Water passes through the tubes in a water-tube boiler. The stack is a passage through which the combustion gases are exhausted to the atmosphere. The gases in the stack are referred to as stack gases or flue gases. Common Boiler Types Boilers can be categorized by any of a number of characteristics. A boiler can combust natural gas, oil, coal, pulverized coal, wood, or wood waste to heat water.
Some boilers can use more than one type of fuel.
Burners deliver fuel mixed with air to the combustion chamber in proportions that allow efficient burning. Fire-tube boilers are generally fueled by natural gas or fuel oil. At the point of use, the steam or hot water either transfers its heat across a heat exchanger or is injected directly into a process. Heat not consumed or wasted during distribution will be returned to the boiler by a feedwater pump in the form of condensed steam or hot water. Re-using condensate or high temperature water allows for more efficient operation than adding colder water from a water main.