Back to Dust Collection for me

[Chuck Trisdale]

Boy this is a boring project (but hopefully good for the lungs).

Since I am not at home where the manual is, I thought I would run this question by you guys. Easy Dust collection question - I hope. I have to go down to 2 and 1/2 inches from 4 for some of my tools (and some 1 and 3 inch for that matter). In you guys' experience is it better to go down to 2 closer to the vac or go 4 inch as far as I can toward the tool? Making sense?

[Jim Becker]

Reduce at or near the tool, but you'll also may want to crack another blast gate to reduce the noise.

[Scott Vigder]

What jim said. I've learned cracking a blast gate away from the reduced spot helps the DC system breathe a little easier to reduce motor stress and (YES) noise!

[Chuck Trisdale]

I didnt think about opening another gate, thanks for that. This has not been a lot of fun, but it will make the wife happier - and my lungs.

[Stephen Dixon]

What jim said. I've learned cracking a blast gate away from the reduced spot helps the DC system breathe a little easier to reduce motor stress and (YES) noise!

In a dust collection system, more air is more load, opening a second valve away from your intended DC point increases the stress on the motor. The high pitched whine of a DC with the air pinched down is the sound of the motor free running at no load RPM. As the collector "loads up" the RPM drops and the whine goes away. Works that way with shop vacs too. Not less stress on the motor but a lot less stress on your ears.

Steve

[Ian Barley]

Steve

I would have to disagree with your summary. From what little I know the amount of air is less of a factor than the amount of friction that has to be overcome and the amount of pressure differential that has to be generated.

Think of an impeller revolving in free air. It has to overcome very little resistance and in will overrun its rated speed if left in this state for any length of time. In this situation the motor is doing more "work" (moving air) but is not being done any favours as far as longevity is concerned. There is also no useful vacuum because there is no pressure differential

At the opposite end of the scale an impeller spinning in a totally enclosed cylindrical space will rapidly move all the air available to it in that space effectively stirring the air with the only thing it has to overcome being the friction of the air moving against the sides of the cylinder. Again generally less work than the design spec of the motor. This is the situation which would arise if you blocked off all of the inlets of a system which was otherwise airtight. I call this a collapsed vacuum. The venturi effect is taking place inside the cylinder but it is of no use because the wall of the cylinder is in the way.

At any point between these two extremes we get some pressure differential between the air inside the system and the air outside the system. How much differential - in a high volume low pressure system - depends mainly on the friction in the ductwork. Too much friction and the vacuum collapses. If you run a lovely big impeller and have 1" square on inlet space effectively the impeller uses most of its energy stirring rather than pulling. On my own system the best performance occurs when I have at least one 6" inlet open. If the machine I am using has a 4" port I open a spare. Nothing to do with noise (I wear ear defenders all day anyway). That is the area that the impeller needs to create the best air movement.

Shop vacs are usually set up somewhat differently - operating on a high pressure low volume basis - and the better ones have bypass motors to deal more effectivly with the collapsed vacuum.

If my underatnding of this is wrong I am very happy fo somebody to correct me but just hope that we will all bear in mind that the ability of the system to control dust is inherently more important than its ability to sound good. Wear ear defenders (or plugs, or something!)

[Chuck Trisdale]

Very informative. I had read on here another day that there were some substantive differences in the shop vac and a Dust Collector. I need to get mine sealed up and do some testing!! You were talking about air resistance. I am considering changing to metal until I get to where I need flexible hose to reduce that. Would say 10 - 15 feet more of flexible hose vs 10 feet of smooth make a lot of difference?

[Ian Barley]

Chuck

I have learned a bit the expensive way but am by no means an expert. Rule of thumb that I use is that good quality flex hose creates about 3 times more resistance than fixed duct of the same length. Lower quality probably more.

Ductwork is complex. Bigger is not always better and in a high volume system maintaining speed is as important as providing volume. Bill Pentz's site is a very good source of information. (Google for it if you haven't got it already) As a generality what Jim says is right - run as big as you can for as far as you can. Reduce as close to the tool as poss and keep flex runs short.

FWIW I would think more than twice before hooking any port smaller than 3" to a high volume system. Most kit with ports that size is gonna do better with a high pressure (shop vac) based extraction. I know there are exceptions but they are few.

[Chuck Trisdale]

I was thinking I may need to stay with the shop vac on the smallest machines (routers actually), but I would love to get away from it if I could.. it is so dang loud!!

[CPeter James]

When I run small hose on my DC, I have a bleeder coupling that I put in at the junction of the 2 1/2" and the smaller hose. It is a coupling with 2 1" holes drilled into it. It bleeds additional air into the system to help with the air flow problem. It works!!

CPeter

[Kent Fitzgerald]

Think of an impeller revolving in free air. It has to overcome very little resistance and in will overrun its rated speed if left in this state for any length of time.

Ian, a synchronous AC motor (like that in a dust collector) will not "overrun its rated speed." Think of it this way: when you use your table saw, do you feel compelled to keep it under constant load to prevent "overrunning." Of course not.

Stephen's analysis is correct: In a dust collection system, more airflow is more load on the motor. No airflow = no load = less work for the motor.

Regards,
Kent

[Phil Maddox]

Stephen's analysis is correct: In a dust collection system, more airflow is more load on the motor. No airflow = no load = less work for the motor.

Regards,
Kent

Ken speaks the truth. More air is more work. Less air is less work.

Good luck.

Phil

[Dick Brown]

Not to sound like an expert, but if you put an amp. meter on your dust collector and start opening gates you will soon find out that the more air it is moving, the more load on the motor

Dick.

[Ted Jay]

Steve

...................................

If my underatnding of this is wrong I am very happy fo somebody to correct me but just hope that we will all bear in mind that the ability of the system to control dust is inherently more important than its ability to sound good. Wear ear defenders (or plugs, or something!)

I think it would just be easier to just box it in it's own area, or put it in another room?

[Randy Denby]

I guess that I am somewhat of an expert in that I've been doing HVAC for 28 years.....this said just to reaffirm that more air=more load and vice versa.

I have had people over the years, question me about problems they had when converting an old air handler blower to use as a shop fan.The cause of their problem is rooted in the answer above,If you ever take a squirrel cage blower wheel and use it for a shop fan, be sure to cover part of the inlet venturi. These fans are designed with some restriction expected. If they have no inlet or outlet resistance then they will cycle on internal overload due to too much air capacity.