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

Originally Posted by BobL

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
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Originally Posted by BobL

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.

Originally Posted by 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

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

Originally Posted by ronboult

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

Originally Posted by BobL

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

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

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.

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.

Originally Posted by LuckyDuck

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

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!