Eductor calculations 2

[CharlieH]

Anyone have experience with eductors?

I am a new engineer and am looking for pointers on calculating the flowrate needed to move a gas.

We have up to a 150# steam supply as the motive stream. The question is what pressure can we regulate this steam to in order to still maintain the flow.

We are passing the steam at some pressure (this is what is to be determined) through the eductor to induce the flow of a stack gas, which is at or near atmospheric pressure (may actually be a slight vacuum). The outlet of the eductor will have a stack gas/steam mixture that will then be returned to the stack. This flow will serve as a "fast loop" that every so often an analyzer will pull a "slip stream" from to test.

Any suggestions as to how to formulate a relation of motive stream to outlet pressure? The eductor used for gases is basically a converging/diverging nozzle from what I understand. The pressure of the motive stream is dropped, to increase its velocity. This in turn sucks the stack gas into the eductor. The two then exit the eductor at an intermediate pressure.

Also, any good sites that thoroughly explain steam usage in refineries? I am looking for something that starts with the basics of steam.

Thank you...

 

[codeeng]

Try Graham:

http://www.graham-mfg.com/

 

[zdas04]

Eductors, Ejectors, Jet Pumps, hose-end sprayers, and sand blasters are all in the family of equipment called "thermocompresors". They're all in the same family, but they're each a different beast.

An eductor is a device intended to move high liquid-cut fluids. The throat has a very short entry transition and a straight section followed by a divergent section.

An ejector is intended to move high gas-cut fluids and has a convergent section for an entry to the straight throat which is again followed by a divergent section.

Steam can be used as the motive fluid in either case.

The reason for the lecture is that if you a trying to pump liquid you don't want to use and ejector (velocities get pretty high). If you are trying to compress a gas then you don't want to use an eductor (the effeciencies are too low, I've done it but it requires a lot of power fluid).

It sounds like you're in the market for an "ejector". For scoping evaluation, you'll need something like twice the steam pressure as your exhaust pressure (i.e. if you're exhausting into 20 psia, you'll need something over 40 psia steam). For practical purposes, you'll want to look at about 2 compression ratios (i.e., for the 20 psia exhaust you'll only be able to pull the suction down to about 10 psia). Your steam demand (in mass flow rate terms) will be about twice your suction requirements (i.e., if you need to move 2,000 lbm/hr of suction gas you'll have to supply 4,000 lbm/hr of steam).

None of these "rules of 2" are exact, but before you waste a bunch of time chasing a marginal project it is worthwhile to apply them to see if you're in the ballpark. Steam is a very good motive fluid, so you'll be able to bump all of the ratios a bit in a good direction.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[katmar]

Perry's Chemical Engineers' Handbook has a nomograph that you can use to get a rough estimate. But I would not design and manufacture one from that basis - rather purchase a proprietary unit.

 

[GL1]

information required:

1. Motive fluid and pressure... given as 150 psig steam.
2. Suction fluid type and pressure + temperature. Say air at 14.5 psia?
3. Expected flow rate of suction gas, in #/hr.
4. Discharge pressure. Say 15.5 psia?

That will enable any vendor to specify the ejector. If you post the details, I can give a rough estimate of steam useage.

"The question is what pressure can we regulate this steam to in order to still maintain the flow. " is not a true question. The ejector will do what it needs with the high pressure steam, you just provide the minimum pressure at the inlet.

 

[eng2hot]

Look at the "HEAT EXCHANGE INSTITUTE" Standards for Steam Jet Vacuum systems.
It has a lot of information on designing steam jet vacuum systems.

 

[Christine2]

Hi, I'm doing a summer intern and I was assigned to work out the suction pressure of an eductor.

The motive media is ink which passes through a converging nozzle, more ink (with air mixed in it) is then sucked up through the suction line and passed to the discharge line. I need to work out what the suction pressure is.

Would anyone be able to help me out on this one please?

Thank you =)

 

[zdas04]

Christine2,
It is bad form to post the same question in multiple places, it is really bad form to post a new question at the end of a thread.

David

 

[Christine2]

I apologise for posting the question in multiple places. I'm pretty new to this website. I'm very sorry.

 

[rmw]

Thanks for your co-operative attitude. Your other thread is noted, and I will respond there if I come up with an answer I think will help you.

rmw