Penberthy 3/8" Injector

[johnp]

Just got my tailpipes and union nuts from Harold Stark, and put it all together tried with the air compressor and more water comes out of the bypass than the main. Is it because I need back pressure from the boiler to keep the flapper closed? Wht the heck is going on with this thing. It looks very simple.

[preaton]

Needs steam not air to work properly.

[preaton]

Needs steam not air to work properly.

[preaton]

Needs steam not air to work properly.

[preaton]

Sorry for the stutter.

[Lopez Mike]

Ah, grasshopper, there is a world of difference between air and steam. Injectors depend on the energy and state changes that happen when steam accelerates through a nozzle.

For me, I just accept that it is magic. Sort of like Steve McQueen in The Sand Pebbles explaining that the water (dead steam) is turned into live steam by the 'devils in the boiler'. One book I read used an usual word in explaining it. The author said that the steam 'chased' the water back into the boiler.

When You get running on steam, as long as the feed water isn't too hot or the lift too high, it will work.

[Lopez Mike]

One of the rules that has stood me well over the years is that if I can explain something in a sentence or two to a person with little background, then I understand it myself. In this case, I have failed. So I guess I don't understand it as well as I thought.

When all else fails (and probably before all else fails!) try the web and Wikipedia:

http://en.wikipedia.org/wiki/Injector

Enjoy,

[Mike Rometer]

For sure, with injectors in general, if you get any air in the water feed, or lack of steam flow, they wont work. Most I've seen will only work in certain pressure ranges. Too high and they wont pick up, too low and they just drop out. It should also be said that the bigger they are, the more reliably they seem to work.

[fredrosse]

The only way an injector can work is if there is absolutely no gas in the mixture issuing from the mixing nozzle. Air is a non-condensable gas, so if there is any of it in the stream, then the injector will not work. Steam is also a gas, but when it mixes with the colder feedwater, it can all be condensed into liquid water, so there is no gas left in the mixture, it is all liquid water.

If the injector suction water is too hot, then it cannot condense all of the steam entering the injector, and hence there is still some non-condensed gas in the mixture (steam), and the injector will not work This is why injectors cannot handle hot water suction.

The physics of an injector are interesting, using 100 PSI steam to produce 120 psi feed pump discharge with no moving parts, but they are also troublesome if conditions are not quite right.

A bilge eductor, which also uses a steam jet to produce pumping of cool water is similar, a jet of steam entrains cold water, and carries the mixture away at a pressure somewhat above suction pressure, but these devices have such low discharge pressure that they can often work ok with compressed air as the motive gas. On steam, full condensation is not necessary for adequate performance with an eductor.