The amount of feedstock that can be cracked in a steamcracker is limited by the amount of heat that can be transferred to the medium in the coil. The decisive factor for heat transfer is the available radiant tube surface on the inside diameter. The larger the surface area, the greater the potential for maximizing heat transfer. Higher productivity, yield and longer service life are associated with this and selectivity is decisively influenced. The concept of enlarging the tube bore surface by profiling is not new. Before the availability of the patented, cast PEP (Paralloy Ethylene Profiled Tube) tube, the only finned tube available on the market was a drawn tube with small outer diameters. PEP tubes are available in sizes up to 150 mm in diameter. Drawn tubes have a size limit as well as a temperature limit caused by the absence of creep hardening secondary carbides. It is the secondary carbides that give the cast PEP tubes the recognised superior creep properties compared to drawn tubes.
The PEP tubes are manufactured by an electrochemical machining process from thick-walled centrifugally cast tubes in segment lengths of up to 4 metres. The process does not change the properties of the alloy and the profiling can be carried out straight or spirally. However, investigations have shown that the considerably higher pressure loss of the medium in the spiral coil is only offset by a relatively small additional increase in heat transfer performance. PEP tubes offer a considerable economic advantage in production and maintenance both for light and heavy feedstocks compared to plain tubes:
