Dust Collector Impeller Direction.

[echnidna]

Hi all,
I'm setting up a dust system and felt the DC flow rate was very poor.
Took the front off and found 12mm (far too excessive) clearance between the impeller and the housing. This must reduce the effectiveness as it would allow a lot of leakback.

But the thing that really struck me as soon as I took the cover off was that the impeller shape suggests it should rotate anticlockwise and not clockwise as manufactured.

I hope someone experienced or qualified in Fluid Dynamics can comment on this.

[echnidna]

sorry , meant to include this pic.

[ozwinner]

Dust Collector Impeller Direction.I think mine goes around and around??

Al :confused:

[vsquizz]

Bob, its hard to tell from the photo where the inlet is. I assume it's on the end cover you have removed to take the photo. whoops...then the correct direction off rotation is clockwise.

On my old DC I fitted a masonite ring (cut out with router) inside the end cover to close up the vane gap. Just make sure there is not excessive end float on the shaft.

Cheers

[bitingmidge]

Bob,

Back to the font of all knowledge on all things dust extraction for this one:http://billpentz.com/woodworking/cyclone/Equipment.cfm#Fans
all appears to be as it should be!!

While you are there, check out the clear plastic cyclone now in production!

Cheers,

P

[gatiep]

Bill Pentz actually says that one can run the impellers backwards as far as I recall. They seem to be shaped incorrectly to run clockwise but apparantly the shape makes them quieter, running backwars they sound more like a siren.

I used a 2 hp economy dustie motor and impeller from Carbies, built a 'cage' according to Bill Pentz's specs and it actually gives me almost twice the inches water (suction) with a 6" inlet/outlet than in its original setup on the unit.

Check out Bills site for all the info.

[echnidna]

From an engineering aspect pointing the impeller vanes backwards may reduce noise but also reduces the volume of air it will pump.

Joe, did you put a cage on the carbie impeller or did you make new housing?

[Andy Self]

The fan is a centrifugal design and should rotate clockwise. Such fans are either forward bladed, radial or backward bladed. The first two generate more pressure and flow for a given rotational speed but are non unloading i.e. the power goes up with increased flow in an uncontrolled manner. The backward bladed design is unloading, power increases to a peak and then falls away after the peak and hence is self protecting. If you went forward bladed, the fan would be capable if overloading the motor in any reasonable design i.e. unless the motor was humungously large. The 12mm sounds excessive, if it is the impellor to front plate dimension. This is effectively the inlet cone clearance and we normally see 25-40mm on fans which are 2-3m in diameter. As you point out, excessive clearance here will result in recirculation, so part of the airflow will simply go round in circles, rather than draw air into the fan. If you are talking about the clearance between the impellor and the casing radially, 12mm is not a problem and in fact too tight a clearance would cause excessive shock loss and strangle the fan. Regarding the efficiency of backward bladed fans, it may appear illogical, but the efficiency will be in the range 75-80%, generally, this being defined as air power generated divided by power at the shaft, near enough input power to the motor. Hence, if you want more flow, there is only 20% of the power available to fund it before you will need a larger motor.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p>

[echnidna]

Thanks Andy

[gatiep]

Thanks Andy


Bob,
I only used the econ 2 hp motor and impeller. I made a new housing with 150 mm inlet as per Bill Pentz's site.

Some interesting figures for those who are interested.

Firstly the 2 hp economy as one buys it.

Current drawn with 1 x 100 mm inlet 4.1 A
2 x 100 mm inlets 4.8 A
With the double inlet plate removed
(150 mm dia) 4.5 A
1 x 1 3/4 " inlet 3.7 A
1 x 2 3/4 " inlet 3.3 A


Now the figures for the project.

motor with no fan ( motor only ) 4.1 A
With fan in housing but inlet blocked off 4.1 A
With 1 x 150 mm inlet 5.6 A
With 1 x 200 mm inlet 5.8 A


Please note that the motor and impeller are from same manufacturer as the 2 hp econ dusty and are replacement parts, but it was not possible to use the same motor and impeller in the project and the dusty, as that would have meant taking the dusty apart. Too much like hard work.

It may have been more useful if I did a 100 mm inlet comparason on the project. May still be able to do it if it will be of interest to anybody.

:)

[bitingmidge]

Joe,
I'm deadly interested, in the hope that one day I'll understand this stuff. Currently my eyes just glaze over a bit like the kid's used to at report card lecture time!

What do the current draw figures mean? If it is higher it is obviously working closer to it's intended capacity, or is it just working harder?

Cheers,

P

[gatiep]

Peter,

Obviously, the higher the Amps drawn, the harder the motor is working and I would say the more air is moved. The 2 hp econo motors are rated at 7.7 Amps.
Unfortunately although the motors compared are identical and from the same manufacturer, I think the figures would have been more meaningful if the same motor was used in both units. Would have been better if I measured the dusty with the fan blocked off as well. I find it strange that the dusty is drawing 4.1 A with 1 x 100 mm inlet open, when that is the no load current draw for the project motor. Maybe I should get my ***se in gear and measure the project with a 100 mm inlet, although not giving the no load comparison that I would like, but at least 100 mm with 100 mm inlet.

[echnidna]

Joe,
Do you recall the cost of the impeller?

[gatiep]

Bob,

I cannot remember off the top of my head but as I bought the motor with it I'll have the invoice somewhere for warranty purposes. I'll look it up and get back to you

Ave a nice weekend

:)