Plate coalescers are skim tanks or vessels that use internal plates to improve the gravity separation process. Various configurations of plate coalescers have been devised. These are commonly called parallel plate interceptors (PPI), corrugated plate interceptors (CPI), or cross-flow separators. All of these depend on gravity separation to allow the oil droplets to rise to a plate surface where coalescence and capture occur. As shown in Figure 7-4 flow is spilt between a number of parallel plates spaced a short distance apart. To facilitate capture of the oil droplet the plates are inclined to the horizontal.
Stokes’ Law should apply to oil droplets as small in diameter as 1 to 10 microns. However, field experience indicates that 30 microns sets a reasonable lower limit on the droplet sizes that can be removed. Below this size small pressure fluctuations, platform vibration, etc., tend to impede the rise of the droplets to the coalescing surface.