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

[Thalmees]

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.

Drew, thank you so much for your input.
Will consider which the best, pull or push air.
That needs further search. But, I could not alter any thing outside the building. It's already covered except the windows.
Plus, it looks that there is an implied agreement in this thread that four(4) fans is too much.
As for tools for air flow management, I already have purchased most things. Doran fans and Doran passive louvres.
But wait, all bathrooms in the same building are equipped by fans like the one you mentioned(hidden in the ceiling). Yes, maybe weaker, but stronger ones are available locally.
The best working thing in my current DR, is the Exhaust Fan. It's much much bigger than the fan in my house kitchen. But, yes it's very noisy.
With an area four(4) times the area of my current DR, I think the noise will be much decreased.
Thanks so much for your input Drew.

[scheinfluger_77]

Great discussion. The one thing not addressed (unless I missed it) is appropriate changes per hour in the darkroom. I am not an HVAC person so can’t speak to this. I have read (sadly I do not remember where) under normal circumstances 5-6 changes/hour are sufficient. Presumably this would mitigate the need for larger, more powerful systems.

[Thalmees]

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

Pere, thanks so much taking the subject to this level of depth.
Please study the following drawing.

In reality, there is big negative pressure around the Exhaust Fan(or Fans). That leads to gradual increase of the Air speed(velocity) toward that area, regardless of directions and air pathway.
It's not the case with your computer drawings!
I think the computer assumed no ventilation or passive light tight louvre, at the Exhaust Area.
As for "enough smooth flow over the trays", specifically, please read my next comment.
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not enough smooth flow over the trays.
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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.
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Assuming your computer drawings are accurate, there is maybe better solution than removing the Intake Fans or just using passive light tight louvres.
How about decreasing the height of Intake Fans(in the DR door) to be just above the level of the sink height. In this case, the Fans should be vertically adjacent(not like before, horizontally adjacent). Then, using an "L" shap 6 inches tube, for each Fan, to direct the air toward the sink area?
There is agreement on using rheostats and individual switches. Calibration of the flow, is another later phase.
I think this can solve the problem of your computer drawings.
For my imagination, I think the corner facing the door(which is the opposite and farthest area from the Exhaust area), should has more air pressure to create gradient toward the other corner(Exhaust corner). That could be achieved by directing the air of only one(1) Intake Fan the same as before(toward the corner facing DR door).
The other Fan, will be as above, an "L" tube toward the sink.
Please let me know what do you think Pere. New drawing of the wall facing the enlarger is below.

The lower Exhaust Fan is effectively around 800CFM or lesser, after making appropriate light tight hood(louver).
The reason I insist on two(2) Intake Fans, is the dust control which could be achieved by Positive Pressure better than any other methods. Any later problem of the air flow, can be managed by individual switches and rheostats.
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Pushing air can be a suitable choice, but one have to be careful with the inlet speed.

My apologies, the words "Pushing" and "Pulling", has been used more than once by more than poster in this thread. It's important.
Used alone, has no user location reference.
I think, both words could be used followed by either "to" or "from", to give a clear meaning for any reader imagination.
Like "Pushing to" or "Pulling from". Likewise for "Pushing from" or "Pulling to".
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Pere, I really appreciate your input and depth.
Thank you so much.
Regards .

[Thalmees]

Great discussion.
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Thanks so much to the selfless, expert forum members.
I really appreciate their posts in all subjects not only this thread.
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The one thing not addressed (unless I missed it) is appropriate changes per hour in the darkroom.
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Thanks so much scheinfluger for your post.
I asked about this issue, or just commented on, in posts 24 & 13 at least.
What I understood from getting No answer or No comments about this specific issue, is that I'm badly under estimating the effect of Intake Fans on the Air Flow.
Hope one of the members could comment about this issue.
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under normal circumstances 5-6 changes/hour are sufficient. Presumably this would mitigate the need for larger, more powerful systems.

Thanks for sharing the information.
With simple calculations, 5-6 changes/hour is 10 to 12 minutes for each cycle.
My primary design, is much faster than that(3.5 minutes per 1 cycle), but honestly do not know if that good or bad?
Please review my previous post.
Regards.

[Jim Jones]

An important consideration is the darkroom worker's tolerance for polluted air. I've printed for hours in a small darkroom with no forced ventilation, but I grew up on a farm where the neighbor's barnyard spewed its own type of pollution when the wind was in the wrong direction. With enough motivation we can tolerate almost anything.

[Tin Can]

All I can add is how many air changes are way too much.

Our factory tested running engines in six 1500 cf cells. Our self designed system did 10 changes a minute. It was very loud and windy. We also had a big Halon tank that would fire with a 20 second delay. We pushed air down to floor and sucked it out from top. We also had floor suction with shutoffs. Gasoline.The whole system shutdown when the Halon fired. Get out now. I set off the Halon 2 times by manual release. Antifreeze burns in contact with 1800 F exhaust.

I don't miss working there.

According my math a 250 cfm Panasonic moves 6 changes per hour. It is very quiet and located in the next room.

I listed my chemistry in a earlier post, as I think safer poison is safer.

Everything we do in a DR is about dilutions. Air and liquid.

[Pere Casals]

An important consideration is the darkroom worker's tolerance for polluted air. I've printed for hours in a small darkroom with no forced ventilation, but I grew up on a farm where the neighbor's barnyard spewed its own type of pollution when the wind was in the wrong direction. With enough motivation we can tolerate almost anything.

Depending on chem, no problem. Hidroquinone and P-Aminophenol (rodinal) are also used in cosmetics. Most fixers are salts, silver content in a used fixer is what's more toxic. The stop bath is a kind of vinegar...

But color chem is not as safe, and for BW I use dichromate for reversal... Depending on what we do in the darkroom we have to be careful...

[Pere Casals]

Please study the following drawing. In reality, there is big negative pressure around the Exhaust Fan(or Fans). That leads to gradual increase of the Air speed(velocity) toward that area, regardless of directions and air pathway. It's not the case with your computer drawings!

If you see the image, at left, Radiobutton "entrada" (input) is checked. This is the lines describe the path of the particles entering, if checking "Salida" (output) then you would see the paths arriving to the output, it's the same but it looks different because the lines that are uniformly selected to be viewed.

For a project, I had been 6 months all day long, becoming crazy, simulating fluids. I near needed therapy after that This is a simplified (Express) version of a simulator... with the full version it would be easier to show it.

should has more air pressure to create gradient toward the other corner

In this application there are no static pressure gradients in the room, only an slight difference in the input/output ports. Air is not substantially compressed, it's all about dynamics, we have fluid particles moving and having inertia.

There is agreement on using rheostats and individual switches

Rather than rheostats this is speed controllers, solid state...

"Pushing to" or "Pulling from"

...it's very well understood, pushing is a fan in the inlet...

For my imagination, I think the corner facing the door(which is the opposite and farthest area from the Exhaust area), should has more air pressure to create gradient toward the other corner(Exhaust corner). That could be achieved by directing the air of only one(1) Intake Fan the same as before(toward the corner facing DR door).

Let me make a simulation...

[Thalmees]

Ok,
It is a great brainstorming to me so far.
Not at all similar to solving problems around Photographic Exposure or selecting the best Methodology for Developing a Film. I confess.
Ilford Photo has answered the question about the air flow rate with greater certainty. Thanks Ilford Photo.
Please visit URL: https://www.ilfordphoto.com/faqs/health-safety-faqs/ , or read the quoted text below.

From: https://www.ilfordphoto.com/faqs/health-safety-faqs/
WHAT VENTILATION DO I NEED IN MY DARKROOM?

Smells and fumes from darkroom chemicals and heat from enlargers and lamps are best eliminated by some sort of extract in the room OR by opening the room up between processes.
If you are going to be in a darkroom for longer lengths of time it is useful to set up an extract fan with some sort of light proofing (baffles/black drapes, long duct) which still allows airflow. Also, cover your photo chemicals when not in use (trays/tanks etc.).
With these precautions, you will be able to remain safe and comfortable within the darkroom.
THE FOLLOWING EXTRACT IS AIMED AT COMMERCIAL/EDUCATIONAL DARKROOMS AND WILL GIVE YOU AN INDICATION OF WHAT IS REQUIRED.

This is the UK Industry recommendation and is from a document called:
"Heating, ventilating, air conditioning and refrigeration. CIBSE Guide B"
( CIBSE = Chartered Institution of Building Services Engineers)
"Darkrooms (photographic) 6-8 ACH (but heat gain should be assessed)
2.3.24.4 Darkrooms (photographic)

Small darkrooms for occasional use or for purely developing processes may often be ventilated naturally
with a suitable light trap, although consideration should be given to providing mechanical extract using an air
change rate of 6 to 8 air changes per hour. For general purpose darkrooms, however, the air change rate should
be ascertained from a consideration of the heat gain from the enlarger, lights etc. plus the occupants, on the basis of
a temperature rise of 5-6 K. In industrial and commercial darkrooms that have machine processing, the machines
will very often have their own extract ducting, the air supply being drawn from the room itself. It will usually be
necessary to provide a warmed and filtered mechanical inlet in such cases. In special cases, involving extensive
washing processes, the humidity gain may be significant and require consideration."
ACH = Air changes per hour.
Calculate this my multiplying HxWxD for the room in metres to give m3.
Measure airflow of extract vent in m/second.
Multiply by Vent HxW in metres to give m3 per second.
Multiply that by 3600 to give m3 per hour.
Divide flow volume by room volume to give ACH.
You can find more information about working safely in our learning zone.

Unfortunately, this source does not consider Positive Air Pressure, neither for Flow Rate Nor for Dust Control.
This certainly need some interpretation.
Will comeback for recalculations.

[Pere Casals]

As you see, with a low flow and adjusting the input/output flow directions we sweep very well over the trays: