Thread: Exhaust Length and Tortuosity - Impact on DC Performance

Originally Posted by GarethR

Could anyone tell me the effect on a DC's performance with regards the length of the exhaust line and the impact of bends in the exhaust line?

Basically I'm wanting to reposition my dust extractor/collector which vents to the outside. I will need to increase the length of the exhaust line and add some bends to do so.

Thanks,

Gareth

Hi Gareth,

At its most basic level the best thing to do with the exhaust line are to make it as big a diameter as practical, within comonsense limits.

Whatever size ducting you are running to your machines try running an exhaust line with at least 2 to 3 times the cross section.

You want the invisible dust to remain in suspension as much as possible, but it is less critical than in the pickup lines.

Be careful of leaks as they are more critical in the exhaust line as it is under positive pressure. That's why it is a good idea where possible to make the exhaust line as short as possible, but of course that is not always practical.

I am sure you will get some more detailed information soon.

Cheers

Doug

Thanks Doug,

Originally Posted by doug3030

Whatever size ducting you are running to your machines try running an exhaust line with at least 2 to 3 times the cross section.
Doug

This is going to be a problem, I'm running 150mm ducting to my machines and at max can get a 26100sqmm area from the outlet, without some serious re-engineering. I need minimum ~35500sqmm to achieve the 2X cross section so I'm a wee way off.

Cheers,

GR

Ok, Tell us a bit more about your system.

If you are running 150mm I assume you are running at least 2hp, maybe 3hp?

The cross section of the outlet from the blower itself is not the issue if that is what you are referring to.

While the blower is sucking in the air in the ducting before the blower, its critical that the ducting is not too big or too small to keep the shavings and invisible dust in suspension.

I am assuming that you have some sort of separator located with the blower (cyclone maybe) and the exhaust is blowing out relatively clean air.

The point is to make sure that any ducting after the exhaust from the blower is not a choke point. You want to maintain at least the capacity to pass as much air as comes out of the blower exhaust. Tats what I menat by 2 - 3 times the cross section. That should account for losses around bends etc.

What type of collector do you have? is it located -

- before the blower
- after the blower but within the exhaust line span
- at the end of the line - just dumped into a bin?

Originally Posted by GarethR

Thanks Doug,
This is going to be a problem, I'm running 150mm ducting to my machines and at max can get a 26100sqmm area from the outlet, without some serious re-engineering. I need minimum ~35500sqmm to achieve the 2X cross section so I'm a wee way off.

The 2X cross section is just a guide and if 26k sqmm (~180 mm diam ducting) is all you can get that that is all you can get and I wouldn't let me stop putting the DC outside.
The benefits far outweigh any loss of flow
My question is where are you going to get ducting that size unless you make it yourself?

If you can tell me the length and number/type of bends involved I can have a crack at working to the actual pressure and flow rate loss.
I'l take a guess that it will be less than 5% of the flow rate.

Thank you both for the feedback.

Doug,

I have a 3hp extractor attached to a separator. The separator is attached before the blower. 150mm line runs from the separator to the tools with a short single line that I disconnect/connect to each machine as needed. There is a splitter 100/150 for the TS . A 150mm exhaust line (couple meters) runs from the blower to the outside (previously a window).

Bob,

The exhaust line is currently 150mm, but I can expand that to approx180mm (outlet size max) if essential. I would need to customize some ducting.

Still need to finalise my layout, hence the query regarding length and bends. In any event I guess less is better?

Thamks,

Gareth

Gareth,

After my dust collection system was upgraded Ronboult was kind enough to bring his clever instruments to my shop to measure the airflow.

The results were clear. The biggest losses were caused by the machines themselves and their hoods/ports. Next came the fittings. Bringing up the rear was the length of 150 mm PVC pipe.

Not suggesting you should not be concerned about pipe and fittings; you should. Just sharing the experience in my shop in case it is useful.