Closed vessels where water or any other liquids are heated are called boilers. The liquid does not necessarily boil in such boilers. The hot or vaporized liquid exits the boiler and is used for different applications and processes, for example, cooking, central heating, sanitation, water heating, boiler based power generation etc.

Materials

 
The material used in the making of such boilers is generally steel, earlier known as wrought iron. Due to stress corrosion cracking usage of stainless steel is avoided in the wetted parts of the boiler. Ferric stainless steel is used in super heating areas which are not exposed to boiling water. Materials like copper or brass are used in live steam models where fabrication of small size boilers is easier. In domestic water heaters, cast iron is used. In some countries, they are known as “boilers”. Although the purpose of such heaters is just to produce hot water and not steam so they work at low pressure to avoid boiling.

Source of Energy

 
Combustion of fuels such as wood, natural gas, oil or coal is the source of heating these boilers. Resistance or immersion kind of heating elements are used for electric steam boilers. Nuclear fission is an additional source for generating steam.

Boilers are Classified Into the Following Types

 
Fire boiler tube

Water boiler tube

Package boiler

These Boiler Tubes are Generally of High Pressure or Low Pressure.

Fire Boiler Tubes:

 
The combustion gas enters the boiler tubes with water surrounded on the outer side of the tubes. This type of boiler is used mostly in steam locomotives. The furnace is surrounded by water to keep the temperature below the boiling point of the heating surface. To elongate the path of hot gases the furnace is placed at one end of the boiler tube, thus elevating the heating surface which can be increased by reversing the direction of the gases through other parallel tubes; on the other end, the gases may travel on the sides and later under the boiler. Wherein a locomotive boiler type the gases may pass through a bundle of such boiler tubes which augments the heating surface as compared to a single boiler tube thus improving the heat transfer.

Advantage:

 
The fire boiler tubes are comparatively less expensive. They can be easily cleaned. They are smaller in size. They are available in various sizes. The boiler tubes can be easily replaced.

Disadvantage:

 
They generate a huge amount of steam which increases their weight per pound. In involves a lot of time for the raising the steam pressure due to the large volume of water. It is also unable to respond instantly to the load changes due to its large volume of water.

Water Boiler Tubes:

 
In a water boiler, tubes are arranged in different configurations in the furnace. They are filled with water. These tubes link with the huge drums, the which has water are the lower ones, and steam and water in the upper ones, whereas in a mono tube boiler water flows through a pump with the succession of coils. In this type of boiler, the production rate of steam is high in contrast to lower storage capacity. These water boiler tubes are made such that they can be used on any heating source and are used in high-pressure applications.

Advantages:

 
These boiler tubes are available in sizes greater than the fire boilers. They are able to take the pressure up to 5000 psi. The have the capability to recover faster than the fire boiler tubes. They have the capacity to reach to very high temperatures.

Disadvantages:

 
Their initial capital investment is very high. The complex design of these water boilers makes the cleaning of it difficult. The boiler tubes have no commonality. It is huge in size.

Package Boilers:

 
As the name goes they are a complete package. It only needs water pipe work, fuel supply, steam, and electric connection to become operational. They are like shell type with the design of the fire boiler tubes to gain high heat transfer rate.

Advantages:

 
They require minimum combustion space. They emit high heat resulting in quick evaporation. It consists of many tubes with small diameter resulting in a good convective transfer of heat. It has high thermal efficiency.