the water-tube, i.e. Field-tube, sets being vertical or substantially vertical

Full 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 > the water-tube, i.e. Field-tube, sets being vertical or substantially vertical