Cement Silo Aeration

Posted in: , on 25. Jul. 2007 - 00:26

Hello All,

In addition to my other thread, I have also been working on a silo aeration project for a 50 ft wide x 50 ft tall flat bottom cement silo. The goal of the project is to improve material flow out of the silo eliminating hang ups and to reduce keetching costs.

We have an aeration pad layout designed for the silo bottom, which utilizes 700 sq ft of fabric. My research tells me that the recommended air requirement is between 5-8 SCFM @ 10 psi per square foot of fabric. Which taking the largest (8 SCFM) would produce an air requirement of 5600 SCFM @ 10 psi.

To me, that seemed like a large air requirement, and that was confirmed by my blower supplier who spec'd an Aerzen GM 240S, which would be the largest blower we have on site.

Are my calculations based on reasonable recommendations, or have I gone a stray somewhere? What is the recommended practice for aeration pad design?


Evan Peters

Re: Cement Silo Aeration

Posted on 25. Jul. 2007 - 07:03

Dear Evan, We have dealt with various jobs related modifications of flat bottom silos in India and in Russia.The number of such projects are in the tune of 30.

Now, the best way to do this is--

1.Install a cone (MS make) at the bottom of silo with special cuts on the generator line of cone.

2. Provide flat open air slides all around the inside periphery of silo @ 8-10 deg.slope up to the outer edge of the MS made cone.

3. Silo discharge point shall be in the centre of silo.

4. Required pessure to be given to these open airslides rom a suitably designed blowers.

5. At the outlet of silo a specially built/designed outlet box to be provide,now depends upon the nos. of outlets /extrcation points suitable equipment like flow control gates and cut of gates shall be provided.

I wuold like to inform you that undersigned has worked with IBAU,HAMBURG'S collaborators in India and also have been to Hamburg for special training on this subject.

We are in a position to provide you the followings--

1. Complete layout of the modification.

2. System description.

3. List of equipment with tech. specifications.

4.Fabrication dwgs.


Director-Libran Engineering& Services,India



Re: Cement Silo Aeration

Posted on 25. Jul. 2007 - 10:48


I belive your air volume calculations are reasonable although you may need slightly higher scfm/ft2 but the 10 psig pressure is also reasonable. Remember, at 10 psig, you wil be "aerating" the material at the bottom of the silo but you will not be "fluidizing" the complete head of material.

Where you have gone slightly astray is assuming ALL the air slides are aerated at the same time for a very large total air volume.

Typically. the air slides will be piped into "zones" and only two or three "zones" are operated at any given time. The aeration to the "zones" is cycled on a timed basis around the silo bottom.

By doing this, the air volume being used is much lower than the calculation of total volume and the aeration factor.

Hope this helps.

If I saw the layout pattern you have on the silo bottom, I can recommend the arrangement of the "zones". You can contact me via the address below.


Re: Layout

Posted on 25. Jul. 2007 - 10:59

Thank you for the informative replies. I think we have the air requirement dialed in, so we can price the required blower. As a related question, with the flat-bottom silo, there has been some debate whether to slope the floor, slope the aeration pads and backfill with gravel, or just leave the aeration pads and the floor flat. Any thoughts? I've attached the layout that we drew up for the "flat bottom" option.

Again, thank you.


stockhouse_#3_airpad_layout model (PDF)

Re: Cement Silo Aeration

Posted on 26. Jul. 2007 - 02:06

Gentleman, I have gone through the layout and the followings are my readings--

1. Location of open air slides mark-2,4,6,8 is not correct,arranging airslides in this way will choke the outlet.

2. There are no cross airslides.

3. There is no internal small cone above all the outlets.

4. If, you will reduce nos. of outlets from 6 to 2,then the airslide disribution and extration can be very easy,nos. of outlets you can increase from 2 to 6when you are out from the silo by providing small aerated collection box with 3 outlets each.

I will suggest you not to play with baby if ,you have never done earlier.Think, if, you have 5,000 t or more cement in the silo and can not take out?.



Re: Cement Silo Aeration

Posted on 26. Jul. 2007 - 09:42

I have never seen cement choking in aerated state in a flat bottom silos! No matter how poorly it is designed.

There might be some dead regions in the silo but choking is not an issue. If sufficient air is use localised fluidization occurs and powder flows like liquid so for cement there is a very little chance of dead regions.


Re: Cement Silo Aeration

Posted on 26. Jul. 2007 - 04:55

Dear Mantoo

I have always looked forward to your postings and held them in high regard and agreed with a lot of your views.

However, on this one I have to say that I partially disagree.

Your comments about aeration and flow of cement in the aerated state are correct -- Anil is also correct in that there can be expected to be a sizeable amount of material left in the silo due to the dead zones between the airslides on the floor.

Hog backs (inverted v-shaped sections) can be installed in those areas to mitigate the residual material.


Re: Cement Silo Aeration

Posted on 26. Jul. 2007 - 05:43

Thank you Jack for your nice words, I think I did say dead regions can occur in between the air slides.

I would personally recommend 6 degree slope in the base. However my comment about dead region was for this situation as in this case the fabric / base area ratio is very generous and in my experience dead regions should not occur with the airflow stated above. But then you can disagree with my opinion.


Re: Cement Silo Aeration

Posted on 27. Jul. 2007 - 01:18

Hi, Dear Jack & Mantoo, Cement flows like liquid when it is fluidised but if, the fluidisation is not done correctly, means,arrangement of fabrick/open airslide is not done correctly inside the silo, then it can create Havoc.Flat bottom silo design is little bit tricky that is why IBAU has invented inverted cone silo,In this you can empty the silo 100% but you losses the capacity but one good thing in this is that the load of the entire silo can be distributed evenly comparing to flat bottom silo.I have got the design for 25,000 t multicompartment silo(4 nos.) in which you can store various types of cement I can share the same with the Hon. Members of this forum.

Regards to both the Gentlemen.


Re: Cement Silo Aeration

Posted on 28. Jul. 2007 - 10:14

OK some loose comments

The maximum aeration area to total floor area I have seen is 60%.

Flat silos will always have dead zones - even with hog backs.

IBAU type silos are not 100% emptying - due to spaces between pads etc.

Suggest bringing each and every aeration supply pipe to outside ie do not manifold inside > allows more flexibilioty and also able to isolate is airslide is holed.

Flowabilioty depends upon cement condtion inside eg false set etc.

Need to seriously consider the silo wall loads - there have been a few silo "failures" in the last 10 years. With some types - the rate of dicharge can be very very high and the wall loads almost unknown....esp with periphery type discharges.

Need to think about manhole access for servcability and also how to get a man out if he twists an ankle walking over airslides.

Have U consider the silo mole/live screw type system - first in/last out and almost 100% reclaim on flat silo - I have never seen one and have heard of issues if it fails when buried under cement.