Originally Posted by
William Whitaker
David,
There's not really any correct or incorrect way to do it as long as the air is exchanged in a manner and timeliness that suits the demands made on the room.
Personally, I have my doubts about the ability of a 400CFM-rated Doran unit to adequately exchange the air in a 1200 cubic foot space. That would suggest an exchange rate of once every three minutes, assuming an 8' ceiling and if everything is working correctly. That may be enough, but for me, I'd rather have a little extra performance built in.
For the system I installed 20 years ago I built the ventilation around a surplus furnace fan that I mounted outside of the darkroom space. It pushed filtered air into that darkroom space which exited on the other side of the sink. I worked to keep flow paths as straight and smooth as possible to try to keep flow laminar and avoid turbulence as much as possible. The air entered the room behind a cabinet. Part of the laminar flow idea was to provide a clean workspace for loading film holders. Air crossed the room and over the sink to some ductwork which then directed it to the exhaust built into an outside window. The ductwork toward the window was painted flat black inside and a couple of 90º bends all but insured that no daylight could get in. Finally an exhaust vent with a spring-loaded flap kept dust, dirt, small animals, etc. outside where they belonged.
That system was great. It ran like it had a Hemi in it (which it almost did). I knew when I bought it that the fan would too powerful for the application. And sure enough it was. So I exchanged the driven pulley for one of slightly larger diameter to reduce the fan speed somewhat. It helped, but the fan was still a serious contender for a wind tunnel (which was what I did not want!). Upon turning on the fan, doors would slam shut due to the sudden air flow. And that certainly seems like overkill. But I must add that the exhaust was [necessarily] on the side of the house that was upwind to the prevailing winds in the area and they could be very strong.
Once the doors to the darkroom were shut (and myself lashed to the ship's wheel) the airflow settled into a well-behaved system. There was a constant exchange of air which felt refreshing on most days. Fumes and dust never seemed a problem.
I don't think a mass-produced, factory-built fan unit would have done what I needed. And my space was similar in size to yours. Of course, I did have the prevailing winds to deal with. But I was very satisfied with that installation and would do the same thing again if I had the opportunity.
BTW, the fan was mounted in a coat closet outside of the darkroom (formerly a bedroom) and bolted to the concrete slab floor. The closet door was replaced with a full-size board into which was mounted a 16x20" filter frame. The filter installed therein is what filtered all the air going into the darkroom. After the fan, the air entered the darkroom through a 12x12" duct cut in the wall, thence distributed into the darkroom. That helped, I'm sure, to keep noise to a very tolerable level.
It was a fun project and it worked very well. I still have recollections of going into the darkroom for an evening's work, Turning on the system and being able to feel right away the change in the atmosphere as the cool evening air pervaded my workspace.
I'm sure not being married helped.
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