Effective cooling water treatment is imperative in ensuring water systems are free from scale, corrosion and biofouling which not only limit the life of the system but also provide niche environments for proliferation of bacteria, including legionella bacteria; the causative agent of Legionnaires’ Disease.

legionella control in cooling water systems

There are several key factors that require controlling to prevent the proliferation of legionella bacteria in cooling water system, please read on the learn more about the checks we make.

Control of corrosion

Corrosion may be the result from a number of different mechanisms such as general oxygen driven attack, galvanic corrosion cells caused by dissimilar metals, chloride attack of stainless steel and microbially induced or influenced corrosion (MIC). As a general rule the more scale forming the less corrosive water tends to be to steel and the LSI and RSI indices can be used to predict this. This means that provided scale formation can be effectively inhibited it is beneficial to operate a cooling system at an elevated concentration factor therefore close control of the cycles of concentration is critical to cooling water corrosion control. In effect you are making the water chemistry work for you rather than having to fight against it. It is also important to control the microbial activity and physical fouling since both can be the cause of severe localised corrosion. Once these basics have been dealt with you need a good corrosion inhibitor which is matched to the cooling water chemistry and system metallurgy, and to control its dosage effectively.

Contact us to protect your cooling system from corrosion problems.

control of scale

Scale or lime-scale is a hard, rock-like deposit of calcium or magnesium salts that forms in heat exchangers and cooling tower packing as a result of heat and increased concentration factor. Scale formation impairs heat transfer, interferes with flow and cooling, and can be a breeding ground for legionella bacteria. The scaling tendency of a water supply will depend on the hardness of the water but if not adequately treated even relatively soft waters can become highly scaling when concentrated by evaporation. Cooling water scale control is achieved by either:

  • Removing the hardness from the make-up water using a water softener
  • Adding a scale inhibitor chemical which extends the solubility of the hardness salts and thus preventing scale formation
  • Dosing sulphuric acid to lower the pH and alkalinity and reduce the scaling potential (this technique is generally restricted to large industrial systems)
  • Limiting the system concentration factor to a range within which the hardness salts can remain soluble 

The scaling tendency of water can be predicted by calculating the Langelier Saturation Index (LSI) or Ryznar Stability Index (RSI) and often a combination of these control techniques is employed to prevent scale formation whilst not causing problems of corrosion.

microbiological control

An evaporative cooling system is often an ideal growth environment for a whole range of microbes such as bacteria, algae, fungi and protozoa which can lead to problems such as:

Microbial slimes which interfere with heat transferHighly localised microbial induced corrosionReduced effectiveness of corrosion inhibitorsFouling caused by suspended solids being trapped in the microbial slimesReduced flow and poor water distribution within the cooling tower

Effective cooling water microbiological control is not only essential to the efficient operation of a system but also necessary to control the growth of legionella bacteria and the risk of Legionnaires disease. In order to maintain microbial control effective biocides need to be added on a regular basis and the results monitored using dip-slides and periodic laboratory analysis.

control of (bio) fouling

Controlling scaling, corrosion and microbial growth will reduce problems of fouling but a cooling system may still become fouled with sediment in the incoming water supply (particularly if it isn’t mains water) and airborne dust and debris washed out of the atmosphere by the cooling tower. In some instances fouling may also be the direct result of process contamination. Deposits tend to settle in low-velocity areas and can cause reduced flow, blockages, impaired heat transfer, under deposit corrosion and encourage microbial growth including legionella. If the incoming water contains a lot of suspended solids then pre-filtration or clarification may be advisable. To keep the suspended solids mobile in the recirculating water the chemical programme should always incorporate a polymer dispersant and if the solids level is high then the incorporation of a side stream filtration system in a of the cooling circuit will help to keep the system clean. Since an evaporative cooling system is in effect an air scrubber so some build-up of deposits with time is inevitable so in addition to these on-going control measures the system should be taken off-line periodically for disinfection and the physical cleaning.

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