Thread: BobL's Dusty setup

Because I have the ability to measure air speeds I decided to do some testing I sort of followed Bill Pentz method using a pitot tube air speed measuring device which is inserted sideways thu a hole in the ducting so that it can measure the air speed in the air stream. The pitot tube I have access to is quite large so it probably interferes with the air stream during the measurement more that some of the small tubes but it will serve for comparative purposes.

testSetup2.jpg

By systematically moving the pitot tube across the air stream and stopping and measuring the air speed at each point I can find the max airspeed and examine things like edge effects and the effect of diameter of pipe on total air flow.
testSetup.jpg

Without the bags the DC can only pull 1700 CFM through a 6" pipe (the DC is rated at 3100 cfm but that will be without any ducting or port attached.

With the (moderately dirty) bags on the DC pulls 1600 cfm thru 1m of 6" diam pipe. If I replace the 6" pipe with a piece of 4" it only pulls 933 cfm. Using 4" ducting will be further reduce air flow as longer lengths and 4" fittings are used. All Ttis is why using 6" ducting is better than using 4" ducting, something I already knew but it's nice to see it for ones self.

Now lets look at the edge effect. Here is a graph showing the air speed as one goes from the wall to the centre of a 6" pipe.

EdgeEffects2.jpg
I can get no closer than 0.5 cm from the wall with the pitot tube but there the air is traveling at less than half the air speed than at the centre of the duct.

As I make changes to the ducting system I can check the airflow and make adjustments accordingly

Bob

Now there is likely to be a run on Pitot tube, and manometers throughout Australia

Regards

Graeme Collins and I have been having a PM discussion about calculating the air flow from the above graph and he rightly points out that one has to "integrate under the curve" on the graph to get a more accurate air flow.

Using a spreadsheet model where he considered each point on the graph as representative of the air speed in a cylinder of air moving down the pipe Graeme gets about 1180 cfm. Without knowing exactly how he did this I calculated using the same approach and I get about 1239 cfm (pipe is actually 154 mm in diameter).

From standard charts on pressure v air flow in pipes, the typical air flow for a 150 mm pipe under pressure differentials generated by a standard 3 HP DC is typically 1250 cfm, so the integrated result is much more consistent than with the max figure (1600 cfm) quoted above.

I also found 2 small leaks in my setup - I will patch these up and restest.

I have completed the ducting to the BS and lathe - I will post some photos later today.

Hi Bob,
If I am right, I note that the pilot is inserted perpendicular to the air flow, would angling the tube to go with the flow, alter the readings and perhaps make them truer?
Regards,
Bill

Originally Posted by zelk

Hi Bob,
If I am right, I note that the pilot is inserted perpendicular to the air flow, would angling the tube to go with the flow, alter the readings and perhaps make them truer?
Regards,
Bill

Hi Bill,
What you aren't seeing is that the tube tip is bent through 90º so that it goes directly into the flow.

Cheers

Here are a few ducting pics. There is nothing particularly special about the arrangement but as there are not many 6" ducting posts on the forum I thought you'all might like to see what one looks like.

Firstly here is a panoramic shot of the ducting so far.


You can see where the DC enclosure (ENC) is located just outside the back door.
The system will have three arms but so far I have only built most of one arm and half of another.
Nothing is glued together except some parts of the blast gates.
Gaffer tape will be used to to seal the joints so it can be modified as required.
The ducting enters from the enclosure outside the shed via 3 x 15º and one 45º bend and then divides at junction 1.
The left hand arm divides at junction 2 which currently serves the Lathe, and BS and eventually further left the DP.
The right hand arm is still under construction and divides at junction 3 to form the 3rd arm which will continue around to the the right and eventually go down a wall and into the floor to serve my TS from underneath.
The fittings marked "X" are 15º bends which I used to gain height and enable the ducting to run up against the back wall of the shed. The right hand arm thus starts with about a 100º bend which is not ideal but I had little choice if wanted the ducting to run up against the wall.

Some close ups.

Here is the first junction. The blast gates are all home made from PVC sheet.


The lathe collection point. Eventually I will build a round enclosure along the lines that RobboHippy showed in the Truning forum.

The mesh on the intake is essential otherwise if a piece of work or wood falls near the opening it is sucked in and minced by the impeller.

The BS has 2 x 4" ports on it and I added a 3rd 4" flexy as follows.

Firstly a 100 x 150 Y junction is modified by adding a second 100 mm inlet. I can provide more details about how I did this if needed.

Two flexys go to the standard outlets on the BS and one goes via a series of metal loops to 100 - 50 mm reducer that clip onto a pair of 1/4" offset bolts mounted just above the blade.



This flexy rides on the door so if the reducer is unclipped from the pair of bolts it can ride out with the door for easy maintenance. At the same time this flexy serves as a vacuum line for cleaning up sawdust on/inside the BS that has escaped the dust extraction system.


This shows where the standoff bolts are located. As the blade guide goes up and down so do these bolts and the reducer.

As the blade guide goes up and down so do these bolts and the reducer.

Visibility is somewhat reduced but if that becomes a problem the reducer can be unclipped and moved aside.

Overall I am very pleased with the way it is working so far. The downside is that I lose some wall space but I would rather lose wall than ceiling space as I have very little height to play with.

Bob great setup.

With the reducer on the bandsaw do you find going to the 50mm size reduces the airflow too much to make it near ineffective or has the use of the 8" pipe helped keep the air flow to a reasonable level.

Normally a 4" pipe reduced to 2" gets you an airflow that makes it almost useless.

Originally Posted by Jim Carroll

Bob great setup.

Thanks - It's been rollin around in my head for ~2 years so there is a lot of planning involved.

With the reducer on the bandsaw do you find going to the 50mm size reduces the airflow too much to make it near ineffective or has the use of the 8" pipe helped keep the air flow to a reasonable level.

Normally a 4" pipe reduced to 2" gets you an airflow that makes it almost useless.

I agree - the 2" reducer has very low air flow - I will change that to a 3" when I can find one, or maybe make or adapt one.

The mind boggles! What an ingenious setup, and the thought and work put into it seems to have paid off for you.

nice setup, all those fittings must have cost you.

Originally Posted by Gaza

nice setup, all those fittings must have cost you.

Yes the fittings are not cheap but it's not as bad as it seems, especially when the DC itself cost just over a grand.

I buy my fittings from Pipeco WA and they average ~ $10 ea, which is as much as half of what Bunnings charges. I worked out that the ducting (pipe fittings, pipe and fixtures to hold the pipe) will cost just under $500 which is just under half the price of the price of the DC. Fortunately SWMBO sees this as part of the cost of the shed so all is good in that regard.

Hi Bob,
Based on my use of my BS and where I see the dust going I wouldn't do a pickup point on the blade guard as you have it, the blade gullets generate a small airflow in the same direction as the blade, essentially down and into the hole in the table, any particle that has made it past the pickup points below the table then travelled round with the blade then has to do a 180° to get into the guide pickup, I would utilize the 3rd take off point mounted somewhere low down on the outfeed side of the table above the motor with a big gulp style of hood facing up to feed free dust into the 3rd hose, we get assistance from gavity that way, this is by far where most of my escaped dust ends up, then also mount a small fan up behind or above the infeed side (out of operator way) and ideally fed with some fresh outside air via some air con ducting or such, operator is then supplied with clean(ish) air and takes dust in the direction of big gulp pickup, this is another one of my to do mods down the track
What do you think?

Pete

Thanks for the info Pete, I'm not a big BS user so I will defer to your experience on this. I decided on the 3rd placement based on what Bill Pentz recommended although his is on the other side of the blade. My understanding about the placement of this 3rd intake above the table is that it is not meant to be for visible dust but fine dust that defies gravity.

Anyway I will try it out and see what happens. The few times I have used it so far - the main thing I like about it is that I can easily use it as free vacuum line to clean up any escaped dust after cutting. Come to think about it why doesn't every machine have that ?

Hi Bob,
When you reduce the size of your duct would there be any advantage in a tapered transition rather than a sharp step down, to try and get a venturi affect?
I used to own a truck and I modified the air intake from the side of the bonnet to above the cab. This entailed an 8in duct down the back of the cab, then under the floor and up to the air cleaner. There wasn't enough clearance for 8in all the way under the cab and had to reduce to about 6in for part of the way. The sheety, who rolled the ducting, and I decided that we would taper the transition to try and accelerate the air through the skinny section. Aka venturi.
I drove that truck happy in the belief that there was a lot of high speed wind going on under my feet. I even extolled the virtues to other truckies. Delusional or what?
Geoff.