Waterproof Cooling Fan The quality of how to distinguish?

I believe that many "Waterproof Cooling Fan" of buyers and cooling fan manufacturers are aware, oil-bearing and ball bearing cooling fan, that first of all we start with the bearing identification, the lower the price of oil-bearing turn voice is small, and some manufacturing providers within the oil-bearing plus Teflon, making the "Waterproof Cooling Fan" also has a ball bearing reliability, and then sell high. Ball bearing fan noise ratio of oil-bearing, the price is also relatively expensive, but more reliable, universal life longer than the oil-bearing. If the five-speed fan blades and seven identical ventilation seven blades will be greater. "Waterproof Cooling Fan" The higher the speed, the shorter the life of the corresponding noise is greater. Further, the thicker the fan blades, the greater the blade angle, the pressure is greater. Inlet angle blades (45 degrees maximum) is also one of the important factors determine the amount of ventilation fan. If start from the design, preferably also the air flow at the inlet and outlet to increase the amount of rotation of the secondary air flow channel.