What size for dust extract flexible hose ?

[Stuartgb100]

Hi,
Advice please:
Coming to the end of my new workshop build (approx 6 by 7 metres internally).
Now planning the layout for bits of kit (all on moveable tables, except table saw and RAS).
Kit as follows:
Wadkin AGS10
DeWalt RAS
Bandsaw
Planer/thicknesser
Morticer
Lathe
Radial piller drill

I have a number of dust extract machines which I plan to connect to the various bits of kit.
All are single phase, with motors rated from approx 1 to 2.5hp.

Please feel free to comment on the following ideas/plans etc :

1. The most powerful extractor will be in a 'fixed' position, and connected overhead to the table saw and RAS via vertical drops using 100mm soil pipe.
2. The remaining kit will be served by the other less powerful extractors, (all of which have 100mm outlets). They will be mobile. I plan to move these close to the piece of kit (say within 1.5 metres) and connect with suitable flexible hoses and adaptors as necessary.

So, if the above is reasonable,
A: I presume that I need to keep the pipework as large a diameter as possible, up until the final connection to the piece of kit ?
2: I assume that the flexible pipe needs to be as short as possible ?
3. Do I reduce the diameter of the flexible pipework as close as possible to the kit ?
4. If I can find flexible plastic/rubber pipe (rather than ribbed tube), would that aid airflow ?

Any help would be much appreciated.

Many thanks.

 

[Sideways]

Your large extractor 2.5hp will move about 1500 cfm of air just looking at the specs of machines with similar power.
CFM figures are usually overstated so derate that a good bit to allow for real world performance with a filter actually fitted to the machine, etc.
But you might expect say 1,000 cfm in the real world ?
100mm diameter duct is far too small to carry 1,000cfm. I'm think I remember @Inspector mentioning that the practical max for a 4" round duct with a HVLP extractor is about 700cfm ?
Googling some internet sources, suggests you should be looking at 6" duct for the big extractor if not 8".
Connecting the big extractor to 4" duct will choke it. Save the 4" stuff for your 1hp mobile extractors.

You are absolutely right to keep flexible hoses as short as possible. Depending on who you read, flex duct can be 3x, 4x, even worse than smooth straight duct.

This previous thread is worth a look
https://www.ukworkshop.co.uk/threads/dust-extraction-flex-hose-smooth-vs-normal.141595/

 

[Inspector]

.......
100mm diameter duct is far too small to carry 1,000cfm. I'm think I remember @Inspector mentioning that the practical max for a 4" round duct with a HVLP extractor is about 700cfm ?
.......

400CFM is roughly the maximum you'll get through a 4" smooth duct.
800CFM is roughly the maximum you'll get through a 5" smooth duct.
1200CFM is roughly the maximum you'll get through a 6" smooth duct.

Pete

 

[Stuartgb100]

hanks both for the replies.
Much appreciated.
After researching the dust extract question, I've come up with a few 'principles' that I'm planning to follow, and would appreciate comment, please:

1. Use smooth bore pipework and fittings.
2. Smooth flexible pipe gives better airflow than ribbed flexible.
3. Avoid 90 degree bends.
2nr 45 degree bends will improve airflow.
I assume that if the 45 degree bends are separated by a length of straight pipe, even better.
4. Use the maximum sized rigid smooth bore pipework (as recommended by the dust extractor manufacturer) until a final connection to the woodworking machine.
5. Use the maximum size of smooth bore flexible pipework for the final connection.

As an example of smooth bore pipework, see:
https://www.airflow.com/
Thanks.

 

[Sideways]

Your list of principles looks sound.
For workshop use you may find it worth googling "galvanised steel spiral duct".
The airflow developments products are intended to be built into houses where they'll never be touched and won't carry heavy dust or chips. But they do understand low noise and low resistance to airflow.
I would be less concerned about using just a couple of smooth radiussed 90 bends for the sake of space saving vs the 2x 45s unless you have the room, very keen on swept tees, but I think you're on the right track.

 

[Inspector]

3. Avoid 90 degree bends.
2nr 45 degree bends will improve airflow.
I assume that if the 45 degree bends are separated by a length of straight pipe, even better.

The two 45º fittings have somehow gained traction through repetition to become fact. It is not true in every case.

The Americans do duct loss calculations by giving equivalent straight duct ratings to the fittings and add them all up. As far as I know you folks have calculation methods but using the American way of illustrating is easier. For example a sharp square cornered 90º fitting has the equivalent resistance of 136' and a smooth 90º fitting with a radius of 1.5R or more has 17' of equivalent drag. A big difference that encourages using the best fittings available and affordability.

Look at this link on page 54. It shows those values for 90º and 45º fittings. So the 90º fitting with the pair of 45º corners and the straight part between (1C) has more drag than all of the smooth 90º elbows and the 5 piece 90º elbows for the same radius. This is basically what you want to do. Now you could use the values they give for the 45º fittings instead and they are only a little better than a smooth elbow when considering a tighter radii. If you use a 1.5R or larger elbow (smooth or 5 piece) you are better off than every pair of 45º fittings and a straight section between. You have to account for the value of the length of that straight piece.

What I am trying to say is the blanket statement that a pair of 45º elbows will improve airflow for the most part is false.

If you are branching off the main line then you would use the straight bit of duct and a 45º fitting to make the turn. You would also use them to duck under a beam and back up. Four 45ºs being less than four 90ºs.

Just to clarify how the radius is measured and annotated. The radius is along the centreline of the duct to its focal point. So a 6" 1R elbow is 3 inches to the insides and 9" to the outside of the elbow. The 1R or 6" being along the centreline. Use the largest radii fittings you can reasonably get. Use what you like but avoid any PVC or sheet metal that doesn't have radius fittings.

Pete

 

[SammyQ]

"Swept" 90° bends offer less resistance than shorter, conventional.ones.