Measurement and Research Analysis of Water Vapor Tolerance of Volumetric Vacuum Pump

This article discusses the method of solving the condensation of water vapor in the pump in the work of the volume vacuum pump, such as the heat pump technology, the combined structure of the slide valve water ring pump and the gas ball principle. A more complete and accurate definition of water vapor tolerance and water vapor extraction relative to international standards is presented. The research and development of water vapor tolerance measurement methods, the measurement principle, measurement procedures, and measurement evaluation of water vapor tolerance are discussed. .

In many occasions, the volume of gas pumped by the volumetric vacuum pump contains more or less water vapor, and there are still many water vapors under some conditions. If nothing is done, the water vapor will be condensed and mixed with the pump oil before being compressed in the pump before it is opened. Evaporation in the suction chamber will not only seriously affect the performance of the pump, such as vacuum and pumping. It may also cause corrosion of the pump.

1, to solve the steam condensation method

There are several ways to avoid condensation of water vapor in the pump when evacuating the gas containing water vapor. One method is to use a heat pump structure, that is, to set an oil heater in the oil tank to keep the pump working at 100 At a high temperature of -105°C, it is characterized by its ability to work under any inlet pressure without condensation in the pump, and is very effective. Its deficiency is the need to use high temperature pump oil, and there will be a local overheating area around the heater, so the oil pump must be used to quickly pass the pump, otherwise even the high temperature pump oil will produce local coking time. Long will seriously affect the normal operation of the pump. In addition, high-temperature oils work under high temperature for a long time, which accelerates the oxidative cracking of oils and requires regular oil changes.

Another method is to use a steam treatment system, which is usually composed of a slide valve pump (rotary vane pump) and a water ring pump. The water ring pump causes the oil tank above the slide valve pump to be in a vacuum state, and its pressure is lower than the pump temperature. The saturated vapor pressure of the water, so that the water vapor will not condense in the pump and the tank, the use effect is very good. Steam treatment system has a maximum allowable inlet pressure, which is related to the pump temperature, the higher the pump temperature, the higher the allowable inlet pressure; also related to the ratio of the spool valve pump (rotary vane pump) and water ring pump, the ratio is small Allow high inlet pressure. However, this system must solve the problem of oil in the slide valve pump (rotary vane pump). Because its oil tank is in a vacuum state, the suction vacuum of the selected oil pump must be higher than the vacuum level in the oil tank. According to our experience, commonly used internal meshing cycloidal oil pumps, oil ring pumps and vortex oil pumps cannot meet the above requirements. Only external gear oil pump, its suction vacuum up to 3 × 10 ^ 3Pa, to meet the needs.

The most common and simplest method is to use a gas ballast, which will introduce quantitative non-condensable gas into the pump's compression chamber through the gas ballast passage, so that the water vapour will be discharged out of the pump before being condensed into water, and the gas ballast channel can be used as a valve. control. The gas volume is the pumping speed (5~10)%, the double pump takes a small value, and the single pump takes a large value. This article discusses and studies the measurement method for the ability of the gas town pump to remove water vapor.

2, the definition of water vapor tolerance

Gas ballast is defined as a method of introducing an appropriate amount of non-condensable gas (town gas) into the compression chamber to prevent or reduce the generation of condensate in the compression chamber. There are two methods to measure the capacity of a gas ballast pump to remove water vapor. One is to measure the water vapor tolerance, the other is to measure the amount of water vapor extracted. The most convenient is to measure the water vapor tolerance, and the amount of water vapor extraction can be measured. The resulting water vapor tolerance is calculated.

In the DIN 28426-1:1983 Acceptance Rules for Rotary Vacuum Pumps, the water vapor tolerance is called the water vapor permissible pressure. It is defined as: the gas ballast vacuum pump can run continuously under normal environmental conditions (20°C and 1013 mbar). The highest inlet pressure for draining steam. In ISO 21360-2:2012 "Standard Practice for Vacuum Pump Performance Measurements Part 2: Volumetric Vacuum Pumps", water vapor tolerance is defined as the maximum value of water vapor pressure that can be transmitted by a vacuum pump without condensation in the vacuum pump.

We believe that these two definitions are not complete. DIN 28426-1:1983 does not stipulate that “condensation does not occur in the pump during operation.” If condensation occurs in the pump during the test, the measurement completely loses its meaning. In ISO 21360-2:2012, there is a lack of "standard environmental conditions" in which atmospheric pressure affects the discharge pressure of the pump. If the ambient temperature is too low, the water vapor tolerance will be too low, or even cause two The curve (corrected exhaust temperature~inlet pressure curve and exhaust saturation temperature~inlet pressure curve) does not intersect, so that the water vapor tolerance cannot be determined; if the ambient temperature is too high, the water vapor tolerance will be high and the actual operating conditions will be disengaged. .

We believe that the definition of complete and exact water vapor tolerance should be: the maximum inlet pressure of the water vapor that can be pumped out without condensation in the pump during continuous operation under standard environmental conditions (293 K, 101325 Pa). Similarly, we have also given a new definition for the amount of water vapor extracted: under the standard environmental conditions (293 K, 101325 Pa), the maximum steam volume that can be extracted per unit time without condensation in the pump during continuous operation.

3, research progress of water vapor tolerance measurement method

Water vapor tolerance is a very important performance indicator of a volumetric vacuum pump, and how to measure the pump's ability to extract water vapor is a very critical issue. PNEUROP had a measurement method for water vapor tolerance as early as the 1967 version of the “Vacuum Pump Acceptance Rules Part 1 Variable Capacity Vacuum Pump”, which was corrected when reprinted in 1979.

In 1976, the German DIN standard vacuum technology professional standards committee revised the “steam extraction quantity” formula and published it as a German standard (DIN 28426-1:1976), which was revised in 1983.

ISO 21360-2:2012 makes a major modification to the “measurement of water vapor pressure allowed” method in DIN 28426-1:1983. It was originally determined by the intersection of the exhaust gas temperature-inlet pressure curve and the B/S curve cluster.” The water vapor allowable pressure" is changed to determine the intersection of the exhaust gas temperature-inlet pressure curve and the exhaust saturation temperature-inlet pressure curve, and the name is also changed to "water vapor tolerance".

Our industry has not paid enough attention to this issue, and only in the appendix of “JB/T 6533—2005 Rotary Vane Vacuum Pump” is attached the “Maximum Allowable Measurement of Water Vapor Inlet Pressure and Water Vapor Allowance”. It is evaluated as a main performance indicator of a pump. Most of the volumetric vacuum pumps in foreign countries have “water vapor tolerance” or “water vapor extraction volume” in the performance data, and almost no such data is found in the volumetric vacuum pumps in our industry. Even if there are only foreign data, it has not been performed. Test and assessment.

When we came into contact with the pharmaceutical industry and the power capacitor industry, we have encountered the problem of pumping vapour from the rotary valve pump. Therefore, as early as 1966, we conducted tentative research on the ability of the volumetric vacuum pump to remove water vapour, as there was no data for this. References, and no experience can be used for reference, so the experiment was very difficult, but also found some problems worth exploring and research. From 1988 to 1990, we studied the water vapor tolerance measurement of 2H-30 and H-150 slide valve pumps in accordance with DIN 28426-1:1983, and conducted trial and error tests under various environmental conditions. Some experience, but also found some problems that must be noted in the implementation. Now ISO 21360-2:2012 has made major changes to the measurement method of water vapor tolerance. When we read the standard text, we found that there are still some areas where the standard text needs to be confirmed, and it is necessary to further implement the standards while In-depth research and discussion, while formulating China's volume vacuum pump "water vapor tolerance measurement" standard.

4, water vapor tolerance measurement principle

Water vapor tolerance, such as direct measurement with water vapor, can be complicated. In 1966 when we did a pure water vapor pumping test for a 70L/s slide valve pump, due to the limited conditions, this test was done for the first time without a test method (standard). Therefore, the test plan and concept were not considered well. The understanding of the gas town was not sufficient, and slight condensation occurred in the pump. However, it did not attract enough attention. Therefore, from the current point of view, the test data obtained at that time had certain deviations.

Later, we did a pure water vapor pump test on a 150 L/s slide valve pump. We also found some problems. One is the problem of steam generators. For large and medium-sized pumps, the evaporation area is large and the heating power is also high. To be large enough to fully meet the needs of the amount of vapor; Second, evaporation (water vapor extraction) can only be calculated from the evaporator water level changes, not easy to measure accurately; Third, may cause difficulties in the pump can not be noticed Condensation, which is noticeable by people, affects the measurement of water vapor removal and water vapor tolerance.

From the above tests, it can be seen that direct measurement of water vapor can make the measurement process extremely complicated. Many factors affect the measurement process. A slight condensation of water vapor in the pump is often difficult to detect. When it is found that there is water in the pump oil, the pump The condensation of internal water vapor is already very serious, so the current international measurement of the water vapor tolerance (standard) uses air instead of water vapor, which is then corrected by conversion.

Gas ballast pumping water vapor consumes relatively high power and also causes the pump temperature to rise, which will result in a higher saturated vapor pressure, which can maintain a higher water vapor tolerance. The temperature rise caused by the considerable air flow introduced into the pump, which is related to the inlet pressure, can be used to determine the saturation temperature of the water vapor. Because there is no direct heat exchange with the environment during compression in the pump, it can be considered as adiabatic compression. Triatomic vapor (k=1.3333) and diatomic air molecules (k=1.4) have different adiabatic indices. From the formula of compressive energy, it can be seen that they have different compression work, so they must be replaced by the use of air. The temperature rise caused by water vapor is corrected, that is, the temperature of the exhaust gas is corrected. The pump exhaust temperature measurement also depends on the inlet pressure.

5, measuring device

1. Test cover; 2. Flow meter (measure gas ballast flow B); 3. Test pump; 4. Intake valve; 5. Hygrometer (measure relative humidity %); 6. Vacuum gauge (measurement inlet pressure p1); 7. Thermometer (measurement of exhaust temperature T2); 8. Pressure gauge (measurement of exhaust pressure); 9. Barometer (measure atmospheric pressure)

6, measurement evaluation

Correct the exhaust gas temperature ~ inlet pressure (T2cr~p1) curve and exhaust gas saturation temperature ~ inlet pressure (T2s~p1) curve on an ordinate (linear coordinate) for temperature and abscissa (logarithmic coordinates) In the graph of pressure, the abscissa of the intersection of the T2cr~p1 curve and the T2s~p1 line is the water vapor tolerance.

7. Concluding remarks

There are several points in ISO/FDIS 21360-2:2012 that need to be corrected. First, the formula pa=φH2O100pS (T1) is incorrect. In the description of DIN 28426-1:1983 on water vapor partial pressure, it is indicated that the partial pressure of water vapour at 20°C and 58% relative humidity pa=13.33 mbar. Based on the above data, the above formula is incorrect. , should be corrected to pa = φH2 O pS (T1), according to our customary writing method should be pa = φpS (φ in %). Secondly, in the exhaust temperature correction factor Wcr in Table 1, p0/p1 should be modified to αp0/p1 because it can be seen from the energy formula (2) that when calculating the exhaust temperature correction factor Wcr, it is inconsistent. . Thirdly, in ISO21360-2:2012, the pressure difference between the discharge pressure p2 and the atmospheric pressure “should be ±1 kPa” is wrong, and the pressure difference directly reflects the size of the exhaust pipe resistance, so it should be revised as “ Should be less than 1 kPa”. For the measurement of water vapor tolerance, we just did some work earlier, made some explorations, and got some experience. We hereby put forward research and discussion with our colleagues concerned about this work.

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