Particle contamination of evaporative cooling loops can be created by many sources, including airborne entry, makeup water, corrosion byproducts and precipitated mineral development. This particle matter commonly fouls heat exchangers, reducing heat transfer efficiency, causing excessive shutdowns and cleaning routines, and posing health and safety concerns.
To effectively protect a cooling loop from contamination, it is important to first identify and define the particle contaminants via a thorough particle analysis. The analysis should show not only what types of particles are in the cooling water but also what particles are most responsible for fouling and lost efficiency. Once the contamination sources are known, it is equally important to select a filtration technique with an understanding of its proper placement, sizing and solution potential to best protect the equipment.
Popular Filtration Methods
Of course, to select the proper filtration method, you must identify which equipment components need protection from the contaminants. The usual candidates include heat exchangers, the cooling tower basin or remote sump, the tower fill and the distribution headers/nozzles. In addition, costs associated with the problems such as increased energy and chemical costs, downtime, cleaning, repairs or replacements, outside services and overtime labor and maintenance should be assessed. The anticipated costs will become important when the cost of the problem is being compared to the cost of the solution.
In general, five approaches are accepted as techniques for controlling solids in an evaporative cooling system.
Full-Stream Filtration. With this approach, the filter is installed at the system’s supply pump discharge, prior to the heat exchangers or chillers. The filter is sized according to the full flow of the pump, filtering all the water that passes on to the heat exchanger or chillers. This approach is estimated to increase the operating cycle of the heat exchanger by 10 times before servicing requirements appear. However, this approach does not address the problem of basin/remote sump solids accumulation.
Side-Stream Filtration. This type of filtration typically diverts approximately 10 to 20 percent of the full-stream flow through a filter and back into the full-stream flow prior to the heat exchanger or chillers. Redirecting the side-stream flow back to the pump suction is not recommended because that would reduce the flow to the heat exchangers or require an increase in 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 occasionally are employed, but they are not recommended. Location and seasonal conditions provide for higher contaminant potential, suggesting a higher percentage side stream to 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 become acute.
This technique most often used is when the full-stream flow is extremely high, causing full-stream filtration to be financially infeasible. Like full-stream filtration, this method does not address the problem of solids accumulation in the tower basin or remote sump.
System Turnover. Often misunderstood as side-stream filtration or basin cleaning, system turnover requires the calculation of the total volume of water in the cooling loop and a once-an-hour rate of turnover. This flow rate often is very similar to that of side-stream filtration, but it accounts for greater system fluid volume due to multiple factors such as extensive piping and enlarged basin size. The estimated increase in the operating cycle of a heat exchanger is three times before the servicing requirements become necessary. Like the other techniques mentioned previously, this approach does not address the issue of solids accumulation in the tower basin or remote sump.
Basin Cleaning. When applying basin cleaning as a means of filtration, water is drawn from the tower basin or sump, to the filter package, and directly back to the tower basin or sump via a pattern of specialized nozzles. The nozzles create a directed turbulence of flow designed to influence any settleable particles toward the basin cleaning package’s pump intake. The size of the filter package is based on the size of the cooling tower basin or remote sump.
Despite concentrating effort on the prevention of basin or remote sump buildup and not directly protecting the heat exchanger, this technique is expected to increase the operating cycle of a heat exchanger by eight times before servicing requirements become necessary. And, of course, this method does directly address basin or sump accumulation.
Makeup Water Filtration. This technique employs a filter at the makeup water intake to keep unwanted particle matter from entering the system. Its value is limited to keeping makeup water contaminants from contributing to the system contaminant problem. Its limitation is that most solids typically come from the incoming airflow 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.
With the knowledge of what parts of the system need protection and what contaminants need to be removed to achieve that protection, a filtration approach selected based on an objective set of criteria for effective comparative evaluation.
Effective filtration can extend the life of any evaporative cooling system. The payback value of filtration can be found by calculating the costs associated without filtration and comparing those costs to the costs of the proposed solution. Assess your system and select the approach that best suits your situation.