About the vacuum
Vacuum pumps
For evacuating the gases and vapors into the chamber and thereby creating vacuum, is used the device called vacuum pump. The operation of these pumps is usually in two ways:
1) Compressive pumps, which base of their working is separating particles from the chamber, compress, as guiding them to the outside. Rotary Vane pumps are the most used of these 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

شکل5 : پمپ چرخشی ون دو مرحله ای
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
Defect
|
Advantage
|
Capacity(m3/h)
|
The vacuum pressure (mbar)
|
The type of vacuum pump
|
Oil mixing with water vapor, during air compression .
|
Low power consumption compared to high capacity of pump
|
1200-1
|
2-10
|
One stage rotary
|
3-10
|
Two stage rotary
|
Rising temperatures, at pressures close to atmospheric pressure
|
Cleaning Performance
Free of oil
|
40000-300
|
3-10-2-10
|
Roots pump
|
Leybold [1] Rotary vane [2] Entrapment [3] Rotary Roots Pump
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
. In phase II, the
chamber (3) is sealed off against both the inlet flange and the
pressure flange. The inlet opening (4) for the cooling gas is opened
by the rotation of the pistons in phase III. The chamber (3) is filled
to the outlet pressure p2, and the gas is advanced toward the
pressure flange. Initially, the suction volume does not change with
the rotary movement of the Roots pistons. The gas is compressed by the
inflowing cooling gas. The Roots piston now continues to rotate (phase
IV), and this movement pushes the now compressed gas over the cooler
(7) to the discharge side (Phase V) at pressure p2
.
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
is the volume flow rate which the pump displaces without
counterpressure. The compression ratio K0 when operated without gas
displacement (inlet flange closed) depends on the outlet pressure
p2. Pumping speeds range from 200 m3 · h-1
to several thousand m3 · h-1. Typical
K0
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.
Refrence
Web: https://www.pfeiffer-vacuum.com
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