A. General

One application of heat transfer equipment in the process industries is to supply vapors to distillation columns. These heat exchangers are called reboilers. Most process reboilers are of shell-and-tube construction. Boiling may take place either on the shell side (outside the tubes) or the tube side, depending on various requirements described in the next section. The heating medium is usually steam, but it may also be a heat transfer service fluid or a gas, condensing vapor, or liquid process stream.

The rate of vaporization in a reboiler is sensitive to the available temperature difference because the boiling heat transfer coefficient itself is a strong function of the temperature difference. In cases where adequate ΔT is available, gross approximations in design are often made without serious consequence because vaporization can easily be adjusted to the required level by adjusting ΔT. However, continuing trends toward more efficient energy utilization tend to permit less flexibility in heating medium operating conditions. This produces a requirement for better reboiler selection, much more accurate sizing of heat transfer surface, and better analysis and prediction of probable performance. Furthermore, even with adequate ΔT available, reboiler performance is always limited by a "critical heat flux" above which vapor blanketing takes place.

Because of the complicated nature of the boiling process, very complex calculations are required for a comprehensive design, and use of computers has become standard practice, at least for the more critical designs. The boiling heat transfer coefficient decreases sharply with decreasing ΔT. Therefore, the trend towards smaller ΔT has prompted the use and further study of various types of enhanced boiling surfaces that provide more surface and/or more bubble nucleation sites at low \(\varDelta T.\) Use of some of these surfaces is described in later sections.

One of the great unknowns in the use of enhanced surfaces and in reboiler design is the effect of fouling. Because of the usually high values of the heat transfer coefficients in reboilers, the fouling resistance assigned can represent a large part of the required heat transfer surface. However, it has been frequent practice in the past to specify high fouling factors to make up for gross simplifications in the analysis of the boiling process. Actually, many process reboilers will operate with a very small amount of fouling if properly designed (e.g., see Gilmour, 1965), and the assigned fouling factor is often just a "safety" factor. Sometimes what was thought to be fouling was actually a failure to take into account the effects of wide-boiling mixtures on the heat transfer coefficient. Another possible cause of poor performance that at first appears to be fouling is the build-up of heavy components due to insufficient liquid flow out of the reboilers. This causes a gradual increase in boiling temperature with corresponding decreases in ΔT and performance. When extremely large fouling factors actually are required it is often a sign that the designer should investigate other geometries, higher velocities, or lower wall temperatures.