Everyone has used a kettle to boil water. Boilers are similar to kettles in that they are also used to boil water. Boilers come in all shapes and sizes. The main difference between boilers and kettles is that boilers are continuously boiling water where kettles get water to boiling point then stop.

Principles of Heat Transfer

When water is in contact with a surface or substance hotter than itself their is a transfer of heat into the water. When liquid water reaches a high enough temperature it 'boils' and turns into steam. The steam can then gain even more heat and become superheated. Superheated steam is used to drive steam turbines.

Calculating the heat transfered to water in a boiler can be complex because the water and the heat source are generally moving, the heat is transfered through pipes and during the transition from water to steam, the flow is 'two-phase' (i.e. partly liquid and partly gas).

Industrial Boilers

Industrial boilers can be divided into two types. One type is very similar to ordinary kettles (except on a larger scale). The water is pumped into a big tank containing pipes carrying the heating fluid. The water boils as it comes into contact with the hot pipes, steam rises to the top of the tank and flows into the rest of the system. In the other type, the water is contained in the pipes while the hot fluid(usually a gas) passes on the outside and heats the water. The water is gradually heated as it moves along the pipe. This type of boiler can be carefully designed and controlled to produce superheated steam of a fixed temperature.