Slot Hood Construction

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I asked this question on another thread, but I thought perhaps it was off topic, so I'll ask here formally. Have any of you ever made a slot hood for your darkroom sinks? I understand that what worked for you, may not work for me however, I'm just interested in construction ideas. So, if you've done this, what materials did you use for hood construction, ductwork, and what type of fan did you end up using? I may also consider doing something along the lines of a canopy hood, but would prefer the slot design. Thanks for any suggestions.

-- DK Thompson (kthompson@moh.dcr.state.nc.us), February 20, 2001

Answers

DK, no experience but I was reading this link

http://darkroomsource.com/ventilation.htm yesterday.

There is a long excerpted comment from Terry Carraway, in photo.net, in there. Obviously, if Terry is willing, that's one person to correspond with (search on Terry Carraway in photo.net).

Also, enter 'slot hood' in wwww.google.com. There is a ton of industrial design specs on slot hoods that come up...

-- Mani Sitaraman (bindumani@pacific.net.sg), February 20, 2001.


DK, I've never made one either, but know a little something about the subject. I also did a search, as Mani suggested; a lot of design stuff does come up. A little background on the general topic might help to get you started.

The nominal 10 air changes/hour often recommended is for general ventilation of the darkroom, not for controlling what comes off a specific tank, etc. The general rule of thumb for fume hoods (from my shaky memory) is that air velocity of about 100 feet/minute is generally a decent "capture velocity". (This may sound like a lot, but it's really just over 1 mile/hour; ie, you probably walk 2 or 3 times faster than this). For common darkroom chemicals, 50 ft/min would probably be ok; but slight drafts in the room can easily blow odors out of the "capture zone".

For the slot to work well, it should be part of a "wall" (sort of a backstop for the slot). Otherwise, the slot can also pull air from the wrong direction and you would require a larger blower. Likewise, it would ideally have a "wall" at each end of the slot. In essence, the whole thing would become a fume hood, of sorts.

You would want to use a centrifugal (ie, squirrel cage) blower as opposed to a conventional "fan" style because they can put out a lot more "static pressure". That means they can keep pushing air out even if the outside wind is fighting against you. Also, they can overcome resistance in the ducting and the slots themselves

You could go through and do the design right, but there's a fair amount of work and maybe more expense than you hoped for. Personally, I'd probably look for a small blower at a garage sale or flea market, then just try it out. (1/4 horsepower is probably more than you need, depending on size of sink). Ducting could be a little larger (say 1 times diameter) than the size of the blower outlet. For the slot, total area should be smaller than the fan inlet; maybe half? This is to make a slight restriction so as to even out airflow along the slot. Rather than using a slot, I'd suggest a strip of pegboard (painted to seal in fibers and be water-resistant). That way, you can adjust airflow by covering holes with tape, etc. Also, you need a supply of (dust free?) replacement air for your darkroom; the direction it comes from may affect airflow patterns over the sink. An Industrial Hygienist like Terry (referenced above) would probably test airflow with smoketubes or the like; you'll probably find that a cigarette works just as well.

Good luck on the project! And remember to make the ductwork light-tight!

-- Bill C (bcarriel@cpicorp.com), February 20, 2001.


Thanks for the info. you all. I didn't do a search before, actually it hadn't crossed my mind, I was thinking I might try to see if anyone outside of an Industrial engineering profession, had actually made a hood for their darkrooms. I do have a pretty good reference book called: "Ventilation, A Practical Guide", so I can get all the formulas and specs. for hood design that I need, but I was looking for a sort of "laymen's" approach to the idea. My father is a retired engineer and I had thought about trying this around 5 years ago when I built my present darkroom, however at the time it just got too complicated. Now, I'm sort of thinking about retrofitting a hood, or even just a local unit into my darkroom somehow. I have really decent dilution ventilation, with filtered fresh air coming in, but was thinking of adding this for some processes. Thanks again for the pointers, I'll try these links out.

-- DK Thompson (kthompson@moh.dcr.state.nc.us), February 21, 2001.

Watch out, you ask for me and I appear. :)

Bill gave some good info, except the pegboard thing. :)

To maximize the performance of a slot hood you need to not only put a barrier at the ends, but they also work better with a flat plate above and below them. The amount of flow is dependent on the distance from the slot to the front of the sink, along with the area of the slot. To even out the flow you need to make a plenum behind the slot that uses tapered sides to reduce the area until you get to the proper size for the duct.

My setup is a 5 foot SS sink with about a 5 1/2" back and a 2 1/2" front lip. My slot is just about full length and 1 1/2" high. What I did was put a board that height over the top of the back lip, and that board supports the faucet and such. I did make the board overhang the sink lip by about 1/2" to help the hood performance. The back of the plenum is about 2 1/2" back from the sink lip. This plenum extends downwards and tapers from both ends of the sink to a rectangular opening that fits a standard rectangular to round duct adapter (from Home Depot). I used 8" flexible duct (not the best for good airflow, but much easier to work with) to connect to the fan. I picked up the fan at a ham fest and it is a centrifugal fan of unknown capacity.

When I hooked the whole thing up I performance tested it with smoke. A cigarette works fine, of course I used fancy smoke generating tubes. What you are looking for is that smoke generated at tray height is captured even at the front of the sink and drawn into the system.

For the air intake I build a light trap box designed to fit a standard furnace air filter to the front. I picked up a 3M high efficiency filter at Home Depot that is much better than a spun glass filter, but still very low static pressure.

I don't have my references here to get the equations I used to help others design their hoods. If I remember correctly, I used about 150 - 200 feet per minute at the slot to handle a 24 inch wide sink. You can figure the CFM required by calculating the square FEET (not square inches) of the slot and multiplying times the velocity required.

You do need to make sure that you fan delivers some extra CFM since most of the time the CFM is based on no ducting losses. I would use 20-40% extra capacity as a good starting point, assumming reasonable duct lengths.

-- Terry Carraway (TCarraway@compuserve.com), February 23, 2001.


Terry, thanks for the details. My sinks, 2 6' deltas, sound like they could be just slightly larger than yours. I'm thinking of just building the slot on the one that I use for toning, and has my deep tank on it. Before I had tried (with my father- retired engineers seem to love to crunch numbers...) to come up with a design for both sinks, but it seemed like it was going to be quite a project, especially figuring out all the resistance in the ducts, and the fan size. So, I opted for dilution ventilation, with an intake filter arrangement similar to yours. This works good, but I'd really like some local ventilation. Maybe I'm just dreaming here, but I'm going to sit down and start sketching out designs again. Bill has been very helpful as well, but is the reason why you don't like particle board because of some sort of resistance to air flow? Your description of your hood matches the design of our custom slot hood here at work. Makes sense, if that's your profession though. This thing is a stainless steel monster, running off a fan 4 stories up on the roof of the bldg. It's just hard for me to scale it down into a home darkroom. But, thanks again for the details, I guess I'll get out the tape measure....

-- DK Thompson (kthompson@moh.dcr.state.nc.us), February 23, 2001.


Regarding pegboard, I think that Terry (Hi Terry!) and I are talking about substantially different plenum designs. The one Terry is using, as per my understanding, is a lot like a large funnel that's been flattened to about 2" thick. The wide end has the slot, the narrow end funnels air directly to a blower inlet. Certainly you would not want to stick pegboard on the front of this! I was thinking of a plenum chamber in the shape of a larger box on the wall; it would run the full length of the sink and might be, for example, possibly 1 foot to 2 feet tall and 4 inches to 6 inches deep. It could be made of masonite, etc, with the entire front being pegboard. So rather than a 2 inch x 5 foot slot, it might be a 24 inch x 5 foot pegboard front. If you were, for example, to mount the blower at one end, most of the inlet flow might be at this end. To even the flow out, you would just cover peg holes until cigarette smoke was evenly suctioned along the whole length of the sink. I really don't know what size peg board would be optimum, but for a start would suggest trying to roughly match total peg hole area to the fan inlet area. This is a less efficient type of hood (due to slot restriction) but seems to me to be easier to construct. However, as I said previously, I have not built one so this is hypothetical on my part.

PS; I may be way off, but Terry's recollection of 150-200 ft/min velocity at the slot sounds way low to me. I would guess roughly 5 times higher (or more) to do the job.

-- Bill C (bcarriel@cpicorp.com), February 23, 2001.


Bill, I think I can visualize it...ah, this is taking me back to when I was building this darkroom...Yes, Terry's hood sounds very close to what we have where I work, which does look like a squashed funnel, although this thing's really big. I left the ventilation book there, so I can't recall the formulas now, although who am I to comment on that?? The problem I think I'm going to have is that my sinks and water panel and all that, are already in place (even though I built this thing, I don't want to do a major remodel here) So, I might end up doing some sort of local ventilation/maybe a hood that I can "set" in place. I understand how Terry built his, but I just don't think I can do that without doing some serious replumbing. I might have to get creative. What you're describing sounds more like the fume hoods I've seen in lab safety catalogs (in fact our conservation lab has a very nice fume hood in it). Thanks again for all the info., I need to dig out the Grainger catalog, and the graph paper. I guess this will be my spring project!

-- DK Thompson (kthompson@moh.dcr.state.nc.us), February 24, 2001.

Bill,

The problem with the peg board is that there are not much in the way of holes versus the amount of blank space. Plus you are trying to get a flow of air over the top of the tanks/trays to carry the vapors away before they get high enough to be mixed with the general air. And if you get enough velocity through the holes to have capture out at the front of the sink, they are going to make LOTS of noise. You could make an entire wall out of pegboard and vent that, but overall there is a LOT of resistance to the air.

If you can't make the squashed funnel (I like that description), you can even out the airflow by tapering the slot. Narrower at the end for the fan and wider at the other end. You can also taper the back of the plenum, with the cross section larger at the fan end. But the best way to get even air flow is the squashed funnel.

WIth doing this, remember the stuff making the squashed funnel doesn't have to be that robust. Foam core or foam board would work, thin plywood, plastic/acrylic sheet, even cardboard (with some water proofing). You can seal around plumbing penetrations with foam in a can.

The 200 feet per minute is a velocity measurement at the slot entrance to capture low velocity contaminents at the front of the sink. My hot wire anemometer is on loan right now, so I can't measure it right now.

-- Terry Carraway (TCarraway@compuserve.com), February 25, 2001.


Terry, one of the big problems I think I'm going to have is that my sinks are already in place. When I was building this darkroom several years ago, my father (a retired engineer) had designed a slot hood for the 2 sinks (12' total), but using the ventilation formulas, this hood became very large, as did the fan that would run it. Now that I'm rethinking this idea again, I've decided to limit it to just one of the sinks. I think I might try to construct it out of Sintra. If you're not familiar with this, it's a sheet of compressed PVC that you'd buy like plywood. You can do anything to it (cut,route.drill, glue) and it's waterproof. But, it's pretty expensive. I may try to build something like a slot (or if I have to, a canopy) hood that I can place in the back of the sink, at the top of the rear lip. I'm still unsure of how I'll work the exhaust ductwork into this and where the placement of the fan will be. Thanks again for both of you all's detailed answers, you've given me some good ideas to start with. Maybe someone else can use this stuff as well. Thanks.

-- DK Thompson (kthompson@moh.dcr.state.nc.us), February 25, 2001.

Try working it out with cardboard. Much easier and cheaper to work with. The fan doesn't need to be at the hood, it can be at the exhaust of the duct work.

-- Terry Carraway (TCarraway@compuserve.com), February 27, 2001.


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