Introduction
Industrial circulating cooling water systems are used across power generation, semiconductor manufacturing, chemical processing, metallurgy and data centers. Their core function is to carry process waste heat away through continuous water circulation, holding process equipment at a stable operating temperature. In China, industrial cooling water accounts for roughly 60–80% of total industrial water use, which makes scaling control both an energy issue and a water-resource issue.
Thermodynamic Basis of CaCO3 Crystallisation
Calcium carbonate is the dominant scale constituent in industrial cooling water, typically 60–80% of total deposit mass. CaCO3 occurs in three common crystal forms: calcite, aragonite and vaterite. Calcite is the most thermodynamically stable and the hardest to remove once formed; aragonite is metastable and prefers higher temperatures; vaterite is the least stable and rarely persists.
In practice the Langelier Saturation Index (LSI) is used to judge scaling tendency. LSI is defined as the difference between actual pH and the pH at which the water would be saturated with respect to CaCO3. Positive LSI indicates a scaling tendency; negative LSI indicates a corrosive tendency. The Ryznar Stability Index (RSI) refines this with a wider usable range and is often preferred for high-concentration systems.
Fouling Mechanisms
Fouling in cooling circuits is normally classified into three types. Sedimentation fouling arises from suspended solids — silt, rust particles, microbial debris — settling under gravity onto tube walls and fill surfaces, forming a primary layer that then accelerates further deposition.
Crystallisation fouling is the dominant type. It proceeds in three stages: ion pairing of Ca2+ and CO3 2− in supersaturated solution to form nanometre-scale nuclei; growth of those nuclei into microcrystals; and attachment of crystals to the heat-transfer surface followed by ageing into a dense hard layer.
Corrosion fouling occurs when metal piping — carbon steel, copper alloys — corrodes electrochemically under the combined action of dissolved oxygen and chloride, producing Fe2O3, FeOOH and CuO. These corrosion products deposit on heat-transfer surfaces and, in turn, create under-deposit corrosion sites.
Limits of Conventional Descaling Methods
Conventional approaches fall into chemical and physical families. Chemical methods rely on scale inhibitors such as HEDP, ATMP and PBTCA (organophosphonates) plus dispersants; these suppress crystal growth but do not remove hardness ions from the water, so concentration ratio stays capped around 3–5.
Physical methods — electromagnetic conditioning, ultrasonic treatment — avoid chemicals but show inconsistent, site-dependent performance and little measurable effect at high hardness and high concentration ratio. Acid cleaning dissolves existing scale but is inherently a shutdown activity with corrosion and waste-liquid handling cost.
Principle of ECD Electrochemical Descaling
Electrochemical Descaling (ECD) applies a low-density DC field across purpose-built electrodes, creating a locally supersaturated zone at the cathode surface. Ca2+ and Mg2+ migrate toward the cathode under the field.
At the cathode, water reduction generates OH− and raises local pH to roughly 9–11. The resulting high local carbonate concentration drives CaCO3 to nucleate and grow on the cathode plate rather than on the heat exchanger tubes. Mechanical scraping or automatic back-flushing periodically removes the deposited layer and discharges it as a solid slurry.
The key distinction from chemical treatment is that ECD physically removes scale-forming ions from the circulating water instead of merely keeping them in solution. This decouples scaling control from concentration ratio.
Advantages and Outlook
Because hardness ions are removed rather than suppressed, systems running ECD can operate at markedly higher cycles of concentration, reducing make-up water and blowdown volume. Chemical dosing drops substantially, and heat-transfer surfaces stay closer to design cleanliness for longer.
Scaling in circulating cooling water is a multi-scale, multi-physics process spanning crystallisation thermodynamics, mass-transfer kinetics and electrochemistry. Conventional chemical and physical routes each hit their own wall under high-concentration, high-hardness conditions; the electrochemical route shifts the problem from suppression to directed removal.