1. Principles and Characteristics of Tube Settlers
1.: Water flows between the plates or within the tubes, where the hydraulic radius is small, resulting in a low Reynolds number. Generally, the Reynolds number (Re) is around 200, indicating laminar flow, which is highly favorable for sedimentation. The Froude number of the flow within the tubes is approximately 1×10⁻³ to 1×10⁻⁴, indicating a stable flow state.
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4.Re-flocculation of Particles: Flocculent particles re-flocculate within the inclined plates or tubes, promoting particle growth and further improving sedimentation efficiency.
2. Structure of Tube Settlers
The structure of inclined tube or plate settlers is similar to that of conventional sedimentation tanks, comprising an inlet, sedimentation zone, outlet, and sludge collection zone. However, in the sedimentation zone, many inclined tubes or plates are installed. Figure 1 shows the typical structure of a tube settler.
, , as shown in Figures 2 and 3.
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3. Inlet Zone
Water enters the sedimentation tank horizontally. The inlet zone often features perforated walls, slotted walls, or downward-flow inclined tube inlets to ensure uniform water distribution across the width of the tank. The design and requirements are similar to those of horizontal sedimentation tanks. To achieve uniform outflow from the inclined tubes in upward flow systems, a sufficient height of the water distribution zone below the tubes is necessary, ensuring the inlet velocity is kept between 0.02 and 0.05 m/s.
4. Inclination Angle of Plates and Tubes
. A smaller results in a lower critical settling velocity (u₀), improving the sedimentation effect. However, to ensure automatic sludge sliding and unobstructed sludge discharge, should not be too small. For upward flow systems, is typically not less than 55 degree to 60 degree . In downward flow systems, where sludge discharge is easier, is generally not less than 30 degree to 40 degree .
5. Shape and Material of Plates and Tubes
To make the most efficient use of the available tank volume, inclined plates and tubes are designed in compact geometric shapes, such as square, rectangular, hexagonal, and corrugated forms. For ease of installation, multiple or even hundreds of inclined tubes are grouped into a single unit, which is installed in the sedimentation zone. The materials used for inclined plates and tubes should be lightweight, durable, non-toxic, and inexpensive. Common materials include honeycomb paper and thin plastic sheets. Honeycomb tubes can be made from impregnated paper and cured with phenolic resin, typically forming hexagons with an inner circle diameter of 25 mm. Plastic sheets, such as 0.4 mm thick rigid PVC, are often thermally molded.
6. Length and Spacing of Plates and Tubes
The longer the inclined plates or tubes, the higher the sedimentation efficiency. However, overly long plates or tubes are challenging to manufacture and install, and further length increases yield diminishing returns in efficiency. If the plates or tubes are too short, the proportion of the inlet transition zone (the turbulent to laminar flow transition zone) increases, reducing the effective sedimentation zone. The transition zone in the inclined tubes is about {{0}} mm. Experience suggests that upward flow plates should be 0.8-1.0 m long, with a minimum of 0.5 m, while downward flow plates are around 2.5 m long. With constant cross-sectional flow speed, smaller spacing between the plates or tube diameters increases flow velocity and surface load, thus reducing the tank size. However, excessively small spacing or tube diameters can lead to manufacturing difficulties and blockages. In water treatment, upward flow settlers usually have a spacing or tube diameter of 50-150 mm, while downward flow plates have a spacing of around 35 mm.
7. Outlet Zone
h Greater than or equal to √3/2L
h
L
Typically, L is between 1.2 and 1.8 m, so h is between 1.0 and 1.5 m.
8. Settling Velocity (u₀) of Particles
The water flow velocity between inclined plates is similar to the horizontal velocity in horizontal sedimentation tanks, usually ranging from 10 to 20 mm/s. When coagulation treatment is used, the settling velocity (u₀) is around 0.3 to 0.6 mm/s.