Exhaust fan or intake fan? How many cfm?

[jaycobar-chay]

Ok, not sure if anyone will see this here, but I'm setting up a

darkroom and I've seen a bunch of posts here about how it is better

to use a fan for intake of fresh air instead of exhausting darkroom

air. I've read in a few books the same thing, how it's better to

bring air in with a fan and out with a vent next to the chemicals.

 

I called B&H to ask thier opinion and they said the fan needs to

suck the air out. The fan manufacturer said the same thing, saying

I'd be making a mistake using it to bring air in, which they said

couldn't even be done with their fans. That seems odd to me, since

putting the fan in the door would suck clean air into the darkroom

just as it would suck dirty air out of the darkroom if it were

mounted in a window...

 

Also, Doran said their D68 was too small for my room, which has 950

cubic feet of space. The D68 moves 150 cubic feet per minute. When

I divide 950 by 150 I get roughly six. That's six minutes to change

the air with that fan, assuming proper exhaust vent size. If it

takes six minutes to change the air once, it will get changed 10

times in an hour, right? Why did B&H AND Doran say I needed the

bigger fan? 150 cfm times an hour is 9000 cubic feet of air moved.

Divide that by the cubic feet of my room which is 950, and it looks

to me like the air will get changed roughly 10 times in that hour.

Do I need the bigger fan?

 

Also, Doran said that their lovers are made to go with their fans,

but they say the lovers use is for intake. Would the louvers allow

the necessary exhaust if mounted in a window, above the chemicals,

across the room from the fan? Is one louver enough or do I need

more? It seems to me that the fan and louver would work as well for

exhaust or intake, so if a louver provides enough intake for a

particular fan to exhaust, it should also provide enough of an

exhaust for that same fan if the fan were used for intake, right?

 

Any clarification will be greatly appreciated.

[jimvanson]

Exhaust the air out high so vapors raising from the trays are being pulled up and away from your face. Cross ventilate with a air intake placed low and opposite from the exhaust.<p>You do not need Doran, Home Hardware can fix you up with all sorts of bathroom fan & louvers. They can give you all sorts of ideas on how to lightproof them. I did it on two darkrooms by offsetting the fan & the louver, others I know have used flex hose to duct the air remotely.<p>In closing...Size does make a difference...Bigger is better! Just use a rheostat to adjust the fan speed.<P>Voice of experience here as OH&S installed a $5000 vent system in the community darkroom I taught in. It was spec'd out for flow under perfect conditions. Two years later the fan has deteriorated, a filter was added to the intake. Today your eyes water if you selenium tone in that darkroom with the doors closed...comming out of the darkroom you can still smell the chemistry on your clothing. But they say they have dropped that much cash already so there is no way they're going to replace the fan unit with a bigger one yet.

[dave_schneider]

You really need to take a step back and learn a little about ventilation. A fan is rated for a volume of flow and a specific pressure on the inlet and outlet. When you install the fan in the door you will actually create a positive pressure on the exhaust side and a negative pressure on the inlet side. The pressure differential will cause a significant reduction in the volume of air that the fan moves.

 

You need to examine the vent arrangement you will be using as either the exhaust or inlet. You will need a light trap of some sort and you should have a filter to avoid dust. I would put a filter on the vent if used as an inlet or an exhaust. Now with the light trap and the filter the vent will have a significant pressure loss which means the fan will be reduced in capacity even further.

 

As to fan in the intake or exhaust there are several things to consider. If you put the fan on the exhaust you will create a slight negative pressure in your darkroom which means that any place air comes into the room will be a acting as the intake vents. That includes door seals, electrical boxes, whatever. All those spots can bring dust in with the air. A large intake vent with a filter may create greater pressure loss than the junction box and conduit which means more air will flow via those routes than the big filtered vent you installed. It also means that any chemical vapors will be exhausted via the fan and not escape into the rest of the house since the darkroom is under negative pressure.

 

Now if you use the fan on the inlet you solve the problems of drawing dust in around every door seal and electrical box. You create a whole new list of problems. The room is now pressurized which means all the chemical vapors are being exhausted through those same leaks in addition to the exhaust duct you installed to the outside (you did that, right, you wouldn't just exhaust into the adjacent rooms). The vapors from sepia toning will certainly get your wife and kids to take notice even if they didn't before. So you really need to seal all the leaks in room, even more so than if you use an exhaust fan. The intake fan has to have a filter on it's intake so you don't draw dust in with the air. The filters required to do this will become very large. You can't simply cut a piece of furnace filter the size of the fan cage and expect it to work. Filters are designed to work at a low pressure and low velocity, sticking one on the fan will probably not work. If you use a metal mesh filter like you have on your cooking range exhaust hood you won't filter out the small particles, only the big chunks. If you wanted to do this right you would have to build a filter plenum, maybe a 2 foot square duct section with the filters installed (think of your furnace filters).

 

The reason the fan makers aren't recommending this is that the fans are designed to operate at lower pressures. Using it as an intake fan with filters that would be needed would push the fan up into a pressure range where it is not efficient. You can see fan performance curves on many manufacturer's sites or look at the ASHRAE publications for examples.

 

I think the best solution is an exhaust fan (or two) exhausting to the outside through a rigid duce. Several large area filters for intake air with the appropriate light baffles. When building your darkroom be sure to seal all walls and joints so they are not entry points for dust. You can seal out light and dust with the same methods, caulk under any sill pieces, caulk around all door frames before moldings are installed, line the walls with black plastic before the wall panels go up. These techniques assure the room is dark and that the only intake air comes through your large filtered vents. It assures that the incoming air is as fresh as the air in your house and positively exhausts any hazardous or annoying vapors.

[frankl]

anyone have any advice on how to splice some kind of fan into a dryer-vent line? My darkroom is in the laundry room and this would make an easy venting solution for me in there.. Light-tightness might not be an issue since it rounds a bend before exiting outside.

 

Thanks

[frankl]

sorry to poke in a question on another question :) and the fan should be pulling the air out of the room above the chemistry.

[jimvanson]

Frank...just go to a home building supply store. They'll fix you up with either a indoor or indoor/outdoor bathroom vent fan and flex hose or if you want to go high end a stainless steel kitchen stove hood unit and flex. Then get a rheostat & filters for the intake and away you go...there are dozens of solutions it only depends on how much cash you want to spend.

[nige_buddy]

I have a D68 and matching louvre (in the door so it sucks air from the house) and they seem to work fine in my darkroom which is probably a little smaller than yours (if my metric to imperial conversion was anywhere near correct). If you go this way, consider mounting the fan at the end of some ducting as it's a noisy little bugger! (I will get around to doing this one day). If you go for a domestic exhaust fan, you'll need to build a light trap over it if you're just pumping the air into a roof space, or if it's going out a vent, ensure you've got enough bends to make it light tight.

[chris_waller]

One point to consider - and I speak here as someone previously involved in industrial drying and cooling systems - if you extract air from a room then air has to enter that room from somewhere and that usually means it pulls in dust. So a light-tight grill with a filter element over it is advisable. Areas such as operating-theatres and clean-rooms are under positive pressure to prevent the ingress of dust etc.

[chris_waller]

Sorry, I meant 'grille', unless you're figuring on hamburgers in the darkroom.

[bruce watson]

What you are missing here, Jacob, is that air is compressible. What happens when you try to blow air into a room is that you compress the air inside the room. The net effect is that you raise the pressure inside the room slightly. This increase in pressure will cause some small air flow out of the room, but it is *much less* air flow than what you would expect from the rating on the fan.

 

One analogy is that pushing air is like pushing a rope. It just doesn't work very well. What you want to do is pull air, like pulling a rope. This works a lot better and is far more predictable. That's why people are telling you to use an exhaust fan - they are telling you to pull the air out because it works much better that way.

[jaycobar-chay]

I've got all the stuff just sitting in a room in my basement and I want to get started, but I don't want to black out the windows before I put ventilation in them, and I don't know what is best, or at the very least, sufficient for my room.

 

I can get a D68 and it's corresponding louver for 76 dollars shipped, so it sounds awfuly attractive, but will it be enough? And which way should it go? I'd like to use the fan for intake, so I would probably need to step up to the next biggest fan, which moves 250 cfm, does this sound acceptable? The D68 isn't desinged to vent directly to the outside, so if I did get that one, it would have to be for intake. The other fans are more expensive and when using them for exhaust you have to buy an outside atatchment to keep bugs, rain, wind and rodents from getting in it, which means more money.

 

What should I do? I really want to start using my darkroom...

[dave_schneider]

Jacob, did you read any of the responses to this posting?

[jaycobar-chay]

Yes I did. Yours was especially informative but noncommital. What is a plenum and how is it made? Also, what sort of filter needs to be used on an inlet louvre if you can't just stick one on?

 

Some suggestions seem to confirm my assumption that my desired setup would be ok, some seem to tell me I need more power, if not a completely different setup. Are there any websites that offer blueprints for this type of situation?

 

I guess I'm just an overly cautious type of guy and don't want to do anything stupid.

[dk_thompson]

The problem is that each room is different, and each process requires it's own type of ventilation. There is no universal solution...to do it right, you'd have to get an industrial engineer to design a system for the room. For a darkroom, depending on what you're doing, you might wind up with a slot hood or some sort of fume hood or you might get away with dilution ventilation to just change the total out by X number of times an hour. It gets complicated though when you try to make everything light tight, because the more ductwork and plenums etc you put in--the larger the fan needs to be because of this increased resistance.

 

I'm not qualified to recommend a solution--I know that for a fact. I'm not an industrial hygenist or engineer. I know what happened in our darkroom where I work--when an onsite OSHA visit wrote us up after some bean counter told us they didn't have the money to fix the problem. I'm not knocking some of the responses to this thread--but if you're in a workplace, and there's a violation or something needs to be fixed to bring you up to compliance with the laws--there are courses of action. Usually safety officers, liasons with labor depts and OSHA etc. It might not make the bean counters happy, but chances are they aren't the ones using the lab.

 

This probably doesn't help much, but there are some trade books on ventilation that you can get approx diagrams and formulas for hood design, fan size, ducting etc. I guess when all else fails, it would be better to aim for a larger fan than needed. You need to move the flow out away from your face & body as well. Kodak has some good safety publications you might be able to find some basic diagrams in...I'll see if I can find a link to one, otherwise good luck. Hope this helps. My opinions only/not my employers.

[dk_thompson]

try this for starters:

 

http://www.kodak.com/US/en/corp/environment/kes/pubs/pubsList.jhtml#education

 

J-300 and J-314 are pdf's.

[kymtman]

An old gentleman photographer friend of mine told me to eliminate dust from entering the darkroom when the door was open was to pull the air through a filter from the outside, creating a higher pressure value inside the room and then when the door was opened dust would leave the room rather that being pulled from another room within into the darkroom. There is a new device being advertized that collects smoke and dust by means of electrostatic inductance. the cost is rather high around $300.///// Dust is a concern of mine in the negative dryer..it is hard to contain a dust free enviroment.

If the amount of incoming air is greater than the exhaust air there is a pressure build up inside the room. Always maintain a clean filter when using incoming air, depending or your location the air is almost always dirty.

[william_whitaker1]

Jacob,

 

Dave and DK both make very good points. You need to understand the basis for fan ratings before you can determine if a particular fan is suitable for your application. Also, every installation is different. So, it really does little good to speculate what is best for you without knowing the specifics.

 

With regard to your comment about B&H: B&H is a good retailer and I purchase from them regularly. But they are not in the business of designing darkrooms. They are a retailer and they make their money by selling stuff they have and not by selling stuff they don't have. If all they stock are standard exhaust fans, then it is in their best interest to encourage you to purchase the same.

 

What is most important in darkroom ventilation is exchanging the air. If the air is changed at a modest rate, then you have less issues with fumes and your workplace will be much more comfortable to work in. Nothing gets me down like a warm, closed-up room full of stale air. It's certainly not conducive to creativity!

 

Because I'm a nut and also because I'm not married, I chose to go the positive-pressure route. I picked up a used furnace blower fan and mounted in the closet outside the bedroom which comprises my darkroom. The door to the closet was replaced with a piece of 3/4" melamine-clad with two 16x20 filters in it. The blower is ducted through the wall and into the darkroom where it enters behind some cabinets which form a plenum. (There's that word again! A plenum is simply a passageway for air.) The air exits under the cabinets looking for a way out which it finds by flowing up and over the sink on the opposite wall. The "way out" is via two large ducts leading to vent flaps in the [boarded-up] window. There's only one way out, so the air has to make its way over the sink enroute.

 

Although the system recalls many of the [mis-]adventures of Tim "The Toolman" Taylor, it works very, very well. My only complaint is that it's a bit noisy. It is designed as a low-pressure, high volume system which keeps the air changed. It may sound like a big blower, but, no, you can't feel a breeze when it's running. You are, however, aware of fresh air entering the room. The other reason for the big blower is that the only practical exhaust route is out the window, which is on the west side of the room. The prevailing wind here (and it prevails a lot) is out of the west. So I'm trying to exhaust my darkroom upwind. It's not the most efficient system, but it works pretty well and wasn't that expensive to install. Efficient would have been to use those prevailing winds as my "fan", but that's another story.... My crazy system works for me, but it's probably not what you need or want to get involved with.

 

I would strongly suggest reading the catalogs from Grainger or McMaster-Carr or looking at their websites. McMaster has an excellent web site with a lot of information. The Grainger catalog also has lots of information on fans and blowers with an explanation of ratings for different static pressure systems. You may find that something economical from Home Depot will suit your needs fine if applied in a suitable manner. Then you can get on with making photographs!

[roger_hicks1]

Try both. My darkroom has a positive pressure fan at one end (drawing from a light trapped plenum, linked to the exterior) and a sucker at the other end (evacuating into a light trapped plenum of wall/ceiling space). Oh: and there's a dehumidifyer in the middle, because the darkroom is in a wine cellar built into the side of a hill. I'm not sure how old the masonry is (it's now plastic lined) but it's between 200 and 900 years. Seems to work!

[andy_hughes]

I agree with the folks above who have talked about using both. I see one additional reason for doing this, and that is at the end of developing, when I want to leave the room. Let me explain.

<p></p>

I have a fan to extract air, built into the wall

<a href="http://www.darkroomsource.net/ventilation.shtml#fantools">shown here</a>

and on the other side of the darkroom there is an air conditioner (it gets warm here in the summer) as which brings 'filtered' air into the room.

<p></p>

When I'm developing, I run the extractor fan (I only run the a/c if it's hot in the room) since the extractor fan flows more cfm than the ac fan, the room is negative pressure, removing the fumes.

<p></p>

When I leave the room I'll turn the ac fan on and turn the extractor fan off, then the room is 'positive pressure' and so as I go in and out the door I won't be sucking dust into the room.