The lack of standardization in CFM test methods makes what you are trying to do very hard. The lack of correlation due to your real life design, with its particular vent aperture area and layout, backpressure, components restricting flow, etc is going to make this even tougher.
You may well find that even within a given fan manufacturer's offerings you can't rely on CFM ratings alone. Differences in the number of blades and their design, RPM, direction of rotation, etc, may make one fan more immune to the details of your design than another. We have tested higher CFM rated fans that have our equipment running hotter than the lower CFM one we were hoping to replace. This is, in my opinion, an area where NRTL's allowance of substitutions based on a paper evaluation of ratings is inadequate. We always test. Regards, Jim Eichner, P.Eng. Group Leader, Engineering Services Xantrex Technology Inc. Mobile Markets web: www.xantrex.com <http://www.xantrex.com> Any opinions expressed are those of my invisible friend, who really exists. Honest. -----Original Message----- From: Doug McKean [mailto:dmck...@corp.auspex.com] Sent: Tuesday, June 19, 2001 4:05 PM To: EMC-PSTC Discussion Group Subject: Re: You won't believe this ... Well, maybe you will. "Rich Nute" wrote: > > Hi Doug: > > The issue for me is: What is the safety requirement > that requires cfm (I presume a minimum cfm)? The issue is a Hazardous Energy ( > 240va). The power output that feeds the board is above the limit. The fan itself is more of an issue of having a baseline with which to allow alternates to be used. If I can prove by way of fan company documenation that the fan is x cfm, then that's the basis for any other fan being used. The NRTLs do only construction review and locked rotor testing. Add to that a plastic housing (that's approved), and then add additional heat to the from the chip in case of fan failure and that does becomes a concern. But the issue with the fan is simply a baseline. > So, I presume the safety requirement is that of > temperature of the PWB. Without the fan, the PWB > temperature would rise above the limit value specified > in the standard. It's possible. > For the purposes of safety, nobody cares whether the > fan is effective at cooling the chip, or even if the > chip gets so hot as to self-destruct. We are only > concerned with the temperature of the PWB. > > Working with these data, I see a number of ways out > of this predicament. > > 1. Control the fan by manufacturer's name and model > number. The cfm is not necessary. We simply > know by test that the cooling provided by this > specific fan is sufficient to keep the PWB from > exceeding the allowable limit. The mfr's p/n is not enough. > 2. Control the fan by electrical ratings and physical > size. The electrical ratings (power) are > proportional to cfm. Good point, but again, it's the cfm as a baseline. Thanks for your input ... Regards, Doug McKean ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall," ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall,"
