Page 71 - Applied Process Design For Chemical And Petrochemical Plants Volume III

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50 Applied Process Design for Chemical and Petrochemical Plants

Table 10-10E A. Number of tubes.
Full Circle Tube Layouts Floating Head Exchanger, 1-in.
1
O.D. Tubes on 1 / 4 -in. Triangular Pitch The actual number of tubes to be installed in the unit.
Manufacturing tolerances may require elimination of
Net Free some tubes that preliminary design layouts and tables indi-
Number of Passes
Size Distance Rows cated might be installed in the unit. Figures 10-25A—K and
(In.) 1 2 4 6 8 2 Passes Across Table 10-9 have considered known fabrication tolerances.
Sometimes extra tie rods for baffles must be added, or in
8 22 20 18 16 12 4.13 9
some cases, eliminated. The outer tube circle limit for each
10 38 36 32 32 28 4.50 13
12 60 52 48 46 44 4.63 15 exchanger is determined by the type of shell to be used.
14 73 68 60 58 56 4.25 19 That is, (1) if commercial pipe, greater out-of-round toler-
16 97 98 86 82 80 4.25 21 ances might be required or (2) if formed on shop rolls, the
18 130 126 118 114 112 6.50 25 out-of-round tolerance will be known, but not necessarily
20 170 164 152 150 144 6.75 27 the same for each diameter shell.
22 212 202 196 188 184 7.00 31
24 258 250 242 232 228 7.25 33
26 304 302 286 278 272 7.50 37 B. Exact distance between faces of tubesheets.
28 361 348 338 336 324 7.75 39
30 421 408 400 394 388 8.00 43 Tubes are usually ordered in even lengths, such as 8, 10,
32 482 472 456 446 440 8.25 47 1 1
12, 16, 24, or 32 ft, and the tubesheets are from / 4 in. to / 2
34 555 538 524 520 500 11.06 49
in. shorter between outer faces.
36 625 618 592 588 572 10.44 51
38 700 688 672 660 640 10.69 55
40 786 776 752 742 736 11.00 59
C. Net effective tube length.
42 872 850 834 824 816 11.32 61
This is the net length of tube exposed inside the shell and avail-
able for contact by the shell-side fluid. This length accounts for
the thickness of each tubesheet (and for the double
tubesheets when used). For design purposes, it is usually
estimated from experience, allowing about

1
1. 1 / 2 in. per tubesheet for low-pressure units.
2. 2—3 in. per tubesheet for high-pressure exchangers, 200
psi—400 psi.

D. Exact baffle spacing.

In some instances the baffle spacing must be rearranged
to allow for a nozzle or coupling connection. It is impor-
tant that changes in baffle location be reviewed, as perfor-
mance or pressure drop can be seriously affected. This is of
extreme importance in vacuum units. Baffle orientation is
sometimes misinterpreted by the fabricator, and this can
cause serious problems where liquid drainage is con-
cerned, or the revised vapor flow path can allow for bypass-
ing the tube surface.

Figure 10-26. Tube spacing layouts for tubesheets.
E. Impingement baffle location.
Exchanger Surface Area
When scale drawings are made, the effectiveness of
The actual surface area available for heat transfer is deter- impingement baffles can be evaluated easily. Sometimes it
mined from the fabricator’s shop drawings. From these is necessary to relocate or make slight size changes in order
details, the following are fixed: to properly protect the tubes and direct the vapor flow.