Water Tube Boiler | Operation and Types of Water Tube Boiler

A water tube boiler is a boiler that circulates water inside tubes heated from the outside by hot gases. A fire tube boiler uses the reverse arrangement, with hot gases inside tubes surrounded by water.

Advantages of Water Tube Boiler

The advantages of water tube boiler construction include fast response and operation at high steam pressure. These types of boiler therefore serve large thermal power station units.

1. Larger heating surface can be achieved by using more numbers of water tubes.
2. Due to convectional flow, movement of water is much faster than that of fire tube boiler, hence rate of heat transfer is high which results into higher efficiency.
3. Very high pressure in order of 140 kg/cm² can be obtained smoothly.

Working Principle of Water Tube Boiler

The working principle of water tube boiler is natural or forced circulation through heated tubes.
The basic water tube boiler shown here has an upper steam drum and a lower mud drum. Downcomers carry denser water towards the lower drum. Heated riser tubes return a water and steam mixture to the upper drum.

Heat produces steam bubbles in the risers. The lower density of this mixture makes it rise to the upper drum, where steam separates from the water. Cooler feedwater entering the upper drum has a higher density and descends through the downcomers. This density difference maintains natural circulation while the boiler operates within its design range.

Natural circulation weakens as pressure rises because the density difference between water and steam becomes smaller. Boiler controls match firing and feedwater to steam demand, while some high-pressure designs use forced circulation. A water tube boiler does not regulate pressure through circulation alone. Safe pressure control for this type of boiler depends on its control and protection systems.

Types of Water Tube Boiler

The following types of water tube boiler are grouped by their tube and drum arrangement.

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

Horizontal straight-tube boilers include these two drum layouts:

1. Longitudinal Drum Boiler
2. Cross Drum Boiler.

Bent-tube boilers include these four layouts:

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

A Babcock-Wilcox Boiler is a Longitudinal Drum Boiler or Horizontal Tubes Boiler. Its cylindrical steam drum lies lengthwise above the heat source. A rear downcomer and a front riser connect through straight water tubes inclined by 5^o to 15^o, as shown.

Working Principle of Longitudinal Drum Water Tube Boiler

Working principle of Babcock – Wilcox Boiler depends on the thermosiphon effect. Cooler feedwater enters the rear of the longitudinal drum. Its higher density makes it descend through the rear downcomer and enter the inclined water tubes, where heating lowers the density.

The heated water and steam mixture rises through the inclined tubes and returns through the riser. Hot gases outside the tubes supply the heat that creates steam bubbles. The drum separates steam from water, and steam collects above the water surface in the longitudinal drum of the Babcock – Wilcox Boiler.

Cross Drum Water Tube Boiler

Construction of Cross Drum Water Tube Boiler

A Cross Drum Boiler is a variant of the Longitudinal Drum Boiler. In a Cross Drum Boiler, the steam drum lies across the heat source. The downcomer connects below the drum, while the riser returns through a horizontal connection near the top. Water tubes inclined by 5^o to 15^o connect these paths in the same general manner as a Babcock-Wilcox boiler.

Working Principle of Cross Drum Boiler

The working principle of cross drum boiler uses the same density-driven circulation as a 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

A Bend Tube Boiler, more accurately called a Stirling Boiler, is a four-drum water tube boiler. Bent tubes link three upper drums to one lower drum and support natural circulation.

Construction of Bend Tube Boiler

Three upper drums sit above the heat source. Bent water tubes connect them to a fourth lower drum in the heated chamber. Equaliser tubes connect the upper drums, and steam leaves from the upper section.

Working Principle of Bent Tube or Stirling Boiler

Feedwater enters an upper drum and descends to the lower water drum because it is denser than the heated mixture. Heat creates steam bubbles in the lower drum and connecting tubes. The lower-density mixture then rises towards the upper drums. This density-driven loop is the working principle of bent tube boiler.

Capacity of Stirling Boiler

Compared with some early Babcock – Wilcox boiler installations, older references quote a Capacity of Stirling Boiler around 50,000 kg per hour and pressure around 60 kg/cm². Actual capacity and pressure depend on the specific design.