Thread: 90mm PVC pipe for dust extractor

1 HP DE's are just about a waste of time even if they're right next to the dust source. By the time you add 15 metres of duct to the inlet it wouldn't have the power to pull the skin off a custard. Like a SCMS the waste from a lathe is extremely difficult to collect. Since you won't be achieving much anyway there probably isn't any reason to not use the 90mm storm water pipe, but an upgrade to a, minimum, 2 HP DE and 6" duct would be the way to go.

My mistake. It is 2hp extractor. So, based on that, what about 90mm pipe?

Twin 90mm ducts would be a significant improvement on a single 100mm. If you have enough 90mm available that is. Even then I'd only consider it an interim measure. Why not upgrade to 6" straight away? If funds are at a premium have a look around the local builders demolition yards, amazing what you can find there at times. A single 6" duct can carry at least 40% more air than two 90mm.

Once you have a DE powerful enough to move the air you're still restricted by the size of the duct. Remember a DE is a low pressure extractor and any restrictions in the system will have a much more significant effect on performance than it would in a high pressure system.

Just add to what aldav said one thing to remember with dust extractors is they work by volume of air they can move not the pressure

Originally Posted by aldav

Twin 90mm ducts would be a significant improvement on a single 100mm. If you have enough 90mm available that is. Even then I'd only consider it an interim measure. Why not upgrade to 6" straight away? If funds are at a premium have a look around the local builders demolition yards, amazing what you can find there at times. A single 6" duct can carry at least 40% more air than two 90mm.

At 8" WC pressure (about the most produced by a 2hP DC) a 90 mm duct will carry ~250 CFM, while a 100 mm duct will carry ~400 CFM, and a 150mm duct will carry about 1250CFM

On a conventional 2HP DC with the twin 100 mm inlets the two idlest fight each other for air so the next result is no 2 x 400 CFM but less than 600 CFM. This assumes no flexy, bends, or more than a couple of metres of ducting.

For small lathe work about 800 CFM is a minimum requirement - the only way to achieve this with a 2HP DC is to modify it to accept 150 mm ducting as per the sticky at the top of the dust forum. This will produce about 850 CFM of flow with a clean bag or about 920 CFM with a clean Pleated filter. This assumes no long runs of ducting, no flexy or bends. Some small improvement in flow can be obtained using a Bell Mouth Hood collection opening at the lathe.

Originally Posted by BobL

At 8" WC pressure (about the most produced by a 2hP DC) a 90 mm duct will carry ~250 CFM, while a 100 mm duct will carry ~400 CFM, and a 150mm duct will carry about 1250CFM

On a conventional 2HP DC with the twin 100 mm inlets the two idlest fight each other for air so the next result is no 2 x 400 CFM but less than 600 CFM. This assumes no flexy, bends, or more than a couple of metres of ducting.

For small lathe work about 800 CFM is a minimum requirement - the only way to achieve this with a 2HP DC is to modify it to accept 150 mm ducting as per the sticky at the top of the dust forum.

he speaks much wisdom, I did try the 90 and it was absolutely bollocks compared to 150mm
Yes its scary butchering your machinery (for the first time and you might gasp at the extra $$ for the bigger pipe, but look around for prices, and dont forget to look at seconds or gumtree etc
IT IS WORTH THE EXTRA

Here's an alternate approach to the analysis. We know that the pressure loss in a duct is proportional to it's length and the square of the flow rate through it, and inversely proportional to the fifth power of it's diameter. The IDs of 100mm and 90mm stormwater pipe are 98mm and 80mm. So, at the same flow rate and length a 90mm pipe will have (98/80)^5 = 2.76 times the pressure loss of a 100mm pipe. If you use two 90mm pipes in place of one 100mm pipe then, if the flow divides equally, the pressure drop in the 90mm pipes will be (1/2)^2 = 1/4 what is was, or 2.76/4 = .69 times the pressure loss in the 100mm duct. With the proposed double 90mm running 15m length compared to the original 100mm duct at 4m we have the new pressure loss .69x(15/4) = 2.59 times the original. That makes it 2.59x4m = 10.3m equivalent length of 100m pipe.

There will be losses involved with separating and then recombining the flow so the result of the proposed change will be worse than adding 6.3m to the existing duct. Probably much worse. How much depends upon your skill at fabricating low loss splitters and combiners. I certainly wouldn't try it.

What was proposed is a single 90mm duct. The OP was asking if that would be adequate. The consensus is clearly that it won't be and that an immediate upgrade to 6" duct is the way to go. This is subject to the OP's ability to shoulder the cost.

Whilst it would be nice if we all had the financial resources to fully indulge our hobby it's not going to happen.