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h   ¼ perpendicular distance between plates of CPI and PPI (in.)
                     ¼ angle of inclination of plate interceptors with the horizontal
                          (deg)

            9.6.1  Sizing Horizontal Cylindrical Skimmers

            The dimensions of the skimmer must be sufficient to allow for separation
            of the smallest oil droplet that need to be separated and for the desired
            retention time. Let D be the skimmer diameter (in in.) and L s be the
            skimmer length (in ft). We shall assume that the skimmer is half full with
            water, which moves horizontally from the inlet to the outlet of the
            skimmer. Due to the presence of various internal components, the effective
            length for separation, L, will be less than L s . To be on the conservative
            side, we shall also assume that the oil droplets will settle vertically upward
            from the bottom of the skimmer to the surface of the water; that is, the oil
            droplets will travel a distance equal to the radius of the skimmer to reach
            the surface of the water. The time it takes the oil droplet to rise to the
            surface, t o , must equal the time it takes the water to move from the inlet to
            the outlet of the skimmer, t w .
                 From Stokes’ law, the terminal rise velocity of the oil droplet, u o ,is
            given by
                                ð
 w   
 o Þd m ft
                              6
                 u ¼ 1:787   10                                           ð1Þ
                                     pw    s
            where d m is the diameter of the oil droplet (in  m). Therefore,
                     ðD=2   12Þ
                 t o ¼         s
                         u o

                                 D  pw
                 t ¼ 2:34   10 4         s                                ð2Þ
                  o
                              ð
 w   
 o Þd m
            The average velocity of the water, u w , is obtained by dividing the flow rate
            by the flow area; that is,

                      ½5:61Q pw =ð24   3600ފ ft
                 u w ¼              2  1
                       ½0:5ð =4ÞðD=12Þ Š  s
            The water retention time is obtained by dividing the effective length, L,by
            the average velocity; therefore,
                              2
                            D L
                 t ¼ 41:9786     s                                        ð3Þ
                  w
                            Q pw





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