(94d) Enhanced Heat Transfer in Radiant Coils by Swirl Flow Tubes

Cracking reactions are endothermic and are driven by the energy supplied to the radiant coils by
the combustion of fuel gas in the radiant box. Ethylene yields are favored by low pressure, high
temperature and low residence time in the coil.
Residence time can be reduced by shortening the length of the coils but this implies higher heat
fluxes through the tube walls and higher tube metal temperatures. Currently, Technip (and other
ethylene licensors) design coils close to the maximum feasible tube skin temperatures. Further
increase in the heat transfer rates is inhibited by the metallurgy of the radiant coils. Therefore, any
technology that reduces the tube skin temperatures of radiant coils is interesting.
The Swirl Flow Tube (SFT) technology of Technip is such a technology. Technip acquired the SFT
technology for application in radiant coils of steam cracking furnaces. The SFT is a helical pipe,
which has a significant technological advantage over other coils with special attributes. This swirl
flow enhances the heat transfer by a more intense mixing in the circumference compared to the
classical straight tubes. SFT’s give 35 to 90 % higher heat transfer rates compared to straight
tubes, at the cost of 40 to 90 % higher pressure drop. For the same feed and feed rate with given
on-stream time, the maximum Tube Metal Temperature (TMT) of SFT’s will be lower than that of
bare tubes. In other words, feed rate can be increased to reach the maximum TMT at the given onstream
time.
Other technologies which have a similar effect are available, among others internally finned tubes.
All these technologies yield significant higher heat transfer rates than normal tubes at the expense
of a higher pressure drop. The latter decreases the yield of ethylene which reduces the benefit of
these heat transfer enhancing technologies. These technologies have higher pressure drops of
approximately 140 % or more, explaining the huge potential improvement using SFT’s which
generate higher heat transfer at lower pressure drop.
This paper will describe the SFT® technology, its validation, its application possibilities and its case
studies to demonstrate its capabilities.