Thread: Something different? Ducting a SHOP VAC – one happy chappy

I have just posted here about my woodworking drill press (and it's vitally connected to this thread).

Hey Bob, I'm thinking about the next phase of ducting over to the other side of the Barn, and there's a couple of things that my wee head can't add up.

You were saying that 50mm, or even better, 60mm pipe would be the go. What I can't make sense of is the drop in air velocity that this would inherently bring (the air speed has to be >=20 mps, or 72 kph). Surely the air speed would drop in linear proportion to the area of the cross-section of the pipe? I'm presuming that setting the vac to the different hose diameters used only changes a scale for the alarm to go off, and that the volume of air moved is still identical, and therefore all that changes is the air speed with different diameter hoses.

Man-O-Meter
Sounds like something to be used in a Mr Universe contest.

I did indeed search the net for some ideas, but came up with very little, other than multi-carburettor synching. I thought it a bit odd that there was no mention of measuring the pressure of a Vac suck. However, the principle is pretty simple, but there's still a brain-blocker there for me. It seems to me that the setup would be something like:

• Get a two metre length of transparent pipe, and attach it in a U to a board that is plumbed vertical.
• Attach a scale (say a one metre rule) to the board
• Put a quantity of dyed water into the pipe
• Attach the vac to one end of the pipe to see how far up the scale it will pull the water, and use some equation to determine the pressure

But this raises some Qs:

• The big one - isn't just going to suck the water into the vac? I can't see how it won't! The CTM is designed as a wet vac, but that's hardly the point! EDIT: one end of the pipe is sealed dummy
• How do I convert the millimeters of movement to pascals/kilopascals (or whatever I need to convert it to)
• Surely the cross-sectional area of the pipe will have a bearing (just like 40,50,60mm pipe will in vac performance)
• Are we talking absolute or differential pressure measuring? Suspect differential, but haven't really got a clue
• The altitude here is 1007m, but I presume this has little, if any, relevance

Cheers
Brett

Brett

I can help with a couple of things here. You don't have to worry about the cross sectional area of the manometer because there is no time or flow aspect involved: Only pressure. The frictional losses due to small pipe diameter are irrelevant in this instance.

This link will help you convert inches of water to KPa. Just scroll down until you find the pressure section.

Online Conversion - Convert just about anything to anything else

It diverts a huge quantity of brain cells to be made available for destruction by alcohol.

You are talking differential and your elevation is purely relative. You would only have to take that into account if you were comparing your system to an identical one on the coast, at Bondi for example, where you would have to adjust for altitude. Although if you lived in Bondi, you would probably not be messing around with wood and standing knee deep in sawdust but swaning around on the beach musing onver the changes in swimming costumes over the last fifty years.

Regards
Paul

Great, thanks Paul. So I guess i can just use whatever size clear tube I can get hold of.

Originally Posted by FenceFurniture

Hey Bob, I'm thinking about the next phase of ducting over to the other side of the Barn, and there's a couple of things that my wee head can't add up.

You were saying that 50mm, or even better, 60mm pipe would be the go. What I can't make sense of is the drop in air velocity that this would inherently bring (the air speed has to be >=20 mps, or 72 kph). Surely the air speed would drop in linear proportion to the area of the cross-section of the pipe? Because area has a squared function (dia^2) any change won't be linear, e.g. if you go from 100 pipe dia. to 200 dia. pipe you have now changed the flow by a factor of 4, e.g. 4 lots of 100 pipe flow will go thru a 200 pipe, (assuming no losses and all things equal) so... 2D=4Q To go from 50 to 60 your vac would need to go up by more than a linear amount, however your vac may have been calibrated to deal with this so switching up a step may be actually producing the required increase in flow for the larger pipe, your vac sounds like a nice bit of gear so chances are it will do that I'm presuming that setting the vac to the different hose diameters used only changes a scale for the alarm to go off, and that the volume of air moved is still identical, and therefore all that changes is the air speed with different diameter hoses.

Man-O-Meter
Sounds like something to be used in a Mr Universe contest.

I did indeed search the net for some ideas, but came up with very little, other than multi-carburettor synching. I thought it a bit odd that there was no mention of measuring the pressure of a Vac suck. However, the principle is pretty simple, but there's still a brain-blocker there for me. It seems to me that the setup would be something like:

• Get a two metre length of transparent pipe, and attach it in a U to a board that is plumbed vertical.
• Attach a scale (say a one metre rule) to the board
• Put a quantity of dyed water into the pipe
• Attach the vac to one end of the pipe to see how far up the scale it will pull the water, and use some equation to determine the pressure

But this raises some Qs:

• The big one - isn't just going to suck the water into the vac? I can't see how it won't! It will if the vac is capable of lifting the height of water in your manometer lets assume you have ~18" of water in each leg (using the 2m length of tube) this is your zero point, attach vac and it will suck the water up one leg while going down in the other (technically not quite right but for ease of explanation) the total difference is your inches of water colum, if it looks like you are going to suck it all the way up then you need more length of tube, there will come a point where it won't be able to lift the water any further, this is the max. static pressure your vac can generate.The CTM is designed as a wet vac, but that's hardly the point! EDIT: one end of the pipe is sealed dummy
• How do I convert the millimeters of movement to pascals/kilopascals (or whatever I need to convert it to) pressure=rho x g x h .......rho density of liquid (water) g gravity 32feet/sec^2 h height of WC units are important so beaware
• Surely the cross-sectional area of the pipe will have a bearing (just like 40,50,60mm pipe will in vac performance) From memory there is something to do with the miniscus (the curve the water makes at the junction of water level to wall of tube) with small dia tubes but use say an 8 or 9 or 10 ish size tube and you will be near enough
• Are we talking absolute or differential pressure measuring? Suspect differential, but haven't really got a clue your just interested in the diff between the hieght of each leg
• The altitude here is 1007m, but I presume this has little, if any, relevance there will be some difference if comparisons are made between 2 systems at different altitudes but like Paul says you are interested in what your system is doing with each change you make

Cheers
Brett

I posted these in a thread a while back but here's a few pics of my static pressure tests
72mm 2.83.JPG100mm 4.JPG112mm 4.5.JPG

Pete

This looks relevant
Pressure

Usually tho all you want is the inches/mm that your system can/is lift/ingand then use that as a number to compare with to see if the change you made (big pipe smaller pipe more flex less flex.........) is better or worse for flow so working out the actual pressure in not really that necessary, once you have the WC height use Bob's chart (somewhere in a thread) to see what flow you have.


Pete

Excellent Pete, thank you. What I meant about linearly proportional was if the cross-sectional area doubles than the air speed would be halved, but totally understand what you mean about squaring.

Originally Posted by FenceFurniture

Excellent Pete, thank you. What I meant about linearly proportional was if the cross-sectional area doubles than the air speed would be halved, but totally understand what you mean about squaring.

Nope, for a given flow of air, then if that same flow is then directed thru a pipe twice the diameter, the air speed will be 1/4 of what it was in the small pipe.



Pete

Originally Posted by FenceFurniture

! EDIT: one end of the pipe is sealed dummy

Cheers
Brett

I am guessing you mean the manometer, one end of the tube remains open and one will be hooked to the vac


Pete

Originally Posted by pjt

Nope, for a given flow of air, then if that same flow is then directed thru a pipe twice the diameter, the air speed will be 1/4 of what it was in the small pipe.

This is only accurate at certain speeds / ducting sizes. At the air speeds and narrow ducting used by vacuum cleaners this relationship is not that accurate.

FF, Pete appears to have answered most of your questions but I will add a few comments

Unless you need to compare with some external values there is no need to use any equations or conversions when measuring pressures with a simple manometer. You can just use the difference in the heights of the two water columns directly in inches or mm directly.

The CTM26 supposed has a maximum pressure generation of 24000 Pa which is equivalent to ~8 ft of water or ~3.5 psi. If your water column is shorter than this it will suck the water back into the vacuum cleaner.

I'm not sure how much of this 8ft or H2O pressure is translated into into real working pressure, you would have to try it out - but start with very tall U-tube. This height will make it very easy to see changes in pressure but may make it impractical to use in a shed. It may be easier to just buy a cheap pressure gauge, maybe even one of those digital tyre gauges would work?

In contrast DCs generate pressures of only around 1 ft of water but most measurements are done in the few inches range. This makes a bench top manometer more practical but pressure changes more difficult to measure.

Pete also refers to a chart that relates pressure differences to flow rates. I can't find it at the moment but I don't believe it applies all that well to the higher pressures and small pipes used in vacuum cleaners as the boundary layer effect will be very large.

Originally Posted by FenceFurniture

Hey Bob, I'm thinking about the next phase of ducting over to the other side of the Barn, and there's a couple of things that my wee head can't add up.

You were saying that 50mm, or even better, 60mm pipe would be the go. What I can't make sense of is the drop in air velocity that this would inherently bring (the air speed has to be >=20 mps, or 72 kph).

OK you need to be very careful about adding large diameter duct to an existing small diameter system that has tight bends and junctions. For example if you already have 40 mm diam ducting with single 90º bends etc from your Vacuum cleaner (VC) to some point in your shed then this as you have already observed will have already have produced significant pressure losses and a reduced air speed. If you now add 50 or 60 mm ducting to the 40 mm ducting the air speed in the 50 mm duct may now be too low and the dust will fall out of suspension.

To get the benefit of the 50 or 60 mm ducting you would have to go all the way back to the VC with that diam ducting.

This means you will have to continue to use 40 mm ducting but this has the disadvantage that too long a length of this will simply throttle the VC. About the longest 40 mm corrugated ducting considered useful for most VCs is only about 5 m. After that, although the VC will still pick up dust the pressure drop will be too high to hold dust in suspension and the ducting will clog very easily. With straight smooth walled PVC pipe the length will be considerably longer but how much longer I'm not sure.

What you have hit is a fundamental problem with VC is that they simply do not move enough air to operate in lengthy ducted mode. The same thing happens with small DCs and long narrow ducting, they look like they are picking up dust OK but after a few weeks or months the ducting slowly fills up with dust and the dust pick up at source becomes feebler and feebler.

Originally Posted by pjt

Nope, for a given flow of air, then if that same flow is then directed thru a pipe twice the diameter, the air speed will be 1/4 of what it was in the small pipe. Pete

. Yes, I said double the area, not diameter.

Bob, this will be a fresh set of ducting coming from the vac, so no problem with the bigger pipe.

Ok, so the other end of the manometer really is open, which is why the tube needs to be, as you say, a very impractical 2.5 metres long for both sides (I knew there was something going on). Clearly if the tube was sealed then it wouldn't drag the water up nearly so far (as a partial vac would be created in the sealed end). That makes me wonder if this may be a better way to be able to use a much shorter length of tube. Or am I up the wrong tree, and the water movement would be bugger all?

Again, as you or someone else said (Paul?), I don't really need to measure the pressure, just compare the following:
height of water with a one metre length of vac hose (for interest)
height of water with a 3.5 metre length of vac hose (for standard)
height of water at other places around the system

Originally Posted by FenceFurniture

Bob, this will be a fresh set of ducting coming from the vac, so no problem with the bigger pipe.

OK.

Ok, so the other end of the manometer really is open, which is why the tube needs to be, as you say, a very impractical 2.5 metres long for both sides (I knew there was something going on). Clearly if the tube was sealed then it wouldn't drag the water up nearly so far (as a partial vac would be created in the sealed end). That makes me wonder if this may be a better way to be able to use a much shorter length of tube. Or am I up the wrong tree, and the water movement would be bugger all?

A simple manometer measures a difference in pressure between both ends so having one open end enables the atmosphere to be used as a ready reference.
If the tube is enclosed at the other end the extent of movement will depend on the volume (and pressure) of the air left at the enclosed end. If there is no air there will be no appreciable movement. If some air is enclosed then the water columns will move to balance the pressures but unless you know or calculate the pressure of the enclosed air then you have no constant "reference point" especially if that air volume leaks. It's just easier to use the atmosphere as the ready reference.

Again, as you or someone else said (Paul?), I don't really need to measure the pressure, just compare the following:
height of water with a one metre length of vac hose (for interest)
height of water with a 3.5 metre length of vac hose (for standard)
height of water at other places around the system

yes for one and two, but not so much for 3 if you are thinking this will allow you to measure pressure losses along a duct. Since the air inside a duct is (apart from turbulent areas) is not significantly differentially pressured, the pressure will be the same more or less anywhere along any a single duct. All you can really measure with an existing fixed (glued) system is the total pressure loss for an individual duct (this also assumes the duct is isolated from other ducts.

To test the pressure losses of the individual components of a system you need a piece of test duct say 1m long and drill a pressure testing point in the middle of that bit of ducting (lets call that press A). Then add a component to the end of the test dust and measure the pressure loss for the combination (lets call this Press B) B-A will tell you the press loss for that component. Pressure testing points need to be in regions of relatively smooth flow, if not turbulence can affect the measurements.

I use the threaded 4 mm micro drop irrigation taps and 4 mm id black plastic tubing for tap off points. The threads are supposed to be 4 mm metric but I have never been able to force one of these micro irrigation fittings into a 4 mm threaded hole in PVC but I find a 5 mm tapered metric tap (with the same pitch) threaded about half way down the tap plus a bit of thread locking glue on the 4 mm micro drop irrigation fitting does the trick.

One very important component to test is the final collector nozzle or tool or machine port as this often generates the greatest pressure drop of all and becomes the rate determining component.

The sum of the pressure drops for each component for a given duct will only tell you if one duct will create a lower pressure drop (and hence greater flow) than another but it will not tell you what the flow is unless all the components are put into a pipe flow calculator or all the components are assembled.

Just how close the total pressure drops can get to the maximum pressure drop has always been difficult to determine. If the sum of the pressure drops starts to comes to within ~75% of the max pressure the VC can generate then one needs to be very careful as a slight blockage will make the dust fall out of suspension and the current air flow may not be sufficient to resuspend the sawdust. hence constricting the duct.

Manometers are indicative rather than absolute devices but they are better than nothing.