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Thread: Pentz blower sizing

  1. #1
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    Pentz blower sizing

    Several years ago I built a Pentz cyclone and blower. Well I'm going over my setup and I'm changing some stuff around and I see that my blower housing is not to the correct size. I believe that when I built it it was correct to the plans listed but now it seems that there was a redo of the info on bills site about the blower. I have a 15" impeller but the plans are for a 14" and 16". It says that for a 15" just print out at 107% scale. I'm having problems getting that to work on windows 10 or OS X. I also don't know if it's scaling correct because I don't have any dimensions to go off. Ideas. I thought about just multiplying the 14" dimensions x 1.07 but it would be much easier to print it off.


    http://billpentz.com//woodworking/cyclone/blower.cfm

  2. #2
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    Bump ..
    cheers

  3. #3
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    He gives dimensions here (for the 16" I believe):
    http://billpentz.com/woodworking/cyc.../budblower.gif

    You can scale the 16" images by multiplying by 0.9375 to get the same scaling as the 14 * 1.07, that might be easier to get to print as you're not running off of the page at least. If you can get all the % digits in there round to the nearest one (16*.94 gives a 15.04" compatible housing which is darn close).

    Between those two things I believe you should be able to get it to print and cross check the scaling.

  4. #4
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    Quote Originally Posted by Ryan Mooney View Post
    You can scale the 16" images by multiplying by 0.9375 to get the same scaling as the 14 * 1.07, that might be easier to get to print as you're not running off of the page at least.
    The spirals are different on the 2 drawings, as in each is meant for a different exhaust size. IIRC, the 14" housing is designed to transition to 8" dia exhaust, and the 16" should transition to a 10".

  5. #5
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    Quote Originally Posted by Ed Thomas View Post
    The spirals are different on the 2 drawings, as in each is meant for a different exhaust size. IIRC, the 14" housing is designed to transition to 8" dia exhaust, and the 16" should transition to a 10".
    Hmmm interesting point. The 14" scaling gives an 8.56 sized exhaust and the 16" scaling gives a 9.375" exhaust which is about an inch of difference. I don't think the original 1.07 looks quite right with that either.

    The scaling should give the right dimensions for the body but you'll likely have to fair the curve to the exhaust by hand a bit to make it work with something you can reasonably hook up to it.

  6. #6
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    Quote Originally Posted by Ryan Mooney View Post
    ... fair the curve to the exhaust by hand a bit to make it work with something you can reasonably hook up to it.
    Better to make a proper transition from either of those outlets to a 10" exhaust pipe. Going to 8" would create some unnecessary back pressure (I think). The instructions to make a transition are also on Bill's site.

    Scaling the drawings is not ideal. To draw the spiral you have to know the offset of the wheel into the exhaust. From there you add the gore clearance to the wheel radius which locates the gore point. Given a specific width of outlet you can calculate a linear expansion of the spiral from the gore point to the outer side of the exhaust outlet.

  7. #7
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    Quote Originally Posted by Ed Thomas View Post
    Better to make a proper transition from either of those outlets to a 10" exhaust pipe. Going to 8" would create some unnecessary back pressure (I think). The instructions to make a transition are also on Bill's site.
    I believe that as long as the exit has as much or (preferably) more cross sectional area as your input you're ok (certainly a little extra won't hurt and gives you some wiggle room for bends, etc.. and I believe that square tubing has some more restrictions than round but don't quote me on that). Not sure what Dave's piping looks like so I can't answer that one definitively but for 6" round pipe only has ~28.25" of cross section and an 8"x5.5" square opening (assuming similar height as the clearvue impeller) has 44" of cross section so assuming he's using the ubiquitous plastic pipe setup even an 8" opening isn't to bad. If the input is an 8" pipe then an 8" outlet (again making assumptions about the height) is indeed a significant point of back pressure.

    Your points about the spiral are well made, that section of the design is indeed quite complicated and has a lot of trade offs in performance versus noise versus etc... I don't claim to understand that bit well enough to make reasoned suggestions. (I do know the less clearance up to some limit the more noise and higher performance until eventually you've built an air raid siren but past that there's a lot of hand waving).

  8. #8
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    Pardon me if I'm a bit rusty on this. FWIW, I drew the 14" housing to a 5.625"H x 7.0"W outlet (Bill's spec) circa '04, for a group build (~20) in Canada. The tiled version on Bill's site is from that endeavor. The 16" was drawn to 6.25"H x 9.5"W outlet (IIRC, for Alan in Magnolia,Tx?, his spec) in '09. He didn't have space/height restrictions so got a freer flowing housing. I don't remember why the height spec was increased. And there's one on the drawing board 6.5"H x 7.5 (or 8.0)"W that I'm having trouble finding the round tuit for.

    Quote Originally Posted by Ryan Mooney View Post
    and I believe that square tubing has some more restrictions than round but don't quote me on that
    square tube has more wall (for the air to rub on) relative to round tube's cross section so yes there is more resistance

    I believe that as long as the exit has as much or (preferably) more cross sectional area as your input you're ok (certainly a little extra won't hurt and gives you some wiggle room for bends, etc.. <snipped>). Not sure what Dave's piping looks like so I can't answer that one definitively but for 6" round pipe only has ~28.25" of cross section and an 8"x5.5" square opening (assuming similar height as the clearvue impeller) has 44" of cross section so assuming he's using the ubiquitous plastic pipe setup even an 8" opening isn't to bad. If the input is an 8" pipe then an 8" outlet (again making assumptions about the height) is indeed a significant point of back pressure.
    Just to clarify (note: I'm not an air engineer): you can design a blower with 12" inlet and 6' outlet and not have a problem, or vise versa. I expect there would be a problem if you had a 12" outlet and choked it down to 8" shortly thereafter. I would expect that as the vane passes through the gore point area the back-pressure would amplify the existing pulse waves. So, what I was trying to say in the prior post was 1) don't mess with the spiral at all (it has a linear progression [Bill writes of the need for a smooth & even spiral just before the section with the drawings]) 2) don't reduce after the outlet.

    The question then becomes does scaling either of the drawings give a outlet with a cross section less than an 8" pipe.

  9. #9
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    Quote Originally Posted by Ed Thomas View Post
    Just to clarify (note: I'm not an air engineer): you can design a blower with 12" inlet and 6' outlet and not have a problem, or vise versa.

    Sure but you become limited to the throughput of the lesser of the two so if you've got an impeller sized for duct size X and and inlet of duct size X it doesn't make a lot of sense to make the outlet smaller than X because it then becomes the bottleneck. Conversely making the output substantially larger than the input doesn't really buy you much (but doesn't hurt either). I think we're all in more or less violent agreement on that part :D
    [1]

    Quote Originally Posted by Ed Thomas View Post
    square tube has more wall (for the air to rub on) relative to round tube's cross section so yes there is more resistance
    Out of curiosity I looked up the math for square vs rectangular ductwork and found http://energy-models.com/piping-and-ductwork-systems-0 (using the the "Frictional Losses for Noncircular Ducts" section) and quickly ran the numbers for those outlet sizes. The numbers are pretty interesting indeed (and a fair bit worse than I'd naively expect). Its interesting to note that for a perfectly square pipe the expected throughput is really really close to what you'd get for a round pipe with the diameter of the squares side (so basically that extra space isn't helpful at all - the 8x8 was added to illustrate that).

    For posterity the formula is:
    Round pipe equivalent diameter = 1.3 * (((x * y)^0.625)/((x+y)^0.250))

    X Y Equivalent Round Pipe
    5.625 7 6.85
    6.25 9.5 8.38
    6.5 7.5 7.63
    6.5 8.0 7.87
    8.0 8.0 8.75

    Quote Originally Posted by Ed Thomas View Post
    The question then becomes does scaling either of the drawings give a outlet with a cross section less than an 8" pipe.
    Indeed, and without knowing the height of the outlet for Daves build I can't answer that . It appears likely though (although to my above point if he only has 6" ductwork then a 6"+ equivalent cross section would be the limit).

    A related question is whether or not scaling the drawing changes the angles of the spiral, I don't ?think? it does.. but am certainly will to be proven wrong there. The two drawings appear to have slightly different spiral angles as well (in order to obtain the desired output duct sizes) so there appears to be some fudge room but how much and in what direction .. I don't know and will once again resort to a vigorous waving of hands


    [1] Correction on the strikethrough part - obviously not just duct diameter but length matters on the loss, the statement above vastly oversimplifies things such as to be not useful.
    Last edited by Ryan Mooney; 04-22-2016 at 06:13 AM.

  10. #10
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    Quote Originally Posted by Ryan Mooney View Post
    A related question is whether or not scaling the drawing changes the angles of the spiral, I don't ?think? it does.. but am certainly will to be proven wrong there. The two drawings appear to have slightly different spiral angles as well (in order to obtain the desired output duct sizes) so there appears to be some fudge room but how much and in what direction .. I don't know and will once again resort to a vigorous waving of hands
    I'm not quite sure I understand your question re: "angles of the spiral". I think that is what I would call "degree of ramp". If you can visualize the "angles" as a line relative to the center of the wheel, you would see a straight line, just one angle, no angles.

    Looking at pages 1-4 of http://billpentz.com/Woodworking/Cyc...werhouse16.pdf you can see how the splral is created. Page 1 establishes the parameters. Page 2: the upper set of lines represents the distance from wheel center to the gore point (9.6") swept through 298º in (1)3º and then (59)5º increments (actual requirement is 297.953º); the lower set of lines represents the ramp from the gore point radius (9.6") as it expands to meet the outlet outer radius (14"). A line drawn across the ramp would show it to be straight. Page 3 shows the ramp's radii rotated as required. Page 4 shows the ramp's radii located to wheel center and the Bézier curve(s) placed thereon.

    The 2 drawings, of course, have different ramps. Scaling a drawing does not change the degree of the ramp.

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