The magnetic technology has been cited in the literature and investigated since the turn of the 19th century, when lodestones and naturally occurring magnetic mineral formations were used to decrease the formation of scale in cooking and laundry applications. Today, advances in magnetic and electrostatic scale control technologies have led to their becoming reliable energy savers in certain applications.
For example, magnetic or electrostatic scale control technologies can be used as a replacement for most water-softening equipment. Specifically, chemical softening (lime or lime-soda softening), ion exchange, and reverse osmosis, when used for the control of hardness, could potentially be replaced by non-chemical water conditioning technology. This would include applications both to cooling water treatment and boiler water treatment in once-through and recirculating systems.
The primary energy savings from this technology result from decrease in energy consumption in heating or cooling applications. This savings is associated with the prevention or removal of scale build-up on a heat exchange surface, where even a thin film can increase energy consumption by nearly 10%. Secondary energy savings can be attributed to reducing the pump load, or system pressure, required to move the water through a scale-free, unrestricted piping system.
About the Technology
The technology addressed in this FTA uses a magnetic or electrostatic field to alter the reaction between scale-forming ions in hard water. Hard water contains high levels of calcium, magnesium, and other divalent cations. When subjected to heating, the divalent ions form insoluble compounds with anions such as carbonate. These insoluble compounds have a much lower heat transfer capability than heat transfer surfaces such as metal. They are insulators. Thus additional fuel consumption would be required to transfer an equivalent amount of energy.
The magnetic technology has been cited in the literature and investigated since the turn of the 19th century, when lodestones or naturally occurring magnetic mineral formations were used to decrease the formation of scale in cooking and laundry applications. However, the availability of high-power, rare-earth element magnets has advanced the magnetic technology to the point where it is more reliable. Similar advances in materials science, such as the availability of ceramic electrodes and other durable dielectric materials, have allowed the electrostatic technology to also become more reliable.
The general operating principle for the magnetic technology is a result of the physics of interaction between a magnetic field and a moving electric charge, in this case in the form of an ion. When ions pass through the magnetic field, a force is exerted on each ion. The forces on ions of opposite charges are in opposite directions. The redirection of the particles tends to increase the frequency with which ions of opposite charge collide and combine to form a mineral precipitate, or insoluble compound. Since this reaction takes place in a low-temperature region of a heat exchange system, the scale formed is non-adherent. At the prevailing temperature conditions, this form is preferred over the adherent form, which attaches to heat exchange surfaces.
The operating principles for the electrostatic units are much different. Instead of causing the dissolved ions to come together and form non-adherent scale, a surface charge is imposed on the ions so that they repel instead of attract each other. Thus the two ions (positive and negative, or cations and anions, respectively) of a kind needed to form scale are never able to come close enough together to initiate the scale-forming reaction. The end result for a user is the same with either technology; scale formation on heat exchange surfaces is greatly reduced or eliminated.
These technologies can be used as a replacement for most water-softening equipment. Specifically, chemical softening (lime or lime-soda softening), ion exchange, and reverse osmosis (RO), when used for the control of hardness, can be replaced by the non-chemical water conditioning technology. This would include applications both to cooling water treatment and boiler water treatment, in once-through and recirculating systems. Other applications mentioned by the manufacturers include use on petroleum pipelines as a means of decreasing fouling caused by wax build-up, and the ability to inhibit biofouling and corrosion.
The magnetic technology is generally not applicable in situations where the hard water contains "appreciable" concentrations of iron. In this FTA, appreciable means a concentration requiring iron treatment or removal prior to use, on the order of parts per million or mg/L. The reason for this precaution is that the action of the magnetic field on the hardness-causing ions is very weak. Conversely, the action of the magnetic field on the iron ions is very strong, which interferes with the water conditioning action.
Literature provided by and discussions with manufacturers described a typical installation for a boiler water treatment scheme as including the device installed upstream of the boiler. Manufacturers vary in their preference of whether the device should be installed close to the water inlet or close to the boiler. Both locations have been documented as providing adequate performance. Generally, the preferred installation location for use with cooling towers or heat exchangers is upstream of the heat exchange location and upstream of the cooling tower. Downstream of the cooling tower but upstream of the heat source was also mentioned as a possible installation location, primarily for the use with chillers or other cooling equipment.
The primary caveat on installation of the magnetic technology is that high voltage (230V, 3-phase or above) power lines interfere with operation by imposing a second magnetic field on the water. (This is most noticeable when these electric power sources are installed within three feet of a magnetic device.) This second magnetic field most likely will not be aligned with the magnetic field of the device, thus introducing interference and reducing the effectiveness of the treatment. Installations near high voltage power lines are to be avoided if possible. Where avoidance is not possible, the installation of shielded equipment is recommended to achieve optimum operation. Some manufacturers also have limitations on direction of installation--vertical or horizontal--because of internal mechanical construction
The primary energy savings result from a decrease in energy consumption in heating or cooling applications. This savings is associated with the prevention or removal of scale build-up on a heat exchange surface where even a thin film (1/32" or 0.8 mm) can increase energy consumption by nearly 10%. Example savings resulting from the removal of calcium-magnesium scales are shown in Table 1. A secondary energy savings can be attributed to reducing the pump load, or system pressure, required to move the water through a scale-free, unrestricted piping system.
Table 1. Example Increases in Energy Consumption
As was discussed above, magnetic and electric fields interact with a resultant force generated in a direction perpendicular to the plane formed by the magnetic and electric field vectors. (See Figure 2 for an illustration.) This force acts on the current carrying entity, the ion. Positively charged particles will move in a direction in accord with the Right-hand Rule, where the electric and magnetic fields are represented by the fingers and the force by the thumb. Negatively charged particles will move in the opposite direction. This force is in addition to any mixing in the fluid due to turbulence.
Figure 2. Diagram Showing Positioning of Fields and Force
The result of these forces on the ions is that, in general, positive charged ions (calcium and magnesium, primarily) and negative charged ions (carbonate and sulfate, primarily) are directed toward each other with increased velocity. The increased velocity should result in an increase in the number of collisions between the particles, with the result being formation of insoluble particulate matter. Once a precipitate is formed, it serves as a foundation for further growth of the scale crystal. The treatment efficiency increases with increasing hardness since more ions are present in solution; thus each ion will need to travel a shorter distance before encountering an ion of opposite charge.
A similar reaction occurs at a heat exchange surface but the force on the ions results from the heat input to the water. Heat increases the motion of the water molecules, which in turn increases the motion of the ions, which then collide. In addition, scale exhibits an inverse solubility relationship with temperature, meaning that the solubility of the material decreases as temperature increases. Therefore, at the hottest point in a heat exchanger, the heat exchange surface, the scale is least soluble, and, furthermore due to thermally induced currents, the ions are most likely to collide nearest the surface. As above, the precipitate formed acts as a foundation for further crystal growth.
When the scale-forming reaction takes place within a heat exchanger, the mineral form of the most common scale is called calcite. Calcite is an adherent mineral that causes the build-up of scale on the heat exchange surface. When the reaction between positively charged and negatively charged ions occurs at low temperature, relative to a heat exchange surface, the mineral form is usually aragonite. Aragonite is much less adherent to heat exchange surfaces, and tends to form smaller-grained or softer-scale deposits, as opposed to the monolithic sheets of scale common on heat exchange surfaces.
These smaller-grained or softer-scale deposits are stable upon heating and can be carried throughout a heating or cooling system while causing little or no apparent damage. This transport property allows the mineral to be moved through a system to a place where it is convenient to collect and remove the solid precipitate. This may include removal with the wastewater in a once-through system, with the blowdown in a recirculating system, or from a device such as a filter, water/solids separator, sump or other device specifically introduced into the system to capture the precipitate.
Water savings are also possible in recirculating systems through the reduction in blowdown necessary. Blowdown is used to reduce or balance out the minerals and chemical concentrations within the system. If the chemical consumption for scale control is reduced, it may be possible to reduce blowdown also. However, the management of corrosion inhibitor and/or biocide build-up, and/or residual products or degradation by-products, may become the controlling factor in determining blowdown frequency and volume.
Aside from the energy savings, other potential areas for savings exist. The first is elimination or significant reduction in the need for scale and hardness control chemicals. In a typical plant, this savings could be on the order of thousands of dollars each year when the cost of chemicals, labor and equipment is factored in. Second, periodic descaling of the heat exchange equipment is virtually eliminated. Thus process downtime, chemical usage, and labor requirements are eliminated. A third potential savings is from reductions in heat exchanger tube replacement due to failure. Failure of tubes due to scale build-up, and the resultant temperature rise across the heat exchange surface, will be eliminated or greatly reduced in proportion to the reduction in scale formation.
Devices are available in two installation variations and three operational variations. First to be discussed are the two installation variations: invasive and non-invasive. Invasive devices are those which have part or all of the operating equipment within the flow field. Therefore, these devices require the removal of a section of the pipe for insertion of the device. This, of course, necessitates an amount of time for the pipe to be out of service. Non-invasive devices are completely external to the pipe, and thus can be installed while the pipe is in operation. The followingg illustrates the two installation variations.
Figure 3. Illustration of Classes of Magnetic Devices by Installation Location
Figure 4. Illustration of Classes of Non-Permanent Magnet Devices
Electrostatic units are always invasive. The other two types can be either invasive or non-invasive. The devices illustrated in Figure 3 are examples of permanent magnet devices.
Most of the devices are in-line--some invasive, some non-invasive--as opposed to side-stream. The invasive devices require a section of pipe to be removed and replaced with the device. Most of the invasive devices are larger in diameter than the section of pipe they replace. The increased diameter is partially a function of the magnetic or electromagnetic elements, and also a function of the cross sectional flow area. The flow area through the devices is generally equivalent to the flow area of the section of pipe removed.
The non-invasive in-line devices are designed to be wrapped around the pipe. Thus downtime, or line out-of-service time, is minimized or eliminated.
The ranking results from the screening process for this technology are shown in Table 2. These values represent the maximum benefit achieved by implementation of the technology in every Federal application where it is considered life-cycle cost-effective. The actual benefit will be lower because full market penetration is unlikely to ever be achieved.
The primary question to be answered is "Does the technology work as advertised?" The history of the technologies, as illustrated through primarily qualitative--but some quantitative--assessment in many case studies, has shown that when properly installed, a decrease in or elimination of scale formation will be found. While the evidence supporting the technologies may be thought of as mainly anecdotal, the fact remains that upon visual inspection after installation of these devices the formation of new scale deposits has been inhibited. In addition, in most cases, scale deposits present within the system at the time of installation have been removed.
The key here is properly installed. By this it is meant that a manufacturer or their qualified representative is responsible for equipment integration. Unlike many other technologies where much of the knowledge has been reduced to a quantitative model, the non-chemical water treatment industry still relies largely on experience as the means of providing quality installation, service and, consequently, customer satisfaction.
Of particular interest to the manufacturer would be physical parameters such as water flow rate, and water quality parameters such as hardness, alkalinity, and iron concentration. These parameters will help determine the optimum size and the extent of treatment.
The manufacturer may also want to know whether the installation is for use in conjunction with a boiler or a cooling tower, and for once-through or recirculating water systems. These parameters will help determine the optimum location within the system.
Other factors of interest may include whether the cooling or heating system is sensitive to particulate matter, and if so what particle sizes. The device works by initiating the precipitation of scale, thus particulate matter will be present in the treated water. If the system is sensitive to particulate matter there may be a need for a solid separation device such as a filter, a settling basin, a cyclone, or a sump to collect solids and to allow for their easy removal from the system.
Previously, the technology can be applied wherever hard water is found to cause scale. Since the technology is a physical process, as opposed to chemical water softening, it is expected to perform best in locations with harder water. In general, only a few locations do not require or would not benefit from some type of hardness control. Hard water is one in which the hardness is greater than 60 mg/L (or ppm) as calcium carbonate. This corresponds to approximately 3.5 grains of hardness per U.S. gallon. The Pacific Northwest states, the North Atlantic coastal states, and the Southeast states, excluding Florida, are locations where naturally occurring soft water is most likely to be found. The balance of the United States could benefit from some type of water treatment to control scale formation, using either one of the traditional technologies such as lime softening or ion exchange, or the non-chemical technology discussed in this FTA.
Where to Apply
Non-chemical scale control technologies can be used for either boiler scale control or cooling tower scale control. Boiler scale control applications are the majority of the installations, but the control of silica scale in cooling water applications is also possible. Experience has been cited with both retrofit installations and in new installations (see References for a brief listing of applicable reports and publications).
Non-chemical scale control technologies are best applied:
What to Avoid
There are a few precautions to be noted before selecting the technology: