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.
