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Who Turned up the Heat? What You Need to Know about Biofilms and Heat Transfer.

 

Whether you work at an oil refinery, power plant, or in an office building, chances are you owe your comfort to a cooling tower.  These engineering marvels are responsible for keeping your process fluids cool, turbines spinning, and even your work environment comfortable by transferring heat from water to air. Of course, where there is water, there will be microorganisms.  Bacteria, fungi, and algae can thrive under the warm and humid conditions commonly found in cooling systems.  Bacteria are particularly problematic, as they can attach and form biofilms on the surfaces of heat exchange systems, limiting their ability to transfer and reject heat from the water.

In cooling towers, a common site for biofilm build-up is on the surface of the cooling tower fill.  Fill is a medium that is used to distribute condenser water over a large surface area, increasing contact with the air and promoting heat transfer through evaporation.  If biofilms are permitted to accumulate within the fill, they can restrict or completely impede flow, reducing the amount of heat that can be rejected from the tower.

Rooftop cooling water systems

In systems with open circuit cooling towers, biofilms can also reduce system efficiency through formation in the condenser.  In this part of the process, heat flows from the refrigerant to the cold condenser water through the heat exchange coil.  If a layer of biofilm has formed on that barrier, then the flow of heat slows down.  If biofilm formation is significant enough, you may even notice your work environment get a few degrees warmer!  To compensate for these effects, we increase air and/or water flowrates.  Unfortunately, that requires more power and increases electrical costs.  For example, it is estimated that approximately 1 mm of biofilm in a centrifugal chiller can reduce heat transfer by up to 35%, which could cost an additional $9,000 per year* for a 200 ton chiller operating at 50% load.   Eventually, the process may need to be shut down for cleaning, resulting in lost production and increased labour costs.

Not only do biofilms in cooling systems reduce efficiency, but they can harbour pathogenic bacteria, such as Legionella, and cause microbiologically influenced corrosion, which can reduce the useful life of the cooling tower by months or years. The cost implications of corrosion are significant. It is estimated that corrosion costs the global economy US$2.5 trillion per year, and research has indicated that 20% of corrosion is microbiologically influenced.

As mentioned in previous blogs, it is impossible to eliminate biofilm risks completely – we can only minimize their impact.  Doing this requires good housekeeping, effective treatment, and a strong monitoring program.  2nd Generation ATP is an ideal first line of defense in this regard. It is accurate, portable, and completely captures all potential microbial contaminants in the cooling system, allowing it to be used as a direct indication of biocide efficacy.  Test methods are also available for measuring sessile (attached) and planktonic (floating) organisms, giving a more complete picture of the system’s microbial activity.  Between reliable and accurate monitoring tools, best-in-class knowledge and support, and revolutionary software products, we’ve got you covered!

Want to learn about how our test kits afford your team the ability to diagnose, troubleshoot, and solve contaminated water problems within a matter of hours compared to days or weeks with traditional methods?  Check out our brochure for Industrial Fluids Treatment.

 

*Cost assumptions: chiller efficiency = 0.6kW/ton, power costs = $0.05/kWh


Further reading:

Using ATP in a Legionella Water Management Plan

 


Brad Mcilwain

Brad McIlwain has over eight years of experience in the water and wastewater industry. Prior to joining LuminUltra, he worked for several years as a consulting engineer, where he helped solve complex water and wastewater treatment process issues. He has a particular interest in water system corrosion. During his Master’s, his research focused on water quality and corrosion in premise plumbing. Brad enjoys being outdoors, spending as many weekends as possible camping, hiking, and cycling.

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