Boiler Construction and Classification

Boiler Construction

The Boiler is a device which generates steam at very high pressure and temperature.  Thus, it should be designed to withstand the mechanical and thermal stress caused by the temperature and pressure.

While selecting materials or boilers, there are also issues such as cost and availability which determine the choice.

In general, Low Pressure Boilers are constructed out of cast iron or steel. 

Miniature Boilers can be made of stainless steel or copper while Power Boilers are designed out of special steels which can withstand very high pressure and temperature.  

Classification of Boilers on the basis of pressure

According to the standards of the ASME (American Society of Mechanical Engineers), Boilers can be classified on the basis of pressure into the following types.

Low Pressure Boilers

Boilers with operating steam pressure not exceeding 1.021 atmosphere and a temperature of 394 K. 

Power Boilers

Power Boilers are those boilers whose pressure rating and temperature are above those of Low pressure boilers.

Classification of Boilers on the basis of fuels

Based on the fuel used to produce heat, boilers can be classified into

Solid Fuel Fired boilers

These boilers are fired by either wood or coal.  The fuel may be crushed into fine powder or cut into chips before being fed into the furnace.  The combustion of these fuels will produce ash which can be used for other purposes.

Liquid Fired boilers

These boilers are fired by a liquid such as Furnace oil or diesel.  The Furnace oil may required to be processed to attain the desired viscosity and the temperature before being fed into the furnace.  The furnaces for these boilers have a higher flow rate and smaller volume.  

Gas Fired boilers

These boilers have gas as the fuel.  The gases have very little residue.  The flow rate is higher and the furnace volumes are very small.  

Waste Heat Recovery Boiler

These boilers do not have a separate heat source.  They recover the heat from another industrial process such as exhaust from an engine or the waste heat in a cement plant.  The recovered heat is used to power the boiler.  

Factors which influence the selection of boiler type

The following are some of the factors which determine the type and Size  of boiler
  1. Power to be Generated
  2. Operating Pressure
  3. Type of Fuel and its quality.
  4. Load Factor
  5. Location
  6. Availability of Water
  7. Availability of Area for the Boiler
  8. Cost of Operation and Maintenance