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Discussion on Energy-Saving Technologies for Circulating Cooling Water Systems in Chemical Enterprises. In chemical enterprises, circulating cooling water is a common and important utility system, characterized by its complexity, large number of users, and high water volume. The energy consumption of the corresponding circulating cooling water system is also very high; its water usage accounts for approximately 85%–92% of the total water consumption of the enterprise, while its electricity demand accounts for about 20%–30% of the enterprise’s total electricity consumption. Therefore, energy-saving optimization of the circulating water system in chemical enterprises has a significant impact on the energy savings of these enterprises. This article discusses energy-saving technologies in aspects such as the selection of circulating cooling water systems, the configuration or selection of equipment, and the choice of water treatment chemicals or techniques, which can serve as a reference for practical projects. 1 Selection of the circulating cooling water system: Generally, industrial circulating cooling water systems are divided into two main categories: open systems and closed systems. An open-system refers to a circulating cooling water system in which the circulating cooling water is cooled by direct contact with air; the return water comes into direct contact with air within an open cooling tower to exchange heat and thus reduce its temperature. The cooling in open-type cooling towers is primarily achieved through the evaporation cooling of water; as a result, there is significant water loss in the system. Moreover, due to direct contact with air, the water quality in the system deteriorates, which increases the amount of wastewater that needs to be removed, and consequently, more water must be added to the system. However, compared to closed systems, it consumes less electricity at the same processing capacity, and the total floor area required for the cooling tower is also much smaller than that of closed systems. A closed-system refers to a circulating cooling water system in which the circulating water is not cooled in direct contact with the atmosphere; instead, the returning water cools down through indirect heat exchange with air and spray water inside the heat exchange tubes of a closed-type cooling tower. The cooling in a closed-loop cooling system is primarily achieved through heat transfer by water; as a result, the amount of water lost in the system is small. Moreover, since the circulating water is usually soft water (or pure water) and does not come into contact with air, the water quality in the system remains good, and there is almost no need for wastewater discharge. Therefore, the amount of water that needs to be added to the system is very small. Due to the higher air volume required by closed systems, along with the installation of spray pumps, their power consumption is greater compared to open systems. Due to limitations in heat exchange efficiency, the maximum processing capacity of a single closed-cycle cooling tower is generally around 300 m3/h. If a higher processing capacity is required, multiple closed-cycle towers need to be connected in parallel, which also increases the floor space occupied by the equipment. The circulating water in a closed system is soft water; therefore, a soft water production system is required to replenish water for the closed-loop water system. Overall, the total energy consumption of open systems is lower than that of closed systems, making them relatively more energy-efficient. However, if the process equipment has high requirements for water quality, or is affected by the conditions at the project site (such as a shortage of water resources or high levels of dust and sand in the air), it is more appropriate to use a closed system. 2 Equipment selection or configuration: Typically, a circulating cooling water system mainly includes the following equipment: cooling towers, circulation water pumps; in open systems, there may also be bypass filtration equipment and chemical dosing equipment. By selecting appropriate equipment based on the operating conditions of the project, energy savings can be achieved. 2.1 Cooling Towers (1) Water-driven fans (turbines) – Cooling towers: To achieve the desired heat exchange efficiency, industrial cooling towers are generally of the mechanically ventilated type driven by electricity. A motor installed at the top of the tower drives the fan to rotate, thereby forcing air to flow within the tower and enabling heat exchange. As awareness of energy conservation and environmental protection grows, water-driven fan cooling towers have begun to be promoted in recent years and have been actually used in some projects. The core technology of water-driven fan cooling towers is the use of a turbine instead of an electric motor (including the drive shaft and reducer) as the power source for the fan. This changes the way the fan in the cooling tower is driven, from electrical power to hydraulic power; the output shaft of the turbine is directly connected to the fan, driving it to rotate, and it is this hydraulic force that propels the fan blades of the cooling tower to achieve ventilation and heat exchange. Currently, the driving force for the rotation of the turbine in the hydrodynamic fan cooling towers used in industrial circulating cooling water systems comes from the excess pressure in such systems (the additional head provided by the circulating water pumps). This approach makes full use of the excess energy available within the system, thereby reducing power consumption and achieving energy savings. For a hydrodynamic fan cooling tower to operate properly, the circulating return water at the top of the tower must have sufficient pressure head to drive the turbine, typically ranging from 0.1 to 0.3 MPa. During the design process, to meet the heat exchange requirements of the process heat exchange equipment, the pressure of the circulating feed water is set relatively high; in addition, a more conservative approach is adopted when calculating the head loss in the piping network. This will result in the pressure at the actual operating point of the system being lower than the rated pressure, causing the entire circulating water system to operate under conditions of excessive flow and low pressure over an extended period of time. Using a hydrodynamic fan cooling tower at this time can not only save electricity used by the motors in traditional cooling towers, but it also serves to regulate the pressure in the pipeline system. Furthermore, in certain industrial projects, flammable and explosive process fluids may enter the circulating cooling water system. When explosion-proof requirements apply to the cooling tower fans, it is more reasonable and cost-effective to use hydraulic fans as a substitute for explosion-proof motors. However, the author believes that water-driven fan cooling towers should not be adopted immediately in ordinary new industrial projects, as the energy surplus of the systems in such projects cannot be calculated accurately nor determined through stable operating parameters. If the pressure requirements of the turbine are taken into account from the initial design stage, this will transfer those kinetic energy requirements to the circulation pump, increasing the pump’s head and thus its power consumption. (2) Spray cooling towers: The spray cooling towers currently used in actual projects fall into two main categories – high-efficiency spray ventilation cooling towers and HJP-type spray cooling towers. The core component of a high-efficiency spray ventilation cooling tower is the spray propulsion atomizer, which utilizes the residual energy of the circulating return water to release energy through a vortex chamber and specially designed converging-diverging nozzles, thereby creating fine water particles, mist particles, and bubbles. The nozzle has an inclination in the horizontal direction; as the mist stream rises away from the nozzle inside the tower, it comes into contact with air over a large surface area, which generates a reverse thrust that causes the nozzle to move rapidly in the opposite direction. Thus, the nozzle flow generates an opposite horizontal force that drives the rotating mechanism and the lightweight blades to spin, creating wind force that induces convection of air within the tower; this helps to reduce the temperature of the circulating water while also achieving energy savings. When the water pressure of the circulating return water is between 0.12 and 0.16 MPa, the flow velocity of the special nozzle can reach around 18 m/s, while the wind velocity inside the duct can reach 28 m/s. Experience from actual projects shows that the cooling efficiency of high-efficiency spray ventilation cooling towers is comparable to that of conventional cooling towers, but it reduces the electricity consumption associated with the operation of fan motors, with only an additional one-time investment required for equipment modifications. The cooling principle of the HJP-type spray cooling tower relies on high-speed rotating water streams ejected from nozzles, which create a vacuum in the vaporization chamber outside the nozzles. This vacuum forces some of the water to vaporize, and the heat absorbed from the remaining water is used to compensate for the latent heat of vaporization; in other words, it utilizes the principle of phase change to significantly cool the hot water. At the same time, when water is sprayed into the space inside the spray tower, it exchanges heat with the air, resulting in a second slight drop in temperature; the total temperature reduction achieved by this device is the sum of the temperature reductions from these two processes. The proportion of water vaporized in the vaporization chamber through the nozzle depends primarily on the special design of the nozzle structure and the temperature of the water before it enters the nozzle. Since the HJP type spray cooling tower uses evaporation-based cooling, it provides excellent cooling performance; moreover, by controlling the proportion of water that enters the vaporization chamber, it is possible to handle circulating water with a high inlet temperature. The H JP type spray cooling tower not only ensures effective cooling, but also eliminates the need for replacing fillers and performing regular maintenance on fans, as it has no fillers, no fans, and no mechanical moving parts inside its structure. This reduces the costs and effort associated with such maintenance tasks. Water loss is also reduced at the water outlet due to the suction effect of the fan, and the drifting mist does not cause any wetting of the surrounding area. However, due to its high initial investment cost, it is not yet widely used in industrial circulating water systems in China. (3) Variable frequency control of cooling tower fans: The processing capacity of industrial cooling towers is typically determined based on the maximum capacity required by the process and the highest heat load during summer. However, in actual operation, various factors such as season, temperature, and workload can cause deviations from the most unfavorable conditions designed for the system, resulting in the equipment operating at lower loads and thus unnecessary energy consumption. Variable-frequency speed control technology is a new technique that has emerged in the field of electrical drives in recent years; this device changes the motor’s speed by altering the frequency of the power supply. In a typical variable-frequency control system for cooling tower fans, an inverter with built-in PID functionality is used, and the outlet water temperature of the cooling tower serves as the control parameter to form a closed-loop control system. A temperature sensor is installed on the circulating cooling water supply pipeline to transmit the cooling water temperature signal to the frequency converter. After passing through the PID controller built into the frequency converter, an appropriate voltage and frequency are supplied to the cooling tower motor, thereby adjusting the fan’s speed and output power, or the number of fans in operation (in the case of multiple fans being used). This creates a closed-loop feedback system: when the temperature of the cooling water decreases, the speed of the fans is reduced or the number of operating fans is decreased, thereby achieving energy savings and reduced consumption. 2.2 Variable frequency speed control of circulation water pumps At present, the circulation water systems in many industrial projects use throttling to adjust the head and flow rate of circulation water pumps. Although this method is simple and easy to implement, it results in significant energy losses due to the artificial increase in resistance during adjustment. Moreover, when the actual load is far from the rated operating condition, the pumps often operate outside their efficient range, leading to a situation where a powerful pump is used for a task that requires a less powerful pump. For systems with large fluctuations in the water consumption for circulating cooling water, the use of variable frequency speed control technology to adjust the speed of the pump motor in order to regulate the pump flow rate can not only avoid the performance losses associated with throttling but also ensure that the pump operates always within its efficient range, thereby enabling significant savings in electricity consumption. When the flow rate of the circulating cooling water changes, it indirectly causes changes in the pressure in the piping network. Based on this characteristic, an inverter with a built-in PID function is used, and a closed-loop control system is established with the cooling water pressure as the control variable. A pressure sensor is installed on the circulating cooling water supply pipeline. Based on the pressure signal from the piping system, the PID controller built into the frequency converter generates an appropriate voltage and frequency to supply to the motor of the circulating water pump, thereby adjusting the motor’s speed and output power, or adjusting the number of circulating water pumps in operation (when multiple pumps are used). This creates a closed-loop feedback system that maintains the pressure of the cooling water supply, improves the startup and operation of the equipment, extends its service life, and reduces power consumption. 3 Selection of water treatment chemicals or technologies 3.1 Increasing the concentration ratio: The water replenishment volume in a circulating water system equals the sum of losses due to evaporation, wind effects, leakage, and wastewater discharge. By increasing the concentration ratio at which the system operates, it is possible to reduce the amount of wastewater discharged, and thus decrease the water replenishment volume, achieving water conservation. However, excessively high concentration ratios will increase the levels of hardness, alkalinity, chloride ions, and other substances in the circulating cooling water, thereby increasing the tendency for scaling and corrosion; this requires enhanced treatment measures to stabilize the quality of the circulating water. This requires effective water treatment methods; while taking environmental protection into account, high-efficiency water treatment chemicals are used to increase the concentration factor. The following water quality stabilization agents have been applied in the circulating water systems of various industrial projects, yielding significant benefits. Sulfuric acid-scale inhibitor treatment: This involves adding sulfuric acid to the water first in order to reduce the alkalinity of the make-up water to a certain level, and then adding scale inhibitors such as polyphosphates and organic scale inhibitors. Thereby achieving the purpose of scale prevention and ensuring the stable operation of circulating water. This method requires little space and has simple technology. However, it should be noted that excessive SO2-4 concentrations can erode concrete. Moreover, treating circulating cooling water with organophosphates will inevitably promote the reproduction of aquatic organisms, thereby increasing the degree of corrosion. Therefore, chemical treatment must take into account multiple effects such as scale inhibition, corrosion prevention, and sterilization; in general, a composite scale inhibitor can be considered. Treatment with weak acid resins: It can reduce the carbonate hardness and corresponding alkalinity in water. The addition of corrosion inhibitors prevents corrosion in the circulating water system, allowing the concentration ratio of the circulating water to be increased without an increase in sulfate ions in the water. This method is suitable for treating water with a high proportion of carbonate hardness. Its advantages include a simple system, favorable operating conditions, high exchange capacity, easy regeneration, and low acid consumption, thereby fundamentally solving the problem of scaling. The disadvantages are high operating costs, large floor space requirements, and high wastewater discharge volumes. Lime softening – acid addition: Appropriate amounts of lime are added during pretreatment to remove Ca2+ and Mg2+ from the water. This method is commonly used in circulating cooling water systems where the calcium content in the water is high and the amount of water to be supplemented is large. Although the carbonate alkalinity of water treated with lime can be reduced, CaCO3 precipitation may occur. To eliminate this instability, a small amount of H2SO4 can be added. The advantage of this method is its high processing capacity and low operating costs. The disadvantages are high investment costs, high requirements for the purity of lime powder, and a significant environmental impact. Reverse osmosis desalination technology: Reverse osmosis is used to soften and desalt circulating cooling water. Its desalination rate is usually around 98%, typically 95%. This treatment method is easy to operate, suitable for automation, and provides good desalination effects, which helps improve the quality of circulating water. The disadvantages are high investment, severe membrane fouling, and frequent cleaning. Ozone water treatment technology: As a water treatment agent, ozone has strong bactericidal capabilities and produces little waste. It helps save water and energy, does not require pH adjustment, and causes no secondary pollution. It also offers excellent effects in terms of corrosion inhibition, scale prevention, and sterilization for recycled water. 3.2 Pipe drag-reducing energy-saving agents: Drag-reducing energy-saving agents are chemical additives used to reduce the resistance to fluid flow and thereby save energy. In recent years, international environmental and energy-saving organizations have launched specialized research projects on drag reduction for energy savings. Countries such as Denmark, the Netherlands, Canada, the United States, Japan, and Singapore have carried out extensive research on surfactant-based drag reduction technologies, achieving significant results; the frictional resistance in pipelines can be reduced by more than 70% in some cases, and certain types of drag-reduction and energy-saving agents have already reached the practical application stage. Basic research on pipeline drag reduction and energy savings has also been carried out in China, along with applied research on drag-reduction and energy-saving agents. To date, three types of drag-reducing and energy-saving formulae have been developed, using cationic surfactants, amphoteric surfactants, and non-ionic surfactants as the main components. When applied in circulating water systems, drag-reducing energy-saving agents not only reduce the investment cost of the piping network but also lower the electricity consumption associated with the daily operation of circulating water pumps. Adding pipeline drag-reducing and energy-saving agents to circulating water systems is a simple and effective way to save energy. Retrofitting existing circulating water systems does not require significant investment in new equipment, offering broad prospects for development as well as substantial economic and social benefits. Note: This article is reproduced from Chemical Engineering Machinery, Volume 39, Issue 07, 2011; author: Xu Sufang, Shanghai Branch of East China Engineering Technology Co., Ltd