AIR CONDITIONING
Typical
components of cooling towers
ambient air, again plus the 'approach' of the heat exchange process.
Where it is sufficient to cool the fluid to a temperature somewhat above ambient temperature, typically a minimum of around 5°C above, a dry air cooler can be used.
A third option is to design an adiabatic system where a fluid temperature equal to or only slightly lower than the ambient temperature is desired. This is in effect a hybrid system that can often work “dry”, using sensible heat exchange only, but is able to work “wet” when necessary at higher ambients to keep the cooling system in bounds.
The size of the cooling tower heat rejection system (in terms of thermal power to be dissipated) directs the choice between two systems: ‘packaged’ type cooling towers, which are pre-assembled at the factory, or ‘field- erected’ which are constructed directly on site. Cooling towers of whatever type require routine maintenance, not least for safety reasons. Cooling towers have, in the past, received bad press because of their association with Legionnaire’s Disease (LD), a potentially fatal form of pneumonia. LD is the most serious of a number of diseases caused by legionella bacteria which also include Pontiac fever and Lochgoilhead fever. However, sensible precautions and regular cooling tower maintenance can eliminate the risk of catching it. The Health and Safety Executive offers a wealth of advice on this (https://bit. ly/3n7vra5).
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Safety and efficiency go hand in hand. Well- treated water, avoiding scale deposits and other deposits in the evaporative tower and in the plant, has a positive influence in minimising the proliferation and diffusion of organic substances such as algae or bacteria.
A clean evaporative cooling tower is always able to offer maximum efficiency with minimum consumption, both in terms of electricity and evaporated and purged water.
It is also often advisable to add water pre- treatment such as softening or osmosis processes to the system. By doing so, it is possible to reduce the consumption of water and dosing products and at the same time, thanks to the higher concentration, to more effectively protect the system and increase safety against the negative effects of failure of part of the treatment. The installation and the proper management of a suitable treatment system ensures stable operating conditions for the cooling system and high efficiency over time. Automatic systems can optimally operate towers and require limited ‘hands on’ operational input.
By evaluating the specific requests of each system appropriately, it is possible to identify the correct management system, with different degrees of complexity, to ensure the required performance at an acceptable operating cost. Despite the higher initial cost, the adoption of more comprehensive treatment and pre- treatment systems reduces system operating costs. These are therefore always recommended.
T
he main components that characterise evaporative cooling towers, whether open or closed circuit, include:
■ Principal containment and support structure for the evaporative tower: Can be made of sheet metal, glass fibre or both materials. For extremely large industrial or hyperbolic towers, cement is also used. ■ •Heat exchange surface (in open circuit towers) or heat exchange coil, generally with bundles of smooth tubes (in closed circuit towers): The heart of evaporative towers, these are the components through which the heat exchange between water and air takes place. ■ Axial or centrifugal fans: The only moving mechanical part, these are the components that ‘force’ the water evaporation required to achieve cooling. The choice of an axial or centrifugal fan, depending on the design conditions, is important in order to achieve the most efficient operation and the lowest energy consumption of the system. ■ Water distribution system: Normally comprising a matrix of pipes and nozzles, this makes it possible to distribute the water to be cooled over the heat exchange surface (open circuit cooling towers) or on the heat exchange coil (closed circuit cooling towers). ■ Droplet (or drift) eliminator: Located immediately upstream of the fan, this serves to retain the water droplets which would otherwise be carried outside by the flow of air created by the fan.
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