Page 317 - Mechanical Engineers' Handbook (Volume 4)

306 Heat Exchangers, Vaporizers, Condensers

MTD and larger plants. A reasonable trial tube length is chosen and the number of tubes
(NT) required for surface A , Section 2, is calculated as follows:
o
NT A o (12)
aL
o
where a the surface/unit length of tube.
o
For plain tubes (as opposed to finned tubes),
a D o (13)
o
where D the tube outside diameter
o
L the tube length
) can be determined from the number of tubes, but also
The tube bundle diameter (D b
depends on the number of tubepasses, tube layout, and bundle construction. Tube count
tables providing this information are available from several sources. Accurate estimation
equations are given by Taborek. 11 A simple basic equation that gives reasonable first ap-
proximation results for typical geometries is the following:
D P 0.5
NT
b t (14)
/4
tube pitch (spacing between tube diameters). Normally, P /D 1.25, 1.33, or
where P t t o
1.5.
The shell diameter D is larger than the bundle diameter D by the amount of clearance
s
b
necessary for the type of bundle construction. Roughly, this clearance ranges from about 0.5
in. for U-tube or fixed tubesheet construction to 3–4 in. for pull-through floating heads,
depending on the design pressure and bundle diameter. (For large clearances, sealing strips
are used to prevent flow bypassing the bundles.) After the bundle diameter is calculated, the
ratio of length to diameter is checked to see if it is in an acceptable range, and the length
is adjusted if necessary.
Baffle Spacing and Cut
Baffle spacing L and cut B (see Fig. 9) cannot be decided exactly until pressure drop is
c
bc
evaluated. However, a reasonable first guess ratio of baffle spacing to shell diameter (L /
bc
D ) is about 0.45. The baffle cut (B , a percentage of D ) required to give good shellside
s
s
c
distribution may be estimated by the following equation:
B 16.25 18.75
L
bc
c
D s (15)
For more detail, see the recommendations of Taborek. 11
Cross-Sectional Flow Areas and Flow Velocities
The cross-sectional flow areas for tubeside flow S and for shellside flow S are calculated
t
s
as follows:
S

NT
2
t
4 D i NP (16)
S 0.785(D )(L )(P D )/P t (17)
o
b
s
t
bc
where L bc baffle spacing.