1. According to Passage of water and hot gases:
    Water Tube Boiler: A boiler in which the water flows inside some small tubes which are surrounded by hot combustion gases.
    Fire-tube Boiler: The hot combustion gases pass inside the boiler tubes, which are surrounded by water.
  2. According to Pressure of steam generated
    Low-pressure boiler: a boiler which produces steam at a pressure of 5 to 10bar is called a low-pressure boiler. This steam is used for process heating.
    Medium pressure boiler: It has a working pressure of steam from 15 bars to 80 bars and is used for power generation or combined use of power generation and process heating.
    High pressure boiler: It produces steam at a pressure of more than 80 to 200 bars.

Water Tube Boiler

A water tube boiler can be defined as a Steam boiler in which the flow of water in the tubes, as well as hot gases, enclose the tubes. This boiler attains high-pressures, as well as high-steam capabilities, can be achieved. This is because of condensed tangential pressure on tubes which is known as hoop stress.

Components of Water Tube Boiler

  • Boiler Shell: This shell is the outer cylindrical part of a pressure container.
  • Mud Drum: This is a cylindrical formed space at the base of the water area. The impurities like mud, sediment, and others will be gathered.
  • Strainer:  This is a type of device as a filter to hold solid elements letting a fluid to supply.
  • Sight Glass: A glass tube is utilized on steam type boilers for giving observable signs of the water level in boilers.
  • Security Valve: A spring-loaded tap that unlocks when force gets the location of the valve. This can be used for stopping the unnecessary force from the construction of a boiler
  • Boiler: This is a surrounded space offered for the fuel combustion.
  • Feed-Test Tap: The high-force water flows via this tap, which releases to the boiler simply and supplies the water to the water type boiler.
  • Steam-Stop Tap: It controls the steam flow supply at outside.
  • Burner: This is a device for the beginning of air and fuel into a boiler at the preferred velocity. This is the most essential apparatus for the firing of gas or oil.

Working Principle of Water Tube Boiler

The working principle of water tube boiler is thermal siphoning (circulation of natural water). Basically, this type of boiler includes two drums namely steam, lower or mud drum.

At first, the water is supplied into the steam type drum with the help of a water pump. Whenever the fuel is burned, then hot gases will be generated that are permitted to supply in the shell part of the boiler. The hot gases which are produced by the fuel will replace heat by the water; the water gets changed into steam. Because, the water temperature increases, the concentration will increase automatically.

Clearly, the concentration of steam will be lesser than the water. Thus in the steam drum, the water, as well as steam, gets divided obviously due to variation in concentration. Here the traveling of steam will be upward because of low concentration as well as water will travel downward because of high concentration.

Types of Water Tube Boilers

  • Simple Vertical Boiler
  • Stirling Boiler
  • Babcock and Wilcox Boilers

Fire Tube Boiler

working of fire tube boiler is as simple as its construction. In fire tube boiler, the fuel is burnt in to the furnace. The hot gases produced in the furnace are passes through the fire tubes. The fire tubes are submerged in water inside the main vessel of the boiler. As the hot gases are flow through these tubes, the heat energy of the gasses is transferred to the water surrounds them. As a result steam is generated in the water and naturally rises up and is stored upon the water in the same vessel of fire tube boiler. This steam is then taken out from the steam outlet valve for utilizing for required purpose

Types of Fire Tube Boiler

  • Cornish Fire Tube Boiler
  • Lancashire Fire Tube Boiler
  • Locomotive Fire Tube Boiler
  • Cochran Fire Tube Boiler
  • Scotch Marine Fire Tube Boiler
  • Immersion Fire Tube Boiler