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9.6.2 Sizing Horizontal Rectangular Cross Section (API)
Skimmer
The same procedure described in Section 9.6.1 is used to determine the
width, W, effective length, L, and height of the water, H, of the skim tank.
In this case, we have

H
t ¼ ð9Þ
o
u o
L
t ¼ ð10Þ
w
u pw
Equating Eqs. (9) and (10) with u o given by Eq. (1) and u pw given by
dividing the water flow rate [5.61Q pw /(24 3600)] by the flow area (HW ),
we obtain
Q pw pw 2
WL ¼ 36:5 ft ð11Þ
ð

Þd m
o
w
For this type of skimmer, the recommended efficiency factor for
turbulence and short-circuiting is 1.9; therefore, Eq. (11) becomes
Q pw pw 2
WL ¼ 70 ft ð12Þ
ð
w
o Þd m
The retention time as obtained from Eq. (10) should also be equal to the
volume of the skimmer occupied by water (HWL) divided by the water
flow rate; therefore,
HWL
t rw ¼ 15401 s ð13Þ
Q pw
The height of the water, H, is usually limited to one-half the width, W;
therefore, substituting 0.5 W for H in Eq. (13) and dividing by 60, the
retention time in minutes is given by
2
W L
t rw ¼ 125 min ð14Þ
Q pw
Rearranging, we obtain
2 3
W L ¼ 0:008t rw Q pw ft ð15Þ
Any combination of W and L that satisfies both Eqs. (12) and (15)
satisfies both the settling and retention time constraints.

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