"24V DC Cooling Fan" In order to meet some special installation requirements - such as a circular duct, circular heat sinks, easy to use, "just like" the fan, but need to frame the diversion effect, circular outer box fan should be born.
Frameless "24V DC Cooling Fan"
In some cases, the frame of the diversion effect is not necessary, or even a negative impact or impede the installation exist, it will be "abandoned". At present, more common in some circular radial radiators, especially "24V DC Cooling Fan" built-in type. Typical for Zalman CNPS7000A, and Intel Prescott boxed mix of "Sunflower."