Roots vacuum pump (mechanical booster pump) for higher vacuum can not be directly into the atmosphere, such as the straight row atmosphere will cause Roots vacuum pump inlet and exhaust pressure is too large, so Roots vacuum pump overload, such as simple Increasing the Roots vacuum pump motor power will in turn cause the Roots vacuum pump to overheat so that the small gap between the Roots vacuum pump rotors quickly becomes stuck due to thermal expansion. In order to ensure Roots vacuum pump can achieve a higher vacuum Roots vacuum pump rotor clearance must be guaranteed. Therefore, Roots vacuum pump must be used before the stage pump, pump with the former stage to pump pressure within the system to a certain range and then start Roots vacuum pump, Roots vacuum pump so to avoid overloading. Pre-stage pump can be used water (liquid) ring vacuum pump, rotary vane vacuum pump, slide valve vacuum pump, reciprocating vacuum pump can be directly to the atmosphere of the vacuum pump.
Roots vacuum unit under normal circumstances, the choice of water ring pump as the fore-pump than other vacuum pumps is more favorable, mainly because it can extract a large amount of condensable steam, especially when the gas-sealed mechanical vacuum pump can be excluded condensable Inadequate steam capacity, or use of solvents that can deteriorate the pump oil and affect performance, or when the vacuum system does not allow oil contamination becomes even more pronounced.
Roots pump - water ring pump unit is widely used in chemical industry, food sublimation drying, high-altitude simulation test vacuum system. Such joint unit, there are roughly the following types.
(1) Roots pump - water ring pump: The role of the water ring pump in the unit is to create the required vacuum for the Roots pump, thus requiring the maximum allowable discharge pressure of the water ring pump, that is, on the one hand, Increase the water pump ultimate vacuum, on the other hand, we must try to improve the maximum allowable exhaust pressure Roots pump.
In general, the single-stage water ring pump ultimate vacuum is not high, but at present China's production of the Roots pump requires pre-vacuum is higher, so in fact not a single-stage water ring pump as the Roots pump pump, and use The double-stage water ring pump, which can increase the ultimate vacuum, can be used as a fore-stage pump. The double-stage water ring pump can also increase the ultimate vacuum of the unit.
The ultimate vacuum of a Roots pump is low, especially when it is operated in combination with a water ring pump. The range of use is limited and the ultimate vacuum of the entire unit may be lower. However, if two Roots pumps are used in series Combined with the water ring pump, you can greatly increase the unit's ultimate vacuum.
Therefore, it is common to see in this type that two Roots pumps are connected in series and then use a two-stage water ring pump for the foreline pump to form a unit.
(2) Roots pump - water ring pump - air pump unit: even with a two-stage water ring pump, the ultimate vacuum increase is only within a certain range, which is limited by the saturated vapor pressure of water. The theoretical ultimate pressure of water ring pump is the saturated vapor pressure of water. Considering the influence of gas backflow and other factors, in fact the ultimate pressure of the water ring pump is significantly higher than the saturated steam pressure at the water temperature. In order to improve the ultimate vacuum level pump, you can also make the water ring pump and atmospheric pump combination.
In this way, the series vacuum pump ultimate vacuum of up to 20 ~ 30Torr, if the water ring pump and the two stage air pump combination, the ultimate vacuum up to 2 ~ 10Torr.
(3) Roots pump - water ring pump parallel mechanical vacuum pump: This unit is mainly used for vacuum treatment system that needs to handle a large amount of water vapor, and the ultimate vacuum requirements are high, for example, in vacuum drying.
In a vacuum system that requires a large amount of water vapor, a water ring pump is suitable, but due to its low ultimate vacuum, the ultimate vacuum of the entire unit is low. Although vacuum systems requiring a higher degree of vacuum require a mechanical vacuum pump with a higher ultimate vacuum to be used as a foreline pump. However, due to water ring pump power consumption, low efficiency, high noise, in a long time vacuum drying system, the water ring pump as the Roots pump is not economical.
In these circumstances, the gas-town mechanical vacuum pump can be connected in parallel with the water ring pump, as the Roots pump pump. Vacuum drying, the first water pump pre-pumping, until a substantial reduction in water vapor, and then open the gas mechanical town vacuum pump, cut off the water ring pump. If you need more time to complete the drying occasion, the required cooling water and power are less, as shown in Figure 3.
Roots pump - water ring pump unit operation
1) The front of the unit condenser
In order to make the unit as small as possible, it is possible to try to condense the steam to be drawn before it enters the pump unit, so that the non-condensable gas and trace residual steam remain. The volume of the gas decreases at the same pressure after cooling. Therefore, the amount of air required after condensing is reduced, and accordingly, the pump can be selected to be smaller.
Which way is more economical? It should be as the case may be, for example as follows:
There are two ways to condense steam: one is to install a cooling device and the other is to install a condenser in the high pressure stage of the unit so that it can be cooled with normal water.
The system requires 50kg of water vapor per hour, with a volume flow of 50000m3 / h at a suction pressure of 1Torr.
1) To suck the amount of water vapor above, three Roots pumps are required in series, and a water ring pump is used to make up the unit of the previous stage. The total power of this unit is 90kW.
2) To condense the steam before it reaches the vacuum pump, a condenser and a cooling device with a power of 30000 kcal / h are installed at A, as shown in Figure 4. In 1Torr suction pressure, the steam condensing temperature is -19 ℃, in order to ensure continuous operation, the condensing unit should take the condensing temperature of -25 ℃, and the installation of two parallel condenser. Based on the composition of the non-condensable gases, the amount of suction in the vacuum pump can be reduced to 1000-2000m3 / h. The power of the main unit (including the condenser's power consumption) is also 90kW.
3) Roots pump out the water vapor, and under 45Torr pressure condensation, the pressure of some condensing temperature is about 36 ℃, so condensing the condenser can be maintained at a temperature between 30 ~ 35 ℃, available ordinary Cooling water cooling. The condenser is located at B. At this time the total power consumption is about 75kW.
Through the comparison of the above three groups shows that the third option is the best, can reduce the power consumption of 15kW.
In summary, only the non-condensable gas is left after the steam is cooled. When the pressure is low, the specific volume of water vapor is quite large. After condensable condensables, the amount of pumping required by the pump is significantly reduced. In addition, whether the steam is condensed or not, the volume flow will be reduced at the same pressure as long as the gas temperature decreases. For example, chemical processes 200 ~ 300 ℃ temperature of the gas is not uncommon. After cooling from 300 ° C to 50 ° C, the volume of dry air is reduced by about 45% so that a smaller capacity suction vacuum pump unit can be selected.
(2) The operating sequence of the unit:
1) When there is no by-pass valve in the unit, the water ring pump should be started first. The gas in the system being pumped is tapped by the Roots pump (the gas drives the rotor of the Roots pump to rotate on its own like a flow meter) Atmosphere, to be water ring suction pressure of the pump (such as the air pump in series, the suction pressure of the air pump) to Roots pump initial provisions (that is, allow the exhaust pressure), start the Roots pump, the unit official operation ,start working.
2) When there is a bypass valve in the unit, as shown in Figure 5, start the water ring pump first and then start the Roots pump. At this moment, the pressure difference between inlet and outlet of Roots pump is large, the bypass valve is opened automatically, Part of the gas in the container through the bypass valve into the water ring pump, the other part of the pump under the action of the Roots through the pump into the water ring pump, evacuation rate is obviously increased, so quickly to Roots pump pre-vacuum into Exhaust pressure is smaller, the valve automatically closed (or manually shut down), the crew officially work. This method can greatly shorten the pre-pumping time, but the equipment is more complicated.
(3) Unit - Roots pump - foreline pump performance relationship
The performance of the unit is closely related to the performance of the Roots pump, while the performance of the Roots pump is different with the previous pump.
1) Roots pump due to the rotor and rotor, there is a gap between the rotor and the housing, so there is a backflow, and this backflow by the inlet pressure and outlet pressure of the impact, even with the same Roots pump , The use of different fore pump, its pumping rate will be different.
Roots pump suction rate can be determined by the following formula:
δ = δ0 (P2 / P1 / K)
Where: δ0-design pumping rate;
P1 - inlet pressure
P2 - outlet pressure;
K-Inherent constant, determined by the shape of the pump rotor, the amount of clearance, the peripheral speed of the rotor, and the outlet pressure.
As can be seen from the above formula, the amount of suction is affected by the ratio of the outlet pressure to the inlet pressure. That is, if the suction rate of the foreline pump is increased, the pumping rate of the Roots pump also increases.
(2) The ultimate pressure is determined by the pumping speed of the pump, the return flow of each clearance, the amount of pump leakage, and the amount of deflation on the high vacuum side. which is:
P0 = (Q1 + Q2 + Q3) / δ Where: P0 - the ultimate pressure;
δ - pumping rate;
Q1 - return flow;
Q2 - leakage
Q3 - deflated volume.
Among these parameters, Q1 is greatly affected by the exhaust pressure, ie, the ultimate pressure of the foreline pump. The pressure of the Roots pump varies with the saturated vapor pressure of the water ring when the foreline pump is used as a water ring.
Is the same Roots pump with different forepump performance comparison.
The higher the ultimate vacuum of the foreline pump, the higher the ultimate vacuum of the unit. The cascade of two cascade Roots pumps can increase the ultimate vacuum of the unit (essentially the former Roots pump is the last one The pump on the front stage), and the performance curve flattens out, ie the range used expands (compare curves 1 and 2, curves 3 and 5). The curves for units 1 and 2 are roughly the same. Similarly, the curves for units 3, 4 and 5 are the same. However, 1,2 units curve and 3,4,5 unit curve is completely different from the two sets of curves. This shows that for the same Roots pump, the choice of different fore pump, the performance curve of the unit is essentially different. This shows that the fore pump on the unit performance has a considerable impact.
(4) the choice of water ring pump
The so-called water ring pump is the use of water as a liquid ring liquid ring pump, liquid for liquid ring has many advantages, such as cheap, easy to get, will not pollute the environment. However, there is also a great disadvantage that the suction pressure of the water ring pump is also high due to the high saturated steam pressure of the water. At this time, such as the use of saturated vapor pressure with low liquid as a ring, you can increase the pump ultimate vacuum. If a unit Roots pump maximum allowable exhaust pressure of 10Torr, the water ring when the pump must be increased as the pump before the pump, if the switch to mineral oil for the liquid ring is not increased Pump can be used as a fore pump, this can simplify the device.
(5) Unit performance and Roots pump allows discharge pressure
The performance of the unit depends to a large extent on the allowable discharge pressure of the Roots pump. The lower this permissible value, the less likely the water ring pump will be as a forepump. If Roots pump this allows the value of 1 ~ 10Torr, regardless of single and multi-stage water ring pump limit pressure is much higher than this value, it can not be used alone with this Roots pump combination, but need to add two Air pump. If Roots pump discharge pressure allowable value of more than 100Torr, the water ring pump can also be used as a front stage Roots pump Roots pump, which greatly expanded the scope of the foreline pump.
(6) Application examples
The production process of a chemical fiber product is as follows: low molecular weight - polymer - belt shape - slicing - drying (using Roots pump - water ring pump vacuum drying) - spinning - drawing, heating - textile - finished.
One of the most important processes is to dry slices of 5x5x5 (mm) size for spinning, ideally in moisture-free conditions during spinning. In fact, this can not be done, Therefore, the moisture content requirements of not more than 0.02%, if the water exceeds this requirement, to silk under high temperature and pressure, will break down the polymer, affecting product strength.
Roots pump - water ring pumping units used in the drying process vacuum system device. The technical parameters of each pump in the unit are shown in the following table.
The function of the main equipment is as follows:
(1) vacuum valve to close the vacuum valve, the unit stopped running, to maintain a certain degree of vacuum drying system.
(2) automatic pneumatic safety valve to prevent sudden stop when the unit water ring pump system to Roots pump and vacuum drying system intrusion.
(3) When the differential pressure valve unit starts to operate, start the water ring pump first and then gradually generate the pressure difference at both ends of the pressure differential valve. When the pressure difference valve reaches a certain value, the valve will be opened automatically to allow most of the gas in the system to flow into the water ring Pump. When the atmosphere gradually through the air pump into the water ring pump, the differential pressure across the valve and then gradually reduced, so off valve, so the air pump immediately began to work normally, the role of pressure valve is to shorten the atmospheric pump before normal work Take time.
(4) start and stop the role of water level check valve to prevent water ring pump water back.
Operation sequence is as follows: first start water ring pump, the system gas diameter by the Roots pump (at this time to promote Rotor Rotor rotor) into the water ring pump from the water separator to the atmosphere. When the absolute pressure of 50Torr pumping start Roots pump 2, when the absolute pressure to 20Torr and then start the Roots pump 1, the final system pressure up to 0.088Torr, under normal circumstances can be maintained below 1Torr.
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