Imagine a rectangular tank with a surface A containing a liquid to a height H (see figure 1). Flow the tank is homogeneous and the flow rate = Q The settling or rising velocity of a particle with diameter d is governed by Stoke's Law:

It follows that the tank surface A is inversely proportional to the square of diameter d of the smallest particle which will completely separate. An increase in the efficiency of a separator can, as a result, only be achieved by a considerable increase in the surface A, which leads to the installation of large tanks with large surface area A. The effectiveness of such large tanks is limited due to factors like:

- Turbulence and short-circuit currents.
- Variation in ambient temperature.
- Temperature gradients in the liquid.
- Surface wind load.

In the modern TPI System, the separating surface area is increased vertically by the addition of a number of parallel plates mounted at an inclination. This not only affords a dramatic space saving but also facilitates very small separating diameter.

The K-pack TPI consists of a plate pack constructed of closely spaced corrugated plates inclined at a 45° angle or more, depending on the application. Waste water flows through these plated either parallel to the corrugations in "Counter-current Flow" or at right angles to the corrugations in "Cross Flow". Under laminar flow conditions, the short distance between the inclined plates is now the only distance over which the pollutants have to rise or sink before they are intercepted and separated from the mother fluid.