vacuum brakes (again)

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Tinsley on 25/06/2020 17:48:45:

An ejector is a simple case of a venturi effect , Simple applied Bernoulli theory. As Stephen says, there is little difference in performance (if any) between running an ejector on steam or compressed air.

…

Andrew.

Does Stephen say that? I see 'A vacuum ejector will work just fine on air, provided the design and manufacture of the ejector is okay.' It's a big 'provided' – are the design parameters for compressed air and steam really identical?

How about this quote:

While the ejector itself can be quite simple, specifying the optimum system to meet specific needs is not simple. Important parameters involved in ejector sizing and staging include pressure of motive gas, required discharge pressure, suction pressure and relative mass flow rates of motive fluid to suction fluid.

For instance, most ejectors use steam as the motive fluid. The quality of the motive steam affects the operation of the unit. The usual requirement is for dry, saturated high-pressure steam.

In operation, it is very important to maintain the design quality of steam. If the quality of the steam is low, suction pressure and capacity will decrease, especially in multistage designs.

Excessive steam superheat can also adversely affect the suction capacity of an ejector. It decreases the energy level ratio, and the increase in specific volume tends to choke the diffuser.

My point – if the quality of motive steam effects operation, wouldn't substituting compressed air be problematic too? But it is useful to know in practice ejectors can be tested on compressed air and at relatively low pressures.

If only it wasn't so hot I'd be delighted to read Andrew's simple explanation of ejectors using Venturi Effect and Bernoulli's Principle! As it is my brain can't cope with the Beano…

[Andrew TinsleyParticipant @andrewtinsley63637]

Dave,

Leaving aside the physics of ejectors. I have carried out tests running ejectors on both compressed air and steam and within the limits of experimental error, steam and compressed air produce the same ultimate vacuum for a given pressure

The steam used was not super heated and a maximum pressure of both steam and compressed air was 80psi. with a minimum pressure of 15psi .I used 3 designs of ejector and all showed the same results on steam and compressed air. Although the efficiency varied between designs (The Doug Brown design was the best).

If you wish to dispute the results then you are welcome to carry out your own tests. I would be interested to see the results.

Andrew.

[Paul KempParticipant @paulkemp46892]

Posted by Andrew Tinsley on 25/06/2020 20:04:24:

Dave,

Leaving aside the physics of ejectors. I have carried out tests running ejectors on both compressed air and steam and within the limits of experimental error, steam and compressed air produce the same ultimate vacuum for a given pressure

The steam used was not super heated and a maximum pressure of both steam and compressed air was 80psi. with a minimum pressure of 15psi .I used 3 designs of ejector and all showed the same results on steam and compressed air. Although the efficiency varied between designs (The Doug Brown design was the best).

If you wish to dispute the results then you are welcome to carry out your own tests. I would be interested to see the results.

Andrew.

Can I ask how you measured the efficiency? I need to make a water lifter (same principle) so interested in the theory. From my brief research on model water lifters the parameters for cone angle and position seem to vary quite widely between 'designs'. I want to move the most water for the least steam consumption as that should result in the minimum temperature rise of the delivered water?

Thanks,

Paul.

[stephen goodbodyParticipant @stephengoodbody77352]

Hi "SillyoldDuffer" (I love the name!) and Andrew,

An interesting dialog between you both!

While I don't want to get embroiled in a practice-vs-theory debate, I think it's worth noting that the gas used to drive a vacuum ejector does make a difference in theory. I believe that this is partly because the density of the drive gas affects the ejector's theoretical performance under ideal conditions.

As you both know, every gas has a different density at a given pressure and temperature and, in the case of steam and other phase mixtures, dryness fraction. If I remember my thermodynamics correctly (someone please correct me if not!) a denser gas will likely give better ejector performance under otherwise identical conditions.

However (and this is a big "however"!), while there is a theoretical difference in performance for an ejector when operated on fully dry steam, versus wet steam, versus air, I personally don't see any measurable differences in practice between air and steam because the effects are swamped by all the other variables, factors, tolerances, errors, inefficiencies and measurement inaccuracies that come into play in my models.

In other words, I think you're both right!

Best regards,

Steve

 

Edited By stephen goodbody on 25/06/2020 21:18:55

[David Bothwell 1Participant @davidbothwell1]

Got the ejector "sucking" but my valve is not "airtight" although it has been lapped, thought of a gasket or outer shroud on moving part

[Howard LewisParticipant @howardlewis46836]

If you are looking for Mr Brown's book, the author is D A G BROWN, (Derek not Douglas) Otherwise you may have problems finding it.

The book is

"Miniature Injectors, Inside and Out". £ 14.95 + postage

It is a good book, Derek knows his stuff is a very experienced practical.engineer.

Howard

Edited By Howard Lewis on 26/06/2020 16:39:31

[David Bothwell 1Participant @davidbothwell1]

Thanks for the info Howard, I have seen the book at two wildly varying prices, My ejector is working, my problem is the valve, but I am going to build/design my own (hopefully)

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