How does a Water Tube Boiler work? | Types of Water Tube Boilers

The main objective of the boilers is to generate steam. There are multiple types of boilers, and a water tube boiler is one of them. A water tube boiler uses to generate high-pressure steam. These are the most common boilers used all over the world for different applications. The water tube boilers are most famous because they produce high-pressure steam than the fire tube boilers. In this previous article, we discussed the fire tube boiler. Therefore, in this article, we will mainly discuss to water tube boiler.

What is a Water Tube Boiler?

A water tube boiler is a type of boiler in which the water flows inside the tubes while tubes are surrounded by hot flue gases. The water tube boiler has high steam generation rate than a fire tube boiler. This is due to hoop stress (the tangential compression pressure on the tube).

This type of boiler has a furnace, water tubes, economizer, feed pump, shells, superheater and convective evaporator.

It uses a furnace to burn fuel. The heat produced due to fuel burning is used to heat the flue gases. As the flue gases get heat, they further transfer their heat to the water, which converts into steam.

Water tube boiler

The water tube boilers have an economize for preheating the water.  This water preheating process uses to increase the efficiency of the boiler.

The main objective of the water tube boiler is to produce high-pressure steam that a fire tube boiler can’t be achieved.  Therefore, these boilers are used for marine, large power plants, textile factories, chemical industries and sugar mills.

Working of Water Tube Boiler

The water tube boiler works on the base of natural circulation. It has two drums:

  1. Mud drum or lower drum
  2. Upper drum or steam drum

The upper and lower drum connect through two tubes:

  1. Riser Tube
  2. Downcomer Tube

water tube boiler working

Fire of all, a feed pump is used to pump the water into evaporator tubes of the boiler.

The solid fuel (i.e., coal) places on the grate of the furnace. When the fuel combustion process starts, the heat is generated, which is transferred to the flue gases. 

When flue gases get heat, they start flowing outside the evaporator tubes. The produced hot flue gases transfer their heat to the water, and the water transforms into steam. Due to this heat transfer process, the temperature of the water increases due to that water concentration also increases.  In this way, the water concentration will be higher than the generated steam concentration.

After steam production into the evaporator tube, the mixture of the water and steam is sent into the steam or upper drum.

Due to the difference in concentrations, the water and steam are separated in the steam drum.

As mentioned earlier, steam has a lower concentration than water; water will flow downward while steam will flow upward.

After separating the steam and hot water, the hot water flows into the lower or mud drum via a downcomer tube.

This hot water is transferred from the lower drum to the upper drum via a riser tube using a natural circulation process.

When the boiler generates extra steam, the force inside the drum increases and the amount of water supplied to the drum decreases, which tends to reduce the flow of steam. Similarly, when the steam generation rate decreases, the force inside the water tube boiler decreases and the amount of water received increases; due to that, the production rate increases. In this way, the water tube boilers control the steam generation rate.

Types of Water Tube Boilers

The types of water tube boiler are given below:

  1. Yarrow boiler
  2. Loeffler boiler
  3. Simple Vertical Boiler
  4. Lamont Boiler
  5. Stirling Boiler
  6. Benson Boiler
  7. Babcock and Wilcox Boilers

1) Simple Vertical Boiler

The simple vertical boiler is a most common type of water tube boiler. This boiler has a vertical axis of direction according to the position. It has a chimney, safety tap, steam stop tap, pressure gauge, manhole, steam space, cylindrical shell, water gauge, fusible plug, hand hole, cross box, firebox, fire hole, feed check tap, grate, and ash pit.

Simple Vertical Boiler

The vertical boiler has a fire hole that uses to place the fuel on the grate. As the fuel burn, the hot gases generate which use to convert the water into steam.

An ashpit is installed just below the grate. This ashpit uses to collect the ash of the burnt fuel. The boiler also has a chimney which uses to eliminate the unused gases and smoke. This boiler installs in the vertical position. The boiler shell links with the furnace bed.  It has 2.0 m in height and 1.25 m in diameter.

These types of water tube boilers are used in steam cranes, Steam shovels, steam tractors, railway steam engines, and steam lorries.

Read More: Different types of Boilers

2) Stirling Boiler

The Stirling boiler is an externally fired, bent tube and stationary water tube boiler.  This is one of the most efficient boilers. It has the capacity to generate steam up to 50,000 kg/hr and steam pressure up to 60 bar.  The Stirling boiler has efficiency from 60% to 80%. In 1888, Alan Stirling invented the first Stirling boiler.

Stirling boiler

The Stirling boiler uses to generate steam in a large land-based stationary plant.  It has 2 mud or lower drum while 3 steam or upper drums.

The mud drums are installed on the base of the boiler so that hot water can naturally flow from the steam drums to mud drums while steam drums install on the top of the boiler. The mud drums are linked with the steam drums via bent tubes.

One of the main advantages of the Stirling boiler is that it has a very high steam generation rate because of additional drums. It also has the capacity to generate a very high amount of pressure with a small amount of pressure drop.

The main disadvantage of this boiler is that it has bent tubes; due to that, the cleaning and inspection of the tubes are very hard.

3) Babcock and Wilcox Boiler

The Babcock and Wilcox boiler is an externally fired, solid fuel fired, high pressure, stationary, multi-tubular, natural circulation, natural draft, horizontal drum axis water tube boiler. In 1967, George Herman Babcock and Stephen Wilcox was invented the Babcock and Wilcox Boiler.

Babcock and Wilcox Boiler

This type of water tube boiler has the capacity to generate steam pressure up to 20 bars. The steam generation rate of the Babcock and Wilcox boiler is up to 20 tons/hr.

The steam drum of this boiler is made of steel. This boiler has steam and mud drums, water tubes, fire door, downtake and uptake header, mud box, superheater, baffles, furnace, grate, pressure gauge and inspection door. The fire door of this boiler uses to supply the fuel into the furnace. This door is installed at the bottom of the boiler.

One of the big advantages of the Babcock and Wilcox boiler is that it has very high overall efficiency. The cleaning, repairing and replacement of the tubes are very easy.

The main disadvantage of this boiler is that it is not best for sedimentary and impure water. It also has a very high maintenance cost.

4) Benson Boiler

It is a supercritical, drumless and high-pressure boiler. In 1922, Mark Benson designed the Benson boiler. It has no drum.

It is a supercritical boiler that compresses the feed water up to a supercritical pressure.  This compression of the water stops the creation of the bubble on the surface of the water tubes. Under supercritical pressure, water and steam have the same density so that no bubbles can be formed.

Benson Boiler

The main advantage of the Benson boiler is that it has 20% less weight compared to other water tube boiler types because it has no drum. It also takes less floor space for installation. The thermal efficiency of this boiler is up to 90%.

5) Lamont Boiler

In 1925, Walter Douglas La-Mont designed the first Lamont boiler.  It is known as a Lamont boiler because of the name of its inventor. This type of water tube boiler has the capability to produce steam up to 5000 kg/hr and steam pressure of more than 150 bar.

This type of boiler is a forced circulator internally fired boiler. It has a flexible design. It uses a centrifugal pump to force the water into the furnace.

Lamont Boiler working

The main advantage of this boiler is that it has a very simple operation and design. There is no problem in the initial start.

The disadvantage of this boiler is that there is bubble formation on the tube surface, which reduces the steam generation rate.

5) Loeffler Boiler

It is an internally fired, forced circulation boiler. The Loeffler boiler has the capacity to generate steam from 90 to 100 tones/hr and steam pressure up to 100 bar.

The major parts of the Loeffler boiler are the evaporating drum, radiant superheater, centrifugal pump, mixing nozzle, and convection superheater.

Loeffler boiler

The major advantage of the Loeffler boiler is that it has the capability to use saltwater to produce steam. The main disadvantage of this boiler is that there is sediment and salt deposition in the tube’s inner surface. This deposited sediment and salt reduce the heat transfer process due to that the steam production rate reduces.

Water Tube Boiler Parts and Functions

The water tube boiler has the following parts:

  1. Furnace
  2. Feed Pump
  3. Grate
  4. Chimney
  5. Mud drum
  6. Fire box
  7. Superheater
  8. Pressure indicator
  9. Economizer
  10. Steam drum
1) Furnace

It is an enclosed area. It uses to burn solid fuel for producing heat for flue gases.

2) Feed Pump

This pump uses to transfer the water into the water tubes.

3) Chimney

This component of the water tube boiler uses to place the fuel inside the furnace.

4) Ash pit

The ash pit is installed just below the grate. It uses to collect the ash of the burnt fuel.

5) Mud Drum

This drum installs at the bottom of the boiler. As the water flows from the steam drum to the mud drum, the mud drum removes the contamination from the water and sends it back into the steam drum.

6) Economizer

It uses to preheat the water. It helps to increase the efficiency of the boiler.

7) Steam Drum

As the steam is generated into the water tubes, it is sent into the steam drum. The steam drum uses to separate the steam and water.

Advantages and Disadvantages of Water Tube Boiler

The water tube boilers have the following advantages and disadvantages:

Advantages of Water Tube Boiler

  • These boilers have a safe operation.
  • These produce more steam pressure than fire tube boilers.
  • The water tube boiler has high steam production rate than a fire tube boiler.
  • They have simple and easy construction and working.
  • These boilers have up to 90% overall efficiency.
  • The water flow in this boiler is higher than the fire tube boiler. Therefore, it has a high heat transfer rate which increases its efficiency.
  • The water tube boilers are the best boilers for large power plants.
  • The steam produced by the water tube boiler has high quality the steam of the fire tube boiler.
  • It needs less floor space.
  • You can easily transport the water tube boilers from one place to another.

Water Tube Boiler Disadvantages

  • The water tube boilers have complex designs.
  • The operation of this boiler is very complex. Therefore, a skillful person requires for its operation.
  • It has a high maintenance cost.
  • These boilers have a high cost than the fire tube boilers.
  • They are not best for small industries because they produce very high steam and pressure.
  • It has complex installation.

Applications of Water Tube Boilers

  1. The water tube boilers use in power plants, chemical industries, and textile factories.
  2. These boilers are also used for sugar mills and paper mills.
  3. They also utilize for marine applications.

FAQ Section

What are the examples of water tube boiler?

The water tube boiler has the following examples:

  1. Lemont boiler
  2. Stirling boiler
  3. Simple Vertical Boiler
  4. Benson Boiler
  5. Yarrow boiler

What is a water tube boiler used for?

The water tube boiler is used to generate steam for different applications. There are also used in chemical industries, textile factories, sugar mills, and paper mills.

How long do water tube boilers last?

The service life of a water tube boiler is from 15 years to 40 years.

Why water tube boiler is more efficient?

The water tube boiler is more efficient than the fire tube boiler. This is because a water tube boiler converts the water into steam more quickly than a fire tube boiler. The tubes of the water tubes boilers hold less amount of water than the water tank of the fire tube boilers.

What is the efficiency of water tube boiler?

The efficiency of a water tube boiler without an economizer is up to 85% for hot water and 81% to 83% for steam.

Why water tube boiler is better than fire tube boiler?

The fire-tube boilers are less efficient than the fire tube boilers. The water tube boilers have a high steam generation rate and produce high steam pressure.

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