Look at the heating element of a washing machine or dishwater in a hard water area and you will see a white encrustation containing hardness salts. This is commonly referred to as limescale and is an example of domestic fouling. The limescale (calcium carbonate) that deposits on the heating element will, if untreated, reduce the efficiency of the machine, induce corrosion of the element, and ultimately lead to appliance failure.

Industrial fouling poses a far greater problem than anything in the domestic sector. Huge volumes of fluids are handled and the systems that contain the fluids can become fouled. The quality of water streams used by industry varies widely and gives rise to numerous fouling problems.

Types of Fouling

Mineral scale deposition occurs as a result of heat transfer or pressure changes. Calcium carbonate scaling from hard water, and calcium phosphate and oxalate formation in sugar refineries are examples. Other types of fouling include the growth of algae and bacteria (bio-fouling), the consolidation of loose particles (particulate fouling, e.g. corrosion byproducts), and the accumulation of "coke" like deposits (an example of chemical reaction fouling).

What Can Go Wrong?

Process managers should be concerned about fouling. Deposits are an insulating layer on heat transfer surfaces. This leads to more power being consumed or to the installation of heavier duty, more expensive heat exchangers to compensate. It is estimated that 40% more energy is needed to heat water in a system fouled with 1/4 inch of calcium carbonate scale. Scaled boiler tubes mechanically fail as a result of overheating and cooling tower plates can collapse due to the weight of scale deposits. Erosion damage can occur as a result of scale particles breaking loose and subsequently impinging upon other surfaces.

Pipework scale reduces the available cross-section area, and fluids are affected by increased pipewall friction. A larger, more power-consuming pump will be required to maintain throughput volumes but this may allow only a temporary solution to the problem. A plant that needs to be shut down for cleaning costs money.

The formation of a thin uniform layer of scale or wax can temporarily reduce steel corrosivity, but eventually stagnant conditions develop under the deposit and electrochemical reactions will corrode the steel surfaces. The result can be fluid leaks and equipment failure, which are potentially very dangerous. In the food industry, the incorporation of even undesirable trace particulates can lead to off-flavors or off-colors, reducing shelf-life, or even making the product unsaleable.

Not only are plant and product integrity at risk but personnel health and safety may be compromised. Safety valves or emergency process sensors that are fouled may not operate in an emergency. Overheated boilers have been known to explode. Failure to control bacterial growth in cooling water can create conditions hazardous to health (e.g. production of legionella pneumophila) or, in anaerobic conditions, may allow the production of toxic hydrogen sulphide from sulphate reducing bacteria.

Recognizing Fouling

Because scales and other deposits generally form inside closed systems, it is not always evident that deposition is occurring. But some clues can provide the necessary evidence. It is useful to try to answer the following questions:

• Are energy/heating bills reduced immediately after cleaning the plant?


• Is it necessary to arrange significant planned and/or unplanned downtime?


• Are heat exchangers performing below design?


• Is corrosion a problem in the plant?


• Are there signs of unexpected deposit formation within the system?

The more times the answer is "yes," the more likely it is that there is fouling. If fouling can be controlled, there is the potential to save energy, prevent equipment failure, and reduce maintenance. Furthermore, a successful treatment strategy will maintain fluid flow, reduce corrosion effects, and provide a safer environment -- in addition to saving money.

Solving The Problem

A process audit would identify the extent of the current problem, the point in the system corresponding to initial fouling, and most useful, why there is a problem. From the evidence collated, it may be possible to suggest a solution without the need for expensive external control measures. Minor changes in the process temperature, pressure, pH or fluids composition could significantly reduce the fouling potential at practically no cost.

Treatment options include inhibitor chemicals, descalers, ion exchange, physical cleaning such as pipeline pigging, or the installation of permanent magnets, or electronic devices such as the patented Scalewatcher® Electronic Scale Control System.

Although it is usually possible to find a chemical solution to a fouling problem, environmental and safety pressures demand that chemical consumption be reduced wherever possible. Increasingly, restrictions are being applied regarding the use of chemicals, due to their environmental impact.

Physical Methods

A range of physical methods can be used to remove fouling deposits. Water jetting, sand or plastic-bead blasting can be used in accessible locations. Such methods are expensive and can cause abrasion of surfaces.

Magnetic & Electronic Descaling

Unlike other preventative techniques, these devices do not stop precipitation but alter the shape of the crystals to reduce the adherence and buildup of deposits on the pipewall. Perhaps the most remarkable observation is that devices can affect descaling downstream of the point of installation; a softening and loosening of existing scale several weeks after installation is commonly reported.

To understand the mechanism, some knowledge of mineral scale precipitation is necessary. We know that in order to form a scale deposit three conditions must be met:

1. The solution must be supersaturated.
2. Nucleation sites must be available at the pipe surface.
3. Contact / residence time must be adequate.

To prevent scale it is necessary to remove at least one of these preconditions. Clearly contact time is not an alterable factor. To be effective any device must therefore affect either the supersaturation value or the nucleation process.

The direct effect of electronic devices is on the nucleation process and in particular to enhance initial nucleation through the creation of new nucleation sites within the bulk fluid flow. Crystal growth then occurs at these points of nucleation and not at the pipewall. Suspended solids increase with a corresponding drop in the level of supersaturation, and these effects have been observed in the field. The localized pH increase near the pipewall, caused by hydroxyl radicals formed by electromechanical interactions, is one mechanism that drives the changed nucleation characteristics.

A Lorenz force F is experienced by charged particles that flow through a field: F = qE + q (V x B) where q is the charge on the particle, E is the electric field vector, V is the particle velocity, and B the magnetic field vector. Electronic devices operate at very small residual magnetic fields whereas magnets need high field strength (>1000gauss) for optimum performance. The flow dependency of magnetic devices is explained by the velocity parameter, V, and E=0. The flow non-dependency of electronic devices is explained by the fact that the magnetic component approaches zero, but the electric component is essentially constant. This suggests that the key performance parameter is the total value of the 'Lorenz' force acting on the charged particles, rather than the individual magnetic and electric field vectors.

Electronic devices are not flow-rate dependent and can be built to fit pipe diameter up to at least 60 in. The units are lightweight, easy to install, can be retrofitted, and produce no significant magnetic field. They are usually effective on calcium carbonate, are claimed to reduce iron fouling, and appear to prevent fouling by various other substances.

The patented Scalewatcher® Electronic Scale Control System, originally launched in 1989, is now sold throughout the world. An environmentally friendly alternative to chemical and mechanical descaling, it provides a non-intrusive, chemical- and maintenance-free method of removing limescale from pipes and equipment. It has successfully treated hard water problems for industrial manufacturers as well as water companies, oil producers, farmers, horticulturists, shipping companies, shopping centers, educational and government establishments.

The Scalewatcher® functions by means of an electronically applied field generated by a solenoid coil wrapped around the outside wall of the pipework to be treated. Using Scalewatcher's patented technology, an induced electric field causes crystals to grow out of the dissolved mineral ions. These crystals remain suspended in solution and no longer contribute to the buildup of hard pipewall deposits. Existing scale deposits are softened and erosion processes then remove loosened scale crystals from the system.

Research in the U.S., Europe and Asia by vendors such as Scalewatcher® has led to the implementation of an effective scale deposit-control strategy, reducing costs and contributing to customers' profits.