Re: How Do I Scale Air Velocity With Changes In Air Density?
Mark,
I have only ever looked at this theoretically, I have no direct experience of operating at these pressures we decided not to proceed with the project but I came to the conclusion that the effect would probably not be that significant.
Fluidisation theory is the closest analogy if you want to look up some references but I came to the conclusion that:
1) The density effect would be based on "differential density". If your particle has a true density of say 1500 kg/cum and your gas went from 1.2 to 240kg/cum, the effect would be similar to conveying with a soid of true density of approx 1260 kg/cum in normal air. A change
2) The other effect is likely to be changes in viscosity, viscosity of a gas tends to be fairly insensitive to to changes in pressure. I haven't seen figures up to 200 bar but from memory the difference was less than 10% at 35 to 40 bar.
Others may have some practical experience to support or knock holes in the above (?) but I remember being surprised at the time when I came to the conclusion that density changes would have much less effect than I initially thought it would. ■
Density Effect On Conveying Velocity
A very approximate answer can be obtained by comparing pneumatic and hydraulic conveying. For the pneumatic conveying of fine granular coal a minimum conveying air velocity of about 15 m/s is required. For the hydraulic conveying of the same coal the water velocity required will be about 1.5 m/s. The density of water is about 800 times that of air. So you have a 10:1 reduction in velocity for an 800:1 density change. At 200 bar the density of air is still only about one quarter that of water.
While I was at Glasgow Caledonian University we conveyed a number of materials in a large test facilty that could be operated at up to 25 bar. The pressure of the receiving vessel could also be varied up to 25 bar. Minimum velocities were slightly lower than those given by the above model. I assume that the pipeline pressure drop is not significant.
For your material the minimum conveying air velocity for dilute phase at atmospheric presusre will be about 12 m/s. ■
Re: How Do I Scale Air Velocity With Changes In Air Density?
Dear Mr mgallimore
As I understand your case, it is about conveying fine particles in dilute phase, in a pipe system, where approx 200 bar static AIR pressure is present.
The pressure drop caused by the pneumatic conveying can be :
a)A separate airpump, working at an intake pressure of approx 200 bar. This has to be an air pump WITHOUT internal compression.
b)The static pressure is reduced in the pipeline by the pressure drop of the pneumatic conveying. The airflow has then to be controlled as if it was a positive displacement pump.
The required air velocity at 200 bar can be estimated as follows :
Average air pipe velocity at atmospheric pressure = approx. 5 times v(suspension)
Suspension velocity
The suspension (floating) velocity of a particle, defined at normal standard air conditions
(T=0 deg C and p(amb)=1 bara is given as :
4 * d * rho(p)
vs = SQRT ----------------------
3 * cw * rho(air)
and
4 * d * rho(p)
cw = ----------------------
vs^2 * rho(air)
The air density at 200 bar(absolute) = 1,293 x 200/1 x (273+t)/273
vs200/vs1 = SQRT (1/200) = 1 / 14
vs200 = vs1 / 14
assumed is t1 = t200 and density fluctuation around 200 bar is neglected.
The material, you want to convey is not known to us nor the suspension velocity, but this should be approx. 1,5 m/sec
Thus:
v(air) at atmospheric conditions = approx 7,5 m/sec
(BUT, here the pressure drop in your installation. due to the pneumatic conveying varies significantly and thereby the air density and the velocity in a constant diameter pipe)
V(air) at 200 bar should (by theory) be approx. 5 x 1,5/14 = approx. 0,55 m/sec
(BUT, here the viscosity of the air is assumed equal to atmospheric)
A possible procedure to follow could be :
1)perform a conveying test at pressures close to atmospheric (6 – 10 bar static)
From these tests a product loss factor can be derived.
2)Calculate the pneumatic conveying at f.i. 30-50 bar static
3)Perform tests at 30 – 50 bar static
4)Check whether the calculations match the test results
5)Design the final installation.
The set-up of those tests will require different air volumes and/or pipe diameters and the set-up needs to be arranged in a very smart way.
Another approach is to try it fully theoretically all the way from the beginning and do the tests later.
If we can be of any help, provide us with some more details and let us know.
I do not know if these type of conveying installations are built often, but to me it seems a technological challenge.
Interesting ■
Teus
Thank You All For Your Replies
As a layperson on this subject you have all posted interesting and quite varied replies. It'll be a challenge to balance one arguement vs the other so if anyone else is able to contribute to the debate then I'd be very grateful. If any replies can be linked to any published work or sources of data that would really assist in picking the best route.
Many thanks again
Mark Gallimore ■
Re: How Do I Scale Air Velocity With Changes In Air Density?
If you all are interested in literature about the subject of pneumatic conveying under high pressure/density, I give the following info:
Doctor thesis by Ulrich Heucke (1998)
Universität Erlangen-Nuernberg, Germany
"Horizontale pneumatische Foerderung bei hohem Druck"
He has measured the influence up to 20bar. For the minimum velocity he suggests the following formula:
v min = K * (product Massflow * g^2/air density)^0,2
with K=5 for many products
regards
Reinhard Ernst ■
Re: How Do I Scale Air Velocity With Changes In Air Density?
Dear Mr Gallimore,
Another thought that crossed my mind about your case is :
Conveying installations under a static pressure close to atmospheric, have pressure ratios between 1,5 to 4.
If you operate a pneumatic conveying under a static pressure of approx 200 bar, then the resulting pressure drop would be :
1,5 x 200 = 300 bar to 4 x 200 = 800 bar
These pressures are difficult to handle, unless the pneumatic conveying system is designed at a pressure ratio of f.i. 1,1 resulting in 1,1 x 200 = 220 bar.
That leaves 20 bar for pneumatic conveying, which is under these circumstances, how strange it may sound, a low pressure system.
I am very interested in your findings.
Question to Mr Ernst : please can you explain the dimensions of the formula of Mr Heucke ? It seems that K has also a dimension.
best regards ■
Teus
How do I scale air velocity with changes in air density?
I need to predict the velocity to transport sub 40 microm mineral particles through a pneumatic conveying line (lean phase) there the line static pressure is very, very high - 200 bar.
I will run some tests in the system but only at pressures between 6 and 10 bar and so the line velocities in the test will be higher than those required at 200 bar.
I'd bevery grateful if someone might advise how gas velocity reduces with increasing air density and hopefully define a relationship between the two parameters.
It's a large extrapolation but any guesstimate is better than none at all.
Best regards,
Mark Gallimore
Mark.gallimore@validata-eng.co.uk ■