Convection Section Layout
Vendors usually design new heaters; Company engineers are responsible for reviewing designs and considering modifications. Here are some guidelines on layout to help you evaluate vendor designs.
Tube Spacing. Center-to-center spacing of tubes along a row (perpendicular to flue gas flow) is usually twice the tube inside diameter, whether tubes are finned or not. Alternate rows are usually staggered (displaced one diameter) to improve the convective heat transfer coefficient. Spacing between rows is usually 0.866 times diameter, to give equilateral triangular spacing. Corbels (shelf-like projections from the wall) are frequently used to block the extra space at the ends of alternate rows.
Figure 100-3 shows corbels and convection tubes arranged in triangular pattern.
Flue Gas Mass Velocity. Assuming for a moment that the number of tubes is fixed, shall the convection section be wide and short (better from the structural design viewpoint) or tall and narrow? Making the convection section taller and narrower increases the following:
• flue gas mass velocity
• outside (controlling) heat transfer coefficient
• number of tube rows (in the vertical direction)
• flue gas pressure drop through the section
• height of the section.
As usual, the choice is an economic one. A typical value of flue gas mass velocity through the available cross-section between tubes (and fins on tubes) is 0.4 lb/sec/ft2. This translates to linear velocity of around 20 ft/sec at the convection section inlet (highest temperature, lowest flue gas density) and around 10 ft/sec at the outlet.
A rough estimate of flue gas pressure drop in convection coils is one-half velocity head per tube row. See Appendix B if a closer estimate is needed.
Order of Convection Services. In general, the process or utility stream that must be heated the hottest will see the flue gas first. The one that enters coldest will see the flue gas last. The principle is to maintain a reasonable temperature difference throughout the convection section. For the same reason, streams being heated usually flow counter to flue gas. There are some exceptions:
• Flow in steam generation coils (isothermal) is upflow, cocurrent with the flue gas.
• Cocurrent flow is sometimes used at the hot end of the convection section to reduce the maximum tube wall or fin temperature.