Energy Saving in Evaporative Towers

An evaporative tower is a thermal machine employed to regenerate cooling water feeding industrial thermoregulation systems, often used in big plants working with high heat amounts to be managed, but also used in minor plants, with smaller and cheaper tower models. In this kind of thermal machine cooling is achieved by mass transfer, a thermal exchange process where water gives heat to air, releasing part of the water in the form of water vapor that draws thermal energy to complete the phase change from the remaining water, which then gets cooled.

External weather conditions and air humidity impact on the process: the more dry ambient air is, the more evaporation will happen and lower water temperatures can be reached, with higher tower performance. Cooling with evaporative towers is employed with quite low water temperature, usually inlet water at 40 degrees and cooled down at 20-25. Minimum water temperature in output, cooled after the evaporation process, can however only be 3 degrees higher than wet bulb temperature, which is the minimum temperature that air can reach after the evaporation process itself, at the maximum vapor water saturation level affordable.

Thermal cooling in an evaporative tower is then achieved by direct contact water/air. Several kinds of evaporative towers exist, but the most common is the one with typical cylindrical shape (or parallelepiped shape in case of small/medium prefab towers) narrowed on top, a shape that causes a natural air circulation from the bottom to the top of the tower. Air comes in at the bottom of the evaporative tower, fans and radiators can be used to force it also in order to increase the cooling effect of the structure. Air coming from the bottom gets sucked toward the top, while water to be cooled is sprayed from the top, coming from a condenser or another water thermal circuit engine, and gets fragmented in tiny droplets using filters and separators. Inlet water has high temperatures, absorbed during the served thermal cycle, and meeting the cool air, sprayed water becomes in part vapor, loosing the gathered heat. Cooled water is then collected at the bottom of the evaporative tower, ready to be re-used.

Evaporative towers have related costs and consumptions, threads to be carefully defined since from the first steps of the plant project, lowering at the best energy and water consumption. Cooling process obtained with an evaporative tower requires water consumption, lost as water vapor in the typical white plume on top, in a quantity directly proportional with the heat amount to be dissipated by the plant. Vaporization of one liter of water draws 600kcal of thermal heat, and so an evaporative tower wastes water depending on the thermal compound it has to dissipate. But also electricity used to activate circulating pumps, bringing water up in the tower, are other costs and consumption factors, and also electric energy used by fans moving inlet air coming in.

Cost optimization and energy saving in an evaporative tower can then be achieved minimizing the electricity requested to make the system work, through an accurate project and design of the structural characteristics of the tower itself, leading immediately to energy consumption reduction that can be higly significant in huge plants. It is then important to choose the correct dimension of a tower, since lowering air capacity and transfer areas on filling packs leads to a higher air demand to be supplied by fans, consuming a lot of electricity, in fact erasing the primary saving on construction costs. Choosing a smaller tower to save on costs is the wrong choice in the longer term, both on economical and energetic grounds.

Strictly depending on thermal and water amounts to work with, the right choice is instead enlarging transfer surfaces and dimensions, making it possible to use the less forced air circulation with electric fans to reach the correct thermal cycle provided by the evaporative tower. A lower ventilation also leads to a lower speed of air in exit, minimizing water consumption usually lost for dragging.

A suitable design and sizing of an evaporative tower can then lead to a minor use of fans and ventilation systems, also lowering the noise of the tower. Evaporative towers can indeed often be quite noisy, and silencers are usually employed to shut down noise coming from fans, and it also increases electric demand.

The right design and dimension of an evaporative tower must be determined in the project phase, firstly to minimize forced air circulation using fans, and then because the natural environment and the plant’s existing obstacles, such as walls and other reflecting surfaces stopping noise diffusion, can be used to avoid demand for silencers. These expedients and project solutions can seriously affect energy consumption and long-term costs of any industrial thermoregulation system enhanced with an evaporative tower.

Please contact Tempco for more information.

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