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 (http://www.airhand.com/designing_charts.aspx#chart2) -- "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%20engineering/20080415/24227.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/articles/Product_guide/perf_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/fans/principle/principle.htm
http://www.epa.gov/apti/bces/module5/fans/performance/performance.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 ?

[Neil Brooks]

Alan and John :

Awesome !

Thanks so much for that. That means I can wrestle with the theory and the explanation for the rest of my life :D, but ... that surely speaks to what happens, in practice.

I guess the non-linear part would be interesting ... that is ... one gate open, two gates open, three gates open, etc.

And ... since we don't know what fan speed is doing all this time, I'll still have to cogitate on this one, at my leisure ;)

[John Lanciani]

Hi Neil,

Fan speed on a dust collector with an induction motor will be between 95% and 100% of rated at all times. When the slip exceeds about 5% for any length of time the motor will not be long for the world as the windings will start heating very quickly due to the higher current.

To save people from having to look it up, slip is the difference in speed between the rotating magnetic field in the motor and the rotor.

[Neil Brooks]

Thanks for that, John.

Okay. Here's where I'm getting confused. Here's a fan performance curve chart. It shows that multiple variables are at play:

http://www.esmagazine.com/ES/Home/Images/es1104-acma-fig7-lg.jpg

Doesn't this say that the direct relationship is TRULY between CFM and BHP, and that -- all things held constant -- BHP will be directly related to amperage ?

It also says -- to me -- that as static pressure increases, CFM increases (certainly a function OF the SP, but is it also related to the motor speed ? You're saying it isn't).

And ... that curve doubles back on itself. After some "sweet spot," BHP goes DOWN.

:confused:

Sorry. HUGE image :eek:

[James Baker SD]

consensus seems to be forming on blocking the intake lessens the work on the motor as it has no air to move. What happens if you block the output? still no air flow from point A (far end of the intake hose) to point B (back into the room) but the motor might be spinning the same air over and over inside its fan housing and maybe still working hard? I've often heard you can reduce power consumption of a pump by restricting the flow thru it, but now I am wondering if that only works if you restrict the intake and not the outflow? James

[Neil Brooks]

Sorry. I'm geeking out, and finding this fascinating.

Here's another chart that tells its own story (googled "fan laws"):

http://www.airturbine.com/images/ms304data.gif

It speaks to Johns point, about constant motor RPM, but ... given that ... as SP rises, so does brake horsepower, which ... should translate into increased amperage, no ?

James: yours is a great question. Dick Thomas is saying ... if I read it right ... input OR output shouldn't make a difference.

[John Lanciani]

I ripped this from Oneida's website, it it their advertised performance for the V-3000. As SP increases, CFM decreases.

Performance:

1285 CFM @ 3” W/C SP
1045 CFM @ 5.4” W/C SP
840 CFM @ 7.4” W/C SP
670 CFM @ 9.2” W/C SP
0 CFM @ 13.1” W/C SP


One issue that I think you're running into is trying to correlate amperage to horsepower. For the small, mostly single phase, motors that we all use in our shops - or any induction motor for that matter- you need to use watts to calculate horsepower since the power factor is such a huge variable in the equation. The power factor could be as bad a .4 for an idling ecconomy motor to as high as .9+ for a high efficiency motor running at full load.

[Dan Friedrichs]

As usual, our profound apologies to the OP for turning a really simple question into an engineering dork-fest :)

Neil, the big fan curve you posted above is not representative of a dust collector. A dust collector's fan curve will indicate that CFM and SP are always inversely proportional. Also, with the induction motor that drives a DC blower, the speed is constant (set by line frequency) - once you eliminate that variable, I think it gets easier.

[Neil Brooks]

It did, Dan, and ... thanks a bunch for that.

I WAS getting tripped up in the multivariate part of "fan laws," generally.

Knowing that ... a few things ARE held constant, in OUR world ... makes all the difference.

And ... yeah ... Sorry, Randy ;)

[Dan Friedrichs]

It is really interesting stuff, though, isn't it? Just imagine how many people get their shop vac hose stuck to a rug (or something) and rush to dislodge it, thinking that they are overworking the vac, when in reality, they were giving it a break!

[Neil Brooks]

It is really interesting stuff, though, isn't it? Just imagine how many people get their shop vac hose stuck to a rug (or something) and rush to dislodge it, thinking that they are overworking the vac, when in reality, they were giving it a break!

Odd you mentioned that.

After our exchange, I tacked an old mouse pad to my shop wall, and have taken to parking my DC AND ShopVac hoses there, in full suction ;)

[Alan Schaffter]

Odd you mentioned that.

After our exchange, I tacked an old mouse pad to my shop wall, and have taken to parking my DC AND ShopVac hoses there, in full suction ;)

Just remember with a shopvac no air flow means no cooling!

[Roger Pozzi]

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.

So,,,,,,,,,,,,,, the OP's answer is,,,,,,,,,,,,no???
Sorry. I got confused.

[Neil Brooks]

So,,,,,,,,,,,,,, the OP's answer is,,,,,,,,,,,,no???
Sorry. I got confused.

LOL !

No problem narrowing the 4" down to a 2-1/2", for use at the lathe.

Apologies :)

[Randy Alkins]

Thank you Roger, I was so confused I was thinking why did I ask?

[Dick Thomas]

Conifer, actually, Neil. And yes, there are a lot of factors involved. The static pressure is essentially the 'back pressure' of the ductwork, or the resistance to airflow. Increasing the ductwork resistance in any way, i.e., longer ducts, smaller ducts, closing gates/dampers, will result in a lower cfm throughput.

With a fixed pipe diameter and length, the static pressure drop in inversely proportional to the air velocity = cfm. Larger pipe = lower static = higher cfm. Smaller pipe = higher static = lower cfm. BUT! It's all dependent upon the fan/blower having high enough capabilities to overcome the system resistance. In designing a fan/duct system, the engineer starts with a building layout and adds the required airflow for each separate space. (In a DC system, that's comparable to an inventory of the tools you'd be connecting to the system and the desired static inlet pressure at each tool.) Using airflows, he'll size the ductwork for a given duct velocity considering space usage and the desired sound levels: hospital rooms need quieter airflow than a machine shop. Now he has air velocity and duct size, which with duct manufacturers data, will yield a static pressure loss for each individual 'stick' of ducting. The longest combined duct length usually yield the highest total static requirement. He then goes to fan catalogs to select a fan wheel/body/style that can provide the total system cfm AT A GREATER STATIC PRESSURE THAN THE HIGHEST PRESSURE LEG IN THE SYSTEM. Thats from the SP vs cfm line on the fan curve. Also, for most industrial size fans the selection step considers the fact that the cfm vs SP line is actually multiple lines, considering using the fan at several different rpm's.
{Note that most lower $$ fans give numbers such as my HF 2HP, which they say produces 1600 cfm and 6.5" Hg = 88" H2O (Surely inflated!!!!). That does NOT mean 1600 cfm AT 88"; It means 1600 cfm at 0 static, open discharge, NO ductwork, NO bags, etc, and 0 cfm at 88". }
Having picked a fan, he'll use the cfm vs hp line(s) on the curve to select a motor to drive the fan.

Thats why the links I gave have so many pages, equations, and charts. My mind is whirling to think I used to work with that stuff every day.

RETIREMENT ROCKS!!!!

[Neil Brooks]

Thank you, Dick !

[OT first: Conifer ... Bailey ... Evergreen. Pretty areas. Don't get there often enough. We're in Fort Collins]

Your summary was excellent, and .... is pretty much in line with the most important thing I've learned in all the time I've been (stuck) looking into HVAC, and ... happily ... into dust collection.

The parallels between the two aren't quite perfect, but are very helpful. In resi HVAC, they ....

Do Manual J calculations, to determine heat gain/loss of the building
Do Manual S calculations to determine the appropriate size of heating and cooling equipment
Do Manual D calculations to properly size the ductwork


With dust collection, however, (you OBVIOUSLY know this. I like to leave these bread crumbs for future Creekers doing searches for dust collection), the "right way" is:


Determine CFM needs for proper dust extraction from each machine
Determine appropriate duct sizing, based ON that CFM and airflow need
Establish a good ductwork layout based on your shop, your machines, your work flow, and about 50 other factors ;)
Determine total static pressure provided BY that ductwork
Pick an appropriately sized DC with a fan curve that will provide adequate CFM at the SP your system design provides


I simply haven't ducted my system, yet, but ... having learned TOO darned much about HVAC, the hard way :rolleyes: -- probably won't make a move until I've Done The Math.

Buy once, cry once ;)

And ... again ... to the OP: apologies, but ... it's not really off topic. I think ... any time a dust collection topic comes up ... it's probably helpful to discuss a basic approach TO dust collection. Once the theory is out there, many parts of designing your system get easier.

And ... it's easier than sending people to Bill Pentz's website, where -- too often -- they're never heard from again :)

[Van Huskey]

Just remember with a shopvac no air flow means no cooling!

Ah, but here like pretty much everything in life a definitive statement usually has caveats...

I think had you written "with a Shop-Vac" you would indeed be correct, without referring to the brand allows your statement to fall dangerously close to implying the more generic "shop vacuum". When speaking about the "species" and not the individual you include the shop vacuums that have separate cooling fans that run independent of the vacuum fan motor and thus aren't negatively impacted by reduced air flow through the hose and vacuum itself. :)

[Neil Brooks]

Ah.

Van.

Welcome to the little Geek-Fest we've got going, here :)

[Ole Anderson]

Heck, I'll join in...


Note that most lower $$ fans give numbers such as my HF 2HP, which they say produces 1600 cfm and 6.5" Hg = 88" H2O (Surely inflated!!!!). That does NOT mean 1600 cfm AT 88"; It means 1600 cfm at 0 static, open discharge, NO ductwork, NO bags, etc, and 0 cfm at 88". }


I am going to assume that HF has a typo and they meant 6.5" H20. Even a lot of shop vacs won't pull 88" of water.

And why does the shop vac with the hose stuck to your pant leg go higher in pitch? Because the motor isn't under load so it speeds up, just like your drill after you pull that big bit out of a piece of Oak.