Darkroom Ventialtion -- Ideas for pulling fumes off trays

[Bruce Watson]

I want to push fresh air in on one side of the DR and pull fumes off the trays on the other.

My HVAC days are long behind me, but I thought I should drop a hint anyway. If you'll quit thinking in terms of "pushing" air you'll be a lot closer to understanding air flow. The thing about air is that it's a compressible fluid (that's fluid, not liquid, before someone asks). If you try to push air into a space with an insufficient outlet, it just compresses. IOW, the pressure rises. With conventional fans it's not unusual to see air flow stop completely (all it takes is a poorly designed elbow or one too many elbows in a pipe to create an "insufficient outlet"), with a slight pressure differential across the fan. This can be dangerous if you think you are moving air but in fact are not.

Think instead of "pulling" air.

This leads you to a natural, if not obvious, conclusion. In this case, it's about fan location. It shouldn't be hanging on your darkroom wall and trying to blow air into a pipe. It should be located on the exterior wall of the structure in question, pulling air through a (rigid) pipe that leads to your darkroom, and blowing air into the natural plenum (our sufficient outlet) of the outside atmosphere. In addition to improving the fan's air moving efficiency this has the added benefit of getting a noisy fan away from your workspace which makes it much more pleasant to work there.

I'm just sayin' that pushing air is about like pushing a rope. Something to think about maybe.

[Brian Ellis]

I put a large fan on a shelf opposite the door to the darkroom (darkroom was about 9 feet long and 6 feet wide, i.e. not very big). I aimed the fan over the sink. I kept the door open all the time with the fan going, except when it had to be dark, i.e. for the roughly 3-4 minutes of time it takes to put the paper in the easel, expose, and run the paper through the developer. Once it hits the stop bath the door comes open (no need to have it dark with paper in the stop or the fix, development stops when the paper hits the stop bath). Since I used the BTZS tubes for film I didn't need to worry about keeping the room dark for film, only for paper, so I just left the door open all the time when processing film except for the minute or two it took to load the tubes.

[Ulrich Drolshagen]

I use a professional venting system in mine. I have shown it already here:
http://www.apug.org/forums/forum43/1...tml#post323539
There is a short discussion of the details in the subsequent posts.

The solution is very quiet in normal use, not comparable to kitchen or bathroom vents.
It's very efficient at full power while still more quiet than a kitchen vent at low speed. The only draw back is, that it sucks heated air aout of the house. But the extra cost of a heat recovery was not justifiable. The system is usually running at low speed. Only when I am using toner I run it at high speed. So I don't bother.

Ulrich

[Rob Vinnedge]

My system actually does push air, contrary to Bruce's well considered idea. I have a powerful roof mounted blower that blows into my darkroom through two HEPA filtered ceiling vents at opposite ends of my 9"X20" space. I have a 12" dia. duct pipe positioned vertically over my stop/fix/chem. mix area in the middle of my 19" sink on the opposite wall from the two ceiling vents. This gives me fresh air at all times, a positive pressure in the room, and extremely efficient venting of fumes and odors. The air moves very quickly out the duct pipe. The positive pressure eliminates the dust problem and the remote roof fan eliminates noise. I also have the fan on a rheostat for greater control over air flow.

[CG]

Best to have a slight, net, push into the darkroom. I write net since, as Bruce Watson notes, air doesn't push as well as it pulls and thus it's a good idea to have a fan pulling air out of the darkroom. But, it is also a good idea to have a slight positive pressure to the darkroom so the one is not constantly pulling dust in under the door etc. So, ideal is a moderate fan pulling out to guarantee a good flow, and a stronger fan pushing filtered air in.

[Mark Carstens]

More great ideas! Sweet...Thanks!

I will have fans at both ends of the system, each wall mounted outside the darkroom. The darkroom interior is about 11.5' x 8.5' with a 10.5' ceiling. Big enough for an 8x10 enlarger, but just barely.

Is there a consensus as to what I height I need to position the intake air vent? I was initially planning on slightly above head height (I'm 6'1"). My sink is about 40" high and sits to the right of the enlarger.

THNX
~Mark

[Rob Vinnedge]

My two intake vents are positioned in each of the 13' high ceiling corners on the long wall (20') opposite the sink wall, which has the 12" diameter duct positioned two feet above the middle of the 19' sink. The fresh air sweeps down and across the room to the center of the sink leaving no dead space in the room. It gathers all odors, fumes, and dust and shoots it out the only place available - the 12" duct.

[Mark Carstens]

Thanks again, everyone!

~Mark

[Toyon]

This leads you to a natural, if not obvious, conclusion. In this case, it's about fan location. It shouldn't be hanging on your darkroom wall and trying to blow air into a pipe. It should be located on the exterior wall of the structure in question, pulling air through a (rigid) pipe that leads to your darkroom, and blowing air into the natural plenum (our sufficient outlet) of the outside atmosphere. In addition to improving the fan's air moving efficiency this has the added benefit of getting a noisy fan away from your workspace which makes it much more pleasant to work there.

I'm just sayin' that pushing air is about like pushing a rope. Something to think about maybe.[/QUOTE]

Thanks for the information Bruce. Based on your suggestion, I repositioned my fan to eliminate outlet bends and now it works much better.

[William McEwen]

Does anyone here wear a surgical mask while printing? Don't laugh -- I like my lungs and I'm not indestructable any more. I've been thinking about doing this.