Side Stream Filtration
5 options and their advantages/disadvantages
Side stream filtration is an important but often overlooked component of an effective cooling water treatment program. Filtration reduces suspended solids in a system by continuously filtering a percentage of cooling water and returning it to the system. This leads to less fouling and biological growth, reduces the amount of scale and corrosion in the system, and improves efficiency. As with anything, there are many options to consider when selecting a cooling tower filtration system. Below are some of the most common technologies available and some the advantages and disadvantages of each. As with selecting any new system we recommend talking to your water treatment partner and, in the case of filtration, run a particle analysis so the proper filter is selected.
Common Filtration Technologies:
Media filters work well for side stream filtration in cooling towers and closed loop systems. A typical system resembles a water softener - it includes a fiberglass mineral tank, control valve and media. Different combinations media can be used depending on the application, often getting as low as 10 micron filtration. A multi-media system offering 3-5 different medias in one tank can allow for a wider range of capture between larger and smaller particles before exhausting the bed. Single media filters such as Filter-AG and Birm are also common for iron and particulate removal.
Media Filters are automatic backwashing by either a set timer or difference between inlet and outlet pressures. The system will go through a backwash to lift the media and send the solids collected to drain. When backwash is complete a rinse of the bed occurs and packs the media pack into place. Since the backwash flow rate is sized up to 5 gpm per square foot of surface area, as a result backwash flow rate can be high. It is recommended to backwash with a clean separate source of water and not the water you are treating.
Cartridge filtration consists of a filter housing and cartridge filter. Water enters the filter housing on the outside of the cartridge filter and passes through the filter media before being discharged from the filter housing. Any particles larger than the micron rating of the filter will plug the holes of the filter media. Once enough of the holes are plugged, there will be a resulting pressure difference of the incoming water and outgoing water. This differential pressure will determine when the filter should be replaced.
Cartridge filters have a relatively low capital cost. However, the cost of replacing the cartridges and the manual labor to install them should be considered when selecting these types of filters. If filters are not changed when plugged then your system will not be filtered.
Turbo Disc Filters
Miller Leaman Turbo Disc filters are fully automatic filters that utilize a stack of polypropylene discs that serve as the filtration media. Under normal operation the discs are compressed together. When the differential pressure exceeds the system setting, the system uses compressed air and a small amount of water to back wash the filter. During backwash the disc stack expands and water is directed through small openings that cause the discs to spin. The combination of the water flow and spinning action cleans the particulate from the filter.
The benefits of the Turbo Disc filters are that they have a relatively small footprint compared to similarly sized multimedia filters. The pods are relatively small in size so there is less surface area where biological growth can occur. Since the backwash uses a combination of both hydraulic and mechanical cleaning there is much less water used during backwash (10-12 gallons per backwash). Replacement of the filter media is a simple process when compared to replacing media in a multi-media sand filter. The main downside of the Turbo Disc is that it can only filter down to 50 microns.
Centrifugal separators rely on high velocity centrifugal force to spin particle contaminants out of the entering water. Benefits of centrifugal separators are that they have no screens, moving parts or filter media, making them a low maintenance option for filtration. A drawback of centrifugal separators is that they can only filter down to 40 microns. Often times these can be used in conjunction with cartridge filters. The centrifugal separator will reduce the load on the cartridge filters, while the cartridge filter will remove the small particles that the centrifugal separator cannot.
Mesh & Weave-wire Filters
These types of filters utilize wire screens inside of a housing to remove particulate. These filters are engineered to be fully automatic, self-cleaning, and operate on can water pressure alone, eliminating the need to remove the screens for manual cleaning. Dirt particles are collected on a screen as water passes through the filter. The dirt collected on the screen causes the water pressure to drop, triggering the cleaning cycle. The Vacuum Screen Cleaner suctions the dirt from the screen and flushes it out the drain. The backwash cycle is accomplished in seconds withoutinterrupting the main flow, providing continuous filtration. This leads to lower backwash flow requirements. These types of filters also have very small foot prints. That means that they can fit into tight spaces and it also has the potential make full-flow filtration possible. In some cases these filter have difficulty removing organic solids which can necessitate manual removal and cleaning of the filter.
Proper Filtration Improves Efficiency and Prolongs Equipment Life
Side stream filtration improves water quality, efficiency, and prolongs equipment life. If you need help assessing which technology is the best fit for your system or would like to have a particle analysis run on your cooling water, reach out to Watertech at 414-425-3339