Water-tube Boilers Explained: An Overview of Different Boiler Tubes

Water-tube boilers, unlike their fire-tube counterparts, are not so simple in design, construction, and maintenance. They are the sophisticated class of well-researched industrial engineering devices known for their applications in the marine industry and otherwise.

If not all, then much of the advancements in boiler construction and technology are made in water-tube boilers due to their extremely high-temperature range, such as above 1300°C, and steam generation capacity as high as near and above 12000 kg/hr.

On account of the cutting-edge development and modifications made in the water-tube boilers from time to time, the relevant literature is rich and somewhat difficult to understand due to the addition of new terminologies and parts.

Much ease in reading can be provided if one builds a sound knowledge related to the key terms of this boiler category. Therefore, the present post is devoted to serving this purpose: It specifically discusses all the types of tubes (from major to minor) used in the water-tube boiler to familiarise the reader with the key components of its construction.

Different Tubes Used in Water-tube Boilers

Let’s discuss the different types of tubes used in water-tube boilers one by one. These tubes include:

  1. Generating tubes
  2. Screen tubes
  3. Water-wall tubes
  4. Downcomers
  5. Risers/Return tubes
  6. Superheater tubes
different types of tubes in water tube boilers

1. Generating Tubes

These are the small diameter tubes that are placed in the way of hot burned gases. Convection usually occurs through these tubes. These are the main tubes of any water-tube boiler, as maximum heat transfer between the flue gases and water occurs in the generating tubes.

The lower the diameter of the generating tube, the higher the steaming rate. But, there should be a lower limit to the diameter of the generating tube to prevent the overheating of the water.

Another drawback is that if the generating tubes are too thin, then there would be maximum heat transfer from the passing over burned gases to the water in the generating tubes, and the temperature of the burned gases would fall below its dew point, thereby causing corrosion of the heating surfaces.

Hence, in modern boilers, generating tubes are not used as such, especially in radiant heat boilers in which water walls are used instead.

2. Screen Tubes

It’s another class of tubes used extensively in water-tube boilers. They are fitted adjacent to the furnace, as shown in the figure, where they act as risers or return tubes as they return the pre-heated water-steam mixture to the steam drum.

They receive heat from the furnace flame as well as from the burned gases. But why are they called screen tubes? What do they actually screen off, and why? They are called screen tubes because they act as a screen between the superheater and furnace flame, thus blocking the direct radiant heat transfer between the two.

Their diameter is chosen to be large enough in order to keep the steam from overheating. They are fitted in rows, for instance, three, eight, and eleven.

3. Water-wall Tubes

Water-wall tubes are used to absorb the heat of the furnace. They are used in boilers to reduce the amount of refractory material in the furnace.

However, in some optimal designs, both features are clubbed: it is made possible with the use of water-cooled refractory walls. These walls consist of water tubes with studs welded onto them to sustain high temperatures and prevent damage.

In some other designs, part of the surface of water-wall tubes is exposed to the radiant heat to convert water into steam. Yet, in other boiler versions, these tubes are connected by welding through fins or straps, thereby eliminating the need for refractory material, as in the radiant heat boilers.

4. Downcomers

These are the large-diameter tubes that are unheated, which means that they are not exposed to the hot gases of the furnace. Their purpose is to supply water from the steam drum back to the water drum and headers. In other words, it acts as a feeder to the water drum and lower header.

5. Riser or Return Tubes

Their purpose is to return the water and steam to the steam drum from the top water-wall headers. If they are encased in special screening tubes, they are called screen tubes as well.

6. Superheater Tubes

These are the small diameter tubes that are placed in the superheating section of the boiler after the screen tubes. They are positioned so as to benefit from the main gas stream at best. Their chief purpose is to superheat the saturated steam receivable from the steam drum for its ready expansion in the steam turbine or steam engine.

They are necessarily protected against their exposure to radiant heat because of the risk of overheating the steam since its specific heat is much lower compared with that of feedwater.

There are many superheater tubes which act as superheater support tubes. They are called support tubes because they are water-cooled and help in regulating steam’s superheat temperature. They include straight tubes, curved tubes, and u-bend tubes.

All the tubes, as mentioned earlier, are proper for water-tube boiler design. Although there are other tubes as well, which are common in both fire-tube and water-tube types such as in economizer, attemperator, steam drum, and others, they do not come in the immediate environment of the present blog post.