Types of fans

Thread Starter

strantor

Joined Oct 3, 2010
5,187
Right now I'm using a shopvac as a dust collector. I'd like to have something more suited to the task but I balk at paying $1k+ for a glorified vacuum cleaner with a sticker that says "dust collector."

Small vacuum motors can be had from surpluscenter for under $20. They resemble Ametek Lamb motors I have dealt with before. But looking at the specs on commercial dust collectors (2000+ CFM), it appears as though I'd need quite a few of those to make it work. Looking for other options...

I ran across some aftermarket performance auto radiator fans rated over 1000CFM, for under $40. Just looking at CFM, I could hit the target with 2 radiator fans. But I'm not sure if CFM from a radial fan designed to blow, is an apples-to-apples comparison to CFM from a squirrel cage unit designed to suck.

One side of my head says that both are prime movers, whose job it is to move air from one place to another; suction and discharge are (must be) equal. Therefore 1000CFM is 1000CFM, no matter what. Then the other side of my head chimes in and says: yeahhbut... the squirrel cage suction unit has the outer duct/shroud to prevent "traction loss" (for lack of the proper term - air slipping past the blades instead of being moved forward by them). But I'm not sure that's true either, after seeing what happens when I block off the inlet of my shopvac.

Someone, please resolve my internal dispute.
 

wayneh

Joined Sep 9, 2010
16,154
The key is the flow rate at a defined differential pressure. The auto fans are not likely rated at a 15psi pressure difference. I would think that you also need a blade designed for low pressure. Moving "nothing" up to 15psi is different than pumping 60 up to 75psi.
 

markdem

Joined Jul 31, 2013
113
Been a wood worker myself, here is my .2.

It all comes down to what you want to collect from where.

A dust collector (high flow low pressure) is great for moving a large volume of air at low pressure from a enclosed area. It is great for things like table saws, router tables and thickenisers. Tools that normally enclose most of the dust. It can move large chips of marital but as it is low pressure, it will not be able to gather it from very far.
A vacuum cleaner (low flow high pressure) is great for things like sanders and most hand held tools. It may not be able to move as much air, but it can gather dust from a larger area. it is far better with tools that "throw" the dust around.

If you ask me, 1000CFM is quite small. I have a 2400CFM unit and it is right on the borderline of what I would call "ok" once you add a bit of ducting.

Also, axial flow fans would be no good here anyway. The way that the fan blades work make them useless if you try to duct the air flow. The only work well in open air.
Have a look here at the 3 stickies at the top for more info.
http://www.woodworkforums.com/forumdisplay.php?f=200

Have fun.
 

#12

Joined Nov 30, 2010
18,167
Not familiar with dust collectors, but...
Manufacturers make charts of CFM, RPM, HP, ultimate vacuum. Find some.
The best way to throttle a squirrel cage is to interfere with its input stream. That's why vacuum cleaners go high rev when you block their intake. They lose grip on the suction side. You can disable them just as well on the exit side, but you're trying to block a higher energy there. You want to insure the tip speed never exceeds the speed of sound. The larger the diameter, the higher pressure you can produce at the output.

As a matter of speculation, it seems to me you should be sucking on a filter bag so most of the crap gets caught before it can hit a fan blade. This brings you to the ultimate suction pressure number. The farther down in the vacuum range it sucks, the longer the bag lasts between cleanings. There is no such thing as a perfect filter, so schedule inspections of the intake side of the blades.

That's all I have.:(
 

Thread Starter

strantor

Joined Oct 3, 2010
5,187
Not to start a war, but, aren't most vacuum cleaners/shop vacs a turbine not a squirrel cage?
sure, maybe I'm using the wrong terms. Here's a cutaway of the Ametek vacuum I'm familiar with:

Is that a turbine or a squirrel cage? looks like a stack of squirrel cages to me; maybe that consitutes a turbine.

Here's the "vacuum" motor from a commercial dust collector:


Is that one (above) a Turbine or squirrel cage?

This is what I think of when I hear "turbine"



and this is what I think of when I hear "squirrel cage":


The vacuum motor pics I posted don't seem (to me) to fall squarely into either of those groups.
 

Lestraveled

Joined May 19, 2014
1,946
What application would require you to need, " ....quite a few of those......" 2000 CFM dust collectors?

Consider this- Compare the filters. The Shopvac has maybe one square foot of filter where as a dust collector has 20 + square feet of filter area. (I used to own a Delta double bag collector.) This makes a tremendous difference in the effective air flow.
 

MaxHeadRoom

Joined Jul 18, 2013
19,671
Generally the definition of Turbine is the impeller is made to move by a flow of energy over the blades by fluid or other means, rather than the blades being the creator of flow as in a fan.
The vacuum motor fan shape is just a compressed form of impeller whether by radial blade fan or V.M. type.
Max.
 

wayneh

Joined Sep 9, 2010
16,154
and this is what I think of when I hear "squirrel cage":
You've got it right. It has an axial blade whereas all the others are radial (and would serve poorly to contain a squirrel!). Just like the distinction between windmill configurations.

FWIW, I've taken apart a number of dehumidifiers and have found both kinds. Same application, different configurations.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,187
Not sure I saw you mention what "dust" you are collecting..
multipurpose sucker. I intent to use it for my sandblasting cabinet, wood sanding/sawing/routing/etc, and as a fume extractor over my lathe and for welding - maybe also for boiling gasoline over open flame [joke]
 

#12

Joined Nov 30, 2010
18,167
I feel a need to explain my primitive approach to fans. In my simple mind, there are only two types of fans, a propeller and a squirrel cage. A propeller is axial to the air flow and its method of inflicting energy on the air is based on the surface area of the blades, the overall diameter and the pitch of the blades. A squirrel cage changes the direction of the air flow by throwing the mass radially. To me, the impeller in a small wet&dry vac is a distorted squirrel cage. When you look inside the engine of a jet airplane, those are called turbines, but I see propellers.

The purpose of this post is to explain why my contribution is so limited and the way I use words is caused by lack of education.
 

markdem

Joined Jul 31, 2013
113
A dust collector will be useless for trying to suck up fumes. (Same sort of goes for a lathe). The area of the "sucking" is just too small. You would be better off with a cheap fan blowing the air away.
 
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