A water tube boiler is a type of boiler in which hot gasses surround heated water inside tubes. This is how a water tube boiler is generally defined. This boiler actually operates in the exact opposite manner to a fire tube boiler, which passes hot gases through tubes that are submerged in water.

Advantages of Water Tube Boiler

Since water tube boilers have so many benefits, big thermal power plants are typically where you’ll find these types of boilers.

1. More water tubes can be used to create a larger heating surface.
2. Convectional flow causes water to move through the air much more quickly than in a fire tube boiler, which increases the rate of heat transfer and boosts efficiency.
3. Very high pressure in order of 140 kg/cm² can be obtained smoothly.

Working Principle of Water Tube Boiler

The water tube boiler’s operation is both fascinating and straightforward.
Let’s sketch a very simple water tube boiler diagram. It mainly consists of two drums, the upper drum being called the steam drum and the lower drum being called the mud drum. As seen in the image, the two tubes known as the riser and down-comer connect the upper drum and lower drum.

Steam is naturally produced when water in the lower drum and the riser connected to it is heated, and it then naturally travels to the upper drums. Steam is naturally separated from water and stored above the water’s surface in the upper drum. Since the hotter water in the lower drum and that in the riser is lighter than the colder water fed from the feed water inlet at the upper drum, the colder water pushes the hotter water upward through the riser. In the boiler system, there is only one convectional flow of water.

As more steam is produced, the pressure inside the closed system rises, obstructing the water’s convectional flow and making the rate at which steam is produced proportionately slower. Once more, if the steam is removed through a steam outlet, the pressure inside the system decreases, which causes the convectional flow of water to accelerate, increasing the rate at which steam is produced. The water tube boiler is able to regulate its own pressure in this way. As a result, this kind of boiler is known as a self-controlled machine.

Types of Water Tube Boiler

There are many types of water tube boiler .

1. Horizontal Straight Tube Boiler.
2. Bent Tube Boiler.
3. Cyclone Fired Boiler.

Horizontal Straight Tube Boiler again can be sub-divided into two different types, such as

1. Longitudinal Drum Boiler
2. Cross Drum Boiler.

Bent Tube Boiler also can be sub-divided into four different types, such as

1. Two Drum Bent Tube Boiler.
2. Three Drum Bent Tube Boiler.
3. Low Head Three Drum Bent Tube Boiler.
4. Four Drum Bent Tube Boiler.

Horizontal or Longitudinal or Babcock-Wilcox Water Tube Boiler

Construction of Babcock-Wilcox Boiler

Longitudinal Drum Boiler and Horizontal Tubes Boiler are additional names for the Babcock-Wilcox boiler. One cylindrical drum is positioned longitudinally above the heat chamber in this style of boiler. As depicted in the figure, the drum’s downcomer tube is filled, and the drum’s riser tube is attached. As depicted in the figure, these down-comer tube and riser tube are connected to one another by 5^o to 15^o straight water tubes.

Working Principle of Longitudinal Drum Water Tube Boiler

Working principle of Babcock – Thermonyphon principle underlies the Wilcox Boiler. As mentioned in the construction of the longitudinal drum boiler, the rear feed water inlet of the drum is fed with colder material. Since colder water is heavier, it drops through a down-comer that is installed at the back of the drum. The water enters a horizontal water tube from a down-comer where it heats up and becomes lighter.

The water rises through these horizontal tubes that are inclined upward as it gets lighter and eventually returns to the boiler drum via a riser. Steam bubbles are produced in these tubes as a result of the heat that is absorbed by the water as it travels through inclined water tubes from the hot gases that surround it. These steam bubbles then enter the longitudinal drum of the Babcock – Wilcox Boiler through a riser, naturally separate from the water, and occupy the space above the water surface.

Cross Drum Water Tube Boiler

Construction of Cross Drum Water Tube Boiler

The Cross Drum Boiler is essentially a Longitudinal Drum Boiler variant. The steam drum in a cross drum boiler is positioned in opposition to the heat source, as depicted in the figure. So Here, the riser is attached to the top of the drum via a horizontal tube, as shown in the figure, and the down-comer is attached to the bottom of the drum. In the same way as a Babcock-Wilcox boiler, 5o to 15o inclined water tubes are connected to down-comer and riser tubes.

Working Principle of Cross Drum Boiler

The working principle of cross drum boiler is same as longitudinal drum boiler.

1. The feed water is fed to the cross drum through feed water inlet.
2. Then this water comes down through the down-comer pipe and enters into inclined water tube placed in hot chamber.
3. Here, the water becomes hot and steam is produced in the water which comes into steam chamber.
4. Here in the steam drum steam is separated from water in natural way.

Bend Tube Boiler or Sterling Boiler

The improved version of a water tube boiler is a bend tube boiler, also known as a sterling boiler. The bend tube boiler uses four drums,. But its operating principle is essentially the same as that of other water tube boilers.

Construction of Bend Tube Boiler

As seen in the figure, three drums are positioned over the heat source. The fourth drum is positioned inside the heat chamber,. And it is connected to the top three steam drums using a bendable water tube. As depicted in the figure, equalizer tubes connect the top three drums. Equalizer tubes are used to extract steam.

Working Principle of Bent Tube or Stirling Boiler

First, the feed water enters the upper drum on the right. The water in the lower water drum descends because it is more dense. Steam bubbles are created as a result of the heating of the water in that water drum and the pipes that connect it to the other two upper drums. So The bent tube boiler’s most fundamental operating principle is as follows.

Capacity of Stirling Boiler

Capacity of Stirling Boiler is much higher than Babcock – Wilcox boiler as high as 50,000kg per hour and pressure as high as 60kg/cm².