Cross-flow microsand filtration improves cooling tower performance, reduces maintenance, and enhances environmental interaction.
Cooling towers absorb heat from HVAC systems and eject it into the atmosphere. They only do it well if properly maintained to reduce bacterial (particularly Legionella) buildup and scaling. Proper maintenance is essential to reduce bacteria and increase heat-exchange efficiency. While proper chemical treatment reduces downtime and protects cooling-tower assets, the growing water scarcity faced worldwide urgently calls for fewer chemical treatments and more effective water management.
To reduce the concentration of contaminants in HVAC cooling towers, a substantial amount of water must be periodically discharged. However, this discharged water, or blowdown, contains anti-scale, anti-corrosion, and anti-microbiological toxic chemicals. Due to the wide variation of chemicals used in cooling towers, it is not possible to quantify the exact number of pollutants discharged globally into the environment.
The blowdown is typically transferred to local sewage or storm-water systems and ultimately into the environment. An added concern is that hazardous materials discharged into local sewage systems must be treated by municipal operations. One technology that can help mitigate this problem is cross-flow microsand filtration.
High-efficiency cross-flow microsand filtration (CMF) captures microscopic and even submicron particles. CMF systems improve the efficiency of chemicals and require fewer chemicals to achieve the same results achieved with more-conventional methods.
While high-efficiency cross-flow microsand filtration (CMF) is a uniquely effective solution for reducing fouling, according to research by Eneref Institute, Philadelphia, only about 15% of all U.S. commercial buildings use CMF systems in their cooling towers. Roughly 25% use centrifugal separators; 20% use screen filters; and 20% use cartridge bags, mono- or multimedia filtration and other systems. Healthcare and university facilities are especially aware of the need to eliminate bacterial contamination and therefore represent the largest share of high-efficiency CMF users.
Conventional deep-bed multimedia, or sand, filters typically capture particles of 20 microns and larger. While such systems can remove as much as 90% of contaminant particles by weight, they leave, unchecked, all fine particles—the particulates that are most responsible for the fouling that supports Legionella and system inefficiency.
By count, fine particles far outnumber larger particles in cooling-tower systems. For example, a single 3-mm particle is equal in weight to as many as 256 billion 2-micron-sized particles. That explains why cooling tower water analysis reports typically show that 95% of particulates are less than 5 microns in size. When measured by mass, however, 80% of the bulk is comprised of particles larger than 15 microns. Traditional filtration systems reduce the quantity of larger particles but not the smaller particles that cause fouling—demonstrating why parts per million (ppm) is an incomplete measure of total suspended solid (TSS) in water.
How CMF Works
CMF systems differ from traditional flow media filters in a number of ways. Rather than a simple vertical flow, water is directed across the top layer of the media bed by an injector. The flow scours the media’s top surface layer, preventing surface binding by lifting larger particles into suspension. The media bed then becomes a clean, free space in which fine particulates are trapped. In this way, CMF technology makes optimal use of the media surface area and removes particles down to submicron levels, thus protecting the cooling tower by reducing the risk of fouling, scaling, and corrosion.
No filtration system can remove dissolved solids or increase the cycle of concentration (the ratio of dissolved solids in the cooling tower to the dissolved solids in the make-up water). Because dissolved solids are removed by purging water and topping up the tower with fresh make-up water, frequent blowdown is the only solution to reduce the concentration of total dissolved solids (TDS). However, as mentioned, frequent blowdown sends toxic chemicals into the environment, perpetuating the cycle.
Unlike total dissolved solids, total suspended solids (TSS) can be reduced by filtration. Reducing TSS makes chlorine and other disinfectants more effective, allowing chemical treatments to be reduced by as much as 35%. Therefore, the rate of corrosion from oxidizing chemicals is reduced, as is chemical odor. Furthermore, it may no longer be necessary to use coagulants to fuse and jettison small particulates. The lessened chemical requirements not only reduce material and labor cost but, more significantly, provide environmental benefits by decreasing the number of toxic chemicals entering the ecosystem.
When suspended solid particles accumulate and settle on heat exchangers, fouling occurs. Fouling reduces heat-exchanger efficiency, increases maintenance, and forces shutdowns. Fouling can also be caused by airborne sediment such as dirt and silt.
With significant microbial growth, microorganisms create a gel-like biofilm. Biofilm is another source of fouling, as it creates a layer that protects microorganisms from disinfectant chemicals, making cleaning difficult. Biofilms also prevent anticorrosion chemicals from reaching the heat-exchanger surface. Moreover, biofilm formation prevents microorganisms from being easily flushed away during cooling-tower blowdown. By filtering particulates less than 5 microns in size, CMF reduces fouling, thereby maintaining the equipment efficiency designed by manufacturers.
Inorganic calcium salts cause scaling, not fouling. While fouling and scaling reduce thermal conductivity, or heat transfer, fouling has a greater impact on conductivity. Every thousandth-of-an-inch increase in fouling necessitates a 10% increase in power from the system because the deposits insulate the heat-transfer surfaces, impeding heat exchange.
While the thermal conductivity of copper pipes can be as high as 398 W/m K, the thermal conductivity of calcium carbonate, the most common cause of scaling, is just 2.26 W/m K, or 1% that of copper. Calcium sulfate and calcium phosphate, also common scaling sources, have low conductivity, similar to calcium carbonate. Yet biofilm, a root cause of fouling, has a thermal conductivity significantly lower than the salts that cause scaling, just 0.63 W/m K. This is why CMF is crucial to maintaining heat-transfer efficiency.
The fouling rate is much faster when smaller particles are present in the water, explaining why fine filtration is essential. As water flows across the metallic surface (which is jagged at the microscopic level) of a heat exchanger, larger particles tend to roll and bounce off. However, the fine particles—those that CMF systems are designed to capture—are the first to cling to surfaces.
For example, if the concentration of 8-micron particles were to double, the fouling heat-transfer resistance in the system would increase by only 5%. On the other hand, if the concentration of 1-micron-sized particles were to double, the fouling heat transfer resistance could increase by as much as 150%.
While no filtration system eliminates Legionella, reducing the number of fine particles can break up the shelters that protect microbes from disinfecting chemicals. In this way, high-efficiency CMF technology maximizes the chemical effectiveness by diminishing the opportunity for microbes—including protozoa, algae, fungi and Legionella—to cultivate and multiply. Reducing fine particles also removes the nutrient source for bacterial growth.
The best CMF systems on the market can achieve filtration flow rates five times greater than those of traditional media filters. These high rates can be achieved because CMF systems require less than 5% of the cooling-tower water to flow into the filtration media at a time. Traditional technologies may require as much as 30% of the cooling tower flow. The shallower bed of CMF systems also requires 50% less water for backwash than traditional deep-bed systems. Also, with reduced space requirements, CMF systems are ideal for existing facilities with a limited capacity for expansion.
A better understanding of water filtration systems will help specifiers and owners reduce the quantity of toxic chemicals discharged into the environment, increase energy and water efficiency, and limit the growth opportunity for pathogens.