The Ofeldt is another naturally circulating boiler and consists of a single vertical drum from which helical coils extend radially. Combustion gases ascend amongst the nest of coils and their heat generates a steam/water mix within the tubes which rises through the coils, exiting into the outer drum. The majority of the steam and water separate in the outer drum, the steam passing up through holes in the conical cap, down into the interior drum and up through the outlet pipe. These rapid changes of direction cause water entrained in the steam to slough off and fall into the bottom of the inner drum where it eventually vaporizes. The outer drum serves as a downcomer, directing the water to the lower end of the generator coils where it is recirculated. Ofeldt  (Natural Circulation) The lesson to take away is that there are innumerable ways to arrange boilers; Thornycroft and Ofeldt having reached drastically different solutions for a small, naturally circulated automotive boiler as an example. Ask any two steam nuts to choose a design and likely you will get at least three passionately held, mutually exclusive answers. Natural circulation can be as simple as the Field boiler to the right; cooler water descends through the inner tube while the fire licks the outer tube, creating a rising steam/water mix.