Expert: Dave Russell - 1/7/2004

Question
This is a question relating to the paint fumes questoion I asked a couple days ago.  He has a wood shop with saws, sanders, plainers, and other wood working equipment that creates a lot of saw dust.  Right now he has a blower that draws air with the sawdust through a series of ducts, through the blower itself and into a forage wagon located outside the building.  I want to install a gravity hopper to catch the saw dust with a dust seperator on top.  Then run the exaust from the dust seperator through a series of filters and finally venting inside the work area.  Right now, all the makeup air from the dust system enters through infiltration via the doors and windows.  The doors are garage type lifting doors that leak by design.  I feel he cannot afford a negitive pressure in the building during the winter time.  Is air filtration a good idea in this case?  Is there a calculation to deturmine the size of the air seperator per volume of air moved?  The priority is to maintain reasonable air quality and stable humidity.  The boards change size and warp if the humidity changes too much.
Thanks

George

Answer
It would appear that two things might be in order.  
1. Get a good book on ventillation and industrial design from ASHRAE and look up cyclones.  It will have cyclone design for removal of particulates.  The cyclone will remove most if not all the saw dust.  Follow that with a filter, maybe even a bag or hopper type which can be easily cleaned and emptied.
2.  Your idea for return air is fine, but you may have to put up with some odors from wood which has been scorched or burned by the finishing process.  If that's not objectionable, there is no problem with the idea.
3.  Humidity control is a bit tougher.  Winter air is notoriously dry because outside air when brought inside has a greater water capacity due to warming.  New York winters can cause the air inside the building to dehumidify everything, cracking wood.  You can overcome this by installing a couple of large scale humidifiers.  Use a psychometric chart to determine the quantity of moisture you will need to replace in the air after first figuring the air infiltrating the building (even a rough estimate is fine), and size the humidifiers accordingly.  Ifyou can get your hands on an Carrier air conditioning handbook, so much the better.  It will have most of what you need, including the psychometric charts.
4.  I believe that you will want to maintain about 50 percent relative humidity in the wood storage and working areas.  Just remember that while that's good for working, when the material leaves the shop, it will shrink and crack because of lower winter humidity in the winter environment.