| 摘要 |
PROBLEM TO BE SOLVED: To be able to easily judge the exchange time of a filter and to simplify the structure by providing an air permeable outer cylinder arranged outside the filter located on the outer circumferential surface of an air permeable inner cylinder extending to the vertical direction with visibility capable of visually recognizing the surface of the filter through the air holes. SOLUTION: An inner cylinder 13 uniformly forming many air holes 16 and a cylindrical filter 12 covering the outer peripheral surface side of the inner cylinder 13 are coaxially provided in an outer cylinder 11 extending the upper side of the outer peripheral edge of a circular bottom plate 17 on a floor and uniformly forming many air holes 15. The total surface area of the air holes 15 of the outer cylinder 11 is made equal to or above that of the air holes 16 of the inner cylinder 13, since a pressure loss to a fan 14 in the inner cylinder 13 becomes large to decrease the dust collecting efficiency when the total surface area of the air holes 15 of the outer cylinder 11 is equal to or below that of the air holes 16 of the inner cylinder 13. Each unit hole of the air holes 15 in the outer cylinder 11 is formed into a shape and a size to observe the surface of the filter 12 with the raked eye. As a result, the change or the degree of soiling on the surface of the filter 12 is visually recognized and the exchange time of the filter 12 is easily judged.
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