Transition casting for boiler with steam cooled upper furnace

A boiler, comprising:

• an upper furnace having steam-cooled membrane walls comprising steam wall tubes;
• a lower furnace having water-cooled membrane walls comprising water wall tubes;
• and
• a transition casting disposed between the upper furnace and the lower furnace, the transition casting comprising:
• a first steam tube having a steam tube diameter and a steam passageway extending between a first steam tube front end pointing in a forward direction and a first steam tube vertical end pointing in a first vertical direction;
• a first water tube having a water tube diameter and a water passageway extending between a first water tube first end and a first water tube vertical end pointing in a second vertical direction opposite the first vertical direction, the water passageway having at least one bend between the first water tube first end and the first water tube vertical end;
• a membrane joining the first steam tube and the first water tube to form a pressure seal therebetween;
• and
• a first heat transfer fin extending from the membrane and abutting the first water tube and abutting the first steam tube, and extending between the first steam tube and the first water tube.