What three types of steam boilers are used in the industry?
Firetube Watertube like David F said, except that he is wrong saying a firetube is the most common. It was the most common in 1910. A better third type is a Once Through boiler. There is no steam drum. It's more efficient, but more complicated controls.
There is Fire Tube which means the fire is in the boiler tubes and the water is around the tubes (most common type). There is Water tube which the water is in the tubes and fire is around the tubes. Then there is a relatively new type called electric arc boiler which is the most cost efficient of the electric boilers.
