Bent-tube

The words are literal , a water tube boiler is water inside the tubes with hot gases surrounding them and a fire tube boiler are got gases within the tubes with water surrounding them.

Straight tube boiler and bend tube boiler

the advantage is the design is complicated

there are two types of boilers water tube and fire tube. fire tube boilers are divided in to two categories vertical and horizontal. this according to the construction of fire tubes. it is said that vertical boilers are inefficient than horizontal boilers.

The vertical boiler is small capacity, and horizontal boiler is higher capacity, well, the horizontal boiler have many types according the drums, there are single drums boiler, and double drums boiler, while, the single drums boiler have fire tube boiler and water and fire tube boiler, double drums boiler is usually water tube boiler.