A "steam injector" is a device used to force water into a boiler that contains pressurized steam.
I will offer an explanation: If you imagine a tea kettle or pot of water on a hot stove, the water will come to a boil and form steam. As this boiling and forming of steam occurs, the level of the water in the pot or kettle goes down. If you forgot about the pot or kettle, it would eventually overheat and possibly turn red hot and be damaged or destroyed.
Obviously, if you were paying attention to the pot or kettle, you would add more water before the water in the pot or kettle was totally boiled off.
Sharp Stewart was a builder of locomotives, I believe. The boiler of a steam locomotive can be likened to an overgrown tea kettle. As the locomotive moves itself along the rails, it does so by letting steam out of the boiler and into the cylinders. As this steam is used in the cylinders, it exhausts up the smoke stack and into the atmosphere. Meanwhile, there is still a hot fire in the boiler firebox. As the locomotive keeps running, the water level in the boiler drops since steam is being released from the boiler to the engine cylinders.
One of several things has to happen:
-no water is added to the boiler. The boiler runs dry, overheats, and explodes.
-water is added to the boiler, and the boiler does not overheat.
-no water is added to the boiler, but the fireman knew his business and had a light fire which he was able to smother with a bank of fresh coal.
In the most likely scenario, water is added to boiler. However, the boiler contains steam under pressure, so some means of forcing the water into the boiler and overcoming the steam pressure is needed. On a steam locomotive, space is at a premium, so a steam driven pump is not necessarily a good choice. A steam driven pump will be able to force water into the boiler, overcoming the pressure. But, the downside is the pump is going to force cold water into the boiler. The introduction of cold water into a boiler containing pressurized hot steam is not healthy for the boiler and also tends to "kill" the head of steam needed to keep the locomotive pulling a train.
This is where the injector comes into the picture. The steam injector is a very ingenious and compact device. It takes steam from the boiler at the pressure contained in the boiler, and uses that steam to draw in cold water, accelerate the cold water to a high enough velocity that it has enough kinetic energy to overcome the pressure within the boiler. In the process, the steam mixes with the cold water, heating it. The result is pre-heated water is fed into the boiler.
The first injectors were invented in France by Giffard in perhaps the 1850's ( ? I am no scholar or historian). The worth of the injector was immediately apparent. It provided a simple means of feeding water into locomotive (or steamboat or other boilers) as well as preheating the water. A host of inventors and firms through the 19th and into the first third of the 20th century worked at improving and manufacturing steam injectors.
The main use of steam injectors was on steam locomotives. A typical locomotive had two injectors, so there was a backup in case on injector failed to operate. A hot boiler with a head of steam and no way to add water had a high probability of resulting in at least some damage to the boiler due to overheating. In the extreme cases, it was a "low water explosion" that could tear the boiler apart and kill the engine crew.
With a coal fired locomotive boiler, it was not like having an oil or gas fired boiler. There was no instantaneous "fuel shutoff" that killed the firing and stopped adding heat. A coal fire, even if the fireman stopped shovelling coal, had a hot bed of coal/coke and was not about to extinguish itself anytime soon. Injectors could be finicky and tempermental and "fail to pick up" for a variety of reasons. It was soon realized that as wonderous an invention as the injector was, it was not 100% dependable, so two injectors (or an injector and a separate feed pump) were always used.
Injectors were used not only on steam locomotives but on steam traction engines, steam road rollers, steam cranes and steam shovels, on steam sawmill boilers, on steamboat or smaller steam ship boilers, on boilers in mills and factories. Once steam locomotives (along with steam road rollers, traction engines, and smaller boilers in sawmills and similar) were phased out, the use of injectors became almost nil.
The design of an injector is a tricky bit of science. The injector uses a specially shaped nozzle to accelerate the steam into the "throat" of the injector. In the throat, the high velocity steam creates a partial vacuum which draws in the water from a tank or other source. This water mixes with and condenses the steam. The steam is moving with a very high velocity, so accelerates the water while condensing into it and heating it. The acclerated and heated water enters the "forcing cone" and decelerates and proceeds through the feed pipe to the boiler. Now it gets more interesting: the steam was at whatever pressure the boiler happened to be operating at, perhaps a tad less due to frictional losses in the piping and valves between the boiler & the injector. The water was at atmospheric pressure. In the injector, the steam is able to combine with the cool water and raise the resulting heated water to a pressure slightly greater than the boiler pressure to force it's way back into the boiler. No pumps, no moving parts. If a person were in tune with what the injector accomplishes, they might say it was a sort of "thermodynamic magic".
I've fired boilers on locomotives, marine boilers, and on a steam crane and in steam sawmills. I've relied on injectors to feed water into the boilers, counting on them to function as designed. I've cussed at injectors that "failed to pick up", cooled them with wet rags, and been thankful for the backup of a second injector or steam feed pump. I've coaxed injectors to "pick up", trying to "sneak up" on them by letting cool makeup water run out their overflows to cool them and throttling the steam slowly, to get them to pick up. Through it all, when I've heard the sound of the steam in the injector nozzle and the "clunk" and dull roar telling me the injector has "picked up" , I've been relieved and at the same time marvelled at the whole design and principals behind the injector. When I hear an injector start to work, I put an eye on the boiler level glass, and think of how squeaked through thermodynamics and transport processes in engineering school. I wonder what the velocity of the steam must be coming out of the injector nozzle, and think of how I sweated through compressible fluid flow, a course in engineering school. I am probably the only person who thinks in those terms, encompassing a combination of cussing the injectors, their ancestry, while marvelling at the injector at all levels.
Your grandfather had to have been a very gifted engineer in my book to have designed steam injectors that were patentable and that worked. At the time he was inventing injectors, so were a horde of other people. This was pre-internet and pre modern high speed communications, so he was often on his own to research and create. However, he might well also find out he had reinvented the wheel. That his patents were defensible and stood on their own says a lot. My hat is off to your grandfather- from an oldtime engineer in today's world.
Joe Michaels
