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Thread: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

  1. #11
    Tin Can's Avatar
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    I use water for film stop, mix my own citric acid for paper stop.

    I use only TF5 mixed with distilled but in separate containers. I don't mix the fix betwixt film and paper.

    Up to now I used Rodinal on all film. Ilford PQ for paper.

    Most LF film is processed in covered tanks with Gas Burst.

    I won't mix any powder inside the DR, I have a shed if that comes up.

    Too many dead friends from Industrial Disease at ye olde factory. Not kidding. At least one refused to use the readily available high power vent hoods.
    where is the monolith

  2. #12
    Thalmees's Avatar
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Quote Originally Posted by angusparker View Post
    ...
    I would say if you are drawing air from inside the house the intake fan is not necessary. What is key is the air path to come in above head height and to be drawn out from just over the trays far below nose height. You will most likely need a soffit and light tight louvers to get the air in across from the trays and uptake vent so you have positive air movement from top of back of your head to tray and out.
    ...
    Thanks so much angus.
    Great help.
    As for passive air inlet(or passive light tight louvres) instead of active Positive Pressure Fans, it could help in better exhaust flow, but I do not think this will help the purpose of the design.
    I adopted the Positive Air Pressure because I believe it's the best way to get rid of dust.
    Then, I chose Inhouse Air(instead of outside Air at window area), because Inhouse Air is clearly much less dust born, plus the fans are high in the door well above the ground level of the house. I do not know so far how can I get filters that could fit in Doran fan?
    As for the pressure gradient, I revisited my primary design, thanks.
    Please see the drawing below, thanks everybody for the comments.
    The intake fans will be at least 72" above the ground of the room, this is the maximum usable height for the door, but will use angled 6" tube(Doran specific size) to direct Positive Pressure Air up higher than the level of the door(up to 100" above room ground level, as needed, but I will consider not to add much of dead space[tube length] to conserve power of positive air flow).
    At the same time, I owered the level of the bigger Exhaust Fan to be 50" from ground level. Planning to make the sink low to approach it on a chair, which the case for all smaller prints.
    Appreciate your input angus.

    Intake door fans can be extended higher than 72"

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  3. #13
    Thalmees's Avatar
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Quote Originally Posted by Peter Collins View Post
    For the space as dimensioned, you have too much fan power (as CFM) and thus air movement. I hope that you contemplate a rheostat/dimmer instead of a simple on-off switch. Thus, if you have too much air flow, you can reduce fan rpm. Consider, as well, separate switches for each fan; for very little extra cost, you can have a huge range of adjustment. Also--You just want to keep your breathing zone in fresh air, not necessarily the whole darkroom, and not your whole body.
    You don't want so much air movement that you have turbulent flow in the darkroom. Something closer to laminar flow is ideal. If you have too much air movement through the fans, it will feel almost windy in the darkroom.
    Peter, thank you so much.
    Thought my self minimalist.
    Do you think the upper Exhaust Fan is needed?
    Thanks for suggesting individual switch for each fan plus rheostat/dimmer. Will consider that seriously.
    As for the volume of air flow, the Intake Fans can provide 800CFM without filters. The lower and bigger Exhaust Fan, can pass 800CFM after making the appropriate louvre. Not considering the upper Exhaust Fan for awhile only.
    But wait! The room volume is above 2800 CF. Which means around seven(7) equivalent fans to clear that volume in one(1) minute. Just divide 2800CFM by 400CFM, for one(1) minute.
    I only have two(2) in each side, which means it takes about 3.5 minutes to clear complete room Air in one cycle. Adding the upper Exhaust Fan, will decrease the time maybe to 2.3 minutes. Of course assuming overload on the intake fans or more sufficient air going into the room.
    Honestly, I do not know if that times are low(fast) for an efficient air flow in a darkroom?
    Please let me know if I have a mistake.
    Thanks so much Peter.

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  4. #14
    Thalmees's Avatar
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Quote Originally Posted by Jim Jones View Post
    I kept darkroom ventilation simple: a filtered louvered small fan forcing air into a small darkroom, and darkroom air leaking out wherever it could. This was satisfactory for many years. If the surrounding area had been living space, I would have sealed the leaks and let positive darkroom pressure vent to the house's outside.
    Thanks Jim for your input.
    That was the situation for me some time in the past.
    Succeeded in Exhaust but failed in creating positive pressure. That resulted in a lot of dust.
    Regards.

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  5. #15
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Quote Originally Posted by Christopher Barrett View Post
    ...
    Ideally, you want a positive air supply in the ceiling with exhaust venting near the tops of the trays.
    ...
    HTH,
    CB
    Thanks so much Christopher for your input and for sharing the photo of your DR and link.
    It's not only correct, it's the ideal.
    Originally, my room was finished constructed as a living room, and do not wish to do more destruction.
    In the next phase after installing every thing and trying what I'll settle on from this thread, will consider flexible ducts to bring the Positive Air to the opposite corner of the Exhaust fans.
    As for recommending against putting Exhaust Fan high in the ceiling, do you think that the upper Exhaust Fan is actually needed? Drawing of the wall that faces the enlarger, is in post# 12.
    Regards.

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  6. #16
    Thalmees's Avatar
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Outside the concern of this thread and as for my current darkroom.
    Do not know actually, who is sharing whom?
    Dust(a lot of it), lizards, cockroaches, snakes, mice , more than one species of ants and other incredible creatures, are continuously(some are on/off hopefully) sharing me(effectively, I'm sharing them) the darkroom, for the past several years.
    It's one(1) meter below the house ground level room, with 3 walls facing empty lands. The ceiling is another horrible story.
    My trick to make a positive air pressure, has failed few weeks after it started.
    Among all enemies and friends sharing the room, dust and static electricity are the biggest.
    With all limitations around the future DR, I wish really to reach the best possibilities for an efficient permanent DR, through this forum.
    Regards.

    The generosity of spirit in this forum is great, its warmly appreciated.
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  7. #17
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Quote Originally Posted by Randy Moe View Post
    ...
    Jim Jones suggests positive room pressure. I have read many that suggest that. Seems good.
    ...
    Hello Randy,
    Thanks so much for your post.
    Thanks for confirming that the best strategy to fight dust, is Positive Air Pressure.
    That's the reason why I did not consider passive louvres instead of active Positive Pressure Fans, in my primary design.
    Regards.

    The generosity of spirit in this forum is great, its warmly appreciated.
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  8. #18
    Drew Wiley
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    The secret is to pull air, not push it. If you get a big OUTDOOR wall-mounted Broan squirrel-cage from Grainger, it will isolate both noise and work far more efficiently than an indoor push fan. If you must do it indoors, an IN-LINE Panasonic squirrel-cage will cost you around $400 but also be quiet. Avoid rotary fans (noisy). I strongly recommend a high-quality unit which accepts a dedicated speed control (around another $40). Of course, you'll need light-tight air intake vents like Doran offers; forget their fans. Air should be pulled from behind across the sink toward a wall vent or suitable fume hood. Duct size and smoothness has to have matching efficiency, though configured to prevent light from entering via the exhaust fan. One good exhaust fan is better than a bunch of cheap ones. Air filters or purifiers won't do it by themselves, though they are nice for dust control in film-handling and enlarging areas.

  9. #19

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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Quote Originally Posted by Drew Wiley View Post
    The secret is to pull air, not push it.
    It's nearly the same, for each one liter of air you pull another one will enter, and for each liter you push one will exit. And the fan will make the same effort in one case than in the other one.

    If pushing the air inside then the flow entering is more turbulent (because the fan), this helps venting better all the room space. If pulling we vent more the area close to the outlet.

    If we push the air in... then the outlet vent (having no fan) will take the air around it in the same way that if we were pulling the air... but the higher turbulence will mix more all the air inside.

    Another difference is that if we push filtered air inside then we have an slight positive pressure inside, preventing some dirt intake from uncontrolled paths, this is the principle used in Cleanrooms: https://en.wikipedia.org/wiki/Cleanroom , this is good for darkrooms.

    In case of Biocontainment rooms it's the counter, negative pressure is ensured to prevent "dirt" from the inside escaping to the outside, so the air is always pulled and filtered (or Sterilized) before reaching the outside.


    So, if we have trays not exactly under the outlet then pushing looks better.

  10. #20
    Drew Wiley
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Here you go again, Pere. You're guessing. I actually have decades of experience distributing fans, including the entire line of Panasonic, and have specified all kinds of applications, including not only private darkrooms but commercial photo labs and all kinds of scientific research facilities. Within walking or bicycle distance of my former office and warehouse were some of the biggest pharmaceutical, biotech, and university research labs in the entire world. An everyday issue for me. My own big squirrel cage outside my sink room has been running quietly and flawlessly for forty years now. A good investment. You need to understand things like hydrostatic pressure related to humidity, air friction in the ducting etc. The only problem with my exterior fan is that every few years wild bees try to make a hive in it - so I have to leave it running a few days to discourage them ! In my house I use a Panasonic in-line duct fan instead, for the bathroom shower steam, which pulls the air all except for the last few feet to a roof exhaust cap. Really nice and super-quiet. I also have an actual clean room in my lab, separate from the sink & processing room. It has a big electrostatic air cleaner. If biohazard is potentially involved, what on earth are you developing in that darkroom anyway, film from the Andromeda Strain satellite ???

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