Spiral ducting diameter for a 3.75HP extractor.

[ModburyJoinery]

Hi folks, as per the title, I am trying to figure out what diameter ducting I should be buying to work with a 3.75HP extractor. The run would be about 10 metres long.

I currently use 110mm grey soil pipe and have been experiencing quite a bit of chips failing to be collected from my P/T, which is at the far end of my existing run.

I have read that 150mm is a good place to start with a 2.5-3HP machine, and so my totally uneducated guess is that I should be looking at 180mm ducting.

Any thoughts?

Many thanks.

 

[Trevanion]

Grey soil pipe as in PVC pipe? You must get horrendous static electric coming off it when you have the planer going on full whack.

I'm no ducting expert. I would say 200mm ducting for 5m and then downsize it to say 150mm ducting for the rest of run and then 125mm or 100mm downpipes to the machines to keep the air pressure up would be ideal in my mind. Your extractor can't physically suck any more air through a 100mm pipe even if you put a 10HP machine on the end, there are limits to how much air can go through the pipes. Also, avoid hard 90 degree turnings if you can, sharp 90s will cut airflow in half.

 

[Inspector]

I made some assumptions because you haven't given all the information and I ain't know engineer. (hammer) but it should get you in the ballpark.

Playing with an online calculator the 110mm (4.375") PVC pipe and 10 meters (33'), a pair of elbows (stamped to be equivalent to PVC) and a guess of 2040CM/H (1200cfm) you have a theoretical static pressure loss of 16" of water column (you'll have to convert that) and velocity of 11,500 feet/minute in the duct. That is actually impossible to achieve as the maximum flow a pipe that size can handle would be 450 cfm (760CM/H) so the speed would be lots lower.

Upping to 6" (150mm) galvanized metal (5piece elbows) and there is a loss of 4.3" of WC and the duct speed would be 6100ft/min. So 6" would be the minimum you want to get the flow if my guess is correct for your DC. Advertised flow rates are exaggerated by as much as twice of what can be achieved in an actual system.

Going to 7" (180mm) galvanized metal (5piece elbows) and there is a loss of 2" of WC and duct airspeed would be 4,500ft/min. The speed in a duct you want to carry dust is 4,000ft/min as a minimum so this would be the maximum you want your duct diameter to be.

The suggested 8" would be a loss of about 1.1" of WC and the airspeed would be 3,439ft/min. This would start being too big and the heavier dust will settle out in the ducts especially if they are stepped down along the way. I calculated it as 8" all the way as the calculator doesn't let you do multiple sizes. You have to do them separately and add them up. I think. :roll:

So you would only want 7" pipes if your runs are basically straight. 6" will give you the flow you need as long as you are not stepping the pipes down. In other words you need to cut the hoods, chutes and ports open to 6" to allow the air to flow. You can use smaller ducting and openings but you will only move the chips and not the fine dust that harms the respiratory system. Basically what you now have.

The calculator I played with if you want to put more accurate info into it is. http://www.freecalc.com/ductloss.htm

Bill Pentz did a lot of research on dust when it made him very sick and among other things his site has a spreadsheet for designing ducting. I suggest you plod through the whole site, even though it is repetitive, and you will understand it all better. http://billpentz.com/woodworking/cyclone/index.cfm

Pete

 

[ModburyJoinery]

Thanks for your input Pete, and Trevanion. It is really appreciated. I will have a good look at Bill Pentz' work!