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Thread: DC chip vs fine dust collection

  1. #76
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    Of course if your verticals are a bit different you don't need to worry as much about the diameter of the drop and velocity to move the dust up the hill.


    I think the longest vertical distance my dust has to go is a tad over 4' . . . . . . . DOWNWARD!! Just couldn't resist!






  2. #77
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    I think I am going to run an experiment on current draw. I posted here some current figures that others said were not correct and have since had nearly exact corroboration of those figures from another system I had no hand in putting together, in fact his were slightly lower so to some extent I feel vindicated. I have a feeling that the 6" inlet on the 1800 fan housing is a choke point for the 15" fan and if the 15" impeller were put in a Max housing I would see a rise in current draw due to the 8" inlet which means that the 16" fan could use a bigger inlet also. For the Max I always recommend that the system be built with an open 6" duct to atmosphere to scrub the air at all times as well as the duct to the machine. To me running an 8" main down to one 6" duct is not a good idea at all and it is clearly impossible to run 8" to a machine so why not scrub the air, every little bit helps.
    Last edited by Chris Parks; 02-15-2012 at 9:05 PM.
    Chris

    Everything I like is either illegal, immoral or fattening

  3. #78
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    Chris, I think you will find the amp increase is exactly what you will find . I'm assuming that when you run a system with 6" open to scrub the air you are diverting from an 8" and then running 6" mains downstream. It would be interesting to not only measure the amp draw but hold a fan type anemometer on the machine port end and measure the velocity with the "scrubber" open and closed. would tell you how open the scrubber port could be without affecting the flow at the machine end. I've found it even matters where the open port is as my system branches out in several directions. Watching the correlation between cfm and velocity as compared to amp draw is pretty interesting. My system varies from about 9.4 amps closed to 12.6 fully open at 63 hz. It seems to max out at about 1600 cfm. The remote location of the blower causes some loss, hence the 63 hz to make up. Seems to add about 11-14% to the flow. Dave

  4. #79
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    Quote Originally Posted by Ole Anderson View Post
    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.
    Ole, I didn't mean to imply the stove pipe duct would fail under a certain negative pressure. I just stated that spiral duct resists buckling better than the crimp together duct. If these two ducts are the same gauge, then the spiral duct will be stronger because of the mechanical strength gained from forming the spirals. In my mind, the spirals would act like stiffeners, but maybe I'm mistaken.

    If the crimp together duct works for you, then by all means use it. I would have probably used the crimp together duct for my system, but the S&D pipe was much easier to work with for me.


    Mike

  5. #80
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    Chris,
    I would offer that if you have a fan that can handle an 8" duct at 4000 FPM, then it would be best used on a system where you have two users or a machine that requires a lot of CFM (1400 CFM). However, I totally understand buying bigger when you get the chance so you are prepared for future needs. You are correct, that it is a waste to use only a 6" if the fan can handle an 8". If you have the bleed, do like David said and choke it down until you get good flow at the machine. If the bleed is close to the fan, it may be nearly closed off, and could make a lot of noise. You could also locate the bleed at the end of an 8" run, and use 6" drops to the machines.

    Another idea might be to duct the bleed up to the ceiling and build a home-made air cleaner. I experimented with the design of one that used a straight duct with numerous holes in it. You could wrap the holed duct with filter media. The design difficulty is getting good distribution of the air through the holes for the entire duct length. Just an idea.

    Mike

  6. #81
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    I've certainly seen the collapse of lightweight ducting reported somewhere Ole (very possibly here), but can't remember the details which anyway may or may not have been the whole story. It does suggest though that care is needed with lightweight ducting.

    Ditto on the leakage Mike mentioned. Noticing that the fits on the spiral I used were a bit sloppy I ran the numbers for an average 0.5mm gap x no. of joints. It ran out to something like 30% of the open area - so taping or effective sealing is definitely needed.

    On blower inlet and duct sizes. Should it be of interest to get some data points Chris (I can take some amp readings) my system has 160mm dia (6 3/8in) spiral Euro ducting all the way on the suction side, including the sleeve at the entry to the inlet connection to the cyclone.

    Apart from the top cylinder being a couple of inches longer to suit the thro' floor installation the cyclone is fabricated from galvanised steel to the dimensions on Bill Pentz's pages. The fan inlet tube is 9in in diameter for an 18in cyclone body diameter, and its entry (the lower end) and the cyclone inlet chute and ramp are positioned relative to each other and to the cone exactly as in Bill's drawing. (the inlet tube that drops down the centre of the cyclone)

    The impeller and blower housing are Clear Vue 16in CV Max.

    The blower outlet is 8in (I used a modified transition that attaches directly to the body - see pic below), and it connects straight to an 8in dia x approx 3ft long attenuator before taking a (2 x 45) 90 deg bend to drop about 2ft into the filter cabinet. The outlet from the cabinet is also a full 8in diameter through a 90 deg bend. The filters are 2x Donaldson nanofibre/HEPA cartridges. The option is also there to vent to the outside through an 8in leg that runs about 3ft further on past the bend. This means that my example is 8in or larger right from the blower inlet to the exhaust.

    On the suction side the 160mm ducting is full bore all the way too - from and including the flexibles to the end of cyclone spiral. The cyclone inlet chute and spiral have about 9x4 = 36sq in of open area - a bit bigger than 6in duct, but smaller than 8in. The distance and number of bends is a bit dependent on which drop is in use. The shortest run is about 30ft, and has about four 45 deg bends in it. The longest is about 45ft with about 6 bends.

    Despite the largish exhaust and lack of restriction in the blower my system still seems to run a bit restricted - even with the drops not connected to a possibly restrictive machine hood. I didn't record the figure, but think it's drawing around 4HP with one blast gate open.

    I wonder is it possible that turbulence/inefficiencies in the blower or the cyclone (a cyclone is in one way a device that uses energy contained in the airflow to separate dust - so by definition it's restrictive) could be a restricting factor too? (as well as duct/inlet/exhaust sizes) My blower has the silencing strip screwed to the outer housing just before the exhaust which is supposedly some sort of attenuator -maybe it restricts the flow a bit.

    ian

    blower exhaust transition.jpgblower assembled on cyclone.jpg
    Last edited by ian maybury; 02-15-2012 at 3:37 PM.

  7. #82
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    Ian, that is one of the nicest BP cyclone executions I have seen.

  8. #83
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    Thank you very much Phil, I really do appreciate the compliment. There's a full album of build photos on the Clear Vue forum gallery here: http://www.gallery2.clearvuecyclones...Max/ondablade/ Since you're encouraging me I've tacked a few more from it below.

    I'd better not hijack the thread, but i should say that i 'cheated' a little. Or at least in order to keep the price down to something I could afford and at the same time get the parts of the fabrication needing proper equipment done right I drew up and had made by a HVAC fab house the basic cyclone cylinder and cone assembly, the inlet chute, the blank for the spiral, and the inlet and exhaust transitions. I handled the labour intensive (expensive) cutting in and assembly, and soldered it up.

    This was my take on what I've been banging on about when i talked about buying strategy. HVAC fab guys with the right equipment are hugely cost effective - all of the above sheet metal parts cost me around $400. (and our costs are very comparable to the US) The other big saving opportunity I've been talking about is similar - think of buying your spiral ductwork from one of these companies that manufactures it. (quite a few have the forming machine)

    They are low margin low overhead establishments, and they won't have fancy web pages and brochures for you to select from - but it's only when the branded retailing OEMs with marketing managers to support get their hands on it that the price sky rockets.

    I'd better stop now....

    ian

    motor & blower assy.jpgfilter cartridge mounting.jpgfilter cabinet gasket detail.jpgcyclone assembled in mounting .jpg35 gal dust drum & drop from cyclone.jpg
    Last edited by ian maybury; 02-15-2012 at 8:02 PM.

  9. #84
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    What about static pressure?

    Doesn't the static pressure capacity of the dust collector's fan play a significant role in its ability to pull in dust or chips? A 400 CFM fan with a static pressure capacity of 1" will not be able to draw as much dust as a 400 CFM fan with 3" of static pressure capacity. This is where the design of the dust collection system (i.e. the static pressure generated by the equipment, hoses and ducting) relative to the CFM and static pressure capacity of the collector is important. Correct?

  10. #85
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    Hi George. Probably the simplest way to put is that a given fan and motor will (presuming it remains in good shape) always produce a specific volume flow of air (in CFM or cubic feet per minute) when asked to suck air through a system that creates a specific pressure resistance (in WG or inches of water gauge) at that particular CFM.

    Put that same fan on another system that creates a higher pressure resistance at that CFM (or on a test rig that can adjust the pressure - the fan responds mostly pressure resistance for a given duct layout), and the result will be that the CFM drops - the fan hasn't got the HP to deliver the original 400CFM at this higher pressure. Increase the pressure again, and it will reduce further, and if you keep on repeating this it will keep on dropping until a pressure is reached at which the output of the fan falls so low as to be pretty much useless.

    The reverse is also the case - keep on reducing the pressure and the CFM output will increase until it approaches a maximum determined by its internal restrictions, or by the maximum power (HP) the motor can deliver. (it reaches its peak amps)

    If you were to every time you repeated this exercise to record the pressure resistance and CFM delivered in a table you would have the information needed to draw a fan curve - a graph showing the CFM the fan will deliver over the full range of pressures it's capable of operating over.

    The point to be remembered is that a fan that can handle higher pressure is only going to deliver whatever CFM the fan curve predicts for whatever pressure it's running against - you can't convert 'surplus' pressure capability to extra CFM. Against that a fan with higher pressure capability will deliver higher CFM on a system that's more restrictive - again as predicted by the fan curve.

    The one option generally available with a fan to get more CFM output from it is to increase the RPM at which it is driven with a different motor, or a VFD. (by up to maybe 10% at most or as permitted by the makers) This increases both the CFM output, and the pressure capability. Meaning that every time you change the speed of the fan it will perform to a new fan curve.

    A given system meanwhile produces a given pressure drop in in WG when you flow air through it at a given CFM. The pressure drop increases if you increase the CFM, and reduces when you decrease the CFM. If you tested your system and made a note of the pressure drop measured for each CFM tested you have the information to produce a graph a little like the fan curve - only this time called a system curve.

    Typical fan and system curves are shown here: http://blog.mechguru.com/machine-des...ng-fan-curves/

    To select the required fan and motor combination to suit our system we have to first decide what minimum CFM or airflow we need. The factor that determines this is the design airspeed in FT/MIN - the 4,000FPM minimum in verticals has been mentioned many times. Multiply the cross sectional area of your duct in square feet by the required airpseed and you get CFM. e.g. 4in duct @ 4000FPM = .0855x4000 = 342 CFM; 6in duct @4000FPM = .1963x4000 = 785CFM.

    To make the selection it's a case of extracting the total system pressure drop in inWG from our system curve at the target CFM. (usually for the 4000FPM above) You then switch to the fan curve and read off the CFM the fan is capable of at this pressure drop. If it's higher than the the target CFM then it has more than enough capability to deliver the required airflow in the system.

    To get to the actual CFM it can be expected to deliver keep on raising the target CFM in small steps (decrease if the original number was below the target CFM), extracting the corresponding (higher/lower as applicable) pressure drop from the system curve, and reading the CFM for that pressure drop off the fan curve. Eventually you'll find a pressure drop for which both the fan and system curves predict the same CFM - this is the actual CFM your fan should deliver on your system.

    I'm sorry if it's all a bit of a theoretical mouthful. In practice it's hard to get reliable data, and these basic calculation methods are to a degree not exact - so it's not an exact science. Fan curves for dust systems produced for hobby and DIY markets are often especially dodgy and misleading in their claims.

    One way of cross checking these often outrageous claims is to look up the fan curves published by a reliable industrial supplier like Cincinatti Fan for a fan at your RPM and with the same size of impeller. (their SPB/3,450 RPM fans seem to be similar to what we use, and they table numbers for sizes from about 1HP to far larger than anything we use)

    The second issue is that unless you have fluids modelling package or are prepared to do a lot of paper work you'll probably like most of us (that are given to numbers) just estimate the likely pressure drop of your system at 4,000FPM - using again the tabled pressure drop data for duct sizes and bend/fitting types published by reliable industrial suppliers like the above do (this time in their Engineering Data booklet), stick it into the fan curve and satisfy yourself that you have plenty of puff in reserve. To get to this number you'll probably find yourself chasing the makers for pressure drop figures for cyclones, filters and the like which have to be added in with that of the ductwork too.

    You might only estimate the total pressure drop in your system at 4,000FPM and satisfy yourself that it's comfortably within the typical 6 - 12in WG operating range and will suit a fan you know from friends can do the job. i.e. that your system is not going to be more restrictive than normal.

    Or you may just rely on the fact that your friend is running and doing well with a similar ducting layout and fan to what you will want to install, or that the maker checks out your proposed duct runs and tells you that it'll be OK....

    ian
    Last edited by ian maybury; 02-16-2012 at 6:29 PM.

  11. #86
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    Quote Originally Posted by Michael W. Clark View Post
    Ole, I didn't mean to imply the stove pipe duct would fail under a certain negative pressure. I just stated that spiral duct resists buckling better than the crimp together duct. If these two ducts are the same gauge, then the spiral duct will be stronger because of the mechanical strength gained from forming the spirals. In my mind, the spirals would act like stiffeners, but maybe I'm mistaken.

    If the crimp together duct works for you, then by all means use it. I would have probably used the crimp together duct for my system, but the S&D pipe was much easier to work with for me.


    Mike
    I agree that the spiral crimp will add a degree of stiffness. Fair enough, thank you.

    Quote Originally Posted by Ian Maybury
    I've certainly seen the collapse of lightweight ducting reported somewhere Ole (very possibly here), but can't remember the details which anyway may or may not have been the whole story. It does suggest though that care is needed with lightweight ducting.

    Yes, floating around here somewhere is a pic of a fully collapsed steel duct along a ceiling. Quite interesting. But if I remember correctly the OP said he went with the thin (30 gauge) stuff for sake of expediency. That is why I always note 26 gauge steel duct and warn against the 30 gauge.
    Last edited by Ole Anderson; 02-16-2012 at 8:54 AM.

  12. #87
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    George,
    The combination of CFM and design of the machine hood is what brings the fine dusts and chips into the ductwork. The fan's job is to provide this CFM at the hood. The fan has to overcome the resistances in the ductwork to provide this CFM. The system resistance depends on the losses in the duct system and hood arrangement.

    As Ian said, if you have an existing system, you can measure the flow and static pressure. Then, if the system does not change, the static pressure can be calculated at any new CFM desired. The static pressure changes with the square of the change in CFM. If you double the CFM, the resulting static pressure goes up by a factor of four (for the same duct system). Also, the BHP (brake horsepower) goes up with the cube of the change in CFM. The BHP would go up by a factor of 8 if you double the CFM on a given system. This can sometimes be a concern with VFD's if the motor BHP is borderline and you drive the VFD too far above 60Hz. The CFM increase is typically proportional to the fan speed change. If you increase the fan speed 10%, then the BHP requirement would increase by 33% (1.1^3 = 1.33) for the given system.

    Also, again as Ian said, you can refer to fan manufacturers rating tables for a comparable fan to get an idea of your fan performance. Take some dimensions of your fan (inlet, outlet, scroll diameter, scroll width, etc.) and find one that has similar dimensions. If your fan is direct drive, use the motor RPM for the fan speed. If it is belt drive, you will need to adjust the motor rpm by the pulley ratio to get the fan speed. Enter a fan rating table for a fan similar to yours and plot the CFM and SP for the points listed near your fan RPM. This will give you a good idea of the fan curve. You can plot the system curve (using the relationship above for CFM and SP to develop more points for it) on top of the fan curve. Where the two lines cross is the system operating point. If the system changes (duct plug, blast gate more closed, step on a hose and flatten it, etc.) the system curve changes. If more reistance at the same flow, the curve is steeper, and it is shallower for less resistance at the same flow. The fan curve doesn't change unless you change the fan speed or damper the fan. As the system changes, the fan will have a different operating point.

    This all sounds very technical, but if you go through this excercise, don't get hung up on absolute accuracy. If you do the calculations for your system, then measure the performance and get within +/-5 or 10% of your calcs, pat yourself on the back, job well done.

    Mike

  13. #88
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    George, Ian and Michael give great advice. I can't find it now but there are charts that calculate the SP of various pipe sizes, generally at 100' lengths and of the various diameter and radius fittings. It is important to also realize that the resistance of pipe goes up fairly quickly with the velocity of the airflow. If you are trying to exceed the 3500-4000 fpm standard you will see how quickly the pipe and fitting resistance adds up. It is worthwhile to study different system curves as you quickly get an idea how impeller, HP, and filter area affect the performance. I assume my system will operate between 8 and 12" SP and look at the cfm in that range. You will find that you are generally operating at the higher end of the SP and lower end of the cfm so anything you can do to reduce resistence within the velocity range you are striving for pays dividends. The law of diminishing returns also applies. the Cincinnatti fan ratings are pretty interesting as well. You can see that while we all talk about fan diameter there are other factors that influence cfm and motor amperage can be exceeded pretty easily if your system resistance is low. Dave

  14. #89
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    I enjoyed and mostly understood the theoretical/technical discussions by Ian and Michael in the last few posts. It makes sense that as you increase the resistance of your circuit, the CFM will go down for a given motor. However, I'm struggling to understand whether there is a constant in a given system. Its clearly not CFM, FPM or static pressure. In other words, is there some parameter (e.g. horsepower, or fan speed) that stays constant despite what we do to circuit resistance? Or do all of these parameters vary together as we change any one of them? This may be drifting too far from the OP, but could you say a little more about what is happening to HP, amp draw, and fan rpm, and make it so easy that even a doctor could understand it?

  15. #90
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    Jim, others will be more coherent but here is my take. Fan speed is the only constant unless you vary it by increasing the rpms by increasing the HZ if using a vfd or changing the pulley size in a belt driven unit. Amp draw depends on the cfm moving through the fan so as resistence brings the static pressure to the limit of the fans ability to pull air the amp draw stops increasing. Increasing rpm can increase the cfm and the amp draw to a point but other factors will interfere so there is a limit to how fast you can run the fan and get a benefit. To get the most from your system you want to fully load the amp draw to the motor specs. In my system the 5 hp 3 phase motor pulls 12 amps with a 1.15 service factor so I try to make sure the farthest ports draw 12 amps when open and the closer ones can be more if I don't exceed the margin built into the motor-13+ amps. Pipe, flex, cyclone and filters all add resistence so it becomes important to minimize that as generally a system is running SP at 8-12" and tops off at 12-14". Pipe size, filter area, and velocity all effect cfm so sizing stuff is important. When I built mine, the farthest drops to the edge sander only pulled 11.6 amps when open so I increased HZ to 63.5 to increase the rpm and draw 12 amps. I can slow it down if using the machines close to the cyclone. The full 12 amps will give me 1500-1600 cfm. Many systems run less than full amps due to the various restrictions in place and if single phase have no adjustability other than changing pipe sizes. At the end of the day the only way to pull more amps is to pull more air and vice versa. Flow readings and amp draw are the only ways to really judge how it all works. Now Ian will give you the technical answer. Dave

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