With the development and progress of the national economy and science and technology, my country's thermal power units have evolved from mainly 300MW per unit before the 1980s to mainly 600MW currently. The water spray area of ​​the supporting cooling systems, i.e., cooling towers, has also increased to 7000-9000m2 (as shown in the Handan Feng Power Plant). The emergence of such large cooling towers has presented new requirements and challenges to traditional mechanized construction technology, and has also provided unprecedented opportunities for innovation and development space for improving the level of construction mechanization in my country. The following is a brief introduction to some typical construction methods that have emerged in different historical periods, to understand the evolution of vertical transportation methods in mechanized construction of cooling towers, and to summarize the future development direction of vertical transportation methods in this field. [b]I. Traditional Scaffolding Methods[/b] In the 1950s and 1960s, limited by the then-backward technical equipment conditions, my country generally adopted traditional internal and external full-span scaffolding and the erection of wooden formwork for the overall construction of small cooling towers. This process has the following drawbacks: inconvenient dismantling and installation of scaffolding templates, high cost of scaffolding erection, high labor intensity, large amount of high-altitude work, and difficulty in handling the holes left on the cylinder wall for the connecting rods of the internal and external scaffolding. Currently, it is only used as an auxiliary method in some parts of the cooling tower construction, such as the A-beam, the ring column, and other equipment attachment systems. In recent years, the hoisting of large precast components such as A-beams and ring columns has usually been completed by crawler cranes. [b]II. Combination Method of Multi-Row Hole Scaffolding and Steel Pipe Vertical Shaft[/b] In the late 1970s, domestic construction units adopted a combination scheme of 120-meter-high ordinary steel fastener vertical shaft scaffolding system and multi-row hole scaffolding trestle bridge in the construction of cooling towers No. 3 and No. 4 of Ningxia Daba Power Plant, which solved the problem of vertical transportation in the construction of large-height cooling towers at that time. The key technology of this scheme is to verify its stability through load calculations based on the mechanical properties of the initially selected vertical shaft frame cross-section, ropes, and combined cross-sections, such as the moment of inertia, bending stiffness, radius of gyration, and structural reduction coefficient of the vertical shaft frame. Its features (as shown in the figure) include: the shaft frame structure has 16 main holes, 20 reinforcing holes, and 51 nodes formed by the uprights and horizontal bars on each layer, with 2 fasteners at each node, totaling 102 fasteners. The guy ropes are installed starting at 20 meters above the ground, with 8 layers of guy ropes at heights of 35 meters, 63 meters, 76 meters, 89 meters, 102 meters, and 120 meters. The trestle bridge has 6 layers of guy ropes starting at 35 meters above the ground. For the guy rope anchoring nodes, a cross brace is added between the four corner uprights, and the rope is anchored to four uprights. The advantages of this type of system are its economy, practicality, safety, reliability, and ease of construction; its disadvantages are that strict joint calculations, structural calculations, and testing are required to ensure the overall construction system is reasonably designed and safe to use; the manufacturing and installation processes are complex, the construction speed is slow, and the labor intensity of construction personnel is high during use. The theoretical verification of this ultra-high scaffolding construction system of over 120 meters represents a breakthrough in the use height of traditional ordinary steel pipe scaffolding systems, and has certain promotional value, becoming a choice for the construction of some small and medium-sized cooling towers in the 1980s. [b]III. Combination Method of Prefabricated Suspension Bridge and Coupler-type Steel Pipe Vertical Shaft[/b] Since the 1960s, Chinese construction units have begun to manufacture prefabricated vertical suspension horizontal suspension bridges for the construction of small cooling towers. In 2000, Liaoning Electric Power Construction Company adopted the traditional prefabricated suspension bridge combined with a metal vertical shaft frame and construction elevator for vertical transportation in the construction of a 3,500-square-meter cooling tower in the second phase of the Fushun Power Plant project. The 3,500-square-meter tower is a cast-in-place reinforced concrete hyperbolic natural draft cooling tower, 90 meters high, with a ring base center radius of 37.497 meters, a throat elevation of +70 meters, and a foundation depth of -4.5 meters. The A-frame columns and water distribution system are prefabricated reinforced concrete structures. To meet the needs of the tower wall construction, Liaoning Electric Power Construction Company, according to the construction organization design requirements, constructed a 135-meter-high, 12-span metal vertical shaft on the northeast side of the cooling tower, equipped with a prefabricated suspension bridge and an external construction hoist (as shown in the above figure), serving as the vertical transportation system and horizontal passage for the cooling tower construction. This vertical shaft has six guy ropes and a 12-span base, with reinforcing rings at the top, bottom, and each layer of guy ropes. The prefabricated suspension bridge has a main bridge length of 20 meters, a secondary bridge length of 8 meters, a self-weight of 15 tons, a design construction load of 6 tons, and a total weight of 21 tons (as shown in the above figure). The SC200/200 ordinary construction hoist is attached to the outside of the vertical shaft frame, with an attachment frame installed every 6 meters. The hoist itself is attached to a 135-meter-high shaft frame composed of 90 standard sections. This elevator has a rated weight of 2.0 tons, a capacity of 18 people, and a rated lifting speed of 34 m/min. This system further reduces the workload of traditional scaffolding and improves construction speed; however, the manufacturing and installation processes are relatively complex. In the construction of large cooling towers, technical limitations increase safety risks and equipment investment. The combination of prefabricated suspension bridges and fastener-type steel pipe vertical shaft frames was a typical vertical transportation method for cooling tower construction in the 1970s and 1980s. Currently, this method has been used to complete the construction of more than 40 small and medium-sized cooling towers. IV. The combination method of using tower cranes as the main body supplemented by traditional construction elevators can be divided into two forms depending on the erection position of the main working machinery—the tower crane. 4.1 The method of using an internal fixed tower crane: In 1998, during the construction of Cooling Tower No. 3 (water spray area 2000m2, tower top elevation 71m) at Daqing Petrochemical Thermal Power Plant, the first successful application of a vertical transportation scheme combining a QTZ25 small fixed tower crane (30m boom length, 80m lifting height) with a traditional construction elevator (as shown in the right and lower figures) was achieved. The key technical aspects of this scheme were: ① determining the attachment scheme of the tower crane inside the cooling tower; ② dismantling and transporting the tower crane after the completion of the tower wall. The eccentrically positioned tower crane employed a flexible attachment method using pre-adjustment with hand-operated hoists, solving the problem of vertical stability of the tower body after exceeding its independent height; the attachment structure used a lightweight truss, ignoring the impact of flexible attachment on the self-weight of the tower crane's wire ropes, reducing the amount of high-altitude work and improving the operational safety factor. The tower crane dismantling and transportation scheme utilizes two pre-fabricated and overload-tested lifting rods and hoisting ropes installed on the tower arm. Through a pre-designed workflow and steps, the tower crane boom smoothly passes through the throat of the cooling duct during construction and is dismantled upon completion (this process is shown in Figures 1, 2, 3, and 4, and will not be described in detail). While the assembly and dismantling costs of this small tower crane scheme are slightly higher, in practical experience with multiple small and medium-sized cooling tower construction projects, the machinery costs are only one-third of those using a medium-sized tower crane scheme, resulting in excellent overall benefits. For example, the No. 1 cooling tower (2500m²) of Jiamusi Thermal Power Company and the No. 1 cooling tower (3500m²) of Mudanjiang No. 2 Power Plant both used the internal tower crane scheme to complete the cooling duct pouring with excellent results. However, for large cooling tower construction, due to the even higher machinery costs and the heavy workload of personnel and material transportation, this scheme proves inadequate. The advantages of combining an internal fixed tower crane with a construction elevator for vertical transportation are high reusability of the machinery, short assembly and disassembly cycles, and wide applicability. This can improve the overall efficiency of enterprises in the construction of small and medium-sized cooling towers. 4.2 External Self-Erecting Tower Crane Method: In 2002, Zhenjiang Fourth Construction successfully implemented the external tower crane method for the first time in China during the construction of a 75-meter-high, 2200-square-meter hyperbolic cooling tower at the Jiangjiang Fertilizer Plant. This method was subsequently successfully applied again in the construction of a 60-meter-high, 1200-square-meter cooling tower at the Shandong Huajin Power Plant Phase III (as shown in the right figure). In the same year, Beijing Electric Power Construction successfully used this method in conjunction with high-altitude work equipment during the construction of a 2500-square-meter cooling tower (75m tower height, 67.7m outer diameter of the ring base, 33.4m throat diameter, and 35.8m head diameter) at the Jinjie Thermal Power Plant Phase II (as shown in the right figure). In the above-mentioned small cooling tower construction, this method represents a breakthrough in the construction of an external tower crane for vertical transportation, departing from the traditional method of using large, multi-hole derricks. This technology is innovative in my country's cooling tower construction methods and is suitable for small cooling towers with lower mechanized construction costs. [b]V. Combination Method of Curved Elevator and Folding Arm Tower Crane[/b] In the 1990s, due to site and technical limitations, the traditional vertical shaft scaffolding bridge method was no longer usable in the construction of large tower walls. To overcome the key technical challenges of vertical transportation construction for the two 150m high, 9000m² water-spraying towers (Asia's largest hyperbolic cooling towers) at the Handan Power Plant, the Building Mechanization Research Branch of the China Academy of Building Research pioneered the use of a large self-erecting folding arm tower crane (folding arm crane) and the Kaibo SCQ60 curved construction elevator in 1997, achieving success and forming an innovative construction method. This scheme deploys a QTZ240 self-erecting folding arm tower crane inside the tower for vertical transportation of steel bars, concrete, and various materials. An SCQ60 VVVF variable frequency curved elevator is installed outside the tower specifically for personnel transportation; the traditional tripod formwork construction method was used for the construction of the tower wall, successfully completing the wall construction. The arrangement of folding boom cranes and curved elevators: The Kaibo SCQ60 curved elevator, developed by the Building Mechanization Research Branch of the China Academy of Building Research, can operate vertically by attaching to the curved surface of the cylinder wall, with a load capacity of 600-1200 kg and a capacity of 8-12 people. During the construction of Tower #2, the research institute applied the most advanced frequency conversion technology to the curved elevator for the first time, increasing the operating speed from 28 m/min to 40 m/min. This reduced the personnel transportation time during peak construction periods from 2 hours and 40 minutes to 1 hour, significantly improving operational efficiency. The advantages of this technical solution are as follows: using a folding boom crane for concrete pouring requires only 18 people per shift, while using a derrick requires approximately 40 people per shift. This significantly reduces the personnel input for material transportation compared to the traditional derrick system, allowing various materials to be directly hoisted to any location, reducing the labor intensity of workers. The folding boom crane has a fast assembly speed, a large lifting capacity (3.5t including the hook), and a large working coverage area, which can significantly accelerate vertical transportation speed; the curved elevator has opened up a dedicated channel for construction personnel to quickly move up and down, resulting in high traffic efficiency. Disadvantages: Due to the obstruction of the eccentric fixed position of the folding boom crane, the construction accuracy control of a portion of the cylinder wall radius is relatively complex; the concrete pouring speed of the folding boom crane hopper is generally 5-6 m³/h, with a maximum of 10 m³/h, which is slower than the concrete pouring speed of the derrick construction; the equipment utilization rate needs to be further improved, and the investment is relatively large. After 1997, domestic units such as Tianjin Electric Power Construction began to promote the folding boom tower crane and curved elevator technology system to improve cooling tower construction in projects such as Jixian Power Plant and Wujing Power Plant, achieving good results. After 2000, Beijing Electric Power Construction adopted the built-in lifting derrick in the construction of the No. 1 cooling water tower project of Datang Tuoketuo Power Plant to replace the folding boom crane, which reduced the construction equipment cost. This construction technology is suitable for the construction of cooling towers of 5,000-9,000 square meters. Currently, eight cooling towers with a water distribution area of ​​over 5,000 m² have been built in my country using this technology, including two cooling towers with a water distribution area of ​​9,000 m². Currently, there are about five large power construction units in China capable of applying this method. This technical solution conforms to the trend of innovation and development in the era of construction mechanization, transforming traditional metal derricks into auxiliary transportation machinery, greatly reducing labor input and intensity, marking a new stage in the mechanized construction of large cooling towers in my country. [b]VI. Multi-machine combination method with Kaibo multi-functional construction hoist as the main body[/b] In 1999, the multi-machine combination method with Kaibo SC200/200 multi-functional hoist as the main body, developed by the China Academy of Building Research, successfully completed the simultaneous transportation of concrete, steel bars and personnel for the first time in the construction of the hyperbolic cooling tower at Tianjin Power Construction Panshan Power Plant, which was a pioneering achievement both domestically and internationally; it became an innovative method to solve the vertical transportation problem in mechanized construction of power plants in my country after the curved elevator and folding arm tower crane methods. This combination method can be divided into the following two types according to the number of main machinery. 6.1 The combined operation method of a multi-functional hoist and a self-erecting tower crane was used in the construction of a 9200m² large cooling tower at the Dezhou Power Plant Phase III in 2000. The hoist and tower crane complemented each other in the concrete pouring process, accelerating the handover of processes and the speed of concrete pouring, thus ensuring the overall construction period. The combination of the self-erecting tower crane and the Kaibo SC200 multi-functional construction hoist technology ultimately formed a mature new method of multi-machine combined construction. The multi-functional hoist can be easily installed by simply connecting it to a scaffolding network. Based on its use in the completed Panshan Power Plant No. 1 cooling tower, the operation of the hoist does not affect the entire scaffolding network or the tower wall. This attachment method makes disassembly of the hoist more convenient than direct attachment to the building. This type of construction hoist is simple to operate, safe, and reliable, increasing construction speed by more than three times compared to the previous method of using a folding boom tower crane for material loading. Practice has proven that this method requires less equipment investment, is simple to manage, and is well-suited for the construction of large and medium-sized cooling towers. 6.2 The Dual-Machine Combined Operation Method Using Multifunctional Construction Hoists: In the construction of some small and medium-sized cooling towers, the scaffolding network erection process is relatively simple, and the existing scaffolding system can fully meet the requirements of the construction hoist attachment and platform material unloading functions. Therefore, generally only two multifunctional construction hoists need to be installed at both ends of the cooling tower diameter for combined operation. The two machines can independently complete the vertical transportation of personnel, concrete, steel bars, and small tools required for cooling tower construction, resulting in faster construction speed and significant savings in equipment investment (as shown in the right figure). This combined construction method is highly efficient, with equipment investment only one-quarter of that of the articulated tower crane solution. It can be asserted that with the increasing demands for construction mechanization, the multi-machine combined method with multifunctional construction hoists as the main body will inevitably become one of the mainstream choices for vertical transportation in the mechanized construction of large and medium-sized cooling towers; the disadvantages are that the scaffolding needs to be professionally designed and calculated, the workload of erecting scaffolding is large, and the erection process requires strict standards. Through the above brief introduction, it is easy to see that the mechanized vertical transportation methods for cooling tower construction have gone through the following stages: the era of traditional scaffolding – the era of metal vertical shaft frames – the era of traditional suspension bridges – the era of tower cranes – the era of multi-functional construction hoists. Although these methods have seen the emergence of mechanized construction methods based on advanced equipment such as tower cranes and multi-functional construction hoists, improving the level of construction mechanization, they still require the erection of a certain amount of auxiliary scaffolding. The safety risks associated with scaffolding erection and dismantling and working at heights still exist. Developing a safer, more reliable, efficient, and practical integrated solution by combining the strengths and weaknesses of these methods has become the development direction for technological innovation in the mechanized vertical transportation of cooling tower construction. Future Prospects: With the continuous expansion of the power market and the continuous development of mechatronics technology in the lifting field, the comprehensive innovation ideas in the current field of construction mechanization research have extended to the field of mechanized cooling tower construction. Developing specialized vertical transportation equipment for cooling tower construction with system functions and modular integration features, which is urgently needed by power construction units, has become a reality. The China Academy of Building Research's Building Mechanization Branch has pioneered an integrated solution for vertical transportation in mechanized cooling tower construction. This solution is a new type of specialized vertical transportation equipment for cooling tower construction, integrating the functions of a multi-purpose elevator, tower crane, and suspension bridge. It has been successfully applied in the construction of the Zhoukou Power Plant in Henan Province. The equipment has a maximum operating height of 150m; the tower crane's maximum working radius can reach 20m; it provides attachment for the multi-functional elevator and a platform for the storage and horizontal transportation of materials such as steel bars and concrete during construction, eliminating the need for scaffolding or other auxiliary support systems. The main features of this new equipment are as follows: ① Good system rigidity, maintaining system balance even with changes in the cooling tower radius, safe and reliable lifting, and convenient use; ② The working radius can be adjusted according to the construction progress; ③ A small lower slewing tower crane is installed at the top for convenient lifting of steel bars and small building materials; ④ Sensitive and reliable torque limiters and alarm devices are installed in various parts to ensure safe and reliable operation; ⑤ The system height can be adjusted according to the construction location and progress of the cooling tower, ensuring the working surface is level with the construction surface. This integrated solution for vertical transportation construction significantly reduces investment in construction machinery and scaffolding, while incorporating the advantages of previous methods. It is highly efficient, safe, and easy to install and dismantle. The method is technologically portable, not limited to mechanized construction of cooling towers, but also easily applicable to the mechanized construction of other irregularly shaped buildings, representing the future direction of mechanized vertical transportation construction of cooling towers towards specialization, modularization, and integration. [b]Author Biographies:[/b] Li Shulin (1958- ) Male, Researcher, from Bayan County, Heilongjiang Province, Director of Beijing Institute of Building Mechanization. Wang Ping (1972- ) Male, Engineer, from Jing County, Hebei Province, Deputy Director of the Science and Technology Development Office of the Institute of Building Mechanization. Contact Address: No. 61, Jinguang Road, Langfang City, Hebei Province, 065000 Tel: 0316-2015684 [b]References:[/b] ﹝1﹞Zhao Zhijin. Construction Handbook [M] Beijing: China Building Industry Press, 1997, Third Edition. ﹝2﹞Li Delong. Improvement and Discussion of Construction Schemes for Large Cooling Towers. Materials from the Fifth Annual Meeting of the Power Industry Committee. ﹝3﹞Ma Xiuwen, Sun Yuqin. Attachment and Dismantling of Tower Cranes in the Construction of Hyperbolic Cooling Towers. Heilongjiang State-owned Farm Construction Corporation No. 1. ﹝4﹞Zhang Huawen, Liang Shuzhong. Materials from the 5th Annual Meeting of the Power Industry Committee on Construction Technology of the 9000m² Cooling Tower at Hanfeng Power Plant [5]; Investigation Report on the "9.19" Major Accident at the 6th Phase of Qinghai Qiaotou Power Plant; Compilation of Materials from the Qinghai Provincial Construction Commission [6]; National Construction Industry 2000 New Technology Application Demonstration Project Materials Album; Construction Industry Management Department of the Ministry of Construction.