Results of dust collection mods to Ridgid contractor saw

tvrgeek

Scott
Corporate Member

wolfsburged

New User
Bill
Got a nifty little gauge.
Can't do it without data!

Interesting, that's pretty inexpensive. It looks like it is ~2" diameter. How do you use it in practice? Do you need to build a 4" shroud/reducer to force all of the airflow through it and do you have to do any correction/math?
 

mpeele

michael
User
Interesting, that's pretty inexpensive. It looks like it is ~2" diameter. How do you use it in practice? Do you need to build a 4" shroud/reducer to force all of the airflow through it and do you have to do any correction/math?
I have one of these PyleMeters - PMA90 - Tools and Meters - Temperature - Humidity - Moisture and you just hold it in the flow stream. I'm a little suspect of it though. I had a 1 1/2 hp Oneida cyclone with an internal filter. I would get only a 2% to 3% drop in flow from a 5" duct when the filter was dirty and showing 4 in/H2O drop across the filter. Right after cleaning the filter I would measure 1 in/H2O drop across filter. I have a little more faith in the filter differential but since I built it it too is probably is off since I was just using copper tubing connected to an electronic pressure sensor.

I replaced the 1 1/2 hp Oneida with a 3 hp V system and the meter measures really high flows, much higher than I had expected. I've ordered another pressure sensor since my old one only measures 5 in/H20 and new system will draw much more than that.
 

Alan in Little Washington

Alan Schaffter
Corporate Member
Wow, a lot of fruit being thrown in this basket! Too many apples and oranges in this discussion. I read through everything but don't seem to understand the goal- better dust collection is way too broad, especially when talking about a table saw! Are your goals (1) improve dust collection around the blade, i.e. reduce the amount of fine and coarse dust thrown off the blade, and (2) improve collection in the cabinet, i.e. reduce the amount of dust collecting in the cabinet (and possibly escaping from the cabinet)?

A quick review- dust collectors need both SP (static pressure or simply suction) and CFM to operate properly. SP to overcome friction loses created by ducting/fittings/duct diameter and CFM to move a large volume of air. You can have high SP at zero CFM (the old vacuum cleaner commercial where they pick up a bowling ball)- doesn't do much good for wood dust collection more than an inch or so from the intake. You can have high CFM but relatively low SP like in an HVAC system, but it won't pick up heavy dust. Both attributes can be plotted on a graph called a fan curve that is unique to each motor/blower unit. The point on the curve where your dust collector operates can vary due to ducting, but it is always along that curve. If you have just a short wide pipe running into the blower, you'll have the system’s max CFM but its lowest SP. If you block off most the pipe the SP will rise to its max but the CFM will be almost nil.

The reasons a dust collector needs SP is to be able to pick up and collect dust and chips. The reason it needs high CFM is because while dust is typically generated from a point source on most machines, it can quickly spread out in all directions, including in the direction of the operator. You must collect from all directions- compare the volume of dust-laden air instantly generated at the source to the much larger volume of dust laden air just a few seconds later!! That requires a lot of CFM.

Let's talk about (1) first. If you do the math, a table saw spinning a 10” blade at somewhere around 5500 rpm (typical for a TS) potentially throws dust and chips from blade tips AND gullets at the operator at somewhere around 160 mph (check my math)! No typical dust collector mounted to the cabinet will provide enough SP or CFM at the blade opening to stop this. You need some sort of over-blade shroud to capture the dust and kill the kinetic energy of the dust until it has slowed sufficiently or is stationary so it can be picked up. If the hose running to the shroud is small (e.g. 2.5” or smaller) you’ll need something, like a shopvac, with high SP to collect the dust. Two issues with shrouds- like the cabinet, they must have a source of make-up air- under the lip or through a skirt (like a CNC router skirt) and there are many operations where it just can’t be used effectively.

As others have mentioned you must have the same square inches of opening in the cabinet for make-up air as you do at the dust collection port. Also, remember the area of small openings don’t add linearly due to friction loss, so you actually need more cabinet inlet area than the dust port. If you don’t, you’ll be killing CFM. To ensure dust doesn’t collect in the cabinet, just make the floor of the cabinet slope relatively steeply towards a dust port on a side or preferably at the bottom.

One more comment- you aren't doing yourself a favor by using "low velocity" wyes on your ducting instead of standard wye fittings. They are cheaper, but low velocity wyes generate turbulence that raise SP at the expense of CFM.
 
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Oka

Casey
Corporate Member
Anemometer (anemonmeter) Is one way but it is not real accurate. a better way would be be to use a Pitot (peeto) tube measure. That is what I use for HVAC it is pretty accurate (pitot handheld).
Problem with the little fan type anemometers is they lose accuracy above about 400 cfm. After that they get real wonky unless you can create an ideal measuring condition. The Pitot measures differential pressure which it converts to either feet per min or cfm.
 

tvrgeek

Scott
Corporate Member
Comments addressing several posts:
It would be easy to build a slack tube monometer to verify the drop across the filter and your gauge accuracy. The turbine not only disrupts the flow, you can move it around and see flow is not even across the trunk. I measure three places and average.

Yes, a pitot tube is more accurate, also over 10 times the cost. Most HVAC folks I have seen use the cheap turbines.
Empirical accuracy is not as important as verifying the basic trends. If the inlet to a duct measures 300 CFM, and I improve the ducts so it measures 400, that is of value. Is it really 290 and 402? Who cares? It is accurate enough to verify sufficient duct velocity to not have dropout in the ducts.

My goal is to reduce the fines in the air. Others may have different goals.

Everything produces turbulence.

The design of TS lower blade shrouds is more than lacking. I have a couple ideas, but do not know if it is possible to retrofit. Basically using a ramp on the left side so the blade itself provides some cross-gullet flow as a passive air knife Fine dust is light so not a lot of momentum. The depth of the shroud may also be a problem. Location and shape of the port I question.

Blade guard ports are a serious problem. Much can be done DIY. Look at the expensive SawStop overarm system for a good lesson on doing everything wrong.

What I know is my modifications greatly reduced the dust above the table. I then completed my duct replacement with 6 inch and it helped even more. I am putting a CV-1800 on order today. I thought about a Harvey collector as it could sit under my outfeed table and be a very direct shot to the TS, but it made for far worse to my BS and it's flow is not that great for the one I could afford.

It would be interesting to see objective measures of a trunk with branches vs a plenum with all tools being home-run. I suspect the tradeoffs to not justify the cost of home-run having no branches causing turbulence.

I have not seen for sale, but a lesson in basic fluids tells us one can vary the cross-section of a bend to help maintain the effective consistent impedance. It is easy to do using a two piece stamping. Probably not worth the development cost.

There are some really good CAD simulations for fluid dynamics. Getting popular in the hot-rod world. Better ones are based on MATLAB which I do not have access to.

I have not seen any data if any advantage in spiral duct over HVAC over PVC and if there are any advantages due to breaking up laminal flow. It would not be difficult to manufacture any duct with a dimpled surface.

If one has sufficient gross flow, they can increase spot pressure difference by the venturi effect. There may be opportunity for both lower shroud and blade guard improvements in how the branches are merged.
 

Rwe2156

DrBob
Senior User
Respecting (maybe jealous is the right word) of all your knowledge & please don't take this to be rude, but I've gone down the blackhole of trying to understand the theory and basically concluded it all adds up to a big enough blower and not too many bends.

I think I can sum up Bill Pentz whole website by saying "if it sucks it doesn't suck." :-D
 

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