# How to calculate the required airflow to dry a batch of sawdust?

Sawdust has to be dried while in storage waiting to be processed.

The time window for the drying is 12 to 36 hours.

The storage room is 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 has to be provided from under the vented steel sheets in terms of volume, temperature and humidity?

I was thinking to use tubes 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 so I know with what equipment to start.

• What heat source will you use? Any agitation for your saw dust bed? Design review re. dust explosion hazard?
– mart
Jan 30, 2017 at 11:01
• Your limiting factor will be the sawdust getting trapped in the vent screens from the flow of air but as much air flow as possible, updraft will allow fluidised bed operation. Temperature needs to be kept below 451 degF to prevent flash point and lower to minimise polymerisation but otherwise as hot as you can afford. The dryer the air the better, fresh ambient at a minimum (no recycle of air, just exit heat recovery) but previously dehumidified air would be best. Feb 2, 2017 at 8:59
• Your questions is rather open ended but you are trying to remove a lot of water. 21000*.5 - 21000*.2 = 6300kg of water. Here is a paper that may offer clues on how much energy you will need in the end. - scholarworks.uark.edu/cgi/… Feb 2, 2017 at 9:08

## 2 Answers

You may come up with various equations that will suggest various more or less (usually more) inaccurate value because the number of obscure factors, like sawdust grain shape, its layout, porosity, ambient air humidity and so on, will make any calculation such a distant approximation it will be useless.

Instead, take a 20x20cm sheet of the same kind that's to be used, with air vent that can be partially closed, sprinkle 21 gram of the same wet sawdust that's to be dried on 3/4 of its surface, set vent to half diameter, blast air or 2 hours, check if sawdust is dry. If not, open vent more. If yes, close it more. Repeat as needed - in 3-4 iterations you'll have a throughput that gets the work done in desired time.

Then measure the air flow (even as simple as known volume plastic bag, check time needed to inflate) and multiply by 10,000. The result may be ~10-20% away from optimal, but certainly better than a simplistic equation would yield.

The Volume of the entire room per hour of airflow at 105 degrees will take 16 hours at 25% humidity.
If you want it done in 8 hours youll have to double the volume of air to pass through the room in 1 hour, this will start to look like a vacuum.

• This is not an answer to the question, which was how to do the calculation. You just gave a numerical answer. What formula(s) did you use, and what assumptions did you make? Jan 30, 2017 at 13:38
• There are 2 questions, the first in the title and then one at the end. The second "what airflow has to be provided" was answered and the OP wants to be able to "have a rough idea" to start choosing equipment. May 1, 2017 at 8:31