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Hard water limescale is a major factor of end user increased energy costs

06 May 2009

Hard water limescale is a major source of industrial fouling and is a primary reason for increasing end user energy bills.

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 ¼ inch of calcium carbonate scale.

Recognising Fouling

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

 Do energy / heating bills reduce 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" then the more likely it is that there is fouling. If fouling can be controlled there is 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 this it will save 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-ENiGMA 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 is 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 the 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 build up of deposits on the pipe wall. 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 pre-conditions. 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 on 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 pipe wall.
Suspended solids increase with a corresponding drop in the level of supersaturation and these effects have been observed in the field. The localised pH increase near the pipe wall, 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 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.

TABLE 1

Scale prevention with the Lorenz Force

• Scalewatcher: F=q(E + vxB)
• Magnet: F=q.vxB Newton
• Alloys: F=q.E Newton

F: Lorenz Force on particle or dissolved mineral ion in the liquid, measured in
Newton.
q: Charge of particle or dissolved ion, measured in Coulomb
E: Electric field generated by alternation magnetic field measured in V/m
(Volt/meter)
v: Velocity of water measured in m/s (meter/second)

Electronic devices are not flow-rate dependent and can be built to fit pipe diameter up to at least 60". 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-ENiGMA Electronic Scale Control System, originally launched in 1989, is now sold throughout the world. It provides a non-intrusive, chemical and maintenance free method of removing limescale from pipes and equipment. It 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 build-up of hard pipe wall deposits. Existing scale deposits are softened and erosion processes then remove loosened scale crystals from the system. Responsible vendors have invested significant sums in the research of electronic systems in the USA, Europe and Asia. The company's research has lead to the implementation of an effective scale deposit-control strategy, directly reducing energy costs and contributing to
customers' profits.

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