Influence of particle size on the capacity of a rotary drier

Posted in: , on 23. Sep. 2006 - 15:29

We are thinking of reducing the feed particle size to our rotary drier in order to increase its drying capacity.

Our current feed is 5-20 mm soft porous limestone (with the humidity all over inside the stone not only on the surface) with up to 4% humidity.

We are thinking of grinding the limestone to less then 3 mm with 50 less then 250 microns in order to make the drying faster in the dryer and increase the cpacity through the dryer.

We have no fears that the fines will be swept from the dryer as we have to remove them anyway. We can put a larger burner if needed.

Is there a good chance that it shall work?

Amit Segev


Amit Segev

Rotary Drier

Posted on 16. Oct. 2006 - 08:28

could you explain your inlet/outlet conditions (temperature, air flow, dimension of drier) to make short drying analyzes.

If you count on higher capacity by the same drier, you need to increase inlet temperature or airflow thru dryer. Higher air stream mean also higher separation of fine particle, what is also your target. Capacity will increase, we can predict level after drying analyzes. Do you need theory background?

About material I suggest make first TGA/DTA analyzes and drying curves (kinetic), to be shure about material properties, namelly it is not necessary that all water evaporate by 105 deg.C, a lot of porous material keep water by higher temperature (zeolite till 500 deg.C). Does have limestone special characteristic by drying?

best regards

Andrej Horvat/Slovenia

Re: Influence Of Particle Size On The Capacity Of A Rotary Drier

Posted on 16. Oct. 2006 - 02:56

Dear Andrej,

Thank you for your reply!

Size of our rotary drier:

1.4 m Diameter. 4.5 m length with lifter (5 m total length).

Co-Current drier type.

Max air flow rate 20,000 Nm3/hr.

Max hot side temp: 700 deg C.

Max cold side temp 200 deg C (Nomex bag house).

105 deg C is high enough to evaporate all the needed humidity.

What does that tell about the capacity with fine limestone feed that I plan?

Thank you

Amit Segev

Negev Industrial Minerals-Israel

Amit Segev


Posted on 11. Dec. 2006 - 10:43

Dear Amit,

taking in account, that direct fired rotary dryer with references for limestone (higher Tinl.) have thermal efficiens ca. 60%, one can calculate that capacity should be up to 100t/h of limestone

namelly heat available to do work ( delta Enthaly of air = Tin - T.out) is ca. 650 kJ/kg of air multiply 20,000 Nm3/h (air density let say 1.2 kg/m3 by ambient T) = 20,000 x 1,2 x 650kJ/kg = 15,600MJ/h, thats equvalent for 7t/h of evap.H2O!

By 60% efficieny thats still 4t/h of evap. H20 or 100t of limestone with 4% moisture content.

By Tinl. =400 degC and 5000Nm3/h (present conditions) one can calculate that capacity is 8x lower, thats 13t/h of limestone.

Exisiting thermall efficiency is nearby 40%, thats pretty lower than average in your industry wher is typical value between 60 and 65% (see Industrial Drying of Solids, chapter Rotary Dryers, author Edwin M. Polsak). Might you have lower Tinl. than 400 deg.C or heat losses are hugher than other).

So, you have potention to increase capacity, what's necessary is to check material behaviour. Let's try in Lab scale first with finer material and than in situ, there is not a big risk, only measurements have to be done as soon as posible. For ca. 15,000 US$ one can get online moisture analyzer with info in 1sec.

Best regards

Drying Fines

Posted on 26. Sep. 2006 - 09:59

It should work - you are increasing the surface area of the product which will help in the heat exchange. But to quantify if you will get 5% or 10% improvement is not so easy.

One way is to set up a rotisserie in a Laboratory oven and do some basic tests. A cruder method is to put a couple of samples in the over, and tumble the containers every ten minutes or so. Then when you have dried the sample for a reasonable time measure the remaining moisture content (that would involve weighing the samples at a set time, then leaving them to dry out completely and weighing again).

Have fun.

Influence Particle Size On The Capacity Of Rotary Drier

Posted on 29. Sep. 2006 - 09:39

4% moisture is not a really high moisture content for a drier. For 10 T /hr feed rate the quantity of water to be evaporated is about 400 kg per hour.

The retention time in drier is important for ensuring the product emerges bone dry.

You can increase retention time by changing design of lifters / latticework in the drier drum. Once the material attains a temp. of + 105 Deg C, all moisture including that in porous stone will evaporate.

You will save on size of dust collector unit and emission of dust by retaining present feed size.

Vinayak Sathe :

vinayak sathe 15, Rangavi Estate, Dabolim Airport 403801, Goa, India

Re: Influence Of Particle Size On The Capacity Of A Rotary Drier

Posted on 9. Oct. 2006 - 07:29

Dear Vinayak Sathe,

I'm trying to get rid of nearly the dust (-100 microns) so blowing the dust out of the kiln with the air stream is my goal.

Amit Segev

Negev Industrial Minerals-Israel

Amit Segev

Kiln Etc.

Posted on 9. Oct. 2006 - 07:40

Greetings and salutations Amit from the soon to be frozen "Eastern Wilderness 1140 feet above mean sea level".

The one thing I did not see in your posting was about prescreening before it enters the kiln, are you screening down to 30 mesh before it enters the kiln?, if not that is a lot of your problem as all the dust will do is act like a heat sink.