Homeowners Professionals

How a steam boiler works

Many large businesses and public buildings need a way to provide a source of heating and hot water, but a standard boiler you might find in your home doesn’t have the power to heat a large building.

Steam boilers  are very popular for industrial use, particularly because steam can travel through a large pipe system much more easily than water, which would require lots of pumps. The steam can also be utilised in other ways, such as for adjusting humidity levels or sterilising equipment in the case of a hospital.

But how does a steam boiler work?

How is steam produced in a boiler?

Steam boilers are very complex, but we’ve tried to explain how they work in an easy to understand way.

There are two main designs of steam boilers: water tube and flame tube/smoke tube (otherwise known as a shell boiler). In a water tube boiler, water is contained in tubes that are surrounded by hot gas. In a shell boiler, hot gas flows through tubes that are surrounded by water. In both cases, the hot gas is able to heat the water. A water tube boiler is able to reach a pressure of about 200 bar, whereas a shell boiler will reach a maximum pressure of around 25 bar. A shell boiler can produce up to 26 tonnes of steam per hour, making it sufficient for industrial use.

The boiler itself is a large horizontal cylinder which has insulation all around it. The boiler will be 75 per cent full of water, allowing a 25 per cent gap for the steam. Both water tube and shell boilers work via a three-pass system, however the following information is only applicable to shell boilers.

Within the boiler, the flame tubes sit in the water. Hot gas enters the flame tubes (first pass) and continues through an internal reversing chamber that reverses the flue gases, leading them into the smoke tube (second pass). An external reversing chamber reverses the flue gas a second time, leading it into another smoke tube (third pass). It’s as if the gas is passing backwards and forwards through the boiler, heating the water as it moves. Once the gas has moved through the third pass, it exits through a chimney. The heated water then turns into steam.

The steam can be transported around the system via pipes and hot water will be pumped to taps. Any steam that condenses within the pipes will be returned to the boiler, where it is heated again. Some water may be lost over time and will need to be re-added.

You can find a cross section illustration of a three-pass shell boiler below:

How long does a steam boiler last?

The average lifespan of a steam boiler is between 20 and 30 years, if it has been maintained correctly. There are many elements of a steam boiler and, in order to make sure the boiler is running safely, there are some checks that should be completely regularly.

Cold water enters the boiler system via the mains. This water won’t be completely clean and may contain dirt and debris. The boiler system should have a filter installed to clean the water and a deaerator that removes the air. Some particles and sludge may remain, and these should be filtered out through the blowdown. The blowdown enables this dirt to be removed from the whole system. When the valve is opened, the suction caused by the opening removes solid matter from the boiler floor. The valve can be opened manually or automatically via a programme and should be released on a regular basis to avoid dirt build up.

Salt can quickly build up in a steam boiler, which could lead to corrosion of the boiler. This is because when water evaporates, the salts that are naturally in the water cannot evaporate. Instead, they form a solid crust. Because of this, each boiler will have a total dissolved solids (TDS) valve. A sensor will allow the TDS valve to be opened when it detects an unusual level of salt in the system. The valve allows water to be drained, taking the salt with it, and new, filtered water can replace what was lost.

Finally, gas and oil can only burn when oxygen is present. Because of this, every burner within the steam boiler will have a combustion air fan installed. To ensure the longevity of the boiler, the temperature of the combustion air should be between 5°C and 40°C.

How does a steam engine work?

Steam engines made use of steam boilers to enable trains to run in the 18th and 19th centuries. The way a steam engine works is extremely similar to a steam boiler.

Instead of gas, coal was applied to a fire to produce lots of heat. A boiler full of water was located just behind the fire so the water would eventually heat and evaporate. This steam would rise and follow a pipe system into a cylinder. Within the cylinder there was a piston. The pressure created by the steam would begin to move the piston in a forwards and backwards motion. This movement would drive the crank and connecting rod, a long extending arm that you could see on the wheels of the train. It forced the wheels to move until the train had reached a particular speed, when momentum could take over.

Once momentum was carrying the train, the piston was no longer working as hard and so the excess steam was pushed back through the cylinder and out of the train’s chimney.

What is a boiler ticket?

A boiler ticket is a safety certificate that’s issued for a steam boiler on a train. Steam trains were withdrawn from the UK in 1968, however the ban was lifted in 1971. Electric and diesel trains are now much more the standard, however steam trains can occasionally be found running on the mainline railway, where they often attract crowds of spectators.

A boiler ticket is required for steam trains that have had a major rebuild or haven’t been checked in ten years. Annual safety inspections are a requirement, and the train can be banned until the boiler has been repaired or replaced. If this is the case, a new boiler ticket must be obtained.

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