What Size Fan Do I Need?

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There are lots of AC-DC power provides and DC-DC converters with output power rankings that may range dependent upon the kind of air cooling supplied. “Convection air cooling” normally refers to conditions the place a power provide or converter is cooled by the prevailing ambient air temperature, adjoining to the power device, with out forced-air-flow from followers or blowers. If the power device has two output power rankings, the “convection cooled” (still-air) power ranking is decrease than the “forced-air convection cooled” ranking.

The power provide pictured above is an open body switchmode provide with two output power rankings. For “convection cooled” functions, this provide can present as much as 151W of output power. Nonetheless, with “forced-air-cooling” it will possibly present as much as 201W of output power. The datasheet for this power provide signifies that for “forced-air-cooled” functions, 1.5 m/s (Meters per Second) should be supplied by the person. 1.5 m/s equals 295 LFM (Linear Ft per Minute). Consult with conversion elements proven beneath.

Most followers are rated in CFM or Cubic Ft per Minute of air “Quantity” circulation. So, what dimension fan do it's good to present 295 LFM of air “Velocity” circulation for the above application?

Most occasions the power provide is cooled by directing the air circulation alongside its longest dimension; for instance, from the enter connector finish to the output connector finish. Nonetheless, all the time learn the power provide’s instruction handbook to find out the producer’s beneficial axis for the cooling air-flow. The same old methodology for figuring out the required fan dimension is to first decide the height and width for the opening or port via which the air will circulation round and thru the power provide. On this occasion the power provide is 3.15” huge and 1.46” high (and 8.2” lengthy). We are able to take into account the availability’s width occasions its height because the minimal space of the inlet port for compelled air cooling of the availability. Then, we have to convert these dimensions from inches to ft by dividing by 12”. 3.15” = 0.26’ and 1.46” = 0.12’. So, the minimal “Space” of the port via which the air should circulation to chill the power provide is 0.26’ x 0.12’ = 0.0312 sq. ft. The method for figuring out the CFM (volume) ranking of the fan when the required LFM (velocity) is understood is as follows:

CFM = LFM x Space (in sq. ft)

Subsequently, on this instance:

CFM = 295 LFM x 0.0312 ft2 = 9.2 CFM (min. fan ranking)

Followers are rated in CFM based mostly upon the anticipated free-flow of the air coming from them, with out obstructions, which trigger back-pressure. After all, actual world functions all the time embody some obstructions. To make sure the least quantity of back-pressure, it's best to have the exit ports within the enclosure about 1.5 occasions the realm of the minimal entry port. In most functions there are different warmth masses and parts that may hinder the trail or free circulation of the cooling air. It's subsequently clever to pick out a fan with a better ranking than is calculated. Maybe a 10 CFM or bigger fan must be used on this application.

Tip: The usage of a bigger fan working at a slower pace can ship the identical airflow as a smaller fan working at a better pace, however the bigger fan will probably be a lot quieter.

Since most followers have spherical air shops and sq. mounting patterns, the air-flow from the fan might require ducting throughout the end-product’s enclosure to direct the cooling air to the high power units together with the power provide.

The identical course of could be used to find out the proper fan ranking for AC-DC power modules or DC-DC converters, with or with out heatsinks that require forced-air-cooling. When heatsinks are used (see photo beneath), all the time direct the air circulation in the identical course because the slots between the fins of the heatsink.
In all conditions, the system should be examined with the chosen fan and all different units in-place to verify that the power provide or converter and the load it drives don't exceed their most working temperature, beneath worst case circumstances (most ambient inlet air temperature, 100% power load, and many others.). If issues are noticed, a better CFM rated fan or dual followers could also be required.

Within the metric world, followers are typically rated in “m3/hr” (Cubic Meters per Hour) and the air velocity is rated in “m/s” (Meters per Second). The next Metric to English conversion elements could also be helpful.

1 m3/hr = 36 ft3/hr ÷ 60 min. = 0.60 CFM (cubic ft per minute)
1 m/s = 3.28 ft/sec x 60 sec = 196.85 LFM (linear ft per minute)

Some followers and power provides have dimensions in mm (millimeters).
Simply keep in mind that 1 inch = 25.4 mm, and 1 mm = 0.04”

There are a variety of excellent online calculators to help you in figuring out the fan dimension and rankings required for numerous forced-air-cooling functions. Listed below are a couple of of these web sites:

http://www.airperformancetech.com/conversion-tools.htm

http://www.aavidthermalloy.com/technical/airflow.shtml
http://www.calculatoredge.com/optical%20engg/air%20flow.htm