New Design Concepts and Effect Analysis of Coiled Tubular Closed-Circuit Cooling Towers

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This post was last edited by youngking77 on 2024-4-23 at 09:41. Key points: 1. Several types of traditional coil-type closed towers; 2. Problems with traditional coil-type closed towers; 3. Updates in design concepts; 4. New benefits brought by new coil-type closed towers. 1. Several representative types of traditional coil-type closed towers: 1) Counterflow closed tower; 2) Cross-flow closed tower; 3) Combined-flow closed tower. 2. Problems with traditional coil-type closed towers: 1) Heat exchange and cooling. As we know, the basic principle of a closed-type cooling tower is that it uses an open-type cooling tower to cool water, which is then used for heat exchange with a tubular heat exchanger in order to reduce the temperature of the fluid inside the tubes. A simple and straightforward explanation is that it consists of an open-type cooling tower plus a shell-and-tube heat exchanger. Therefore, any closed-type cooling tower must take into account both the cooling and heat exchange processes. The two processes should be synchronized in a timely manner. In traditional closed-type cooling towers, cooling and heat exchange occur simultaneously, but neither process achieves optimal results. In cross-flow and mixed-flow systems, approximately 40% of the cool air enters the coil area to undergo heat exchange with the coils; however, due to the poor efficiency of heat exchange between air and water, this results in a waste of air volume, and the desired outcomes are not achieved. Since 40% of the cool air is diverted to the heat exchange area in the cooling section, sufficient additional air volume must be supplied to ensure proper cooling in that section. To provide this additional air volume, an extra 40% in energy consumption is required, which represents wasted energy. Regarding counterflow, there are two scenarios: one involves the use of packing, where the packing is used for cooling and the coiled tubes facilitate heat exchange; the coiled tubes also serve a partial function of distributing water flow. However, since the material used for the coiled tubes is expensive, using it for this purpose represents a waste of resources. Therefore, when designing, only the required heat exchange area needs to be taken into consideration. However, due to structural constraints, during ventilation it is difficult for the cooling water to completely cover the coils, making it hard to achieve adequate heat exchange. Meanwhile, the coils can increase air resistance and raise static pressure; as a result, with the same volume of air flow, energy consumption increases. This is even more apparent in counter-current closed towers without packing. First of all, it is necessary to ensure the cooling efficiency of the closed tower; the cooling section requires sufficient water dispersion area and air volume. Secondly, the heat exchange section needs enough heat exchange surface area. The materials used for heat exchange in closed towers are usually stainless steel, galvanized carbon steel, or copper. In fact, even the cheapest option, galvanized carbon steel, costs far more than packing. If a sufficient water dispersion area is required, the cost of such a closed tower would be exorbitant. Therefore, manufacturers opt to increase the air volume as much as possible in order to reduce the need for a large water dispersion area. And the cost of increasing air volume is higher energy consumption. In fact, for a counterflow closed tower with 100% efficiency, the heat exchange area of the coil is almost equal to the wetted area of the packing; at such costs, its price is about twice that of a segmented counterflow closed tower used for cooling heat exchange. 2. Review: Overall, the biggest problem with traditional closed towers is their low energy efficiency; there are issues with their design philosophy – the essence of cooling towers is not properly understood, nor are the basic principles of cooling and heat exchange taken into account. 3. Maintenance and cleaning: Another issue with traditional closed towers is freezing and scaling. The design of existing closed cooling towers has a fixed structure, which makes it impossible to address these problems; effective cleaning is not even possible, and when issues arise, the only option is to disassemble the equipment. III. Update of Design Concepts 1. Segmented cooling and heat exchange in cooling towers to maximize efficiency. In fact, it is quite simple to address the issues mentioned above. First, cooling and heat exchange are carried out in separate stages, with each stage making full use of energy efficiency; subsequently, these two stages are combined. The cooling stage is handled using the principles of open-type towers, while the heat exchange stage is addressed through the use of heat exchangers. There are already very mature technologies and methods available for these processes, so there is no difficulty in implementing them, and it is possible to achieve the lowest energy consumption ratio along with the highest efficiency. 2. New designs under new concepts: A. Cross-flow closed tower; B. Counter-current closed tower. 4. New operational benefits of the new designs: 1. The energy consumption of a closed tower is equal to the energy consumption of an open tower plus the energy consumption of the spray pump – this represents the lowest possible energy consumption for a closed tower. 2. Efficiency: Firstly, the cooling section ensures optimal cooling performance without any compromise. Secondly, the heat exchange section follows the design principles of shell-and-tube exchangers, ensuring proper flow rates and temperature differences inside and outside the tubes, so that the cold fluid can fully surround the heat exchange tubes and facilitate efficient heat transfer. In this way, the cooling requirements of a closed system can be met with lower energy consumption and heat exchange area. 3. Additional benefits: By separating cooling and heat exchange processes, we were surprised to find that we were able to resolve the issue of difficult cleaning due to scale buildup on the coils. At the same time, this approach also solved the problem of sudden shutdowns in winter caused by ice formation and cracking of the coils. More importantly, it paves the way for future mechanical cleaning of the coils. The coil is placed above the water collection tray (tank) and below the air intake area. When cleaning is required, the machine is shut down, and a large tank is formed below the air intake area; this also serves as a cleaning device. There is no need to disassemble the coil, as a separate cleaning area and equipment are provided. It is so convenient that maintenance can be carried out at minimal cost; cleaning can be done annually, or even every six months or on an even shorter schedule. There is no persistent scaling on the surface of the coils – just as Buddhism teaches, one should wipe it clean daily to prevent dust from accumulating. Another freezing issue is that during sudden shutdowns in winter, if the fluid inside the coils is water, ice formation leading to tube rupture is a common phenomenon; the new design introduced here can prevent such tube rupture due to freezing. When the system is shut down and the circulating water inside the pipes cannot be drained immediately, water can be poured into the water tray to raise the level and fully submerge the coils; at temperatures above -20°C, this approach ensures that freezing does not occur inside the pipes for 24 hours (this value is currently only empirical). As for the mechanized cleaning of coils, this is a new topic for us, which will be introduced at an appropriate time.