Heat exchangers, baffles passes

Several descriptions have been pubUshed of the continuous tar stills used in the CIS (9—11). These appear to be of the single-pass, atmospheric-pressure type, but are noteworthy in three respects the stills do not employ heat exchange and they incorporate a column having a bubble-cap fractionating section and a baffled enrichment section instead of the simple baffled-pitch flash chamber used in other designs. Both this column and the fractionation column, from which light oil and water overhead distillates, carboHc and naphthalene oil side streams, and a wash oil-base product are taken, are equipped with reboilers. 

Figure 1.7 Simple detail of shell-and-tube heat exchanger. The water box may be designed for as many as eight passes, and a variety of configurations of shell-side baffles may be used to improve heat transfer, (a) Several water box arrangements for tube-side cooling, (b) Assembly of simple two-pass exchanger with U-tubes. [Fig. 38.2, The Nalco Water Handbook, 1st ed. (1979), reprinted with permission from McGraw-Hill, Inc.)...

Figure 10-8. Single-pass shell and tube Teflon tube heat exchanger, countercurrent flow. Tube bundles are flexible tube Teflon joined in integral honeycomb tubesheets. Shell-side baffles are provided for cross-flow. Standard shell construction is carbon steel shell plain or Teflon (LT) lined. Heads are lined with Teflon . Tube diameters range from 0.125-0.375 in. O.D. the temperature range is 80-400°F pressures range from 40-150 psig. (Used by permission AMETEK, Inc., Chemical Products Div., Product Bulletin Heat Exchangers of Teflon . )...

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. 

TWo tubular heat exchangers are available each with a 0.44 m i.d. shell fitted with 166 tubes, 19.0 mm o.d. and 15.0 mm i.d., each 5.0 m long. The tubes are arranged in two passes on 25 mm square pitch with a baffle spacing of 150 mm. There are two passes on the shell side and operation is to be countercurrent. With benzene passing through the tubes, the anticipated film coefficient on the tube side is 1000 W/m2K. 

Ammonia Vaporizer A shell and tube-type heat exchanger with two passes per shell on the tube side. This unit should contain internal baffles. Operating pressure is 1240 kPa, with a design pressure of approximately 1400 kPa. This exchanger is made from mild steel. 

FIGURE 3 Sectional view of a typical fixed tubesheet shell-and-tube heat exchanger (A) tubes (B) tubesheets (C) shell (D) tube-side channel and nozzles (E) channel cover (F) pass divider plate (G) stationary rear head (bonnet type) (H) tube support plates (or baffles). 

Problem Below is a cross-sectional view of a heat exchanger consisting of a multi-pass configuration with baffles and five pipes running down the middle. The pipe diameter is small compared to the combined width of the baffles ( of dimension 1 each) and the distance between them. The baffles are spaced at a distance 4d apart with the five pipes positioned as shown in the diagram. The central cooling pipe is at distance 0.51 from the heat exchanger wall. The other four pipes are positioned at a distance... 

For shell-and-tube heat exchangers with cross-flow baffles, the preceding methods assume that an adequate number of baffles has been provided. If the shell-side fluid makes less than eight passes across the tube bundle, the mean temperature difference may need to be corrected for this cross-flow condition. Appropriate curves are presented in Caglayan and Buthod [20]. The curves in this reference may also be used to determine correction factors for cross-flow exchangers with one shell pass and more than two tube passes. 

It was seen from the discussion of heat exchangers that the fluid streams are not strictly countercurrent. Baffles on the shell side induce crossflow, and in a two-tube-pass heat exchanger both countercurrent and cocurrent flow occur. To account for deviations from countercurrent flow, the logarithmic-mean teri5)erature difference is multiplied by a correction factor, F. Thus,... 

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