Shell-side Reynolds number, (NR

FIG. 11-15 Correction of friction factors for ideal tube banks. (a) Triangular and rotated square arrays. (b) In-line square arrays.

Shell-side Reynolds number, (NRe)s

FIG. 11-15 Correction of friction factors for ideal tube banks. (a) Triangular and rotated square arrays. (b) In-line square arrays.

2. Calculate the pressure drop for an ideal cross-flow section.

p sm nb where b = (2.0)(10~3) (SI) or (9.9)(1Cr5) (U.S. customary).

3. Calculate the pressure drop for an ideal window section. If

where b = (5)(10"4) (SI) or (2.49)(10~5) (U.S. customary).

Nw ls

where b1 = (1.681)(10-5) (SI) or (1.08)(10-4) (U.S. customary), and b2 = (9.99)(10~4) (SI) or (4.97)(10~5) (U.S. customary).

4. Find the correction factor for the effect of baffle leakage on pressure drop Rl from Fig. 11-16. Curves shown are not to be extrapolated beyond the points shown.

5. Find the correction factor for bundle bypass Rb from Fig. 11-17.

6. Calculate the pressure drop across the shell side (excluding nozzles). Units for pressure drop are lbf/ft2.

AP, = [(Nb - 1)(APbk)Rb + Nb APwk]Rl + 2 APbkRb + ^j (11-25)

The values of hs and APs calculated by this procedure are for clean exchangers and are intended to be as accurate as possible, not conservative. A fouled exchanger will generally give lower heat-transfer rates, as reflected by the dirt resistances incorporated into Eq. (11-2), and higher pressure drops. Some estimate of fouling effects on pres-

sure drop may be made by using the methods just given by assuming that the fouling deposit blocks the leakage and possibly the bypass areas. The fouling may also decrease the clearance between tubes and significantly increase the pressure drop in cross-flow.

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