What are deposits?
Accumulations of deposits in cooling water systems diminish the efficacy of heat transfer and the capacity of the water distribution system. Furthermore, these deposits prompt the creation of oxygen differential cells, expediting corrosion and leading to failures in process equipment. The deposits vary in form, ranging from thin, firmly attached films to thick, gel-like masses, contingent upon the substances causing the deposition and the deposition mechanism.
The formation of deposits is significantly impacted by various system parameters like water and surface temperatures, water flow speed, duration of contact, and the materials used in the system. The most severe deposition occurs in process equipment with elevated surface temperatures and/or reduced water velocities. The advent of high-efficiency film fill has raised concerns regarding deposit accumulation in cooling tower packing, which is broadly classified into scale or foulants.
What is scaling?
Scale deposits form due to the precipitation and crystal growth at a surface that comes into contact with water. This precipitation occurs when the solubility limits are surpassed, either in the bulk water or on the surface itself. The salts that commonly form scale on heat transfer surfaces are those exhibiting retrograde solubility with temperature.
These compounds might be entirely soluble in the cooler bulk water, but they become supersaturated in the hotter water near the heat transfer surface, leading to their precipitation onto the surface.
The phenomenon of scaling isn't solely linked to temperature. For instance, calcium carbonate and calcium sulfate scaling can occur on unheated surfaces when their solubilities are exceeded in the bulk water. Surfaces made of metals provide ideal sites for crystal nucleation due to their rough textures and the low velocities near the surface. Corrosion cells on the metal surfaces create regions of high pH, encouraging the precipitation of various salts found in cooling water. Once scale deposits form, they facilitate further nucleation, leading to an accelerated crystal growth rate.
Controlling scale can be achieved by operating the cooling system under subsaturated conditions or by using chemical additives.
The features and benefits of deposition and scale control include:
