The droplet diameter is the most important single parameter to control to aid in water settling since this term is squared in the settling equation. A small increase in diameter will create a much larger increase in settling rate.
It would be extremely rare to have laboratory data of droplet coalescence for a given system. qualitatively we would expect droplet size to increase with retention time in the coalescing section, and with heat input, which excites the system leading to more collisions of small droplets. Droplet size could be expected to decrease with oil viscosity, which inhibits the movement of the particles and decreases the force of the collision.
The coalescence equation indicates that the oil-water interface zone is where nearly all of the coalescence occurs. Except for providing some minimal time for initial coalescence to occur, increasing retention time in a crude oil treating system may not be very cost effective. Consequently, in most systems, if one does not design the hydraulics properly at this zone, the opportunity can be lost to grow water droplets of sufficient size to settle in the vessel.