Fan selection for cooling electronics

[nickagian]

Hi guys,

Does anyone have experience in selecting fans for cooling electronic systems?

I have read several online guides from the various manufacturers but there is something that remains unclear to me. They all have this formula where you can calculate the minimum required air flow created by the cooling fans. This calculation is based on the total heating power of the system and a parameter called "temperature rise".

Well I don't actually understand what this "temperature rise" parameter means or how could I set a limit for it. It seems to be the allowed temperature rise inside the enclosure in comparison to the ambient temperature. But this is somewhat confusing to me. I mean, I have the system design specification that my electronics should work at a max ambient temperature of 70 degC. So what about the temperature rise?

Could anyone help in this?

Thanks,
Nikos

 

[jiripolivka]

Hi guys,

Does anyone have experience in selecting fans for cooling electronic systems?

I have read several online guides from the various manufacturers but there is something that remains unclear to me. They all have this formula where you can calculate the minimum required air flow created by the cooling fans. This calculation is based on the total heating power of the system and a parameter called "temperature rise".

Well I don't actually understand what this "temperature rise" parameter means or how could I set a limit for it. It seems to be the allowed temperature rise inside the enclosure in comparison to the ambient temperature. But this is somewhat confusing to me. I mean, I have the system design specification that my electronics should work at a max ambient temperature of 70 degC. So what about the temperature rise?

Could anyone help in this?

Thanks,
Nikos

You can find software to design a cooling system. In many of my projects I gradually worked as follows:

1. You must know the full dissipation of your system. In my case the systems were installed in NEMA metal boxes.
If the dissipation was over 10 Watts I chose aluminum boxes, for lower power, steel was OK.

2. Use a car light bulb(s) as a power resistor, put it in the box you chose, and power it to the full dissipation you determined. Insert thermistor in the box to measure internal temperature. Locate the thermistor away from the bulb, o indicate air temperature only. Use a watch to measure time. Sit next to it all and record a table, degrees C versus time.
Plot a graph between the times taking data. The graph will show a rapid temperature rise, then going to a steady state.
Most semiconductor components and some capacitors cannot operate above 60, 85 and 100 deg.C . So check how your graph is rising. If it does not go steady above some critical temperature like 60 deg.C, you may need a fan to push the temperature down. If the graph still rises after one hour, sit and watch until it stops.

3. Get a fan and install it inside the box so the air can freely circulate. Repeat the above test and check if the plot has improved. If not or not enough, use a stronger fan. Repeat till you get a reasonable solution. Sometimes I used two small fans, better than one strong fan. Use also heat sinks on sensitive components so the circulating air cools them.

4. Once it happened to me that two fans could not give me a good inside temperature. Then I had to use an aluminum ridged block installed in NEMA box lid. Ridges on both sides allowed to dissipate more than 25 Watts inside and the inner temperature did not exceed 55 deg.C after two hours of operation. Air circulation is important.

5. if you need to dissipate a large power in a small volume, you can now use liquid coolers made for powerful computer processors. They are more costly than simple fans but offer a solution not available before say 2010.

 

[chuckey]

Temperature rise is the difference between the ambient (inlet) air temperature and the outlet air temperature. Beware of internal hot spots and make an allowance for dirt and dust obstructing the air flow. Include over temperature shut down if highly stressed. Could be worth spraying the inside and out side of the cabinet matt black.
Frank