Originally Posted by
ian maybury
To Rod's point on fan HP. Bill went for a 5hp motor, but most installations even with the big 16in impeller don't in practice seem to pull much more than something approaching 4hp in practice. Which implies from the fan curves that they probably run at around 9 - 10in WG pressure drop.
Reduce the duct size, or lengthen the runs (increase the pressure drop) and the HP will drop even further. (fans are counterintuitive in this regard)
He (Bill) says that it was either that or a 3hp with a high service factor (the next stock size up after 3hp is 5hp), and since even the 14 in or so impeller he then was using was already pulling a bit over 3hp steady state he went for the 5hp motor. He was cautious as well about the overly optimistic rating of motors by some makers, and didn't want people to suffer failures.
The logic was (a) there is no significant running cost penalty with the larger motor because the power consumption is similar (it does create the issue of higher start up current though - although that's seemingly not such a problem for you guys in the US), and (b) that if a system is accidentally run with a lot of blast gates open or a section of ducting missing the big motor isn't going to immediately overload. It's also been able to handle increases to 15in and then 16in in impeller size without problems.
My guess based on my own system and with the help of feedback from others is that the big 16in impeller is quite likely somewhat restricted on 6in ducting - that if for example somehow 7in ducting was available to allow the pressure drop in your typical system to be reduced that it would deliver quite a bit more CFM, but at the expense of pulling more amps - it'd bring it closer to fully using the 5hp motor.
Which of course may be heading towards being more than is needed for effective dust collection on a single machine at a time system - I've not seen any information on this. One problem in this case is anyway going to be that few machine hoods will allow these flows - the result of very large ducting in that case may simple be to reduce the air speed more than is ideal for good transportation.
The advantage of running a big impeller slightly restricted is that it'll cope better with machines with smaller hoods, partially blocked filters and the like. A larger impeller has a higher tip speed, and as a result creates a higher level of suction/depression at a given rpm than a smaller one. Put another way - slower tip speed seems to result in a fan curve which drops off more rapidly at the high pressure end of the fan curve.
Building in this extra cushion for reliability of course makes good sense in the context of the DIY style high CFM system that Bill had in mind - most of us will happily pay a little more for a rock solid solution....
ian
The story I got when I became their distributor in Australia went like this. The 16" fan was produced for export as most export markets run the fan at 2800/50hz not 3450/60hz as happens in the US and the larger fan overcame the drop in RPM and restored the airflow to equal what the US enjoys with the 1800/60hz. I recently did some current draw tests on the 1800 at 220/50hz and with no ducting it drew 12.8 amps full load which is nowhere near 5hp and at no load it was about 4 amps. This was through a VFD operating at 60 hz secondary into the motor and measured at the primary supply into the VFD at 50hz single phase. We run the three phase motor from a single phase input into the VFD to restore the speed.
Chris
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Don't think anybody has dogs in, it's more a matter of trying to understand what could be going on Jim - if only for the selfish reason that we're running these systems.