AQUANILE

Deposit and Scale Control

Intense Calcium Carbonate scale. Intense Calcium Carbonate scale.

Deposit accumulations in cooling water systems reduce the efficiency of heat transfer and the carrying capacity of the water distribution system. In addition, the deposits cause oxygen differential cells to form. These cells accelerate corrosion and lead to process equipment failure.  Deposits range from thin tightly adherent films, to thick gelatinous masses, depending on the depositing species and the mechanism responsible for deposition. Deposit formation is influenced strongly by system parameters, such as water and skin temperature, water velocity, residence time, and system metallurgy. The most severe deposition is encountered in process equipment operating with high surface temperatures and/or low water velocities. With the introduction of high-efficiency film fill, deposit accumulation in the cooling tower becomes an area of concern.  Deposits are broadly categorized as scale or (microbiological) foulants.

Scale deposits are formed by precipitation and crystal growth on a surface which is in contact with water.  Precipitation occurs when solubilities are exceeded either in the bulk water or the surface. The most common scale-forming salts that deposit on heat transfer surfaces are those that exhibit retrograde solubility with temperature. Although they may be completely soluble in the lower temperature bulk water, these compounds (e.g., calcium carbonate, calcium phosphate, and magnesium silicate) supersaturate in the higher-temperature water adjacent to the heat transfer surface and precipitate on the surface. Scaling is not always related to temperature. Calcium carbonate and calcium sulfate scaling occur on unheated surfaces when their solubilities are exceeded in the bulk water.  Metallic surfaces are ideal sites for crystal nucleation because of their rough surfaces and the low velocities adjacent to the surface.  Corrosion cells on the metal surface produce areas of high pH, which promote the precipitation of many cooling water salts. Once formed, scale deposits initiate additional nucleation, and crystal growth proceeds at an accelerated rate.

Fouling occurs when insoluble particles suspended in recirculating water form deposits on a surface. Fouling mechanisms are dominated by particle-particle interactions that lead to the formation of agglomerates. At low water velocities, particle settling occurs under the influence of gravity.  Parameters that affect the rate of settling are particle size, relative liquid and particle densities, and liquid viscosity. The most important factor affecting the settling rate is the size of the particle. Because of this, the control of fouling by preventing agglomeration is one of the most fundamental aspects of deposition control. Foulants enter a cooling system with makeup water, airborne contamination, process leak, and corrosion products.


 

 

AquaNile Chemical Industries available treatment programs (AquaNile ANXXXX Series) 

  • Organic Phosphonates based Scale Inhibitors
  • Polymeric Dispersants
  • Synergistic Antiscalants

 Custom designed programs are available upon customer request, after completing a site survey by one of our field experts. For more information, contact ANCI sales team.