(under subclass 51) Devices so arranged that the products of combustion pass through the boiler in opposite directions two or more times before they enter the outlet flue.
SEE OR SEARCH THIS CLASS, SUBCLASS:
54, 72+, 81, and 85, for other structure which includes return features.
87, for horizontal fire tube boilers having U-shaped couplings for the fire tubes.
149+, for horizontal flue boilers having return fire tubes.