travel the disposition of the nozzle members 22 with reference to each other is illustrated in fig2 and fig3 illustrates the detailed design of a typical one of the nozzle members 22 as shown in fig3 the nozzle member 22 is constituted by an outer member 50 having an upwardly and outwardly extending frusto conical surface comprising the carrying surface 24 and defining a central aperture and which terminates in a downwardly extending skirt 27 and an inner guiding member 23 located in the aperture defined by the carrying surface 24 also having a frusto conical configuration the carrying surface 24 has an integral portion having a radius of curvature r which communicates with the connecting pipe or gas supply tube 26 of the nozzle member 22 which itself is in fluid communication with the header 12 an annular nozzle slit s is defined between the inner surface of outer member 50 and the outer surface of the inner member 23 as noted above the carrying surface of the nozzle member is frusto conical its angle u03b1 being preferably between 1 u00b0 and 10 u00b0 the radius of curvature r of the outer member 50 is preferably considerably larger than the radius of curvature r of its outer margin the inner guiding member 23 guides the entering flow and as shown in fig3 this member is plate like so that it has on its margins upwardly turned edges defining a frusto conical surface having an angle u03b2 preferably about 45 u00b0 the central part of the guiding member 23 has been provided with perforations 25 which communicate with a pipe 28 running inside the tube 26 the guiding member 23 may also be.