Naked Engineering - Steam Engines

Dave -   Earlier this week, I met Meera at Cambridge's Museum of Technology to explore the engineering of an iconic bit of coal fired power, the steam engine...

Meera -   This week Dave and I are aboard a Foden steam wagon, chugging around the Museum of Technology here in Cambridge.  It is a scaled down model of a wagon and this site is the old sewage pumping station in Cambridge.  Now Dave, why have you brought me here?

Dave -   Well steam engines historically are one of the most important pieces of technology.  They took us from an economy which was entirely based on animals and stuff you'd grow off the ground to basically a modern world.  Even today they produce over 70% of the energy which we use in this country.

Meera -   Joining us this week aboard the wagon, or in fact driving it, is the main engine driver here at the Museum of Technology, David Gates.  Now David, how exactly are we moving along?  How does a steam engine work?

David -   Well you take your fuel source.  In this case it's coal which is burnt and heats the water to create steam. The steam then operates the pistons to create the motion to move the engine.

Dave -   Because water is going from a liquid to a gas it expands hugely at about 100 degrees Celsius.  It's roughly 1500 times greater at high temperatures, or even more, and that huge expansion can be used to drive a mechanism.  So from a physicist's point of view you're just causing water to boil, getting a large amount of expansion, then you somehow connect that to the wheels.  The actual engineering practicalities of that are a lot more complicated though.

Meera -   Yeah.  I imagine there are many steps involved, David.

David -   There are indeed, yes.  In fact, we have a model here which shows those various steps.

Meera -   So we've come inside to the model now and in fact, this represents a Hedley steam engine which is located just behind us.  It's a cross section so we can see what's really going on inside.  There's a cylinder towards the back of the steam engine into which the steam enters.  There's also a tube coming out and a valve on top. So what's really happening here to control the movement of steam into this?

David -   Well your valve on top allows the steam to go into one end of the cylinder which will then push the piston inside until it gets to the end.  Then the valve on top would change direction, this will allow steam to go into the other end of the cylinder and push the piston in the opposite direction.

Dave -   So there's more pressure on the side of the piston where the steam is coming in than on the other side.  That means there's an overall force in the piston which pushes it along. Then the piston is attached by a series of linkages to the wheel which is actually doing the work.

Meera -   Okay.  Well we can actually turn this model on then to see the piston movement in action.

Dave -   The piston is then attached to a long rod which is then connected to a crank, allowing it to turn the wheel.  This wheel must be somehow attached back to the valve so that it can be opening and closing at the right time.

David -   Yes.  There is basically a cam, off-centre, which changes the direction of the valve.

Meera -   It's all a very smooth motion and the wheel is just continuously turning here.

David -   That's correct, yes.  You have to have rather a large fly wheel in a simple engine like this one just to create the motion required to keep up the momentum.

Dave -   I guess the other problem you've got is that you can get steam into the cylinder, but where does it go once it's in there?

David -   With this particular engine, when the steam comes out of the cylinder it just passes to atmosphere and that is a complete waste.  Later engines used the steam more than once.

Meera -   So we've now entered a very large room with two extremely impressive engines here. These are the main sewage pumping engines on the site and firstly, from the cylinder down to where the fly wheel is, it's about 11 metres in length.  This engine, and the fly wheel itself, is about 7-foot high.  It's very grand, David.

David -   These engines were built in 1894 by Hathorn Davey of Leeds. They have two cylinders on these.

Meera -   This is where the whole fact that steam is being used twice really comes in.

David -   Yes.  So when steam leaves the small high pressure cylinder there's a lot of useful energy in it, so it's used a second time in the low pressure cylinder.

Dave -   And so, because the steam has already expanded maybe by a factor of two or three in the small cylinder, it's going to be a lot bigger. It's going to take a lot more space and you need a bigger cylinder to let it expand again, a second time, and get some more useful work out of it.

Meera -   What's the benefit of letting it expand again?

Dave -   Well essentially, you've still got steam under quite at a pressure coming out of the first cylinder. You can't let it expand all the way to atmospheric pressure, the cylinder would be immense and it would be very expensive to build the engine.  So you can use a second cylinder to let it expand again and get more of the energy out of it.  This means you get more work for the same amount of fuel.

Meera -   So these two processes of expansion are all still basically working though to push the piston, David.

David -   That's right.  Yes, you have a piston in each cylinder but they're connected to the same rod which operates the pumps.

Meera -   And another way to get even more energy out of the steam is to use a condenser, which this engine also uses.

David -   That's right.  When the steam leaves the low pressure cylinder there's still a lot of useful energy there, so it's passed into a condenser where the steam is then turned into water to be used for feeding the boiler. It also creates a vacuum, so it's actually sucking the piston along as well as it being pushed. This makes it more efficient.

Dave -   So again, you're getting more pressure difference across the piston so you're getting more work out.  Again you get more energy for free essentially.

Meera -   And what about the amount of say, energy being consumed? The amount of coal being burned and so on?

David -   Well with the way we operate these engines at present, we will burn round about 3 quarters of a ton of coke per day.

Dave -   So that's equivalent to hundreds of litres of petrol you're burning, just to run an 80-horsepower engine for maybe 3 hours in the day.  So, the efficiencies are nothing like even a modern car.

David -   Steam engines, really one of the reasons they went out of use, was low efficiency.

Meera -   They really did change industry a great deal though.  We've seen it in motion for example in a wagon, it's used here for pumping in the sewage plant.  As they developed, they had more and more applications.

David -   Indeed, yes.  It really was the steam engine that turned the world around. With the spinning mills as well you could create virtually anything you wanted.

Dave -   And in fact, even today, most of the electricity you use at home is generated using steam engines. Not reciprocating engines like this but a steam turbine.  They basically work by blowing steam pass a load of vanes.  It's essentially very specifically designed windmills.  They spin incredibly fast and you get a huge power density from your steam engine, allowing coal power stations, nuclear power stations, oil power stations all to produce lots of steam and generate electricity from it.  You use this electricity every time you turn on a light switch.

David -   So essentially, although people think steam is old technology, it still powers the modern world that we live in.

Dave -   That was David Gates, the main engine driver at Cambridge Museum of Technology.