Ventilation question for people smarter than me..

That pretty much qualifies everyone!

I have a 6" inline with a 4" reducer on it. The 4" PVC pipe will come straight down from the ceiling and run the 6.5' length of my sink with holes drilled into it about every 3 inches or so. My question is, how big should the drilled holes be? I am wanting to make sure that I am maximizing the suction power of the fan, but I have no understanding of "airflow dynamics" or whatever that is called.

I appreciate any input!

Thank you,

Chris

And to add to the OP's question, should the size of the holes vary along the length?

I'm not a physicist either, but I'd consider bigger holes nearest the most unpleasant chemicals, probably fixer and maybe stop bath. (Toners can be worse, but I don't use the sulphide types in the darkroom)

Also, based on my experience with a not-quite-optimal extractor in my kitchen, fumes won't rush directly to your air suction points. Significant amount will go straight up or dissipate in all directions.

The main issue is to mininze resistance in the airflow. The less work the fan needs to do in oulling air through it, the more efficient it will be. So large holes and short pipes are in principle better. Also make sure there is an easy supply of fresh air; ie not only pay attention to the extraction holes, but also make sure there are places where fresh air can easily enter the room. This is especially important if you have snugly sealing doors in your darkroom; a low resistance air inlet needs to be provided for.
The important thing is to keep in mind you're essentially refreshing the air; replacing old air with new, clean air. The higher the turnover of air in your room, the cleaner it will be. The exact location of vents etc is a little less important, but it's beneficial if the air outlets and inlets are not directly adjacent but instead are eg in opposing corners. This prevents dead pockets of air in the room.
Of course it's nice if the outlet is positioned close to where most of the fumes are, but even if this is not feasible or easy to achieve, a high turnover will still be quite effective.

Essentially you're creating a forced draught in the room; much like opening opposing windows in a room.

Amador said:

but I have no understanding of "airflow dynamics" or whatever that is called.

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I made the darkroom vent design with CFD airflow simulation... but this is totally innecessary.

> place adjustable intakes in your holes, sliding doors to adjust apertures is ok.

> Use a Cohiba type cigar to throw "static clouds" of smoke to see how you have to adjust apertures.

Your holes may depart from totalling the same aperture area when added than the pipe section area.

Sadly Behikes are not available right now, beyond importation issues to USA, not every year they can be produced and they are aged for 7 years IIRC.


A dominican Cohiba also will adjust your apertures, of course.

The size or capacity of the fan necessary also depends on the size of the room. The specs should be available from the fan manufacturer. If possible I would ask them about how you plan to set it up and what size vent for fresh air is necessary for the fan to have adequate air exchange, You may want to over do it as well. Better to have too much than not enough.

http://www.jeffreyglasser.com/

http://www.sculptureandphotography.com/

Amador said:

My question is, how big should the drilled holes be?

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Hi, it should probably depend on several things, of which the air velocity is one. But offhand you might not be too far off if the total area of the holes is roughly the same as the crossection of the duct. For a 4" diameter duct that would be about 12 square inches (formula is (diam/2)^2 x pi). So to match this you'd want about 250 1/4" holes, or about 115 3/8" holes, for example.

If there is a fairly high air velocity in the 4" duct then the air coming through the holes could disrupt the flow. If so the things to look at would be increasing the number of holes (perhaps double?) and perhaps increase the size of the drilled pipe.

User 138S suggests "adjustable intakes in your holes." One possibility could be to use sliding sleeves to block holes. For example, say that in your drilled pipe you leave 6 inches bare, then 6 inches of drilled pipe, etc. Then you could have a 6 inch-long sleeve that normally sits in the bare area. If you want to restrict flow in that area you could slide the sleeve to block some, or all, of those holes. This lets you "tune" the area where the exhaust picks up from. As 138S also suggests, the use of smoke lets you see what is going on.

Ilford's web site says 6 to 8 air changes per hour for darkrooms (industrial standard - probably more than you'd need). Figure how much volume your extractor can move in an hour and divide by the volume of your darkroom to figure the air changes per hour you'd be getting.

I asked about varying the hole sizes because it occurs to me that if the holes nearest the extraction fan are too large you might end up with more airflow at that end than at the other end.

138S said:

A dominican Cohiba also will adjust your apertures, of course.

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thank you for that video ! near me they grow wrappers it is a beautiful thing to see an artist at work

MattKing said:

I asked about varying the hole sizes because it occurs to me that if the holes nearest the extraction fan are too large you might end up with more airflow at that end than at the other end.

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Yep. In a "proper" setup a well-designed plenum would be used to even things out. The main idea, typically, would be to have the air velocity through the holes to be relatively high, which means they are more restrictive. So the air, looking for the path of least resistance, tends to be more or less balanced across all the holes. There's more to it, but I think the "adjustable" holes would be a simpler alternative to more elaborate design. Plus, this would let the OP "target" certain trays, or whatever. Even to reduce the overall airflow if desired.

Thanks for the feedback, everyone! I really appreciate it. I do like the idea of a sliding mechanism to allow me to customize the air flow. I will definitely look to incorporate that. I have a 14"x14" air inlet opposite the outlet, so "new" air is not an issue.

Thanks again!

Chris

Shouldn't you have air filters on the intake venting to eliminate dust getting sucked in?

jnantz said:

it is a beautiful thing to see an artist at work

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Yeah

She is a true artist doing that, a very good one.

138S said:

Yeah

She is a true artist doing that, a very good one.

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What is the wet stuff she uses to "glue" the leaves?

Alan Edward Klein said:

Shouldn't you have air filters on the intake venting to eliminate dust getting sucked in?

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Yes, and I do. Two of them in fact.

Chris

Alan Edward Klein said:

What is the wet stuff she uses to "glue" the leaves?

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There are many vegetable Gigar Glues, they use either gum Taracanth, gar gum or cellulose glue (Bermocoll 320). Honey and maple syrup are also choices, IIRC Montecristo Nº4 (Che Guevara choice) uses honey, honey may add some flavor which can be good or not for a kind of cigar. (https://www.amazon.com/s?k=cigar+glue&__mk_es_US=ÅMÅŽÕÑ&ref=nb_sb_noss_1)

That "torcedora" in the video is famous Norma Fernandez, she personally rolled the cigars smoked by Fidel, he died 90 years old, Marlboro is more toxic I guess . Norma rolled 100 promotional Behike boxes at $19,000 per 40 cigars box !!! regular production Behikes are 1/10 that per cigar price.

Well, those dominican cohibas sold in the USA are also well made, but different...

If your pipe is coming straight down, how are you going to prevent light piping down too? But let me interject some basics. Air pulls better than it pushes. So ideally you want either an inline fan or an externally mounted one; that also helps isolate fan noise. You want a bigger fan than you think. For one thing, to keep light out, you need a duct line that isn't straight. If you use flexible corrugated ducting, it introduces a certain amount of air friction. But the largest factor is hydrostatic pressure. If there is high relative humidity outside or rainy conditions, it can take twice as much force to exhaust air against that kind of pressure. How big is your workspace? That is very important. Anything other than a tiny bathroom will probably need at least a 250CFM fan. You should get one that is quiet and for which the speed can be electronically controlled. You might want a gentle draw in dry conditions and a very rapid draw if you accidentally spill something noxious like acetic acid. Think of a legitimate fume hood. It doesn't necessarily have to be overhead. In fact, it's better if it is a series of intake vents like you're contemplating arranged in a row to draw air away from you toward the back of the sink. But if this is too close to sink level, it will act like a cooler and make tray temperature hard to maintain. Worthy small fans typically run around $300 or more. Panasonic makes an excellent very quiet inline fan for slightly more than that. Avoid cheap noisy propeller-style fans. Now, how big your subsidiary vents along the wall, and ducting manifold behind that? - the bigger the better. If your main duct is 6 inch, you might think about 4 inch for your secondary system. And if your main fan is variable-power, you won't need to get horribly nitpicky about perfectly balancing your subsidiary ducts.