There 2 major types :
1. Fire tube boilers
These are for small capacities under 10 Tons/hr,in which the flue gases flow in tubes submeged in the water .
2. Water tube boilers
Capacitiies more than 10 tons ,and the water and steam are in the drums,connected by riser tubes,where the flue gases give their heat to generate steam in the upper drum.
For details ,consult Perry ,and boiler manufacturers.
