Neglecting Fan Heat Dissipation in Electroncis Enclosure Analysis?

[spongebob007]

I have always included the power dissipated by the fans in thermal calculations and CFD models of electronics enclosures. Recently I had a customer ask me to justify why I assumed that the fans dissipate 100% of the input power as heat in the CFD model I did of their system. So I sat down and did some math. I calculated the fan efficiency and determined that the actual power dissipated by the fan at the predicted operating point was only around 42% of the input power. Next I calculated the air temperature rise through the fan using delta_T=Q/m_dotCp. Regardless of whether I take the fan efficiency into account or not, in each case the predicted temperature rise across the fans is less than 1C. From this the conclusion that I came to is that taking the extra time to include the fan power dissipation has a negligible effect on the overall accuracy of the analysis. Basically the fans don't heat the air up before it enters the electronics enough to be concerned about. Even if the case were such that the fan heat causes a temperature rise on the order of 4 to 5 degrees C, our company design policy is to have 20% margin on allowable temperatures, so that any error would be likely swallowed up by the margin anyway.

So do others doing this type of work exclude fan heat from the analysis? Ha sanyone ever run into a situation where neglecting the heat introduced by the fans has resulted in significant errors?

 

[IRstuff]

Input power as VA or actual power? That would be the only question in my mind.

Actual, real, power must be dissipated either as heat or as mechanical energy, which manifests eventually as heat.

Conservatism is not really a bad thing.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[spongebob007]

These are DC fans, so with my limited EE understanding I am assuming P=VA is it. When speaking of actual power (dissipation) I mean Q_loss=efficiency*P_input rather than simply assuming 0% efficiency. At the end of the day, neither approach has meaningful impact on the downstream temperature predictions and if I neglected the heat dissipated by the fans completely I don't see any issues with that either.

 

[IRstuff]

All of the blenders demo'ed at Costco, Vita-Mix, Blendtec, and Ninja, can boil soup solely from mechanical agitation, so both electrical and mechanical power is converted to heat. If the overall system can deal with that, then the system is designed correctly.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[hendersdc]

I have always included the full rated power of the fans in the CFD analyses. Any CFD analysis is full of assumptions that lean to either the conservative or aggressive side of things and hopefully we come out a little conservative in the end. CFD of an electronics enclosure is at best an approximation that can get you the right number of right sized fans and heat sinks with decent flow over your components of concern. Anything that pushes the envelope enough to require that level of detail around fan power dissipation requires heavy testing to validate the results anyway.

 

[gruntguru]

The answer to the focal point of your question is - your customer is wrong. Electrical energy going into the fans should be added to the total heat load applied to the ventilation air.

je suis charlie

 

[willard3]

If the fan energy doesn't go into the airstream, where does your customer propose that it goes?

 

[lilliput1]

If the fan blows air into the enclosure then include fan heat. If the fan exhaust air from the enclosure, do not include fan heat.