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Thread: Sherwood FM400 3hp extractor
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21st September 2017, 11:16 PM #1Woodworking mechanic
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Sherwood FM400 3hp extractor
I've been looking around at purchasing a 3hp unit and this one came up on special for $449 so not a bad price.
What I have are questions regarding the specs. Timbercon quotes.
1900cfm
14" H2O static pressure
If you use the formulae kindly provided by BobL and apply it to 6" ducting, the Potential cfm comes to 1556 cfm, somewhat less than the 1900cfm quoted.
Reading further in their description, they say that all their static pressure testing is done on 4" ducting, so using the formulae for 4", it arrives at a potential of 518 cfm
Hoey, in the thread " Hafco W328 dust extractor" very kindly provided some measurements and it seems it has a 12" impellor ( same as my 2hp) with 7 blades that appear to have a more aggressive design with a deeper blade.
Question is, looking at these figures, is it worth upgrading from my modified 2hp. If anyone else has one, I'd be more than happy to ship my Testa Hot wire anemometer so they can do their own flow testing. I'd also like some static pressure figures if possible.
it would be nice to get some actual specs. on a range of machines as it would help people with future purchases.
Cheers
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22nd September 2017, 12:08 AM #2.
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The 1900 CFM will be
a) For the impeller alone, no long ducting, no filters
b) performed using a single middle of the test duct measurement i.e. no cross sectional flow profile measurement.
The wording on the Tmbecon ad relating to Static pressure is incorrect.
Static pressure is measured with NO air flowing through the system so the size of the ducting used is irrelevant and clearly they misunderstand what it means.
I could be wrong, but struggle to believe the 14" WC Static pressure.
Nevertheless it should still pull a genuine 1000 CFM with clean filters, short ducting runs and machine ports opened up.
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22nd September 2017, 09:32 AM #3
@ Lappa, the model DC I have is almost universally listed with a 12" impeller, almost every variation of this and even the original UFO-102B documents list either a 12" or 13" impeller, in only one case did I find a 14" impeller listed. Even some of the 4 HP units from the same company list 12" or 13" impellers - really makes it difficult to fathom the quoted specifications, unfortunately nowhere have I seen that critical fan curve data. I've seen WC static pressure quoted as high as 16" for these things. The original listing with a 12" impeller was 9.5" of WC - I think the marketing people just pick a figure from where the sun don't shine.
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22nd September 2017, 10:01 AM #4.
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Originally Posted by MandJ . . . .[COLOR=#333333
It sounds counter intuitive but for a given size and speed of impeller, the static pressure and flow rate are basically trade offs. By reducing clearances inside the impeller it is possible to generate a higher static pressure but this can limit the flow rate at higher back pressures and it also increases noise.
An example of a high static pressure and low flow rate is a vacuum cleaner, 30" of static pressure and 100 CFM with no restriction dropping to < 50 CFM with restrictions, PLUS a lot more noise. Even more extreme in this regard is an air compressor.
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22nd September 2017, 12:45 PM #5
Thanks Bob, that's what I was thinking, I realised a few years back that although these different clones of the same basic design look the same at first glance, the dimensions of the Impeller housing and impeller are different, I have a lot of clearance (compared to others I've seen) around this impeller and there is little chance of even a relatively large chunk of timber getting jammed in this thing, and a few have gone through before I fitted the intake screens, that also make sense as my static pressure in a fraction under 10" both on a digital manometer and my big sloping tube home made unit.
Cheers.
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