7 replies to this topic

[yclee99]

""How do you pump water into a boiler using only steam pressure from that boiler? ""

I got this from following website
http://www.electroni...-let-me-be.html

I just know the basic principle of how ejector works. But never think of the this (question above).
Can someone explain to me how it works (outlet pressure is higher than inlet pressure)?

[djack77494]

I may be mistaken, but I do not think it is possible to achieve a pressure higher than the motive fluid's pressure. For "pumping" a liquid, the device should be called an eductor. Using steam from a boiler, the steam's velocity is increased per Bernoulli's Law at the expense of static pressure. The energy contained in the high velocity steam can be used to "pump" the water, but I don't believe you could ever get above the motive fluid's initial pressure.

[yclee99]

QUOTE (djack77494 @ Jun 16 2009, 10:04 AM)

I may be mistaken, but I do not think it is possible to achieve a pressure higher than the motive fluid's pressure. For "pumping" a liquid, the device should be called an eductor. Using steam from a boiler, the steam's velocity is increased per Bernoulli's Law at the expense of static pressure. The energy contained in the high velocity steam can be used to "pump" the water, but I don't believe you could ever get above the motive fluid's initial pressure.


Thanks for correcting me.

I think we cannot get above the motive fluid's initial pressure just with a eductor. But, how about if they connect in series (using same motive fluid)?

[latexman]

But, how about if they connect in series (using same motive fluid)?

The answer is still no. You can approach the motive fluid's pressure if you use an infinite amount of steam, but it cannot be exceeded. Consider the simple momentum balance that assumes the liquid water has very little velocity:

m₁v₁ = (m₁ + m₂)v₂

[katmar]

The old steam locomotives did most definitely use their own steam to pump water into the boiler using an injector - i.e. you can have a delivery pressure higher than the motive pressure. It is all a question of the ratio of the area of the steam nozzle to the area of the combining nozzle (throat).

In the process industry we are usually more interested in using ejectors to create vacuum, and in this application the steam nozzle is generally small compared with the main throat. Compare that with the nozzles in the diagram at <a href="http://www.steamengi...tor/index.html" target="_blank">http://www.steamengi.../index.html</a>

The injector cannot create energy. But energy is the product of pressure and volumeric flowrate. You will see that the injectors had an intermediate overflow where excess water was removed at low pressure and low volumetric flow (after condensation). I suspect (have not done the calcs myself) that this is important in getting the high outlet pressure with low volumetric flow. This would allow the conservation of momentum equation presented by latexman to be satisfied.

[latexman]

Excellent post katmar!

[grimster85]

I have not had that much experience with actual injectors/ejectors but my understanding is that the momentum balance isn't really applicable in this situation. This is because there are significant reaction force on the body of the ejector (ie recoil) but as it is fixed in place you don't notice the acceleration (though the structural guys will want to know about it!).

Liike Katamar has said, I also believe that the injectors on steam engines work on the principal that the water is subcooled and as it mixes with the motive steam there is a reduction in volume due to cooling/condensation of the motive fluid. Thus the system behaves much like a compressor would.

Just my 2 cents

[djack77494]

Excellent post katmar!

I agree. Interesting too. Live and learn.