Generally, the boiler is an equipment that is used for the production of steam. The steam generated in the boiler is further utilized to drive a turbine attached to a synchronous generator. The synchronous generator converts the mechanical energy of the turbine to electrical energy. This generated energy is supplied to the bus bars further it is transmitted to different areas. These boilers are classified into different types among them water tube and fire tube boilers play a crucial role. In this article, we shall discuss what is a water tube boiler, mountings, and accessories required for the operation, types of water tube boilers, advantages, and disadvantages.
What is a Water Tube Boiler
Water Tube Boiler is a high-pressure boiler that operates at a pressure of about 165 bar. Basically, in this boiler, water will be flown inside the tubes surrounded by flue gases. More steam is generated by using this boiler so, the overall efficiency of the boiler is high. The water Tube boiler is shown in the figure below.
The boiler requires different types of mountings to be placed on the shell.
Mountings are those which are used for the functioning of the boiler.
- Safety valve
- Feed check valve (a kind of non-return valve)
- Water level indicator
- Pressure gauge
- Steam stop valve
The pressure gauge is mounted on the top of the shell. It indicates the amount of pressure observed inside the boiler. It is a dial-type instrument that indicates pressure in different units such as kgf/cm2 or K.pa or PSI. Now a day’s digital type instruments are also used where it senses the pressure by some transducers and sends the signal to the display unit.
Water Level indicator
During the operation of the boiler, the operator must know the level of water. Without knowing the level inside, the boiler cannot withstand the pressure and may lead to an explosion. So, a water level indicator is mounted to the shell of the water tube boiler.
Water at higher pressures has a lot of energy so, safety precautions should be taken to prevent any loss. For this purpose, we install a safety valve to the boiler. Whenever the pressure is above the prescribed limit I,e withstand capability then safety valve will be opened. At-least two safety valves are to be provided.
Steam Stop Valve
A stop valve is provided to operate the steam control according to the requirement. The wet steam in the shell is sent to the superheater to reheat and is resent to the shell.
Accessories are provided to increase the efficiency of the boiler
- Air pre-heater
Without mountings, boilers cannot operate but without accessories, boilers can operate. But, the efficiency will be low without accessories.
There are different types of Water Tube Boilers. They are
- Stirling Boiler
- Babcock – Wilcox Boiler, and
- Benson Boiler
The entire arrangement is enclosed in brickwork. A Stirling boiler has a number of drums and also a mud drum. Depending upon the requirement the number of drums is chosen. A Stirling boiler can work up to a pressure of 60 bar and the production of steam would be around 50000 kg/hr. The efficiency of this boiler will be around 60 – 80 %. The figure which depicts the Stirling boiler is shown below.
There are three drums placed at the top and each of these three tubes is connected to the mud drum by some bent tubes. The reason for using bent tubes is to avoid thermal stresses. The topmost three drums are connected by circulating and equalizing tubes. Circulating tubes are used to circulate water equally among the three drums and equalizing tubes to maintain an equal amount of steam in the drums. The first drum is connected with a steam stop valve. Similarly, the second drum with a pressure gauge and third drum with a feed check valve. The mud drum is connected with a blow-off valve. The feed water is provided by the feed check value which comes to the mud drum. The path of flue gases from the combustion of fuel is increased by using Baffle plates.
The Feed Water is allowed inside the drum by a feed check valve. The water travels to the mud drum where any unwanted particles will get settle at the bottom of the mud drum, which will be eliminated by a blow-off valve. The rest of the water will move towards the other two drums. The water circulation is maintained by circulating tubes and steam pressure is maintained by equalizing tubes. The fuel that is burnt in the firebox over the grate produces hot flue gases that travel over the water tubes giving its temperature. For the proper passage of hot flue gases over the water tubes baffle plates are arranged. The heated water generates high-pressure steam used for the production of Electricity.
- The steam production rate is very higher.
- Pressure drop inside the boiler is less.
- Great flexibility in the design.
- Cleaning is difficult.
- The size of the boiler is large.
- Large in space (occupies more space).
- A very high level of superheating is not possible.
It is used in
- Paper Mills
- Sugar Mills, and
- Steam Engine.
Babcock – Wilcox boiler
It is a type of water tube that is horizontal, externally fired and the circulation is natural.
The entire arrangement is enclosed in an Ironed structure (welded steel drum). The water tubes are inclined at an angle of 15°. This boiler is made of steel shell whose both ends are welded. A water level indicator, pressure gauge, safety valve, and manhole are the mountings used.
The water tubes are placed below the shell by support from the bottom at an angle of 15°. Baffle plates are arranged to increase the area of contact of flue gases with the water tubes.
An anti-priming pipe reduces the moisture (wet steam) in the boiler and sends the remaining content to the superheater. Superheater reheats the wet steam with the help of flue gases and sends it over the water tubes. Babcock – Wilcox boiler is shown in the figure below.
The grate is made of cast iron which is used for the burning of fuel. A mud drum is arranged at the bottom which is used to collect any unwanted material. A passage is provided at the bottom to send the gases out after its usage. Manhole is provided which is used to clean the shell in order to avoid corrosion.
Working of Babcock and Wilcox Boiler
The fuel is burnt over the grate such that flue gases will be produced. These are naturally circulated over the water tubes. The heat of flue gases is transmitted to the water flowing inside the tubes. The water is sent into the tubes from the lower head whose density is higher as the water gets heated its density becomes lower and travels upwards to the upward header. The evaporated water is converted into steam which is sent into the shell.
This boiler has a horizontal shell at the top and the water tubes are inclined at a particular angle. The feed water is fed to these inclined tubes. As the fuel is burnt in the burning area, heat is generated along with the flue gases. The flue gases will spread all over the surroundings. In order to increase the efficiency and steam production baffle plates are used which increase the spread of flue gases all over such that more steam will be produced.
If the steam produced is not sufficient then we can superheat by using a super-heater. The steam is re-circulated in the area surrounded by flue gases to increase the temperature. The steam production is around 20 – 40 tons/hr and pressure 11- 17 bar.
- The pressure loss is minimum as it works on natural convection.
- Damaged tubes can be easily replaced because of the large space inside the boiler housing.
- High operating cost.
- Weight is heavy.
- High Maintenance cost.
- There is a chance of burning.
- It is used in steam power plants
It is a high-pressure water tube boiler and it works on the principle of critical pressure. Critical pressure, both liquid, and gas are under the state of equilibrium(balanced condition). Thus, the water entering at critical pressure will be converted to steam immediately.
It consists of a Benson chamber in which radiant superheater, convective superheater, economizer, and air preheater are placed. Feed blower is connected to the economizer which is fed to the radiant superheater. The radiant superheater is fed to the convective superheater which is connected to the prime mover or turbine to generate electricity. The hot flue gases will travel from bottom to the top. The air preheater is connected to the cold air blower where cold air will be heated by hot flue gases inside the air preheater.
The cold air blower connected to the air preheater supplies cool air which is heated by hot flue gases in the air preheater. The hot air will be used inside the furnace. The feed water blower pumps water into the economizer where the water will be heated and sent to the radiant superheater. Then, radiant superheater partially water will be converted to steam which is sent to the convective superheater. Here, the partially converted steam is completely converted to steam. The steam thus produced drives the prime mover which generates electricity.
- It has no drum
- It is lightweight.
- High efficiency up to 90%.
- The chance of an explosion is less.
- Salts gets deposited inside the boiler.
Thus, in this article, we can conclude that the higher the rate of steam generation, the higher the efficiency of the power plant. As the water tube boiler is a high pressure operating boiler, we can generate more steam. In this article, we had an overview of what is a water tube boiler, its types, construction, working, advantages, and disadvantages. Here is a question for the readers, what is the difference between a fire tube and water tube boiler?