March 2020

passed over the cooling tower is always much greater than the amount of evaporation that occurs, which is typically about 1.6% of the circulating flow, so the opportunity for impurities to build up is reduced, as the water has a constant washing affect. Couple this with the fact that fouling of the heat exchange surface does not significantly reduce the surface area of the water until it is very severe, and you have a heat exchanger that maintains its performance for much longer than a dry cooler. One of the cheapest components in a cooling tower is the fill pack (heat exchange surface), which is designed to be easily removed and replaced in most cases. Compare this to a dry cooler, where the most expensive and difficult part to remove and replace is the coil. Freezing coils can be catastrophic, crippling plants in a matter of hours, and while cooling towers can also freeze, they often require little or no thought throughout the winter months. Lowest cold water temperatures and highly manageable There are many arguments as to why cooling towers are not the ideal choice in some locations - visible plume, high water usage, and the cost of chemicals are the main three - but just consider the following before dismissing them. We maintain cooling towers that are 40 years old, providing the fill is clean, and the air moving equipment and water distribution systems are working, then they still deliver the same cold water temperature that they did 40 years ago. Regular cleaning is simple to undertake, and replacement of the main components is also relatively simple. In the UK, water treatment is at the top of its game using intelligent dosing systems to monitor the condition of the circulating water, to protect not only humans from harmful bacteria, but also the process equipment from scale and corrosion. Cooling towers are a more than manageable risk. Probably most importantly, cooling towers achieve the lowest cold water temperatures compared to dry and hybrid coolers, which can directly affect your productivity, or process performance. Justification of using alternative methods of cooling often centres around payback, which focuses on the cost of water treatment and the cost of water in evaporative systems. These justifications are based on a fixed baseline performance of a given system, in adiabatic dry air coolers and hybrid coolers this baseline performance will deteriorate, and may never return to the original design performance, so it may be true for the first 3 months, but 3 years after installation it almost certainly won’t be, which makes these justifications not altogether realistic. Cooling towers, some of which were built in the 1960s, dominate areas of the countryside, and some of the towns and cities around the globe, they offer a long term effective solution to cooling, they can run on sea water, river water, effluent water and borehole waters, they can be built of wood, plastic, metal and concrete, some are big, some are small, but with the right systems in place they are capable of doing their job safely and effectively delivering cold water to everything from data centres in frozen landscapes to metal smelting plants in the deserts, for many, many years. There’s still a lot going for the trusty cooling tower. when it comes to heat rejection. Finned coils need regular cleaning, but they are hard to clean due to the trade-off between the water pressures needed to clean the coils versus the potential to fold the fins during cleaning. Other methods of cleaning exist, but in many cases it is the mechanical action of the water dislodging impurities that has the greatest effect. Regular cleaning also weakens and eventually will cause the break-down of the fins which is terminal for the fin coil. Cooling towers maintain performance for longer Cooling towers on the other hand, exchange the heat contained in the circulating water by spreading out the water over a large surface area and bringing air into contact with the surface of the water. The volume of water 30 | Plant & Works Engineering www.pwemag.co.uk March 2020 Process, Controls & Plant Focus on: Process Heating & Cooling