Introduction

In our previous blog, we discussed how does steam power plant works now let’s discuss the classification of boiler such as Water-tube boiler, Fire-tube boiler, and the difference between water tube boiler and fire tube boiler as well as Size selection of the boiler.

The boiler is a closed container that converts water into steam by using hot gases

A boiler is the most important component in a power plant as it generates the steam to rotate the turbine.

There are only a few major components in a power plant turbine boiler and alternator. Now you must be thinking about the other ones. The remaining components are either used to increase the efficiency of the plant or to complete the close cycle.

In the late 1700s and early 1800s kettle boiler was first developed to convert the boiled water into steam. As you can imagine, a clear picture of that boiler cause we all know the kittle, in this type of boiler, water is just placed above the fire and converted into steam.

Around 1867,  conventional boilers are evolved and after that, the steam generation industry started to develop. Later on, George Babcock and Steven Wilcox became the founding father of steam-generating boilers. After that, till this date, new inventions and modifications are going on.

classification of steam boiler

Boilers are classified into two categories

1. Water-tube boiler
2. Fire-tube boiler

Water-tube boiler

As the name suggests, water is inside tubes in the water tube boiler, and hot gases are outside the tubes.

This means you have to image a close container containing several tubes n which water flows and hot gases are passed outside those tubes.

A Water-tube boiler is specifically used where high temperature, as well as high pressure, is needed.

Working principle

Water flows in the tube, and hot gases enclose the tube.

Water is supplied to the steam drum form, where it is circulated in the tubes with the help of a water pump. Fuel is burned inside the boiler furnace to generate hot gases. These hot gases are permitted to supply in shell part of boiler where the hot gases produced by fuel replaces the heat by water. And hence the water is getting converted into steam which is the only function of the boiler.

Components

Boiler shell

The boiler shell is the external cylindrical part pressure container. This shell forms a case that contains water and steam, often including the flue gases and the furnace.

In a short boiler, a shell is a cylindrical drum that protects the boiler’s internal parts and gives the covering.

Mud drum

As the name suggests, it is a drum where impurities like mud, sediments, etc., settle. Mud drum is basically located near the water storage of boiler where all mud is collected

Strainer

We know that water is required to be converted into steam. Therefore, this water should be free from impurities and any solid elements.

So basically strainer is a filter to hold solid elements letting a fluid supply.

Sight glass

As the name suggests, it is a glass placed on the boiler container from where we can observe the water level in the boiler.

Types of water tube boiler

1. Simple vertical boiler
2. Stirling boiler
3. Babcock and Wilcox boilers

Fire-tube boiler

Hot gases of combustion are inside the tubes surrounded with water.

The whole operation of the fire tube boiler is opposite to that of the water tube boiler.

Some fire tube boilers are vertical, multitubular, internally fired, and natural circulation boilers.

The fire-tube boiler was popular in the 18^th century and mainly used for steam locomotives.

Working principle

Fuel is burned inside a furnace. When we talk about fuel, coal is the primary fuel for a steam power plant. This burning of fuel generates hot gases; these hot gases are passed through the tubes. These tubes are covered with water from all sides. When the hot gases pass through the tubes, the heat energy of gases is transferred to the water surrounding them. As a result, steam generated in water naturally comes up. It is stored upon water in the same vessel of fire tube boiler.

The generated steam is taken out from the steam outlet to rotate the turbine.

Types of fire tube boiler

1. Simple vertical fire tube boiler
2. Cornish
3. Locomotive boiler
4. Cochran
5. velcon

difference between water tube boiler and fire tube boiler

Sr.No.	Basis for comparison	Water-tube boiler	Fire-tube boiler
1.	Construction	Water is inside the tubes, and hot gases are outside.	Hot gases of combustion are inside the tubes surrounded with water.
2.	Evaporating capacity	High due to availability of large heating surface.	Low due to less available heating surface.
3.	Operating pressure	High working pressure due to the small size of the drum. (more than 125 bars)	Less (limited to 25 bar)
4.	Suitability	Suitable for large capacity power plants.	Suitable for small and medium capacity power plants.
5.	Space requirement	Less space is required.	More space is required.
6.	Safety	Water-tube boiler provides higher safety than fire tube boiler.	A Fire-tube boiler is less safe compared to a water tube boiler.
7.	Example	Babcock and Wilcox boiler.	Cochran boiler, Lancashire boiler.

Size selection of boiler

The selection of size and type of boiler depends upon:

1. The most important factor when selecting the size and type of boiler is output required in terms of the amount of steam required per hour, operating temperature, and pressure.
2. Availability of fuel and water.
3. The probable load factor.
4. The other factors that influence the choice of the boiler are availability, initial cost, maintenance cost, labor cost, fuel cost, and space requirement.