There are many different types of boiler systems on the market today. From heat recovery boilers and solid fuel boilers to watertube boilers, they all serve their own unique purpose. Watertube boilers come in different types and are typically used as an energy source when an emergency or any other occasion strikes. A water tube boiler usually comes as a longitudinal drum boiler, cross drum boiler or bent tube (Stirling boiler). Read on for the ways in which these boiler systems differ. Cheap Boilers