Use of rectangular duct profile for dust extraction?

[hugov]

Hi – new here,

Wondering if anyone has tried using kitchen extractor style rectangular ducting for dust extraction? I have a low ceiling in my workshop and don't want to have large round ducts hung from it, and I had a scrap of rectangular PVC ducting (204x60mm) left over from my kitchen refurb that got me thinking. This ducting is available from Screwfix (and also no doubt others) and is designed to have a similar cross-sectional area to 5" ducting (12,240 mm^2 versus 12,272 for the 5" round). The main drawbacks I can see are the lack of available fittings and adapters (though being rectangular you could make them from sheet materials fairly easily), and the fact that some of them have quite sharp corners that would create more backpressure than a nice smooth fitting for round ducting, but I think it should still work. My fluid dynamics knowledge is a bit rusty but I think the shape will have more skin friction than a round duct, reducing its effective size, perhaps to be comparable to a 4" round duct.

Anyone tried, any drawbacks I haven't considered? I'd probably just run the rectangular across the ceiling and then maybe have some round on the wall.

Thanks,
Hugo

p.s. I'm well aware of PVC ducting electrostatic considerations, let's not discuss them again in this thread.

 

[Inspector]

Square is better than rectangular but your going to bump your head on it if round is too big. Generally a square duct of XxX flows about the same as a round duct of X. Later I will post some links to equivalency charts and calculators for you. Can't do it now as I have to go into town for some stuff and I have to get the links from my backed up files as I haven't reloaded them back since the last great system crash which needed a new hard drive to fix. It can be done but you will not want to do it if your DC is marginal.

Pete

 

[jimmy_s]

Rectangular will have a higher friction loss but the main losses are likely to be due to fittings which vary with velocity presssure or 1/2 x density of air x velocity^2 which is about 0.6xV^2. You need a decent velocity in the ductwork to prevent the dust etc settling in the ductwork and ideally its designed with all branches running at a similar velocity. Straight duct might be 1 or 2 pascals per metre but a bend could be 15 or 20 pascals for example. Other thing would be whether it can withstand the negative pressures without collapsing.

 

[Rorschach]

Check out the small shed youtube channel, Peter (?) used it in his workshop.

 

[Inspector]

Here are the promised links.
https://www.engineeringtoolbox.com/equivalent-diameter-d_443.htmlhttps://www.cdicurbs.com/ductcalhttp://www.freecalc.com/ductloss.htmhttp://www.lorencook.com/PDFs/Catalogs/Cookbook_Catalog.pdf Rectangular/round equivalents on page 71 and you can compare losses of round/square radius elbows on pages 54 and 56.

Pete

 

[hugov]

Thanks very much Pete, Jimmy and Rorschach. I found the Small Shed Youtube videos mentioned, #58 is the start of the dust extraction/ducting series, and a year later (#214) the problems emerge, very interesting! Not directly comparable to what I'm trying to do though, since he's using a LVHP vacuum and commensurately lower velocities in the duct, causing particles to drop out of suspension and accumulate in the duct.

Using the different estimation formula linked from Pete's post I get estimates between ~90mm and ~110mm as the round duct equivalent. Assuming 1000 m^3/hr (which I know won't be achieved, but it's a good number since many tools are specified as requiring that number), the Engineering Toolbox calculator gives ~30 m/sec velocity (seems fine) and 10-12 kPa pressure friction loss per 100m (seems manageable, around a tenth of an atmosphere) but I think where the rectangular ducting falls apart is the corners which are limited by available sections to quite sharp bends that will lose a lot of pressure. I think the sharp corner sections are equivalent to around 30m of ducting according to the estimation formulae/tables, but it depends markedly (ten times) on how sharp the corner is.

I'm wondering if I could make some corner sections myself (out of MDF or perspex or something) that are more gradual than the available ones. That might just about make my plan viable – otherwise I think I have to just make do or tolerate larger round ducting. I'll have to think about it.

Thanks again for your help!

Edit to add: I plan to keep using my shopvac for handheld tools and general cleaning, and only using the ducting and an HVLP extractor for the table saw and thicknesser (and a bandsaw if I get one of them).

 

[jimmy_s]

Sounds like a plan Hugov. Flowrate is about 270l/sec with a duct velocity of about 23metres per second which is in the range recommended 20-25m/s. Friction losses will be a few pascals per metre, looking at CIBSE guides its about 3pa/m but your fitting losses are the main issue at those velocities as the velocity pressure is about 320pa. So a normal radius bend may have a pressure drop of about 100pa and branches much more, so you would want to try to make up long radius bends etc where you can to reduce your pressure losses. I don't know how robust the PVC duct is - the negative pressure may also still be an issue.

Jimmy

 

[mindthatwhatouch]

Could make gradual bends by cutting the ducting into wedge shapes and gluing?

 

[Sandyn]

Slightly different installation, but I have just installed an extraction system for my kitchen using 204X60 rectangular ducting. Because I didn't want to knock any holes in the kitchen wall, I decided to run the ducting in the loft, then down through loft floor, along 0.7m and down into my workshop to use an existing vent. I doubled up on the 150mm fans, because the complex route the ducting had to take, ie two parallel ducts. I ran a 150mm circular vent for about 5m, from the fans then converted the 150 round to 204X60, followed by two flat right angle bends and two right angle bends.

The bends really kill the flow significantly. You would have to somehow fabricate gradual bends. I used my mitre saw to cut the ducts, but it would be difficult to cut wedge shapes. The ducting shatters in the middle if not supported properly.

I just did a very crude test with a 1m length of 204X60 ducting and a vacuum cleaner. The ducting sealed reasonably well at each end. The ducting collapsed by about 50% at the centre of the duct, but did recover when the vacuum was removed.

 

[Sachakins]

The small shed YouTube video link

 

[TRITON]

Use of rectangular duct profile for dust extraction?

Dear God No !!

The Earth will break