Randy Delenikos, LAKOS Separators & Filtration Systems
Evaporative cooling has largely withstood the test of time, both in terms of cost efficiency and overall performance. Various makes, models, styles and configurations are available, each offering specific advantages and benefits. Generally speaking, from a durability point of view, cooling towers fall into three categories. The first is packaged towers, which are made of metal construction and have a maximum life expectancy of about 15 years. The second is field-erected towers, which are made of wood and have a maximum life expectancy of about 15-20 years. Third are towers made of ceramic or cast-in-place concrete, which are more durable (and more expensive) and often are guaranteed to last 15 years. The reality of this life expectancy, however, shows that packaged towers commonly do not last beyond nine years and field-erected towers reach only about 10-15 years.
The issue is not so much a matter of construction as environment, operation and care. Incidents such as lightning strikes, hurricanes, fires and other significant events take a toll. Chemical imbalance is also a clearly traceable culprit. And unwanted contaminants pose a surprisingly high risk to cooling towers, defining the subject of this article and offering the potential to regain years in terms of life expectancy and performance.
To be sure, cooling towers act as effective air scrubbers, drawing dust, dirt, organic matter and airborne particulates into the tower environment. Make-up water also has its share of sand, silt and particle matter (even if it’s city water, which is often reservoir or well water) leaving it susceptible to at least some concentration of particle matter. Lastly, and most importantly, the process of evaporative cooling is its own worst enemy, creating a potentially significant amount of precipitated grit and scale by way of heating-cooling, acceleration-dormancy and the treatment of water.
The most obvious threat from such contaminants to a cooling water system is at the heat exchanger. These devices rely on small, controlled velocity pathways that can easily clog and foul with particle matter. And long before the painstaking shutdown and cleaning process comes excessive pass-through fluid velocity (beyond design parameters) and reduced heat exchanger efficiency.
 Figure 1. A separator installed on the flow from the cooling tower thus protecting the heat exchanger from fouling. |
No less problematic today is the effect of contaminants in the cooling tower itself. Particle-laden water that passes through a tower’s fill inevitably deposits unwanted particles into crevices and across surfaces, affecting both flow and efficiency. Water distribution headers and nozzles can become clogged, limiting proper flow and wetting design. Perhaps most problematic of all is the certainty that troublesome solids will find their way to the tower’s basin or remote sump, settling there to create still more problems.
Even with routine blowdown and proper chemical treatment, particle accumulation in a tower basin or sump is still a problem and a concern. Blowdown can be a problem, given its water loss and the need for both make-up water and renewed chemical treatment. Beyond that, however, is the increasingly apparent problem of bacteria and disease, created when solids are allowed to accumulate in a damp, dormant environment. And once the water is contaminated, the air soon follows, leading to potential respiratory problems such as tuberculosis, staph pneumonia and even Legionella.
Identifying Contaminants
You can expect several contaminants in your cooling tower water.
Inorganic solids - Sand, silt, dust and dirt can come from the water supply/make-up water and via the air intakes (cooling towers are a great air scrubber).
Organic solids - Leaves, grasses, cottonwood seeds, algae, pollen, insects and other floatable debris find their way into the tower environment.
Precipitated solids - As water evaporates, it leaves behind minerals, which become grit and scale.
In general, these contaminants appear as either settleable solids or as floatable solids. The distinction is important, given that settleable solids are largely the type that accumulate in a tower basin or remote sump, whereas both solids types can be a problem to the heat exchangers, tower distribution headers, nozzles and fill.
 Figure 2. A separator installed on the flow from the process back to the cooling tower, thus protecting the cooling tower from unwanted debris. |
Particle size is another issue. While a broad range of particle sizes may appear in a cooling tower environment, only those that can accumulate to troublesome levels should be of concern. To put it in perspective, 40 microns is about the smallest particle that can be seen by the human eye. Human hair is about 30-120 microns. Pollens range from 20-200 microns. Virus spores range from 0.1 to 0.01 micron. And most bacteria are graded at 5 microns or less. According to the Water Quality Association, drinking water standards range from 0.5 to 5 microns and only bacteria is a concern at that size, typically removed via disinfection, not filtration.
In essence, attempting to filter extremely fine particles may be more costly and less effective. Affordable protection makes more sense than absolute perfection.
Determining System Needs
Put simply, you need to identify what needs protection from the contaminants. This can include the heat exchangers, the cooling tower basin or remote sump and the tower fill and/or distribution headers/nozzles.
At the same time, assess the costs associated with the problem. Typical costs can include downtime, cleaning, repairs and/or replacements, the cost of outside services and overtime labor and maintenance. These factors will become important when comparing the cost of the problem with the cost of the solution.
In general, five approaches are widely accepted as the techniques for controlling solids in an evaporative cooling system. Each addresses the problem in a different way and each has its own value and benefit.
Full-stream filtration - This technique calls for installing the filter at the system supply pump’s discharge (from the tower basin or remote sump) prior to the heat exchangers/chillers. The filter is sized according to the full flow of the pump, filtering all the water that passes on to the heat exchangers/chillers.
The primary value of this approach is to protect the heat exchangers/chillers, which is estimated to increase the operating cycle of the heat exchanger by eight times before servicing requirements (based on experiences with users who have kept good “before and after” records). This technique does not directly address the problem of basin/remote sump solids accumulation. Although effective filtration can reduce overall solids concentration, the tower environment itself does attract and create unwanted solids that can settle in the basin and never pass on to the heat exchanger.
Side-stream filtration - The typical practice is to divert 10 to 20 percent of the full-stream flow through a filter and back into the full-stream flow prior to the heat exchangers/chillers. Redirecting the side-stream flow back to pump suction is not recommended, since that would reduce flow to the heat exchangers or require an increase in the pump output. The logic of this technique is filtering the water at a rate greater than the anticipated input of contaminants.
Lower side-stream percentages are occasionally employed, but not recommended. Location (such as near open fields or windy, dusty conditions) and seasonal considerations (such as pollen or spring blossoming) may effect higher contaminant potential. Using a low-percentage side-stream may not overcome these conditions.
This approach is estimated to increase the operating cycle of a cooling tower’s heat exchangers by three times before servicing requirements (again based on experiences with users keeping good records). It is commonly used when the full-stream flow is extremely high, making full-stream filtration impractical, cost-wise. Like full-stream, this technique does not address the problem of solids accumulation in the tower basin or remote sump.
System turnover - Sometimes misunderstood as side-stream or basin-cleaning, this technique requires calculating the total volume of water in the cooling loop (in the basin/sump, piping, heat exchangers, and so on) and selects a once-an-hour turnover rate (measured by total water volume divided by 60 = U.S. gallon per minute [gpm] flow rate). Often, this flow rate is similar to that of side-streaming, but accounts for greater system fluid volume, due to extensive piping, enlarged basin size and so on.
With system turnover, the estimated increase in operating cycle of a heat exchanger is three times before servicing requirements. Like the techniques above, this approach does not address the issue of solids accumulation in the tower basin or remote sump.
Basin cleaning - Filtration directed specifically to control solid accumulation in the cooling tower’s basin or remote sump is new to the heating, ventilating and air conditioning industry. However, its success and value make it among the most popular approaches today. Basically, water is drawn from the tower basin/sump to the filter package and sent directly back to the tower basin/sump via a pattern of specialized nozzles to create a directed turbulence of flow designed to influence any settleable particles toward the basin cleaning package pump intake. The size of the filter package is based on the overall size of the cooling tower’s basin or remote sump. A formula of 1 to 5 U.S. gpm per square-foot is the common rule, determined by the severity of solids built-up in the basin.
Though its technique is directed specifically at the tower basin/sump, the value is nonetheless predictable and is estimated to increase the operating cycle of a heat exchanger by five times before servicing requirements. Unlike the other techniques above, this technique is the only method that directly addresses basin/sump accumulation.
Note that this approach requires the appropriate use of a venturi-like nozzle system to increase the total flow activity without the need for a high-volume pump, thereby keeping equipment and pump energy costs to a minimum. Known as eductors or hydroboosters, these nozzles increase the flow that passes through them by a factor of five to six times, enabling the filter package to use a smaller filter and pump, while still achieving the flow activity necessary to sweep the settleable solids across the basin/sump to the filter package’s pump intake.
An important element to making this approach work effectively is adhering to the flow and pressure requirements of the chosen nozzles to achieve the necessary flow to sweep the solids in the basin/sump and prevent troublesome accumulation. Inadequate flow/pressure to these nozzles minimizes the flow-increase capability of the nozzles, reduces the overall flow activity necessary to sweep solids toward the pump intake and into the filter and, in essence, achieves not much more than the equivalent of the “system turnover” technique already discussed.
Make-up water filtration - This technique uses a filter at the make-up water intake to keep unwanted particles from entering the system. Its value is limited to keeping make-up water contaminants from contributing to the system contaminant problem. Its limitation is that most solids typically come from the incoming air flow and the creation of solids via the evaporation-precipitation process. To date, no protection factor has been identified with this approach, although a water supply with significant sand, silt or organics could certainly create equally significant problems if not properly filtered.
Filtration Payback
New equipment proposals are often met with resistance and claims of “it’s not in the budget”. However, equipment with cost-saving implications simply suggest that money being spent to correct the problems can be re-directed to the solution, with the promise of payback savings in a reasonable time period. Such is the case with proper filtration for an evaporative cooling water system. To figure the benefits, calculate the costs associated without filtration and compare them to the cost of the proposed solution. That’s payback value.
Another way to identify whether filtration can become a solution is to apply these criteria:
- Reduced Maintenance Costs: 60-90 percent savings - Calculate the costs currently associated with contaminants in the water system. These can include downtime, labor, overtime, lost productivity and heat-exchanger cleaning/punch-out services.
- Reduced Energy Costs: 10 percent savings - When contaminants foul a heat exchanger or chiller, the result is longer operating cycles to achieve the proper cooling factor. This savings estimate is conservative, at best.
- Reduced Water Costs: 5-10 percent savings - Proper filtration can effectively reduce the frequency of blowdown. Consider not only the cost of the make-up water, but also the cost of sewage/waste discharge fees.
- Reduced Chemical Costs: 5-15 percent - With make-up water and solids fouling comes higher chemical treatment input. Without it, the savings are obvious.
Consider these criteria as a formula for calculating the basic payback associated with proper filtration. Add to these costs the wear and replacement of the tower and heat exchangers/chillers, the damage to pumps a nd the fouling and repair/replacement of valves and control instruments.
Selecting Filtration
Once the problem is defined and the solution is chosen, all that remains is to select the proper filter or separator to achieve the desired results. The following criteria place the proper means for analysis in the control of the buyer. Consider:
- Particle removal capabilities - Given what’s required, can the filter remove the troublesome solids?
- Flow range - Will the filter be able to handle the required flow rate?
- Pressure loss - How much pressure loss will the filter require? Is it variable or steady?
- Liquid loss - How much water is needed to clean the filter? How will you handle this water loss?
- Solids-handling - How will the filtered/separated solids be handled?
- Replacement parts - What routines and costs can be expected?
- Maintenance requirements - What procedures will be necessary? At what intervals and with what potential effect on availability and duplicate hardware?
- Space requirements - How much space is available? Will the filter fit in the area designated?
Be sure to take a solution orientation approach to selecting a filter or separator. A complete solution provides the proper filter, a proper means for automating it, an appropriate technique for capturing and handling the filtered/separated solids and the capability to package this solution to minimize engineering, purchasing, installation and operating start-up.
Filtration and separation systems are available for a wide range of problems. Use care in determining the real nature of a problem. It is possible to achieve absolute perfection, but it is prudent to seek only affordable protection.
Author: Randy Delenikos is Vice President of Sales for LAKOS Separators & Filtration Systems based in Fresno, Calif.