Shell Sizes Heat-exchanger shells are generally made from standard-wall steel pipe in sizes up to 305-mm (12-in) diameter; from 9.5-mm (%-in) wall pipe in sizes from 356 to 610 mm (14 to 24 in); and from steel plate rolled at discrete intervals in larger sizes. Clearances between the outer tube limit and the shell are discussed elsewhere in connection with the different types of construction.
The following formulae may be used to estimate tube counts for various bundle sizes and tube passes. The estimated values include the removal of tubes to provide an entrance area for shell nozzle sizes of one-fifth the shell diameter. Due to the large effect from other parameters such as design pressure/corrosion allowance, baffle cuts, seal strips, and so on, these are to be used as estimates only. Exact tube counts are part of the design package of most reputable exchanger design software and are normally used for the final design.
Triangular tube layouts with pitch equal to 1.25 times the tube outside diameter:
C = 0.75 (D/d) - 36; where D = Bundle O.D. d = Tube O.D.
1 Tube Pass: Nt = 1298. + 74.86C + 1.283C2 - .0078C3 - .0006C4 (11-74a)
2 Tube Pass: Nt = 1266. + 73.58C + 1.234C2 - .0071C3 - .0005C4 (11-74b)
4 Tube Pass: Nt = 1196. + 70.79C + 1.180C2 - .0059C3 - .0004C4 (11-74c)
6 Tube Pass: Nt = 1166. + 70.72C + 1.269 C2 - .0074C3 - .0006C4 (11-74d)
Square tube layouts with pitch equal to 1.25 times the tube outside diameter:
C = (D/d) - 36.; where D = Bundle O.D. d = Tube O.D.
1 Tube Pass: Nt = 593.6 + 33.52C + .3782C2 - .0012C3 + .0001C4 (11-75a)
2 Tube Pass: Nt = 578.8 + 33.36C + .3847C2 - .0013C3 + .0001C4 (11-75b)
4 Tube Pass: Nt = 562.0 + 33.04C + .3661C2 - .0016C3 + .0002C4 (11-75c)
6 Tube Pass: Nt = 550.4 + 32.49C + .3873C2 - .0013C3 + .0001C4 (11-75d)
Shell-Side Arrangements The one-pass shell (Fig. 11-35E) is the most commonly used arrangement. Condensers from single component vapors often have the nozzles moved to the center of the shell for vacuum and steam services.
A solid longitudinal baffle is provided to form a two-pass shell (Fig. 11-35F). It may be insulated to improve thermal efficiency. (See further discussion on baffles). A two-pass shell can improve thermal effectiveness at a cost lower than for two shells in series.
For split flow (Fig. 11-35G), the longitudinal baffle may be solid or perforated. The latter feature is used with condensing vapors.
A double-split-flow design is shown in Fig. 11-35H. The longitudinal baffles may be solid or perforated.
The divided flow design (Fig. 11-35/), mechanically is like the one-pass shell except for the addition of a nozzle. Divided flow is used to meet low-pressure-drop requirements.
The kettle reboiler is shown in Fig. ii-35K. When nucleate boiling is to be done on the shell-side, this common design provides adequate dome space for separation of vapor and liquid above the tube bundle and surge capacity beyond the weir near the shell cover.
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