# Plate Settler Theory

The settling pattern of a discrete particle in a rectangular basin is that as the particle settles it is carried forward by the velocity of the liquid flow through the basin. Thus if Vf is the velocity of fluid, Vs the settling velocity of the particle, L the length of the basin, and d its depth, then a particle at the influent will settle to the bottom of the basin only if:

$V_f max = V_s L/d$

Therefore, the flow velocity at which a basin can operate successfully is directly proportional to the length of the basin, and inversely proportional to the depth.

This scenario applies to horizontal plates as well as rectangular basins. In horizontal plates, a particle must fall a few inches. In a rectangular basin, a particle must fall 10 to 20 feet. The more even the flow the more efficient the settling process.

Horizontal plate settlers are spaced 1 – 2 inches apart; therefore, the particle only needs to travel vertically 1 – 2 inches. The settled solids must be removed mechanically from the horizontal plates.

Inclined plate settlers are positioned on a 55-60° angle to allow the settled solids to slide down the plate and to the bottom of the basin. In this case the distance the particle must travel vertically is:

 D = distance the particle travels vertically d = plate spacing Cos µ = Cosine of plate angle (55°) D = d , D = 2 Cosµ .574 D = 3.48″

Since the particle sees the horizontal area, horizontal plates are calculated as the actual settling area. Inclined plate settlers use the horizontally projected area and therefore the total plate area may be calculated by:

 At = HP Cosµ

HP = Horizontally projected area

Cos µ = Cosine of plate angle (55°)

At = Total ft2 of plate surface required

Because each plate settler provides an effective settling area equal to that of its horizontal projection, MRI Plate Settlers will increase a basin’s effective settling area by up to ten times.

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