The radial clearance of the axial flow fan refers to the clearance between the tip of the blade and the inner wall of the casing, and the axial clearance refers to the clearance between the two cascades of the impeller. The size of the two gaps has an impact on the performance of the fan.

The radial clearance of the axial flow fan is too large and the negative pressure is high, which will inevitably cause a large air pressure difference between the tip of the blade and the two sides of the casing clearance, which is the main reason for axial air leakage. The leaked air is drawn back by the blades and discharged to the atmosphere, forming a "circulating airflow". The work of this part of the airflow is consumed in vain, so the efficiency of the fan is reduced.

When the rotor tells it to rotate, the gas spiraling along the axial direction, due to the weight of the air itself and the moisture and dust particles in the air, is thrown by the rotating blades to the surroundings. The particles of the air are densely packed near the casing and close to the inner wall The front spiral flow increases the airflow density and negative pressure along the radial direction. In this case, the radial gap is too large, and it turns back to produce a large amount of cylindrical airflow along the gap ring around the casing to flow in the axial direction and reverse direction, which impacts the normal airflow, causing the airflow to waste power, thereby reducing efficiency .

When designing an axial fan, it is necessary to maintain a certain axial gap between the blade cascades, mainly to ensure that the subsequent blade cascades have uniform air intake and eliminate the mutual influence, otherwise the respective working conditions will be deteriorated. Uneven airflow may act as a source of vibration, causing the following blades to vibrate. On the contrary, if the axial gap is too large, the axial dimension will be too long. Therefore, the size of the axial gap has an impact on the noise, vibration and axial size of the fan.