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Rotary vane pump: operating characteristics and working principle

Update:15-03-2019
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What are the operating characteristics and internal wor […]

What are the operating characteristics and internal workings of a rotary vane pump?

Single and two stage pumps

One limiting factor for 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 top rotor and the stator of the pump. In a single-stage rotary vane pump, the seal The pressure difference can be close to 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 (Figure 8). This produces backflow, ie pumping oil Return to the movement of the vacuum furnace chamber.

To create a higher vacuum using a rotary vane pump, a two-stage pump design is used. The two-stage pump uses 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 significantly lower than atmospheric pressure, this design results in a lower pressure at the exit of the high vacuum stage, as opposed to a single stage design that is subjected to atmospheric pressure at the outlet. This reduces the differential pressure between the Duo Seal and the high vacuum stage blades, allowing them to operate at higher inlet pressures. The two-stage rotary vane pump can achieve inlet pressures 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 an vent valve at the outlet of the low vacuum stage.

Mode selection
Some two-stage rotary vane pumps have the ability to operate in high throughput mode or high vacuum mode. Select the mode by turning the knob on the pump control panel. The mode selector controls the flow of pressurized oil to the high vacuum stage of the pump, which changes the characteristics of the pump. In high-flux mode, oil pressure (and therefore flow) increases, and in high-vacuum mode, oil flow decreases. This feature overcomes the problem of higher pressures at insufficient pressure differentials during the low vacuum phase, 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 can be started 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 ultimate vacuum. The high-throughput mode is also used to pump condensable (dirty) steam and to clean the pump oil if 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 (see below). By selecting these two modes in combination with (high, low or no) gas ballast (Table 1), various pumping characteristics (ie, pressure and flow performance) can be achieved. The mode selector switch can be activated when the pump is turned on or off, while some larger pumps can 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-back suction or vacuum relief valve). As the name suggests, when pumping stops, the unit is turned off, 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 in the pump to the pressure outside the pump outlet. This prevents the oil in the housing from filling the stator chamber. When the pump is reopened, 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 (eg, oil seal rotary vane pump Part 1) is hydraulically actuated. In a two-stage rotary vane pump, the isolation valve is located in a high vacuum stage.

Gas town

Moisture and vaporized contaminants (usually from dirty work introduced into the vacuum chamber) will enter the pump oil and interfere with the efficient operation of the pump. As a result, when the oil loses its ability to provide a seal between the blade and the stator and at the Duo seal, it becomes difficult to reach the ultimate vacuum and takes longer and longer, resulting in lower pumping efficiency. In addition, the nature of the oil changes, resulting in insufficient lubrication and the possibility of introducing internal corrosion. To avoid these problems, simple but efficient gas ballast (also known as gas ballast) operation is used.

Gas ballasting is the injection of non-condensable gases (such as nitrogen or air) into the rotary vane pump during the compression phase, resulting in reduced condensation. The ballast gas is injected through a one-way (aka "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 warm up and drive oil and other volatile vapors out of the oil to the vent stack.

The theory behind this is that the injected gas dilutes the vapor in the pumped gas so that the partial pressure of the vapor never reaches saturation during compression. 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 until sufficient dilution occurs. As the rotor continues to rotate, the pump discharge valve is forced open and discharges the pumped gas, a mixture of ballast gas and steam.

In addition to diluting the condensable vapor, the gas ball also raises the temperature of the process gas by 10-20 ° C (18 - 36 ° F), which further inhibits condensation. In addition, to prevent condensation of steam during normal operation, the gas ballast is also used to purify pump oil that has been contaminated with condensed steam. 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 load. This should last at least 30 minutes. In some critical applications, or when dirty work is performed and significant degassing is expected, it is best to compress the pump for 20 to 30 minutes after each cycle. This helps to purify 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 pumped from the vacuum chamber. As the inert gas, nitrogen is used when moisture, oxygen or hydrogen contained in the air reacts with the process gas. In most other cases, air is the preferred ballast gas.

The main disadvantage of gas ballast is that it reduces the ultimate vacuum of the pump during use (Figure 11). It also increases the speed of the oil discharged from the pump. The amount of gas produced by ballast can be selected on most pumps with low flow and high flow characteristics. The negative effects of ballasts on ultimate vacuum and oil loss are smaller in low flow mode than in high flow mode.

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 to a lower condensation temperature than the steam (moisture and others) carried in the gas. The steam becomes liquid and collects on the inner surface of the heat exchanger inside the condenser, preventing them from entering the pump. The resulting condensate was collected and removed. The inlet condenser can be water cooled using a shell and tube heat exchanger or cooled with a refrigerant or a cryogen such as liquid nitrogen.

The condenser also helps to minimize backflow of oil vapor from the pump into the vacuum chamber. Even with the inlet condenser, the rotary pump can accumulate condensation contaminants in the oil. Therefore, inlet condensers and gas ballasts are typically used to achieve maximum 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, i.e., the flow of oil vapor to the pumped gas, and back to the vacuum chamber. Reflux (see oil seal rotary vane pump part 1) is the result of evaporation of oil at low pressure. It can cause contamination because the oil deposits on the surface of the furnace and can interfere with the ongoing process.

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

Alumina will also remove moisture from the front line and collect it as liquid water. Over time, this will slow down the evacuation rate when the alumina is blocked by water. Therefore, when there is moisture in the pumped gas, it is recommended to use the inlet condenser with the front stage trap.

When using a foreline trap, the trap must be bypassed during roughing, which is the period of high initial flow evacuation at higher pressures. The problem can only be reflowed after the roughing has been completed and a higher vacuum has been achieved. At this point, the gas passes through the pre-stage trap. The bypass device prevents rapid and unnecessarily clogging of the high flow gases and vapors that alumina pumps during roughing.

Although frontline traps are common, the first line of defense against backflow is the use of low vapor pressure pump oil, which is less prone to evaporation and therefore less likely to flow back.

In addition to the foreline traps, 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 collectors. The choice of trap is based on the specific application and composition of the pumped gas.

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