What is scale?
As rainfall descends, it picks up CO2, which makes water slightly acidic. By the time it reaches the earth, it is now a natural solvent. As it seeps down to the underground aquifers, it dissolves mineral along the way, becoming “hard.” Hard water is generally concentrated with four hard minerals: calcium, magnesium, carbonate, and sulfate. Every water supply has some dissolved mineral content.
How does scale form?
When energy is applied to hard water, the minerals can drop out of the solution and settle on surfaces. This is called scale, the most common of which is limescale. These minerals can form a hard crust that can cause many problems with equipment, from clogging to increased energy requirements.
To understand the process that allows dissolved mineral content to rebuild solid rock, it is important to understand the condition of pH. The scale for pH is measured from 0.0 to 14.0, with 7.0 as a perfect neutral. In general terms, water supplies with a pH below 7.0 have a greater acid content and tend to dissolve rock into minerals. Water supplies with a pH above 7.0 have a lower acid content and tend to build mineral scale.
There are two key conditions for scale formation:
- The pH level must be neutral or above
- There must be an energy transfer, cooling or heating, to act as a catalyst.
How does scale affect water-using equipment?
Mineral deposits such as limescale create major problems for foodservice operators with ice, coffee, espresso, steam and warewashing equipment. Mineral scale can clog tubing and small orifices, coat heating and cooling elements, and result in increased detergent usage. Scale also causes reduced energy transfer and efficiency loss, resulting in increased energy demands for cooling or heating, and increased operating costs.
Many water-using appliances, from coffee brewers to ice makers, are susceptible to limescale build-up. Steamers and combi ovens are among the most susceptible. As water boils and evaporates, minerals remain and become concentrated. Because of these high concentrations, steamers can require frequent deliming—an acid cleaning process that removes mineral scale. This process is harsh to the equipment surfaces and decreases equipment life.
Freezing water can also cause scale to form. Commercial cuber-type ice makers require more service to correct scale build-up than any other equipment commonly used in foodservice. Like steamers, commercial icemakers leave a high concentration of minerals as most of the water becomes ice. The resulting residue is a murky mixture full of sediment and growing crystals that restricts tubes, fouls pumps, clogs orifices, scores valves, and causes ice to hang in clumps.
Fortunately, scale growth can be reduced by adding small amounts of polyphosphates to water. Polyphosphates are completely safe and nontoxic, and many occur naturally in foods or are added during processing. They are also used in the treatment of drinking water to combat corrosion and scaling. Most scale-producing situations can be resolved more effectively with a point-of-use water treatment system that couples fine filtration and a polyphosphate feed. Fine filtration reduces particles that act as nucleation sites for scale formation. These particles speed up the scaling process and can add as much as 60 percent to the weight of the scale, depending on the particles in the source water. Everpure produces many systems meeting these specifications for home use, vending and office applications, and commercial foodservice.
Types of scale
The appearance of scale varies infinitely and depends on the impurities that are present in the water. For example, pure limescale is pure white, but sediment and turbidity due to dust, dirt and mud may color it. Common contaminants have these color properties:
There are three methods for reducing scale: