Water-tube boilers built-up from sets of spaced double-walled water tubes of return

Official title: Water-tube boilers built-up from sets of spaced double-walled water tubes of return type in unilateral abutting connection with a boiler drum or with a header box, i.e. built-up from Field water tubes comprising an inner tube arranged within an outer unilaterally-closed tube

F22B23/00 (Water-tube boilers built-up from sets of spaced double-walled water tubes of return) appears on 6 US grants and 3 published applications in this corpus.