Water Tube Boilers

This guide considers the use of water tube boilers and how they are used in industrial steam generation applications. Developed by the boiler specialists at Water Treatment Services the guide explains how to recognise tube boilers, how they work and where they are used. It also looks the main components of theses systems and the advantages and disadvantages of this type of steam generator.

Water tube boilers are a special type of steam generator that tend to be used in large scale industrial and commercial applications.

They are different from shell boilers because they circulate hot water around a series of tubes to allow the heat transfer process to take place.

The heat source used to heat the water in a tube boiler encapsulates these tubes and can heat the water to very high temperatures (and pressures).

Water tube boilers are often preferred to other types of steam generator as they can run at much higher pressures than other types of water boiler.

Where are water tube boilers used?

Water tube boilers are typically used in very large commercial and industrial applications including power stations, or in other situations where a very high steam output of is required – over 500 kilos per second, high pressure steam up to 160 bar, or steam which is “superheated” to temperatures up to 550 degrees centigrade.

With new heat transfer technologies water tube boilers are now no longer just used in large scale industrial settings, however.

They are also available in smaller sizes which are designed to compete with conventional shell boilers.

This type of smaller tube boiler is made and put together in a single unit.

Larger water tube boilers are usually made in sections, and put together when they arrive on site.

How do tube boilers work?

Water tube boilers use the process of water circulation and heat transfer to operate… the process is also sometimes referred to as thermosiphoning.

If you’re thinking of investing in water tube boilers, it’s important to know how this works.

Firstly, cold feedwater comes into the steam drum behind a baffle.

As the incoming cold water is denser than the warm water, it sinks towards the bottom of the steam drum, and this drives the warmer water up into the tubes.

As the water is heated, this creates steam bubbles in the tubes… the job of the steam drum is to separate these bubbles from the hot water.

When the pressure in the water tube boiler increases, the difference between the density of the hot water and colder water decreases, and this reduces the water circulation.

In order to keep the same steam output levels at higher pressures, operators need to increase the distance between the lower drum and the steam drum, or think about introducing some other way of forcing the water to circulate.

How is a water tube boiler constructed?

Here we’ll consider the main parts of a tube boiler.

The furnace

The furnace is also known as the radiant section and is the area of the boiler where you find the primary heat source.

The design of the furnace makes sure that the boiler tubes are kept well away from any naked flames that may damage the boiler.

If the two come into contact, this can cause erosion, and ultimately cause the boiler tubes to fail.

Boiler walls or panels

The furnace part of the boiler is usually lined with components called membrane panels.

It is the role of these panels to absorb the heat which is given off by the flame from the furnace.

Convection area

The convection area of the tube boiler is designed to allow efficient heat transfer from the hot gases through the processes of convection and conduction.

This is done in order to get the most energy possible from the hot gas.

A particularly large boiler might have several banks of tubes one after the other.

Typical water tube boiler configurations

There are three main ways in which a water tube boiler can be configured… the selected design will tend to depend on the capacity of the boiler.

Longitudinal drum boilers

Longitudinal drum boilers were the first type of tube boiler, invented to work on the thermosiphon principle.

Cool feed water is fed into a drum, placed lengthwise above the primary fuel source.

The cooler feed water passes to the rear circulation header and then flows into heated tubes.

The temperature of the water is gradually increased as it flows up through the tubes where it eventually begins to boil.

The boiling process has the dual effect of decreasing the waters density and creating steam, which pushes hot water and steam into the front circulation header and back into the drum… the steam is separated off from the water.

Most tube boilers of this design tend to be in the range of 2,250 kilos per hour to 36,000 kilos per hour.

Cross drum boilers

The cross drum boiler design is a variation on the longitudinal drum design described above.

The drum is placed at right angles, or crossways, to the heat source and works in the same ways as the longitudinal drum but has the advantage of providing a more even temperature across the length of the drum.

The downside of this design is that is can sometimes become faulty when working at higher capacities.

If the tubes in the upper part of the boiler are allowed to dry, they can overheat or fail completely.

Most boilers of this type operate between 700 kg/h and 240,000 kg/h.

Stirling boilers

Stirling boilers (sometimes called bent tube boilers) build on previous designs, refining the set ups to create improved heat transfer.

The Stirling boiler also works on the core principles of density of water, and temperature – however, it uses four separate drums to heat the water.

Cold water flows into the upper levels, it then falls down to the lower drums.

The water in the lower drums is heated and steam rises back into the upper drums to be removed.

This style of water boiler has a greater hear transfer area, and is better at promoting natural circulation than other types of boiler.

What are the advantages of water tube boilers?

There are a number of reasons why businesses might consider installing tube boilers over other styles and designs. These include:

The volume of water used in a tube boiler tends to be relatively small, and therefore they are quick to respond when you change the heat intensity.
The design offers the flexibility to incorporate burners on any of the walls of the boiler. This gives options to control the temperature in various parts of the boiler using both vertical and horizontal firing. If the boiler has a superheater, this is the best way of controlling the superheated steam which it produces.
As the diameter of the tubes and the steam drum are smaller, this means they can withstand much higher pressures. Tube boilers can operate at pressures of up to 160 bar in power stations and other large scale steam generating applications.

Are there downsides to using tube boilers?

As with everything, there are also some disadvantages to using water tube boilers. These include:

Although multiple burners provide improved flexibility, this also means that the control systems needs to be more complex too.
Tube boilers are more complex to manufacture, and with larger units there is more on-site assembly required than with other types of boiler.

Other systems to consider

There are other systems to consider as follows:

Combined heat and power

This type of system directs the hot exhaust gas produced from a gas turbine.

The gas, at around 500 degrees centigrade, produces saturated steam, and is used in operating the plant.

Often, this type of set-up is used on sites where there is equal demand for both electricity and steam.

This type of unit typically operates at around 90% efficiency.

Combined heat and power plants (CHP)

These sorts of water tube boilers are usually designed to operate at high to very high capacity.

However, there are also smaller heat boilers on the market and these are usually designed to run alongside a land-based gas turbine unit.

Combined cycle system

This is a variation on the combined heat and power (CHP) system – it takes the saturated steam which then flows through a superheater to create superheated steam.

The superheater usually fires separately, due to the lower temperature in the exhaust of the gas turbine.

The steam which is produced by a combined cycle system can then be used to turn turbines and generate electricity.

In a conventional boiler, the furnace produces higher heat than the exhaust of the gas turbine exhaust, and because of this, this style of boiler needs a much larger heat transfer area.

As a result, in many settings a standard water tube boiler will provide a more compact solution.

Many boilers are fitted with a feedwater heater to make the boiler run more efficiently.

Superheaters may also be added into this style of design, however, this usually means adding in extra burner to ensure the steam is suitable for steal turbines.

Industrial steam boiler specialists

Water Treatment Services offer a comprehensive range of water treatment solutions for the management of industrial boilers and steam generating systems.

Our experts can help you optimise costs and operational efficiencies, reduce downtime, achieve water and energy savings, and increase plant reliability and safety.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained technicians and consultants we can offer professional, cost effective boiler water treatment and engineering services throughout the UK and Internationally.

Contact us today to learn how our industrial steam boiler solutions can help you and the environment.