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

Originally Posted by LuckyDuck

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/ad...t-Used/128017/

doesnt hurt to put another RL in the system?

Originally Posted by Albert

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/ad...t-Used/128017/

doesnt hurt to put another RL in the system?

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!!!!!!

Bummer, just saw that you're in Auckland!

Originally Posted by LuckyDuck

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.

Originally Posted by LuckyDuck

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

Originally Posted by Albert

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.

Originally Posted by BobL

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...

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

Originally Posted by 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?

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.

Originally Posted by 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

I've butted in again 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

Originally Posted by BobL

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.


photo 1.jpgphoto 2.jpgphoto 3.jpg

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

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

Originally Posted by Albert

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.