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Thread: cfm
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17th February 2013, 11:11 PM #1Intermediate Member
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cfm
Hi
Just installed my supposedly 2300 cfm dusty. Followed bills page, 6" pvc 45deg ys to machines . straight run and 3 45deg elbows into the impeller. Air speed in the main duct is about 6000 ft / min . Does not equate to 2300 cfm. Research suggests that is about all i can expect with 6" duct.
If i increased the speed of the impeller from the 2800rpm would i increese the vacuum and the cfm or would the primitive aerodynamics of our impellers just stall or indeed fall apart.
Kev
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18th February 2013, 07:42 AM #2.
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That sounds like one of these?
The 2300 cfm will be for the impeller only ie no bags and no twisted ducting between the impeller and bags, all of which cause restriction.
Followed bills page, 6" pvc 45deg ys to machines . straight run and 3 45deg elbows into the impeller. Air speed in the main duct is about 6000 ft / min . Does not equate to 2300 cfm. Research suggests that is about all i can expect with 6" duct.
If you measured 6000 ft/min accurately that is equivalent to 1178 cfm.
If you only measured the air speed in the middle of the pipe it won't be that air speed all the way across the pipe and the CFM will be even less. I get about 8330 ft/min in the middle of my 6" pipe but the CFM is about 1260.
1250 cfm is about the limit for a 3HP motor and those impellers.
If i increased the speed of the impeller from the 2800rpm would i increese the vacuum and the cfm or would the primitive aerodynamics of our impellers just stall or indeed fall apart.
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18th February 2013, 08:41 PM #3Intermediate Member
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Yep Bob it's the big blue one. Looks like i need some schooling in measuring I assembeled all the blue stuff. left the bags off. Attached a piece of 6" PVC about 300mm long sealed with duct tape to the fan housing. Cut a hole 150mm from the end and positioned fan of the air speed meter in the center of the pipe.
I am concerned that if the fan is not producing what it should that i may have a ecectricial supply problem as i have burnt out 2 3hp motors in my saw in 6 months.
Kev
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18th February 2013, 09:20 PM #4.
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Yes it's not as straightforward as it seems. It actually takes a fair bit of experience and the right instruments to get accurate readings.
Have you read my post which showed how I did mine?
I assembeled all the blue stuff. left the bags off. Attached a piece of 6" PVC about 300mm long sealed with duct tape to the fan housing. Cut a hole 150mm from the end and positioned fan of the air speed meter in the center of the pipe.
The size of the hole in the duct wall needed to insert a measuring fan will also screw up the reading. Even the very small fans will give incorrect readings. Also the reading position should be at least 750 mm and preferably 1200 mm) from any opening or changes in the duct. Hence you really need to use a 1500 (and preferably 3 m) length of ducting and the measurement position should be half way long that pipe.
I am concerned that if the fan is not producing what it should that i may have a ecectricial supply problem as i have burnt out 2 3hp motors in my saw in 6 months.
A simple to assess power problems is with an AC voltage meter and measure the voltage at the power point with and without the DC running.
If the voltage drops and remains dropped more than a couple of volts then you might have a problem.
BE CAREFUL.