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how to find suction pressure in ejector

how to find suction pressure in ejector

how to find suction pressure in ejector

(OP)
hi guys have just joined the forum ,i have a problem -i need to find out suction pressure in an ejector with following data

load fluid flow rate   1931 kg/hr(air pressure 1.012 kg/sq cm abs)
motive fluid flow rate(steam 4.8 kg/sq cm abs)  1448kg/hr
discharge  pressure 1.24 kg/sq cm abs

can i apply equations for converging diverging nozzle for finding it out

RE: how to find suction pressure in ejector

The problem you're facing is that most ejectors are designed for supersonic power fluid flow.  As a gas approaches 0.6 Mach, the density begins changing rapidly, invalidating the "constant density" assumption in the Euler Equation (and by reference the Bernoulli Equation).  Any nozzle calcs based on Bernoulli will be significantly wrong.

I designed an ejector earlier this month.  I used an old FORTRAN program that I purchased from IHS.  I did the preliminary sizing with a VBA program I wrote using sub-sonic nozzle arithmetic (i.e., based on Bernoulli).  The FORTRAN result had little comparison to the output of my VBA program.  We'll be building this gadget in the next few weeks and then we'll see which program came closer.  Earlier this year I designed and built an ejector using the same programs--the FORTRAN predicted values that were within 5% of measured parameters and my VBA program was over 35% off.

The short answer is that I've always found it impossible to predict performance on an ejector that someone else designed, there are just too many "small" things that make a huge difference in performance.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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RE: how to find suction pressure in ejector

(OP)
hmm .. but we can apply equations on converging diverging nozzles for compressible gases

also, is there any way to find out how much air will produce same suction as steam

air pressure 7 kg/sqcm
steam        4.8kg/sq cm

i know it has to be roughly arnd 1.4 times but i need to be a bit more  accurate...and lastly how can i possibly find optimum temperature for motive fluid

RE: how to find suction pressure in ejector

Yes there is a way--the description of the equations runs to a couple of hundred pages of very esoteric prose.  Your 1.4 number is truly nonsense.  Depending on very small changes in various parameters your power fluid to suction fluid ratio can run from 0.9 to 2.5.  Compression rations can be anywhere from 1.2 to 10. etc.

David

RE: how to find suction pressure in ejector

(OP)
hi again

I can't find anything about multi nozzle ejectors...can a 3 nozzle ejector maintain a vacuum of 40 mm Hg with entrainment ratio 1.65.

 

RE: how to find suction pressure in ejector

gaurav348:

I suggest that the best thing you could do is to contact one of the experienced ejector manufacturers like say Croll-Reynolds at www.croll.com

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

RE: how to find suction pressure in ejector

Milton,
It's all you.  I haven't gotten through to him, maybe you'll have better luck.

David

RE: how to find suction pressure in ejector

zdas04: any chance you could post a reference to text you mentioned above?

I2I

RE: how to find suction pressure in ejector

Sure, go to www.IHS.com and search for "ESDU" (or maybe its EDSU, I never can remember, but it is some combination of those four letters with an E at the beginning and a U at the end) and "ejector".  The thermo compressor section of this HUGE library is the source code for a couple of pretty capable programs and hundreds of pages of documentation.  They want you to run it there through CITRIX (with their front-end to save you the pain of a text editor), but your $800 only buys a one-year access to the site.  I found a free FORTRAN compiler and compiled the source (legal, but very cumbersome).

Good luck with it.

David

RE: how to find suction pressure in ejector

There is so much empirical knowledge required to design a good ejector that I would have to agree with Milton on going to an established vendor.

However, short-cut methods have been published which allow you to make estimates - even if they are not good enough to manufacture from. A graphical method is included in Perry (Chemical Engineers Handbook). I only have access to a 1963 4th Ed where I am at the moment and in that version it is on pages 6-30 to 6-32. That method does include a way to predict how much air you would need as motive fluid in the place of steam.  A critical factor that you have not mentioned is the ratio of the areas of the motive nozzle and the venturi throat. You will have to measure this to be able to make the prediction you need.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

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