Thread: Pressure drop off in a shop vac hose

Have you read Bill Pentz's website yet?

When I asked the vacuum cleaner shop people a similar question recently, they told me that they don't really want to sell more than 5m of hose.

U did mean shop vac not dust extractor didn't u?

No, haven't read Bill's site because I'm talking a shop vac, not a big DE. I have got a 3.75 hp DE (unused as yet) but it will be connected directly to the one or two machines it can be used on (it's a 5x3 metre shed, so not a lot of machines can fit in).

It's not a single number because it depends on too many variables.

Pressure drop is proportional to
- velocity squared
- fluid density which varies with temp
- pipe length
- a friction or viscosity factor

And inversely proportional to 2 x diameter of the pipe.

If you know what all those are for your system and keep your units under control you can work it out. The easiest thing to do is measure it. Manometers are easy to make.

Thanks Bob. I can't measure it because I'm planning the system of pipes. Perhaps the question should be "how long is too long for a 27mm (ID) pipe, before the pressure is not enough to extract efficiently".

Would it be easier if assumptions were made, such as constant temperature, friction, and speed? I suppose an indication could be that Festool make a 7 metre hose for the CTL 36. Perhaps that's the longest distance they recommend.

From the Festool website the maximum airflow is 3900 litres per minute, and the maximum vacuum is 24000 kp (probably measured directly at the intake with no hose). Hose ID would be 27mm.

Festool supply (or used to supply) 50mm and 36mm hoses for their vacs.
I think the intake on the machine is still 50mm

If you really want to plumb for the vac, I suggest using 50mm or 36mm main lines and only necking down to 27mm at the tool.

BUT, the Festool vacs have big wheels for a reason. AFAICT, the intention is that you move the vac to the work area.
Don't know about you, but I do nearly all of my power sanding on one bench so having the tool beside the bench is the way I approach the issue.

I doubted the inlet pressure is 24000 kilo pascals (since that is 2.4 bar) so I went and looked up the festool site and it is 24 kPa.

This is a useful on-line pipe pressure drop calculator.
Pressure Drop Online-Calculator

Pipe roughness is difficult to estimate because I don't know what sort of hose is used.
If it is corrugated hose then 0.1 mm would be about right
For dynamic viscosity I used 81 x 10^-6 Pa s
Air density is ~1.2 kg/m^3.
The rest you should be able to work out for yourself.

When I plug this into the calculator I get a pressure drop of 11 kPa/m which means the vacuum cleaner hose can't be much longer than 2 m.

Using a pipe roughness of 0.01 I get 7.7 kPa/m, which means that 3 m of pipe is the max.
Using a super smooth walled pipe (ie pipe roughness of 0.001) the press drop is still 7.2 kPa/m.

This all assumes the vacuum actually maintains 3900 L/min with a hose attached, which of course it does not. The 3900 L/min will be without any hose attached. What happens is, as soon as the hose is attached, the flow rate drops, it has to otherwise no air will flow.

A 5 m long 27 mm smooth wall pipe running at 50% flow rate (1950 L/min) will drop a total of 12 kPa. A corrugated pipe loses 19 kPa ie nearly all of the 24kPa.

What this clearly shows is that 27 mm pipe and 1200 W motor is not designed to pull air through long runs. This is why if you want to move a lot of air a big diameter pipe is needed.

That's a big help Bob, thank you. It pretty much tells me what I thought the answer would be - bigger hose tapered down, as Ian suggests, and no longer than about 3m. That more or less puts me back to the current situation of moving the vac around, if necessary. It's only a 5x3 metre shed, so it doesn't have to move very far! More a change of orientation, than movement.

What I'm hoping to do is have the vac tucked out of the way, and just change one of the hoses that goes into it. What I won't be able to do is have the hoses go up the wall, along the ceiling etc, because the run will be just too long I strongly suspect.

I'll have a play with that calculator and see what results I get from a 50mm hose.

If I want to persist with up the wall etc, would there be any virtue in using plastic plumbing pipes of a larger diameter? Would there be a maximum diameter, beyond which it would be pointless (using this "small" machine)?

Last question (I think): if I have no intention of using the vac on metal grinding, is there any real need for anti-static hose (it is much more expensive)? I read in a post yesterday that there have been no reported fires from DEs less than 1500 cfm, and 3900 litres per minute is way less than that. Or is it a case of static can affect wood particles to the point of potential combustion, therefore anti-static is required, regardless?

Btw, sorry about the 24000 kp, a case of a reading typo! Even my Orved Vacuum Packer only goes to 1 bar (and never really reaches that).

Originally Posted by FenceFurniture

If I want to persist with up the wall etc, would there be any virtue in using plastic plumbing pipes of a larger diameter? Would there be a maximum diameter, beyond which it would be pointless (using this "small" machine)?

What will happen if you double the cross sectional area of pipe is the air speed will drop by a factor of 4. The danger then is the air speed will be too low to hold the dust in suspension and the pipes will clog up. This happened to me a few years ago using a small (1HP) DE and the pipes filled up by about 1/3 of their volume with fine dust.

Last question (I think): if I have no intention of using the vac on metal grinding, is there any real need for anti-static hose (it is much more expensive)? I read in a post yesterday that there have been no reported fires from DEs less than 1500 cfm, and 3900 litres per minute is way less than that. Or is it a case of static can affect wood particles to the point of potential combustion, therefore anti-static is required, regardless?

The only reason for antistatic hose is if you do not wish to get zapped occasionally when using your DC/Vacuum cleaner. The physics of electrostatic charge and plastic pipes means there is always ZERO charge on the inner surface as all the charge migrates to the outer surface of an insulator.

Btw, sorry about the 24000 kp, a case of a reading typo! Even my Orved Vacuum Packer only goes to 1 bar (and never really reaches that).

No worries.

Great Bob, that's very helpful info. Thanks very much.