The cross-flow fan is used extensively in the HVAC (Heating, Ventilation, and Air Conditioning) industry. The fan is usually long in relation to the diameter, so the flow approximately remains two-dimensional away from the ends. Unlike radial machines, the main flow moves transversely across the impeller, passing the blading twice. The popularity of the crossflow fan in the HVAC industry comes from its compactness, shape, quiet operation, and ability to provide high pressure coefficient. Effectively a rectangular fan in terms of inlet and outlet geometry, the diameter readily scales to fit the available space, and the length is adjustable to meet flow rate requirements for the particular application. One phenomenon particular to the cross-flow fan is that, as the blades rotate, the local air incidence angle changes. The result is that in certain positions the blades act as compressors (pressure increase), while at other azimuthal locations the blades act as turbines (pressure decrease).