We almost always use coalescing plates to remove oil from water. The oil entering the system is in the form of a spectrum of small droplets. These droplets rise according to Stokes's Law, a principle of physics which says that larger oil droplets rise more quickly than small ones and when the water is warm the droplets rise more quickly as well. Small droplets may rise at a very slow rate. Because we don't want to have to wait for very small droplets to rise all the way to the surface of the separator (and provide a large and therefore expensive separator), we install closely spaced coalescing plates so that the rise distance is very short - as little as a quarter inch. In effect, we substitute coalescing plates for more residence time within the vessel for the droplets to rise to the surface. This makes the separator smaller and less expensive. We have two coalescing plate spacing designs available - nominal ¼" and nominal ½" and offer coalescing plates in calcium carbonate-filled polypropylene for most uses and other plastics for use in aggressive chemical environments. The operation of Stokes's Law also requires laminar flow within the vessel, and large empty tanks do not meet this requirement - large vessels are subject to channeling flow and convection currents, so by installing our coalescing plates we have also improved the separation by making the flow conform to the requirements of the physics of removal. The droplets rise up and meet the underside of the plates where they are captured. Subsequently, more droplets are captured and they coalesce into very large droplets. These droplets then migrate up the sloping surfaces of the plate to the top where there are holes that allow them to come to the surface. The diagram shows this process. The same principles and plate can also be used to separate water from oil. While the principles of oil removal are simple, effective application of these principles is not as easy. It is our careful attention to design that allows us to use gravity to such advantage in removing small droplets of oil from water. If there are special problems in the separation or an extremely low effluent is required, we may suggest other technologies to supplement the operations of the coalescing plates. For a more in-depth discussion of the technology and how they work, please see "How They Work " on our Technical Information page. Also, for a complete description of the operating principles of coalescing plate modules, please see "Arc Coalescing Plates".