How Airflow and Static Pressure Specifications relate to real world use.
Airflow specification is the maximum airflow of a fan with no resistance to the airflow
Not a real world use of our fans.
But we use the fan in a case with a grill (restricting airflow) and often a filter (also restricting airflow) and often blowing into a HDD cage (still more restriction to airflow).
End result is that the fan specified airflow is more than twice what is actually flowing .. in fact often even more.
Static pressure rating is the pressure level when air stops flowing
Again, not a real world use of our fans.
The reality is that if we have 2 fans, one with a higher airflow and lower static pressure rating and the other with lower airflow and higher static pressure rating .. but in almost every case (no pun) the lower airflow / higher static pressure rated fan will flow more air than the higher airflow / lower static pressure rated fan will
Let us look at fan A with 70cfm & 0.8mm H2O rating compared to fan B with 50cfm and 2.0mm H2O rating.
Now let us assume the grill, filter, HDD cage, cables etc. create a pressure level of 0.35mm H2O .. half of fan A's rating .. which drops the cfm by about 60-70% giving us 21 -28cfm of airflow. But for fan B the 0.35mm H2O resistance is dropping the airflow by 20-30% leaving us with 35 - 40cfm of airflow.
Fans A and B with 0.35mm H2O resistance
- Fan A 70cfm & 0.8mm H2O fan is flowing 21-28cfm.
- Fan B 50cfm & 2.0mm H2O fan is flowing 35-40cfm.
But what if the grill, filter, HDD cage, cables, etc create 0.8mm H2O? This is the static pressure rating of fan A .. so now it is flowing no air.. But for fan B the 0.8mm H2O is less than have it's static pressure rating and iy will still be flowing about half of it's rating leaving us with about 25cfm airflow.
Fans A and B with 0.8mm H2O resistance
- Fan A 70cfm & 0.8mm H2O fan is flowing 0 cfm.
- Fan B 50cfm & 2.0mm H2O fan is flowing 25cfm.
These are estimated resistance and airflow to resistance to give an example. Hopefully it helps you understand how static pressure and airflow relate to each other
In most cases a fan with a real static pressure rating of about 1.5mm H2O will do a good job of moving air.
Actual Airflow Versus Peak CFM & Static Pressure:
Testing is done for:
- Airflow; done in free airflow .. not resistance at all .. fan hanging in open space. All of our fan use has resistance.
- Noise; done in free airflow .. no resistance means less noise. Again, all our fan use has resistance.
- Static pressure The amount of pressure it takes to stop fan's airflow. All of our use requires airflow .. so not really applicable to our use. P-Q curve graph is more appropriate.
Here is a P-Q graph and chart of fan performance at 0.5 & 1.0mmH2O to show how far from specs the actual airflow is in use .. like intake through a grill, filter or exhaust through a grill, or on a cooler / radiator. Our fans are working with from 0.1 to 1.0mmH2O .. usually 0.2 - 0.7mmH2O. .. meaning 0.5mmH2O resistance is a quite appropriate for our uses. Also keep in mind this is with fans at full speed. The flow / ability to overcome resistance drops dramatically at fan speed slows down.
Please remember I am not recommending any of the fans. Some are great, some are not. This is only an example of airflow with resistance similar to what we have compared to their published specifications.
Please take note of how many of the fans are flowing no air at all at 0.5mm H2O and how many more are flowing nothing at 0.75mm H2O.
Please take note of how many of the fans are flowing less than 20CFM at these pressure.
Notice how much airflow the other fans are flowing at 0.5 and 0.75mm H2O compared to what they flow with no resistance (0.0mm H2O). I put a line in to show 20 CFM, but if that is all the case is flowing per fan and the CPU cooler has a 60-70cfm rated fan, it will take 3 case fans to just keep up with the CPU cooler .. and most GPU coolers use as much air as the CPU coolers .. meaning 6x case fans each moving 20cfm into the case in order to keep up wiht the component demand.
Also, this data is based on fans running at full speed. It is important to remember that as fan speed decreases, so does it's CFM of airflow .. and also it's static pressure rating. In other words a fan that will flow 65cfm in open air and flows 35cfm at 0.5mm H2O at full speed is lowered to half speed it will flow about halve as much. Add pressure (resistance) and it is much lower still.
Linear FPM is the air speed. CFM is calculated by using the area of the hole the air is flowing though and the air's speed. For example a 140mm fans is 0.1656 sq feet times 300 FPM (feet per minute) is 49.68CFM
- Scythe GT-15 ratings are 58cfm / 2.03mm H2O
- Cougar Vortex ratings are 70.5cfm / 2.2mm H2O
- GT-15 is flowing 7.8-7.9cfm more air at 1.02-1.19mm H2O
- The area of pressure we use fans in normal applications on radiators, coolers, grills and filters.