Control Scale to Optimize HVAC Equipment Energy Efficiency

Heat exchangers that have become fouled with limescale deposition lose their effectiveness and require intensive energy consumption to operate.

It costs U.S. industry billions of dollars a year to control and remove the limescale that builds up in water-fed industrial heating, venting, and air-conditioning (HVAC) equipment such as boilers, chillers, heat exchangers, and evaporative coolers.

Limescale not only increases downtime and maintenance costs and causes the early replacement of capital equipment, it also increases energy consumption.

Descaling can minimize problems related to limescale accumulation on HVAC equipment by conserving water, reducing energy, and minimizing unexpected shutdowns. Electromagnetic descaling devices are environmentally friendly

alternatives to chemical and mechanical descaling.

Scale Deposition Types

Scale usually refers to a mixture of sparingly soluble mineral salts. Mineral scale deposition, or fouling, occurs during heat transfer or pressure changes. Calcium carbonate (CaCO₃) scaling from hard water, as well as calcium phosphate and oxalate formation in sugar refineries, are examples.

Other types of scale include the growth of algae and bacteria (biofouling), the consolidation of loose particles (particulate fouling), the accumulation of coke-like deposits (for example, chemical reaction fouling) (see Common Fouling Mechanisms sidebar), and sometimes a combination of several different materials. Over time they create changes to the water composition.

How Scale Increases Energy Use

Figure 1
Scale in pipe work reduces the available cross-sectional inside diameter (ID) and the flow resistance, requiring more power to propel the fluid.

CaCO₃ is the predominant component of the hard and tenacious scale deposit from water and is particularly present in processes involving heat transfer. A concentration of dissolved solids by repeated partial evaporation of the water is the main factor that causes calcium carbonate scale. Eventually even soft water will become scale-forming when minerals are present in low
concentrations.

Process, maintenance, and facility managers should be concerned about scale deposition. Deposits become an insulating layer on heat transfer surfaces that inhibits efficient heat transfer. This causes more power consumption.

It is estimated that 40 percent more energy is needed to heat water in a system fouled with ¼ inch of limescale.

Often manufacturers install heavy-duty, expensive heat exchangers to compensate. Scaled boiler tubes mechanically fail as a result of overheating. Cooling tower plates can collapse under the weight of scale deposits. Scale particles breaking loose and subsequently impinging upon other surfaces can erode the surfaces.

Scale in pipe work reduces the available cross-sectional inside diameter (ID) and therefore the flow resistance (see Figure 1). To maintain throughput volumes through this narrower ID, a larger, more powerful—and power-consuming—pump will be required, but this may help only temporarily. It costs money to shut down a plant for cleaning.

Other Ill Effects of Scale

At first the formation of a thin, uniform layer of scale or wax temporarily reduces steel corrosion, but eventually stagnant conditions develop under the deposit and electrochemical reactions corrode the steel surfaces. This results in fluid leaks and equipment failure.

In the food and beverage industry, even the undesirable trace particulates in the piping can contaminate the product and foul the flavors and alter the colors, making the product unsalable.

Oil wells have significant scaling problems from the highly mineralized water that is extracted with the oil.

Not only are plant and product integrity at risk; 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 (for example, production of Legionella or pneumophila) or, in
anaerobic conditions, may allow the production of toxic hydrogen sulfide from sulfate-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 and heating bills lower immediately after cleaning the plant?
• Is it necessary to arrange significant planned and 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 scaling or fouling is occurring. Controlling deposition will save energy, prevent equipment failure, and reduce maintenance. Furthermore, a successful treatment strategy maintains fluid flow, reduces corrosion effects, and provides a safer environment—all of which save money.

Solving the Problem

Figure 2
Electromagnetic scale removal devices work by producing a complex frequency-modulated waveform, inducing an electromagnetic field inside the pipe. Scientific research shows that a specific electromagnetic field initiates a colloidal cluster regrouping in the first stage of crystallization. The resulting CaCO2 will be in the form of scattered particles that will be flushed out by the
medium.

A process audit identifies 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 collected, it may be possible to find 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, and the installation of electromagnetic devices.

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

Physical Mechanical Methods. A range of physical methods can be used to remove fouling deposits. Water jetting or sand and plastic-bead blasting can be used in accessible locations. Such methods, however, can cause abrasions on the surfaces, which in turn can lead to an increase in scaling because the abrasions provide a suitable platform for concentrated limescale.

Physical Electromagnetic Method. Electromagnetic scale removal devices alter the shape of the CaCO₃ crystals to reduce the adherence and deposit buildup on the pipe wall by changing nucleation in such a way that the particles do not stick to any surface (see Figure 2). These devices can affect descaling downstream of the point of installation. A softening and loosening of existing scale several weeks after installation is commonly reported.

This treatment is effective when the water flow is extremely low or zero, as the term qE is independent of the flow velocity. Because scale no longer builds up, the flowing water will remove existing layers of scale over time, caused by diffusion according to the law of mass action. By electromagnetic treatment, hard water is capable of dissolving and removing existing scale layers.

The direct effect of an electronic device described above is on the nucleation process and colloids. This approach operates on a phenomenon known as the Lorenz force.

F = qE + q (V x B)

Where F is excited by charged particles that flow through a field

q = Charge on the particle
E = Electric field vector
V = Particle velocity vector
B = Magnetic field vector

Electromagnetic devices operate at very weak alternating electromagnetic fields, where not so much the strength is important; rather, the frequencies used determine a successful treatment.

Scale prevention and removal are achieved in several ways:

1. Electromagnetic devices influence the initial nucleation, resulting in crystals that do not stick together. While untreated water builds up matted structures that grow more and more, electromagnetic treatment creates idiomorphic, scattered crystals that will not form matted structures. They have a roundish shape, which means that they have a rather big volume in relation to a rather small surface. This feature makes them sensitive to water currents and they will be flushed out of the pipe easily.
2. Colloidal clusters in water may have positive and negative charges (Janus model) at different places. Only a little energy is needed to reposition the colloids if an electromagnetic field with the correct frequency is used. Once these colloids have been repositioned, their tendency to stick is much lower.
3. The scale on a pipe wall acts like any process at a junction of two substances; the law of mass action applies. This means that there is an action and a reverse action simultaneously. In this case, while some particles build up scale, other particles will go into solution again. In untreated water, the tendency for particles to build up scale is stronger than to dissolve. After treatment, the situation changes: As the particles in water stop depositing, scale particles dissolve.

Besides the chemical law of mass action, there is a physical aspect that supports scale removal. Scale builds up layer by layer, piling up more and more. These layers, however, reflect changing water composition over time. So while they are quite homogenous in the layer itself, the surrounding layers have a different composition. The interlayer bonding force between the layers is weaker than inside a layer. The last layer, being in contact with the water, will be attacked first by the treatment. When it is removed, the intermediate layer will be removed quickly, exposing the next inner layer. Layers differ in composition and thickness, which determines the time it will take to remove them.