Is my ventilation doing anything?

[Sean]

Ok was working in the darkroom all night (again), and tonight put in my light tight intake vent. So one end of the room has the intake vent and the other side of the room has a big Doran darkroom fan sucking air out. Now it's supposed to move a lot of air, and when I go outside i can feel a steady flow of air coming out of it. It's not like a blower vac or anything but you can feel a decent amount. What I'm not sure of is how much air is being moved. Since I was doing construction in there all night it got very dusty. So I cut the lights and shined a flashlight near the exhaust fan. There were very fine dust particles flitting all around it but not even getting sucked out. I thought everything fairly close to the vent would be getting sucked out big time but it felt like it was doing absolutely nothing. Is this normal or have I done something wrong? Is it moving air in a way that I'm not realizing? I also thought I would feel a light air coming in through the intake vent but even with my face right to it I felt nothing. The room is air tight and the intake vent was exposed to the outdoors. Can someone shine some light on this for me? Thanks

 

[Laurent]

Sean,

I guess that if the light proof intake is build with several "gates" (sorry, my English is not good enough for the darkroom vocabulary) (let's say that as light -usually- does not bend, it would be enough to have a kind of Zs in the path of the air to let air in and keep light out) then the dust would "fall" between the gates and would therefore not cross the intake.

If you feel a steady flow, then I'd assume the air goes through your darkroom by some path.

Did I miss something ?

 

[noseoil]

Sean, try using a lighter and "flick your bic" in the darkroom in front of the intake to see which way the flame is moving. You should see a movement of air. Similarly, there should be the same movement, in the opposite direction, near the exhaust fan.

P.S. Don't use this trick to test for gas leaks, use soap suds. Don't use soap suds to test for darkroom leaks. tim

 

[rogueish]

If your standing outside and you feel the air coming out, then the fan is working. If your standing inside and don't see the dust leaving, then it's possible the air being "sucked" out is not all from inside. Can air from inside the walls and outside the room get "pulled" out? Check on air tightness of fan to wall seal. Is the exhaust port bigger or smaller than the intake vent? If the intake vent is large and the exhaust port is small (or smaller) then you will notice a difference in "pressure" between the two. I would say that if it's not pulling out the dust in front of the exhust port, you have an air leak (as opposed to light leak ).
Wet your hand or hold a light ribbon on the outside of the exhust port, then do it again on the inside of the room in front of the exhust port. you should feel/see the same amount of air. If not, your fan is pulling air from another source.

 

[mark]

Get some type of perfume and spray it into the room without the fan moving. Turn on the fan and see how long it takes to remove the smell. It is what we used in the Darkroom I used to work in when the new hood was installed. Worked like a charme.

Dust is a nasty beast. It resists everything that has been created to move it or get rid of it. It's only purpose is to ruin what you are doing. The only time it will actively head in any one uniform direction is when you are printing or you are loading a film holder. Other than that it just sort of swirls working up energy in the great microbial mosh pit.

 

[Robert Hall]

You might open the door to the room and see if the air flow increases out the fan exhaust.

 

[Flotsam]

Get some type of perfume and spray it into the room without the fan moving. Turn on the fan and see how long it takes to remove the smell.

And then try to convince your wife that you aren't entertaining ladies during the hours that you spend locked in that room.

 

[rbarker]

And then try to convince your wife that you aren't entertaining ladies during the hours that you spend locked in that room.

Yeah, I think I'd use aftershave instead.

Oh, wait. That might be even worse!

As others have suggested, Sean, it's probably a matter of making sure that there is enough available air in-flow to match or exceed the capacity of the exhaust fan. If the in-flow is restricted, the fan will pull air from wherever it can find it. In addition to doing the flic-your-bic test at the exhaust port inside the darkroom, you might do the test on the darkroom side of your air intakes, too. Essentially, I think you want to make sure that the air in-flow is well filtered, and sufficient to satisfy the exhaust fan.

Additionally, you may want to compare the volume of the darkroom to the fair-flow rating for the fan. They usually have a rating measured in CFM (cubic feet per minute). A 10' x 10' darkroom with a 7' ceiling would have a volume of 700 cubic feet, for example. A 100CFM fan would then take 7 minutes to theoretically exchange the entire air volume in the room. Depending on the location of the air intakes, however, you might have "stagnant" air in corners and such that never gets moved.

 

[mark]

And then try to convince your wife that you aren't entertaining ladies during the hours that you spend locked in that room.

Hush up!!...She's buying it so far.

 

[John McCallum]

need more direction

Just another thought. With a large darkroom, the volume of air could dilute the affect of the fan.

The air going out of the vent, could just as easily be coming from the ceiling area as from across the chem trays. Or even a two deg change in air temp could keep the fan occupied for minutes (see where this is going?) It may be necessary to consider a hood above the trays to direct the airflow exactly where it's needed? (I know this would be more hassel!!)

 

[Sean]

Thanks all, well I made some progress. I found a few remaining air leaks coming from the bottom of one of the walls. Nothing some spray foam couldn't handle. After sealing those leaks I could then feel a gentle air coming through the intake vent. The fan is rated for a room 20x20ft and with my new wall in the darkroom in now 17 1/2 x 11ft, however the ceiling is on a slope up to 9ft so that ads extra volume. I'm not sure if I'll need a vent hood or not? Is there a trick to determining this? Would like to hear from people with a darkroom my size to see what they do. Also the fan's out nozzle had a stamped metal cover with holes in it (I assume to keep critters out), I got a nibbler tool and cut that out and I think it's probably increased airflow atleast 20%.

 

[John McCallum]

In my last company, we did a lot of work with an air-flow/extraction system that we used. For OSH and general comfort issues it was fairly important that we got it right, so we did quite a lot of investigation. The general steps to improving the efficiency were ...

1. check the volume of air is not restricted at any point in the system. So the inwards vent should have the same size opening as the extractor fan. The idea is to keep the air flowing at a constant speed (i.e. not slowing down OR speeding up anywhere). Restricting the size of an airway will speed up the air; conversely increasing the size of an airway will slow down the speed of the airflow. A big darkroom will slow down the air so much that it would have difficulty being efficient (sorry).

2. check for leaks that could bring in air from where you don't want it to.

3. specific extraction points sometimes need ducting directed to those points (eg like the chem trays).

The first two steps were considered necessary; and the last by assessment. First steps are probably easier, but if there is still a problem, then it might be more affective to draw the chem vapours off the trays before they are distributed into the general airspace of the darkroom.

I have an aquaintance who recently set up her home darkroom after 20yrs professional B&W wet printing in a lab. She is elderly now, doesn't use gloves and her darkroom is relatively small. She did go to the effort of putting a hood over the trays.

But like all things, just a suggestion! It may not be necessary.

best,
john.

 

[rbarker]

Just give us a day or two, Sean, and we'll design a system of localized ducting that will require a 15hp, 3-phase motor to do the air handling. Then, you'll have better justification for installing your own power sub-station to handle that other big enlarger.

 

[Sean]

Maybe I can purchase a used helicopter engine and place it on the roof

Well, another long day working on the darkroom and 1st coat of paint is on, everything is sealed and spray foam under two walls that had big gaps (the joys of converting a garage!). The fan seems to be doing well, plenty of air coming out of it and when I put my face infront of the intake which is placed on the opposite wall/opposite side I feel a good breeze blowing in. I have a filter on the backside of the intake and the intake is a doran lightproof vent. Now eventhough a good breeze is coming out of the vent, when I open my darkroom door I can hear the exhaust fan rev up quite a bit. I suppose my light tight vent + filter media is decreasing the efficiency. Would it help much if I added another vent or maybe I need a better filter media that allows more air flow? hmm..

 

[Nige]

Sean. how noisy is it? I've got the smaller Dorian with matched inlet and it seems to do a decent job (I can tell the difference between having it running or not!) but it's very noisy. One day I'll move the thing somewhere further away by putting some ducting between it and my darkroom.

 

[noseoil]

Sean, you might want to look into a residential kitchen cook top hood for the chemical trays (fixer area) as these are inexpensive and easy to install. Remember to keep the front edge where it is hard to bang your head on it. Do a mock-up by bending at the trays to make sure you have room for clearance. Paint the inside of any duct work with black paint and put in a couple of 90 degree bends to cut down on light, and a grill on the outside with a boxed cover and open bottom for air escape. You can get a small louver from a building supply which goes in a gable end (roof vent in the side wall).

 

[glbeas]

The fact that the fan revs up tells me you are working a negative pressure balance in the darkroom and will tend to suck dust in from the outside every time you open the door. What you might eventually do if this bothers you is to put a fan on the intake side with maybe 50% more cfm capacity than the existing exhaust fan. This will reverse the pressure balance in the room, even a fractional psi on the positive will blow dust away from any openings in the darkroom. Both fans working together will insure an efficient air exchange.

 

[BruceN]

I did mine just the opposite - I put the fan/filter combination in the interior wall and over near the trays is just a vent to the outside. That way it sort of keeps the room pressurized and seems to help get rid of dust instead of sucking it in. I still avoid the flashlight test, though. I might see a dust particle and feel compelled to re-engineer the whole thing again, my wife would shoot me!

BTW - I didn't want to pay the outrageous prices for the doran fans and grills, so I bought a (much quieter) 300cfm bathroom fan and constructed a light proof grill for it out of oak and 2" roof flashing. It works quite well and allows a good airflow. If anyone is interested I can work up a web page explaining the construction.
Bruce

 

[John McCallum]

I think Bruce and Gary's suggestions are good too fwiw. The noise of the fan is something to be aware of.
Generally the motors are designed for a particular airflow capacity. We previously attached ducting and airbags that were too big (and allowed too much air-flow) for the motor on an extraction system. As a consequence the motor speed up and came close to overheating. So, it may be more important to keep the air intake close to design levels rather than making it very much bigger. The documentation should have suggestions about that.

 

[Sean]

Thanks for the info everyone

I still avoid the flashlight test, though.

I wish I would have never picked up that flashlight! hehe

 

[BarrieB]

Others are correct, the CFM rating of the fan is important,however, I feel that the Exhaust should be trapping ( collecting) the fumes from your wet-bench, so the ductwork (pipes ) should be adjacent to where the fumes come from. To this end the fresh air intake should be lower than the fumes thus permitting the collection of the fumes.