With drought and water shortages looming large over our communities, and the resulting increased focus on water conservation, industrial water users are looking for ways to reduce their water usage. Industrial cooling applications represent one of the largest water uses in the nation, so focusing on reductions in this area can have a significant impact on an area’s water usage. Fortunately, reductions can be the result of some simple changes. Incorporating filtration systems on recirculating cooling towers is an effective treatment tool to ensure efficient cooling tower operation.
Particulate Buildup and Effects
The water source for cooling tower feed and makeup is the first point of introduction for particles into the system. Surface water sources contain both organic and inorganic particulate. Most intake systems only include coarse straining, which does not remove the particulate that causes issues within the cooling system. If recycled wastewater (municipal or internal) is used, this also may contain significant particulate, depending upon the treatment level before use. As systems feed water to open cooling towers, there will inevitably be introduction of organic and inorganic particles as the ambient air is passed through the cooling water. The ASHRAE Handbook published by the American Society of Heating, Refrigeration and Air-Conditioning Engineers reports that, “A typical 200-ton cooling tower, in a season, may assimilate upwards of 600 pounds of particulate matter.”
This introduced particulate, in addition to system-born particles such as rust, pipe scale, biological growth and mineral deposits, causes issues for all of the typical cooling system equipment, including heat exchanger and condenser fouling, plugging of spray nozzles, sediment accumulation in piping and valves, and abrasion of pumps and seals. The particulate also provides additional surface area in the system for the proliferation of pathogens such as Legionella bacteria. Additionally, increased concentrations of particulate in the water will negatively affect the chemical treatment program. Suspended particles may react with corrosion inhibitors, polymers and anti-scalents, which will result in increased chemical demand. Buildup of particles on the system equipment prevents corrosion inhibitors from reaching their targets, resulting in increased corrosion and scaling. It also increases the potential for bio-corrosion directly from the particulate layer.
And, let us not forget the energy-water nexus of cooling tower operation. A layer of fouling or scale measuring 1/1000-inch thick can reduce heat transfer and increase energy costs by as much as 10 percent. By filtering the water to remove potentially fouling particles, the filtration system is impacting energy costs as well. Those reduced energy costs benefit the end user by providing additional cost savings and by decreasing water usage (and any related additional energy production).
The Right Filtration Degree for Industrial Cooling Water
An often-confusing question is how to best reduce the particulate loading in the cooling system while also considering the most effective and efficient filtration system addition. The first step is to determine the ideal filtration level — also known as the “filtration degree,” which is typically indicated as micron rating — to efficiently and effectively reduce the particles in the system. When looking at a typical cooling tower water analysis of particles sizes, the most common information provided is particle count. Particle counts will show that there are high counts of very small solids (less than 20 micron) with decreasing counts of larger solids. A simple review of this data will indicate that very fine filtration is necessary in order to impact water quality.
However, simple particle counts do not take into account the actual volume that these particles take up when saturated — especially when dealing with organic solids. In the majority of cases, the focus of cooling water filtration should be on reducing the aggregate particulate volume in the cooling tower because it is the voluminous solids that are responsible for the vast majority of cooling tower issues. These include heat exchanger fouling, nozzle plugging and bio-growth. In these cases, using a water analysis that calculates particle size by volume will generally show that the high counts of very small solids are not significantly contributing to particulate volume. It also generally will show that the solids in the 40 to 100 micron range (which have lower counts) are the main contributor to particulate volume. This particle range should be the focus of removal except in some more specific cases (ultra-pure facilities, for example) where the focus may continue to be on reduction of total particle counts. (In those limited applications, finer filtration may be appropriate.)
After evaluating the particle loading in a cooling tower system and reviewing the particle volumes or counts, it is important to next consider what the protection goal is with the treatment system. A combined evaluation of both the best reduction of particulate and a focus on the specific technologies requiring protection will help to establish the best filtration degree to provide comprehensive system protection.
With water conservation as a focus for most new installations, a cost/benefit analysis should be conducted on the treatment system options. As the filtration degree of the filter system is reduced, more filtration area will be required. This will require a larger system footprint and will increase the wastewater produced by the filter system. If it is determined that particle count is the focus of removal and very fine filtration is required (typically around 5 micron), media filters will be required. These will have a significantly larger footprint and wastewater volume than the variety of options available if particulate volume is the main focus of removal (40 to 100 micron).
Filter Technologies for Industrial Cooling Water
Various filter technology options can be successfully added or incorporated into cooling tower systems to improve efficiency. Determining which solution is right for each system requires an analysis of the pros and cons of each type of technology and an evaluation of each cooling tower’s specific requirements.
Bag or cartridge filters will provide effective filtration and are offered in a range of micron sizes to suit each system’s needs. One drawback is that prompt maintenance is required to replace the filters when they reach clogging capacity. This results in higher labor costs than a self-cleaning filter option. Bag and cartridge filters typically are not changed until they reach a high differential pressure (as high as 30 psi), so there are considerable energy costs associated with their use. Used bags or cartridges must be disposed of, adding additional operational costs to the system.
Hydrocyclones or centrifugal filters have been used in many cooling tower systems to remove heavy particulate from the water stream. These generally have a small footprint and are easy to add to existing systems. While effective at removing solids with a high specific gravity, these filers are not effective at removing lighter particles and organics that can be a main culprit in nozzle plugging and system fouling. If a system has a high concentration of lighter particles or organics, this is not a suitable solution.
Sand media filters are effective at removing small particles. It is a well-known and simple-to-use technology. In many cases, however, reaching the fine filtration level provided by media filters is not necessary to remove a substantial volume of particulate accumulating in the tower. Sand media filters have a large system footprint, interrupt the flow during backwash (or require an N+1 design), and require a high volume of clean water for backwash. This can reduce the water efficiency of the tower and increase system wastewater volume. These systems should be considered when particle count reduction is the filtration goal.
Self-cleaning screen or disc filters are an effective option for filtration in the 40 to 100 micron range when particulate volume is the focus of removal. The screen or disc element ensures removal of all solids (organic or inorganic) larger than the micron rating of the screen or disc. Many technology options are available, so system footprint and wastewater volume can be minimized. Energy requirements are low because the self-cleaning operation is based on a pressure differential and only operates when needed. In most technologies, filtration is not interrupted during the self-cleaning cycle.
Filtering for Real Cost Savings
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Finding the right cooling tower filter solution requires analysis of the particulate composition in the tower to allow for specification of the appropriate filtration degree. This leads to selection of the best filtration technology to effectively and efficiently provide particle removal for each specific system. But, reducing your environmental footprint is not the only benefit of introducing filtration to your cooling tower system. Reductions translate into real cost savings by reducing energy usage, wastewater production, makeup water, pumping, heating/cooling and by reducing chemical costs related to both non-target chemical reactions and loss in blowdown water. Recirculating cooling tower systems with appropriate filtration treatments offer a simple and smart solution to improve industrial environmental footprint while offering tangible cost savings. Incorporating a filter solution into cooling tower systems is a project all industrial users should seriously consider.