Multiple pass design for once-through steam generators

Claim 1

IN A FORCED-FLOW VAPOR GENERATOR WHICH COMPRISES AN ALL-WELDED BOUNDARY WALL CONSTRUCTION FOR A RECTANGULAR FURNACE ENCLOSURE HAVING OPPOSITE FIRST AND SECOND WALLS AND SIDE WALLS THEREBETWEEN, THE ENCLOSURE INCLUDING UPPER AND LOWER PORTIONS AND BURNERS IN THE LOWER PORTION IN AT LEAST ONE OF SAID FIRST AND SECOND WALLS, A CIRCUIT ARRANGEMENT FOR THE BOUNDARY WALL COMPRISING: PASS MEANS DEFINING AT LEAST THREE FLOW PASSES IN SERIES IN SAID LOWER PORTION FOR THE FLUID BEING HEATED, SAID PASSES INCLUDING FIRST AND LAST PASSES AND AT LEAST ONE INTERMEDIATE PASS, THE TOTAL HEATING SURFACE IN SAID PASS MEANS ACHIEVING A PREDETERMINED ENTHALPY PICKUP; ONE OF SAID FIRST AND LAST PASSES CONSTITUTING A SUBSTANTIAL PORTION OF THE WALL OPPOSITE SAID BURNER MEANS, THE OTHER OF SAID FIRST AND LAST PASSES CONSTITUTING A SUBSTANTIAL PORTION OF THE WALL WHICH INCLUDES SAID BURNER MEANS; THE INTERMEDIATE PASS OCCUPYING A SUBSTANTIAL PORTION OF THE SIDE WALLS TO PROVIDE A MEANS FOR TRANSITION FROM THE EXPANSION AND CONTRACTION OF THE FIRST PASS RELATIVE THAT OF THE LAST PASS; INTERMEDIATE MIXING POINTS BETWEEN SUCCESSIVE FLOW PASSES LIMITING THE FLUID ENTHALPY PICK-UP IN EACH PASS TO A PREDETERMINED AMOUNT; EACH PASS COMPRISING A PLURALITY OF PARALLEL UP-FLOW TUBES AND HAVING TUBES DISPOSED IN AT LEAST APPROXIMATELY HALF OF THE PERIPHERY OF THE FURNACE WHEREIN THE TUBES OF THE FIRST AND LAST PASSES ARE INTERSPERSED WITH THOSE OF THE INTERMEDIATE PASSES; EXTERNAL DOWNCOMERS BETWEEN THE LOWER PASSES; THE TUBES OF THE PASSES BEING ARRANGED FOR THE UPFLOW OF THE FLUID BEING HEATED; AN ADDITIONAL PASS IN THE UPPER PORTION OF THE FURNACE; AND AT LEAST ONE RISER CONNECTING SAID ADDITIONAL PASS IN SERIES WITH THE LAST PASS OF THOSE IN THE LOWER PORTION OF THE FURNACE; EACH OF THE PASSES OCCUPYING A SUFFICIENT PART OF THE FURNACE ENCLOSURE LOWER PORTION TO ACHIEVE THAT ENTHALPY PICK-UP BY WHICH THE DIFFERENCE IN TEMPERATURE BETWEEN ADJACENT TUBES IN THE ALL-WELDED ENCLOSURE DOES NOT EXCEED APPROXIMATELY 100* F.