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30th March 2014, 08:55 PM #1SENIOR MEMBER
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Ducting, should I reduce the main trunk after divert valve?
Due to possible future expansion, I will be using Dantherm ducting, very smooth and lip-locked, the system will serve 2 machines for now, one is my Robland combination and the other will be the SCM sander.
The extractor is a Holytek 4kw 3200cfm 24 socks extractor ( similar to Dantherm s500) the main trunk will be 250mm, from the extractor inlet it will:
1. Go up 1m
2. 90 degrees bend(1.5x radius)
3. 2 m across the workshop.
4. Diverter valve
4a1. 90 deg bend down (1.5 x radius)
4a2. 250mm to 3x 150mm branch
4a3. Connect 3 x 150mm to the sander hoods
4b1. 240mm trunk carry on for another 2m (or add a reducer so trunk is 250mm to 150mm)
4b2. 90 deg bend (1.5 x radius)
4b3. Y branch from 250mm to 4 x 125mm ( or from 150m to 3 x 100mm)
4b4. Connect all the 125mm to the combo hoods
Comments and thoughts please?
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30th March 2014 08:55 PM # ADSGoogle Adsense Advertisement
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30th March 2014, 09:32 PM #2.
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Albert, I'll comment if you put all that in a diagram or two.
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30th March 2014, 09:49 PM #3SENIOR MEMBER
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30th March 2014, 11:51 PM #4
A pic of your diverter valve would be good also.
Pete
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31st March 2014, 07:56 AM #5SENIOR MEMBER
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Here is the plan and the diverter valve, please click on the image to get larger pic.
SKMBT_C28014033109430.jpg
SKMBT_C28014033109510.jpg
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31st March 2014, 10:04 AM #6.
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I wouldn't use that diverter valve. It will impede the flow in two ways. Firstly it represents a small but significant physical obstruction to air/sawdust , and second it will introduce extra turbulence into the path way.
If you ever use a machine that makes shavings it will easily get caught up in something like a diverter valve.
A better setup will use a Y junction with two blast gates.
I see you are planning to use 250 mm pipe as part of your system but then it will be reduced down to 150 mm.
The most the Holytek unit can pull through 150 mm will be 1250 CFM, that's the limitation of 150 mm ducting.
A 250 mm (10") pipe has a cross section of 0.545 ft2.
This means that 1250 cfm will have an air speed of 1250 /0.545 = 2291 ft/min (fpm).
This is well below the recommended 4000 fpm needed to keep sawdust in suspension.
This means you will need to build in a inlet into the side of the 250 mm duct that you keep mostly open to maintain the air speed in the larger duct.
I think you are doing this to service you combination machine and think that 2 x 125 mm will equate to the same flow in 1 x 250 mm?
125 mm ducting can at most carry about 800 CFM so 2 x 125 mm ducts will represent 1600 cfm.
In a 250 mm duct, 1600 cfm represents a linear air speed of 2900 fpm - also below the 4000 fpm speed needed to hold sawdust in suspension.
This also assumes your combination machine is not restricted in any way and most machines are severely restricted. Short of measuring the flow you won't know if the machine is choking the flow.
Once again a port you can hold open most of the time in the 250 mm duct will be needed
Even reducing the 250 mm back to 225 mm won't help the reduction to 150 mm ducting case.
The 2 x 125 mm comes out to 3600 fpm which is borderline and will probably be OK but I wouldn't do it without building an extra port into the side of the 250 mm ducting.
The Holytek 4kw 3200cfm will be more like 1600-1800 cfm so it is far better to use my figure to do any calculations with.
Manufacturers all make a single point measurement in the middle of the air stream on the naked impeller. Compared to real systems with filters, junctions and ducting it best to half their claims.
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31st March 2014, 12:27 PM #7SENIOR MEMBER
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OK, thanks Bob for the detailed reply....
so in conclusion, would you recommend the following:
200mm for the entire setup
no diverter valve, use 2 blast gates and y branch
say 1700CFM from the Holytek, 8" duct is 0.33ft2, the airspeed will be 1700CFM/0.33 = 5100 fpm, I should be fine then?
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31st March 2014, 01:30 PM #8SENIOR MEMBER
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Hi Bob, How come my calculation shows the 150mm has capacity only to do 900CFM? but yours 1250CFM? I am assuming 24m/s for the speed of the dust.
In a 150mm duct:
dust speed per second x cross section of duct x seconds in an hour x m3/hr to CFM factor (0.5885)
24 x 0.075 x 0.075 x pi x 60 x 60 x .5885 = 900CFM
in a 100mm duct:
24 x 0.05 x 0.05 x pi x 60 x 60 x 0.5885 = 399CFM
If I use your CFM to get the speed of the dust in a 150mm duct, it will be 33m/s....?
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31st March 2014, 01:49 PM #9.
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31st March 2014, 02:03 PM #10SENIOR MEMBER
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31st March 2014, 02:10 PM #11.
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I'd still look at adding a second Y on the 8" line just before the other one. The branch of that Y that goes nowhere (i.e. that is not attached to a machine) should obviously have a blast gate on it and that way if you want clear a dust drop out if a machine chokes or you want to ben the shed quickly you open that blast gate and the DC will fair rip any residual dust out of the shed via that unrestricted pathway.
I vent my shed via the duct that sucks from behind my lathe. It is a naked (i.e. not restricted by a machine) duct so max air flows.
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31st March 2014, 08:40 PM #12Woodworker
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Bob, this sentiment makes me uneasy, because in life most things are not "all" or "none". Take the Felder RL units as a possible exception to the rule. According to John Renzetti, friend of Bill Pentz and owner of the Felder Owner Group (FOG) in the US, the Felder RL units are tested using 5m of ducting with 4 x 90 degree bends, and the addition of quartz dust into it for 1hr.
The filters on the RL units are integrated to the extractor so testing on the "naked" impeller is unlikely. This is particularly the case as the impellers on these units are located after the filters, not in front, as for most extractors, such that RL units "suck" the dust-laden air through the filters, rather than "push" it through after the fact.
Do you have different information on how Felder arrived at the published fan curves for RL units?
(Apologies if this is getting too far off topic. I'm happy to start a new thread if necessary.)Warm Regards, Luckyduck
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31st March 2014, 08:59 PM #13SENIOR MEMBER
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all good, I was once considering a RL200 myself, but when my current unit came up for sale for $1000 AUD, its hard to say no, its a Holytek 24bag 4kw unit, filter media is 40m2, advertised m3/hr is 5440, I think if I take 2/3 of this value it should be ok, I dont want to underestimate it.
The RL is similar to the Donaldson Torit Unimaster system, the fan is after the filter, below are the fan curve and the cross section of the Donaldson Torit unit, although the Donaldson Torit is about 3.7m high....
unima.JPG
DT.JPG
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31st March 2014, 09:10 PM #14.
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20 m/s = 4000 fpm which is the air speed necessary for the sawdust to stay in suspension but I would hope that the Holytek can deliver far more air speed than this for 150 and 100 mm ducting.
If it can't then it stay away from it.
Dust Collectors don’t operate on a fixed or even a nominal air speed but their air speeds vary from zero up to a maximum value determined by the pressure gradient that can be established by the impeller/motor. The pressure gradient is dynamic (i.e. it changes) due to leaks,lengths of ducting, types and numbers of junctions, filters blocking, blockages in ducting from sawdust falling out of the air stream etc.
Even more complicated is the fact that the speed inside a duct is not uniform but varies from near zero at the wall to a maximum value in the middle.
In most cases we work with the best possible pressure an impeller/motor combo can generate and it's all downhill from there.
Under these maximum pressures, the most a 100 mm duct can transfer is around 420 CFM, that translates to an average linear air speed inside the duct of 4583 FPM
In a 150mm duct the max flow is 1250 cfm which translates to a 6266 FPM
This is for a zero duct length pipe. Inside that pipe the actual air speed will be about 20% higher in the middle of the pipe and 20% lower at the outer edges of the pipe.
Taking an average of the two is incorrect as the air flow in the out areas dominates the summed flow.
So to summarise it's better to work with air flow (CFM) and calculate FPM from that than the other way around.
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31st March 2014, 09:37 PM #15SENIOR MEMBER
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