MACHINES positive displacement rotary or oscillating piston liquid and elastic fluid or elastic fluid liquid only reciprocating piston or other liquid and elastic fluid or elastic fluid liquid only non-positive displacement liquid and elastic fluid or elastic fluid liquid only ENGINES positive displacement rotary or oscillating piston liquid and elastic fluid or elastic fluid liquid only reciprocating piston or other liquid and elastic fluid or elastic fluid liquid only non-positive displacement liquid and elastic fluid or elastic fluid liquid only PUMPS positive displacement rotary or oscillating piston reciprocating piston or other non-positive displacement In this class, the following term is used with the meaning indicated: "steam" covers also other condensable vapours, e.g. mercury, diphenyl, diphenyl oxide. This subclass covers only methods of, or apparatus for, the generation of steam under pressure for heating or power purposes. METHODS FOR STEAM GENERATION , STEAM BOILERS General characteristics having drum; having furnace tube; having fire tube; having combined fire tube and water tube; having fire-box ; ; ; ; having water tubes auxiliary tubes horizontal; horizontally-inclined; combined horizontally-inclined and vertical; vertical or steeply-inclined ; ; ; formed of sets of spaced double-walled water tubes or of return tubes; water tubes with internally-arranged flue tubes ; Special characteristics , Modifications or arrangements; details of general application ; PLANTS; CONTROL SYSTEMS ;