The first question that comes up when discussing a water based cooling system, such as evaporative cooling, is always about its safety and in particular about the control of legionella bacteria. In this article we will explain what you should be looking at if you are considering such a system, illustrating with examples of our approach to ensuring that our evaporative cooling systems are safe.
Knowing that legionella is transmitted through inhalation of aerosol droplets containing the bacteria, there are two main risk areas that water based systems need to address:
- Waterborne: legionella bacteria multiply at temperatures above 250C
and 500C, so water temperature is a key factor in controlling the growth of the bacteria in the system. Minimising the quantity of legionella bacteria in the system’s water is the first step in controlling the risk of infection.
- Airborne: risk of breathing in legionella bacteria if the cooling system atomises water bearing the bacteria.
Does this mean that evaporative cooling is not safe? No, but it does mean that if you are considering such a system for your factory or warehouse it is important that you understand how it manages these risks and make sure it has the right certification.
What makes an evaporative cooling system safe
Water quality is critical: the system must ensure it is maintained and monitored. The system should be designed to maintain the water temperature below 200C and include safety mechanisms that kick in if it rises above 250C. Regular maintenance is also critical and a system designed to make inspections easy is preferable.
For example, our Colt CoolStream system uses only drinking water, exchanges the water regularly, monitors its temperature and foresees a drying cycle that kills all legionella bacteria once a day. In addition, there is an additional safety feature that ensures that in cases of an error the unit bleeds off all water and dries itself, and in case of a power outage the water empties automatically by a failsafe drain valve.
The system should be designed in a way that prevents the release of water aerosols during the cooling process. This can be done in three ways with direct evaporative cooling:
- Designing the system so that it produces macro droplets bigger than 5 micrometres (μm). This solution, however, is very risky, as micro droplets will also be created, increasing the risk of breathing in the bacteria. It also requires frequent maintenance and automatic disinfection systems.
- Preventing the formation of droplets using a modern rigid wet medium, which is the most common solution. In this case, however, it is worth looking at the type of fan used: axial fans ensure a very smooth and even air distribution over the face of the wet medium, offering more effective protection. The large centrifugal units used by many units, on the other hand, tend to provide a more uneven distribution. Look out for systems using Aspen wood pads as desorption media, as they allow the passage of droplets even at low air velocities below 1.7 m/s. These are sometimes found in older or simpler, low-end units.
- Accepting the formation of micro droplets, protecting from contamination by ensuring complete disinfection of the water. This solution can be expensive to install and operate.
Going back to our example, the way we have addressed this in our CoolStream system is by controlling the maximum speed of the air as it goes through the wet desorption medium, so that it is too slow to carry water droplets. The axial fans are controlled to keep the air speed below 1.7 m/s, well below the 2.0 m/s minimum speed where water droplets can be caught in the air stream, leaving a safety margin.
How do I know an evaporative cooling system is safe?
The first thing to do is to check which hygiene tests the system has successfully completed and that it has achieved a hygiene certification issued by a standards organisation recognised by the Health and Safety Executive (HSE). You can find detailed guidance and practical advice in the HSE L8 Approved Code of Practice.
Our CoolStream system has a VDI 6022 certification, which we recommend, as it adopts a broad approach, dealing not only with legionella, but also E.coli and all hygiene aspects of air handling systems. This certification is issued by the German Association of Engineers (VDI) and is recognised as a global standard by the HSE.
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Paul Langford is an Engineering Director experienced in product development, manufacturing & testing for HVAC, solar shading, louvre systems and smoke control.