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

  1. #21
    Thalmees's Avatar
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Quote Originally Posted by Luis-F-S View Post
    You can invert a fan to blow into the room. I did that in one of my darkrooms. If you want to use 4 fans, I would put the on individual switches. I have rheostats on mine, also made by Doran and seldom run them full speed. I would certainly do this on the exhaust fans. On the intake fans, you can run both of them off one rheostat. I got mine at KHB in Canada and they've worked like a charm.
    On the exhaust fans, if you're not exhausting to the great out doors (lots of light) consider removing one of the set of louvers as it doubles the output.
    Just don't go crazy with it and don't loose any sleep over it. There is a thread in here on my fan setup. See:
    http://www.largeformatphotography.in...1ft-room/page2
    You can see photos of my three darkrooms on:
    http://www.largeformatphotography.in...arkroom/page51
    L
    Thank you so much Luis for your kind followup reply. Appreciate it.
    I encourage other kind posters to reply to my queries and thanks so much.
    Few important points has been confirmed essential. Individual switches, rheostats, highest possible level from the ground for the Intake Air and lowest level possible for the Exhaust Fan.
    But still few points also.
    1. Why I should not use Positive Pressure Fans "On" all the time?
    I know it has a limited life span, but I do not think it's short.
    Plus, how can I make sure that no dust(or minimal) will inter the DR?
    2. How many fans are enough?
    Operated one new Doran fan in my hands, just to see if it's working or not and to test the Air Flow Speed/Volume. I found it not that powerful! The fan is new, but the air flow speed was not that much!
    Knowing that my room is more than 2800CF, I concluded it's too weak to create the required Positive Pressure for the given room air volume.
    That's the reason why I used two(2) Positive Air Pressure fans in my primary design.
    Could not expect appropriately what's the effect of one fan on 2800CF after it installed in place?
    3. Is the upper Exhaust Fan necessary?
    As far as I know, if intake air is 800CFM, Exhaust should be a little more.
    Is that correct?
    Appreciate your input.

    The generosity of spirit in this forum is great, its warmly appreciated.
    ------------------------------

  2. #22

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

    Quote Originally Posted by Drew Wiley View Post
    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 ???
    Drew... tell me, what advantage has pulling air compared to pushing air ? don't tell me that's not exactly the same flow amount...

    Single difference is that if pushing you may have the turbulence generated by the fan inside the darkroom, while if pulling the that turbulence is generated in the outside.

    But when placing an HEPA filter in the intake (after the fan) that turbulence is reduced to a minimum. Well, you may know that all cleanrooms are pushing air, or not ?

  3. #23

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

    This is a simulation of what will happen with the air flow in the Thalmees' darkroom.

    Today it's quite straight to make an accurate flow simulation that will tell the trajectories of the air and the air speed in each spot of the room.

    This is usefull to see if nasty loops are formed or if critical areas are well cleared from fumes without contaminating the rest of the room.













    What I see in that design is that inlet flow has too much energy in the inlet flow and this ends in a not enough smooth flow over the trays.

    No problem with non toxic fumes, but for hazardous fumes the flow should go more direct from the trays to the outlet.

    So in that case it would be better to remove the fans in the inlet.

    Pushing air can be a suitable choice, but one have to be careful with the inlet speed.

  4. #24
    Thalmees's Avatar
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Hello Pere,
    Appreciate your input in the subject, thanks so much.
    Quote Originally Posted by Pere Casals View Post
    ...
    a basic HEPA air purifier solves the dust.
    ...
    That's great idea really.
    In my current dusty DR, I use two(2) small Honeywell plus one(1) bigger but slimmer Panasonic air purifiers. Each one of them has ionizer in addition to HEPA filter. My current DR is about 600CF, BTW!

    But, if your DR is not well air tight and surrounded by small but frequent whirlwinds, from three(3) sides, this trick will turn like a joke. Even if you controlled the few meters in front of DR door.
    In my next DR, I believe it will do a great job.

    Quote Originally Posted by Pere Casals View Post
    ...
    If we want to vent the darkroom I'd only place a fan (of the necessary power) with an HEPA filter introducing air in the room, ...
    The HEPA filter in the input fan should be covered with a regular foam (easy to clean) to remove the large particles, in this way the HEPA filter will last more.
    ...
    This is the best idea in your post Pere. Thanks.
    Please let me know more about HEPA for intake fans, from where can be provided?
    Then, does the HEPA filter reduce the Air Flow? I think so.
    It's already 800CFM for 2800CF room!
    Which means 3.5 minutes to clear all room volume in one cycle.
    I can imagine that HEPA filter could reduce air flow from 800 to maybe 200CFM.
    I do not know what's the best air exchange volume(flow rate) in a typical DR?
    But I feel it's more than to be covered by 800CFM fans, let alone by 200CFM.
    .
    Quote Originally Posted by Pere Casals View Post
    ...
    and the exhaust would be trought a light tight vent.
    ...
    Thanks for your suggestions.
    That is the passive(fan free) louvre.
    But, honestly, I could not answer my self:
    Why no fan in the exhaust area in the first place?
    Why just passive louvre is enough?
    .
    Quote Originally Posted by Pere Casals View Post
    ...
    I don't see the need to install a second fan in the exhaust outlet.
    ...
    Considering volumes and flow rates mentioned above, How did you reach to conclude that one(1) Exhaust Fan is enough?
    When I built my current DR, several years ago, the situation was just arbitrary. I chose the biggest possible fan for the Exhaust and made the biggest hood(light tight louvre) in Mankind history for that fan!
    In my next DR, I hope to do everything methodological as possible.
    .
    Appreciate your comments Pere.
    Regards.

    The generosity of spirit in this forum is great, its warmly appreciated.
    ------------------------------

  5. #25
    Thalmees's Avatar
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Quote Originally Posted by Pere Casals View Post
    This is a simulation of what will happen with the air flow in the Thalmees' darkroom.
    ...
    What a happy surprise after I upload my last previous reply.
    Thanks so much Pere.
    Please use: https://postimages.org , to upload the images again.
    It's free and provide bigger images.
    After upload, select the copy sign of: "Hotlink for forums", in the right side, then paste it like text, each code/image in a line.
    My apologies if you find inconveniences.
    Regards.

    The generosity of spirit in this forum is great, its warmly appreciated.
    ------------------------------

  6. #26
    Drew Wiley
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Pere - there's an entire trade devoted to this kind of work. Any of the major fan manufacturers will offer some sort of catalog or website primer in the ABC's. In most situations you need about twice the CFM rating you think you will in order to get the net amount after ducting friction and hydrostatic pressure losses. And in terms of installation fuss and maintenance/replacement cost, one serious fan is going to be a bargain compared to a bunch of so-so ones on toy rheostats. And until you understand certain basics, all those hypothetical charts you give are meaningless. A large squirrel cage on a solid state control will run efficiently at even very low speed - very helpful is one is doing ordinary developing versus, for example, an accidental spill of glacial acetic acid requiring a sudden dramatic increase of exhaust. A push propeller fan suitable for this would be at least twenty times as many decibels of noise as a pull or inline squirrel cage. Again, just go to a big HVAC catalog like Grainger to compare specs and prices (their pitiful website gives only a partial selection). Questions to
    ask in advance : 1) cubic footage of the darkroom space to be vented; 2) general climate (rain, humidity,etc); 3) how is the space heated or air conditioned; 4) how is your anticipated duct network going to be configured, duct type, diameter, and length ?

  7. #27

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

    Quote Originally Posted by Thalmees View Post
    Please let me know more about HEPA for intake fans, from where can be provided?
    I'd use an spare filter in a DIY casing, for example Amazon has ($23) an Surround Air MT-8400SF spare filter, larger than 10".

    In fact I do that in a drying cabinet for dry plates.

    Quote Originally Posted by Thalmees View Post
    Why no fan in the exhaust area in the first place?
    I'd only place a single fan, for input or for output. If placed in the input then IMHO the duct should have a large section to not generate too much turbulences, my guess is that a more laminar flow is better, also an HEPA filter will help with the turbulence.


    Quote Originally Posted by Thalmees View Post
    Considering volumes and flow rates mentioned above, How did you reach to conclude that one(1) Exhaust Fan is enough?
    In a kitchen you can generate a lot of smoke and that's solved with a single 250 CFM extractor, but the flow it's quite smooth and well designed and placed just abobe the smoke generation. Perhaps a kitchen extractor is an option...

    A too high flow would bring on problems with air heating/conditioning/filtering...
    Attached Thumbnails Attached Thumbnails 61SD30zBE7L._SL1500_.jpg  

  8. #28

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

    Quote Originally Posted by Drew Wiley View Post
    1) cubic footage of the darkroom space to be vented; 2) general climate (rain, humidity,etc); 3) how is the space heated or air conditioned; 4) how is your anticipated duct network going to be configured, duct type, diameter, and length ?
    Drew, those were the preliminary calculations...

    When we have potentially toxic emissions we may want to know if the flow in the room moves the emissions directly to the outlets or if the trajectories mix the emissions with all room air before making its path the exterior. In the former case we can completely avoid operator exposure to the chem with a lower flow, in the second case operator will have some exposure even in the case we use a high flow.

    We can use smoke from a bare cigar to see in practice if fumes lighter than air are making its path directly to the outlet, and we can use dry ice fumes to see is heavier than air fumes are also making it's path directly to the outlet.

    Click image for larger version. 

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    This is a way to know if our operator is exposed to toxicity hazards, there are more ways based in samplig the air to fullfill labour regulations...

    But when air samples have contamination (in case of labour regulations) then exploring the actual trajectories (with heavy and light smoke) and simulation software is the way to solve the problem.

  9. #29
    Drew Wiley
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    Re: Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

    Give it a bit of heavy rain or subtropical humidity, and an ordinary kitchen range hood would be quite weak, esp given the fact the duct might need a diverted path to keep light out. It's obviously better than nothing above a small sink, but nowhere near powerful enough to split into an extraction manifold or boost in case of an emergency spill. 250 CFM is marginal. At the 50% typical resistance loss I referred to, that's no more powerful than a cheap shower fan. With a 10 ft sink, I like around 750 CFM of pull with dedicated speed control. Or around 450 would be suitable for a smaller station. Again, pull, don't push air for maximum efficiency. Industrial rules for toxic emissions are a completely different subject. Real HEPA filtration is in an utterly different price league than that toy stuff referenced at Amazon, and you certainly don't need it in a typical home darkroom. It is nice to have in a clean room where film is loaded into either film holders or enlarger carriers.

  10. #30

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

    Quote Originally Posted by Drew Wiley View Post
    an ordinary kitchen range hood would be quite weak / esp given the fact the duct...
    Drew, a kitchen extractor is heavy duty gear, sometimes my wife roasts an entire bison under it... of course it clears an small surface...

    The duct can go directly to a light tight trap and exit...


    Quote Originally Posted by Drew Wiley View Post
    250 CFM is marginal. At the 50% typical resistance loss I referred to, that's no more powerful than a cheap shower fan. With a 10 ft sink, I like around 750 CFM of pull with dedicated speed control. Or around 450 would be suitable for a smaller station.
    I agree with those numbers, of course, but as I want filtered air I want to keep the flow at a minimum while breathing only fresh air, with those requirements a more refined design is needed, then it's necessary to understand how the flow travels inside the room, flow lines have to go from trays to the outlet, without intermixing inside the human populated area.



    Quote Originally Posted by Drew Wiley View Post
    Again, pull, don't push air for maximum efficiency. Industrial rules for toxic emissions are a completely different subject.
    Push and Pull should have exactly the same flow volume efficiency, the difference is that if pushing (without care) we may generate turbulence that wont allow a direct flow from the trays to the outlet...

    Industrial rules enlights what we should do in a darkroom to breath fresh air only. I'm not speaking about environmental emissions, but about safety in the workplace.


    Quote Originally Posted by Drew Wiley View Post
    Real HEPA filtration is in an utterly different price league than that toy stuff referenced at Amazon, and you certainly don't need it in a typical home darkroom. It is nice to have in a clean room where film is loaded into either film holders or enlarger carriers.
    My cheap Honeywell 16000 purifier is HEPA class H13, this is removing at least 99.97% of airborne particles of 0.3 micrometers (Ám), this is an amazing filtration for a darkroom, any commercial HEPA spare filter intended for home air purification is quite good because it's intended for people suffering allergies (pollen...).

    But IMHO we should use a double system, this is filtrating entering air, and and recirculating the inner air with a purifier to deal with particles we generate from clothes and skin.


    _____


    To check the quality of our ventilation system we only need a Puro Habano, best suited gear for calibrations is a Cohiba Behike. It will show how the flow behaves and if the flow takes the fumes directly to the outlet or if the fumes walk around in our darkroom.

    I'd would warn to not smoke it. That's specialized gear for calibrations only.

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