A little theory

[TomL]

My kid brother (mechanical engineer) told me that the 5 HP motor & the 15 inch air-foil blade does NOT suck air thru the 6 inch PVC pipes in my shop!

Instead the motor assembly creates a partial vacuum in the blower housing itself, and outside air pressure pushes more air thru the pipe(s) to fill that void.

Sounds reasonable to me.

Based on the above, I ran the figures as follows:

A 2 inch pipe has a radius of one inch, giving an cross-sectional area of 3.14 sq inches at the inlet.
So at sea level (what else?) the pressure exerted (pushing)on the air in that pipe is 46.158 pounds. (14.7 x 3.14)
(Think of pressure exerted on a piston of the same cross-sectional area )

A more common 4 inch pipe would have a surface area at the inlet of 14.56 sq. inches.
Air pressure there would be 184.6 pounds ! (Getting better!)

A 6 inch pipe such as I have installed has an area of 28.26 sq inches resulting in a pressure exerted at the inlet of 415.4 pounds! No wonder that sucker seems to SUCK! (or should I say "blow")

For the few that have a motor/blade large enough to accommodate an 8 inch pipe, the entrance area comes out to 50.24 sq inches and an inlet pressure of 738.5 pounds.

Regards
TomL

[dwdrury]

The general gist of the analysis is OK, Tom, but there's one small fly in the ointment: you can't pull a perfect vacuum with an impeller, so you'll never make 14.7 lbs suction. There's a chart of pressure and flow under the Drawings & Specs subheader of the Product Info menu off ClearVue's home page. It appears with flow nearly choked off, the blower will develop just under 13 inches of water (atmospheric pressure at sea level is about 34 ft of water, or just under 30 inches of mercury).

You need a displacement pump to create near vacuum here on earth, or 14.7 PSI pressure differential between inside and outside the pipe. Otherwise, when the delta P gets too great, blower airflow reverses locally, sneaking around the blades as it were.

Regards,

DWD

[TomL]

DW
You are correct, of course!
A good definition of an absolute vacuum is:
quote-
Absolute Vacuum
A volume which contains no matter, also called perfect vacuum. An absolute vacuum is not obtainable.
end quote.

I should have stated that the topic was go give more info for those that are considering going with a 4 inch pipe just to save money.
Which it would sure do! The complete 6 inch piping in my shop set me back just over $500.

Got the Parks & Parks barrel installed and saw that it worked beautifully.
Don't have the two filters connected yet, but noticed that I could see NO DUST coming out of the exhaust port when dumping hands-full of sawdust into the inlet. Couldn't even see any that settled on flat surfaces.
Thanks for the post!

[Erik]

My kid brother (mechanical engineer) told me that the 5 HP motor & the 15 inch air-foil blade does NOT suck air thru the 6 inch PVC pipes in my shop!

Instead the motor assembly creates a partial vacuum in the blower housing itself, and outside air pressure pushes more air thru the pipe(s) to fill that void.

Sounds reasonable to me.

Based on the above, I ran the figures as follows:

A 2 inch pipe has a radius of one inch, giving an cross-sectional area of 3.14 sq inches at the inlet.
So at sea level (what else?) the pressure exerted (pushing)on the air in that pipe is 46.158 pounds. (14.7 x 3.14)
(Think of pressure exerted on a piston of the same cross-sectional area )

A more common 4 inch pipe would have a surface area at the inlet of 14.56 sq. inches.
Air pressure there would be 184.6 pounds ! (Getting better!)

A 6 inch pipe such as I have installed has an area of 28.26 sq inches resulting in a pressure exerted at the inlet of 415.4 pounds! No wonder that sucker seems to SUCK! (or should I say "blow")

For the few that have a motor/blade large enough to accommodate an 8 inch pipe, the entrance area comes out to 50.24 sq inches and an inlet pressure of 738.5 pounds.

Regards
TomL

Tom,
I found this engineering note on the web. Very, very interesting. Lots and lots of data.
Everybody should download it and READ it. It's not long - 16 pages - but it sure changed my view on how to install ducting.
http://www.cincinnatifan.com/catalogs/EngData-203-internet.pdf
It's on the page:
http://www.cincinnatifan.com/product-catalogs.htm under Enginering Data

Just my 2 cents
Erik

[TomL]

Thanks for the link, Erik!
I now have that document safely tuck away in my library.