AIR CONDITIONING


Towers of strength


Cooling towers are heat rejection devices for buildings, typically disposing of unwanted heat from chillers. However, they can also be used in other HVAC equipment such as heat pumps. Tim Mitchell, sales director of Klima-Therm explains.


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ven though their working principles are similar, cooling towers come in all shapes and sizes and can be used in a variety of applications. These include power plants, oil refineries and manufacturing facilities. As in these applications, in the HVAC sector, cooling towers remove heat from buildings, usually by lowering the temperature of a stream of water using the principle of evaporation. The main design considerations for cooling towers are the cooling load of the building and typical weather patterns surrounding it, in particular the wet bulb temperature of the ambient air.


Cooling towers for HVAC installations generally employ one of two heat transfer methods: direct evaporation from the working fluid (water), or evaporation of water from the surface of a heat exchanger in which the working fluid is circulating. In both cases, the air to which the heat is rejected is moved by an electric motor- driven fan.


Cooling


tower types T


here are several types of cooling towers designed for different applications.


They include open and closed-circuit cooling towers, evaporative condensers and adiabatic systems. ■ Open-circuit cooling towers – Exploit a simple and natural physical principle: forced evaporation of a minimal water quantity lowers the temperature of the main water mass. A well-sized cooling


30 February 2021


Open circuit cooling towers remove heat from the circulating water by evaporating a small portion of it to the ambient air using the heat in the water to do so, thus reducing the temperature of the fluid. These are the most efficient cooling towers. Open circuit cooling towers could often historically be seen producing what are known as ‘drift’ emissions - visible clouds of water vapour drifting off the top of the tower.


These drift clouds are rarely seen nowadays due to the installation of very efficient drift eliminators, to minimise water loss and reduce the Legionella risks associated with warm, fine water droplets.


Closed circuit cooling towers circulate the working fluid in a coil inside the tower such that it is not directly exposed to the ambient air. Recirculated cooling water is then pumped over the surface of the coil and the heat to evaporate it is taken out of the working fluid. They are typically used where the system needs to be clean and free of contaminants. If the working fluid


is water, it must be mixed with glycol to form a fluid that will not freeze at the project minimum design ambient temperature.


With so many variants of these two types of tower available, it is important to design bespoke, project-specific cooling tower solutions, examining a range of issues including environmental conditions, energy efficiency, maintenance and legionella prevention. If the temperature of the cooled fluid needs to be below the ambient temperature, an evaporative system is the only way to do it: in this case, the minimum theoretical limit of the cooled fluid is, as we have seen, the wet bulb temperature of the air plus a small “approach” related to the efficiency of the heat exchange process.


In contrast, dry systems are based on only sensible heat exchange which is less efficient than the exchange of the latent heat of evaporation. The limit in this case is imposed by the temperature of the cooling fluid, namely the


tower can cool circulating water down to a temperature 2-3°C above the wet bulb temperature.


■ Closed-circuit cooling towers – Use the forced evaporation of a small quantity of water to lower the temperature of the main water mass. Process fluid to be cooled enters the upper header of a coil which is constantly drenched by water recirculating in the small, pre-assembled evaporative circuit of the unit. Evaporation of a fraction of recirculating water removes heat from the coil exchanging surface.


■ Evaporative condensers – The refrigerant to be condensed enters the upper header of a coil. As in closed loop cooling towers,


the coil is constantly saturated by water recirculating in the small, pre-assembled evaporative circuit of the unit. Evaporation of a fraction of recirculating water removes heat from coil exchanging surface.


■ Adiabatic coolers and condensers – A thermodynamic phenomenon by which the air temperature is decreased through humidification. Adiabatic coolers can be used for water/glycol mixtures cooling or refrigerant condensation.


■ The temperature that can be reached is called the ‘adiabatic saturation temperature’: it can be much lower than the ‘dry’ air temperature, depending on humidification system efficiency.


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