7 replies to this topic

[nanohydrogen]

Dears,

I am going to find out the required pressure for the air to pass through a bed of stone in a cooling tower.

I have read the book: Handbook of pneumatic conveying engineering (by David Mills) and it said that it can be determined by permeability of the stones (15-20mm size). The equation is:
U = C (deltaP/L)
Here it said that:
U is superficial air velocity
C is the permeability
delta P is pressure drop
L is bed length

However, when i try to find out more information on permeability in the internet (actually can't find much information), I find someone just simply mention velocity rather then superficial velocity......Thus, I am frustrated on which velocity should I used in the calculation and experiment determination....

Also, I also find that most ppl calculate the pressure drop by Ergun equation (and is related to superficial velocity this time).

Which model is more suitable for my case indeed??

Thank you very much!!!....

[breizh]

Hi ,
Consider this resource :

http://web2.clarkson...ackfluidbed.pdf

Hope this helps

Breizh

[nanohydrogen]

Thank you.

I have read through that information.

And I will probably perform an experiment first to examine the Ergun equation.

However, I am not sure about the measurement of superficial velocity (I am not a physicist indeed....so never heard of this term before...).

In case if i only have a wind speed meter [expressed in ms-1] and a simple blower/fan [which can't control or measure the air flow rate m3s-1], how should i measure the superficial velocity? Is that I should first put the probe of wind speed meter into the pipe, near the outlet of blower/fan, without any stones inside the pipe first? After putting stones into the pipe, the air velocity measured at the inlet of the pipe is no longer the superficial velocity, am i right???

plx plx, thank you so much for you help!

[breizh]

HI ,

superficial Velocity = flow rate /cross section of the bed (1) , refer to the document attached

From the measurement of the velocity at the outlet and knowing the cross section of your pipe you will get the flowrate .

Should be fine !

Breizh

[nanohydrogen]

HI ,

superficial Velocity = flow rate /cross section of the bed (1) , refer to the document attached

From the measurement of the velocity at the outlet and knowing the cross section of your pipe you will get the flowrate .

Should be fine !

Breizh

From you reply, " the velocity at the outlet" is refer to the outlet of the blower in the absence of stone in the pipe. am i correct???

becoz i read from source that the superficial velocity is the flow velocity base on empty column....

thank you for your help

[breizh]

nanohydrogen,
The superficial velocity is related to the bed , it 's the flow of gas divided by the cross section of the bed : m3/s divided by m2 >>> m/s ( surpercifial velocity ) .
but
To get access to this superficial velocity you have to know the flow rate of gas passing through the bed , which is equal to the velocity of your gas multiply by the section of the pipe where you are doing the measurement ( equation of continuity with the assumption of density = cte).

In other words the cross section where you are doing the measurement of the velocity to determine the gas flowrate could be different from the cross section of the bed.

Is it better?

Breizh

[nanohydrogen]

Breizh,

Thank you for your reply.

Thus, if the cross-section area of the measurement i take is equal to the cross-section area of the bed (actually same diameter pipe), the velocity measured after the blower outlet & before the stone inlet is already the superficial velocity? Thank you!
(my set up is just have one pipe collecting to the outlet of the blower. I put stone into the middle part of the pipe. I have drilled holes between the stone and blower to measure velocity)

I still not so clear why some place state that the superficial velocity is: the velocity of the gas would have through an empty bed.

thank you!

nanohydrogen

[Root]

Breizh,

Thank you for your reply.

Thus, if the cross-section area of the measurement i take is equal to the cross-section area of the bed (actually same diameter pipe), the velocity measured after the blower outlet & before the stone inlet is already the superficial velocity? Thank you!
(my set up is just have one pipe collecting to the outlet of the blower. I put stone into the middle part of the pipe. I have drilled holes between the stone and blower to measure velocity)

I still not so clear why some place state that the superficial velocity is: the velocity of the gas would have through an empty bed.

thank you!

nanohydrogen

Nano,
For superficial velocity, you need cross-sectiona area of vessel not the volume of bed (here stone).
Toor