A typical example of principle-based innovation: An overview of innovation cases in liquid ring vacuum pumps

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In 1890, the German Karl Hoffmann invented the principle of the liquid ring pump. In 1904, the German company Siemens obtained a patent for the liquid ring vacuum pump, which marked the beginning of its efforts in developing and producing such pumps through practical technical research. The liquid ring vacuum pump patent obtained by the American Nash Engineering Company in 1914 (Figure 1) marked the beginning of competitive rivalry in terms of technological development as well as market competition between the two leading approaches to liquid ring vacuum pump technology on a global scale. For over 40 years since the introduction of the liquid ring vacuum pump, the market has been dominated by designs featuring circular inlet and exhaust channels (Figure 2). However, when dealing with air containing a large number of water droplets or when replenishing the working fluid, this structure causes continuous impact stress on the blades, as the flow enters perpendicularly to the rapidly rotating blades. In severe cases, this can lead to cracks at the base of the blades as well as pitting. To address such engineering challenges, a more robust design for the impeller is required. Is there then a more appropriate technology that could be applied in the design of liquid ring vacuum pumps? Based on this idea, an engineer at Nash Company invented around 1948 the principle of axial intake and exhaust for liquid ring vacuum pumps – namely the vertebral-type liquid ring vacuum pump (Figure 3). The invention of the vertebral-shaped inlet and outlet channels means that even when the inhaled gas contains more liquid droplets, the liquid that reaches the root of the impeller still moves perpendicularly to the direction of the impeller’s rotation; however, due to the lower linear velocity at the root of the impeller, the stress generated by its impact on the impeller is much smaller. Meanwhile, when a vacuum pump is used to draw in large amounts of saturated steam, based on the general gas state equations and heat transfer principles, by installing nozzles at the suction inlet, it is possible to achieve an evacuation rate higher than that obtained during testing (Figure 4). Installing nozzles at the suction inlet has become one of Nash Company’s most important solution designs based on a cone-shaped design (the actual effect is limited, as the water sprayed in absorbs heat and enters the pump, thereby reducing the pump’s own suction capacity). For liquid ring vacuum pumps, whether they have a vertebral-shaped inlet and outlet design or a disc-shaped one, there is one common feature: in order to give the water ring within the pump chamber greater kinetic energy and a flow velocity distribution that is more suitable for a liquid ring, the eccentrically mounted impeller needs to be designed with a forward curvature. This, however, subjects the impeller to a radial load; although this radial load is smaller compared to that of an impeller with a backward curvature, it still exists. Meanwhile, the pressure difference between the inlet and outlet ports creates another type of load. As a result of this, based on the vertebral-shaped inlet and outlet channels, Nash Company developed another invention – the double-action casing (Figure 5). Obviously, the double-acting casing design results in greater kinetic energy losses of the working fluid; as a result, its isothermal compression efficiency is relatively lower compared to that of conventional liquid ring vacuum pumps. However, in certain specific processes, a design that provides balanced loading in all directions becomes the decisive factor in making a choice. For example, in the chlor-alkali industry, chlorine pumps – namely the well-known Naisch pump mentioned in university textbooks – were the only type of pump available at the beginning of the country’s establishment. Although chlorine pumps from the Soviet Union and Italy were introduced later, they still operated on the principle of a double-acting casing. Since 1833, when Michael Faraday discovered that chlorine could be produced by passing an electric current through an aqueous solution of sodium chloride, namely 2NaCl + 2H2O = 2NaOH + H2↑ + Cl2↑, this discovery has become the most fundamental cornerstone of the modern chlor-alkali industry. In modern chlor-alkali industry, chlorine and hydrogen are produced by electrolyzing saturated brine in an electrolyzer; depending on the electrode polarity, the chlorine gas contains a certain amount of water vapor and salt mist, while the hydrogen gas contains a certain amount of water vapor and alkali mist. In the chlor-alkali industry, this is commonly referred to as wet chlorine and wet hydrogen. For wet chlorine, processes such as cooling, demisting, and drying are required in order to obtain pure chlorine or liquid chlorine that meets standard specifications through subsequent steps. In this process, given the physical and chemical properties of chlorine produced by electrolysis and the working principle of vacuum pumps, rotary vane vacuum pump compressors become an important choice for chlorine transportation processes – this is due to the approximately isothermal compression process and their simple structure. However, there is a major issue with using conventional rotary vane vacuum pump compressors: when concentrated sulfuric acid is used as the working fluid, its very high density results in greater radial loads on the pump without reducing its speed; a double-stage casing can help to counteract these loads. Although, to date, chlorine transfer pumps have become more diverse as a result of technological advancements and Naeff pumps have gradually been phased out, they still played a very crucial role in driving the development of the chlor-alkali industry in China. Although the vertebral-type liquid ring vacuum pump boasts superior principles, the disk-type liquid ring vacuum pump benefits from the absence of such vertebral-shaped contact surfaces, which allows for reduced backflow of compressed gas. This is evident in the ultimate vacuum capabilities of disk-type single-stage impellers and vertebral-type single-stage impellers; although the difference in values is not significant, in certain specialized applications, the disk-type single-stage impeller offers clear advantages. For example, when it comes to vacuuming under the back pressure of power plant condensers, to address this limitation, Nash Company developed a completely new product design: the AT series of two-stage liquid ring vacuum pumps – essentially, two pumps with different pumping capacities connected in series to achieve two stages of compression. In this context, as a representative of disk-type liquid ring vacuum pumps, Siemens has focused its research and development efforts on more profound principle-based improvements. One of the most important inventions in this regard is what is known as the flexible exhaust valve (Figure 6). Compared to the small ball-type over-compression relief techniques commonly used in previous disk-type liquid ring vacuum pumps, the flexible exhaust valve can more effectively reduce over-compression at high vacuum levels, thereby improving its isothermal compression efficiency. By the 1990s, although Siemens was a giant compared to Nash at that time, this seemingly did not diminish Nash’s drive for innovation and competition. During this period, Nash established the fundamental principles and structural advantages of liquid ring vacuum pumps through patents for single-stage dual-compression technology (Figure 7) and GSV technology (Figure 8). Whether using single-stage or dual-stage impellers, this remained the case until Siemens sold its vacuum business... However, no matter how strong the Americans’ spirit of innovation might be, liquid ring vacuum pumps would eventually be overshadowed by the passage of time; the future belongs to screw vacuum pumps... Yet, this American spirit is one that we should learn from and emulate!