An increasing number of cooling tower users are utilizing a filtration system to control particulate levels and maintain cleanliness of the cooling tower and overall water circulation system.
The cascading water in a cooling tower continuously "scrubs" airborne contaminants from the atmosphere. These contaminants, in conjunction with particulate in makeup water, find their way into the cooling tower sump and ultimately can flow downstream into the system, where they accumulate in heat exchangers, condenser tubes and other critical water-cooled process equipment.
In conjunction with corrosion, particles contribute to the following problems:
Reduced operating efficiency.
Increased downtime for cleaning and repair.
Increased cost of water treatment.
Shortened equipment lifespan.
A cooling tower filtration system does not replace the need for water treatment, or vice versa. Chemicals cannot remove dirt just as filters cannot control water chemistry. A filtration system will complement the water treatment program. If particulate is not removed from the system, it provides an excellent breeding ground for algae and bacteria. Many water treatment methods, including traditional chemical treatment and ozone, can be used in your process equipment.
Full-Flow vs. Side-Stream Filtration Approach
Two basic approaches are used to filter your cooling tower water: full-flow and side-stream. Full-flow filtration continuously strains the entire system flow. For example, in a 100-ton system, the flow rate would be roughly 300 gal/min. A filter would be selected to accommodate the entire 300 gal/min flow rate. In this case, the filter typically is installed after the cooling tower on the discharge side of the pump. While this is the preferred method of filtration, for higher flow systems, it may be cost prohibitive.
Side-stream filtration, although popular, does not provide complete protection, but it can be effective. With side-stream filtration, a portion of the water is filtered continuously. This method works on the principle that continuous particle removal will keep the system clean. Manufacturers typically package side-stream filters on a skid, complete with a pump and controls. For high flow systems, this method is cost-effective.
Properly sizing a side-stream filtration system is critical to obtain satisfactory filter performance. There is some debate over how to properly size the side-stream system. Many engineers size the system to continuously filter the cooling tower basin water at a rate equivalent to 10% of the total circulation flow rate. For example, if the total flow of a system is 1,200 gal/min (a 400-ton system), a 120 gal/min side-stream system is specified.
A more accurate approach is to calculate the system's total water volume and filter it once per hour. A six-step calculation can be used.
A vast array of full-flow and side-stream cooling tower filtration technologies are available, including strainers (screen filters), centrifugal separators, disc filters, sand media filters, automatic screen filters, bag filters and diatomaceous earth (DE) filters. Two of the most common full-flow filters are strainers and centrifugal separators.
Also referred to as screen filters, strainers commonly are installed as full-flow filtering devices. They are available in different degrees of automation and range of screen micron (mesh) options. Typically, strainers are used to remove particulate sizes ranging from 50 to 1,000 micron.
One such strainer employs a cone-shaped screen element (figure 1). As water enters the strainer, heavier particulate such as sediment is accelerated downward into the debris reservoir at the base of the strainer. The particulate then is flushed from the strainer reservoir via the debris flush port. Strainers should be installed with bypass piping in case the screen element must be removed for cleaning. If water quality is expected to be poor, strainers should be oversized to provide greater screen surface area and reduce maintenance frequency.
Like strainers, centrifugal separators commonly are used as full-flow filtering devices. These devices employ a centrifugal spinning action to separate particulate from the cooling tower water (figure 2). Because separators work on the principles of velocity and gravity, it is critical that they are sized properly. When they are, separators are effective in dealing with heavy particulate. Because of the turbulent spinning action that occurs inside a centrifugal separator, they tend to operate with a substantial pressure loss (5 to 10 psi).
By definition, cooling towers pull in airborne contaminants -- sometimes called floaters -- that are fairly light. Separators are less effective at removing these lighter contaminants because floaters do not have a high specific gravity relative to water. They tend to pass through a separator and flow downstream. Depending on water quality, it may be a good idea to add a strainer on the downstream side of the separator.
Two of the most common side-stream filtration alternatives are disc filters and sand filters. Both tend to be fully automatic and self-cleaning, and side-stream filter systems commonly are packaged with a pump and controls. The water is pumped from the cooling tower sump, through the side-stream filtration system, and returned back to the cooling tower sump.
Side-stream disc filtration systems are relatively new to process cooling water applications. The media comprises injection-molded (polypropylene) plastic discs that form a three-dimensional filter media. As cooling water passes through the depth of the discs, the discs capture particulate and fibrous contaminants. After passing through the disc media, the filtered water is returned to the cooling tower sump. An on-board backwash controller monitors the differential pressure across the filter system. As the discs get dirty, an automatic backflushing cycle is initiated, sequentially backflushing each disc filter cartridge until the entire filter is clean. The dirty water is piped down the drain. When the discs are clean, the filter goes back into filtration mode. Disc filters are available in the 50 to 200 micron range and are becoming popular because they use a minimal amount of backflush water.
Side-stream sand filters commonly are used on cooling tower applications. Depending on the media, they can be used to remove particles as fine as 10 to 50 micron. With this setup, dirty water enters the top of the filter through the over-drain assembly and is distributed over the sand media bed (figure 3). The sand bed traps the particles and allows the filtered water to pass through the under-drain assembly and back to the cooling tower sump. As dirt accumulates, it causes a pressure differential across the filter. When the differential pressure reaches setpoint, flow through the sand bed is reversed, backwashing accumulated dirt out of the top of the filter and down the drain. After the media is cleaned, the filter goes back into normal filtration mode.
Although sand filters provide clean water, they use a high volume of backwash water, and over time, the sand media must be replaced. This can be a labor-intensive maintenance procedure.
Selecting a System
When selecting a cooling tower filtration system, a potential user should consider many factors. Manufacturers can provide an application questionnaire that will help you define your filtering needs and assist them in making recommendations. When approaching filter manufacturers, be prepared to answer the following questions:
Is a full-flow or side-stream system desired?
What is the system's flow rate?
What size particles are in the system? What are the characteristics of the particles (sand, algae, etc.)?
What is the budget for the system?
After installing your cooling tower filtration system, the system should be properly maintained so it can contribute to clean cooling tower water for many years.