For user's safety and energy efficiency, which maximizes airflow air multiplier.
Facing two or four cross fan is rotated in the opposite direction, to the flow for air amplification.
Using this principle, the low output of the motor can also get the wind is moderate, compared with the existing fans about 40% to 60% of power can be reduced. 
And blades does not exposed to the outside it can minimize the risk of accidents.

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