Below: A Newcomen haystack boiler, and Robert Stephenson's early locomotive,
Today, steam boilers for the new steam engines. The
University of Houston's College of Engineering presents this series about the
machines that make our civilization run, and the people whose ingenuity created them.
Richard Hills interrupts his history of the steam engine, to
give us a chapter titled Good Servants but Bad Masters. He's finished
describing the early engines. Now he pauses to say something that cuts to the
bone of inventive development: Steam engines were fine servants but, as their
creators concentrated on the engine, they neglected the boiler. A steam engine
needs a steam boiler -- and a simple teapot will not do.
Engine builders before James Watt had worked with both high and low pressure steam.
For high pressure, Thomas Savery tried to build small boilers of soldered copper.
The solder softened and he had a lot of trouble. Thomas Newcomen built low pressure
engines, so his boilers could be much larger -- typically a five-foot diameter riveted
lead dome resting on a cylinder of riveted wrought iron. They called his boilers
haystacks because of their shape.
But both types of boilers were just big teapots -- pools of water heated from below.
Watt's engines went only to modest pressures. He started out using haystack boilers,
then switched over to something called a wagon boiler. It looked like a long
covered wagon. But it was still just a big teapot.
All the while, people struggled with materials -- soldered coper, wrought iron, cast
iron. Not until the mid-19th century would boilers be made of Bessemer steel. But
engine builders somehow kept dodging the larger question: How to expose water to heat
When they needed more steam they just built bigger teapots. In Watt's time, one maker
built a spherical boiler twenty feet in diameter. It's no surprise that something that
large blew up. It flew 150 feet through the air and many people died.
Not 'til the early 1800s did engineers start seeing more than a simple teapot when they
thought about boilers. Steam locomotives came into their own, then steamships. They
needed small engines, and that meant high pressure. Engineers finally had to start
thinking about how to manage a great deal of heat flow in boiler that was small,
very strong -- and still safe.
A few 18th-century visionaries had talked about carrying flame through a bath of water
in a pipe -- or carrying water through the flame in a pipe. But Watt kept using big
teapots. He had to have twelve square feet of heating surface for every horsepower.
It took the stimulus of the railroad to change that. Today, only a small fraction
of a square foot produces one horsepower.
It's an old story: a big old out-of-sight tank had little curb appeal. All the fun
drama lay with the engine. The abstract problem of manipulating heat flow in a boiler
seemed distant from power production. You see the same thing today. We focus on the
car, and forget its exhaust pipe, or the gas pump. It sometimes takes a long time
for a tail to catch up with a dog.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
R. L. Hills, Power from Steam: A History of the Stationary Steam Engine.
(Cambridge: Cambridge Univ. Press, 1989).
For a discussion of heat flow as it occurs in boilers, see J. H. Lienhard IV and
J. H. Lienhard V, A Heat Transfer Textbook, Ch. 3. This is available free of charge
A few codicils to this episode: Newcomen started out using soldered copper, then he switched
to wrought iron. Watt first used haystack boilers then switched to the wagon type.
On the matter of square feet per horsepower, modern power plants require only around
0.02 square feet per horsepower. That looks like it's 600 times better than Watt's boilers.
But Watt's engines were much less efficient. They typically required 20 or 25 times as
much steam. That means modern heat exchanger design makes our boilers around 25 times as
effective as Watt's.