Airflow Rate

[sabu31]

Hi all,

I am having a datasheet from Ohmite which mentions the thermal resistance of their heat sink with respect to air velocity (ft/min). The datasheet of the fan is giving in cfu (38cfu/min). How can I correlate between the velocity and cfu.

Thanks

 

[KlausST]

Hi.

It's a rather basic physics / mathematics problem (still hard to get a reliable result)

Most probably "cfu/min" means "cubic feet per minute".
Volume per time.

Now if you want to know the speed of air in ft/min you need to know the cross section of air flow.

Let's say you have a square pipe with 0.5 ft x 0.5 ft in size, then you have a cross section of 0.25 square feet.

Now divide the volume over time by the cross section to get the speed:
38 cfu/min / 0.25 square feet = 152 ft/min.

*****
Air flow across a PCB:
Imagine you hold your PCB outside the window of a driving car. Now the speed of the car equals the (relative) speed of air across the PCB. But since the PCB is not in a closed pipe now, you neither can estimate the effective cross section, nor the volume per time.

All in all it depends on "how controlled" your airflow is from fan to PCB. If in some kind of pipe you can do some calculations. If "open" you just can guess.

Klaus

 

[Easy peasy]

very good post above - if you channel the air from the fan to the point of cooling, then if you restrict it too much the head or pressure goes up and the flow goes down a bit - all good manufacturers show graphs for this

then there may be extra back pressure from tightly finned heasinks and tortuous exit pathways - build and test