A boiler is essential to drive a steam engine.
It turns water into steam - thats all it does...
...But there is more to it than just a kettle.

How a boiler works

An ordinary cup of coffee may be hot and on a cold day you might see a wisp of steam rising from it but hardly enough to run an engine. To understand better we'll have to look inside...

What we see is water sitting in the cup. What did you expect it to do? Even if we look closely the only thing there is hot liquid. To get bubbles and then steam we have to heat it up...

Now the water is boiling. The liquid is turning into gas. Mostly this is happening under the surface and the gas (that is the steam) rises to the surface and escapes to the atmosphere. Note the following:
  1. The pressure is just 'one atmosphere' ie room pressure
  2. No matter how hot you make the flames underneath the water never gets hotter than 100 degrees
  3. If you take the heat away it stops boiling
To turn a kettle into a boiler we need to seal in the water...

When water is boiled in a closed container the steam being boiled-off from the water has nowhere to go. It builds up inside the container and causes the pressure to rise. (If the container is not strong enough it will more than likely kill somebody as it explodes.) As the pressure rises so the temperature needed to make more steam increases. If some of the heat can't be used to make steam then it has to go into heating up the liquid. The result is:-
  1. High pressure steam
  2. High temperature steam
  3. Liquid just waiting for a drop in pressure to flash into steam.
We now have just the sort of steam engines like. But the third item is what makes a boiler different to other pressure vessels...

If you take away the heat from a boiler it doesn't get any hotter and the pressure won't increase. It just sits there with say a quarter of the volume being steam and three quarters water above boiling point.

But look what happens if we start to take some of the steam out of the boiler. As soon as we remove a bit of steam, the pressure drops slightly. The superheated water now boils a little bit - replacing the steam just removed. This is the secret of boilers. We can keep removing steam - lots of it - many times the volume of the boiler in one go.

Of course you normally keep a boiler fired-up while you're using it, but the wonderful property of being able to generate so much steam in a short period without extra heat means that a small but steady heat is able to drive something very powerfull for a short time. When the demand for steam is less the boiler is storing energy for future use.

Different types of boiler

The general construction of a boiler is a furnace where a fuel is burnt surounded by water. The hot gases from the burning then either pass through tubes surrounded by water (known as a fire tube boiler) or have to pass through a nest of water filled tubes (known as a water tube boiler.) In both cases the object of the tubes is to give as much heating surface as possible so that the maximum amount of heat can be extracted from the hot flue gases. Fire tube boilers are used on locomotive engines and smaller static boilers. Water tube boilers are used for higher pressures and larger installations such as powerstations.


As well as the fuel which in the past was coal, but nowadays in most everyday industrial applications is oil or gas, air has to be drawn into the furnace for combustion. Nowadays air is forced with fans, but in times past other methods had to be found. At a factory for example the products of combustion would be sucked out of the boiler by the column of hot flue gases rising in the chimney. This pulled fresh air into the fire as it did so. On a railway or traction engine where you can't have a tall chimney another method is used. When steam has been used in the cylinders it still has some pressure left. This is blown up the chimney by a special nozzle called a blast pipe. As it shoots out of the blast pipe it draws flue gases with it thus drawing air into the fire. This is what makes the chuff-chuff sound of a locomotive. Because of the way, when working under heavy load, the exhaust steam is still very fierce, a huge blast will not only draw air into the fire but also suck large cinders from the fire and throw them out of the chimney - nice for photographers but not for the fire brigade.

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