Squirrel cage Venting fans

I have a small fume hood under which I do some welding and other assorted metal work, spray painting and soon, hopefully some blacksmithing. The fan that vents this fume hood is a small squirrel cage fan which has a max flow rate of about 600 CFM but I cannot run it at that speed for long because it over heats and trips out so I can only run it at about half that flow rate. The motor is nominally rated at 1/6th of a HP but it draws about 0.8A at 350 cfm so its more like 1/4 HP. Being a squirrel cage fan it's a very low pressure device and I measured it at 0.7" of WC.

350 CFM is OK for most of the small spray painting and welding I do but soon I will be starting blacksmithing and I would really like a little more grunt. I have a nominally 1/3rd HP squirrel cage fan that I know can output 800 cfm I was originally going to use for the fume hood but thought it might be too big. Anyway I decided to get it out and measure the specs on. It only generates 2" of WC but it can really move some air (1960 CFM !). At that flow rate the motor draws 3.A so 1.15 HP. I don't know how long this can run for at this current but I will give it a whirl. Even if I run it at half that speed (ie ~1000 CFM) it will be significantly better than the smaller unit.

This is what the two fans look like, small fan is on the right, bigger one on the left. BTW I am going to be giving the smaller fan and fan controller away to anyone that wants it. more details in the market place.
BTW it's not as good as looks in the photo :D


https://www.woodworkforums.com/attach...d=272689&stc=1

I installed the big squirrel cage fan in the fume hood and ran it flat out for about half an hour and although it got quite hot it has not tripped out.
This indicates it can run flat out for a few minutes on an irregular basis.

When I measured the flow rate yesterday the 1960 cfm I calculated was using some measured readings on or over the max of 30 m/s that my air speed meter could measure, so it could have been more than this but hey I just recorded these readings as 30 m/s which would give me a minimum flow rate. When I measured it again today it occurred to me that if I closed all openings to the shed I could just leave the main 1.2 x 2 m door open and measure the air speed across the doorway, which being a large orifice would have a much lower air speed that I should easily be able to measure.

The average air speed across the doorway is 100 FPM, times the X section of the doorway of 26 sqft = 2600 cfm, . . . . . WAIT! that's more than twice my DC!
That's still a minimum as it assumes I have no leaks anywhere else in the building which I know is not correct.

Anyway now I can serious vent the shed very quickly if I need it, way faster than my DC and at about 1/3rd the power.
When I do my metal work I'll probably be running it on half speed (1300 CFM - still not too shabby)

This fan is pretty old as it has 1986 stamped on the control box and I know it ran continuously from 1986 till 2001 so they are incredibly reliable.
It is a Reece Fan - made in Adelaide.
I don't know if these are still available but I can really recommend these as a venting fan as they move huge amounts of air.
I'll post some pics of everything once I get organised.

Here is the large fan installation.

Previous fan
https://www.woodworkforums.com/attach...-fumehood4.jpg


New Fan takes up more room but can grab gunk air more easily.
The outlet to the outside is so large daylight can be seen through it!
ON-off switch is on top of the white control box.
https://www.woodworkforums.com/attach...d=273257&stc=1

The funny looking double topped wooden stand is a turn table on which I place things on to spray paint them.

https://www.woodworkforums.com/attach...d=273258&stc=1

Old vent was a 6" diam pipe - this one is twice the cross sectional area
https://www.woodworkforums.com/attach...d=273259&stc=1

Oh yeah, noise wise using the iPhone sound level meter

Inside shed 1 m from fan
Flat out (2600 cfm) 74 db, (1000 cfm) 57 dB

Outside 2 m below outlet
Flat out (2600 cfm) 67 db, (1000 cfm) 48 dB

Looks good and I wish I had a shop with that kind of room.

Have you considered turning a bigger bell mouth opening, say with a 3 to 6 inch radius at the edge of the opening, to see if it would improve the airflow and by how much?

Is the fan one sided or does it draw from both sides? If it is double sided would centring it in the hood improve the airflow?

Pete

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Originally Posted by QC Inspector

Looks good and I wish I had a shop with that kind of room.

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Room? there's not that much room, just a figure of 8 racetrack around the inside of the shed.

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Have you considered turning a bigger bell mouth opening, say with a 3 to 6 inch radius at the edge of the opening, to see if it would improve the airflow and by how much?.

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Not really as I am not worried about the amount of flow, in fact it generates too much flow and as a result the motor gets very hot. Normally these fans are designed to blow through HEPA filters, ie under some restriction, so it will not draw as much current and get so hot. Fortunately there is a speed control which can slow the motor down and I am running it at half speed which provides more than enough flow (1300 cfm) although I can boost it up to max momentarlly while for example, doing a long welding run.

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Is the fan one sided or does it draw from both sides? If it is double sided would centring it in the hood improve the airflow?
Pete

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Fan does draw from both sides but the side nearest the side of the hood has the motor in it so draws only about 1/3rd as much air from that side. If I was installing it from scratch I would have installed it in the middle but as there was already a hole in the wall at that location I just enlarged the existing hole.

Thanks Bob.

Pete