The furnace effluent quench system generally consists of two stages. Primary
quenching occurs in close-coupled, roof-mounted SLE exchangers, one per radiant pass. The SLE is a
double-pipe linear exchanger that generates SHP steam [up to 130 bar(g)]. Coke laydown in the SLE
is minimal, online decoking is very effective, and mechanical cleaning is essentially eliminated. For
heavy liquids feed cracking, the cracked gas from the SLEs is further cooled in a quench fitting by
direct injection of quench oil.
Boiler feed water (BFW) is supplied to the steam-generating quench exchangers from a highpressure
steam drum under thermosyphon. BFW is preheated in the upper zones of the convection
section. Balance of convection heat recovery preheats/vaporizes feed and superheats both dilution
steam and SHP steam. The steam drum is equipped with internal separators to produce high-quality
steam (>99.9 percent vapor fraction), minimizing turbine and superheater fouling. Furnace export
SHP steam is supplied to recovery-area compressor turbines.
The pyrolysis of hydrocarbons forms a large quantity of coke that deposits in the radiant coil, thus
increasing the average metal temperature and pressure drop, particularly in the outlet tube. The
furnaces must be decoked periodically using steam and air. This procedure oxidizes and spalls the
coke from the coil and quench exchanger surfaces. The furnaces are designed to allow for the
injection of the decoke effluent into the radiant firebox for incineration. Waste gases from various
recovery section services are also incinerated in the firebox.