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 & 0.7mm 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 0.7mm H2O? This is the static pressure rating of fan A .. so we now it is flowing no air.. But for fan B the 0.8mm H2O is less than have it's static pressure rating and 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.

by doyll