proposed unit would consist of one tube side pass and a tube length of 3.05 m. 

Adopting a standard tube length of 4.88 m, number of tubes = (3125/4.88) = 640. With the large flow of water involved, a four tube-side pass unit is proposed, and for this arrangement 678 tubes can be accommodated on 25 mm triangular pitch in a 0.78 m i.d. shell. Using this layout, the film coefficients are now estimated and the assumed value of U is checked. 

There is one tube-side pass and one shell-side pass, and the tube material is stainless steel with a thermal conductivity of 10 Btu/(h)(ft)(°F) [17 W/(m)(K)]. 

Calculate the outlet temperature. Assume that there are several tube-side passes. The outlet temperature can then be calculated using Fig. 7.14. This figure can be employed to directly calculate the outlet temperature of the hot fluid (which is of interest in this example) in such a case, the... 

Calculate the pressure drop for the water flowing through the air-cooled heat exchanger designed in Example 7.37 if the number of tube-side passes is 10. The density of the water is 60 lb/ft3 (961.1 kg/ m3), and the viscosity is 0.74 lb/(ft)(h) (0.31 cP). Assume that the velocity in the nozzles is 10 ft/s (3.05 m/s) and that the viscosity change with temperature is negligible. 

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