Because "220V AC Cooling Fan" parameter is usually calibrated in free air transmission, that in the absence of pressure circumstances. In practical applications, the fans tend to bear given back pressure resistance system. And this pressure is related to many factors, it is difficult to determine its size. For this reason, in order to compensate for this influence of the pressure difference, generally to selected "220V AC Cooling Fan" parameter derating 60% to 80% use. Under the air-cooled heat sink selection is actually the case for the purposes of a specific system, may have a variety of radiator solution, you can choose a small radiator under a large air velocity conditions, flow velocity can also choose a smaller environment working under the big radiator. "220V AC Cooling Fan" gas flow velocity, the greater the noise generated by a fan, but these drawbacks can be obtained by a small system size to compensate. It is worth noting: When the flow rate is greater than 7m / s, almost never to reinforce the cooling effect. When the system power is only required cooling equipment, you can also use a radiator with a fan.