This is because a leaf "24V DC Cooling Fan" To be a great centrifugal force, in the case of high-speed rotation, it will cause a very unstable operation, there is the risk of "fly" the vast majority. The two blades are allowed two blades in order to maintain a certain angle high speed centrifugal rotary offset each other, so there is 2 "24V DC Cooling Fan" to ensure that the fan blades blades "will not fly!" But not only the efficiency of the two blades is extremely low, and the motor and bearings, which are great demands "three blades," the fan just to meet it all! 4,5,6,7 .... all OK, but "24V DC Cooling Fan" 3 blades are standard minimum limit.