"Axial Fan" illustrates a fan unit condition is still widely used method at present. Fan P-Q characteristic curve showing the relationship between the energy and the fan flow volume of the gas supplied to the unit; line P-Q characteristic curve showing the relationship between gas flow required per unit volume of the piping system and the flow of energy, which is different from the two curves concept. However, a fan unit, the two curves and interrelated, mutual restraint, the device is mass-balance condition that is the result of the fan and piping; energy balance is the fan and piping results.