Chris, I'm not sure it is all that complicated. It's really that few actually test the airflow in their system. True, a 6" pipe works with a 15" impeller but it wastes a lot of capacity when coupled with a 5 hp motor. I can get 6000+ fpm through a 6" pipe in my system, but I get the same velocity in a 7" main and almost the same in the 8" line. Originally I thought that 30' from the cyclone I would reduce down to 6" as necessary to keep the cfm up. Using an anemometer at each port showed me the error in my "keep it simple approach" and I tore out the 6 and replaced it with 7" with no reduction in velocity. Through sheer brilliance in stupid planning, my Oakley edge sander sits 50' away and needs all of the 1500 cfm I bring to it. The original 6" made me wish for a 7.5hp system which would have been a failure with the same pipe. I run old machines that have mediocre DC with hood modifications so my needs are different than most, but if you choose the clearvue max for example and use 6" pipe you are wasting the upgrade. Dave
I thought you might come in David - it was you I was thinking of when i mentioned some running larger ducting on big fans and not running into problems.
I agree Chris too on not confusing people, and am very loath to do anything to undermine the 4000FPM vertical/3,500FPM horizontal/6in duct size guideline for design of 3HP plus dust systems - as Michael suggested you don't get medals for designing systems that may block up. There's as I've posted before advantages to running a large fan slightly restricted in terms of its response to increases in pressure drop.
Against that I've a sneaking suspicion that the CV Max on 5HP has the grunt to run more than 6in ducting if higher CFM is needed. That (very tentatively) maybe Bill P did a lot of his original work with smaller impellers, and that perhaps the Max was never fully tested. My own CV Max/Pentz based system uses 160mm spiral ducting, and feels very like it could handle more. I've no need of it right now, and am dead happy with what I have....
Ya gotta remember on this side of the pond the choices were, and pretty much still are, 4" or 6" PVC or metal. While it is possible to find local sources of metal ducting that aren't outrageously priced, most of the stuff, especially fittings, available from the usual suspects is much more expensive than PVC (S & D). It think too, if you run the numbers for an average size hobby woodworking shop with a 3 - 5 hp DC, 6" works out quite well. It also works well for running a constant diameter from WW machine to blower. It is probably not perfect in all situations, but satisfies most of the criteria without breaking the bank.
Hi Ian. Yes the Clearvue website recommends an 8" main with their max system and most of Pentz work was done in the 14" impeller years. It is much easier for you and I to know whether our systems are maxed out or restricted because we run three phase motors on a VFD. Using the continous amp display shows amps at fully closed and increases as gates are opened until until the restrictions cause the draw to level out. Then it doesn't matter how many more gates open up. If that draw is less than the FLA rating on the motor you know the pipe size or the filter area are maxing out the cfm. Reducing the motor rpm a little at that point won't reduce cfm until a certain point. If you are a one man shop with no friends like me, you adjust the rpm so that the various separate machine ports reach optimal cfm or velocity. I need 63.5 hz to get 12 amps for my Oakley sander and Oliver planer that are far far away and 60 hz is just fine for the shaper right next to the cyclone. Most of the time I just find a compromise setting and live with it. Higher in the winter as the motor is in the unheated and uncooled attic. The VFD works so well I wish Clearvue marketed it. I know the new efficiency ratings mandated for single phase motors are adding cost and weight so the three phase option isn't that much more as the price of vfds has declined. Dave
David, If I understand correctly, when the duct was upsized, the velocity was almost the same versus the smaller duct. Therefore, because you increased the duct area, the CFM increased if the velocity is the same. You were probably also pulling more fan motor amps after installing the larger duct.
I would suspect that what is happening is you reduced the system losses with the installation of the larger duct. This allowed your fan to operate at a point where it can develop more CFM at a lower inlet pressure. This would be a similar concept to me installing a 6" duct on my 1.5HP system versus the current 4" duct and generating more CFM at the hoods.
The larger the duct, the better, but you will reach a point where the velocity is too low. For example, I would probably have a lot of issues if I tried to use an 8" duct.
This is a good example of how the fan and collector interact with the duct system to affect overall performance at the hoods.
Michael, I believe you are correct and saying what I was only in a much more coherent way.
Alan, I agree that pvc has cost benefits but when looking at what is spent to equip a shop I'm not sure that an extra $500 or so for spiral and long radius fittings isn't a good trade off given the money spent for the cyclone. Given, I did and would opt for a used Torit or equivalent and save enough for a cartridge and vfd and consider the pipe a necessary evil. I'm also running 60' runs with big machines so what seems like overkill to many is just adequate. By the way, the auto gates you helped me with are becoming a favorite unnecessary expense as well. Dave
That's precisely the way I'd see it too David. i.e. if opening more blast gates doesn't significantly up the current drawn/HP, then the the header or the cyclone/filter set up is restricting flow. Against that there may be some advantage to that too as I've said like protection of the motor, less reduction in CFM for a moderate increase in pressure drop etc.
Not only Michael will a point eventually be reached when enlarging duct sizes where the velocity is too low, the risk with running with a duct size which more or less maxes out the CFM the fan is capable of is that the motor is probably (because it's already running closer to full load amps) more at risk of running into problems as a result of the opening of extra blast gates, or other reductions in pressure drop.
It's a bit like tuning an engine. You can get more power/CFM out of it, and it's a perfectly valid route to take when it's needed - but don't expect a revvy 150BHP/litre motor to be as tolerant of misuse or to have the same low RPM pulling power (aka response to increased pressure) as your maiden aunt's old chugger. ( an engine trades better 'breathing' for more HP, our system do the opposite)
It's been said before, but Bill P it seems from his pages went for a 5HP motor for this reason - because by not maxing it out (we've mentioned that CV systems often pull only around 4HP on 6in ducting) the risk of motor issues was reduced. (think he reckoned the choices in the US in motors were either 3HP or 5HP, and the 3HP would have been a bit stretched and liable to experience issues)
6in ducting seems to give excellent performance for a pretty wide range of applications provided you have enough motor HP to run it. (unless e.g. you're running big 'old iron' like David with very high CFM requirements) What all this does seem to suggest though is that it's probably wise to ease your system by degrees up to your worst case situation (most gates open/highest flow) while checking that it doesn't draw too many amps for the motor - especially if you're down around say 3HP. Or if you are running larger/very short ducting with a 5HP. An airspeed (FPM) reading if you can manage it might not be a bad move either - to make sure it's not gone too low for good transport.
As David has said running a 3 phase motor off a VFD adds a whole other dimension to your options by opening the possibility of increasing or decreasing your fan RPM - but only if your motor is specified so that it can handle the extra RPM, and deliver the extra HP while remaining below full load amps. (Fan HP is proportional to the RPM X pressure)
Extra RPM (within the limits the fan and motor are specced for) produces more CFM and better fan output at higher pressures - albeit at the expense of what can be quite marked increases in noise levels - but it can also make possible running a reduced RPM with a large impeller to reduce noise levels.
This sort of discussion is of course pushing the envelope a little, and most of us would prefer to get any bad news on any particular set up before rather than after building our system. Which suggests that it's best to either (a) use a well proven ducting size and fan combination (e.g. follow the recommendations of Clear Vue or one of the other established system suppliers), or (b) find the means to do the engineering before we buy or build: i.e. run the pressure drop calculation for your system and match it to a fan with an appropriate curve.
Last edited by ian maybury; 02-13-2012 at 4:38 PM.
We probably have gotten off course a little, but it is good to have an understanding of the DC systems both when installing and troubleshooting an existing system, or selecting collection equipment for a new system.
That's right about overloading the motor. I had that happen to me on a 50,000 CFM, 250HP system last year in a foundry. We shut the fan down to clean out the bottom of duct to take an airflow reading. The reading was downstream of a wet scrubber and there was about 3-4" of mud in the bottom of the duct (low transport velocity). The maintenance guy opened an inspection door near the fan inlet to clean out the duct. He didn't close the door before trying to start the fan back. The motor got almost up to speed befor tripping out. He would let the fan coast down, and try again, failing and heating up the electrical equipment. Convinced him to hit the start button before the fan coasted all the way down, and we got it started back (after closing the inspection door).
Yes Michael, there is no way I could fiddle with my system were it not for the VFD. Trying to figure the amp draw by putting a meter on the wires as I opened the gates, increased the pipe, filters, or hz would cause certain failure in my world of screw ups. It has been surprising how much more cfm I got by playing around though. Granted my measurement is less than perfect with a hand held anemometer at each machine port. The readings do help to quantify whether I'm helping or hurting. We have gotten off base but hopefully we have done more good than harm. Dave
It's fun to play with ideas though. Thanks guys...
I am curious to know if anyone has ever taken a Dylos count reading under normal conditions after someone has walked across a carpeted room (i.e.-living room).
Just a reminder, you don't need expensive spiral duct. 26 gauge snaplock works fine and is close to the price of S&D PVC. In fact snaplock may move a bit more air compared to spiral as it is smoother inside. Quoting from Bill;
" If you do the math for the 1000 CFM we need at our larger tools and the 4000 FPM airspeed we need to keep our vertical runs clear, most small shops should run at least 7” duct. This sized duct is rare and the more commonly available 8” duct is so large we end up with the airspeed falling so low our vertical runs plug. My personal solution has been to use 6” duct which will normally only carry about 790 CFM..."
In my neck of the woods 7" 26 gauge is a stock item at any HVAC supply house. Yes a 7" x5" wye may be special order, but then so will a 6" x4" wye in PVC at the BORG.
As is often the case Bill has been there, done that....
It certainly tends to confirm the view that a full 15/16in impeller and 5HP motor is capable of handling more than a 6in duct...
Two thoughts on ducting which you are probably aware of Ole. (1) As discussed before there seem to be very big differences in the pricing of spiral ducting depending on whether you buy from a low overhead manufacturing HVAC fabricator type supplier, or a brand name retailer. Spiral if bought right is for example definitely cheaper than PVC over here. (2) I'm not sure where the limit lies, but these big fans can develop pretty decent levels of suction, and the spiral seam must play a big part in resisting pressure. Care may be needed with some of the lighter ducting that seems to be about.
[QUOTE=Ole Anderson;1873275]airspeed falling so low our vertical runs plug. [QUOTE]
Not to disrespect Bill P, but it is rare that the vertical runs plug. If the velocity is not sufficient, the material will fall back to the bottom of the vertical and plug the horizontal.
I struggled with duct material selection. Most spiral duct, especially the affordable stuff, is also light gauge and made for supply air systems. Spiral duct leaks usually 2%-10% depending on the amount of negative pressure, quality of the construction, etc. It does resist buckling better under static pressure than the "stove-pipe" duct (thin gauge crimp together duct). Plastic duct can be a static electricity pain in the but, you need to ground it. The best is welded seem duct and the clamp together duct (Nordfab and KB Duct), but it can be pricey for a home shop. If in an industrial woodshop, non-conductive duct materials are not allowed per code.
I used plasic S&D pipe, then wrapped it with bare coper wire in my home shop. I attached the copper wire to ground on both ends. The duct doesn't grow any "hair", so I assume the copper wire is at least helping.
Just quoting Bill, doesn't mean I always agree with everything he says. I try to keep my verticals an inch smaller than the similar horizontal run in order to move heavier chips and small offcuts up the pipe.
I keep seeing implications that 26 gauge snap lock (stovepipe) steel will fail in a dust collection situation and frankly I feel it is doing a disservice to those looking at all options for duct material selection. Please provide substantiated failure examples in non-commercial situations or refrain from casting a shadow on that material. It works fine for me and many others. My 2 hp Oneida DC, with all of the gates shut and joints sealed, has not and will not collapse. Much thinner 30 gauge snap lock will fail. If mine ever collapses, and I don't see how that would happen short of beating it into submission with a hammer, I will let you know and I will be prepared to "eat crow". If you thunk the 2 materials (26 and 30 gauge) you will see a huge difference.
Last edited by Ole Anderson; 02-14-2012 at 11:27 PM.