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#1 2007-07-26 02:45:25

Site Admin
From: Philippines
Registered: 2007-06-20
Posts: 586

Guide to Blower Fans

PMAers Guide to Blower Fansrev 1.0c


Blower Fans (or just plain fans), are part and parcel of our daily computing PC habits. From the near silent HTPCs (Home Theater PCs), to the roaring and raging OverClocker's delights, pushing their PCs to the hardware limits.

This is due to the fact that most modern-day PCs need to be actively cooled using air, which is the easiest and most hassle-free way to cool down those components within. Passively-cooled heatsinks don't just cut it anymore. So a cool breeze running through one's case and heatsinks would be the cheapest and most effective way to tame the roaring temps of modern processors and video cards.

This brings me to this discussion about blower fans. Their most commong properties, characteristics, proper usage, and tips on buying the right one for you.


1. Types And Sizes

Blower fans for PC use normally come in a few pre-determined sizes. Since these fans have a square frame, the measurement that comes with each refers to the length of it's side.

The metric unit used in this measurement is the millimeter (mm), with 1 inch being roughly equal to 27mm. Sizes then range from:

1. 40x40mm - or 1.5 x 1.5 in - Often seen in chipset heatsinks and in old videocards.

2. 60x60mm - or 2 x 2 in - Often used as exhausts of mATX cases, old ATX mid-towers, and as heatsink fans for Pentium III and old Athlon XP heatsinks.

3. 70x70mm - or 2.5 x 2.5 in - This fan size is what AMD uses as the stock fan for all their Athlon64, Sempron 64 and X2 stock heatsinks.
4. 80x80mm - or 3 x 3 in - This is the typical case fan size. Almost all PC cases nowadays have holes or provisions for at least one or two 80mm fans on them. Some aftermarket heatsinks also sport this fan size.

5. 92x92mm - or 3.5 x 3.5 in - Some high-end cases, noticeably the ones coming from Casetek use this fan size for case cooling. this is also the favorite fan size of high-end aftermarket heatsinks for the past years, only as of late have they been replaced by 120mm-fan heatsinks.

6. 120x120mm - or 4 x 4 in - This is the current best choice for enthusiasts. High-end cases often sport at least one 120mm in them, with the really huge towers accommodating up to 3 or 4. The best aftermarket heatsinks also use this fan size for cooling due to it's extremely large airflow. The Scythe Ninja and Thermaltake big Typhoon are examples of such.     

2. Speed, Airflow And Noise

As with all things in life, nothing comes free. This is the same with fan technology. The faster a fan spins, the more air it pushes, and consequently, the noisier it will get. That's one thing I have repeated dozens of times when responding to inquiries. Different people have different tolerances for noise, thus, it's important to obtain the fan that suits your noise-levels, while not scrimping on performance.

Speed is measured in Rotations Per Minute (RPM). The higher the RPM of a fan, the more air it pushes, which leads me to...

Airflow is measured in Cubic feet per Minute (CFM). This literally is a rate of how much cubic feet of air a fan can push while running for one minute. CFM can vary anywhere from 5-15cfm (for 40mm sizes) to as much as 150-200cfm (for those 120mm monsters). As such, the higher the CFM of a fan, the higher its noise level will be.

Noise is measured in Decibel Amperes, or Decibels for short (dBA). The most often problem in measuring noise is that some manufacturers grossly "exaggerate" their claims on the noise levels (and sometimes airflow rate) of their fans, especially those catering to the general PC user populace. Industrial fan manufacturers meanwhile, cannot scrimp on this information since mission-critical applications and workplaces depend on these specs of theirs.

Thus, the baseline I'll use is that of fans made by Delta Electronics, a well known industrial blower fan manufacturer whose fans often cater to high-end, high-speed PC enthusiasts.

3. General Airflow Categories

5-10cfm - Only applicable to small 40mm fans. Hardly felt at all with larger fan sizes.
20-30cfm - The normal range for most generic 80mm fans. Pretty weak, hardly felt on one's hand if exposed directly to the fan's airflow.

45-50cfm - Fast 80mm fans are at this speed. Not too noisy, but very good for case cooling.
- Medium speed 92mms.
- Very slow and silent 120mms. Generic, Quad LED and UV Reactive 120mms are also found here

60-75cfm - Ultra-fast 80mm fans, usually for aftermarket heatsinks. The Thermaltake Volcano 12 fan and Smart Case Fan 2 are at this level. Extremely noisy too.
- Fast 92mms are found within this airflow rates. These are best used as heatsink fans. Good airflow at a tolerable noise level.
- Slow-Medium speed 120mms are found here, as well as the top-end QuadLED and UVRs.
75-90cfm - Extremely Fast 92mms.
- Medium-speed 120mms. Best for cases or silent aftermarket heatsinks.

90-110cfm - Ultra-speed 92mms. Extremely Fast. Extremely Noisy.
- Fast 120mms. Medium noise levels. Best for high-performance heatsinks. Could be used as an alternative desk fan too. lol.

120-150cfm - Very Fast 120mm fans are the only ones generally found in this bracket, as no readily available 92mm or lower-size fan can push such tremendous amounts of air. High Noise Levels at extremely fast speeds. Recommended only for those serious air-cooling addicts.

4. Air Pressure, Fan Blades And Fan Depth

I grouped these three properties together since they're often the factors that affect one another.

Air pressure - In layman's terms, air pressure is the measurement of how strong the airflow coming from the fan is, or how strong a fan can suck/push air. High air pressure is important when cooling restrictive environments, such as heatsinks with a lot of fins, or when trying to suck up air through fan filters. Having a higher air pressure will increase a fan's noise level a bit, with the tradeoff of having generally better performance.

Fan blades - If you notice, fans often come with a multitude of blades to push air. The number of blades connected to the rotor mechanism ranges from 3 blades up to 9 blades (for standard 80-120mm models), and even up to 20 or more blades (for smaller size models). The number of blades directly affect how much air a fan can push and how much air pressure its airflow exerts. Having a lower number of blades would increase a fan's air pressure, while decreasing the fan's total airflow. While conversely, having more blades would increase a fan's airflow, while decreasing the air pressure it exerts.

Fan depth - Fans aren't just stuck with the normal depth (or thickness) of 25mm which is the norm for case fans. The possible values range from 10mm (for those thin 40mm fans) up to 38 or even 76mm (for 120mm high-speed ones). Aside from the convenience of mounting thinner fans, the depth measurement also has a direct effect on the air pressure the fan exerts. The deeper a fan is, the more air pressure it exerts. The trade-off would be a little additional noise, as well as some mounting incompatibilities due to the bigger fan.

So the rule of thumb would be, assuming equivalent rpm:

A. The more blades a fan has, the less air pressure it exerts, while providing more airflow.
B. The deeper a fan is, the more air pressure it exerts, while having a slightly higher noise level.

As one could see, there's trend in all these.. with airflow, speed, air pressure and noise all balancing themselves out...

I only drink diluted water.


2007-07-26 02:45:25

pcModz Bot
Registered: 2015-04-29
Posts: 8

Re: Guide to Blower Fans


#2 2007-08-14 09:50:24

Registered: 2007-07-21
Posts: 4

Re: Guide to Blower Fans

Additional info by beelzebub

If I may add, one can also consider the Amperage Rating of a fan when selecting fans of the same size and depth- apart from selecting them according to the blades to determine the one with the higher airflow.

Amperage Rating


The Amperage Rating is directly proportional to how powerful the fan is in terms of rotational speed. Fans are more or less simple electric motors with Fan Blades attached. The more powerful this electric motor is, the greater the RPM will be if both the fan size and the fan blade design (or number of blades) are identical/similar. It follows that the greater the RPMs the fan motor will be spinning, the greater the airflow will be. Similarly, the greater the RPMs the fan motor is spinning, the noisier the fan whine will also be- since the fan blades will be spinning faster and therefore will be whining at a higher pitch. Fans with higher Amperage Ratings are always noiser if Fan Depth and the number of Fan Blades are identical.

So as a rule of thumb, the higher the Amperage Rating of a fan then the higher the Airflow Capacity will be if both Fan Depth and Fan Blades are similar. Also, the higher the Amperage Rating then the higher the Fan Noise will be. The fan's Amperage Rating is normally specified on the rear of the fan and written on the sticker citing the fan's make and model as well as its Voltage Rating. For fans used for general cooling requirements of computers, this Amperage Rating is usually specified as "0.xxA" wherein the letters "xx" are a specified number in hundreths of an amp. Thus note that the fans used in computers are normally below 1.0A in rating, such as "0.30A" for example.

Examples Illustrating Effects of Amp Rating

To give some idea on the correlation of all of these, we might as well cite a examples to illustrate. For instance, a 120mm x 120mm x 25mm 0.22A 7-Blade Fan Design will very likely pump out in the region of 34CFM of Air Flow and 24 to 26dBA of Noise. That's more or less the usual ratings for 120mm Silent Fans currently available.

For more power and yet still bearable noise levels, 120mm x 120mm x 38mm 0.40A 5-Blade Fan Design will pump out maybe about 90+CFM Airflow Capacity while emmitting about 30dBA worth of noise- making this both still bearable in terms of noise and much more powerful.

In contrast, a 120mm Fan with 25mm Fan Depth and 7 blades (or same as the silent fan) but with a 0.40A Amp Rating (or same as the medium fan) will roar at about 42 to 44dBA!!! It will have higher Airflow Capacity than the 120mm Silent Fan though- about 88CFM or just short of 90CFM (approximated)

Although it would pump out about 120CFM worth of Airflow Capacity, a 120mm Fan with 38mm Fan Depth and 7 blades and 0.72A Amp Rating will most certainly be even noisier... as noisy as a 220V Desk Fan running on the highest speed setting!!!

Note This: High Amp Rating + Multiple Blade Design = Higher Pitched Fan Whine or Noise

With that hopefully we have given you a guide to blower fans that can serve as Rules Of Thumb when selecting the right Blower Fan for your needs.


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