Once in a while on here we get peaple that want to put the Dust Collector in one room (or even outside the building) and suck air from another.
So I would like to have this idea looked at once and for all. It has been a LONG time sense I did anything with caculating air flow and such (I leave that to the MEP guys) but the following is a place to state.
If we have a shop (say 20′ By 10′ by 8′) and we have a Delta 1.5 HP Dust collector that moves (in a Ideal world) 1200 Cubic Feet per minute (what Delta claims, and NO I do not want to go into that)
So we have a room that has 1600 Cubic feet. So in an ideal world (and not this would never happen) we would change the air out of the room in 1.33 Minutes.
Now in fact that will never happen but just to have a point to discuse lets say we have a change of air about ever 2 Minutes.
Now if we put this outside the room we will need to allow the SAME amount of air (in our case we are calling it 600 Cubic Feet per min) back into the room. If we do not we will either reduce the airflow and thus the effective use of the dust collector or we will start to suck air in in places it was not ment to (such as back down chimenys)
So if we take our 600 cubic feet per min and say we have a window (that has an opening of 2 foot by 3) we would need to move the air at a steady 100 feat per minute Or about (if I have all these number correct) 1.2 miles an hour.
And that is with a hole 2′ by 3′ in the wall. Cut that in half and double the speed of the air.
So if you think about this you need to have a pretty big opening to get the air to move back into the room on its own.
At this point I will hope that I have not forgoten to much of this in the (about 20 years) time that I have not used it after being shown it in school.
So if one of you that may know something about Airflow and Make up air and air preasure and static preasure and all that fun stuff could add into this it would be a big help.
Like I said I would just like to have this conversation once so that we can point to it whenever anyone wants to move the dust colector away from the rooms air source. That way we can point them at this thread and they can decide for themselves what they want to do. But I would hate the idea that someone hurt themselves or others because of this issue.
Perhaps the powers that be would get one of thier peaple from the FHB side to do a short article about this in FWW if they look at the numbers and think that it is a hazzared (or they could look at it decide it is not a problem and tell me to shut up 🙂 At which point outside the garage the old DC goes!)
Thanks for any and all help.
Doug Meyer
Replies
Great timing, Doug, as my husband is suggesting mine go outside, since the lathe and "new" scroll saw have pushed the space past the limit! Looking forward to hearing what people have to say.
Great post Doug. Very thoughtful and logical. It always pays to "do the numbers".
I don't know if you remember, but you were going to come see my "small shop" with a FULL complement of iron. I live north of Ann Arbor.
Frosty
"I sometimes think we consider the good fortune of the early bird and overlook the bad fortune of the early worm." FDR - 1922
Yeah I keep meaning to do that. But it seams like my time is more and more busy. Perhaps the end of the month or more likely the beginning of Oct (say the first weekend maybe?
As for the issue with the DC I hope a true expert chimes in. I am an Arch Designer not and MEP guy but I do work with these guys a bit and the issues with what they call Cold Air return is a big one so that is what makes me scared of the idea of putting the DC out of the room. I would hate to suck the fumes from something into the Shop (or the house, the air has to come from some place. Besides it will slow the DC down I would imagine quite a bit.
Doug Meyer
Doug,
Great discussion!
You can be sure I will be following this one as I plan, some day, to outboard my DC. I do however have a 44" x 72" door just in back of my TS that should allow a significant volume of fresh air into the woodshop, so I don't anticipate any problems.
Thanks again.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Yeah, it would be nice to get some "one size fits all" answers, but I don't see how that's possible. There are way too many variables in the equation(s).
The single biggest variable would be the specific arrangement of your DC system. Length of your duct run(s), type of duct material, number and location of elbows, sweeps, tees, blast gates, etc. will affect the performance of your DC. Change any one of these parameters, and you'll probably change the system performance.
I don't have a problem with the maunfacturers flow ratings - as long as they're measured under identical circumstances. It's my understanding that these ratings are determined using new machines with clean bags (or filters), and no ducting. It's sort of like the EPA mileage ratings on cars and trucks - a baseline test under identical conditions to provide relative comparisons. Your real world mileage will vary.
If you and I had the same DC, we would probably see substantially different performance levels depending on the ducting systems we use, the cleanliness of our bags or filters, and even the "tightness" of our ducting connections. (After I wrapped all my duct joints with 2" electrical tape, the air flow at my blast gates increased substantially).
Now, having said all of the above, I can assure you that you're going to need makeup air when you're running your system. The amount you'll need, however, isn't determined by the CFM rating of your DC, but rather by the actual flow rate through whatever blast gate(s) you have open at that time. That flow rate will, in turn, be dependent on your duct system, the cleanliness of your bag or filter, and even the age of your DC.
Personally, I would love to move my DC outside. It isn't terribly noisy, but I really don't want to annoy the neighbors more than I already do when I fire up my surface planer or jointer in the driveway. - lol
The systems I have seen outside have insulated ductwork and a return air register back to the shop. This way your heated or cooled shop air is returned in full/equilbrium.
Edited 9/9/2007 10:15 am ET by DonC
My DC is in an outside closet, not realy a room. I have vents for air movement in the floor of the closet. There isn't any heat or air to my shop so what is the issue here.
Even a dinky little 200-cfm kitchen ventilator needs a make-up air source in a tight modern house, so yes, you absolutely do need to consider make-up air when you have that much air flowing "through the wall," so to speak. Unless you live in a climate that's temperate all year round, I think the only solution that makes sense is to put the DC in a closet, with return air coming back out of that closet, or to at least enclose the DC exhaust in a plenum of some sort that is routed back to the shop.
-Steve
Which is the point. In my new house I have to have a source of Make up air (required by code) for a High Efficiancy Water Heater.
So moving upwards of 1000 cfm out of the building has got to be bad. And While I realise that once size will not fit all. if it is bad to move 800CFM then anything above that would be worse, and while the older houses are not as tight as the newer ones they still would have issues. Air will return down the easy route if it can (say down the fire place flue vs up.
This is the type of thing that an expert can tell us.
Doug Meyer
Doug -
You're exactly right about makeup coming from somewhere, but the questions become:
how much makeup air is needed?
how much of that will come from "natural" sources?
The answer to the first question is dependent on your specific DC system and how you're using it. If you only open one blast gate at a time, your makeup air flow rate (in cfm) is equal to the flow rate (in cfm) at the open blast gate. Changing blast gates will probably change the system flow rate - and the makeup air requirement.
In a garage shop, I would guess that "natural" air inflow would be more than sufficient. Garage doors aren't usually "weather tight" and do little more than keep out the rain. You will probably pull as much makeup air as you need from around your garage door.
"In a garage shop, I would guess that 'natural' air inflow would be more than sufficient."
That's all well and good in a temperate climate, but I can tell you from personal experience that you really don't want to do that when it's 10°F outside.
-Steve
Steve -
I knew that someone would eventually bring outside temperature into the discussion. - lol
Although it's a very valid point, it's secondary to the basic problem of getting sufficient makeup air to compensate for the amount of air being removed from the shop space. The occupant(s) of the shop will care about the temperature of the incoming air, but the DC system couldn't care less.
Once you know the amount of makeup air you need, then you can decide where it will come from. Since garage doors are seldom "weather tight", the amount of air flow around them isn't easily controlled so you're gonna get makeup air at whatever temperature you have outside - whether it's 110* or -10*. In my experiemce, the air flow around a garage door more than makes up for the volume of air being pulled out by the DC system. It isn't always comfortable, but it's enough.
If, on the other hand, you can seal up the garage door (and any other uncontrolled sources of airflow), you would need some kind of makeup air system. It could be as simple as a gated duct to the outside, but if temperature control is necessary, you would need an air-to-air heat exchanger to pre-heat (or pre-cool) the incoming air. The size of the system (and heat exchanger) will depend on the "worst case" air flow of your DC system.
The Issue I have (and unless and expert can chime I I dont think we will settle) is that I think that make up air (if the unit is out of the room) is a LOT more then most peaple think. Yes the make up air will egual the exact amount of air being pulled at any open blast gate and thus be a variable. But when you are talking about a target number of 600 CFM and up I think this is something that moves past what can be assumed to be coming into the building by normal means (gaps in doors and such)
And if it is more then what is available normally it will start to either starve the DC or it will pull it from places you do not want (like exhaust from a Furnace or fire place) and that is where it will get nasty.
Doug
It is for all of these reasons that I think venting the DC to the outside world is a non-starter in a not-so-temperate climate, and maybe not so good an idea even in a temperate climate, unless you can provide a known make-up air source (rather than relying on "leakage").
But there's a simple solution: Enclose the DC exhaust in a plenum, and route that back into the shop. With a few baffles and some sound insulation, you should be able to get the air back without the noise.
-Steve
The only reasons I would move my DC outside would be because of the noise or to gain some valuable floor space.
Everything I've been saying is only applicable to a garage shop. Unless the garage door is somehow sealed off, adding some kind of makeup air supply (and heat exchanger) is probably a waste of time and money. That garage door is going to supply (i.e. leak) all the makeup air a DC system can use and the temperature is gonna be whatever it is outside.
As a resident of one of those "temperate climates", my garage door is wide open 90% of of the time - year round. But, even when I lived in less temperate areas (MO and NY), my garage/shed was never very much warmer than the outside air.
In my experience there seems to be a rapid degradation of suction the further away from the source of a DC duct that you get. Also their are air leakage areas around the point of entry to the DC hose.
I'm curious as to how this effects return air requirements?
Regards,
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Edited 9/11/2007 9:22 pm ET by KiddervilleAcres
It's all about airflow. And airflow depends on pressure differential.
Airflow through a pipe depends on pressure differential from one end of the pipe to the other. The greater the pressure differential, the greater the airflow. Likewise, airflow through any given DC depends on pressure differential between the input face of the DC blower and the output face of the DC blower. The pressure differential across the DC is actually a negative number, since the DC is acting like a pump and pumping the air "uphill," but the rule still holds: The greater the pressure differential (where in this case "greater" means "less negative"), the greater the airflow.
Let's first attach a simple pipe to the input of a DC. The airflow through the pipe must, of necessity, be exactly the same as the airflow through the DC (barring leaks). The pressure at the input end of the pipe is atmospheric pressure, since the pipe is open to the room. Therefore, the pressure at the input of the DC must be lower than atmospheric pressure (or else there wouldn't be any pressure differential across the pipe, and thus no airflow through it). And when you lower the pressure at the input of the DC, you decrease the pressure differential across it, causing the airflow through it to go down.
That's why adding a pipe to the input of a DC will decrease the airflow, all by itself.
Now let's consider a slightly different scenario: We have the DC plumbed through the wall of our garage, so that the input is inside the garage and the exhaust is outside. We've removed the pipe on the input, so we're looking at raw DC airflow.
First, we run it with the garage door open. The garage acts like a really, really big pipe, so large in fact that it requires only an infinitesimal pressure differential to support a very large airflow. So we can assume that the input of the DC is, for all practical purposes, at atmospheric pressure, and the output of the DC is also at atmospheric pressure, since it's vented outside the garage. The pressure differential across the DC is thus zero, which is the maximum (best) case for a DC: It will actually pump as much air as the manufacturer claims it will.
Next, we close the garage door. This is like closing off the end of the pipe with a blast gate, converting it from a really, really big pipe into a not-so-big pipe. In order to support the same airflow, we need a larger pressure differential. (To look at it another way, the DC starts to suck air out of the garage, until the pressure inside the garage is enough lower than the pressure outside to sustain the airflow through the gaps around the garage door.) This reduces the pressure at the input of the DC, and therefore reduces the airflow.
Finally, we seal the garage door (and any other leaks) extremely well. So well, in fact, that the pressure inside the garage goes down so much, and consequently the pressure differential across the DC goes down so much, that it stalls--the airflow goes down to zero.
Bottom line: Relying on leaks to provide make-up air reduces airflow through the DC, because the garage itself starts to act like an additional pipe added on to the system. Is it enough to worry about? That depends on the size and shape of the leaks. If the total cross-sectional area of the leaks is large compared to the cross-sectional area of a typical duct, then it's of no concern. It only becomes important when the size of the leaks is small compared to the cross-sectional area of a duct, which wouldn't happen unless you had a very tight seal around the door.
-Steve (former physics professor, sorry)
Steve,
Holy bat sh*&man!
I actually understood that! Makes all the sense in the world. Thank you for the great explanation.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
>It's all about airflow. And airflow depends on pressure differential.
Uh oh...
>Airflow through a pipe depends on pressure differential from one end of the pipe to the other. The greater the pressure differential, the greater
Airflow would be a product of pressure diff and cross section, minus the effects of turbulent flow.
>the airflow. Likewise, airflow through any given DC depends on pressure differential between the input face of the DC blower and the output face of the DC blower. The pressure differential across the DC is actually a negative number, since the DC is acting like a pump and pumping the air "uphill," but the rule still holds: The greater the pressure differential (where in this case "greater" means "less negative"), the greater the airflow.
How is the sign of this arbitrary pressure measurement relative?
>Let's first attach a simple pipe to the input of a DC. The airflow through the pipe must, of necessity, be exactly the same as the airflow through the DC (barring leaks). The pressure at the input end of the pipe is atmospheric pressure, since the pipe is open to the room. Therefore, the
If that were true, no airflow would exist TO the input end of the pipe from the room and no dust would be collected and no makeup air would be required. ;) At steady state, the pressure at the input end of the pipe would be little more than that at the DC input. That is what supports the actual collection part of Dust Collection. The atmospheric pressure exists outside the tool, on the other side of the dust.
>pressure at the input of the DC must be lower than atmospheric pressure (or else there wouldn't be any pressure differential across the pipe, and thus no airflow through it). And when you lower the pressure at the input of the DC, you decrease the pressure differential across it, causing the airflow through it to go down.
How does the pressure diff decrease when you lower the pressure at the input of the DC relative to atmospheric conditions at the output? That would increase the pressure across the DC.
>That's why adding a pipe to the input of a DC will decrease the airflow, all by itself.
It should not. If the pipe is clean and straight and the same cross section as the DC input opening, it offers no resistance and will not decrease the airflow. No such pipe exists in a woodworker's shop. In practise, they all have defects: not straight, obstacles on the inside, ribbed, reduced size, etc.
>Now let's consider a slightly different scenario: We have the DC plumbed through the wall of our garage, so that the input is inside the garage and the exhaust is outside. We've removed the pipe on the input, so we're looking at raw DC airflow.
>First, we run it with the garage door open. The garage acts like a really, really big pipe, so large in fact that it requires only an infinitesimal pressure differential to support a very large airflow. So we can assume that the input of the DC is, for all practical purposes, at atmospheric pressure, and the output of the DC is also at atmospheric pressure, since it's vented outside the garage. The pressure differential across the DC is thus zero, which is the maximum (best) case for a DC: It will actually pump as much air as the manufacturer claims it will.
If the pressure differential was zero, there would be zero air flow. A DC works by creating a large differential between the input of the blower and the output (thanks to metal 'hands' pushing scoops of air from one side of the unit to the other). This creates a large pressure differential between the intake of the pipe and the intake of the blower. There is another pressure differential on the output. Obstructions on the incoming air (including smaller pipes, bends, cats, etc) on the input will reduce the airflow. The metal hands can only move air they can reach. Obstructions on the outgoing air (including filter bags) will also reduce the airflow. The metal hands can only push so hard on the outgoing air (function of impeller and motor). The DC will push claimed air flow when there are no obstacles to air flow.
>Next, we close the garage door. This is like closing off the end of the pipe with a blast gate, converting it from a really, really big pipe into a not-so-big pipe. In order to support the same airflow, we need a larger pressure differential. (To look at it another way, the DC starts to suck air out of the garage, until the pressure inside the garage is enough lower than the pressure outside to sustain the airflow through the gaps around the garage door.) This reduces the pressure at the input of the DC, and therefore reduces the airflow.
Quite simply, closing the garage door will provide an obstruction to air flow. With gaps still around the door, ANY pressure in the garage lower than atmospheric will support air flow through those gaps.
>Finally, we seal the garage door (and any other leaks) extremely well. So well, in fact, that the pressure inside the garage goes down so much, and consequently the pressure differential across the DC goes down so much, that it stalls--the airflow goes down to zero.
The pressure in the garage would drop to that of the input of the blower. That would not be zero, and most likely, you will not stall a 1.5hp+ motor. The inefficiencies of the impeller will allow it to turn. Try blocking the input to your DC and see what it does. Same thing, different time scale. You'll just reach steady state conditions faster.
I don't understand why you would say that the pressure differential goes down when the pressure inside the garage goes down and atmospheric outside remains the same.
>Bottom line: Relying on leaks to provide make-up air reduces airflow through the DC, because the garage itself starts to act like an additional pipe added on to the system. Is it enough to worry about? That depends on the size and shape of the leaks. If the total cross-sectional area of the leaks is large compared to the cross-sectional area of a typical duct, then it's of no concern. It only becomes important when the size of the leaks is small compared to the cross-sectional area of a duct, which wouldn't happen unless you had a very tight seal around the door.
Any garage door I've ever seen in an unheated garage has gaps much larger than a typical DC input. In my climate, there are people who heat their garages (wood stove, gas) and add insulation and seals to the big doors. That would obstruct the air flow of a DC.
-Steve (former physics professor, sorry)
The reduction of air flow (at the tool) the further from the DC can be blamed on piping leaks and obstacles. Does the air have to change direction? Does it have to negotiate screws, partially closed gates, wires, irregular pipe walls? I'd bet my cheapo, wire-lined PVC pipe would work better if I stretched it *longer* since there would be fewer obstacles. That would mean greater cross section and less turbulent flow fighting the laminar flow.
Andy (never a professor and would rather see people get straightforward answers they can understand and use, sorry)
Damn or is it DAMKHIT (What does that mean?)
Just when I thought it was safe to cross the street, here comes Andy in an 18 wheeler full tilt! :-) Just kidding.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Keep a few things in mind like water air wants to find its own balance. In general we call this atmospheric preasure, and in fact the DC creates a negative number to get airflow (vs your compressor that creates a positive number) and when given a chance the numbers try to balance out.
Thus when you put the DC outside the shop (with no return) you have to figure the differnece between inside pressure and outside (what the DC is running in) and then take what the DC can generate and that gives you the differnece and will effect the air flow.
So in an ideal world if you get to the point that thier is little to know difernece between the DC and the opening it is sucking at it will not move air. Please keep in mind that the resistance of the pipe will effect this also.
Now the issue is, that the room that you are creating a vacum in (well trying to anyway) will try to suck air in from any place it can. This will happen in the cracks in the wall and around the door and even (a vary small amount) will flow in around any loose fitting wall structure. But it will pull from the path of the least reistance the most (untill it equals the resistance of the other paths) now when you have all sorts of other options for air flow like say a wood bruning oven this can be an issue.
Doug
Just when I thought it was safe to cross the street, here comes Andy in an 18 wheeler full tilt! :-) Just kidding.
Yeah, well I hadn't taken my chocolate that morning... just gave me a flashback to all the lecturers I had that overcomplicated the subject. In their case I guess it was job security. Not sure why people do it here. ;)
Andy
Andy,
This is a very informative discussion for me, I think. I'm not an engineer so am having difficulty understanding some of the technical jargon.
What I think I'm hearing is that shorter is better, with respect to the ducting in the DC? Also, that smoother or less angular turns are better as is smoothness of inside walls all around?
Regards,
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Edited 9/13/2007 9:10 am ET by KiddervilleAcres
Edited 9/13/2007 9:10 am ET by KiddervilleAcres
"What I think I'm hearing is that shorter is better, with respect to the ducting in the DC? Also, that smoother or less angular turns are better as is smoothness of inside walls all around?"
Shorter and smoother are better, yes. But you have to be careful how you define "shorter." You can't just take a measuring tape to your ductwork; you have to multiply the measurements by factors that depend on the geometry. From an efficiency point of view, a 90° elbow is much "longer" than the length you measure with your tape, for example.
You can sometimes find "effective length" values listed in ductwork catalogs. A 4" 90° elbow that's physically only 10" long might be listed as having an effective length of 35", for example.
-Steve
Steve,
In the case of a 90° turn I've seen, what I would call an elongated turn (Told ya I ain't no engineer!) that is supposed to lessen the negative impact on a 90° elbow, if you will.
Also, if I remember correctly it also makes the effective length less than the elbow but gets you around the turn so to speak.
Next question: What effect does a reduction in diameter have in terms of its length, i.e. the reduced diameter length should be kept as short as possible?
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
"In the case of a 90° turn I've seen, what I would call an elongated turn (Told ya I ain't no engineer!) that is supposed to lessen the negative impact on a 90° elbow, if you will."
Right. All else being equal, the larger the radius, the better. Ideally, you don't want "bends" at all, but rather long, sweeping curves. (If only you could get the ductwork manufacturers to fabricate long-radius curved ducts to spec, at a reasonable price.) You can sometimes approximate such a long, sweeping curve by replacing a 90° bend with two 45° bends separated by a length of straight pipe, or maybe go even further and use three 30° bends.
Although the physics is more complicated because of turbulence, it's analogous to rounding a corner in your car. Race car drivers learn to take the proper "line" through a curve, which happens to be the one that maximizes the radius of the path of travel.
"What effect does a reduction in diameter have in terms of its length, i.e. the reduced diameter length should be kept as short as possible?"
Right. The basic rule of thumb that you can apply here is that for a given flow rate, you need a minimum diameter of pipe, but once you reach that diameter, there's no real advantage to increasing the diameter any further. I did some more calculations:
Example 1: Round pipe, 10 ft long, 1000 cfm airflow
pipe diameter
pressure drop
3"
51.1" WC
4"
12.8" WC
6"
2.2" WC
8"
0.7" WC
10"
0.3" WC
Example 2: Round pipe, 10 ft long, 500 cfm airflow
pipe diameter
pressure drop
3"
11.2" WC
4"
3.2" WC
6"
0.6" WC
8"
0.2" WC
10"
0.1" WC
So you can see that "big enough" depends on the airflow. At 1000 cfm, the cutoff is around 6-7" diameter. At 500 cfm, it's around 4-5" diameter.
-Steve
Steve,
I'm sorry but even if I knew Greek I don't think I could understand your post. Not a reflection against you as I ain't no ingneer but more likely a reflection on me. That said, let me try to clear the air. With respect to the topic, pun IS intended!
The DC I am getting has a 6" Y connector with 2 4" inlets. One of the 4" inlets which will be blast gated, will be dedicated to the TS/Router (housed in ext. wing of TS). No active duct will be longer than 7'.
The other 4 " will have a length of hose (~ 8' ) to connect to Planer & Jointer (all mobile) on an as used basis,otherwise gated shut.
My concern is the 2 4" inlets going into the 6" inlet on the DC. I rationalize this to be a potential blivit (10 lbs. of dung in a 5 lb. bag). I'm thinking that I would be better off running a 6" hose from the DC to each machine on an as used basis. The connectors on the Planer & Jointer being 4" so a 4" to 6" adapter will be necessary of course.
Does this make sense? Also, my woodshop is small (16' X 20') and all machines are in an island arrangement in the center of the shop.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
"I'm thinking that I would be better off running a 6" hose from the DC to each machine on an as used basis. The connectors on the Planer & Jointer being 4" so a 4" to 6" adapter will be necessary of course."
It's possible that you'd see some improvement, but my bet would be that it would be minimal. The fact that the manufacturer installed 4" ports on the machines suggest that neither one can use more than 500 cfm. If that's the case, then you won't get much improvement by going to 6", unless you're running a really long line.
What is the rated airflow of the DC?
-Steve
Steve,
It's rated at 1,200 CFM.
http://www.newwoodworker.com/reviews/d50-760rvu.html
Regards,
Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Edited 9/13/2007 2:56 pm ET by KiddervilleAcres
"It's rated at 1,200 CFM."
In that case, I'll bet you could have both machines connected with (short) 4" lines and running at the same time, and have performance that's not significantly worse than a single machine connected with the same length of 6" line.
-Steve
What I think I'm hearing is that shorter is better, with respect to the ducting in the DC? Also, that smoother or less angular turns are better as is smoothness of inside walls all around?
You're hearing it exactly right, Bob. In any piping or ducting system, flow rate is dependent on the length of the pipe (or duct), the "straightness" of the system (changes in flow direction will eat your lunch), and the "smoothness" of the system walls. Maximum efficiency (i.e. max flow rate) is obtained when the flow is as laminar as possible. Turbulence within the system reduces the fluid flow.
Just for fun, Google Bernoulli's Equation. It completely describes the factors involved in fluid flow. I haven't used it for several years, but back in the day, I could make it sing - which helped me pass the mechanical P.E. exam on the first try - lol
Hi Dave,
I'm curious to know if air swirls in a spiral fashion the same as water in a drainpipe where gravity is invilved with the movement of the water.
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Good Grief, Bob, how would I know!!? - lol
I've never looked into that question, but I would doubt if would. Liquids have a property called surface tension which (I suspect) is a significant factor in the "swirling" action we see.
Since gases (e.g. air) don't have this property, and their density is much less, gravity would have little effect. In fact, without some outside force, air won't "drain" out of a pipe.
Dave,
Gotcha!
I'm hoping to use clear ductwork so I can see where/if there is any clogging up in the lines. I'll let you know if it swirls. :-)
Regards,Bob @ Kidderville Acres
A Woodworkers mind should be the sharpest tool in the shop!
Air certainly swirls (picture the smoke rising from a cigarette sitting on the edge of an ashtray). And gravity can be the force that causes the swirling. But gravity doesn't have much effect in a DC duct. Instead, the force that causes the swirling there is produced by the pressure differential.
-Steve
"...a flashback to all the lecturers I had that overcomplicated the subject. In their case I guess it was job security. Not sure why people do it here."
Let's say you want to build an airplane. You could follow someone else's plans, and if you were careful enough (and assuming the plans were decent!), you would end up with a working airplane, without having to know anything at all about how airplanes work.
Or, you could learn how airplanes work, and then you would have the knowledge to modify an existing design, or design one from scratch.
Either way, you end up with a working airplane. Neither way is "better" than the other; they're just different means to an end. The second way does have one notable advantage, though, in that it better prepares you to cope with the unexpected. (On the flip side, it has the disadvantage that it can make you overconfident.)
From Bob's question, I got the impression that he was trying to understand the problem better, not just get a "Do this and you'll be fine" answer. That's why I gave the reply that I did. It's not a matter of overcomplicating the subject--the subject is plenty complicated to begin with! Rather, it's always a matter of deciding how much simplification to apply. The balance one has to reach is to simplify enough, but not oversimplify.
-Steve
You're talking about the difference between trade and design. That's not what I meant by overcomplicated. You can explain things two different ways and the student could learn the same principles. There is always a *long* way to describe the same principles. I've seen many profs describe things in a very roundabout way that is very difficult to follow and there were more direct, faster ways to get to the same place.
With respect to the fact that you were trying to help Bob (I've taken my chocolate today), I think you overcomplicated the answer to the point of inaccuracy. That does not help the person trying to understand the principles involved in fluid dynamics. That was my point. I don't even like the topic. Let 's talk about power tools.
Andy
"Airflow would be a product of pressure diff and cross section, minus the effects of turbulent flow."
I'm not sure what you mean by "minus the effects of turbulent flow," but sure, of course. However, that is completely orthogonal to what I said. The magnitude of airflow certainly depends on lots of factors, but the fact that the air flows at all depends on pressure differential. And for a given piece of plumbing, the greater the pressure differential, the greater the airflow (up to a limit which is is far beyond what could ever be acheived in a DC setup).
"The atmospheric pressure exists outside the tool, on the other side of the dust."
Again, I'm talking about a plain ol' piece of pipe. No machine (and no dust). But you did catch me in an oversimplification. When I said the "input end" of the pipe, I really meant in the far field, beyond the turbulent flow region. (And the same goes for the output end.) To get an idea of what that means, try this simple experiment: Sprinkle some sawdust more or less uniformly across the floor. Take the hose from your shop vac and lay it down on the floor. Now turn on the vacuum. The sawdust in the vicinity of the end of the hose will get sucked up, but beyond a few inches, the dust won't budge. This boundary is what I mean by the "input end" of a pipe (to a very rough approximation, it corresponds to the laminar/turbulent flow boundary).
"How does the pressure diff decrease when you lower the pressure at the input of the DC relative to atmospheric conditions at the output? That would increase the pressure across the DC."
I misstated that. I meant to say that by decreasing the input pressure, you're decreasing the pressure differential across the upstream (low pressure) side of the blower, not across the entire blower. I could draw some pictures to show how this all works, but I won't do that unless people are really interested.
"If the pipe is clean and straight and the same cross section as the DC input opening, it offers no resistance and will not decrease the airflow."
No, that's not true. Think about it. What is the motive force that makes the air flow? Answer: pressure differential. If there were no pressure differential, the air would just sit there.
I did some calculations, based on an airflow of 1000 cfm, through a "perfect" pipe, 6 ft long and 6" in diameter. The Reynolds number under those conditions is about 300,000, so we're well into the turbulent flow regime. And the pressure drop is about 0.3" WC. Not a huge amount, but certainly measurable, and enough to reduce air flow by a few percent. At that Reynolds number, the boundary layer is quite thin, about 0.006", and the pipe has to be smoother than that in order to be able to make the "hydraulically smooth" approximation. Of course, a real pipe is nowhere near that smooth, and even under good conditions you're going to have a pressure drop of 2" WC or more. And that's clearly enough to make a significant difference in what the DC will pull.
To get a better idea of how much pressure drop even a "perfect" pipe produces, I looked at the fan curve for the Oneida Pro 1500, and did another calculation: A 500 ft length of "perfect" 6" pipe would reduce the airflow from a wide-open 1600 cfm to about 600 cfm. A 4" "perfect" pipe would only have to be 75 ft long in order to have the same effect. Not enough to be worrisome in most practical situations, but there nonetheless. (And it does illustrate how important it is to correctly size the pipe to the air flow.)
"If the pressure differential was zero, there would be zero air flow."
In a completely passive system (like a pipe), yes. But a DC isn't passive. And again, I'm talking about the pressure differential outside of the turbulent flow regions, in what you might think of as the "environment" of the input and output.
"The pressure in the garage would drop to that of the input of the blower. That would not be zero, and most likely, you will not stall a 1.5hp+ motor."
I didn't say that the motor would stall. I said that the airflow would stall (that is, go to zero). Even the wimpiest DC motor can't be stalled, even if you completely close off the airflow.
"I'd bet my cheapo, wire-lined PVC pipe would work better if I stretched it *longer* since there would be fewer obstacles. That would mean greater cross section and less turbulent flow fighting the laminar flow."
You're never going to be in the laminar flow regime in a DC setup. The flow rate through a 6" pipe would have to be less than 8 cfm in order to reach laminar flow.
-Steve
Edited 9/12/2007 9:24 pm ET by saschafer
One thing I have not seen mentioned is that "Air is compressible." Liquids are not."Frosty"I sometimes think we consider the good fortune of the early bird and overlook the bad fortune of the early worm." FDR - 1922
Liquids are compressible -- just not as much as gases. Otherwise, we wouldn't have mercury thermometers. Not sure what this has to do with woodworking though.
Andy
"One thing I have not seen mentioned is that 'Air is compressible.' Liquids are not.'"
Yeah, it's difficult to conduct an entire fluid dynamics course in a Knots thread....
Short answer: It matters when you're worried about the actual numbers, such as when you're calculating the pressure drop in a pipe. You'll get the wrong answer for air if you don't consider compressibility. (Although in the case of dust collection, it won't be wrong by much, since dust collection works at very low pressure differentials.) It doesn't matter much at all when you're comparing two or more scenarios; e.g., for a given fluid, a longer pipe is always worse than a shorter pipe, as long as all the other parameters remain the same.
-Steve
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