Page 331 - Mechanical Engineers' Handbook (Volume 4)

320 Heat Exchangers, Vaporizers, Condensers

4.2 Vibration
A problem with shell and tube heat exchangers that is becoming more frequent as heat
exchangers tend to become larger and design velocities tend to become higher is tube failure
due to flow-induced tube vibration. Summaries including recommended methods of analysis
51
3
are given by Chenoweth and by Mueller. In general, tube vibration problems tend to occur
when the distance between baffles or tube-support plates is too great. Maximum baffle spac-
ings recommended by TEMA were based on the maximum unsupported length of tube that
will not sag significantly. Experience has shown that flow-induced vibration can still occur
at TEMA maximum baffle spacing, but for less than about 0.7 times this spacing most
vibration can be eliminated at normal design velocities (see Section 2.4). Taborek 11 gives
the following equations for TEMA maximum unsupported tube lengths (L ), inches.
su
Steel and Steel Alloy Tubes
3
For D ⁄4–2 in., (59)
o
L 52D 21
o
su
1
3
For D ⁄4–⁄4 in., (60)
o
L 68D 9
su
o
Aluminum and Copper Alloy Tubes
3
For D ⁄4–2 in., (61)
o
L 46D 17
o
su
For D ⁄4–⁄4 in., (62)
1
3
o
L 60D 7
su
o
For segmental baffles with tubes in the windows, Fig. 9, the maximum baffle spacing is one-
half the maximum unsupported tube length.
For very large bundle diameters, segmental or even double segmental baffles may not
be suitable, since the spacing required to prevent vibration may produce too high pressure
drops. (In addition, flow distribution considerations require that the ratio of baffle spacing
to shell diameter not be less than about 0.2.) In such cases, one commonly used solution is
to eliminate tubes in the baffle windows so that intermediate support plates can be used and
baffle spacing can be increased; see Fig. 17. Another solution, with many advantages is the
rod-type tube support in which the flow is essentially longitudinal and the tubes are supported

Figure 17 Segmental baffles with no tubes in
window.