Home » Public Forums » GMCnet » Fan CFM???? (Lazy Boy Dash)
| Fan CFM???? [message #115514] |
Fri, 18 February 2011 12:04  |
g.winger
Messages: 792
Registered: February 2008
Location: Warrenton,Missouri
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Any body know the CFM rating of a 78 AC fan??? Brought fan to work. Took it to the vibration MW and Elec. Put a dot on the cage. Thought it was 1355rpm. But that seemed kinda fast. Used a "strobe" type tach. Problem was we put the dot on the outsde. So if it blinked twice as fast you can't see the second image when its on the other side. Put the dot on the inside face and that works better. So,,,,674 rpm x 5.25 dia.= 21.64" or 14,607" per min. 1217.25ft. per min. 73,035 ft. per hour.= 13.83mph,,,,,does this sound correct??? 675rpm,,,,,5.25 inch dia. Now I don't remember the area of the fan discharge. But whats the formula for mph of air thru a ??? size hole???,,, If you'r wondering why??? Stonger motor, more cfm could equal more air,,,better AC/Heat,,thanks,,,,,,PL
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| Re: Fan CFM???? [message #115521 is a reply to message #115514] |
Fri, 18 February 2011 12:55  |
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wally
Messages: 643
Registered: August 2004
Location: Omaha Nebraska
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| g.winger wrote on Fri, 18 February 2011 12:04 |
Any body know the CFM rating of a 78 AC fan??? Brought fan to work. Took it to the vibration MW and Elec. Put a dot on the cage. Thought it was 1355rpm. But that seemed kinda fast. Used a "strobe" type tach. Problem was we put the dot on the outsde. So if it blinked twice as fast you can't see the second image when its on the other side. Put the dot on the inside face and that works better. So,,,,674 rpm x 5.25 dia.= 21.64" or 14,607" per min. 1217.25ft. per min. 73,035 ft. per hour.= 13.83mph,,,,,does this sound correct??? 675rpm,,,,,5.25 inch dia. Now I don't remember the area of the fan discharge. But whats the formula for mph of air thru a ??? size hole???,,, If you'r wondering why??? Stonger motor, more cfm could equal more air,,,better AC/Heat,,thanks,,,,,,PL
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Howdy Paul,
I think I see where you are going. We can't change the duct work or blower wheel diameter very easy. More rpm and hence pressure would be the easy way to increase air delivery. Here is a calculator that shows effects of increased rpm and power required.
http://www.engineeringtoolbox.com/fan-affinity-laws-d_196.html
Now to find a motor that would fit.
Wally Anderson
Omaha NE
75 Glenbrook
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| Re: Fan CFM???? [message #115522 is a reply to message #115514] |
Fri, 18 February 2011 13:06 |
g.winger
Messages: 792
Registered: February 2008
Location: Warrenton,Missouri
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I think I can come up with a formula form cfm,,,,gues I could look if up too. But area x V x time would be the same as a really long rectangle!! Hey,,,I think I just got it,,,,,PL
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| Re: Fan CFM???? [message #115664 is a reply to message #115514] |
Sat, 19 February 2011 07:53 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
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| g.winger wrote on Fri, 18 February 2011 13:04 |
Any body know the CFM rating of a 78 AC fan??? Brought fan to work. Took it to the vibration MW and Elec. Put a dot on the cage. Thought it was 1355rpm. But that seemed kinda fast. Used a "strobe" type tach. Problem was we put the dot on the outsde. So if it blinked twice as fast you can't see the second image when its on the other side. Put the dot on the inside face and that works better. So,,,,674 rpm x 5.25 dia.= 21.64" or 14,607" per min. 1217.25ft. per min. 73,035 ft. per hour.= 13.83mph,,,,,does this sound correct??? 675rpm,,,,,5.25 inch dia. Now I don't remember the area of the fan discharge. But whats the formula for mph of air thru a ??? size hole???,,, If you'r wondering why??? Stonger motor, more cfm could equal more air,,,better AC/Heat,,thanks,,,,,,PL
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Paul,
Are you trying to calculate CFM from fan RPM?? The two don't have a whole lot to do with each other over the entire spectrum of centrifugal fan designs. And that is just for a single wheel width.
For a little automotive HVAC fan 12~1400 is about right. Less and the motor gets to big (expensive), much more and it gets real noisy fast.
As to what the fan actual CFM is, you should first tap the housing with some static probes and find a delta P for the fan and work your way along from there. I have a feeling that the HVAC design (now that I know how mine is supposed to work - Thanks Ken) could be vastly improved with a little bit of refinement that was omitted 30old years ago.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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| Re: [GMCnet] Fan CFM???? [message #115685 is a reply to message #115664] |
Sat, 19 February 2011 11:50 |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
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You need to be carefull not to increase the velocity of air through
the cooling coil as moisture will be pulled downstream instead of on
the pan.
On Sat, Feb 19, 2011 at 5:53 AM, Matt Colie <mcolie@chartermi.net> wrote:
>
>
> g.winger wrote on Fri, 18 February 2011 13:04
>> Any body know the CFM rating of a 78 AC fan??? Brought fan to work. Took it to the vibration MW and Elec. Put a dot on the cage. Thought it was 1355rpm. But that seemed kinda fast. Used a "strobe" type tach. Problem was we put the dot on the outsde. So if it blinked twice as fast you can't see the second image when its on the other side. Put the dot on the inside face and that works better. So,,,,674 rpm x 5.25 dia.= 21.64" or 14,607" per min. 1217.25ft. per min. 73,035 ft. per hour.= 13.83mph,,,,,does this sound correct??? 675rpm,,,,,5.25 inch dia. Now I don't remember the area of the fan discharge. But whats the formula for mph of air thru a ??? size hole???,,, If you'r wondering why??? Stonger motor, more cfm could equal more air,,,better AC/Heat,,thanks,,,,,,PL
>
> Paul,
>
> Are you trying to calculate CFM from fan RPM?? The two don't have a whole lot to do with each other over the entire spectrum of centrifugal fan designs. And that is just for a single wheel width.
>
> For a little automotive HVAC fan 12~1400 is about right. Less and the motor gets to big (expensive), much more and it gets real noisy fast.
>
> As to what the fan actual CFM is, you should first tap the housing with some static probes and find a delta P for the fan and work your way along from there. I have a feeling that the HVAC design (now that I know how mine is supposed to work - Thanks Ken) could be vastly improved with a little bit of refinement that was omitted 30old years ago.
>
> Matt
> --
> Matt & Mary Colie
> '73 Glacier 23 Chaumiere (say show-me-air)
> SE Michigan - Twixt A2 and Detroit
> _______________________________________________
> GMCnet mailing list
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--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
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