Page 324 - Mechanical Engineers' Handbook (Volume 4)

3 Rating Methods 313

hot process fluid from which heat is to be recovered, ranging from chemical reactor effluent
to geothermal hot brine.

Selection of Reboiler Type
A number of different shell and tube configurations are in common use, and the first step in
design of a reboiler is to select a configuration appropriate to the required job. Basically, the
type of reboiler should depend on expected amount of fouling, operating pressure, mean
temperature difference (MTD), and difference between temperatures of the bubble point and
the dew point (boiling range).
The main considerations are as follows: (1) fouling fluids should be boiled on the
tubeside at high velocity; (2) boiling either under deep vacuum or near the critical pressure
should be in a kettle to minimize hydrodynamic problems unless means are available for
very careful design; (3) at low MTD, especially at low pressure, the amount of static head
must be minimized; (4) for wide boiling range mixtures, it is important to maximize both
the amount of mixing and the amount of countercurrent flow. Often, fairly clean wide boiling
mixtures are wrongly assigned a high fouling factor as a ‘‘safety’’ factor. This should not be
done, and these fluids normally should be boiled on the shellside because of greater mixing.
These and other criteria are discussed in more detail in Ref. 25, and summarized in a
selection guide, which is abstracted in Table 4.
In addition to the above types covered in Ref. 25, falling film evaporators 26 may be
preferred in cases with very low MTD, viscous liquids, or very deep vacuum for which even
a kettle provides too much static head.

Temperature Profiles
For pure components or narrow boiling mixtures, the boiling temperature is nearly constant
and the LMTD applies with F 1.0. Temperature profiles for boiling range mixtures are
very complicated, and although the LMTD is often used, it is not a recommended practice,
and may result in underdesigned reboilers unless compensated by excessive design fouling
factors. Contrary to the case for condensers, using a straight-line profile approximation al-
ways tends to give too high MTD for reboilers, and can be tolerated only if the temperature
rise across the reboiler is kept low through a high circulation rate.
Table 5 gives suggested procedures to determine an approximate MTD to use for initial
size estimation, based on temperature profiles illustrated in Fig. 16. It should be noted that

Table 4 Reboiler Selection Guide
Process Conditions Suggested Reboiler Type a
Moderate pressure, MTD, and fouling VT/E
Very high pressure, near critical HS/K or (F)HT/E
Deep vacuum HS/K
High or very low MTD HS/K, G, H
Moderate to heavy fouling b VT/E
Very heavy fouling b (F)HT/E
Wide boiling range mixture HS/G or /H
Very wide boiling range, viscous liquid (F)HT/E
a
V, vertical; H, horizontal; S, shellside boiling; T, tubeside boiling; (F), forced flow, else natural convection; /E, G, H,
K, TEMA shell styles.
b
True fouling demonstrated. Not just high fouling factor (which is often assigned for ‘‘safety’’).