Thread: Exhaust Fan Revisited

This is my update to this thread:


Shed Ventilation


Four years ago I installed a couple of hydroponics fans to provide a negative pressure in my work area, specifically to take away fine dust and paint smells.


These work very well - usually depending on which end of the area I am working, I have only one turned on while using dust making gear and leave it on for about an hour afterwards.


The fans were sized to give around 15 air changes per hour each, and with no means of testing other than observation and sniff, I'd say they've been a complete success. By way of example: after letting an awful lot of smoke out of one of my gadgets, the room was clear of smell in fewer than five minutes after turning on one fan.


But it could be better.


After four years there is a build up of dust on the fan casing and within about a square metre of the fan, yet not elsewhere in the room.

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I have no real technical knowledge of this stuff, but suspect this is due in part to "blow back" from my duct installation, which isn't sealed around the fan outlet, and in part due to "spitback" because the fan itself has a barely rounded entrance and is pretty close to a square mouth.


Here's what I think needs to change, and would welcome any input.


1)Angle of installation.


I don't know what to call this, but the fan is located at the rear-most corner of the room, however it is square to the side rather than the long axis, meaning air has to make a sharp right angled turn to enter the fan.


By changing the mounting angle of the fan closer to the direction of flow, ie pointing it more down the room, some improvement should result.




2) "Blowback" (my term)


The fan is sold as a 250mm fan because that is the diameter of the outlet. but the reality is that the fan inlet is 165mm diameter with a central axle of 90mm. This gives a very approximate inlet area of 15,000mm2. I split the outlet into two 150 diameter pvc pipes with gravity louvres to weatherproof the outside. The net outlet area is about 15% larger than the outlet of the fan, but clearly there is some air flow disturbance caused by the louvres and the junction of the duct which reduces this and causes some back pressure and subsequent leakage.


The duct is made from 3mm MDF which I projected manually, scored and shaped, but it is not a tight fit around the fan exit and you can clearly see the result in some places.

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3) "Spitback"


Because there are a million references on the web, I have chosen just one - it concerns much higher air flows that we are talking about here, but the principals are the same, and I was surprised to see the parabolic bellmouth design is almost identical to the one I have been using, taken from another source.



These diagrams are from:
http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

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And I think show what is happening. Refer to the middle example: as the disturbed air containing dust approaches the fan "spitback' creates a pocket of still air around the fan and the dust can settle. This should be pretty much eliminated by the use of a bellmouth to the opening as in the bottom drawing.

PROPOSED SOLUTION:

Time to give the 3d printer another workout I think - and if I proceed, don't expect an update to this for some weeks - there will be a fair bit of lead time for this one!

1) Increase the length of the outlet duct to give a smoother transition from 250 to two 150 diameter outlets.
2)Angle the fan more towards the centre of the room.
3)Make a bellmouth extension to the inlet side.
4)Extend the centre boss for better airflow. I am flying completely blind on this one, so have assumed a similar profile to the bellmouth, and created a parabolic cap that "looks right" with no science involved - I'd love to know what the correct shape is or how to find it.
5) A bellmouth exit would also improve things, but since we are going through a brick wall here and this is inside our house after all, the gravity louvres are going to stay in an effort to minimise intrusion by small creatures and water.

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As always, robust discussion, criticism and assistance will be welcome!

By the way, these solutions are not for everyone, and the fan does work very well, it's just that refitting my shed/workspace is currently happening in the same way some guys restore cars, with ridiculous attention to detail, because for now at least my shed IS my hobby, and because I have copped such a hiding over twenty years from you lot because you thought I'd never finish my dust extraction system, I WILL have the last laugh!

Cheers,

P