CiscoRay Vacuum Technology Insights – The Impact of Different Pre-vacuum Pumps on the Performance of Rotary Vacuum Pumps

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This post was last edited by Catalyzing a Grain on 2018-9-9 at 21:57. The Roots vacuum pump is a vacuum device that utilizes rotors rotating in opposite directions synchronously within its pump chamber to achieve suction and exhaust functions. Due to the gaps existing between the rotating components in the flow path, no lubrication is required there, and there is no mechanical friction; as a result, clean gas transfer can be ensured from intake to exhaust. This excellent property enables rotary vane vacuum pumps to be widely used in industries such as petrochemicals, fine chemicals, pharmaceuticals, and food processing. In actual use, differences in the performance of the preceding vacuum pumps result in varying performance levels of the Roots vacuum pumps, which in turn affects user experience. Moreover, since most vacuum equipment manufacturers in China do not possess this expertise, it leads to differences in prices and performance among different suppliers. This article will focus on describing the differences in performance by using a Roots vacuum pump with an evacuation rate of 150 L/S, along with water ring vacuum pumps of various performances and dry vacuum pumps of various performances. First, let’s take a look at the volumetric efficiency curve of the 150L/S Roots vacuum pump (the volumetric efficiency of rotors with 8 blades, as well as those with 3 blades or multiple blades, or twisted multi-blade rotors, is all lower than that of this curve). As shown in the figure, the vertical numbers 1, 2, 3… on the left represent the compression ratio (exhaust pressure/inlet pressure), while the numbers 0.1, 0.2, 0.3… at the bottom of the graph represent the exhaust pressure, in units of mbar (1 mbar = 100 Pa). The lines at 0.9, 0.8, 0.7 in the graph represent the volumetric efficiency levels, namely 90%, 80%, 70%, etc.; the line △t in the upper right corner represents the temperature rise ； I. Performance comparison of the 150L/S Roots vacuum pump when used with two water ring vacuum pumps of the same type but with different pumping capacities as pre-stage pumps: Assuming the temperature of the working water is 22°C (25°C should be used in the case of a closed-loop design): 1.1 At this temperature, the ultimate vacuum capacity of the water ring vacuum pumps is only 4.7 kPa.a. Considering the erosion damage that occurs during operation, the actual operating pressure must not be lower than 5.5 kPa.a. At this pressure, due to the influence of the water vapor partial pressure, the pumping efficiency of the water ring vacuum pumps is only 72.5% of the theoretical pumping capacity, as shown in the figure. 1.1.1 The nominal pumping rate of the water ring vacuum pumps, taken as 400 m3/h, is used as the value for the pre-stage pumps. At an inhalation pressure of 5.5 kPa.a, the pumping speed of this model of water ring vacuum pump, as shown in the figure, is as follows: By combining this with the performance data from 1.1, we can determine that its actual pumping speed is 360 x 0.725 = 261 m3/h. (In fact, the volume of gas indicated in table 1.1.1 represents only 90% of the actual value, due to an allowable error of -10%). At an operating water temperature of 22°C, this water ring vacuum pump can only function at a pressure of 5.5 kPa.a; under these conditions its maximum actual pumping rate is only 261 m3/h. By referring to the efficiency curve of the rotary vane pump discussed in this article, we can determine how the performance of the rotary vane pump changes under actual operating conditions, as shown in the figure below. As can be seen from the figure, the stable operating pressure for a 150 L/S rotary vane vacuum pump with this type of water ring vacuum pump is 1.83 kPa.a. Its actual pumping rate is limited by a volumetric efficiency of 79%, so the actual pumping rate of the 150 L/S rotary vane vacuum pump is 150 * 0.79 = 118.5 L/S. Based on this, it can be determined that the water ring vacuum pump, operating at 5.5 kPa and a water temperature of 22°C, must have an air handling capacity of at least 196 m3/h. Since the actual pumping rate of the pre-stage pump used under the assumptions outlined in this article is 261 m3/h, the actual compression ratio is only 118.5 * 3.6 / 261 = 1.63. As can be seen from the figure, the adiabatic compression temperature will decrease, which in turn increases the allowable inlet temperature and enhances the stability and reliability of operation of the rotary vane vacuum pump ； It should be noted that this article only assumes a water temperature of 22°C to determine the performance of a rotary vane pump as a vacuum pre-pump for a water ring vacuum pump; however, in actual design, considerations must be taken into account regarding local summer conditions and the actual utility services available, and such temperatures are generally much higher than those assumed in this article!