The axial impeller is, in fact, a propeller, known to all and invented (in the form of a water screw) by Archimedes and improved to air by Lomonosov. In media with low viscosity, the maximum efficiency of such an impeller - both in terms of efficiency and power per unit volume - is achieved at high speeds and a small number (3-4 pieces) of long and narrow almost flat blades operating at low angles of attack, varying from 5–10° at the far edge to 20–30° at the attachment point. An example is the propellers of propeller-driven aircraft, snowmobiles, etc., where silence is sacrificed for the sake of maximum performance with minimum dimensions, weight and fuel consumption. To reduce noise, almost round or saber-shaped blades are used, they are made shorter and wider (these are often called blades), their number is increased, reducing speed and increasing angles of attack. The latter become variable not from the edge to the center of the blade, but along the line of flow around it and at the trailing edge, they can reach 40°. An example is submarine propellers, instrumental (in particular, computer) fans. But this form reduces efficiency. When designing HVAC systems, compromise solutions are found: in outdoor units and industrial hoods (not so critical in terms of noise characteristics), efficient but noisy impellers are used; in condenser coolers, etc. - large blades both in length and width at low speeds; and in fan coil units - directly in rooms with people - as a rule, radial impellers.