An open circuit cooling tower is a special heat exchanger where the air comes into direct contact with water to decrease the temperature of the water itself.
Cooling towers are based on a natural principle that is as simple as it is effective: evaporative cooling. When air and water meet in the thermal fill pack, a small amount of water evaporates, resulting in a natural decrease in the water's temperature circulating in the cooling tower. The cooled water is then reintegrated into the industrial process to absorb the heat again. The water will then be pumped back into the cooling tower to start the cycle again and be cooled again.
Cooling towers represent a really simple heat disposal system that ensures low power consumption, technological innovation with low environmental impact, and the best energy efficiency. This has also been confirmed by Assoclima,the 'Association of Manufacturers of Air Conditioning Systems of ANIMA(Confindustria), among others:
"Modernly designed cooling towers have achieved such high levels of energy efficiency and technological innovation with the low environmental impact that they are now the best plant solutions for heat disposal in both civil and industrial environments. With minimum water consumption that is easily managed, evaporative systems guarantee low electrical consumption with considerable energy savings, extremely reduced overall dimensions and low noise levels. The limited use of resources and energy allows the towers to establish themselves today as the cooling technology with the lowest carbon footprint, in full compliance with EU directives on eco-design and reduction of greenhouse gas emissions.”
For more information on how cooling towers works, please refer to the article "What is and how does a cooling tower"where we have analysed the entire process for both open and closed-circuitcooling towers.
That being said, what are the main factors that can affect the performance of cooling towers? The following are the main factors to consider:
The wet-bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water in the air at constant pressure. The wet-bulb temperature, measured by an instrument called a psychrometer, is always lower than the dry-bulb value. This is always true except when the air is completely saturated with water, i.e. when the wet and dry bulb temperatures are equal (in this case we are talking about 100% relative humidity).
The wet-bulb temperature is an extremely important parameter in the selection and design of cooling towers because it allows the temperature and humidity conditions to be accurately defined according to the location of the system. Therefore, the wet bulb temperature is the primary basis for the thermal design of any evaporative cooling tower.
The temperature difference between the inlet of (hot) water and the outlet of (cooled) water from the cooling tower is called the Range. By increasing the cooling range, the initial investment cost and energy consumption of equipment are reduced.
The heat load or heat output of the cooling water is determined by the standard heat transfer equation: Q = m Cp ∆T for which
- Q= the heat load
- m=the mass of the cooling water
- Cp=the specific heat of water
- ∆T= inlet/outlet temperature differential
Heat output is thus the amount of heat to be dissipated from the water circulating inside the tower. This is a determining factor in selecting the right cooling tower in terms of both size and initial investment.
The approach is the temperature difference between the cooled water entering the basin and the wet-bulb temperature of the air. It is one of the most important factors in the selection of the cooling tower: the more "centred" the approach, the better the performance of the equipment will be. For this reason, the best choice for the design is the cooling tower with an approach neither too far nor too close to the reference wet-bulb temperature. However, it should be considered that a smaller approach guarantees cooler water, but at the same time requires a larger tower, necessarily leading to higher investment costs and higher energy consumption.
Given all the above aspects, we can say that an approach between 3°C and 5°C is the best choice for optimum performance.
Without a doubt, the design, selection and integration of the various components that make up the cooling tower, as well as the use of original spare parts, are essential to ensure efficient performance and long service life for the equipment.
For example, in terms of component selection, Decsa has optimized the efficiency of heat exchange through the development of DecsaPack, an fill pack evaporator that can improve the turbulence of countercurrent water
To maintain the performance established in the design phase, it is also essential to carry out correct maintenance of the cooling tower. For more details on maintenance activities, we recommend reading the dedicated article: Cooling tower maintenance: what to do?
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