# Heat Sinks and air cooling

by DHF
Tags: cooling, heat, sinks
 P: 52 As russ has just pointed out, increasing the air flow rate doesn't necessarily increase the "heat transfer rate". This is a common misconception. The heat to be removed from a CPU is a function of the CPU's operating condition. For example, for a CPU operating at nominal load the heat dissipation will be fixed according to that condition and will depend on the number of calculations, etc that the CPU is handling at that instance. Increasing the fan speed increases the air flow rate and accordingly increases the heat transfer coefficient (essentially due to reduction in boundary layer thickness). In this case, if the convection equation is to remain balanced, the heat source (CPU) temperature must reduce. Convection equation: $\dot{Q}$=hA($T_{source}$ - $T_{ambient-air}$) where Q = fixed by CPU = constant A = heat transfer area = constant Tambient = air temperature = relatively constant Therefore, an increase in h (heat transfer coefficient) must lead to a reduction in Tsource