Page 437 - Handbook of Energy Engineering Calculations

exchanger of single tube-pass and single shell-pass design, with
countercurrent flows of tubeside and shellside fluids. Often, it will be
necessary to use two or more passes for the tubeside fluid. In this case, the
LMTD is corrected with the Bowman, Mueller, and Nagle charts given in
heat-transfer texts and the TEMA guide. If the correction factor for LMTD is

less than 0.8, multiple shells should be used.
Bear in mind that n in all equations is the number of tubes in parallel
through which the tubeside fluid flows; N is the number of tubeside passes
PT
per shell (total number of tubes per shell = nN ); and L, the total-series
PT
length of path, equals shell length (L ) (N ) × (number of shells).
o
PT
The above procedure can be used for any shell-and-tube heat exchanger
with sensible-heat transfer—or with no phase change of fluids—on both sides

of the tubes. Also, N on the tubeside must be greater than 10,000, and the
Re
viscosity of the fluid on the shellside must be moderate (500 cp. maximum).
As pointed out, the designer should assume as part of his job the

specification of tube arrangement that will prevent the flow in the shell from
taking bypass paths either around the space between the outermost tubes and
the shell, or in vacant lanes of the bundle formed by channel partitions in

multipass exchangers. He should insist that exchangers be fabricated in
accordance with TEMA tolerances.
By using the appropriate equations from Tables 6 and 7, the technique
described for rating heat exchangers with sensible-heat transfer can be used
also for rating exchangers that involve boiling or condensing. The method

can also be used in the design of partial condensers, or condensers handling
mixtures of condensable vapors and noncondensable gases, and in the design
of condensers handling vapors that form two liquid phases. However, for

partial condensers and for two-phase liquid-condensate systems, a special
treatment is required.
In addition to designing exchangers for specified performances, the
method is also useful for evaluating the performance of existing exchangers.
Here, the mechanical-design parameters are fixed, and the flow rates and

temperature conditions (work factor) are the variables that are adjusted.
The two process variables that have the greatest effect on the size (cost) of
a shell-and-tube heat exchanger are the allowable pressure drops of streams,

and the mean temperature difference between the two streams. Other