in series, or a combination of both. Parallel installation would be fans mounted side by side. Series installation would be a second fan in line with another fan such as an inlet fan and an exhaust fan. To simplify the discussion, it is assumed the fans are the same model. Parallel fans will provide double the free air flow but no additional driving pressure. Series installation, on the other hand, will double the available static pressure but not increase the free air flow rate. The adjacent illustration shows a single fan versus two fans in parallel with a maximum pressure of of water and a doubled flow rate of about . Note that air flow changes as the square root of the pressure. Thus, doubling the pressure will only increase the flow 1.41 () times, not twice as might be assumed. Another way of looking at this is that the pressure must go up by a factor of four to double the flow rate. To determine flow rate through a chassis, the chassis impedance curve can be measured by imposing an arbitrary pressure at the inlet to the chassis and measuring the flow through the chassis. This requires fairly sophisticated equipment. With the chassis impedance curve (represented by the solid red and black lines on the adjacent curve) determined, the actual flow through the chassis as generated by a particular fan configuration is graphically shown where the chassis impedance curve crosses the fan curve. The slope of the chassis impedance curve is a square root function, where doubling the flow rate required four times the differential pressure. In this particular example, adding a second fan provided marginal improvement with the flow for both configurations being approximately . While not shown on the plot, a second fan in series would provide slightly better performance than the parallel installation. Temperature vis-à-vis flow rate The equation for required airflow through a chassis is where CFM = Cubic Feet per Minute () Q = Heat Transferred (kW) Cp = Specific Heat of Air r = Density DT = Change in Temperature (in °F) A simple conservative rule of thumb for cooling flow requirements, discounting such effects as heat loss through the chassis walls and laminar versus turbulent flow, and accounting for the constants for specific heat and density at sea level is: For example, a typical chassis with 500 watts of load, maximum internal