with heat flux permitting a higher u03b4t for a given wall or an increased wall thickness for a given u03b4t fig3 44 show various methods of accomplishing this each of these is shown in connection with the drum 300 of fig3 fig3 and 38 illustrate a drum or drum shell in which there are circumferential grooves in both the inner and outer surfaces of the wall the inner grooves 303 are offset from the outer grooves 304 and they may overlap in the center of the wall at 305 the outer grooves 304 may be filled with a resilient material having less strength than the drum material the material would be a softer metal and would allow the drum to present a smooth face to the web fig3 and 40 illustrate a drum in which there are only inner circumferential grooves 303 these grooves may extend as near the outer surface as possible the only requirement is that there be enough material between the groove and outer drum surface to hold the drum together fig4 and 42 illustrate another modification of the design shown in fig3 and 40 in this the wall sections 310 between the grooves 303 are tapered on their inner ends 311 to provide greater heat transfer surface these inner ends 311 are grooved at 312 to reduce stress fig4 and 44 illustrate another modification of the structure shown in fig4 and 42 in this one the entire wall of groove 303 is tapered so that there is less material in wall section 310 and greater heat transfer surface the sections 310 are also grooved at 312 to reduce stress fig4 57 illustrate novel means of using circulating fluid such as steam as a.