I think each cell to cell heat transfer is deleting the 1.5C worth of heat even though it should only happen once. Try changing the diamond window tile to insulated insulation and see what happens, assuming the steam to water cell transfer and/or conveyor bridge is sufficient to boil the water.   My test: 95C water interacting with 141C thermium, transfer is capped at 1/4 of the delta or 11.5C. Minus the 1.5C for phase change and the resulting steam should be 10C higher or 1