FX Multipass Fired Tube Boiler design software is developed to size and design multipass boilers. Fire­tube steam boilers are used for the purpose of converting water to saturated steam. Fire­tube steam boilers are composed of a cylindrical drum in which a series of tubes are contained. Hot air from a burner is passed from a furnace to a series of tubes at different passes. Finally, the hot air exits through a chimney. The boiler is filled with water up to the top level of the final pass pumped from the base of the boiler. Steam is then pumped from the top of the boiler to wherever heating is needed.