Ducting, should I reduce the main trunk after divert valve?

[Albert]

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?

[BobL]

Albert, I'll comment if you put all that in a diagram or two.

[Albert]

Albert, I'll comment if you put all that in a diagram or two.
Will do that tomorrow morning, thanks Bob

[pjt]

A pic of your diverter valve would be good also.


Pete

[Albert]

Here is the plan and the diverter valve, please click on the image to get larger pic.


308929

308928

[BobL]

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.

[Albert]

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.

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?

[Albert]

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.


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....?

[BobL]

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....?

Where did the 24 m/s come from?

[Albert]

Where did the 24 m/s come from?

oops it should be 20m/s equivalent of 4000fpm....

if I use 20 in that equation it will be 350CFM for 100mm duct, and 785CFM for the 150mm duct... still far from 1250 CFM...

[BobL]

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?

:2tsup:

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.

[LuckyDuck]

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.

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.)

[Albert]

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.)

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....

308993

308991

[BobL]

oops it should be 20m/s equivalent of 4000fpm....

if I use 20 in that equation it will be 350CFM for 100mm duct, and 785CFM for the 150mm duct... still far from 1250 CFM...

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.

[Albert]

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.

ok. it makes sense not to use an arbitrary number and determine the CFM
but where did you get the magic 1250CFM for the 150mm duct?

the Holytek catalogue says the air speed is 35m/s

[LuckyDuck]

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....


No worries. But 3.7m high? Wow that's big. I thought my RL250 was tall at 2350mm. It's all relative hey?

It it is interesting to compare the advertised numbers on the Holytek compared to the RLs and other extractors, because there seems to be quite a discrepancy between motor size and airflow. The RL250 has a 5.5kW motor, star delta start, 50m2 of filters, and yet only has an advertised airflow of 3540m3/hr at 20m/s. Why so much less than the Holytek?

I know Bob has pointed out that motor draw is more important than the stated size of the motor. That makes sense. Also, the size of the impeller is exceptionally relevant too. No one seems to know how big the impellers are on the RL units. I'm still waiting on the Felder rep to get back to me on that one. Anyway, I am absolutely convinced that Bob and co are right about modifying our machines to reduce resistance.

More to the point of this thread, I also have a 250mm mainline with 200mm and 160mm verticals. I am only just now finalising my ducting installation but even without instruments, airflow dramatically increases (and duct noise decreases) when I open up more than one gate, so that I obtain roughly the same cross section as the 250mm mainline. Cheers.

[Albert]

No worries. But 3.7m high? Wow that's big. I thought my RL250 was tall at 2350mm. It's all relative hey?

It it is interesting to compare the advertised numbers on the Holytek compared to the RLs and other extractors, because there seems to be quite a discrepancy between motor size and airflow. The RL250 has a 5.5kW motor, star delta start, 50m2 of filters, and yet only has an advertised airflow of 3540m3/hr at 20m/s. Why so much less than the Holytek?

I know Bob has pointed out that motor draw is more important than the stated size of the motor. That makes sense. Also, the size of the impeller is exceptionally relevant too. No one seems to know how big the impellers are on the RL units. I'm still waiting on the Felder rep to get back to me on that one. Anyway, I am absolutely convinced that Bob and co are right about modifying our machines to reduce resistance.

More to the point of this thread, I also have a 250mm mainline with 200mm and 160mm verticals. I am only just now finalising my ducting installation but even without instruments, airflow dramatically increases (and duct noise decreases) when I open up more than one gate, so that I obtain roughly the same cross section as the 250mm mainline. Cheers.

I've just measured my impeller, its 255mm from the edge of the impeller to the centre of the impeller, diameter is 510mm.

I think the airflow of the Holytek is measured at the centre of fan when its not connected to anything? ie, no static loss? the figure Felder has used probably has taken consideration of 8inch of water and stuff like that.

thought you maybe interested that there is a RL160 on machines4u.com.au, $3500 AUD..
http://www.machines4u.com.au/view/advert/FELDER-RL-160-Clean-Air-Dust-Extraction-Unit-Used/128017/

doesnt hurt to put another RL in the system?:wink:

[LuckyDuck]

I've just measured my impeller, its 255mm from the edge of the impeller to the centre of the impeller, diameter is 510mm.

I think the airflow of the Holytek is measured at the centre of fan when its not connected to anything? ie, no static loss? the figure Felder has used probably has taken consideration of 8inch of water and stuff like that.

thought you maybe interested that there is a RL160 on machines4u.com.au, $3500 AUD..
http://www.machines4u.com.au/view/advert/FELDER-RL-160-Clean-Air-Dust-Extraction-Unit-Used/128017/

doesnt hurt to put another RL in the system?:wink:

That's a great sized impeller. Will have to post the RL sizes if I ever find them out. Had to laugh about your mention of the RL160 for sale. That's my outgoing machine! I'll do it even cheaper if you need a backup for your Holytek!!!!!!

[LuckyDuck]

Bummer, just saw that you're in Auckland!:)

[BobL]

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.)

I've seen these fan curves before and they show nothing special in terms of impeller performance.
I'll bet they are still single point mid-stream air flow measurements of naked impellers - it's the industry standard way of doing it, why measure and publish accurate figures and look bad amongst your competitors?
Like most large DCs they generate ~10" of WC which, when attached to 6" ducting will generate no more than ~1250 cfm "full stop" - the physics of air flow simply cannot be defeated.
If bigger ducting is used, a bigger impeller/motor combo will move more air, but give how much these things cost then they should do that.
I'm not saying the curves are wrong - they are what they are but like all DC manufacturers they are very misleading because people look at the impeller flow rates and think "WOW I want that" but hook it up to a real working system especially with a choked machine and it ain't happening.

The specific test you describe in your post above has little to do with flow and is a test for meeting an OHSA "mg/m3" of duct in the air coming out of their DC filters i.e. what comes out of their filters meets the spec.
While a very worthwhile test and it would be useful if all DC testing included this test, the real testing and specifications should be on total workshop air quality but that is much more expensive, and also to be fair, effectively out of a manufacturers control, which is why they understandably don't get involved but that also conveniently gives them a lovely "out". While this applies less to some manufacturers, if manufacturers were serious about total workshop air quality they would not be making WW machines with 100 mm outlets and gizzards so throttled that owners have to get out their angle grinders to help them breathe. Big machines in particular should come with 200 mm outlets and a set of adapters than allow the purchaser to decide what ducting size they can use.

Anyway the elephant in the corner is not the total "mg/m3" but the "mg/m3" at specific particle sizes, It's relatively easy to get "mg/m3" well below (especially US and AUS) OHS requirements but if the emissions are all fine dust particles then there may still be a serious health risk. I have not seen anything pubished by anyone about the PM10, PM5 or PM1 dust emissions from dust extractors because manufacturers don't have to, but this is what should be done. PM10 refers to the "mg/m3" of particles of less than 10 microns. Enough ranting - we all still have a way to go in this area.

[Albert]

That's a great sized impeller. Will have to post the RL sizes if I ever find them out. Had to laugh about your mention of the RL160 for sale. That's my outgoing machine! I'll do it even cheaper if you need a backup for your Holytek!!!!!!

will be good to know the size of the impeller of the RL... given the external dimension of the RL250 and RL200, I suspect there is a large jump between the impeller size between RL250 and RL200...

the world is a small place, good price for a RL160 though :)

[BobL]

ok. it makes sense not to use an arbitrary number and determine the CFM
but where did you get the magic 1250CFM for the 150mm duct?
the Holytek catalogue says the air speed is 35m/s

The physics of air flow is what matters - not what the manufacturers say

The 1250 CFM for 150 mm and 420 for 100 CFM figures are THE physical limits to air flow based on the maximum pressures of around 10" of WC that conventional impellers used in DCs can generate.
If you look in post #5 of the first sticky you will see a chart that displays the MAX volume transfer for a given duct size for a given pressure.
Provided the motor/impeller combo can already move that amount of air any increase in motor power and impeller size is moot unless the motor/impeller combo can generate more pressure.
The starting point for any calculations involving DC operations should be this chart. The manufacturers flow data must come secondary to this chart.

The reason manufacturers build DCs with bigger motors and impellers is not to move more air through a single duct but to move more air through more pipes to serve multiple machines simultaneously.
Buying large DCs to service a one person shed operation thus becomes increasingly wasted unless we can work out how to use more than one machine at the same time.

Once we are into the greater than 5HP/16"+ impeller size of DCs, the duct and machine outlet sizes and the degree of throttledness of a machine dominate the air flow from from a singe machine and the amount of fine dust that evades capture at source.

One way to get rid of more fine dust from the vicinity of a machine is to locate an extra naked duct near the machine. Then having a bigger motor/impeller and bigger trunk lines start to become very useful.

[Albert]

The physics of air flow is what matters - not what the manufacturers say

The 1250 CFM for 150 mm and 420 for 100 CFM figures are THE physical limits to air flow based on the maximum pressures of around 10" of WC that conventional impellers used in DCs can generate.
If you look in post #5 of the first sticky you will see a chart that displays the MAX volume transfer for a given duct size for a given pressure.
Provided the motor/impeller combo can already move that amount of air any increase in motor power and impeller size is moot unless the motor/impeller combo can generate more pressure.
The starting point for any calculations involving DC operations should be this chart. The manufacturers flow data must come secondary to this chart.

The reason manufacturers build DCs with bigger motors and impellers is not to move more air through a single duct but to move more air through more pipes to serve multiple machines simultaneously.
Buying large DCs to service a one person shed operation thus becomes increasingly wasted unless we can work out how to use more than one machine at the same time.

Once we are into the greater than 5HP/16"+ impeller size of DCs, the duct and machine outlet sizes and the degree of throttledness of a machine dominate the air flow from from a singe machine and the amount of fine dust that evades capture at source.

One way to get rid of more fine dust from the vicinity of a machine is to locate an extra naked duct near the machine. Then having a bigger motor/impeller and bigger trunk lines start to become very useful.

This is great, thanks for the detail, the graph in the sticky looks awesome and makes sense. Will need to consider how to enlarge the hood...

[ronboult]

Hi BobL
Thanks to the pointer for the max flow though pipes an any static pressure. I have been looking for such data.

However this and your last post raises another Q in my mind.

What is it about the fan design used in DC's that limits the effective static pressure they can generate to about 250mm (10") H2O?
On inspection the clearview fan has considerable clearance above/below and around the periphery of the impeller. This must allow considerable leakage and reduction in Static Pressure generated.

I understand that larger fans and bigger motors serve to move more CFM but how could the static pressure be increased in a practical manner? Is noise generation an issue?

Ron

[BobL]

Hi BobL
Thanks to the pointer for the max flow though pipes an any static pressure. I have been looking for such data.
However this and your last post raises another Q in my mind.
What is it about the fan design used in DC's that limits the effective static pressure they can generate to about 250mm (10") H2O?
On inspection the clearview fan has considerable clearance above/below and around the periphery of the impeller. This must allow considerable leakage and reduction in Static Pressure generated.
I understand that larger fans and bigger motors serve to move more CFM but how could the static pressure be increased in a practical manner? Is noise generation an issue?

Noise is an important factor in impeller design - think giant vacuum cleaner noise both from the impeller, and the hissing noise of fast air moving through ducts. It's not just the volume, the white noise generated by a high velocity gas is quite disorienting and dangerous for a workshop situation. Vacuum cleaners (VCs) can generate around 100 CFM using 30" of WC. Thing about the noise 20 VC would generate to move 2000 CFM.

Next is ducting. If the pressure gets too great then ducting needs to be stronger and at too great a pressure (or vacuum) implosions becomes a possibility especially if ducting is knocked about.

The elephant in the corner is the power needed to move a large amount of gas through a small pipe at high speed. If a 3HP motor can moves 1250 CFM through 150 mm ducting at 10" of WC then (from the chart in the sticky) to move 2500 CFM through 150 mm pipe requires 30" of WC. The power requirements required to generate increases in pressure are such that to double the pressure roughly requires 4 as much power, so to get 3 times the pressure requires 9 times more power, so for a 3HP starting figure, that's ~27 HP!
A 1HP DC using a conventional impeller can generates about 6" of WC, a 2HP should generate about 1.4x more (i.e. 8.4") and a 3HP should generate about 2x more than a 1HP or about 12".

Compare all that to an 200mm duct which can theoretically move 2500 CFM using just 10" of WC. This of course assumes the impeller/motor combo can do that.
This is why the solution to moving more air has always been to use bigger ducting.

What I am presenting is still a very much simplified set of scenarios described above because overall performance of an impeller is not limited to what happens under a static pressure but how well it can maintain flow under load/restriction. The usual way of describing this is by way of a graph of flow rate V pressure drop which is called a fan curve. Some impellers have high flow rates when unrestricted but have very poor flow rates when placed under load or restrictions. An example of that is a squirrel cage fan. These move a lot of air when unrestricted but struggle even under light loads. They are great for ventilation but hopeless for moving sawdust.

[pjt]

Hi BobL
Thanks to the pointer for the max flow though pipes an any static pressure. I have been looking for such data.

However this and your last post raises another Q in my mind.

What is it about the fan design used in DC's that limits the effective static pressure they can generate to about 250mm (10") H2O?
On inspection the clearview fan has considerable clearance above/below and around the periphery of the impeller. This must allow considerable leakage and reduction in Static Pressure generated.

I understand that larger fans and bigger motors serve to move more CFM but how could the static pressure be increased in a practical manner? Is noise generation an issue?

Ron
I've butted in again:p I'm sure Bob won't be to :~

In general terms the diameter of fan determines the pressure generated and width of fan detemines flow, a skinny large diameter fan can generate a higher pressure with x flow when compared to a wide fan and lesser diameter, your house vacuum cleaner is typical of a skinny large diameter fan (high neg. press low flow) the VC is working under conditions (long small diameter corrogated hose, bags, filters, fittings) that are the complete opposite of what we try and do for dust collection in the workshop (short large diameter smooth hose, short lengths of duct, no filters, etc)

In comparison to a VC your air con fan is more likely to by wide when compared to it's diameter (high flow low pressure).

It's not too uncommon to place fans of the same size in parallel when higher flows are required, large industrial evaporative air conditioners for e.g.
To increase pressure one way is to place fan wheels in series, the outlet becomes the inlet of the next, high pressure boiler pumps for e.g. but not really practical for the average DC, inertia starts to have a greater negative impact when diameter increases, starting currents, also balance, noise, high tip speed

The maximum pressure that a fan generates is a non flow test (outlet blocked) so it doesn't matter how much space is around the fan in a housing in a no flow test, housing design does play a role when flow is allowed tho, a housing with plenty of space around it will tend to be quieter, compare this with an air raid siren with blades that are positioned close to it's housing, the woodgears guy made an air raid siren which is worth a look. Also for further reading google velocity triangles in relation to fan design.



Pete

[Albert]

Noise is an important factor in impeller design - think giant vacuum cleaner noise both from the impeller, and the hissing noise of fast air moving through ducts. It's not just the volume, the white noise generated by a high velocity gas is quite disorienting and dangerous for a workshop situation. Vacuum cleaners (VCs) can generate around 100 CFM using 30" of WC. Thing about the noise 20 VC would generate to move 2000 CFM.

Next is ducting. If the pressure gets too great then ducting needs to be stronger and at too great a pressure (or vacuum) implosions becomes a possibility especially if ducting is knocked about.

The elephant in the corner is the power needed to move a large amount of gas through a small pipe at high speed. If a 3HP motor can moves 1250 CFM through 150 mm ducting at 10" of WC then (from the chart in the sticky) to move 2500 CFM through 150 mm pipe requires 30" of WC. The power requirements required to generate increases in pressure are such that to double the pressure roughly requires 4 as much power, so to get 3 times the pressure requires 9 times more power, so for a 3HP starting figure, that's ~27 HP!
A 1HP DC using a conventional impeller can generates about 6" of WC, a 2HP should generate about 1.4x more (i.e. 8.4") and a 3HP should generate about 2x more than a 1HP or about 12".

Compare all that to an 200mm duct which can theoretically move 2500 CFM using just 10" of WC. This of course assumes the impeller/motor combo can do that.
This is why the solution to moving more air has always been to use bigger ducting.

What I am presenting is still a very much simplified set of scenarios described above because overall performance of an impeller is not limited to what happens under a static pressure but how well it can maintain flow under load/restriction. The usual way of describing this is by way of a graph of flow rate V pressure drop which is called a fan curve. Some impellers have high flow rates when unrestricted but have very poor flow rates when placed under load or restrictions. An example of that is a squirrel cage fan. These move a lot of air when unrestricted but struggle even under light loads. They are great for ventilation but hopeless for moving sawdust.

Bob, do you happen to know where I can check if my impeller is too big for my motor? do you think 20 inch impeller is too large for a 4kw motor? motor is 4 pole 1725RPM

below are the images of my impeller.


309119309120309121

[ronboult]

Hi BobL
Thanks for an excellent explanation of the issues involved. What you have written makes great sense when one stops and thinks about it. While my intention was not to increase the static pressure markedly any small increase would help to offset the inevitable duct losses even in a 150mm setup.

Hi PJT

Your last sentence worries me "The maximum pressure that a fan generates is a non flow test (outlet blocked) so it doesn't matter how much space is around the fan in a housing in a no flow test, housing design does play a role when flow is allowed tho, a housing with plenty of space around it will tend to be quieter, compare this with an air raid siren with blades that are positioned close to it's housing, the woodgears guy made an air raid siren which is worth a look. Also for further reading google velocity triangles in relation to fan design."

I would think that even under static conditions with the outlet or inlet blocked the impeller is still moving air to maintain the static pressure in the outlet. If there was no air movement then there would be no static pressure. The fan has to continue rotating to maintain the pressure. Therefore it seems to me that any leakage back past the impeller must decrease the static pressure at the outlet even in a no flow situation. Have I got it wrong?

Ron

[ronboult]

Hi BobL

Just looked at the fan picture in Alberts latest post. His fan construction appears to be naked on both sides. One would expect a large amount of wind shear at the edges of the blade on both sides. Would this increase the noise level? I notice that his motor runs at 1750rpm which may help. My Clearview fan is built on a disc on the side opposite the inlet presumably for strength but may assist noise reduction.

Would a Fan design that has a complete disc opposite the inlet and a annular disc on the inlet side allow the fan to rotate close to the sides of the fan housing without an increase in noise?
Ron

[BobL]

Bob, do you happen to know where I can check if my impeller is too big for my motor? do you think 20 inch impeller is too large for a 4kw motor? motor is 4 pole 1725RPM


Probably not for a 4 pole motor. Are you sure it is 1725, it's more likely 1425 RPM.
The reason the impeller is so big is because the RPMs are half that of other DCs.
The big advantage of a slower impeller is reduced noise.
Air flow is proportional to RPM so a VFD connected to that motor could produce more flow but from what I can tell, air flow is the least of your problems.

The standard way to check if the impeller is too large is
- start with power disconnected
- look at the motor name plate and check what current it should draw
- connect an AC ammeter inline to the motor
- remove all inlets and outlets (removing the bags is fine) to the impeller
- Watching the Ammeter, turn on the motor and check the current.
Unless you are connected to say a VFD, initially for the first few seconds there will be a a large start up current, don't worry about this but then watch the ammeter over about 10 seconds and see that the current does not run over the current specified on the nameplate. It can run a little over maybe 10 - 20% but not 50 or 100%. Depending on its rating, chances are if that happens the circuit breaker may trip.

If it does draw more current then all is not lost, it just means it must always run under a loaded state, i.e. filters/ducting connected.
A potential problem comes about if a filter is damaged or becomes disconnected then the motor may draw too much current.
Most motors have a thermal trip-out mechanism on then to protect them from this but it's still not good for the motor if they are constantly tripping out.

[BobL]

Hi BobL
Just looked at the fan picture in Alberts latest post. His fan construction appears to be naked on both sides. One would expect a large amount of wind shear at the edges of the blade on both sides. Would this increase the noise level? I notice that his motor runs at 1750rpm which may help.
The 1750 RPM is probably a mistake (Its a 4 pole motor) so it should be around 1425 Rpm OR its a 60Hz value OR he has a pulley gearing the RPM up.
Yes a fan like that will make more noise not just because the blades are naked but because they are straight, Backwards curved blades make less noise.
But the lower RPMs will definitely help and probably overcome these issues.
One issue with these paddle type fans is that stuff can tangle around the shaft more easily and they can become a PITA to untangle. OK for sanders and stuff than makes chips and dust but watch out for shavings and streamers.

Albert it would be interesting to know the noise level of the motor/impeller. Most smart phones have free Sound Pressure Level (SPL) apps available for them. OK they are not that accurate but it will give us an idea of what noise it makes.
SOP is at 1m from the unit at the same height as an operators ear (i.e. ~1.8m)


My Clearview fan is built on a disc on the side opposite the inlet presumably for strength but may assist noise reduction.
Would a Fan design that has a complete disc opposite the inlet and a annular disc on the inlet side allow the fan to rotate close to the sides of the fan housing without an increase in noise?
Ron Sorry I don't know.

[Albert]

Probably not for a 4 pole motor. Are you sure it is 1725, it's more likely 1425 RPM.
The reason the impeller is so big is because the RPMs are half that of other DCs.
The big advantage of a slower impeller is reduced noise.
Air flow is proportional to RPM so a VFD connected to that motor could produce more flow but from what I can tell, air flow is the least of your problems.

The standard way to check if the impeller is too large is
- start with power disconnected
- look at the motor name plate and check what current it should draw
- connect an AC ammeter inline to the motor
- remove all inlets and outlets (removing the bags is fine) to the impeller
- Watching the Ammeter, turn on the motor and check the current.
Unless you are connected to say a VFD, initially for the first few seconds there will be a a large start up current, don't worry about this but then watch the ammeter over about 10 seconds and see that the current does not run over the current specified on the nameplate. It can run a little over maybe 10 - 20% but not 50 or 100%. Depending on its rating, chances are if that happens the circuit breaker may trip.

If it does draw more current then all is not lost, it just means it must always run under a loaded state, i.e. filters/ducting connected.
A potential problem comes about if a filter is damaged or becomes disconnected then the motor may draw too much current.
Most motors have a thermal trip-out mechanism on then to protect them from this but it's still not good for the motor if they are constantly tripping out.

Yes you are right Bob, it is 1450rpm drawing 8 amp
309175

[Albert]

The 1750 RPM is probably a mistake (Its a 4 pole motor) so it should be around 1425 Rpm OR its a 60Hz value OR he has a pulley gearing the RPM up.
Yes a fan like that will make more noise not just because the blades are naked but because they are straight, Backwards curved blades make less noise.
But the lower RPMs will definitely help and probably overcome these issues.
One issue with these paddle type fans is that stuff can tangle around the shaft more easily and they can become a PITA to untangle. OK for sanders and stuff than makes chips and dust but watch out for shavings and streamers.

Albert it would be interesting to know the noise level of the motor/impeller. Most smart phones have free Sound Pressure Level (SPL) apps available for them. OK they are not that accurate but it will give us an idea of what noise it makes.
SOP is at 1m from the unit at the same height as an operators ear (i.e. ~1.8m)

Sorry I don't know.

Ok I will measure it once the 3 phase installation is completed, should be around the end of next week. My power provider asked if I want to upgrade from 60amp to 100amp for 2500aud, to get 60amp 3 phase supplied to the fuse box at the boundary is 250aud. 60amp should be enough as I have star delta for the sander, it's operating current is 19.1amp.

[pjt]

Hi BobL
Thanks for an excellent explanation of the issues involved. What you have written makes great sense when one stops and thinks about it. While my intention was not to increase the static pressure markedly any small increase would help to offset the inevitable duct losses even in a 150mm setup.

Hi PJT

Your last sentence worries me "The maximum pressure that a fan generates is a non flow test (outlet blocked) so it doesn't matter how much space is around the fan in a housing in a no flow test, housing design does play a role when flow is allowed tho, a housing with plenty of space around it will tend to be quieter, compare this with an air raid siren with blades that are positioned close to it's housing, the woodgears guy made an air raid siren which is worth a look. Also for further reading google velocity triangles in relation to fan design."

I would think that even under static conditions with the outlet or inlet blocked the impeller is still moving air to maintain the static pressure in the outlet. If there was no air movement then there would be no static pressure. The fan has to continue rotating to maintain the pressure. Therefore it seems to me that any leakage back past the impeller must decrease the static pressure at the outlet even in a no flow situation. Have I got it wrong?

Ron

If we have the inlet completely open and the outlet completely blocked, then in some amount of revs of the fan after initial start the fan will move air into the housing until it can't anymore, the fan has reached it's limit of moving air against the rising pressure, it cannot move anymore air so we have no flow, it is at this point where I mean it doesn't matter what size the housing is, a bigger housing will just mean it might take a few more rotations to move air into the larger space before the max pressure is reached.

In an ideal machine we would have no leaks but there undoubtably will continue to be some air moved by the fan due to leaks, say the motor shaft where it penetrates the housing and any gap or misalignment between the stationary part of the housing and the inlet to the fan, both these leak points can be reduced or eliminated by design, the usual best is to have these gaps at a minimum and therefore give the best max static pressure test value.



Pete

[BobL]

Your last sentence worries me "The maximum pressure that a fan generates is a non flow test (outlet blocked) so it doesn't matter how much space is around the fan in a housing in a no flow test, housing design does play a role when flow is allowed tho, a housing with plenty of space around it will tend to be quieter, compare this with an air raid siren with blades that are positioned close to it's housing, the woodgears guy made an air raid siren which is worth a look. Also for further reading google velocity triangles in relation to fan design."

I would think that even under static conditions with the outlet or inlet blocked the impeller is still moving air to maintain the static pressure in the outlet. If there was no air movement then there would be no static pressure. The fan has to continue rotating to maintain the pressure. Therefore it seems to me that any leakage back past the impeller must decrease the static pressure at the outlet even in a no flow situation. Have I got it wrong?

Ron

Pete has explained this well but I just wanted to relate the following.

Many years ago when I first started messing around with impellers I thought the same way and spent ages mucking around changing gaps and putting in baffles etc and found it made no difference to the static pressure test. What gaps etc will effect is the fan curve and the ability to deliver air under load. This is why fan curves are important

[John Samuel]

Pete has explained this well but I just wanted to relate the following.

Many years ago when I first started messing around with impellers I thought the same way and spent ages mucking around changing gaps and putting in baffles etc and found it made no difference to the static pressure test. What gaps etc will effect is the fan curve and the ability to deliver air under load. This is why fan curves are important
Yes ... had a couple of ah-ha moments myself here as I wandered through the literature.

Static pressure only tells part of the story. Two impellers might have a similar static pressure, but one might be vastly superior at pulling air through obstructions such as machines and duct work.

Cheerio!

John

[pjt]

During the making of my cyclone I decided a flange mount would be a better option than a footmount, an added benefit was the prevention of air escaping through the motor shaft hole and as Bob pointed out in another thread some heat transfer.
Another method to slow down air escaping thru the motor shaft hole is to have the backing plate of the fan as close as possible to the fan housing, 1 or 2mm, this closeness starts to act like a labyrinth seal and thus preventing air escape.

Also in my setup the center pipe in the cyclone has a gap of about 1mm between it and the inlet shroud of the fan, pipe and fan inlet are the same size so basically it's a butt join with a gap, one part rotating the other not, this was done to separate the positive side from the negative side, an improvement might be to have the center pipe fit inside the fan shroud by 15 or 20mm so making it a bit more like a lab. seal, the aim here is to reduce these leak points to help dynamic flow as much as possible not so much for static flow, the ratio of air leaking in a static (outlet closed) to air flowing (outlet open) comparison I would think would be quite small and it would be a poor fan if it couldn't maintain these leaks in a static situation.



Pete

[LuckyDuck]

The physics of air flow is what matters - not what the manufacturers say

The 1250 CFM for 150 mm and 420 for 100 CFM figures are THE physical limits to air flow based on the maximum pressures of around 10" of WC that conventional impellers used in DCs can generate.
If you look in post #5 of the first sticky you will see a chart that displays the MAX volume transfer for a given duct size for a given pressure.
Provided the motor/impeller combo can already move that amount of air any increase in motor power and impeller size is moot unless the motor/impeller combo can generate more pressure.
The starting point for any calculations involving DC operations should be this chart. The manufacturers flow data must come secondary to this chart.

The reason manufacturers build DCs with bigger motors and impellers is not to move more air through a single duct but to move more air through more pipes to serve multiple machines simultaneously.
Buying large DCs to service a one person shed operation thus becomes increasingly wasted unless we can work out how to use more than one machine at the same time.

Once we are into the greater than 5HP/16"+ impeller size of DCs, the duct and machine outlet sizes and the degree of throttledness of a machine dominate the air flow from from a singe machine and the amount of fine dust that evades capture at source.

One way to get rid of more fine dust from the vicinity of a machine is to locate an extra naked duct near the machine. Then having a bigger motor/impeller and bigger trunk lines start to become very useful.


Been busy lately and haven't had a chance to join in. This is an excellent thread and I'm continuing to learn plenty! Bob's post number 22 provided a big "Ahah" moment for me, as he describes the relationship between bigger HP, pressure, and the physical limits of airflow through different sized ducting. I do have a DC with larger HP, but not higher pressure (only 10"), so my best use of the DC (in a one man shop) is to employ a naked duct to scrub the air. That now makes sense to me. I've been working on ducting and opening up my machines. Pics to follow in other threads. Thanks guys.

[BobL]

Been busy lately and haven't had a chance to join in. This is an excellent thread and I'm continuing to learn plenty! Bob's post number 22 provided a big "Ahah" moment for me, as he describes the relationship between bigger HP, pressure, and the physical limits of airflow through different sized ducting. I do have a DC with larger HP, but not higher pressure (only 10"), so my best use of the DC (in a one man shop) is to employ a naked duct to scrub the air. That now makes sense to me. I've been working on ducting and opening up my machines. Pics to follow in other threads. Thanks guys.

You could always invite few friends in to work alongside :D

[LuckyDuck]

Indeed, but my problem with having friends over is we invariably drink too much beer while we check out the latest changes or acquisitions -- not conducive for much work getting done! :p