Thread: cfm

Originally Posted by Kevin-R

Hi
Just installed my supposedly 2300 cfm dusty.

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.

6000 ft/min measured how?

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.

You will get some improvement in both vacuum and flow - if your motor is 3 phase you could use a VFD - I would not go above 3600 rpm.

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

Originally Posted by Kevin-R

Yep Bob it's the big blue one. Looks like i need some schooling in measuring

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.

Air speed meters that use fans are not designed for use with high speed flow in ducting - they are designed for open air measurements. The fan will block way too much of the air flow in the duct and give a lower reading that when it is not there. Fans also do not have a hope of measuring the air flow close to the surface of the pipe so cannot give an accurate reading full stop.

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.

If you want to burn out the motor in your DC just run it with out any bags and little or no ducting. The motor will draw the most current when it is doing the most work which is when it moves most air which is when it is least restricted. Bedsides, you want to know how much air it moves with the bags and ducting.

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.