Thread: The Generic 2HP DC

Large radius electrical conduit PVC bends from Pipeline.

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The problem with these is they take up a lot of valuable space in small DIY sheds.

According to BP's static calc,

At 665 CFM the loss of pressure due to
7m of 6" ducting is .49" WC
2.8 m flex is 0.60" WC
Y and 3 elbows and a 45º is 0.49" WC
All up ~ 1.6" of WC or 22% of your total static pressure.

The drop from 665 to 450 CFM when the system is connected to the TS perhaps is saying more about the way it is connected to the TS than the ducting and the junctions?
Can you post photos of the air inlets and outlets at the TS?

Bob, there is no drop from 665 in the main duct to 450 when the table saw is connected - not sure where you got that idea from?
My posts said there was 456 in the main duct BEFORE I changed the pleated filter to a needle felt filter then it went up to 665.

Bohdan, re the impellor - any idea where you can purchase one from? Searching says Oneida used to have some but its no longer on their website. Other searches don't come up with much locally or for dusties.

Cheers

Since the pressure drop for the 7m of duct is 0.49 and for the bends and elbows is 0.49 then you have effectively doubled your duct length to 14m and as I understand your recommendation is no more than 5m for such a setup or is that because you are allowing for poor bends and elbows.

I agree that the replacement bends will also cause a drop but wouldn't getting rid of the flexy or replacing it with smooth hose give you maximum improvement for the least effort.

Originally Posted by Lappa

Bohdan, re the impellor - any idea where you can purchase one from? Searching says Oneida used to have some but its no longer on their website. Other searches don't come up with much locally or for dusties.

Cheers

Maybe BobL can make a suggestion as I don't have much experience of what is out there or their individual performance figures.

P.S. Maybe contact some one like

Centrifugal Fans, Custom Engineered | Aerotech, Australia

for more information.

Originally Posted by Lappa

Bob, there is no drop from 665 in the main duct to 450 when the table saw is connected - not sure where you got that idea from?
My posts said there was 456 in the main duct BEFORE I changed the pleated filter to a needle felt filter then it went up to 665.

Post #162

Just fitted a seasoned needle felt filter and ran the tests again with the table saw drop disconnected.
3.5A. 665.4 CFM

Maybe I am misunderstanding what you mean by "table saw drop"?

RE: Impellers
Stand alone impellers will probably cost more expensive than a larger used DC.
I wouldn't be putting a bigger impeller on that motor anyway, it sounds like 1.5HP motor.

All my flow tests are done with the last elbow, blast gate and flexy (which goes to the table saw ie the drop) disconnected from the end of the long ceiling run. This gives me a standard so I can make changes and compare results. If the flexy bent a different way etc. the results may alter.

Saw drop on

IMG_0514.jpgIMG_0511.jpg



Saw drop off

IMG_0513.jpgIMG_0512.jpg

The static pressure taken at the new 6" inlet to the impellor (without the filters) was 7.28" H2O so I guess with the filters it is probably lower but I'm not about to dismantle my shed again to measure it

The static pressure taken at the end of the ceiling run AND at the end of the flexy that connects to the table saw (includes elbow, blast gate and flexy) is the same - 156mm ( give or take 0.5mm or 6.141 " H2O (give or take 0.02") = a difference of 1.139" which is less than the 1.6" BP expects. I believe it would indeed be even a lower difference if I could measure at the inlet with the filters attached.

Flow measurement taken with the saw drop off (my standard) then drop connected to ceiling run, then connected to the saw table saw a drop of 2% in flow then a drop of 4.6% flow giving a total from when comparing the ceiling run to the saw table hooked up at 6.6%.

Re the impellor cost - my thought also.

BTW I made up a static tube and stagnation tube out of diesel injector pipe. Nice thick walls so you can get a nice tip shape on the stagnation tube. Took some m/sec measurement with the anemometer at approx 3/4 across the duct then set-up the stagnation tube at the same point and static tube at the wall. Ran a series of measurement and the results of the manometer readings were slightly lower than the anemometer reading by only 2%

IMG_0515.jpg

Originally Posted by Lappa

The static pressure taken at the end of the ceiling run AND at the end of the flexy that connects to the table saw (includes elbow, blast gate and flexy) is the same - 156mm ( give or take 0.5mm or 6.141 " H2O (give or take 0.02") = a difference of 1.139" which is less than the 1.6" BP expects. I believe it would indeed be even a lower difference if I could measure at the inlet with the filters attached.

By static pressure I assume you are blocking off the duct and measuring the pressure in the duct?
Ot do you mean pressure at right angle to the flow?

Blocking off duct

Good work on the pitot tube!

Originally Posted by Lappa

Blocking off duct

OK in that case there should be no air moving in the duct so the static pressure should be the same no matter where you measure along the duct or how long the duct is.
The length of ducting or junctions of flexy have nothing to do with it.
The pressures being measured are low and the pipe radii are so large you should not see any static pressure changes.

The pressure drops that BP and I refer to happen when air is moving - its a dynamic pressure loss due to friction, restrictions, turbulence etc

Now if you do see a static pressure change as you add pipe etc that is most likely because there are small leaks somewhere.
The longer the duct, the more junctions there are the more chances there are for leaks.
In fact that is one of the best tests for leaks.

I find dynamic pressure measurements very difficult to make reproducibly which I why I only usually worry about flow.

So the difference, when blocked, must be due to leaks at joints which everyone said weren't a problem? Yes, I understand that during operation, the pipe should not be blocked so any effect of a leak will be minimal in normal operation.

Anyway - to measure what BP is talking about, do I drill holes in the pipe and insert my static pipe, level with the wall, and measure the pressure along the ducting.

Obviousy it is important that the amount/position of the tube should always be the same at each measurement port.

Cheers

Originally Posted by Lappa

So the difference, when blocked, must be due to leaks at joints which everyone said weren't a problem? Yes, I understand that during operation, the pipe should not be blocked so any effect of a leak will be minimal in normal operation.

Yep - thats right.

Anyway - to measure what BP is talking about, do I drill holes in the pipe and insert my static pipe, level with the wall, and measure the pressure along the ducting.
Obviousy it is important that the amount/position of the tube should always be the same at each measurement port.

BPs pressures are the same as that used to measure air flow with a Pitot tube - air stream side pressure and forward pressure differential.
BP only measures these pressure in the middle of air flow - technically its supposed to be integrated across the radius like the air flow measurements to take turbulence and wall friction into account.
However most pressure curves are made with long test ducts and the uses are usually relative so they don't bother with the integration.

The nomenclature is a bit confusing because I have always though of it as Dynamic pressure losses but its still called static pressure.
The measurement with the inlets blocked is Max Static pressures

If you have time read BPs section on this on his site
Dust Collection Research - Measurement

The only real use for this is to establish the fan curve which is itself very useful because it tells you how the fan will perform under different pressures but I always work direct with flow after all that is what we want to know. What flow does it generate under what configuration.

Don you know BPs StaticCalc Spread sheet?

Originally Posted by BobL

Yep - thats right.



BPs pressures are the same as that used to measure air flow with a Pitot tube - air stream side pressure and forward pressure differential.
BP only measures these pressure in the middle of air flow - technically its supposed to be integrated across the radius like the air flow measurements to take turbulence and wall friction into account.

So, taking anemometer readings along the duct will accomplish the same thing? I can reverse calculate m/sec to Pa to " WC with a spreadsheet Dwyer sent me.


However most pressure curves are made with long test ducts and the uses are usually relative so they don't bother with the integration.

The nomenclature is a bit confusing because I have always though of it as Dynamic pressure losses but its still called static pressure.
The measurement with the inlets blocked is Max Static pressures

How true re nomenclature!!

If you have time read BPs section on this on his site
Dust Collection Research - Measurement

The only real use for this is to establish the fan curve which is itself very useful because it tells you how the fan will perform under different pressures but I always work direct with flow after all that is what we want to know. What flow does it generate under what configuration.

Don you know BPs StaticCalc Spread sheet?

The one where you enter CFM, duct length, various fittings etc. and it calculates the drop in "WC? If so, mine should drop 4" to the end of the ceiling duct by his calcs.

Originally Posted by Lappa

The one where you enter CFM, duct length, various fittings etc. and it calculates the drop in "WC? If so, mine should drop 4" to the end of the ceiling duct by his calcs.

Yep - What starting CFM do you use?