Homemade Ventilation Hood - Fins? or No Fins?

Hello,

I've decided to build some good ventilation to my darkroom. I want to add a wide shallow 60" hood just at the back of the sink to draw the chem. vapors off the top of the trays like the ones shown here: Dead Link Removed My question to anyone who has one of these hoods (or enjoys guessing) is: Are there fins inside the hood that link each intake opening to a central air collector area towards the top of the hood? I think there must be something to even out the suction across the the width of the hood. Does that question make sense?

I plan to use the laminated plastic material (looks like plastic cardboard) typically used in yard-signs for political candidates during elections. Hopefully, I can eventually purchase one of these hoods from Dr. Vent but I figured I should spend my current funds available on the exhaust fan and the duct work and upgrade to a non-homemade hood later if need be.

Any ideas or concerns with this plan?

Thanks very much,
Travis McCauley
Toronto, ON

Travis -

I can't answer your question about fins - but I will share a related thought.

The problem with a hood is that it interferes with your ability to work over the trays. You could elevate the hood to create space to work, but the space that you create between the trays and the bottom of the hood would be contaminated with fumes from the trays. That would defeat your objective of trying to keep those fumes away from the person working at the sink.

I recall seeing plans in one of the photo magazines many years ago for a darkroom ventillation system based on ordinary plastic pipe. The pipe was run across the back of the sink just above the level of the chemical trays, and had a series of holes drilled at an intervals on the side facing the front. Somewhere behind the scenes there was a fan that drew air out of the pipe and exhausted it outside the darkroom (outside the house?). The idea was to create a flow of air across the chemical trays and away from the darkroom worker that would draw away any fumes given off by the chemicals.

If you use a citric acid stopbath and a neutral or alkaline fixer there is not much need for a complex ventilation system. Toning can be done seperately using only a single tray.

Yes Travis,

As Monophoto describes, you don't want the hood drawing the fumes across the face of the darkroom worker, you want them to evacuate BEFORE they have a chance to hit their face.

There are plans somewhere on the net where someone took regular PVC drain pipe, framed their sink and cut slots in the face of the pipe to exhaust the fumes. The also had a "Y" valve that allowed them to turn off the slotted pipes and go strictly to a dryer hose with a small metal hood on the end that could be placed directly over a tray or near a smelly process.

I will post the link if I can find it...

I assume that you are referring to the narrow slot at the back of the sink, not the hood over the sink.

It sounds like a good plan. Better than a lot of darkroom vent systems. My take on the fins/slot would be that the fins do little to help with even air distribution - in fact they could hinder it if badly designed. I’d be more inclined to have fins/guide vanes running along the length of the slot. You would get an even velocity across the slot if the inlet loss (at the slot) was higher than the losses inside the hood (plenum). Therefore one of the key design features is to keep the slot width smaller than the hood depth, and to have a short parallel section of hood above the slots before the taper begins.

The usual way of balancing along a slot or series of inlets would be to restrict the inlets nearest to the exhaust duct and leave the furthest inlets fully open. With a slot that would mean restricting the slot width at the centre (assuming that the duct is central on the hood) by trial and error. This technique also works for a series of slot inlets in a long, large pipe. You may not need the hood.

Inlets have very little 'sphere of influence'. The air velocity at any significant distance from the inlet depends more on where the air is coming from, rather than where it is going to. The inlet slot can just as easily draw from the wall above the sink as from over the trays: it all depends on where the air is coming from. JonE covers that issue on the page you mentioned.

Best,
Helen

Thanks Monophoto

I think the hoods shown on the Dr. Vent site are at the back of the sinks so shouldn't obstruct the workspace. I had that concern about using kitchen stove hoods.

I like the plastic pipe idea since it's so simple. I've seen pictures of such a setup . But my concern with that is that with a 60" run of that you'd lose a lot of suction at the end of the pipe. Maybe with a powerful enough fan it would be fine.

-Travis

travismc said:

Thanks Monophoto

I think the hoods shown on the Dr. Vent site are at the back of the sinks so shouldn't obstruct the workspace. I had that concern about using kitchen stove hoods.

I like the plastic pipe idea since it's so simple. I've seen pictures of such a setup . But my concern with that is that with a 60" run of that you'd lose a lot of suction at the end of the pipe. Maybe with a powerful enough fan it would be fine.

-Travis

Click to expand...

I would think that by the time you got enough fan power going, you'd be moving a lot of airborne dust as well...

Darkroom ventilation is very important, regardless of whether you smell the fumes.

I designed a vent hood that forms the back splash for my sink. Start with an external fan for a kitchen hood (mine draws about 1000 cfm, and is on a dimmer control) and duct it to the center of the sink, below the level of the sink. I built a plenum along the full length of the sink, and used small blocks of plywood to restrict the opening—more restriction close to the exhaust, less farther away. I tested the design with smoke as I built it. Finally, I bent a piece of stainless steel into an “L”, about 12 inches x 12 inches x the full length of the sink, and capped the ends. The plenum forms the bottom, the house exterior wall the back side, and the ss “L” forms the top and front. The “L” extends about 2 inches over the sink (but the trays can be tucked under it), and pulls the fumes off the trays before I breathe them.

And don’t forget to provide for make-up air. I filter my make-up air, and I do not have a problem with dust.

Helen B said:

Inlets have very little 'sphere of influence'. The air velocity at any significant distance from the inlet depends more on where the air is coming from, rather than where it is going to. The inlet slot can just as easily draw from the wall above the sink as from over the trays: it all depends on where the air is coming from. JonE covers that issue on the page you mentioned.

Click to expand...

Before I answer Travis' questions, I want to address this very important point that Helen raised. A point that very few dark room ventilation systems adequately take into account yet. To better understand the sphere of influence she mentions, take a simple drinking straw and blow air out of it onto your hand. You should be able to feel the air as far away as you can hold your hand. Keep your hand there, now try sucking through the straw. Can't feel anything ? How close do you need to place your hand from the straw before you feel a suction? Maybe a few cm ?

So applying this to our darkroom sink, how do we make sure that the vent only sucks most of the air from over the trays rather than from everywhere else? Aside from placing a physical barrier between you and the fumes (like a big perspex/plexiglass sheet a foot above the trays) a practical way of creating a barrier over the trays is to generate an air sheet. Without an air sheet, your ventillation system is not as effective as it could be.

Here is a prototype setup I built for my sink.
http://www.users.on.net/~tallowood/images/DSCN3926_air_sheet_tube_image2.jpg
http://www.users.on.net/~tallowood/images/DSCN3933_air_sheet_tube_image1.jpg
A sheet of air is blown across the sink from a 20mm PVC pipe running the length of the sink containing small evenly spaced holes. A sufficiently strong flow source is used so that the a minimal pressure drop across the length of the pipe results in an evenly distributed air flow in the air sheet. I use an old CPAP flow generator.

Now onto Travis' post/questions.

You asked about fins. As others have mentioned you will need to balance the airflow somehow. You could do this using fins (on the inside of the plenum), but why make life harder for yourself ? Consider instead the idea of making variable size holes/slots along the plenum's length
My plenum comprises two large diameter round pipes connected with a Y coupling and diagonal slots still to be tuned by placing tape over them.
http://www.users.on.net/~tallowood/images/DSCN3921_prototype_darkroom_sink_vent_image1.jpg
http://www.users.on.net/~tallowood/images/DSCN3924_prototype_darkroom_sink_vent_image2.jpg

The images show the prototype using stiff cardboard tubing. I have purchased 100mm PVC tubing for the final version.

Tuning the size of each hole/slot is a little trickier. Short of designing your vent using finite element analysis CAD software, I would recommend using either smoke, fog (from a fog machine) or a flow meter.

I use an electronic flow meter with a low impedance (e.g. a spirometer as used by asthmatics or respiratory physicians).
http://www.users.on.net/~tallowood/images/DSCN3946_spirometer_calibrating_air_flow.jpg

At the moment I have a very powerful extraction fan (>100[L/s] when loaded) on a speed control http://www.users.on.net/~tallowood/images/DSCN3935_exhaust_fan_speed_control.jpg , so I haven't spend the time yet to fine tune an even vent flow. But my intention is to place more tape over the diagonal slots closer to the centre so that the flow readings for each slot are equalised.
Now once your vent is balanced, it doesn't mean that it will effectively suck vapours from across the trays yet. So one final test should be done to prove to yourself how effective the ventilation is. Using your fog machine, release some fog from the front edge of the sink. Does it get sucked up? If not maybe you also need to install an air sheet (see the start of this post for how this is done.) BTW, using fog rather than smoke for this latter test is better, because the warm smoke has the unfortunate property of rising up too fast before it makes its way across the width of the sink.

I forgot to mention that the air intake into the room is provided by an air condiitoning vent inlet on the ceiling. Because I wanted positive pressure in the room it provides >>100 [L/s] of airflow. When using an air sheet in the sink, the exact location of this inlet vent is much less critical because the air velocity above the trays resulting from the air sheet is much greater than what could be provided by any intake vent. The position of the CPAP flow generator (and hence its air intake port) providing air for the air sheet is not crucial either.

regards
Peter

Hello all, I just checked back in with this thread and, wow, thanks for the excellent information. The "inlet sphere of influence" issue got me thinking about air currents in the room. I like the air sheet idea to create air flow across the trays. Seems simple and I never would have thought of it.

Thank you, everyone, for the great input. I'll let you know how it goes.

-Travis

I tried the tube with holes idea as it seemed an elegant solution. In practice it did not work all that well. It made a pleasing kind of noise, but did not have enough suction to move the fumes to the back of the 8' sink. A more powerful fan could have made the difference, but, my patience at an end for further fabrication, I simply purchased two of the most powerful and quiet bathroom ventilators I could find and screwed them on to low shelves at the back of the sink and ran the ducting up and over to dryer vents I installed in a window I blocked up. This has proven to be a satisfactory solution, moving enough air away from me and out of the room, and doing it quietly. I also purchased a large Honeywell air filter with a charcoal element and hepa. I don't know how much residual fumes the charcoal takes care of, but dust problems have decreased significantly. Well worth the price.

I've not read all of this thread and maybe this was brought up but you may also consider that you need somewhere to let air into the space. A tight room w/ only an exhaust fan will create a vacuum, but its pretty unlikely as most walls are not really all that tight. I have a seperate air intake hole in my dkrm wall ( just a light tight L shape) and an exhaust fan port that sits at the end of the sink. The fan is a recycled kitchen hood exhaust fan that sits only a few feet above the sink right in the wall opening. Obviously the shorter the run for your exhaust stack the better it will move air. I wanted the exhaust port to sit in the center of the sink but the extra distance hampered the fans power. I should get a more powerful fan but what I have now works ok. I also cover my trays w/ plastic laminated cardboard sheets to contain fumes when toning. Sometimes I use them when printing as well as even simple fumes have caused alot of problems for some people I know.

Peter thanks for the photos in your drkrm and the good ideas

dave in vegas

JBrunner said:

I tried the tube with holes idea as it seemed an elegant solution. In practice it did not work all that well. It made a pleasing kind of noise, but did not have enough suction to move the fumes to the back of the 8' sink. A more powerful fan could have made the difference

Click to expand...

The beauty about using an air sheet is that you don't need to buy a really powerful extraction fan. Even if you did get a powerful extraction fan it will never be as effective as also using an air sheet.

BTW the fan I had installed is the following.
Dead Link Removed

A lot of the standard bathroom fans have very poor suction when under load (i.e. when ducting is connected, or when the pressure differential across the fan blades in the direction of airflow increases). Unless you can view the fan's load curve then don't assume anything. Once you have the load curve, you can use it to verify that it will provide the required no. of room air exchanges per hour.

regards
Peter

On the hoods that we dealt with for grease vapor removal, we sized the exhaust CFM for the perimeter dimension of the hood. No fins were involved. We usually dealt with 150 CFM per linear foot of hood.

If the same considerations held true for you (they don't) you would need a fan capable of moving 750 CFM plus the dimension of hood ends.

In your instance I would assume a fraction of our requirement would meet your needs. I would guess about 150 to 200 CFM.

When considering exhausting tempered air from inhabited space, it pays to consider the effects on the air balance in your enclosed space. There must be allowance for introduction of make up air for the air that is removed if the fan is run for extended periods. Negative pressure conditions will develop if this is not done. In extreme cases, one can experience down draft conditions on gas fired flues such as water heaters and furnaces. This can lead to the introduction of CO (carbon monoxide) into inhabited space.

Donald Miller said:

On the hoods that we dealt with for grease vapor removal, we sized the exhaust CFM for the perimeter dimension of the hood. No fins were involved. We usually dealt with 150 CFM per linear foot of hood.
If the same considerations held true for you (they don't) you would need a fan capable of moving 750 CFM plus the dimension of hood ends.
In your instance I would assume a fraction of our requirement would meet your needs. I would guess about 150 to 200 CFM.

Click to expand...

Without knowing the internal layout of Travis' hood then even these numbers of 150 or 200 CFM won't mean much. The initial driving requirement for a darkroom is to try and change the air in the room 8 to 10 times every hour. This is controlled by adjusting the flow rate of the air.

The numbers of 150CFM per linear foot of hood attempt to set the air velocity at the hood (units of m/s or ft/s). Whereas flow rate is a measure of volume of air per unit time (usually measured in CFM, L/min or cm^3/s).

regards
Peter

Donald Miller said:

There must be allowance for introduction of make up air for the air that is removed if the fan is run for extended periods. Negative pressure conditions will develop if this is not done. In extreme cases, one can experience down draft conditions on gas fired flues such as water heaters and furnaces.

Click to expand...

Donald, I saw proof of this in a most extraordinary way. The basement space where my darkroom is has a fireplace in it. It has its own flue to the second floor, and two flues come out the top of the chimmey. The second is for the ground floor fireplace. I turned the window AC (in fan mode) on to exhaust one time there was a fire in the other fireplace. Coming back down to the space, I found it filled with smoke... Quite surreal. The exhaust sucked the smoke from the second flue down the first and into the space...

Seeing that the total space is about 400 sq/ft, I have quit worrying about active air exchange, instead using a passive system. Darkroom smells are not an issue. If they were, I would do digital...

Peter, very interesting pictures and links. I already have one of those roof exhaust type fans in my small factory.

One of the issues is noise, or to be precise, air being forced 180 degrees as it hits the hood. This is the only thing I would change if I add one to my darkroom, which I'm thinking of doping.

I would have one with a wind vane ending, instead of a cap. One, there would be less noise, two, there should be less resistance (slight) therefore more air moved.

My darkroom, is a free standing bungalow in the backyard, no chance of any under floor ducting with that.

Mick.

Hi, I read the whole thread with interest, as I'm about to build my first darkroom... quite exciting But I'm wondering if ventilation is such an issue if I only plan to do B&W with resin paper and standard chems? Aside from removing the smell, should I worry that much about chemical vapors?

Thanks!

Smoky05

Yes, you should use adequate ventilation

Smoky05 said:

Hi, I read the whole thread with interest, as I'm about to build my first darkroom... quite exciting But I'm wondering if ventilation is such an issue if I only plan to do B&W with resin paper and standard chems? Aside from removing the smell, should I worry that much about chemical vapors?

Thanks!

Smoky05

Click to expand...

Yes you should use adequate ventilation. Plenty of literature exists to back this up. Check out this publication from Kodak

Minimum recommendations for general ventilation for buildings and processes are provided by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) is for
3.75 air changes per hour

Past recommendations from Kodak have been as high as
10 air changes per hour

regards
Peter