A process for creating a wetted wall in a cooling conduit for use in quenching gaseous effluent from a hydrocarbon pyrolysis furnace, the process comprising: (a) introducing said gaseous effluent into a cooling conduit, said cooling conduit comprising; (i) an inner wall for contacting said effluent, said inner wall defining a bore extending a length of said cooling conduit, said inner wall including a perimeter opening along said bore; (ii) an outer wall external to said inner wall and substantially coaxial to said inner wall; (iii) a substantially annular cavity external to said inner wall and including at least a portion of said outer wall, said annular cavity fluidly and remotely connected to said perimeter opening, said annular cavity externally surrounding a perimeter of said inner wall; and (iv) a peripheral channel extending around a perimeter of said inner wall, said peripheral channel fluidly connecting said annular cavity and said perimeter opening, along said perimeter of said inner wall; (b) introducing a liquid quench fluid through a liquid quench fluid introduction port tangentially into said annular cavity, substantially along a first portion of said outer wall, whereby said peripheral channel functions to provide hydraulic impediment or resistance to fluid exiting from the annular cavity and said introduced liquid quench fluid fills said cavity; (c) passing said introduced liquid quench fluid from said annular cavity through said channel to said perimeter opening along a channel flow path; and (d) passing said liquid quench fluid from said perimeter opening onto said inner wall for distribution of said quench fluid along at least a portion of said length of said inner wall as a quench fluid film, while concurrently passing said gaseous effluent along said bore of said cooling conduit to produce a quenched gaseous effluent stream. (a) introducing said gaseous effluent into a cooling conduit, said cooling conduit comprising; (i) an inner wall for contacting said effluent, said inner wall defining a bore extending a length of said cooling conduit, said inner wall including a perimeter opening along said bore; (ii) an outer wall external to said inner wall and substantially coaxial to said inner wall; (iii) a substantially annular cavity external to said inner wall and including at least a portion of said outer wall, said annular cavity fluidly and remotely connected to said perimeter opening, said annular cavity externally surrounding a perimeter of said inner wall; and (iv) a peripheral channel extending around a perimeter of said inner wall, said peripheral channel fluidly connecting said annular cavity and said perimeter opening, along said perimeter of said inner wall; (i) an inner wall for contacting said effluent, said inner wall defining a bore extending a length of said cooling conduit, said inner wall including a perimeter opening along said bore; (ii) an outer wall external to said inner wall and substantially coaxial to said inner wall; (iii) a substantially annular cavity external to said inner wall and including at least a portion of said outer wall, said annular cavity fluidly and remotely connected to said perimeter opening, said annular cavity externally surrounding a perimeter of said inner wall; and (iv) a peripheral channel extending around a perimeter of said inner wall, said peripheral channel fluidly connecting said annular cavity and said perimeter opening, along said perimeter of said inner wall; (b) introducing a liquid quench fluid through a liquid quench fluid introduction port tangentially into said annular cavity, substantially along a first portion of said outer wall, whereby said peripheral channel functions to provide hydraulic impediment or resistance to fluid exiting from the annular cavity and said introduced liquid quench fluid fills said cavity; (c) passing said introduced liquid quench fluid from said annular cavity through said channel to said perimeter opening along a channel flow path; and (d) passing said liquid quench fluid from said perimeter opening onto said inner wall for distribution of said quench fluid along at least a portion of said length of said inner wall as a quench fluid film, while concurrently passing said gaseous effluent along said bore of said cooling conduit to produce a quenched gaseous effluent stream.