A series of lessons by David C
Dec 2010
Steam Engines
Part 10
The little engine
Now perhaps I’ve given you the impression that Newcomen’s steam
pumps created a revolution in the way things were done in England
in the 1700s.
If that’s the case, I’d better make
it very clear that the steam pump
was very limited in what it could
do, and was adopted only very
slowly
by the businessmen of the time.
It costs a lot of money to build a
steam pump, and if you’ve been
successfully running a business that
draws power from a water wheel for
all of your working life, why should
you want to change to steam?
It was generally the new startup
companies that bought Newcomen’s
pump, and these generally were
textile factories that made easy use
of the pumps’ up-and-down
thumping action.
Newcomen’s pumps were more like hammers than
engines. They’re good for hitting things, or pulling water
out of the ground, but they’re not very good for turning
things.
There were some attempts to get the
rising and falling piston to operate a
crank that would turn a wheel, but
these were always unsatisfactory
because of the pump’s jerky action.
Joseph Cugnot in France no doubt experienced this
problem when trying to drive his steam tractor in 1769.
The tractor would have lurched forwards staccato fashion
with each pulse from the two pistons; turning the vehicle
would have required disengaging one of the pistons
while the other continued to thump away on the other
side of the vehicle.
No wonder he crashed the tractor into a wall.
The strictly up-and-down motion of the
pump
was a major limitation to what it could
do.
Compare that with water wheels,
which had been in operation for
centuries. Whole industries had evolved
around the smooth, circular motion that
these wheels created.
So who really needed a steam pump,
other than the mine owners for whom
the machine had been invented?
Well, as you’ve seen in the last
lesson, there were a number of
people who saw possibilities for the
steam pump if only it could be made
a good deal smaller than it presently
was.
It would have been good for example
to have a small steam pump in a boat
to operate a set of paddles.
Still others saw the possibility of
driving a carriage across the ground,
as long as the pump was small
enough to sit on the wheels.
But Newcomen’s steam pumps were big for a reason.
First, they had to be strong enough to lift water out of the ground.
But just as important was their efficiency.
Large piston chambers don’t lose heat
to the outside world as quickly
as small chambers do.
Let’s suppose you had a small steam pump,
consisting of a small boiler and a small cylinder and a small
piston.
Let’s suppose the cylinder is only 10cm in diameter.
That means the steam in the cylinder can never be
more than 5cm from the wall of the cylinder.
Now steam is a pretty good insulator,
but it still means that about 20 percent of the steam
is within a centimeter of the cylinder wall
and can be affected by the outside temperature.
Compare that with a cylinder that is a metre in diameter.
Now only 2 percent of the steam is in contact with the cylinder
wall,
and that means only 2 percent of the steam is going to be
affected.
That’s just the beginning. Now consider what happens when
you inject a spray of cold water into the cylinder.
Of course, you want to cool the steam down so that it will
condense, but you don’t want to cool the cylinder wall down. If
the cylinder is a metre across, you have 98 percent of the
steam condensing without touching the cylinder wall.
But if the cylinder is just 10cm across, only 80 percent of the
steam can do that. The rest is either trickling down the
cylinder wall or picking up heat from the cylinder wall and not
condensing.
But if the cylinder is just 10cm across, only 80 percent of the
steam can do that. The rest is either trickling down the
cylinder wall or picking up heat from the cylinder wall and not
condensing.
Whichever is the case, you have a
machine that is going to start off well
but within a few beats of the piston is
going to reduce to a pathetic little
wobble that has no power in it at all.
Consequently there was a drive to
make steam pumps ever bigger, not
smaller, and this was becoming
increasingly easy to do with each
passing decade as iron-casting and
iron-cutting techniques improved.
To make steam pumps smaller didn’t
make an awful lot of economic sense.
This fact would have been well known
to the university students in Glasgow in the
1760s.
By this stage, 50 years after
the invention of the steam pump
it was possible to go to university
and learn how to build one.
The University of Glasgow had a
scale model of a Newcomen pump
that they would show to the
students, and anyone observing
this little thing in action got a very
clear lesson that little steam
engines simply didn’t work
because … well ... it didn’t work.
It was a nuisance to the lecturing
staff, however, because they wanted
this little model to demonstrate how a
steam engine ‘should’ work.
Rather than build a bigger model, they
asked the university’s lab technician if
he could tinker with it and make the
thing work a little better.
The lab technician was a young
fellow named James Watt, and by the
time he’d fixed that damned thing,
he had an idea that was going to
make him fabulously rich and make
him perhaps the best known
engineer who ever lived.
Young James’s solution was to fit a
second tank into the design; one kept
perpetually cold so that any steam
directed into it would condense very
quickly.
Naturally, this tank was to be called the
condenser.
Well, when you introduce steam to the
piston chamber, it of course pushes the
piston up as you’d want it to do.
But when you want to condense the
steam to bring the piston down again,
you no longer spray water into the
piston.
Now you open a valve that lets the
steam escape to the condenser, where
of course it condenses.
The advantage of this design is that the
piston chamber can be kept hot at all
times; you no longer lose heat by
spraying water into it.
If the piston is to be kept hot at all
times,
it makes sense now to put a blanket
around it so that no heat can
radiate off into the open air.
These and other minor
modifications
made the steam pump some 5
times more fuel-efficient than it had
ever been,
which inspired Watt very quickly to
visit a lawyer and slap a patent on
his modifications before anyone else
discovered them.
He couldn’t see it at the time
– nobody could –
but young James’s modifications made
all steam engines efficient, no matter
what size they were. That meant (in
theory) it was possible to build a steam
engine small enough to fit onto a set of
wheels and drive it down a road.
There would have to be a few more
modifications, however, before
‘locomotion’ was truly possible.
Interestingly, Mr Watt would be
responsible for all of those essential
modifications.
And most interesting of all, he
wouldn’t even know what those
modifications were leading to.
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