Boilers & Types Of Boilers

Boilers are the enclosed vessels in which the water is heated and circulated as hot water or as steam for the generation of power or heat energy. It consists of arrangement of tubes where the water is heated and supplied for the steam for energy production. It acts as a storage tank for hot water. These boilers consist of tubes that are divided into chief classes.

In the closed vessel the water or any other liquid is heated where steam or vapor is generated under pressure or else vacuum for external use by a direct application of energy from the combination of fuels from a nuclear or electrical energy.


These centralized boilers are just similar to an engine of a car. It provides adequate heat to warm up itself. The size of a boiler is to be matched to the size of the purpose and facility.

Working principle:

A boiler is essentially a closed vessel where water is stored. In a furnace fuel is burnt and hot gases are produced, come in contact with the water vessel where the heat for these gases get transferred to the water and this give rise to the steam production in the boiler. This produced steam will be piped in to a turbine of a thermal power plant.

Major components:

  • Feed water heater:

These are energy recovery devices that are used in steam generation plants where the steam user does not reduce the steam to condensate. The waste steam is reduced to condensate for the return in the feed water heater to the boiler. This is mainly used as a cooling medium that reduces the steam to condensate with an increase in the temperature of the feed water that finally increases thermal efficiency of the boiler.

  • Fuel heater:

Fuel heaters are required to reduce the fuel viscosity. With this the fuel can be atomized by the burning system for a complete combustion.

  • Deaerators:

It acts as a special case in feed water heater that is specially designed for the promotion of the removal of non-condensate gases that are present in the boiler feed water. The main gas components are oxygen, carbon dioxide and ammonia that contributes the boilers as well as steam condensate related corrosive problems. In case of small steam plants the generated steam by the boiler is used in the operation of deaerators. The maintenance failure of this device will lead to an early failure of the boiler, steam condensate piping.

  • Pumps:

The circulating pumps are electric motor driven in all the hot water system and are end suction centrifugal pumps. In a steam system the condensate returns pumps that are typically of the above mentioned type. Feed water pumps are generally multi stage motor driven centrifugal pumps. The operating pressure of the steam or the hot water system is to be more than the shut off head.

  • Combustion air blowers:

Combustion air fan is designed and is an integral part of the boiler hosting. Low pressure centrifugal blowers are used in the stand lone fans. The boiler can maintain relatively constant air pressure over wide range of air flows.

  • Flue:

It is a boiler discharge stack or firebox exhaust duct that is to be large enough in order to conduct the combustion of products far away from the boiler and also with minimum friction loss of the duct. These are to be prepared from the materials that suit the operation under all temperatures and pressure. Commonly the materials used are carbon steel or stainless steel.

  • Economizer:

It is an energy recovery device that uses the hot exhaust gases from the boiler in order to heat the combustion air and the feed water.

  • Steam traps:

These are installed throughout the steam systems in order to remove the condensate, air from all the steam systems. These are of five types as:

  • Balanced pressure thermostatic trap
  • Liquid expansion steam trap
  • Float and thermostatic trap
  • Inverted bucket steam trap
  • Thermodynamic steam trap

Piping :

Piping is used in between the hot water systems as well as steam systems. Piping larger than two inch and of typical standard weight ASTM standard grade steel pipe that is integrated with flanged joints carbon steel fittings.


Boiler or steam boiler efficiency is given by:     formula2This efficiency includes thermal, combustion, fuel to steam efficiencies. It completely depends on size of the boiler. In general the efficiency varies in between 80% to 88%. Some losses are included as incomplete combustion, radiating losses that occurs due to the wall that is surrounding, defective combustion of the gas etc. results in the efficiency range.

Types of boilers:

There are mainly two types of boilers as water tube boiler and fire tube boiler.

  • Fire tube boiler:

fire tube boiler

In fire tube boiler there are number of tubes through which the hot gases are passed and where the water surrounds these tubes. The tube consists of hot gases passing through them with their immersion into water in a vessel that is closed. In this boiler the closed vessel or shell consists of water through which the hot tubes are passed. The boiler cannot produce steam at very high pressure as the steam and water are in the same vessel. Its maximizes up to 17.5kg/ with a capacity of 9 metric ton in one hour.

  • Types of fire tube boilers:
  • Horizontal Return Tabular Boiler
  • Short Fire Box Boiler
  • Compact Boiler
  • Advantages:
  • Its construction is quite compact.
  • The fluctuation of stem demand can be met very easily.
  • The maintenance is also very cheap.
  • Disadvantages:
  • It requires long time to raise steam at required pressure as the water requirement is very high that is used for the purpose of operation.
  • The very high pressure is not possible with the same vessel for water and steam.
  • The stem state will not be so dry that is received from this boiler.
  • Water tube boiler:

water tube boiler

A water tube boiler is a kind of boiler that uses the water that is heated inside the tubes and the surrounded by hot gases around them. It is just reverse to the fire tube boiler where water is heated inside tubes and hot gases surround these tubes.

  • Types of water tube boiler:
  • Horizontal straight tube boiler
  • Bent tube boiler
  • Cyclone boiler

Horizontal straight tube boilers are classified as:

  • Longitudinal drum water tube boiler
  • Cross drum water tube boiler

Bent tube boilers are classified as:

  • Two drum bent tube boiler
  • Three drum bent tube boiler
  • Low head there drum bent tube boiler
  • Four head bent tube boiler
  • Advantages:
  • More number of water tubes facilitate larger heating surface.
  • The movement of water is much faster than that of fire tube boiler because of the conventional flow, which results in high rate of heat transfer with higher efficiency.
  • The pressure obtained is very high about 14okg/.
  • Disadvantages:
  • The construction is not so compact.
  • The maintenance cost is high.

Difficulty is faced in transportation and construction due to its size.

Click Here To Know More About: Classification of Heat Exchangers