How to Prevent Bacterial Growth in Dry and Wet Cooling Systems

In cooling systems, airborne debris, such as dust and leaves, or water depositions, such as scales or inorganic/organic particles, can accumulate on the surfaces of cooling tower fills or the fin coils of air blast coolers. These deposits, when accumulated over time, can significantly reduce the cooling efficiency of a system and lead to the growth of legionella bacteria in the case of cooling towers.

Contrary to popular belief, a small accumulation of debris has only a minimal effect on cooling tower performance. In fact, small amounts of debris can aid the system by increasing the surface area of the water and the air to water contact time without increasing the airside pressure drop. A slight performance improvement may result.

With this in mind, there is still a fine balance to be struck between build-up that is beneficial and build-up that can cause critical issues. A heavy build-up of debris can result in significant weight gain (up to 20x), which can lead to collapse and expensive repair costs. Guidance documents such as HSG274 Part 1 published by the Health and Safety Executive for cooling towers used in the UK, provide a scale that can help to grade levels of fouling within cooling fill.

In contrast, debris accumulation on an air cooler coil can cause extremely fast and noticeable performance issues. Foreign matter creates a thermal barrier between the air and the cooling fins, which affects heat transfer through the finned coils. This is known as thermal contact resistance. While dry cooling systems tend to negate many of the risks of bacterial growth, preventative maintenance should still be performed at regular intervals to ensure maximum performance can be maintained.

Cooling towers that operate in warmer climates often run at temperatures of around 30C to 40C, which are ideal conditions for the proliferation of harmful bacteria that is already present in tap water, but needs things like heat, nutrients, and sunlight to thrive. Algae, rust, scale, and biofilms not only provide a source of nutrients for bacteria to grow, they can also act as protective barriers preventing water treatment chemicals from acting upon the bacteria, meaning that even a well-treated system can have areas of unacceptable levels of bacterial growth. These contaminants can also lead to internal fouling of heat exchange media, as well as blocked nozzles and pipework, which will also have a negative effect on water flow.

With the abundance of sunlight, nutrients, and moisture in warm weather, algae growth in cooling towers can quickly get out of control in the summer months. A dedicated water treatment system will need to be created to address this, but often the best solution is preventative—debris filtration units that will keep out the majority of foreign material and will make the job of algae control much easier.

The fastest and simplest way to analyse debris accumulation in cooling towers is to monitor the weight of heat exchangers over a period of time. When the threshold is reached, action must be taken to reduce it. Alternatively, endoscopic photography of the internals of the media is an effective way to assess fouling.

With air-blast coolers, the fin coils will show evidence of fouling on the individual cooling fins, and this assessment can be conducted externally with no downtime. On the other hand, evaporative cooling systems will often need to be assessed offline.

If debris-related bacteria is already present within your cooling tower, then we suggest the following approach:

As with all systems, a regular cleaning schedule is one of the key elements to preventing bacterial growth, along with an effective water treatment program. Once an assessment has been made, cleaning should be undertaken to ensure that scale, rust, biofilms, and algae do not have a chance to build up in the system. Maintaining a tight schedule will ensure the accumulation of debris never becomes excessive, and bacterial growth can be kept to a minimum.

A cost-effective preventative option would be to invest in debris filters that can minimise the exposure of water to external conditions, and can catch deposits before they have time to build up. Of course, filters won't be able to catch everything, and cleaning will still be required, but everything it does stop will help to reduce costs and help to keep cooling system performance high. Installing debris filters is also a far easier process than cleaning coils or fill packs, which often require specialists and will ramp up costs over time. When dealing with growth-encouraging sunlight, also consider shading the hot water basin of a cross-flow tower.

—Jeff Ebert ([email protected]) is Galebreaker's senior business development manager for North America. He has 30 years of experience in the evaporative cooling and dry heat transfer industry, serving power, petrochemical, and HVAC customers.

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