Boiler with serpentine tubes

A boiler comprising:

• a housing having a top provided with a gas outlet, a bottom, opposite side walls and front and rear walls;
• said housing containing an upper manifold and a lower manifold substantially parallel to the top, bottom and side walls;
• first and second sets of tubes each comprising a plurality of tubes, one set joining the upper manifold to the lower manifold on one side, and the other set joining the upper manifold to the lower manifold on the other side, the tubes of the first set rising from the lower manifold upwardly along their side wall, crossing the housing to the centre recrossing the housing rising therealong and eventually joining the upper manifold, the tubes of the second set rising from the lower manifold upwardly along their sidewall crossing the housing to be substantially in contact with the tubes of the first set so as to form a plurality of superimposed passages, certain adjacent passages being separate one from the other by a passage isolator;
• individual tubes being bent differently so as to form access openings upwardly at the back and laterally at the front from passage to passage;
• the openings being offset so as to require the hot gases flowing through said passages to traverse one passage from front to back and the next passage from back to front, means of introducing liquid into lower manifold and for withdrawing liquid or vapour from the upper manifold and means for introducing hot gases into the lowermost of the superimposed passages which they successively and alternately transverse laterally from back to front until they exit through the gas outlet at the top;
• the liquid flowing through the manifolds and tubes being heated by the hot gases.