Performance evaluation indicators of Roots pumps and the impact of water temperature

{1}

The performance evaluation index of Roots pump based on pumping speed is not accurate enough, as it cannot truly reflect the quality level of Roots pump. It is recommended to use zero flow compression ratio as the performance index to evaluate the pumping capacity of Roots pump.

From the surface, the pumping speed and zero flow compression ratio of Roots pumps are two completely different concepts, seemingly unrelated performance indicators. But in reality, they are not only performance indicators that represent the pumping capacity of Roots pumps, but also closely related to each other. Compared with pumping speed, the zero flow compression ratio can truly reflect the design and manufacturing quality of Roots pumps, and has unparalleled advantages in testing and application. Therefore, we believe that the zero flow compression ratio should be used as an evaluation index for the pumping capacity of LC Roots pumps

Generally, Roots pumps can only work when matched with the front stage pump, so their pumping speed is closely related to the front stage pump. Roots pump, liquid ring pump, rotary vane pump, slide valve pump, piston pump, etc. can form various types of vacuum units, and various ratios can be used according to different needs, so the vacuum units formed have different pressures and pumping speeds. Therefore, the so-called Roots pump pumping speed is actually the pumping speed of the vacuum unit. It largely depends on the type and size of the front stage pump, that is, the pressure and pumping speed of the front stage pump. Although the "Roots Pump Product Quality Classification" standard specifies the model of the front stage pump, due to the standard for the front stage pump (rotary vane pump, slide valve pump), the geometric pumping speed of the pump is (1.1-1.2) nominal pumping speed, so the measurement error of pumping speed alone reaches 2%

The change in ambient temperature will inevitably have an impact on pumping speed measurement, especially when the vacuum degree is high. It indirectly affects pumping speed by affecting the vacuum degree of the preceding pump. This point was elaborated on in the author's previous article "Performance Evaluation Indicators for Roots Pumps - Discussion of Pressure". The "Roots Pump Product Quality Grading" standard specifies the use of the pre stage pump recommended in the "Roots Pump" standard when assessing pumping speed. This is not a big problem for Roots pumps below ZJ-300, but a big problem for Roots pumps above ZJ-600. One of them is the pre stage pump recommended by the standard, which includes both Roots pump and rotary vane (slide valve) pump. It is not necessarily required by the user, but is only designed and manufactured for measuring pumping speed, specifically for a vacuum unit, or even a very complex unit. For example, when testing a ZJ-10000 Roots pump, it is equipped with front stage pumps such as ZJ-2500, ZJ-600, and H-150 pumps. On the other hand, such a matched unit can only test pressure and pumping speed, and cannot measure large allowable pressure differences and zero flow compression ratio curves, because the Roots pump as the front stage pump cannot withstand the pressure differences generated during testing. In this way, in order to test the larger allowable pressure difference and zero flow compression ratio curve, and to equip a front stage pump, another set of units is manufactured, which is less desirable for Roots pump manufacturers

{2} Understanding the impact of water temperature on Roots pumps

Water temperature is an important factor affecting the efficiency of use, but the changes in water temperature during equipment use are also different. Therefore, in order to avoid equipment usage decline caused by water temperature changes, we need to understand the possible effects on vacuum pumps.

1. Excessive water temperature will inevitably cause water to vaporize and evaporate, and the negative pressure working conditions will further exacerbate this vaporization and evaporation, resulting in excessive water vapor generated in the Roots pump. In this way, too much water vapor is inhaled during inhalation, and there is little external inhalation, resulting in a low vacuum degree in the system. There are two main factors that cause high water temperature, one of which is high water supply temperature; The second is that the temperature of water increases during the working process

2. Due to the correlation between water temperature and suction volume, when the working fluid temperature is high, the air volume will decrease. As the temperature increases, the evaporation rate of the work also increases, occupying the original displacement. The suction rate decreases, which will prolong the vacuum pumping time. In addition, under vacuum conditions, the evaporation rate of the sealing liquid will also increase. With the increase of temperature, the percentage of occupied displacement increases, resulting in the inability to evacuate the vacuum. As for the involvement in the data, there are too many differences between the evaporation of sealing liquid at different vacuum degrees and temperatures, and specific data cannot be provided. Only under a certain vacuum degree can temperature changes be reflected and cause changes in evaporation. When using, it is necessary to combine the performance curves of temperature and vacuum degree with the performance curve of the Roots pump to see it

In short, during the use of Roots pumps, the equipment often uses water as the medium, so there is a high requirement for water temperature. Therefore, in our use, we must control the water temperature changes of the equipment to avoid changes in the pumping volume caused by water temperature, which will affect the use of the equipment