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

[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.

[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

[Thalmees]

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

Thank you so much Luis for your kind followup reply. Appreciate it.
I encourage other kind posters to reply to my queries and thanks so much.
Few important points has been confirmed essential. Individual switches, rheostats, highest possible level from the ground for the Intake Air and lowest level possible for the Exhaust Fan.
But still few points also.
1. Why I should not use Positive Pressure Fans "On" all the time?
I know it has a limited life span, but I do not think it's short.
Plus, how can I make sure that no dust(or minimal) will inter the DR?
2. How many fans are enough?
Operated one new Doran fan in my hands, just to see if it's working or not and to test the Air Flow Speed/Volume. I found it not that powerful! The fan is new, but the air flow speed was not that much!
Knowing that my room is more than 2800CF, I concluded it's too weak to create the required Positive Pressure for the given room air volume.
That's the reason why I used two(2) Positive Air Pressure fans in my primary design.
Could not expect appropriately what's the effect of one fan on 2800CF after it installed in place?
3. Is the upper Exhaust Fan necessary?
As far as I know, if intake air is 800CFM, Exhaust should be a little more.
Is that correct?
Appreciate your input.

[Luis-F-S]

Just don't go crazy with it and don't loose any sleep over it.
L

From post #9............you may want to disregard a lot of the next 40 posts.
L

[Luis-F-S]

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

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.
.

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.
.

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.

Doran fans have 2 louvers to make light tight. If you remove one it doubles the fan output.
Read the instructions on the fan.

[Tin Can]

I wish at least one of our 'experts' would document sources with hyperlinks. Saying 'google it' is not much help. The only source we are not supposed to link to is live eBay.

Most of us are not competitors, nor in business any longer.

Share with the class.

[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.


Sent from my iPhone using Tapatalk Pro

[Thalmees]

...
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.
...

Thanks so much angus.
Great help.
As for passive air inlet(or passive light tight louvres) instead of active Positive Pressure Fans, it could help in better exhaust flow, but I do not think this will help the purpose of the design.
I adopted the Positive Air Pressure because I believe it's the best way to get rid of dust.
Then, I chose Inhouse Air(instead of outside Air at window area), because Inhouse Air is clearly much less dust born, plus the fans are high in the door well above the ground level of the house. I do not know so far how can I get filters that could fit in Doran fan?
As for the pressure gradient, I revisited my primary design, thanks.
Please see the drawing below, thanks everybody for the comments.
The intake fans will be at least 72" above the ground of the room, this is the maximum usable height for the door, but will use angled 6" tube(Doran specific size) to direct Positive Pressure Air up higher than the level of the door(up to 100" above room ground level, as needed, but I will consider not to add much of dead space[tube length] to conserve power of positive air flow).
At the same time, I owered the level of the bigger Exhaust Fan to be 50" from ground level. Planning to make the sink low to approach it on a chair, which the case for all smaller prints.
Appreciate your input angus.

Intake door fans can be extended higher than 72"