Considerations for Automatic Filtration of Cooling Tower Water Systems

Waste reduction and process efficiencies are key for companies wanting smoother operations. As various industries, including power generation, seek ways to improve operational efficiency, cooling water treatment practices also have evolved to meet the challenge.

Cooling towers remove waste heat, and cool circulating water, for systems at power stations (Figure 1), chemical plants, and oil refineries. With sometimes tens of thousands of gallons of water per minute flowing through cooling towers continuously, filtration plays a critical role in keeping systems clean, efficient, and operational.

1. Automatic filtration solutions support continuous improvement programs for industrial companies of all types, including power generation stations. Courtesy: Eaton/Getty Images

Cooling towers constantly cascade water to scrub airborne contaminants from the atmosphere and carry them into the cooling system. These systems can also be contaminated by water ingress, or by entrained solids resulting from corrosion processes. Many cooling water towers also incorporate a chemical treatment regime to combat microbiological growth and contamination.

Deposits and other damaging particles can reduce the efficiency of cooling system components such as the heat exchanger and pipeline equipment. Cooling water—and other working fluids commonly used in coolant streams—must be kept clean to enable effective process heat removal. Fouling of the heat exchanger surface by scale or contamination reduces energy efficiency and can adversely impact the performance and reliability of other cooling system assets.

Industrial filtration helps to ensure continuous flow and worry-free operation for cooling water systems. The market offers many types of manual and automatic solutions with automatic filtration, often based on backwashing and mechanical cleaning technologies. Automatic filtration has advantages over manual filtration methods, which can be labor-intensive in their application. A first step in determining the filtration need is to consider the following questions:

■ How much dirt or particulate is being collected? If the filtration method in place requires only occasional maintenance—if cleaning and changeover occur every few weeks—an automatic system may not be necessary (in general, an upstream strainer is the best choice for noncritical applications where 50-micron, or smaller, particles will not adversely impact the process or equipment). Conversely, if the filtering element loads up quickly or requires frequent manual cleaning, an automatic system should take priority.

■ What degree of filtration is needed? A particle size distribution, or PSD, analysis can determine the size and range of particles representative of a given cooling water system. Determining the particle size range makes it easier to select a filtration solution for optimal performance and reliability.

■ What is the flowrate? The filtration solution will need to handle the maximum flowrate to produce the desired effect. Filtration rates are commonly expressed in gallons per minute per square foot of filter area (gpm/ft 2). Also, will pressure and flow be steady or variable?

■ How much loss is tolerable? Backwashing cleans filters to remove accumulated contaminants. Some filtration solutions require a considerably greater backwash flow, causing increased water consumption and potential media loss. Mechanically cleaned filters typically do not need backflushing and will generate far less water for disposal.

■ How much space will the system occupy? Industrial filtration systems vary greatly in size and profile depending on the filtration requirement and flowrate. It is wise to consult a partner with experience in designing and installing customized filtration solutions.

■ How much effort will be required? Whether the solution for your system is a high-performance filter, a durable maintenance-free strainer, or both, investing time to explore the full range of options is worth it, as the right industrial filtration solution can pay dividends for years.

■ What is the return on investment? The right filtration solution can significantly reduce operating expenses and labor costs while helping companies abide by regulations and reach their goals for responsible water consumption and disposal.

Often the approach is to adopt a multifilter system with enough extra capacity to handle the process stream while one or more filters are taken offline for cleaning or replacement. An alternative approach is a self-cleaning automatic filter that does not require downtime for maintenance.

Advantages to automatic filtration include reducing water loss and energy use, combating particle fouling and corrosion, and increasing process uptime. It also reduces maintenance demand and extends the life of cooling system components, and aids in achieving industry/environmental compliance measures.

Industrial strainers and filters perform essentially the same function, but filters can remove particles of much smaller size. The general rule is: "If you can't see it, you can't strain it." This means that particles down to 0.004 inches (0.1 millimeter)—or in other terms, 100 microns, or 150 mesh—can be removed effectively with an upstream strainer. For anything smaller, a filter will be necessary.

Regardless of whether a strainer or filter is used, built-up particles in the filtering element must be periodically removed. Stopping the flow and manually cleaning the element means the process must be shut down or the strainer/filter bypassed during maintenance. As neither scenario is optimal, this often begins the search for a self-cleaning filter. There are two standard automatic filtration designs: automatic backwashing and mechanically cleaned filters.

Automatic backwashing systems accomplish cleaning through an integral backwash function that provides an uninterrupted flow. These commonly use a rotating hollow internal arm to collect debris deposited on the filter media. As trapped particles build up, the pressure drop through the filter increases until it reaches a predetermined value at which point a valve is opened, allowing fluid and accumulated debris to exit through the rotating arm.

An upstream strainer can also be a valuable addition to any cooling water tower filtration system. Coarse upstream filtration provides protection for pumps and for more delicate downstream filters by removing large unwanted solids from the cooling stream. Often, this solution will pay for itself in terms of extended filter element life, less downtime, and reduced maintenance need.

Automatic backwashing filters are also self-correcting during and after upsets. During upset conditions, the filter will start to backwash continuously. Once the process returns to normal, backwash interval times will also normalize. Cleaning frequency can be based on time, differential pressure, manual selection, or other application-specific criteria.

Typically, this type of filter is used to remove particles larger than 50 microns and can handle debris loads of about 200 parts per million (ppm). Automatic filters are best used in high-volume situations where fluid losses up to 5% of total flow during cleaning are acceptable.

The second automatic filtration design option uses a mechanical cleaning disc to scrape accumulated debris off the filtering media (Figure 2). Typically, a pre-timed cleaning cycle helps to ensure a stable run of flow. Additionally, should trapped debris increase the pressure drop across the filter (thus creating an upset condition) the scraper can be actuated at a predetermined value (note that a reliable differential pressure override function is critical to any well-functioning filtration system). Debris is then deposited at the bottom of the filter housing where it can be removed without interrupting the flow through the filter.

2. In this automatic backwashing filtration example, incoming flow passes through the inner cylinder. Particles collect on the inside surface of the screen chamber while filtered liquids flow down and out of the chamber. The backwashing phase is set to activate on a timed cycle with a differential pressure override. Courtesy: Eaton

This cleaning action makes mechanical cleaning filters (Figure 3) suitable for removing particles larger than 25 microns in diameter. Some of these filters are able to handle higher debris loads and more frequent purge cycles than conventional automatic filters. Most also use only a small amount of filtered liquid to carry away and dispose of the debris.

3. In this mechanical cleaning filtration example, a cleaning disc is used to remove contaminants from the filter element. Compressed air at the top and bottom of the center column is used to drive a magnet block within the tube to which the cleaning disc is magnetically coupled. As the magnet moves, the disc is drawn along in close contact with the filter media. This scraping action supports a thorough cleaning cycle as debris is collected at the bottom of the unit. The low volume discharged versus the volume of fluid treated also helps reduce product waste. Courtesy: Eaton

Note that either a strainer or a filter creates a pressure drop and also a flow restriction. Both factors must be accounted for when designing the system. Adding either as an afterthought may require upsizing pumps to maintain adequate flow volume and pressure.

Where even finer filtration is required, bag or cartridge filters can be considered down the line. Bag or cartridge filters use disposable media, which allows for finer retentions and particulate removal. A choice needs to be made between a simplex system, where the filtration process is interrupted when exchanging the filter, or to avoid this by opting for a duplex or run-and-standby system. This multiple filter housing configuration can allow continuous filtration allowing one side to be cleaned while the other is being maintained. Bag or cartridge systems will require regular maintenance and system designers must also consider the cost of the consumables’ purchase and disposal.

Choosing the optimal industrial filtration solution is rarely a simple process. There are many factors and approaches to consider, and even facilities with similar functions can have significantly different design requirements including the need for chemical or biological treatment. The prudent path is to discuss process requirements with a knowledgeable expert or solutions provider and to involve them early in the design stages, which helps to spot challenges and opportunities that may not be obvious to the untrained eye.

Automatic filtration can substantially reduce entrained solids and potential deposits, improving cooling water system operating efficiency. Installing an automatic self-cleaning filter in a cooling water system, for example, can protect heat exchangers, pumps, valves and spray nozzles, while continuously removing particulates and providing continuous flow, even while the system is being backwashed.

—Ulrich Latz is global product manager, Industrial Filtration, for Eaton Technologies GmbH in Nettersheim, Germany. Wim Callaert is senior product manager for Eaton Technologies GmbH in Sint-Niklaas, Belgium. Learn more at eaton.com/filtration.

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