The present invention provides a curvilinear transition duct for use with a centrifugal fan that affords optimal fan efficiency without undue duct length. The duct comprises a top section of generally concave planar shape when viewed from above and a bottom section. Side sections join the top and bottom sections to form a four sided structure having an inlet opening at one side and an outlet opening at the other side. The preferred bottom section is of a generally longer longitudinal length than the top section. Further, the preferred duct has a plane in contact with the edges of the inlet opening which intersect a plane in contact with the outlet openings at a level above the duct. The angle formed between the planes would be 1.degree. to 60.degree..
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