Fundamentals of Tube Settler Design
W. A. BEACH, Research Associate
Permutit Research Center
Princeton, New Jersey
INTRODUCTION
Gravity separation of solids from a liquid is one of the most important processes in
water and waste treatment. Hazen first recognized the need for short settling distances and
suggested the horizontal plate clarifier in 1904111. For the next 50 years, various attempts were made to market devices of this type, but none were very successful due to
poor flow distribution and the difficulty of sludge removal. In 1946 Camp reemphasized
the great improvement that was possible and stressed the need for research and development in the field of sedimentation121. Finally, about 20 years later, Culp and his associates(31. began reporting on practical settling tubes featuring the low settling heights
suggested by Hazen and Camp.
In the past five years, some data have been published relating tube settler performance
to tube depth and angle of inclination but relatively little data are available on the influence
of other relevant parameters. This paper presents the results of tube settling tests on a wide
range of cross sectional shapes, cross sectional sizes, tube lengths and angles of inclination.
Tube performance was judged by measuring effluent turbidity at various flow rates when
settling an alum-lime-kaolin floe.
The first part of this paper will present a brief theoretical analysis of different types of
settlers. To simplify this analysis, ideal settling of dilute, non-flocculating solids will be
considered and density currents, wind currents, inlet and outlet influences will be ignored.
VERTICAL CLARIFIER
First consider settling rates in an up/low clarifier where product is removed from the
top of the clarifier. Clearly, for acceptable results, the upward velocity of the fluid may be no
greater than the settling velocity of the slowest settling particles that must be eliminated
from the product. For most solids we deal with, including natural turbidity, alum floes and
the like, these velocities set the limits for production rates for vertical clarifiers at one or at
most two gallons of product per-minute per-square foot of clarifier surface area.
HORIZONTAL CLARIFIER
Next consider a particle settling in a horizontal clarifying device, as illustrated
schematically in Figure 1. A particle carried forward by the velocity of the liquid flow must
settle at a rate fast enough to reach the bottom before it reaches the end of the device.
Thus in Figure 1 particles beginning at a point a must traverse some route lying
between routes ab and ah'in order to settle before being ejected. If V is the velocity of the
fluid, u the settling velocity of the particles, L the length of the horizontal settling device and
d its depth, then particles at point a will settle to the bottom of the device only if V does not
exceed
t-(u)
since   V max.    L or   V max. - L ,.
u d d
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