About the vacuum
Vacuum pumps

2) Trapping pumps, which by creating trap in different ways, such as cold trap, causing particles condensation, on the metal body. Cryogenic pumps are of this category.

In identifying vacuum pumps, there are several factors, which among them the most important are two following factors:

1. The Pumping rate.

2. The ultimate available vacuum by the pump.

These features being displayed usually at a graph, which is known as the performance graph. For example, the performance chart of a rotary pump vane, DS 602 EDWARDS, is shown in Figure 1. Low vacuum and high vacuum pumps are two main types of vacuum pumps, and as is clear from their names, first one is used to achieve a low vacuum, approximately 10^-3 mbar, and the latter is also used for ultra-high vacuum.

The pumping rate of high vacuum pumps is lower than the first one, but because of the type of process, are used to achieve high vacuum. The most common low vacuum pumps are the rotary blowers, rotary piston, rotary vane, and the most common high vacuum pumps, can be note, cooling pumps, ion pumps, turbomolecular, and diffusion pumps.

Figure 1: The performance graph of the rotary vane pump.

These pumps, as shown in Figure 2, includes a set of used blades on a shaft, in a way that, when the shaft rotates by an external motor, all gases of reservoir pumps, compressed and pushed towards the outputs, and as a result, vacuum is created in the pump reservoir.

The gases of inside the chamber accelerated toward the pump inlet, and fill the created vacuum in the pump, and while the shaft is rotating, these gases are also compressed, successively. These pumps can build as a single-stage or two-stage.

                                                  

Figure 2. Single stage, rotational van pump.

Figure 3: a. real view of a rotary pump, manufactured by Alcatel, b. Schematic view of the rotary pump

The process of chamber pumping is done in four steps, using single-stage pump, as shown in Figure 4:

In step 1, the air of chamber flows into the pump, due to the difference between pressure of chamber and inside of pump.

In step 2, the entered air is isolated from the environment.

In step 3, the entered air is compressed.

In step 4, the compressed air is evacuated to the outside and finally, to the pump outlet.

The reservoir such a pump is filled of oil, which causes lubrication, cooling and sealing. Among the disadvantages of these pumps can be noted to creating limitation of vacuum, because the oil vapor pressure of, and oil vapor flow into the chamber, which in addition to reducing the gap, causing the creation of impurities and getting dirty inside the chamber.

Figure 4: four steps performance of a rotary pump

To achieve a higher vacuum is used, of two-stage rotary pumps. That means, compressed gas, instead of leaving, to pump outlet, is entered into the same part of the previous section, and condensed again, and finally directed towards the outside of the pump. Figure 5 shows a schematic of a two-stage pump.

Roots rotary pumps:

In many cases, it’s not enough the created speed and the vacuum by a rotary pump. In these cases, to increase the vacuum and speed is used a Roots pump, connected directly to a mechanical pump.

These pumps, as shown in Figure 6, are including two arc (lob) piece, which rotate on two independent shafts, one clockwise and the other counterclockwise. As a result of this operation, the inside gases of pump are compressed between rotors by themselves, as well as between the rotor and the stator and guided to the rotary pump.

For example, the pumping rate of a rotary pump increases up to 42 liters per second, but with a roots pump increases to 240 liters per second. In Figure 8, it can be seen, the real view of a rotary roots pump.

As mentioned above, the used oil in such pumps is a serious problem, which causes dirty compartment.

Figure 8: the real view a rotary roots pump, manufactured by German company

Table 1: the comparison of rotary van pumps

Design / Operating principle

The principle of operation of single-stage Roots pumps corresponds to the operating principle of multi-stage pumps as described in Chapter 4.5. In the Roots vacuum pump, two synchronously counter-rotating rotors (4) rotate contactlessly in a housing (Figure 4.16). The rotors have a figure-eight configuration and are separated from one another and from the stator by a narrow gap. Their operating principle is analogous to that of a gear pump having one two-tooth gear each that pumps the gas from the inlet port (3) to the outlet port (12). One shaft is driven by a motor (1). The other shaft is synchronized by means of a pair of gears (6) in the gear chamber. Lubrication is limited to the two bearing and gear chambers, which are sealed off from the suction chamber (8) by labyrinth seals (5) with compression rings. Because there is no friction in the suction chamber, a Roots vacuum pump can be operated at high rotation speeds (1,500 – 3,000 rpm ). The absence of reciprocating masses also affords trouble-free dynamic balancing, which means that Roots vacuum pumps operate extremely quietly in spite of their high speeds.

Design

The rotor shaft bearings are arranged in the two side covers. They are designed as fixed bearings on one side and as movable (loose) bearings on the other to enable unequal thermal expansion between housing and rotor. The bearings are lubricated with oil that is displaced to the bearings and gears by splash disks. The driveshaft feedthrough to the outside on standard versions is sealed with radial shaft seal rings made of FPM that are immersed in sealing oil. To protect the shaft, the sealing rings run on a protective sleeve that can be replaced when worn. If a hermetic seal to the outside is required, the pump can also be driven by means of a permanent-magnet coupling with a can. This design affords leakage rates QI

of less than 10^-6 Pa m³ s^-1.

Pump properties, heat-up

Since Roots pumps do not have internal compression or an outlet valve, when the suction chamber is opened its gas volume surges back into the suction chamber and must then be re-discharged against the outlet pressure. As a result of this effect, particularly in the presence of a high pressure differential between inlet and outlet, a high level of energy dissipation is generated, which results in significant heat-up of the pump at low gas flows that only transport low quantities of heat. The rotating Roots pistons are relatively difficult to cool compared to the housing, as they are practically vacuum-insulated. Consequently, they expand more than the housing. To prevent contact or seizure, the maximum possible pressure differential, and so also the dissipated energy, is limited by an overflow valve (7). It is connected to the inlet side and the pressure side of the pump-through channels. A weight-loaded valve plate opens when the maximum pressure differential is exceeded and allows a greater or lesser portion of the intake gas to flow back from the pressure side to the inlet side, depending on the throughput. Due to the limited pressure differential, standard Roots pumps cannot discharge against atmospheric pressure and require a backing pump. However Roots vacuum pumps with overflow valves can be switched on together with the backing pump even at atmospheric pressure, thus increasing their pumping speed right from the start. This shortens evacuation times.

Figure 4.16: Operating principle of a Roots pump

Backing pumps

Single-stage or two-stage rotary vane pumps or external vane pumps are used as oil-lubricated backing pumps. Screw pumps or multi-stage Roots pumps can be used as dry backing pumps. Pump combinations such as these can be used for all applications with a high pumping speed in the low and medium vacuum range. Liquid ring pumps can also be used as backing pumps.

Gas-circulation-cooled Roots pumps

To allow Roots vacuum pumps to work against atmospheric pressure, some models are gas-cooled and do not have overflow valves (Figure 4.17). In this case, the gas that flows from the outlet flange (6) through a cooler (7) is re-admitted into the middle of the suction chamber (4). This artificially generated gas flow cools the pump, enabling it to compress against atmospheric pressure. Gas entry is controlled by the Roots pistons, thus eliminating the need for any additional valves. There is no possibility of thermal overload, even when operating at ultimate pressure.

Figure 4.17: Operating principle of a gas-cooled Roots pump

Figure 4.17 shows a cross-section of a gas-circulation-cooled Roots vacuum pump. The direction of gas flow is vertical from top to bottom, enabling the liquid or solid particles entrained in the inlet stream to flow off downward. In phase I, the chamber (3) is opened by the rotation of the pistons (1) and (2). Gas flows into the chamber through the inlet flange (5) at pressure p1

.

Gas-cooled Roots pumps can be used in the inlet pressure range of 130 to 1,013 hPa. Because there is no lubricant in the suction chamber, they do not discharge any mist or contaminate the medium that is being pumped. Connecting two of these pumps in series enables the ultimate pressure to be reduced to 20 to 30 hPa. In combination with additional Roots vacuum pumps, the ultimate pressure can be reduced to the medium vacuum range.

Pumping speed and compression ratio

The characteristic performance data of Roots pumps are the pumping speed and compression ratio. The theoretical pumping speed Sth=S0

values are between 10 and 75.

Figure 4.18: No-load compression ratio for air for Roots pumps

The compression ratio is negatively impacted by two effects:

• By the backflow into the gaps between the piston and housing
• By the gas that is deposited by adsorbtion on the surfaces of the piston on the outlet side and re-desorbs after rotating toward the suction side.

In the case of outlet pressures of 10^-2 to 1 hPa, molecular flow prevails in the seal gaps,which results in less backflow due to their low conductivities. However the volume of gas that is pumped back through adsorption, which is relatively high by comparison with the pumped gas volume, reduces the compression ratio.

K0

is highest in the 1 to 10 hPa range, since molecular flow still prevails due to the low inlet pressure in the pump’s sealing gaps, and backflow is therefore low. Since gas transport through adsorption is not a function of pressure, it is less important than the pressure-proportional gas flow that is transported by the pumping speed.

At pressures in excess of 10 hPa, laminar flow occurs in the gaps and the conductivities of the gaps increase significantly, which results in declining compression ratios. This effect is particularly noticeable in gas-cooled Roots pumps that achieve a compression ratio of only approximately K0

= 10.

The gap widths have a major influence on the compression ratio. Due to the different thermal expansion of the pistons and the housing, they must not, however, fall below certain minimum values in order to avoid rotor-stator-contact.

Diaphragm vacuum pumps

Diaphragm vacuum pumps are dry positive-displacement pumps. A crankshaft-driven connecting rod (4) moves the diaphragm (1) that is tensioned between the head cover (2) and the housing (3). The space between the head cover and the diaphragm forms the suction chamber (5). Diaphragm pumps require inlet valves and outlet valves (6) to achieve targeted gas displacement. Pressure-controlled shutter valves made of elastomer materials are used as valves. Since the suction chamber is hermetically sealed off from the drive by the diaphragm, the pump medium can neither be contaminated by oil nor can aggressive media corrode the mechanics. The dead volume between the outlet valve and the suction chamber results in a restricted compression ratio which means that with just one pumping stage it is only possible to achieve an ultimate pressure of approximately 70 hPa. Connecting multiple pumping stages in series makes it possible to attain an ultimate pressure of 0.5 hPa. Lower pressures cannot be achieved, as in this case there is no longer sufficient force to open the inlet valve. The principle of the diaphragm pump is particularly well suited for low pumping speeds of up to approximately 10 m³· h^-1.