Dust collector hose

[Randy Alkins]

Will connecting a 2 1/2" hose to a 4" input cause excess heat to the dust collector by restricting the air flow? Want to use the 2 1/2" at the lathe.

[Chris Fournier]

In my experience I would prefer as large a diameter hose at the lathe as possible. I would stick with 4" as I can't imagine an advantage in using 2.5". Of course I am not you at your lathe either!

[Dan Friedrichs]

No. The blower motor will actually be doing less work since it is moving less air.

[Neil Brooks]

No. The blower motor will actually be doing less work since it is moving less air.

Is that right, Dan ?

Smaller diameter hose should increase the static pressure -- "frictional" losses that the blower motor has to overcome.

In theory, then, increased static pressure should *result in* fewer CFM, but ... I'm not sure it's any less work for the motor. If anything, I'd think the opposite.

[Dan Friedrichs]

Well, the only "work" the blower does is move air. I don't think the work it does is dependent on the SP, only the CFM moved. Obviously, SP and CFM are inversely related, but I don't think there is any way that the blower "knows" the SP being produced. Someone correct me if I'm wrong, but I think the blower just "sees" some air available to be moved, it moves the air, and it took a finite quanta of energy to move said air - independent of SP.

Taking the argument to the extreme, if you completely block the DC inlet, you are moving no air, thus no work is being done, even though a high SP exists. You may think that work is being done to maintain that SP difference, but it isn't - if you draw a vacuum on a bottle and seal it shut, it doesn't take any work to maintain the vacuum, right?

You could verify this yourself with an ammeter, though - just measure the current drawn by the dust collector with 4" hose, 2.5" hose, and with the hose totally blocked. You'd see less current (thus, less power) each time.

[Tad Capar]

I'd side with Neil, but will leave the proper explanation to experts.

[Neil Brooks]

While I do understand what you say, in theory, Dan ... so far ... I still agree with me [].

Quick f'rinstance.

Light up your shop vac.

Place your hand over the end of the hose. Listen to the motor "wind out, in 2nd gear."

While I haven't hooked an ammeter to it ... I have a pretty fair notion of what it's doing: working harder to overcome significant static pressure caused by ... the idiot whose hand is blocking the flow.

I know that ... at a given CFM, higher static pressure = higher amp draw. I don't see any reason for a cheap motor (squirrel cage/low-end furnace blower/ShopVac induction motor) to modulate CFM in the presence of increased static pressure.

My (high-end) furnace, for example, DOES do exactly this. When it sees excessive static pressure, it rolls back CFM.

But ... that's part of the smarts of the system. I KNOW my ShopVac doesn't do this, and I'm pretty sure my DC doesn't, either.

[Neil Brooks]

I got curious, and am Googling a bit.

First room full of engineers -- mostly -- sides with Dan:

http://www.214m.com/html/HVAC%20R%20...415/24227.html

But I'm going to look further. Fascinating, and -- if Dan's right -- not what I would have called intuitive !

[Neil Brooks]

I found this article:

http://met.spsu.edu/dhorton/3343/Fans/Fans%20II.pdf

And ... after five minutes with it ... am pretty convinced that the solid answer is ... "it depends."

Depends on the kind of fan, the kind of motor, and the particular fan performance curve.

So ... Dan ? I call it a tie. Fair ?

[james glenn]

I believe Dan is on the right track.

The shop vac isn't actually working harder when you block the hose because it isn't pulling air
and therefore is louder (higher pitched) because it is under less stress than when actually sucking air.

Just sounds funny to us because we aren't used to hearing that way.

Similar idea with a DC. The motor will spin easier if no air is to be moved. Like it is working in a vacuum.
It's actually easier on the DC.

I found this all someplace before and have heard others talk about it. But can't seem to find the website now.

[Neil Brooks]

James,

I found another site that seems to address this directly:

http://www.greenheck.com/media/artic...erf_basics.pdf

Lemme' know what your take is, after looking at this ... if you're so inclined.

It is pretty cool stuff !

[Dick Thomas]

RE: " While I haven't hooked an ammeter to it ... I have a pretty fair notion of what it's doing: working harder to overcome significant static pressure caused by ... the idiot whose hand is blocking the flow."

Actually, Neil, it's the reverse!! Blocking the inlet - or outlet, no difference - REDUCES the load on the motor (it's working EASIER), because the fan is not moving any air - just spinning it around inside the casing. If you'd like some technical info on the subject, check these EPA websites
http://www.epa.gov/apti/bces/module5.../principle.htm
http://www.epa.gov/apti/bces/module5...ance.htm#brake
As a basic summary, any fan develops maximum static pressure at zero airflow, which is also the lowest HP draw on the motor. Power draw/required HP/current increases with the mass of air moved (= pounds of air). Changing the rpm of a fan wheel also changes airflow. The websites above (and several related ones) gives a whole series of charts (plus those nasty formulas) explaining the relationships among fan speed, pressure, cfm, and power requirements.

When you get into serious design work (done in a past life, retired engineer) you also have to account for altitude - I live at 8300' above sea level, and fans really loose their 'pull' up here!!!

[Alan Bienlein]

I have a 5 hp cyclone and the motor is rated at 20.8 amps. Once I had it and the ductwork installed I connected an amp meter and with all the blast gates closed it only drew about 10.5 amps. With one blast gate open I think it was about 15 amps and with three open it maxed out at about 18.5 amps. I think this should answer your question. The reason a shop vac increases in speed when you block it off is because you remove the load thats being put on it to move the impeller thru the air it's trying to suck in.

[John Lanciani]

I just checked my V3000, results as follows; running with all blast gates closed I get 12.5" SP at the cyclone inlet (Oneida rated at 13") and 8 amps, running with all gates open I get 5" SP and 12.5 amps. The Leeson 3hp motor is rated at 14.5 amps full load.

If a fan is not moving any air, it isn't doing any work. The reason a vac speeds up when you dead-head it is because the fan wheel is turning in a partial vacuum. It has less air to move so it turns faster due to decreased resistance.


Edit - Oops, I guess I type slower than the other guys.

[Neil Brooks]

Dick:

Fascinating !

8,300 ... sounds like Aspen.

I've been having HVAC issues, and -- sadly -- looking into this. I know a furnace needs to be de-rated by 2% for each 1,000' above sea level (I'm roughly a mile up).

Thanks for those links. I'm (not arguing, but) still not entirely sure it's as simple as that, even after looking at those links.

I think one thing we're dealing with, and ... help me if I'm wrong ... is an "all things being equal" equation, but ... in this sort of system, all things are very hard to hold constant.

For instance, on an average DC, DOES the CFM of the fan decrease as SP increases, or ... does the CFM at the 2.5" port, 30' FROM the DC decrease ?

The charts you linked me to seem to bolster the notion that there are a lot of variables at play here, no ?