Oil-Free (‘‘Dry’’) Pumps
Many different types of oil-free pumps are available. We will emphasize those that are most useful in analytical
and diagnostic applications.
Diaphragm Pumps:
Applications: Diaphragm pumps are increasingly used where the absence of oil is an imperative, for example, as
the forepump for compound turbomolecular pumps that incorporate a molecular drag stage. The combination renders oil contamination very unlikely. Most diaphragm pumps have relatively small pumping speeds. They are
adequate once the system pressure reaches the operating range of a turbomolecular pump, usually well below
10-2 torr, but not for rapidly roughing down a large volume. Pumps are available with speeds up to several
liters per second, and base pressures from a few torr to as low as 10-3 torr, lower ultimate pressures being associated with the lower-speed pumps.
Operating Principles: Four diaphragm modules are often arranged in three separate pumping stages, with
the lowest-pressure stage served by two modules in tandem to boost the capacity. Single modules are adequate for subsequent stages, since the gas has already been compressed to a smaller volume. Each module uses a flexible diaphragm of Viton or other elastomer, as well as inlet and outlet valves. In some pumps the modules can be arranged to provide four stages of pumping, providing a lower base pressure, but at lower pumping speed because only a single module is employed for the first stage. The major required maintenance in such pumps is replacement of the diaphragm after 10,000 to 15,000 hr of operation.
Scroll Pumps :
Applications: Scroll pumps (Coffin, 1982; Hablanian, 1997) are used in some refrigeration systems, where the
limited number of moving parts is reputed to provide high reliability. The most recent versions introduced for
general vacuum applications have the advantages of diaphragm pumps, but with higher pumping speed. Published speeds on the order of 10 L/s and base pressures below 10-2 torr make this an appealing combination. Speeds decline rapidly at pressures below ~2 X 10-2 torr.
Operating Principles: Scroll pumps use two enmeshed spiral components, one fixed and the other orbiting. Successive crescent-shaped segments of gas are trapped between the two scrolls and compressed from the inlet(vacuum side) toward the exit, where they are vented to the atmosphere. A sophisticated and expensive version of this pump has long been used for processes where leaktight operation and noncontamination are essential, for example, in the nuclear industry for pumping radioactive gases. An excellent description of the characteristics of this design has been given by Coffin (1982). In this version, extremely close tolerances (10 mm) between the two scrolls minimize leakage between the high- and low-pressure ends of the scrolls. The more recent pump designs, which substitute Teflon-like seals for the close tolerances, have made the pump an affordable option for general oil-free applications. The life of the seals is reported to be in the same range as that of the diaphragm in a diaphragm pump.
Screw Compressor: Although not yet widely used, pumps based on the principle of the screw compressor, such as that used in supercharging some high-performance cars, appear to offer some interesting advantages: i.e., pumping speeds in excess of 10 L/s, direct discharge to the atmosphere,and ultimate pressures in the 10-3 torr range. If such pumps demonstrate high reliability in diverse applications,
they constitute the closest alternative, in a singleunit ‘‘dry’’ pump, to the oil-sealed mechanical pump.
Molecular Drag Pump:
Applications: The molecular drag pump is useful forapplications requiring pressures in the 1 to 10-7 torr range
and freedom from organic contamination. Over this range the pump permits a far higher throughput of gas, compared to a standard turbomolecular pump. It has also been used in the compound turbomolecular pump as an integral backing stage. This will be discussed in detail under Turbomolecular Pumps.
Operating Principles: The pump uses one or more drums rotating at speeds as high as 90,000 rpm inside stationary,coaxial housings. The clearance between drum and housing is ~0.3 mm. Gas is dragged in the direction of rotation by momentum transfer to the pump exit along helical grooves machined in the housing. The bearings of these devices are similar to those in turbomolecular pumps (see discussion of Turbomolecular Pumps, below). An internal motor avoids difficulties inherent in a high-speed vacuum seal. A typical pump uses two or more separate stages, arranged in series, providing a compression ratio as high as 1:107 for air, but typically less than 1:103 for hydrogen. It must be supported by a backing pump, often of the diaphragm type, that can maintain the forepressure below a critical value, typically 10 to 30 torr, depending upon the particular design. The much lower compression ratio for hydrogen, a characteristic shared by all turbomolecular
pumps, will increase its percentage in a vacuum chamber, a factor to consider in rare cases where the presence
of hydrogen affects the application.
Sorption Pumps:
Applications: Sorption pumps were introduced for roughing down ultrahigh vacuum systems prior to turning
on a sputter-ion pump (Welch, 1991). The pumping speed of a typical sorption pump is similar to that of a small oilsealed mechanical pump, but they are rather awkward in application. This is of little concern in a vacuum system likely to run many months before venting to the atmosphere. Occasional inconvenience is a small price for the ultimate in contamination-free operation.
Operating Principles: A typical sorption pump is a cannister containing ~3 lb of a molecular sieve material
that is cooled to liquid nitrogen temperature. Under these conditions the molecular sieve can adsorb ~7.6X 104 torrliter of most atmospheric gases; exceptions are helium and hydrogen, which are not significantly adsorbed, and neon, which is adsorbed to a limited extent. Together, these gases, if not pumped, would leave a residual pressure in the 10-2 torr range. This is too high to guarantee the trouble-free start of a sputter-ion pump, but the problem is readily avoided. For example, a sorption pump connected to a vacuum chamber of ~100 L volume exhausts air to a pressure in the viscous flow region, say 5 torr, and then is valved off. The nonadsorbing gases are swept into the pump along with the adsorbed gases; the pump now contains
a fraction (760–5)/760 or 99.3% of the nonadsorbable gases originally present, leaving hydrogen, helium, and
neon in the low 10-4 torr range in the vacuum chamber.
A second sorption pump on the vacuum chamber will then readily achieve a base pressure below 5 X 10-4 torr,
quite adequate to start even a recalcitrant ion pump.
Many different types of oil-free pumps are available. We will emphasize those that are most useful in analytical
and diagnostic applications.
Diaphragm Pumps:
Applications: Diaphragm pumps are increasingly used where the absence of oil is an imperative, for example, as
the forepump for compound turbomolecular pumps that incorporate a molecular drag stage. The combination renders oil contamination very unlikely. Most diaphragm pumps have relatively small pumping speeds. They are
adequate once the system pressure reaches the operating range of a turbomolecular pump, usually well below
10-2 torr, but not for rapidly roughing down a large volume. Pumps are available with speeds up to several
liters per second, and base pressures from a few torr to as low as 10-3 torr, lower ultimate pressures being associated with the lower-speed pumps.
Operating Principles: Four diaphragm modules are often arranged in three separate pumping stages, with
the lowest-pressure stage served by two modules in tandem to boost the capacity. Single modules are adequate for subsequent stages, since the gas has already been compressed to a smaller volume. Each module uses a flexible diaphragm of Viton or other elastomer, as well as inlet and outlet valves. In some pumps the modules can be arranged to provide four stages of pumping, providing a lower base pressure, but at lower pumping speed because only a single module is employed for the first stage. The major required maintenance in such pumps is replacement of the diaphragm after 10,000 to 15,000 hr of operation.
Scroll Pumps :
Applications: Scroll pumps (Coffin, 1982; Hablanian, 1997) are used in some refrigeration systems, where the
limited number of moving parts is reputed to provide high reliability. The most recent versions introduced for
general vacuum applications have the advantages of diaphragm pumps, but with higher pumping speed. Published speeds on the order of 10 L/s and base pressures below 10-2 torr make this an appealing combination. Speeds decline rapidly at pressures below ~2 X 10-2 torr.
Operating Principles: Scroll pumps use two enmeshed spiral components, one fixed and the other orbiting. Successive crescent-shaped segments of gas are trapped between the two scrolls and compressed from the inlet(vacuum side) toward the exit, where they are vented to the atmosphere. A sophisticated and expensive version of this pump has long been used for processes where leaktight operation and noncontamination are essential, for example, in the nuclear industry for pumping radioactive gases. An excellent description of the characteristics of this design has been given by Coffin (1982). In this version, extremely close tolerances (10 mm) between the two scrolls minimize leakage between the high- and low-pressure ends of the scrolls. The more recent pump designs, which substitute Teflon-like seals for the close tolerances, have made the pump an affordable option for general oil-free applications. The life of the seals is reported to be in the same range as that of the diaphragm in a diaphragm pump.
Screw Compressor: Although not yet widely used, pumps based on the principle of the screw compressor, such as that used in supercharging some high-performance cars, appear to offer some interesting advantages: i.e., pumping speeds in excess of 10 L/s, direct discharge to the atmosphere,and ultimate pressures in the 10-3 torr range. If such pumps demonstrate high reliability in diverse applications,
they constitute the closest alternative, in a singleunit ‘‘dry’’ pump, to the oil-sealed mechanical pump.
Molecular Drag Pump:
Applications: The molecular drag pump is useful forapplications requiring pressures in the 1 to 10-7 torr range
and freedom from organic contamination. Over this range the pump permits a far higher throughput of gas, compared to a standard turbomolecular pump. It has also been used in the compound turbomolecular pump as an integral backing stage. This will be discussed in detail under Turbomolecular Pumps.
Operating Principles: The pump uses one or more drums rotating at speeds as high as 90,000 rpm inside stationary,coaxial housings. The clearance between drum and housing is ~0.3 mm. Gas is dragged in the direction of rotation by momentum transfer to the pump exit along helical grooves machined in the housing. The bearings of these devices are similar to those in turbomolecular pumps (see discussion of Turbomolecular Pumps, below). An internal motor avoids difficulties inherent in a high-speed vacuum seal. A typical pump uses two or more separate stages, arranged in series, providing a compression ratio as high as 1:107 for air, but typically less than 1:103 for hydrogen. It must be supported by a backing pump, often of the diaphragm type, that can maintain the forepressure below a critical value, typically 10 to 30 torr, depending upon the particular design. The much lower compression ratio for hydrogen, a characteristic shared by all turbomolecular
pumps, will increase its percentage in a vacuum chamber, a factor to consider in rare cases where the presence
of hydrogen affects the application.
Sorption Pumps:
Applications: Sorption pumps were introduced for roughing down ultrahigh vacuum systems prior to turning
on a sputter-ion pump (Welch, 1991). The pumping speed of a typical sorption pump is similar to that of a small oilsealed mechanical pump, but they are rather awkward in application. This is of little concern in a vacuum system likely to run many months before venting to the atmosphere. Occasional inconvenience is a small price for the ultimate in contamination-free operation.
Operating Principles: A typical sorption pump is a cannister containing ~3 lb of a molecular sieve material
that is cooled to liquid nitrogen temperature. Under these conditions the molecular sieve can adsorb ~7.6X 104 torrliter of most atmospheric gases; exceptions are helium and hydrogen, which are not significantly adsorbed, and neon, which is adsorbed to a limited extent. Together, these gases, if not pumped, would leave a residual pressure in the 10-2 torr range. This is too high to guarantee the trouble-free start of a sputter-ion pump, but the problem is readily avoided. For example, a sorption pump connected to a vacuum chamber of ~100 L volume exhausts air to a pressure in the viscous flow region, say 5 torr, and then is valved off. The nonadsorbing gases are swept into the pump along with the adsorbed gases; the pump now contains
a fraction (760–5)/760 or 99.3% of the nonadsorbable gases originally present, leaving hydrogen, helium, and
neon in the low 10-4 torr range in the vacuum chamber.
A second sorption pump on the vacuum chamber will then readily achieve a base pressure below 5 X 10-4 torr,
quite adequate to start even a recalcitrant ion pump.
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