Sidestream Filtration of Cooling Systems

Problems with corrosion, fouling, and poor heat transfer can be helped with properly designed filtration.
By Dan Lingen | February 09, 2010
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Cooling towers scrub large volumes of air and effectively remove solids consisting of dust, microbiological organisms and airborne debris resulting in suspended solids in the form of corrosion products, microbiological growths and wood fibers from the tower. If allowed to settle out, these solids can strain even the best treatment program. Under these circumstances, sidestream filters might bring significant improvements, benefiting the cooling system through reduced corrosion rates, increased equipment life, better system efficiency, reduced maintenance costs and better chemical control.

When to Consider Sidestream Filters
The installation of a sidestream filter is a capital expense which may be hard to justify in most plants. Consider the following as indications that a plant should consider installing a filter:
>The primary makeup is from an unclarified water source (river, sewage treatment, etc.) that is high in suspended solids and/or iron.
>The system is having a difficult biological problem even though a good biocide program is in effect.
>Heat exchangers are opening dirty even though a good anti-foulant program is being used.
>Excessive corrosion rates can be traced to fouling.
>Loss of heat transfer is attributed to deposition rather than corrosion.
>High levels of solids are building up in the sump.
>Heat exchangers require frequent mechanical cleanings.
By identifying potential threats to plant systems and solutions to those threats, one can determine the paybacks of installing a sidestream filtration system. Often the payback time is less than one would think and will vary widely from system to system. The final decision will always rest with plant management, and will always be decided on economic grounds.

Benefits of Using Sidestream Filters
Not all potential benefits from sidestream filters apply to all systems, and good judgment must be applied before making any claims. Expected benefits from sidestream filtration include:
>Since solids are removed from the system, the corrosion inhibitor will lay down its protective film on clean rather than dirty surfaces, thereby reducing corrosion rates and increasing equipment life.
>When used with good chemical treatment, the filter will keep the system much cleaner, and, as a result, the need for mechanical cleaning of exchangers and sumps is reduced.
>A cleaner system means better heat-transfer rates for longer periods of time.
>In some cases, the removal of suspended solids from the circulating water allows higher cycle.
>Large biological growths and dead organisms are removed with a sidestream filter. This reduces chlorine demand and makes nonoxidizing biocides more effective.

Filter Design
It is strongly recommended that a particle analysis be done on the representative stream that is to be treated when planning a sidestream treatment for a particular plant. For most sidestream filtration systems, a centrifugal-style filter is optimal, removing major problems and requiring little maintenance or repair. However, a number of filter designs are available, each with advantages and limitations.
Centrifugal filters typically remove a minimum particle size of 80 micron, but only when the particles are all 1.2 times the density of water or higher. These types of filters are more likely used in a mixed stream where 200-micron particles are anticipated.

Auto-backwashable screen systems can be customized to the desired particle size because the screen size is relatively absolute. Screens as small as 10-milicron are available. (A 10-milicron screen is also available but is not durable for most tower applications). If the particle size goes too low it can create a problem because the amount of water needed to backwash the screen can be excessive. Also, debris can stick to the screen, requiring manual cleaning.

Bag filters require labor in changing out the bags when full. However, bag filters offer the flexibility of adjusting the micron filtration provided when needed (for instance in spring cottonwood-seed time).
Cartridge filters are comparable to other filters in initial expense, however, the ongoing expense is much higher. Cartridge filters offer flexibility like bag filters, but with little or no bypass.

Sand filters are the most expensive of all the filters listed here but they also are proven effective and reliable. When set up to backwash on pressure differential, they can handle load variations with manpower.

Filter Media
The size of the filter medium is important, since it must stop suspended solids from passing through, hold solids loosely for easy removal during backwashing and be capable of holding a given quantity of solids without clogging.

A medium-size filter media is defined by two figures:
>Effective size: Particle size above which 90 percent of the medium is larger
>Uniformity coefficient: Effective size divided into the particle size above which 90 percent of the medium is larger.
In most cases, sand or anthracite is the filter medium used, and sometimes the two will be used together (mixed media). The filter beds contain different-size layers of the media. Fine sand or anthracite will be on top, with larger and coarser grades underneath. These will be supported on graded gravel or heavy anthracite.

Most filters operate at a filtration rate of 2 gallons to 3 gallons per minute per square foot of filter surface. Backwash rate is enough to expand the filter bed by 50 percent, usually 10 gallons to 20 gallons per minute per square foot for sand and 5 gallons to 10 gallons per minute per square foot for anthracite.

Filter media will usually be selected to remove those particles will usually be those with the greatest tendency to settle, be the least expensive option for a given water throughput and match the minimum required backwash rate.

Filter Sizing
Historically, filters were designed to handle approximately 2 percent of the circulating rate. This rule of thumb has proved inadequate for the design of sidestream filters in modern industrial plants, and a more sound engineering approach is now being used.

If the suspended solids entering the system are constant, and known, and the solids desired are also known, and these two conditions are in equilibrium states (little variation of either), the filter size can be determined with Equation 1.

Again, this requires the existence of a steady-state condition. When the contamination entering a system is erratic because of varying solids entering the system with the makeup (for instance, after a heavy rain), or with the air (for instance, wind shifts changing the dust level in the air), this equation cannot be used.

The second formula for the calculation of filter size assumes that the varying solids loading must be removed from the system in a period of time that is practical, yet short enough so that serious deposits will not form. In practice, a 95 percent reduction of a sudden solids load in one to three days will prevent excessive fouling and allow a filter size that is not prohibitively expensive. The formula used is shown in Equation 2.

The information on filter design and sizing is provided only as a guide. The specifics are better left to manufacturers of the filtration equipment.

Sidestream filtration is an effective tool for the control of deposition and fouling in a cooling water system, yet because of the capital investment required, its use should be carefully considered. It should not be recommended or used as a substitute for good engineering practices, good control or good programs, nor should it be used to overcome basic design flaws in the system. EP

Dan Lingen is product development manager at US Water Services. Reach him at dlingen@uswaterservices.com or 763-553-0379.