by Mike Brown
Electric power generation with steam
at the individual household level is making a comeback. At the
commercial power plant level, it never left. Even nuclear power plants
run on steam.
What is new is the fairly recent
phenomenon of household-size steam power units for standby power
generation. Unfortunately, most people today have no idea how a steam
engine works or the things you have to keep in mind when setting up a
home steam power system.
The easiest way to deal with a
technology unfamiliar to you is to introduce one concept at a time.
Let’s introduce the basic concept or outline and then go back and flesh
out the details.
A home steam system consists of a
boiler with a furnace to turn water to steam, a steam engine to convert
the steam energy to rotary motion to drive a generator, and a system to
re-circulate the water once the steam has turned back into water. During
the re-circulation of the water utilizing the exhaust steam heat
(providing hot water and/or heating the home itself) increases the
efficiency of the system. The design of a home steam system is dictated
by the circumstances of the home where it will be installed and the fuel
Now here is what you have to keep in
mind while designing your system.
The design of your furnace is based
upon the type fuel you are going to use. Fuel can be solid, liquid, or a
gas (vapor). It should be fairly obvious that a furnace built to burn
logs and twigs is not going to work well with used motor oil or natural
gas, or vice versa.
Boilers come in sizes and shapes as
varied as the colors of the rainbow. However, there are only two basic
The firetube boiler is what you see on
the old farm tractors and locomotives. A firetube boiler basically
consists of a tank full of water with hollow tubes running through it.
The hollow tubes allow more heating surface, in order to turn the water
to steam more rapidly and efficiently.
A firetube boiler will normally not
withstand steam pressure in excess of 250 psi. This is one of the
reasons so many of these devices went into orbit during the last century
and the early days of this one. Our metals are much stronger now.
Once in awhile you will still hear of
a firetube boiler exploding, even when built with modern materials.
Today’s explosions can almost always be traced back to lack of
Even this potential danger can be
largely eliminated by proper construction. Skip Goebel of Sensible Steam
in Branson, Missouri, builds his boilers so that, in the unlikely even
that one of his boilers "goes," the inside of the boiler, the
tubes, give way first. The result is that the water goes down
and puts out the fire in the furnace.
Late in the nineteenth century some
unknown genius came up with the idea of putting the water in the tubes
instead of a water tank. The fire in the furnace then turned the water
in the tubes into steam. Thus was born the watertube boiler. The
watertube boiler had advantages.
The first advantage was that steam in
a tube is much more easily contained than steam in a box or a drum.
Steam pressures in a tube can reach up to 5,000 psi before anything
The second advantage is that water in
a tube turns to steam much more rapidly than it does in a drum. It may
take 20-30 minutes to "get up steam" in a firetube boiler. A watertube
boiler will give you steam in 1-3 minutes.
The third advantage is that a
watertube boiler is cheaper and easier to build. The simplest of the
watertube boilers is called a monotube boiler, which in essence is
nothing more than a coiled copper tube (like a moonshine coil) with
water in it and a fire underneath it.
The fourth advantage to a watertube
boiler is that they are really hard to explode. Normally, all a
watertube boiler will do is spring a leak.
There are a couple of disadvantages to
First, a watertube boiler will not
allow for the fluctuations in pressure that a firetube boiler will. A
monotube requires a fairly constant load.
Second, if a watertube boiler springs
a leak and lets steam escape in an enclosed space, you could have a
problem. If you breathe in 300º to 400º steam, your lungs could
collapse. This is one reason you do not put a boiler inside your
A steam engine is known as an external
combustion engine. That is, the power or energy is produced outside
of the engine. That is, the steam has power before it is
introduced into the engine.
An automobile engine, in contrast,
produces power or energy inside the engine by inhaling a fuel-air
mixture and then igniting it with a spark.
A steam engine is also quite often
lubricated externally. A device called a hydrostatic oiler is placed
between the boiler and the steam engine. Steam picks up the oil and
carries it into the engine.
The Steam Chest
The first part of the engine the steam
enters is called the "steam chest." The steam chest contains the valve
system. On smaller steam engines (10 horsepower and under) the usual
valving system consists of a block of metal that slides over ports (or
holes) cut into a portion of the interior of the steam chest. No springs
are necessary. This valve is called a "D-valve." The D-valve uncovers a
hole or passageway to allow steam to push against the piston head. At
the other end of the D-valve’s travel, the valve uncovers another
passageway that allows steam to push against the bottom of the
piston. The exhaust passageway is in the middle. Such an engine is known
as a "double-acting" steam engine. The piston is alternately pushed by
steam in both directions.
Engines of this type turn fairly
slowly. 600 rpm is not an unusual or "slow" turning speed. Don’t let the
speed mislead you. 600 rpm in a steam engine isn’t comparable to 600 rpm
in a gasoline engine. 600 rpm in a gas engine is an "idle speed" that
produces very little torque (or twisting force). A steam engine can
produce maximum torque at almost 0 rpm. If you have ever seen an old 10
to 16 horsepower steam tractor at a "tractor pull" pulling against our
modern 400+ horsepower gas engines, you will understand. The steam
tractor always wins.
The cylinder, piston, connecting rod
and crankshaft are not what you are used to in an automobile engine. The
connecting rod doesn’t move in a circular motion: it moves straight up
and down (or back and forth). The straight movement is changed to rotary
motion at the crosshead.
A slider moves back and forth in the
crosshead. A second connecting rod connects the first connecting
rod to the crankshaft. Crankshaft rotation drives whatever you want it
to drive—electric generator, water pump, grain grinder, or other device.
An eccentric mounted on the crankshaft
operates the D-valve. The eccentric and the D-valve are connected by a
valve rod. As the eccentric rotates the valve rod is moved back and
forth, so does the D-valve.
If you saw the movie "The Titanic" you
may recall the size of the connecting rods going up and down in the
engine room. That illustrates just how large a steam engine can be made
as compared to one providing standby power for a home which can weigh as
little as fifty pounds (not counting the furnace and boiler) and be
carried around on the front seat of a pickup truck. Notice that you’ll
never see gasoline engines as large as those powering an ocean liner.
As steam engines get larger, they
become more sophisticated (and complex). D-valves become spool (or
cylinder) valves, engines become faster by becoming uniflow (as opposed
to double-acting), engines become more efficient by becoming double or
triple expansion, and so on. Boilers become more efficient as pressures
and temperatures rise and size increases.
There is a trade-off. As steam engines
become more fuel-efficient and sophisticated, they also become more
expensive, more complicated, and harder to maintain. The key word here
An ocean-going freighter with a
triple-expansion engine is practical. The vessel must carry enough coal
to get it from point A to point B with the lowest possible fuel
consumption. Coal must be paid for and there are no "coal stations" in
the middle of the ocean.
A small steam engine used for home
power generation needs to be as simple as possible to facilitate ease of
operation and maintenance, and to keep manufacturing costs down. When
your fuel economy consists of throwing another log into the furnace once
every couple of hours, who cares what the fuel efficiency is? This is
especially so during times when you can’t buy gasoline or diesel fuel.