It is as if "220V Cooling Fan" even when the number of leaves, and a symmetrical arrangement, then not only makes the fan itself is difficult to adjust the balance, and easy to make fans produce more resonance at high speeds, causing "220V Cooling Fan" blade could not withstand prolonged fatigue resonance, and eventually blade fracture and so on. And when the fan blade number is odd, it is impossible to achieve symmetry, "220V Cooling Fan" blade odd combination can bring more performance advantages over Twisted even.