A boiler is a device for generating steam, which consists of two principal parts: the furnace, which provides heat,

usually by burning a fuel, and the boiler proper, a device in which the heat changes water into steam.

The steam or hot fluid is then recirculated out of the boiler for use in various processes in heating applications.

The water circuit of a water boiler can be summarized by the following pictures:

 

  The boiler receives the feed water, which consists of varying proportion of recovered condensed water (return water)

and fresh water, which has been purified in varying degrees (make up water).

The make-up water is usually natural water either in its raw state, or treated by some process before use. 

Feed-water composition therefore depends on the quality of the make-up water and the amount of condensate returned to

 the boiler.

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  The steam, which escapes from the boiler, frequently contains liquid droplets and gases.

The water remaining in liquid form at the bottom of the boiler picks up all the foreign matter from the water

that was converted to steam. The impurities must be blown down by the discharge of some of the water

from the boiler to the drains. The permissible percentage of blown down at a plant is strictly limited by running costs and

 initial outlay. The tendency is to reduce this percentage to a very small figure.

 

  Proper treatment of boiler feed water is an important part of operating and maintaining a boiler system.

As steam is produced, dissolved solids become concentrated and form deposits inside the boiler.

This leads to poor heat transfer and reduces the efficiency of the boiler.

Dissolved gasses such as oxygen and carbon dioxide will react with the metals in the boiler system and lead to boiler corrosion.

 In order to protect the boiler from these contaminants, they should be controlled or removed, trough external or internal treatment.