A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heat and vaporised fluid exits the boilers for use in various process.
Classification of boiler:-
 (1) Nature of boiler-
          (a) Stationary boiler
          (b) Mobile boiler
(2) Contents in tube:-
          (a) Fire Tube boiler
          (b)Water Tube boiler
(3) Location of furnace:-
          (a) Externally Fired
          (b) Internally Fired
(4) Circulation of water:-
          (a) Natural
          (b) Forced
(5) Position of shell:-
          (a) Horizontal
          (b) Vertical
(6) Number of Tube:-
          (a) Single Tube
          (b) Multitube