Single stage pump and two stage pump

One of the limiting factors of the rotary vane pump is the Duo seal, which is an oil-filled non-contact seal in a small 0.025 mm (0.001") space between the rotor and the stator at the top of the pump. In a single-stage rotary vane pump, the entire seal The pressure difference between the pieces can reach 100,000:1 (1000 mbar vs. 0.01 mbar). Above this, the double seal will begin to leak oil from the high pressure side to the low pressure side. This creates a backflow that will pump the oil back to the vacuum. Movement in the oven cavity.

In order to generate a higher vacuum using a rotary vane pump, a two-stage pump design was used. The two-stage pump utilizes two rotary vane pumps in series. The outlet of the high vacuum stage is piped to the inlet of the low vacuum stage. Since the inlet of the low vacuum stage is much lower than atmospheric pressure, in contrast to the single stage design, the single stage design is subjected to atmospheric pressure at the outlet, so this design results in lower pressure at the high vacuum stage outlet. This reduces the pressure differential between the Duo seal and the vane in the high vacuum stage, allowing it to operate at higher inlet pressures. The two-stage rotary vane pump achieves an inlet pressure of 3 x 10-3 Torr (4 x 10-3 mbar). There is no vent valve between the high vacuum stage and the low vacuum stage, but there is only one at the exit of the low vacuum stage.



Mode selection
Some two-stage rotary vane pumps have the ability to operate in high-flux mode or high-vacuum mode. Select the mode by turning the knob on the pump control panel. The mode selector controls the high vacuum level of the pressurized oil flowing to the pump, thereby changing the characteristics of the pump. In high-flux mode, the oil pressure (and therefore the flow) increases, and in the high-vacuum mode, the oil flow decreases. This feature overcomes the problem of insufficient pressure differential at low vacuum levels at higher pressures, thereby ensuring sufficient oil supply to the high vacuum stage (later in the lubrication circuit). This problem does not occur when running at higher vacuums. The pressure differential is sufficient to provide adequate lubrication during the high vacuum phase.

The high throughput mode is used to provide a faster pressure drop at inlet pressures greater than about 38 Torr (50 mbar). A typical cycle may begin in high-flux mode to evacuate the vacuum chamber as quickly as possible and then switch to high vacuum mode at 38 Torr (50 mbar) to achieve maximum vacuum. The high-throughput mode is also used to pump condensable (dirty) vapors and to purify the pump oil when necessary. The high vacuum mode can only be used when the pumped gas is clean.

Pump performance can be optimized through a combination of mode selection and gas ballast. By selecting these two modes combined with (high, low or no) gas ballast, a wide range of pumping characteristics (ie, pressure and flow performance) can be obtained. The mode selector switch can be actuated when the pump is turned on or off, and some larger pumps automatically switch between modes.

Isolation (anti-back suction) valve
Rotary vane pumps are usually equipped with an inlet isolation valve (also known as an anti-backup valve or vacuum relief valve). As the name suggests, the device shuts down when pumping is stopped, preventing gas (or air) from being drawn back into the vacuum chamber through the pump. When the pump is stopped and the valve is closed, air enters the pump outlet, equalizing the pressure inside the pump to the pressure outside the pump outlet. This prevents oil in the housing from filling the stator cavity.

When the pump is turned back on, the valve does not open immediately, but is delayed until the pressure in the pump reaches the approximate pressure in the vacuum chamber, preventing back suction when the pump reaches pressure. The isolation valve (see section 1 of the oil seal rotary vane pump) is hydraulically driven. In a two-stage rotary vane pump, the isolation valve is located on a high vacuum stage.

Gas town

Moisture and gasification contaminants (usually contaminants due to contamination introduced into the vacuum chamber) can enter the pump oil and interfere with the efficient operation of the pump. As a result, it becomes difficult to reach the ultimate vacuum, and the time to do so is longer and longer because the oil loses the ability to provide a seal between the blade and the stator and at the Duo Seal, resulting in a decrease in pumping efficiency. Moreover, the characteristics of the oil change, resulting in insufficient lubrication and the possibility of introducing internal corrosion. In order to avoid these problems, a simple and efficient gas town (also known as gas town) operation was used.

Gas ballasts inject a non-condensable gas (such as nitrogen or air) into a rotary vane pump during the compression phase to reduce condensation. The ballast gas is injected through a one-way (also known as "air-to-the-air") valve located at the top of the pump. One way to consider the use of a gas ballast is to open the gas ballast valve to deliberately destroy the pump's efficiency, which in turn causes the pump oil to heat up and drive away moisture and other volatile vapors from the oil. Was sent to the vent stack.

The theory behind this is that the injected gas dilutes the vapor in the pumped gas, so the partial pressure of the vapor never reaches saturation during the compression process. Injection begins at the beginning of the compression cycle.

After startup, the pump rotor continues to rotate, increasing the pressure generated in the pump, which forces the one-way ballast valve to close, but does not close until sufficient dilution occurs. As the rotor continues to rotate,

The discharge valve of the pump is forced to open and discharges the pumped gas, a mixture of ballast gas and steam.



In addition to diluting the condensable vapor, the gas ballast also increases the temperature of the process gas by 10-20 ° C (18 – 36 ° F), which further inhibits condensation. In addition, the gas ballast used to prevent vapor condensation during normal operation is also used to purify pump oil contaminated with vapor that has been condensed. For heavily polluted pumps, this can take several hours.

It is recommended that the vacuum pump be ballasted at least once a day, usually at the start of the equipment and before the first run. It should take at least 30 minutes. In some critical applications or where dirty work is being performed and a large amount of outgassing is expected, it is good practice to ballast the pump for between 20 and 30 minutes between each cycle. This helps to clean the oil after each cycle of operation.

The choice of air or nitrogen as the ballast gas depends on the characteristics of the process gas extracted from the vacuum chamber. When moisture, oxygen or hydrogen in the air reacts with the process gas, nitrogen can be used as the inert gas. In most other cases, air is the preferred ballast gas.

The main disadvantage of gas towns is that they reduce the ultimate vacuum of the pump when in use. This also increases the ratio of oil discharged from the pump. On most pumps with low flow and high flow capabilities, the amount of gas produced by ballast is optional. Ballasts in low flow mode have a less negative impact on ultimate vacuum and oil loss than low flow modes.

Cold trap

In addition to degassing, another method of pumping a gas containing condensed steam or moisture is to remove it before entering the pump. This is done by a cold trap (also called an inlet condenser) located at the pump inlet.

The condenser operates by cooling the pumped gas below the condensation temperature of the vapor (moisture and other gases) contained in the gas. The vapor becomes a liquid and collects on the inner surface of the heat exchanger inside the condenser, thereby preventing it from entering the pump. The generated condensed water is collected and removed. The inlet condenser can be water cooled using a shell and tube heat exchanger, or it can be cooled using a refrigerant or a refrigerant such as liquid nitrogen.

The condenser also helps to minimize the return of oil vapor from the pump to the vacuum chamber. Even with the imported condenser, the rotary pump will accumulate condensed contaminants in the oil. Therefore, it is common to use both the inlet condenser and the gas ballast to maximize steam handling while minimizing pumping capacity.

Frontline trap

In any vacuum system with a pressure below 0.75 Torr (10-1 mbar), there is a possibility of backflow, which is the migration of oil vapor against the pumped gas stream and backflow into the vacuum chamber (return (see oil seal rotation) The vane pump part 1) is the result of evaporation of the oil at low pressure. It can cause contamination because the oil deposits on the inner surface of the furnace in the form of a film and can interfere with the process being performed.

One way to prevent backflow is to use a pre-stage trap, which is a molecular sieve mounted on the pump inlet. It is filled with activated alumina (also known as an adsorbent) to capture and collect oil vapor. The alumina media is replaceable and must be replaced at the same intervals as the pump oil, usually every 6 months, although this depends on the frequency of use. The pre-stage trap will block 99% of the oil vapor.

Alumina will also remove moisture from the front wall and collect it as liquid water. As time passes, this will slow down the pumping speed as the alumina is blocked by water. Therefore, when moisture is present in the pumped gas, it is recommended to use an inlet condenser in the foreline trap.

When using a pre-stage trap, it is necessary to bypass the trap during roughing, which is the period of high-flow initial evacuation at higher pressures. Reflow is only required after the roughing is completed and a higher vacuum is obtained. At this point, the gas then passes through the pre-stage trap. This bypass device prevents the alumina from being quickly and unnecessarily blocked when a large amount of gas and steam pumped during the roughing process flows.



Although pre-stage traps are common, the first defense against backflow is to use pump oil with a low vapor pressure, which is less prone to evaporation and therefore less likely to reflow.

In addition to the pre-stage trap, other accessories are used on the pump inlet side to capture moisture, steam and solid contaminants. These include desiccant traps, zeolite traps, catalytic traps, trap tanks, and dust traps. The choice of trap is based on the specific application and composition of the pumped gas.

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