CFM

Another way to increase the amount of heat dissipated or removed from network equipment is by increasing the airflow, expressed in cubic feet per minute (CFM). An increase of CFM—the amount of airflow across a given area in a given time—results in increased heat removal.
CFM can be achieved through the use of fans. The larger the fan, the more CFM it provides. An increase in RPM (the speed at which the fan circulates) as well as the size or quantity of fan blades results in a higher CFM, but as these factors increase, so do several others that can be detrimental to the equipment in data centers. For example, acoustic noise is one consequence of high levels of CFM. Additionally, at very high CFM, physical forces also come into play that can cause damage to electrical equipment. Plus the higher the CFM, the higher the upfront capital costs, as well as ongoing operational expenses.

• Most data centers operate between 10 and 20 ΔT. Using this and the CFM of the fan, the necessary quantity of cooling (watts) can be determined.
• The same equation also works for equipment outside the data center. Again, the intake air can only be so cold and at a certain CFM before other issues arise.
• The bigger the fan, the more airflow that moves across equipment. The amount of airflow and the temperature of the air determine the amount of heat dissipation that can occur.
• CFM_in = CFM_out on all passive systems.
• Total CFM_in = the sum of all equipment (servers) mounted in the cabinet.

An increase in fan size will increase acoustic noise at the same motor RPM.