I like these discussions about steam engines. I have always been a bit puzzled as to how injectors work. The boiler is under high pressure, but water must be added or it blows up. The injector must use the steam pressure to force the water into the boiler but how does it do it? - Nevin
Same way pistons work on hydralics.
Large area with low pressure can make the same amount of work as a small area with high pressure.
Feed the water to a tiny t-point intersection then release the steam across it. The movement of the steam create a low pressure area at the t-intersection causing the water to be sucked up.
As long as the pressure is greater than the boiler, then you are good to go. Sometimes heating the injector steam, or pumping it raises the pressure of steam to necessary levels.
If you are interested in the mechanics of it, you’ll want to look up “Superheaters” and “Supersaturation/Superheated (thermodynamics)”
First, an injector and a superheater are two different things, and are found at opposite ends of the water boiling process. The first two paragraphs in Don’s reply come close to describing a feedwater pump.
The closest analogy for the injector is a jet pump. A stream of steam is shot from a small nozzle into a narrowing passage in the pipe, called a venturi. This creates a suction. A jacket of water surrounds the steam jet and is drawn through the venturi with the steam, accelerating it to a speed where it will be forced past the check valve and into the boiler.
A superheater adds heat to the steam just before it is sent to the cylinders, but thats an explanation for another question.
Here is a link to a source providing a lot of detail on live steam injectors:
Well, it isn’t quite like pistons and cylinders, which don’t rely on compression of fluids, but displacement, but it does have to do with pressure. It’s Bernoulli’s principle. Increasing the cross-section area of a fluid stream increases its pressure and decreases its velocity; decreasing the area does the opposite.
There’s another thing which often gets left out of explanations, I think. To do work, you have to put at least as much energy into something as you get out in work, and actually more, because some energy is always wasted. So how do you take a mass of steam from the boiler, add a mass of water, and then force a larger combined mass of steam and water at a higher pressure? The extra energy comes from the heat energy lost by the steam in expanding through the nozzle, which is converted to kinetic energy, of the combined water and steam stream.
http://www.kesr-operating.org.uk/injector.htm
Injectors do work with saturated steam, too.
Right you are. Thanks Tom. I had a brain “laspe” Sorry. The rest I said was correct though (but poorly worded). Some injectors do have “pressure” boosters on them. I’ve seen injector pumps on a number of boilers. Narrowing nozzle or not, you have to have a higher static pressure inside the nozzle than the boiler pressure, or the whole thing doesn’t work. This is why a booster or pump is often needed.
BTW: The superheater lowers the relative humidity of the steam, so when it expands in the piston, it doesn’t condense back to water.
Very educational for me. I’m diesel, but glad I looked in. Broaden my knowledge.Thanx for the postings!!
there are two forms of injectors. Lifting and non lifting, sometimes called flooding injectors, both types are used on locomotives. Non lifting injectors are located usually below the cab in a location where the supply hose from the tank runs down to the injector. Lifting injectors are usually located above the bottom of the tank and must draw water from the tank to their location in order to work.
The principle in both types is the same. steam passes over the end of the supply pipe pulling the water through a nozzle and into the delivery pipe. The steam mixes with the water and by performing work loses some of its pressure converting its energy into velocity. The slug of water moves at great velocity down the delivery pipe and uses its mass to overcome the static pressure in the boiler and force its way through the check valve into the boiler.
tt:
Right, you trade speed for pressure, although the conversion from a fast, low-pressure stream to a slow, high-pressure stream actually takes place in the delivery cone of the injector…Bernoulli again.
I seem to recall, perhaps erroneously, that when the steam draws water into itself via the venturi, that it becomes cool, and at the same time causing a powerful vacuum in one part of the cone…which, wanting to correct this abhorrent condition, induces the following water column to rush in-wards under intense acceleration, and the rest is as described above. So, is it a trade-off between thermal and kinetic energy, or Nature wanting to correct an intense pressure imbalance?
-Crandell
well, actually its the other way around, you trade pressure for velocity
if anybody’s really interested the best book on injectors that i’ve ever come across is “Locomotive Injectors” by J W Harding, while its a bit technical it has lots of detailed drawings of several different but common types of injectors
Before asking the question, did you type “steam loco injector” into Google? Google is your friend.
Tom’s got it right. An injector on a steam locomotive is very much a “pull yourself up by your bootstraps” operation, and does indeed seem counter-intuitive, until you understand the Venturi principle. You can check it out on the net - try Googling “Venturi”.
If you’re lucky, however, you won’t ever have to try to get a cranky “lifting” type of injector to “pick up” and put water in your boiler when all 3 trycocks and the water glass are dry… Trust me - a scary job!