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Ventilation debate!?

PaulMB

PaulMB

pfm Member
I'm hoping pfm can act as supreme court in a difference of opinion between my wife and I. I have to fit a ventilation pipe in a darkroom. I think that it makes no difference if the actual motor of the ventilator is at one end of the 5 meter long pipe, she says that it is more efficient if it is at or near the sucking end.
Any thoughts?
 
No difference, no, not at all, esp. for such a low-precision use.

What will make a difference - mount the thing where you can get at it easily in case of a problem! That's the real reason most small fans (bathroom extract and the like ) are mounted at the 'suck' end - to put them in the wall or ceiling where you can get at it.

(As ever, a larger fan running more slowly is always both quieter and more efficient. Go for the biggest fan suited to the job you need done.)
 
My thought experiment.

Imagine you want to move a huge amount of air through a very narrow pipe. If you put the fan at the sucking end, the lowest pressure difference in the pipe would be 1 atmosphere (there could be a vacuum in the pipe, with 1 atmosphere outside attempting to fill the pipe). If instead you 'blow' the air through the pipe, the pressure in the pipe could be raised much more than 1 above atmospheric, so the flow rate would be higher.

So, I conclude that blowing is more efficient than sucking if you are anywhere near the limit for the amount of air you can shift down a given pipe.

Are you going to pull air into your darkroom through the pipe, or vent out of the darkroom? I've never worked out which is the right way. Getting fresh air in from outside would mean you would have a positive pressure out of the darkroom which means other dust and muck would keep out of the room. However, this will send smells through the rest of your house, and bring cold air in during the winter which might be unwanted :-(
 
All good points, but the real question is - how much air do you really need to move?

Suppose you have an 8 x 10' room with 8' ceiling, a fair chunk of space spare in a home... anyway, 10 airchanges an hour (sort of rate you'd aim for in the highest category of clinical use) is 6400cubic ft an hour, or say 100cfm (45l/sec) Which is overkill, frankly, but easily in the scope of a 4'' extract fan - so put it where you want it, it's more than enough.
 
The amount of air you can shift is greatly determined by the air-tightness of the enclosed space, more so than where the motor is positioned or how powerful it is. In our bathroom, the extractor fan does absolutely nothing if the door is closed, and the mirror fogs completely. You might need to factor this into your design considerations.
 
Thank you all very much! Some very interesting aspects that I had not thought about, apart from answers to the original query. As suggested, I'm getting a fairly powerful fan that can be slowed down if it proves too hurricane-like, to reduce noise. The one I'm looking at is made by Vortice and at top speed sucks 235 cubic meters per hour. This might be too much, we'll see.
The sucking end, distributed in four smaller pipes or a single horizontal pipe with holes along the front, will be located along the wall just behind the trays with developer, stop, fixer, so only the smell/fumes will be gently aspirated away from me, rather than shifting air all around the darkroom. Which is about 4 meters by 2.3 meters by 3 meters high. It blows out into the open air.

Thank you all again for excellent thoughts!
 
An in-line centrifugal fan will operate more efficiently with ducting on both suction and discharge sides.

Presumably your fan is complete with a speed controller in the room?
 
The fan should be a Vortice CA 100 V0 D, which is described as "centrifugal axial" which I assume is the same as "in-line". It will be ducted on both sides. It has, I believe, two ways of connecting the wires for full speed or half speed. There is also the option of a controller, an extra £30-35, so I'll only buy that if I find I need it.
 
Cesare said:
My thought experiment.

Imagine you want to move a huge amount of air through a very narrow pipe. If you put the fan at the sucking end, the lowest pressure difference in the pipe would be 1 atmosphere (there could be a vacuum in the pipe, with 1 atmosphere outside attempting to fill the pipe). If instead you 'blow' the air through the pipe, the pressure in the pipe could be raised much more than 1 above atmospheric, so the flow rate would be higher.
Click to expand...

Good points, Dave. I also think the blowing would be better, but I was thinking of other reasons:

If the air is sucked into the pipe, there will be a swirl of air around the pipe entrance, and that fume laden air carrying fumes that hit the wall around the pipe will be just recirculated back into the room. That is why on an old carburettor inlet you would have a bell-end (oh..err) to prevent this and also squeeze and accelerate the air in the pipe.
If the fan was at the entrance to the pipe you would increase the captive area and get more fumes out, and recirculation of fume laden air won't happen. That may result in getting the overall fume level down, rather than getting the fume level really low in only one concentrated space in front of the pipe.

I may be wrong, but that's my thinking on it.

Your wife may be thinking in terms of a vacuum cleaner, but that's a contact suction and not the same thing at all.
 
The thread topic is issues between Paul & his wife, a long length of 'pipe', & where to put his motor for best efficiency re: sucking.
Is the OP one massive euphemism or what?
 
Still said:
The thread topic is issues between Paul & his wife, a long length of 'pipe', & where to put his motor for best efficiency re: sucking.
Is the OP one massive euphemism or what?
Click to expand...

Well, yes, there may be subtle undercurrents. Aren't there always? Or maybe not...?

However it was a genuine question, and I'm very grateful for all the response. My conclusion is that blowing INTO the darkroom is no good because it will simply spread the smelly fumes all over the place before they eventually get out. While aspirating them just behind the trays will take the smelly fumes and shoot them outside before they get into the general atmosphere of the darkroom.

The extractor fan advertised at £90 is actually very expensive for what it is. A good quality Vortice fan with the same performance costs half as much.
 
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