Injectors

For ignoramuses like myself can you explain in "simple" terms

1) What does an injector do

2) How does it do it

3) What does it mean if you have water and steam coming out of the overflow.

1) it squirts hot water into a boiler

2) a jet of very fast moving steam condenses in water and the resulting very fast moving jet of 'water' is converted to a slower moving jet of sufficient pressure to overcome the boiler pressure.

3) it isnt working.

cheers,

julian

I'm glad someone was brave enough to ask that question

At last, an explanation without any mathematics, none of which helps anyway when the darned things just won't work.

Mike.

Injectors are designed to work at different pressure ranges and should be selected to suit your operating pressure. They often do not work when too hot. (turn the water on first, then the steam. Then steam off first and water off last) (unless it is a lifting type injector with only a steam valve). If not working they may need cleaning by dismantling and soaking the cones in vinegar to remove calcium deposits.

Apart from trawling back through ME over the years for Lawrie Laurence's and others' articles, "Miniature Injectors Inside & Out" by DAG Brown is a mine of information on the subject. He also covers fault-finding and the design of water valves. Even if you don't intend making your own, it's probably worth a punt.

Want a non-mathematical description of the physics?

Hot steam is squirted out of a nozzle, sucking up cold water. The water has to be cold enough it doesn't boil, but mixes with the steam to form a powerful jet of hot water, collected by another cone, that 'punches' its way into the boiler past a valve.

Neil

Ah, but how does the non-mathematics explain the following? We have two input streams; a mass of steam at boiler pressure, so it has stored energy due to the pressure, and a greater mass of water at atmospheric pressure, so no stored pressure energy. We end up with the combined mass of steam and water at, or more than, boiler pressure, so lots of stored pressure energy. Where does the extra energy come from?

Andrew

I know this is a trick question, but I deliberately didn't complicate my original answer with the high efficiency of injectors...

What extra energy?

If you turned the boiler heat off and ran the injector the boiler would cool down, and lose pressure, more quickly than with the injector off.

It's a very efficient process, but some heat will always be lost through the injector pipework and the injector itself.

If you tried to stop this by insulating the injector, it would get so hot that the incoming water would boil and the injector would stop working.

Neil

Presumably the "heat of condensation"?

I'm sorry I asked now. Never mind, I'll run through the literature and sort it out for myself.

Andrew

When it works, I just call it man made magic !

The cold water condenses the steam this causes a vacuum sucking in more water and starting the whole process off, this is why they don't work very well with hot water:- hotter the water the less steam it can condense the lower the vacuum, also this is why its important that none of your joints leak as it will just sucks in air.

I was lucky enough to have been tutored by a very knowledgeable and skill-full model engineer in their manufacture, with his help I made a batch of five that all worked I was chuffed to death, making your own injectors certainly give you bragging rights on the track.

Stew

hi Stew,

in the interests of accuracy may i be permitted to correct you on a few points?

it is not the case that 'the cold water condenses the steam this causes a vacuum'. when the steam leaves the end of the steam cone nozzle it is already below atmospheric pressure. the steam nozzle plus first half of the combining cone acts as a sort of ejector. if the proportions and details of these parts and the check valve are correctly designed and made, the injector will be of the lifting type. it will also then be 'automatic' and self-restarting.

it is not the case that 'the hotter the water the less steam it can condense the lower the vacuum'. hot water will be lifted as much as cold water. what does happen with hot water (above 46 degrees celcius) is that in the combining cone the steam's considerable latent heat causes the water to boil, and so the injector wont work.

cheers,

julian

Practically all modern aircraft use the same principle for fuel tank 'draining' (i.e. getting the dregs out) known as jet pumps. I used to explain the operation as being similar to egg blowing but that only worked with the 'wrinklies'. The young F/Os just looked blank, so, much dry marker expenditure and hand-wagging ensued. How do they work?? Yes, jolly good question, well formulated, nicely presented, thank you!!! The parallel is asking an Italian to describe Sophia Loren with his hands tied behind his back.

rgds

Bill

Never been convinced I understood exactly how these are connected up. I understand the use of venturis to draw water in with the steam and also for slowing the stream down again to allow the pressure to overcome the boiler pressure. Obviously the water (and overflow) connections must be at low (atmospheric) pressure otherwise the steam wouldn't come out in the first place.

My question is - is there a significant fraction of the steam lost to the overflow connection? I always imagined there must be but haven't ever heard it explained. When using the injector, is a fair bit of steam dumped up the stack?

Murray

No.