Steam generator configurations

Claim 1

A VAPOR GENERATOR COMPRISING AN UPRIGHT SHELL; A VAPOR-LIQUID DRUM AT THE UPPER END OF SAID SHELL; MEANS FOR CIRCULATING A HEATING FLUID THROUGH SAID SHELL; A DOWNCOMER WITHIN SAID SHELL COAXIAL THEREWITH, SAID DOWNCOMER LEADING FROM THE INTERIOR OF SAID VAPOR-LIQUID DRUM TO A POINT ADJACENT THE BOTTOM OF SAID SHELL; RISERS CONNECTED TO THE LOWER END OF SAID DOWNCOMER EACH COMPRISING AN INLET TUBE, AN OUTLET TUBE, POLYGONAL HEADERS FOR SAID INLET AND OUTLET TUBES, AND A PLURALITY OF STRAIGHT INTERMEDIATE TUBES OF SMALLER DIAMETER THAN SAID INLET AND OUTLET TUBES EXTENDING BETWEEN SAID INLET AND OUTLET HEADERS, THE ARRANGEMENT OF SAID RISERS BEING SUCH THAT SAID INTERMEDIATE TUBES ARE REGULARLY AND UNIFROMLY DISTRIBUTED OVER A CROSS-SECTON OF THE SHELL OCCUPYING A GREATER PART OF THE VOLUMN OF SAID SHELL IN THE ANNULUS SURROUNDING SAID DOWNCOMER; A COLLECTING RISER IN THE FORM OF AN ANNULUS SURROUNDING THE UPPER END OF SAID DOWNCOMER AND COMMUNICATING WITH SAID VAPOR-LIQUID DRUM, SAID RISER OUTLET TUBES BEING CONNECTED TO SAID COLLECTING RISER SUCH THAT THE CIRCULATION NEEDS OF THE GENERATOR ARE SATISFIED WITH ONLY A SINGLE PIERCING OF THE UPPER END OF SAID SHELL AND OF THE VAPOR-LIQUID DRUM.