Air moving devices for electronic equipment are generally described as being either a type of fan or a centrifugal blower. The main difference between fans and blowers is in their flow and pressure. Fans deliver air in an overall direction that is parallel to the fan blade axis and can be designed to deliver a high flow rate, but work best against low pressure. Blowers usually deliver air in a direction perpendicular to the blower axis at a relatively low flow rate, but can handle higher pressure.
A fan, commonly a “tubeaxial” fan, used in the typical chassis gets aerodynamic lift from the blades to move the air. Fans that interest us are available ranging in size from 40 x 40 mm with a 10-mm thickness yielding about 5 CFM with a max pressure of 0.11 inches of water, up to 120 x 120 x 38-mm units blowing upwards of 200 CFM and 0.6 inches of water. Some of the 40-mm fans, running at 13,000 rpm, can yield up to 24 CFM at an inch of water — but they’re very noisy.
Blowers are available from little 30 x 10-mm 1.66-CFM, 0.370-inch water devices to 159 x 40-mm 61.8-CFM, and 1.4-inch water behemoths.
Let’s compare a 120 x 38-mm fan to a 120 x 32-mm blower.
| CFM | Pressure | Noise (dB) | RPM | Power (W) | |
| Fan | 190 | 0.700 | 59 | 4,000 | 24 |
| Blower | 39.55 | 1.299 | 56.5 | 3,100 | 15 |
Note: Table from a 2014 white paper by Stewart Austin.
If you are using a chassis where there is plenty of vertical room, such as a 4U, and air flow impedance is low (no filters, multiple inlet/outlet openings, air control dampers, or louvers), then a fan (or multiple fans) will move much more air. On the other hand, if you have a 1U chassis, there’s not much height for a fan, and a blower (though it takes up a lot of chassis space) may be a better choice. The blower would be less expensive and quieter than multiple small fans.
A fan will blow a lot more air, four times as much, than a blower. On the other hand, a blower will generate much higher static pressure.
Some operational considerations for fans and blowers
A realistic specification for operating temperature rise must be determined. Then fan/blower curves can be analyzed to find the one that will move the right amount of air to keep the chassis within the allowable temperature limits and keep the noise level reasonable. One factor to keep in mind is altitude. For most designs, a factor of about 1.25 should be used to provide for altitude correction. This would compensate for thinner air at about 6,000 feet.
Fans and blowers using ball bearing have a lifetime of about 18% longer than those using a sleeve bearing when used at room temperature. At higher ambient temperatures, the difference gets larger. Another factor that can impact fan longevity is fan mounting. Orientation does not affect the longevity of ball bearing fans, as they all have preloaded bearings. Sleeve bearing fans can maintain life spans comparable to their ball bearing counterparts when mounted in a vertical position. Any position other than vertical decreases life span.
Some example blowers
The Delta Electronics BFB0612H DC blower has ball bearings and operates from 5 to 13.8 V providing 6.71 CFM with just 34 dB noise level. It runs at 3,200 RPM and takes 0.18 A.
The BG0703-B042-000 from NMB Technologies runs from 12 Vdc and will provide 7.40 CFM while taking 0.12 A. Its noise level is a low 29.5 dB and it runs at 2,200 RPM.
Comair Rotron makes three basic types of DC blowers, all with interesting names: the Whiffet (8.7−15 CFM, 3.14 x 3.14 x 1.26 inch), the Biscut (25−30 CFM, 4.75 x 4.75 x 1.60 inch), and the Spinnaker (60 CFM, 6.35 x 6.35 x 1.98 inch). Their Biscut BD24B7 runs from 24 Vdc taking 0.48 A and providing 26.0 CFM, with a noise level of51.20 dBA running at 3,100 RPM.
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