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 available.
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 types.
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 maintenance.
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 gives way.
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 watertube boilers.
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 home.
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 is "practicality."
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.
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This page was updated on 3 November 2011