Abstract: A downwardly opening generally circular plan-shaped horizontal housing is provided including opposite side maximum width portions and having a driven rotary vacuum blade assembly journalled therein about a central upstanding axis. The lower end of the downwardly opening housing is at least substantially closed throughout an annular zone extending about and inwardly from the outer periphery of the housing lower end and a depending skirt structure is provided establishing a substantial barrier to the longitudinal flow of reduced pressure air inward toward the center of the annular zone immediately beneath the latter from all horizontal directions, except those horizontal directions extending inwardly toward the center of said annular zone from said opposite side maximum width marginal portions.