Abstract: A natural circulation water tube package boiler is assembled in a factory in two self-contained steam generating units. The first unit has a combustion chamber, but no convection tubes. The second unit has convection tubes, but no combustion chamber. Each unit has a mud drum, a steam drum, and tubes connecting the drums. Each steam generating unit is transported separately by truck or railcar to the operating site, where the rear end of the first unit is connected to the front end of the second unit by a large connecting duct; the mud drums of the respective units are connected to each other by water piping; the steam drums of the respective units are connected to each other by water piping and steam piping located above and below the waterline respectively; and the steam drum of the second unit is connected to the water drum of the first unit by water piping.