Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?

[Thalmees]

Darkroom Exhaust Fan(400 CFM) For Positive Pressure, Plus DR Design?
Hello everybody,
Thanks so much for providing your input.
This project is so important to me, appreciate all help in this matter.
Details:
I'm in the process of transfer to a new darkroom.
Just contemplating to fit two(2) exhaust fans, REVERSED, in the door of the darkroom, to create positive pressure. Each will be connected to a separate one(1) "On/Off" switch. Door will use the Automatic Closing Mechanism.
The other side of the door, is leading to a small corridor that leads to the rest of a very big apartment(house).
Will use another two(2) fans as exhaust(not inverted), above each other, separated by 30 inches, on the other side of darkroom, but on the other extreme of the same wall of the positive pressure fans.
Hope the drawing(below) is easier to understand.
One exhaust fan is equivalent to one of the positive pressure fans(400 CFM). The other is bigger(may be 1200 CFM). The bigger exhaust fan will be at bout 60" above the ground level, at the end of my sink.
Exhaust fans are pulling air into a vertical vent area inside the building(not house), that open at the top of the building.
Exhaust fans will work as a passive louvers when they are "Off". Both will be connected to one(1) "On/Off" switch.
Positive Pressure Fans, will work permanently. Or at least one of them.
Please assume every thing is light/air tight.
The room has two(2) windows, but will close them to prevent dust, unless it is seriously needed(do not know, so far).
I'm afraid of pressure gradient to be effective enough to do the job?
Room area: 21 ft X 15 ft X 9 ft(6.5mX4.5mX2.8m). BTW, 9 feet = 108".
Capacity of each each regular fan(2 positive pressure & 1 exhaust): 400 Cubic Feet Per Minute(400CFM).
Capacity of the bigger fan(1 exhaust): 1200 Cubic Feet Per Minute(1200CFM). But, I think this will effectively be 800CFM after making an efficient light tight hood around it.
Room Volum: 2835 Cubic Feet.
Which theoretically means, 7 exhaust fans Plus 7 positive pressure fans(or louvers), of the same capacity.
The original two(2) windows of the room(for air and light), will be closed, to be air/light tight, to prevent any possibility of dust accumulation. Any louvre or fan there, will destroy the shape of the building from outside.
Photos before summary:
Room Design including Inlet and Outlet of Air:

.
Positive Air Fans & one Exhaust Fan Model:

.
Tried to be organized, My apologies being long.
Summary:
So, I'm describing a darkroom design providing fresh air from inside house air(house is designed to provide maximum air/light ventilation, on demand), minimal or no dust could inter the darkroom.
And ventilating safely to a vertical vent area that connected at the top with air above the whole building. No other air ventilation leads to that same vent area in the building.
My query:
Drawback, potential problems, that I may missed?.
Or it's just a crazy idea?
Really wish to know how it will work, before doing more destruction.
Appreciate any input.
Best.

[Luis-F-S]

I think you are way over thinking this. The doran fan you're using, if you remove one of the louvers it goes from a 400 CFM fan to 800 CFM. If you're not discharging directly outside, you probably don't need the second louver, and you can always add it if needed.

Don't understand why you have the need to put two fans in series. I have two exhaust in parallel in my dark room, they both discharge to the same exhaust duct which goes outside. I have two intake louvers at the opposite end of the darkroom for cross ventilation.

If you want to put a positive pressure fan, I would switch it on when you're in the darkroom, and not leave it on all the time. At most I would put it in a timer if you feel you'd have humidity issues if not on. My darkroom has an AC vent, so it is always cool and humidity is not an issue. Hope this helps.

L

[angusparker]

My set up is two fans one for intake that pushes outside air from outside the house through a filter and then through louvers into the darkroom and the other to draw air out to an external vent on the roofline. The are both on the same switch and the same CFM.

I would say if you are drawing air from inside the house the intake fan is not necessary. What is key is the air path to come in above head height and to be drawn out from just over the trays far below nose height. You will most likely need a soffit and light tight louvers to get the air in across from the trays and uptake vent so you have positive air movement from top of back of your head to tray and out.


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[Peter Collins]

For the space as dimensioned, you have too much fan power (as CFM) and thus air movement. I hope that you contemplate a rheostat/dimmer instead of a simple on-off switch. Thus, if you have too much air flow, you can reduce fan rpm. Consider, as well, separate switches for each fan; for very little extra cost, you can have a huge range of adjustment. Also--You just want to keep your breathing zone in fresh air, not necessarily the whole darkroom, and not your whole body.

You don't want so much air movement that you have turbulent flow in the darkroom. Something closer to laminar flow is ideal. If you have too much air movement through the fans, it will feel almost windy in the darkroom.

[Jim Jones]

I kept darkroom ventilation simple: a filtered louvered small fan forcing air into a small darkroom, and darkroom air leaking out wherever it could. This was satisfactory for many years. If the surrounding area had been living space, I would have sealed the leaks and let positive darkroom pressure vent to the house's outside.

[Thalmees]

... Hope this helps.
L

Thanks so much Luis. It is of great help.
Let me please try to understand what you mean exactly.

...,
if you remove one of the louvers it goes from a 400 CFM fan to 800 CFM.
...

There are four(4) fans, only, No louvres.
Two(2) Fans are fit on the door for Positive Pressure(intake). The other two(2) are on the other side of the same wall, for exhaust(draw air out). No passive louvres.
If I understand you correctly, you mean to take one Positive Pressure Fan from its sealed place in the door, leaving only the opening, beside the other working Positive Pressure Fan.
My apologies Luis if my understanding is not correct.
I think that will disturbs the Air Pressure Gradient. Plus, some light will leak from the corridor to the darkroom.
The same case for Positive Pressure Gradation, even if I replaced the Positive Pressure Fan, with a light tight passive louvre.
.

...
Don't understand why you have the need to put two fans in series. I have two exhaust in parallel
...

What actually happen, is connecting each two(2) fans in each area, to an extension power cable with individual "On/Off" switches. If I need two(2) fans to work together, will use the main switch, if I need only one(1) to work, will use the individual switch.
.

...
If you want to put a positive pressure fan, I would switch it on when you're in the darkroom, and not leave it on all the time.
...

I may settle on this solution.
But before that, I wish I can validate my primary design(or any better alternative) through this thread.
Until now, I do not know if inverting exhaust fans(to work as Positive Air Pressure), is appropriate or just arbitrary experimental?
Appreciate your input.
Regards.

[Christopher Barrett]

Good air exchange is definitely a good thing, but more importantly, you should make sure that the direction of air flow keeps the chemistry out of your nostrils and out of your lungs. For example, if you just install louvred exhaust fans into the ceiling, you'll be drawing contaminated air upwards, making it easy to inhale.

Ideally, you want a positive air supply in the ceiling with exhaust venting near the tops of the trays.

This is a good read on the subject...

A shot of my in-progress darkroom... positive supplies across the room in the ceiling with exhaust venting 20" over tray tops, ducted out of the bldg.



HTH,
CB

[Tin Can]

I am about to put ventilation in my 'spare bedroom DR' and this thread is making me rethink my plan.

Jim Jones suggests positive room pressure. I have read many that suggest that. Seems good.

My DR is 140 sq ft, 1400 cubic ft. In a very leaky house.

My last DR was in a corner of a 750 sq ft and 9000 cubic ft, open space. A very airtight loft on sealed cement slab. 6 years ago I installed one Panasonic 240 cfm fan venting from the top of my fixer tray area and outside.

It vented, but seemed weak. I added floor level intake air opposite the sink. That increased exhaust air volume a lot. Fresh air came in, pulled up off the floor, over the sink edge and across the fixer out to the street. Tested with cigarettes. I don't smoke.

I plan to buy the same fan again and install it similarly but my intake air will come from the main house. I currently run a Merv 16 filter on the house HVAC, will add another Merv 16 intake air filter for the intake house air supplying the DR. And as Pere suggests a Honeywell Hepa filter inside the DR.

Dust is the enemy.

I don't see a good way to use positive pressure.

Let's discuss OP further.

[Luis-F-S]

Thanks so much Luis. It is of great help.
Let me please try to understand what you mean exactly.

I may settle on this solution.
But before that, I wish I can validate my primary design(or any better alternative) through this thread.
Until now, I do not know if inverting exhaust fans(to work as Positive Air Pressure), is appropriate or just arbitrary experimental?
Appreciate your input.
Regards.

You can invert a fan to blow into the room. I did that in one of my darkrooms. If you want to use 4 fans, I would put the on individual switches. I have rheostats on mine, also made by Doran and seldom run them full speed. I would certainly do this on the exhaust fans. On the intake fans, you can run both of them off one rheostat. I got mine at KHB in Canada and they've worked like a charm.

On the exhaust fans, if you're not exhausting to the great out doors (lots of light) consider removing one of the set of louvers as it doubles the output.

Just don't go crazy with it and don't loose any sleep over it. There is a thread in here on my fan setup. See:

http://www.largeformatphotography.in...1ft-room/page2

You can see photos of my three darkrooms on:

http://www.largeformatphotography.in...arkroom/page51

L

[Pere Casals]

Darkroom Exhaust Fan(400 CFM) For Positive Pressure

Honeywell Hepa filter inside the DR.

This is my view

IMHO we have two goals for the darkroom air:

> removing odors and potentially dangerous fumes.

> removing dust from air

If we use "safe" chem like Xtol and Dektol then a basic HEPA air purifier solves the dust. But if a pocess provocate toxic fumes then the HEPA won't remove it.

If we want to vent the darkroom I'd only place a fan (of the necessary power) with an HEPA filter introducing air in the room, and the exhaust would be trought a light tight vent. I don't see the need to install a second fan in the exhaust outlet.

The HEPA filter in the input fan should be covered with a regular foam (easy to clean) to remove the large particles, in this way the HEPA filter will last more.

IMHO the air intake should be adjustable (a cheap $10 voltage regulator for motors), delivering an small flow when we work with safe chem, because excessive flow will only bring on problems(noise, dirt in fiters), in that way when we work with more dangerous chem we can still vent what necessary.

IMHO internal dust has to be removed by a regular HEPA air purifier, and we have to input an HEPA purified fresh air flow depending on the kind of chem we work with.


We have to remember that the most common hazard in darkrooms is fumes from acetic stop bath, if using plain water or a citric bath then our venting requirements may decrease a lot.


https://silveronplastic.files.wordpr...er-2008-ww.pdf

page 103: "The fumes which emanate from acetic acid stop baths are perhaps the single greatest health hazard in the darkroom"
page 3: speaks about ventilation