| 摘要 |
Pre-cast concrete beams for supporting a filterbed have deep downwardly extending flanges for providing an underchannel lengthwise of the beam. The beams generally extend across a flume. For backwashing with an air-water mixture, air should be supplied at substantially the same rate to all of the channels. Cross passages between the channels are an aid to this even distribution. Heretofore, special molding procedures for providing the passages or limbers through the beam flanges have increased manufacturing costs. According to the present invention, such passages are provided without extra molding steps and without extra cost by following a delightfully simple concept of merely omitting the end portions of the flanges where beam strength is not required. The end portion projecting beyond the flanges may have a leg across its end to ensure support and aid sealing.
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