Fan setup for drying in small storage hall? 3

[drystuff]

Hello

I would like to dry sawdust while in storage waiting to be processed.
Time window for drying 12 to 36 hours.
Storage room: 20m x 20m x7m (L x W x H)
The sawdust is stored on 300m2 of the 400m2 available. It is placed on tent vented steel sheets at 0.5m above ground.
This gives a sawdust volume of 100m3 and an aproximate weight of 21000kg. Sawdust humidity 50%.
I would like to take it as low as possible in the available time window. Let's say 30% would be acceptable. 20% would be great.

What airflow do you guys think I need to provide from under the vented steel sheets in terms of volume, temperature and humidity?
I was thinking to use pipes to distribute the air under the steel sheet.
The air in will be matched with roof fans taking the air out.
I would like to have a rough idea of what size heated fans and roof fans I would need.
An approximation would be greatly appreciated, as well as any other suggestions.
Thank you

 

[trashcanman]

Go read Chap 22 of 1999 ASHRAE Handbook of Applications, page 22.8 (Hay drying, which is close enough).

 

[drystuff]

great, thank you for your advice
book is online here in case anyone needs it:

http://www.academia.edu/11207373/Ashrae_1999_HVAC_Applications_Handbook

 

[ScottyUK]

Be careful. In the UK there was a large explosion at a wood flour plant in Cheshire where wood dust or 'wood flour' found an ignition source. Four people were killed and the plant was destroyed.

Under the European ATEX Directive wood dust is classed as an explosive dust under certain conditions. The rules for hazardous area design are complex, and those for dusts are quite different to those for fluids because dusts can form layers where fluids don't. I don't work with combustible dusts so I can't offer you much advice on dust-specific aspects of hazardous area design. This document from Stahl gives an introduction to the subject.

edit: updated link

 

[trashcanman]

Yes, safety first. I would advise NO OPEN FLAMES. Use hydronic heating coils for your drying. I would shy away from electric coils too, in case of a short. See NFPA 61 also.

 

[miningman]

Yep there's strong similarities here to two separate wood dust explosions in saw mills in British Columbia about 4-5 years ago. Coal dust explosions in underground coal mines kill dozens if not hundreds every time they occur. The energy in coal or sawdust blasts make regular explosives look like sunday afternoon in the kindergarden. Sawdust has a lot more in common with coal than with hay. There have been a number of grain elevators totally destroyed by grain dust explosions over the years. Unless you could somehow eliminate oxygen from the drying area... flood with nitrogen perhaps....this would be very risky.

I have had some success using calcium carbonate to minimise the occurrences of sulphur blasts underground when the mineralogy is conducive to sulphide ore dusts detonating which would be another line of research if you go down this path.

 

[drystuff]

Ok guys, read a bit about the safety part.
Great advice, thank you.
Will have open windows and fans in the top of the room.
May not even heat the air used for venting from underneath.
And if heated, it will be done indirectly.

 

[waross]

I am more comfortable with imperial measure. 21,000 kg = 46,200 lbs.
50% water content = 23,100 lbs of water.
Heat to raise 23,100 lbs of water from 70 F to 212 F = 328,200 BTU
Heat to vapourize 23,100 lbs of water = 970 BTU per lb. = 22,407,000 BTU
Total heat required = 22,735,200
If you want to do this in 30% of 36 hours that is 10 hours.
You need a heat source capable of over 2 million BTU per hour before you start worrying about fans.
If you are still serious about drying this amount of sawdust you may consider placing the sawdust on heated trays and using a Nash Vacuum pump or other liquid ring pump to pull a vacuum on the sawdust.
Hint; Use the heated trays to also brace the storage container so as not to collapse it.

Re safety; The explosions of sawdust or grain dust occur when the dust is in a fine suspension in the air. It forms a Fuel-Air bomb.
Any fans are a bad idea unless the sawdust is contained in ductwork and protected from ignition sources..
To remove a significant portion of that much water will demand a lot of energy, heat, running a vacuum pump or a combination of heat and vacuum.

Bill
--------------------
"Why not the best?"
Jimmy Carter