The "cross-flow" or "tangential" fan, sometimes known as a "tubular" fan, was patented in 1893 by Paul Mortier, and is used extensively in heating, ventilation, and air conditioning (HVAC), especially in ductless split air conditioners. The fan is usually long concerning the diameter, so the flow remains approximately two-dimensional away from the ends. The cross-flow fan uses an impeller with forward-curved blades, placed in a housing consisting of a rear wall and a vortex wall. Unlike radial machines, the main flow moves transversely across the impeller, passing the blading twice.