Page 329 - Mechanical Engineers' Handbook (Volume 4)

318 Heat Exchangers, Vaporizers, Condensers

V W a (57)
ƒ
LW v
d
where W a air rate, lb/min
L tube length, ft
W bundle width, ft
d
v air density, lb/ft 3
Fan Power Requirement
One or more fans may be used per bundle. Good practice requires that not less than 40–
50% of the bundle face area be covered by the fan diameter. The bundle aspect ratio per
fan should approach 1 for best performance. Fan diameters range from about 4 to 12 ft (1.2
to 3.7 m), with tip speeds usually limited to less than 12,000 ft/min (60 m/sec) to minimize
noise. Pressure drops that can be handled are in the range of only 1–2 in. water (0.035–0.07
psi, 250–500 Pa). However, for typical bundle designs and typical air rates, actual bundle
pressure drops may be in the range of only ⁄4–1 in. water.
1
2
Paikert gives the expression for fan power as follows:
V( p p )
P s d (58)
ƒ
E ƒ
3
where V volumetric air rate, m /sec
p static pressure drop, Pa
s
p dynamic pressure loss, often 40–60 Pa
d
E fan efficiency, often 0.6–0.7
ƒ
P fanpower,W
ƒ
3.5 Other Exchangers
For spiral, plate, and compact heat exchangers the heat-transfer coefficients and friction
factors are sensitive to specific proprietary designs and such units are best sized by the
manufacturer. However, preliminary correlations have been published. For spiral heat ex-
3
changers, see Mueller and Minton. 37 For plate-type heat exchangers, Figs. 9 and 10, rec-
ommendations are given by Cooper 38 and Marriott. 39 For plate-fin and other compact heat
4
exchangers, a comprehensive treatment is given by Webb. For recuperators and regenerators
6
the methods of Hausen are recommended. Heat pipes are extensively covered by Chisholm. 40
Design methods for furnaces and combustion chambers are presented by Truelove. 41 Heat
transfer in agitated vessels is discussed by Penney. 42 Double-pipe heat exchangers are de-
scribed by Guy. 43

4 COMMON OPERATIONAL PROBLEMS
When heat exchangers fail to operate properly in practice, the entire process is often affected,
and sometimes must be shut down. Usually, the losses incurred by an unplanned shutdown
are many times more costly than the heat exchanger at fault. Poor heat-exchanger perform-
ance is usually due to factors having nothing to do with the heat-transfer coefficient. More
often the designer has overlooked the seriousness of some peripheral condition not even
addressed in most texts on heat-exchanger design. Although only long experience, and nu-
merous ‘‘experiences,’’ can come close to uncovering all possible problems waiting to plague