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     The need for filtration of any cooling tower system is somewhat understood and accepted by most building professionals and plant operating engineers. That need is finally driven home and acted upon, however, only after the discovery of a serious corrosion and pipe deposit problem. With all the documented benefits, the reality is that few building managers or process plant operators will pre-empt a condenser water problem with the installation of a capital expense water filter.

     Various changes in pipe quality, chemical water treatment effectiveness, engineering design, and greater system demands, have together created operating problems almost unheard of two decades ago. In addition to an effective water treatment and corrosion monitoring program, the addition of a filtration system is often the key element to maintaining a smooth and trouble free condenser or process water service. Review a summary of piping quality, operating, and design changes which have occurred.

     Choosing the correct type and size filter then becomes the central issue, and often follows the personal preference of existing decision makers. This is generally in contrast to the logical consideration of available facts and needs in addressing the corrosion problem.


     The first step in any filter selection process must be to analyze the type, severity, origin, and extent of the problem. Even under relatively low corrosion rate conditions, substantial deposits are inevitably created. As the below photo exhibit shows from many of the ultrasonic piping investigations CVI has been involved, some level of deposits can be expected given sufficient time and lack of proper precautions. See Technical Bulletin # C-1 for the actual pounds of metal lost and deposits created at various corrosion rates.

     An ultrasonic test report showing a mild and slightly elevated 3 mils per year (MPY) corrosion rate would certainly not equal the importance of a system identified with a 15 MPY high pitting condition caused by under deposit corrosion or MIC. Cutting out one or more samples of the pipe for metallurgical testing is extremely valuable in identifying the deposit problem and actual corrosion mechanism. A particle size distribution study will show the range of the particles involved, though it is the least important of the three investigative options.


     Selecting an appropriately sized filter in relation to the deposits is extremely important. Too small a unit or a high corrosion rate environment will mean that its removal rate will never even meet the rate of new corrosion products created, much less fulfill its intended purpose of cleaning up a fouled system. Too large a unit may mean a wasted expenditure, or worse - pushing a critically needed unit beyond budgetary means and onto the back burner.

     While difficult to estimate prior to installation, measuring the weight of foreign debris removed per backwash cycle or captured within each filter bag, and comparing it against an estimate of corrosion product created per year, will provide some rough approximation of the time it will take to clean the system. With thousands of pounds of iron oxide easily attached to the walls of any older and larger piping system, and far less than one pound of material typically removed per filter cleaning cycle, it is likely to take many months and perhaps years before producing a noticeable improvement.

     Even though a corrosion condition should be addressed relevant to its severity, it usually is not, and budget figures typically predetermine future success in correcting the problem. Often, the economic consequences of a severe corrosion problem may not be fully realized, or the thought of only partially addressing the problem deemed appropriate.

     Of course, the fundamental objective of any filtration system should be to remove the internal deposits which have accumulated, rather than just clean the water itself. Significant difference exists between both objectives - although it may not always be viewed as such. See Technical Bulletin M-10 regarding this important distinction.


     With that in mind, the first major consideration is whether to install full flow or side stream filtration. Without debate, any major corrosion problem is best corrected by full flow filtration. A deposit composition analysis showing significant outside airborne particulates would also support the need for full flow filtration, as would an extremely critical facility or process piping system serving critical equipment.

     Full flow or high capacity filtration units will capture particulates they are designed for on their first pass, whereas side stream units can capture only a 10% or lesser volume. With full flow filtration, the water has no alternative but to enter the filtering device, whereas with a side stream filter, only a small percentage of that water is captured.

     For side stream units, therefore, installation design becomes critical. A poor installation and piping layout, and specifically the wrong take-off point from the main line to the filter inlet, can make any side stream filtration unit almost useless. Conversely, the well designed installation of a side stream bag filtration unit can often offer outstanding results. See Technical Bulletin W-3 regarding some filter installation suggestions.


     Small and light enough particulates missed by the side stream filter may be captured on a subsequent pass, although heavy particulates of iron oxide and scale are more likely to settle out quickly and remain. Chemical additives do provide significant assistance in loosening and maintaining the lighter particulates in suspension, but are often limited by system size, flow velocity, pipe orientation, and the concentration and strength of the chemicals themselves. In many cases, the existing vulnerability of the system to leaks or failure will prohibit the use of the stronger more effective chemical agents - and therefore limit cleanup effectiveness.

    Full flow filtration unit must be able to handle the full volume of water obviously, and due to the high particulate load which should be expected, must automatically backwash. This fact eliminates any serious consideration of bag filters or sand filters as an option. Due to the typically high flow rates involved, which can well exceed 2,000 GPM for any larger installation, full flow filtration will rarely capture particulates down past 50 micron, which can be a disadvantage in some applications.

     CorrView International generally recommends full flow filtration for condenser or open water applications, followed by installing the largest side stream bag or sand filtering unit possible. In certain applications, a secondary by-pass bag filter to polish the water to a low 10 micron rating may be a worthwhile option to consider. While full flow filtration can be very expensive, and often rests in the $50,000 to $100,000 range, such cost must be considered in respect to the greater expense of replacing an entire piping system.

     Many different water filtration manufacturers exist offering a wide variety of products. Most fit into the following general categories:


     Each filtering system offers some benefit over the others in terms of cost, efficiency, particle retention, size, installation, ease of operation, reliability, and maintenance, etc. Once the extent and severity of a corrosion problem is identified, and the need for corrective measures understood, the choice of water filtration product narrows. We offer below the pros and cons of different filtering types based upon our experience with such units, and the success we have seen at their correcting various corrosion problems.


 Bag filters offer a good overall filtration option and are simple in design and operation. Large low rates can be accommodated with individual multiplex units, or by installing multiple smaller units along a header. They are commonly installed across the suction and discharge headers of the pumps, although this option rarely offers the highest effectiveness. Supplemental pumps are sometimes supplied.

Particle retention is determined by the micron rating of the bag element itself and not the filter housing - which makes it extremely adaptable to different deposit problems. Larger 100 micron bags can be initially used to clean the heaviest particulate loading, and then reduced to perhaps 25 micron as the problem condition is controlled. Bag elements provide an immediate indication of effectiveness of both the filter and the chemical treatment program. Magnetic inserts can be installed to help capture iron based particulates.

Bag filters are easy to install and require no power or backwash connections. Their lower installation cost, however, is offset by the need to change replacement bags - thereby requiring maintenance on a regular basis. The dirtier the piping system, the more frequent the bag changes. Since bag filters are usually intended for side stream use, their location and piping into the system is critical to achieving good results.



Separators are suitable for moderate to large size particulate removal, with their effectiveness is greatly dependant upon installation. High efficiency particle removal below 75 micron should not be expected. For many open condenser water distribution studies showing 40-50 micron average size for suspended particulates, a centrifugal separator will remove approximately 40-50% - still a significant benefit. Any particulates which can easily settle out of solution will be removed by centrifugal separation, and they are ideally suited for removing the chips of rust and scale typically found in older systems at the strainers and cooling tower pan.

These units are not truly filters, since no restricting element or media is used. Rather, it is the centrifugal spinning action of the water that forces the particulates, depending upon their mass, outward and then into a central collection point. For that exact reason, high water velocity is critical, and their placement at the bottom of the downfeed supply piping is often the key to a successful installation.

Centrifugal separators have no moving parts, no real energy requirement, and need little maintenance. They automatically backwash usually based upon a timer, and because of the generally high volume of water and large particulates captured in any fouled system, should be backwashed into a large settling tank rather than to a drain. A settling tank also provides in indication of filter and chemical treatment effectiveness.

While some separator manufacturers have claimed a low particle retention down to the 25 micron range and below, we have not seen such success in actual use. Centrifugal separators offer a good option primarily because they provide full flow filtration at some reasonable degree of particle retention, and provide such benefit in a low cost, low maintenance package. Supplemental filtration is often needed, however, to remove the smaller particulates.

An economical and highly effective hybrid filter can be design by installing a full flow centrifugal separator into the condenser water line, and then continuously drawing the higher particulate containing water from its discharge line through a large basket strainer. This eliminates the need for a settling tank, eliminates blowdown loss, plus other benefits.



Sand filters have the advantage of immediate recognition from their widespread service in the pool industry. They provide the lowest particle retention, often at less than one micron. Such fine particulate capture, however, is generally not necessary, and filtering a higher volume of water at 10-20 micron may product better end results. A supplemental circulating pump is always required. Maintenance is generally dependant upon manufacturer, the type of filtering media, and dirt loading.

Flow rates are relatively low, however, and generally do not handle the volume necessary to clean a heavily fouled system beyond maintaining the water itself clean. In that respect, however, sand filters can provide an effective solution to dirty or turbid water. See Technical Bulletin M-10 regarding clean water vs. clean pipe.

Sand filters are only suitable for side stream use, and therefore like other side stream units - installation and piping arrangement becomes critically important. Custom piped installations, pulling water from specific high dirt areas such as the cooling tower pans, have been shown to be very effective at reducing a rust deposit problem. Customized headers, designed to wash bottom pan sediment to the inlet point of the filter, are common additions at many cooling tower pans, and can greatly increase filter efficiency.

In heavily fouled systems, backwashing frequency and water use will be high. Replacement of the sand media is occasionally required depending upon operating conditions. And the always present possibility of an escape of sand or diatomaceous earth media into the piping system can create an entirely new set of operating problems.

While likely the most popular of all condenser water filtering devices, sand filters generally address too small a flow of water at an unnecessarily fine particulate retention. Higher flow at less particle retention, once again, is recommended.

Various manufacturers offer different media, pre-filtration via centrifugal separation, and a wide variation of designs. In most cases, sand filtering equipment is promoted based upon the lowest particle size captured - when overall deposit removal effectiveness should be the primary concern.

With drinking water rarely filtered to below 20 microns, the interest to clean an open condenser water system to a degree 30 times greater is hardly necessary.



A wide range of full flow filter configurations exist with each having specific advantages. All benefit the piping system by providing the greatest protection possible, and offer the fastest clean-up of a deposit laden condenser water system.

Automatic backwash filters vary greatly in complexity, and may therefore create special maintenance problems. Fixed element screen or slotted filters tend to gradually clog and therefore backwash with less and less efficiency over time. At some point, their removal and manual cleaning will be necessary - which can become a large maintenance item depending upon the actual unit.

Rotating arm type filters offer many advantages and can provide full flow filtering at moderate cost. Such units are generally complex in design, and require factory servicing. Their smaller size offers significant advantages, although multiple units may be required to fulfill a flow rate demand.

Due to the large size of most drum type full flow filters, floor space is often a concern, and a major piping revision is always required involving shutdown and welding. Like all other filtering devices, it is a general rule that the smaller particles retention or flow rate required, the larger and more expensive the unit.

Depending upon the model chosen and degree of the deposit problem, backwashing volume may be high, and therefore we always recommend installing a settling tank. Moving a deposit problem from the piping to the drain and sewer ejection system is clearly something to avoid. Secondary filtration of the effluent through a small bag filter housing for return to the system is a viable option should low cycles of concentration result.

CVI has often made the point that any water filter not clogging the drain line with deposits is, by definition, not operating effectively.

A unique full flow automatic backwashing unit based upon an expanding filtering element offers tremendous benefits in terms of efficiency, low backwash rate, reliability, and low maintenance. We consider it to be the best automatic backwashing filter available. See Technical Bulletin W-2 for further information on the Cross filter.


     A carefully planned monitoring program is extremely important to any filtration effort in order to judge results and make any needed modifications. This is best achieved through the use of removable spool pieces - which enable a periodic view into the pipe to judge clean-up effectiveness. See Technical Bulletin C-11 about the information available from spool piece monitoring.


     Overall, supplemental filtration is becoming almost mandatory if reliable and trouble free cooling or process water is desired. A low corrosion rate is often dependent upon effective filtration, and without it, problems are almost inevitable. A filter installation as part of a new cooling plant may prevent problems from beginning, whereas installation after a problem has established itself may be too late to affect any real benefit.

     Critically important to any filtration selection is the water treatment program itself. Poor water treatment protection is usually the fundamental source of a corrosion and deposit problem, and no filtration system should ever be viewed as an alternative or fix to well maintained corrosion control.


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