Ever since once through boilers were adopted in the steam cycle there has been a requirement to construct them simply of vertical tubes as opposed to the common spiral design. The Doosan Babcock Posiflow™ design incorporates a low mass flux furnace design into a once through boiler with vertical tubes.

Virtually all current once through boiler designs use furnace tube mass fluxes >1500 kg/m²s / 1106007 llbs/ft²h. By reducing mass flux levels to below 1200 kg/m²s / 884805.8 llbs/ft²h, Doosan Babcock 'Posiflow™' boilers minimise furnace dynamic pressure losses and the thermo-hydraulic behaviour of the furnace becomes akin to that exhibited by natural circulation boilers. The use of optimised internally ribbed tubing is a key ingredient in the success of these designs. The vertical orientation of the furnace tubes simplifies the structural support system and shortens manufacturing and installation times. In addition, with furnace pressure losses typically only 30% of an equivalent high mass flux design, thanks to significant feed pump power saving, these boilers represent the best available once through technology.