In the world of vacuum, there are significant differences between those at the lower end of the spectrum and those that occupy the higher (i.e. high vacuum) levels. In terms of definitions: vacuum ranging between atmospheric pressure and 1 mbar is known as “rough” vacuum, whilst pressures from 1 to 10-3 mbar are known as “medium” vacuum. Thereafter, the vacuum definitions progress from high to ultra-high (UHV) through to extreme high vacuum (XHV), and range from 10-3 to <10-12 mbar.
Working in Rough and Medium Vacuum Conditions
When working in rough and medium vacuum conditions there is a fundamental truth that needs to be accepted: no single pump will match all your requirements or expectations.
Therefore, it is imperative to list the requirements that absolutely must be met, alongside those that would be desirable (but not essential). It would be logical to assume that one needs to achieve a certain vacuum level and throughput. Thereafter, a range of other criteria should be considered including noise and vibration considerations, ease of maintenance, capital and ongoing costs, the size (i.e. footprint) of the pump itself, its resistance to shock, tolerance to particle intrusion and, whether oil contamination would be an issue.
Rough and Medium Vacuum Generation
When compared to HV through to XHV, the types of pump employed for rough and medium vacuums are fairly simple in terms of operation. However, that is not to underestimate the exacting or precise engineering required (or indeed the science) behind their operation. Furthermore, it should not be forgotten that many of these pumps are employed as fore (or backing) pumps, which are employed to “vacuum-charge” or support, the higher-level vacuum pumps. Without the benefit of such fore pumps, these higher vacuum units would--at best--operate sluggishly and slowly, and--at worst--not at all.
Diaphragm pumps operate in the low vacuum regime. Due to their design, they do not achieve high compression ratios at a single stage. Therefore, one often finds two-, three- and even four-staged diaphragm pumps. Such configurations make them useful as compact and environmentally-friendly units, for example, in laboratory applications and for backing turbomolecular pumps (TMPs). Diaphragm pumps can produce a standard operating range from 103 down to the low mbar range.
These pumps employ a diaphragm (which forms one side of the chamber) which is moved backwards and forwards by a rod. This oscillating motion compresses the gas and activates the valves. The gas moves in through an inlet valve and (when the diaphragm moves back), the inlet valve is closed and the gas is pressurised before being expelled through the exit valve.
The diaphragm and the valves are usually made of PTFE, which make them resistant to corrosives, and less vulnerable to vapour damage. Since diaphragm pumps are “dry” by design, they provide a hydrocarbon-free vacuum. Further advantages of diaphragm pumps are that they are easy to clean and maintain, are suitable for pumping many gases and laboratory chemicals, and as they do not use any oil, their operational and maintenance costs are low.
Scroll pumps, which have a pressure range of 103 to 10-2 mbar, use two inter-leaved Archimedean spiral-shaped scrolls to pump or compress gases. One of the scrolls is fixed, while the other orbits eccentrically within the chamber without rotating, which traps and compresses pockets of gas between the scrolls. This in turn moves the trapped gas from the outside part (i.e. inlet) to the inner part (i.e. outlet) of the chamber.
Scroll pump internals
Scroll pumps are used in a diverse range of applications such as in analytical instruments (e.g. mass spectrometry and in electronic microscopes) where a clean, dry and quiet vacuum is required. Furthermore, scroll pumps are frequently used as backing pumps for TMPs.
Scroll pumps have many advantages over other vacuum pumps: of which the most significant is that their operating costs are low because they do not require oil (which also makes them environmentally friendly). In addition, their maintenance needs are low. However, the wearing of tip seals can lead to particle emission.
Rotary Vane Pumps
Rotary vane pumps, which have a range of 103 to 10-4 mbar, are the most common type of positive displacement vacuum pumps. They work in the following manner: an offset rotor (fitted with vanes that slide in and out of their housing) rotates within a chamber. The vanes, which seal against the inside of the circular chamber, “trap” a quantity of gas which enters through an inlet port. As the rotor rotates, the volume contained between the vanes and the inside surface of the chamber decreases, so the pressure of the “captured” gas likewise increases, until it exits through the outlet port.
Rotary vane pump
Rotary vane pumps offer excellent reliability, robustness, compact design and low investment costs, which make then ideal for numerous industrial and coating applications, including for analytical instruments as well as R&D and industrial applications.
Additionally, their operating pressure range makes them ideal backing pumps for any kind of medium and high vacuum pumps. Whereas the oil-sealed operation is a disadvantage for some applications, the use of oil enables higher compression ratios, a better internal cooling behaviour and makes the pump compatible with dirt, dust and condensation. Of course, the need for servicing the pumps regularly (i.e. oil changes) means higher costs of ownership (compared to dry pumps of a similar size), and they do not provide an oil-free vacuum (hydrocarbons or PFPE etc).
Screw pumps, which have a range of 103 to 10-2 mbar, operate using two counter-rotating screw rotors which are engineered so that they rotate “in on each other”, thereby trapping the gas in the volume between the “screws” of their rotors. As the screws rotate, this trapped volume (as it travels towards the exit port) decreases which not only compresses the gas but moves it towards the exit. Screw pumps are frequently used as fore pumps for Roots pumps.
Screw pumps have numerous important characteristics: despite the micro-space between the two rotating screws, there are no contacting parts nor is there any need for lubrication and, as a result, there is no contamination of the medium being pumped. Furthermore, rotor wear is eliminated, they have a high tolerance against particles, employ high pumping speeds and, are highly efficient due to internal compression. However, they are less suitable for pumping light gasses, and cannot be scaled-down to small pumping speeds. Operational costs and maintenance requirements are also relatively low. Screw pumps are suited to a wide range of applications, such as industrial furnaces, metallurgical systems, packaging and coating.
Roots booster Pumps
Roots pumps, which have a pressure range of 10 to 10-4 mbar, are commonly employed as ‘booster’ pumps to improve ultimate pressure and pumping speeds. Roots pumps employ two counter-rotating interconnecting units rotating within a chamber. Gas enters through the intake flange and is “pinched” between the two rapidly rotating units and the chamber wall and is then expelled through the exit port.
The advantages of Roots booster pumps are that they are very quiet and compact, enjoy a long service life, have no contacting parts, and they provide clean pumping (i.e. there are no particles or oils to contaminate the vacuum system).
A multi-Roots fore pump working in conjunction with a HV, UHV or XHV pump is typically a more economical option for achieving high vacuum, compared to a bigger sized discrete fore pump due to its improved pumping speeds and ultimate pressures.
Roots pumps are frequently used in industrial applications (i.e. the laser industry, furnaces, metallurgy etc.) due to their high pumping speeds, in space R&D, and the fabrication of semiconductors and solar panels.
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Rough and Medium Vacuum Measurement
Rough and medium vacuums are usually measured by what are referred to as “direct gauges”, which measure pressure irrespective/independent to the composition of the gases involved.
Direct gauges fall into two categories: those that rely upon some form of mechanical deformation, such as the diaphragm, the Bourdon tube, the piezoresistance or electrical capacitance; and those that employ the height of a liquid column, which are known as “hydrostatic” gauges.
Mechanical gauges employ metallic internal workings which change their shape depending on pressure, with this deflection linked to a needle-gauge. A variation of these is the capacitance manometer, in which the diaphragm (which makes up part of the capacitor) flexes with pressure change, resulting in a (measurable) capacitance change.
In terms of the challenges associated with measuring pressures, it must be remembered that the physical properties of gases change with pressure. For example, the thermal conductivity and the internal friction of gases are such that their usage depends upon a wide range of factors including: pressure range; what gases are involved (which will determine any correction factors, media compatibility and potential for chemical reactions), the accuracy required; the operating conditions (dirty vs. clean, vibrations, temperature, shock - possibly due to pressure venting - radiation and magnetic fields); installation position of the gauge; and, how the pressure is to be read (and recorded).
Leak Detection in Rough and Medium Vacuum
The detection of leaks, as well as their elimination, management and/or accountability, are as important in vacuum as they are in pressurised systems. As gas is compressible, the pressure (or vacuum) influences the amount of the leak which is quoted in mbar.litre/sec, with the leak rate being the amount of gas that “out-flows” through a leak in a given pressure differential per unit-time.
There are several generalised ways of measuring leaks, each of which depends on the lowest detectable leak rate applicable: the bubble test; differential pressure measurement; pressure decay; pressure rise tests; the helium sniffer mode; and, the helium vacuum mode. These two latter test methods are also referred to as the “tracer gas detection” methods. All methods can be used in rough and medium vacuum.
The bubble test involves pressurising the system, smearing a potential leak point with soap and seeing if it froths, whilst the differential pressure measurement involves gauging the loss of pressure over a set period of time.
However, the most interesting leak tests involve the helium “sniffer” and the helium “vacuum” test.
In simple terms the helium “sniffer” test involves a sniffer probe being passed around the unit under observation with the “sniffed” gas being passed through a mass spectrometer, for helium identification and measurement.
The sniffer test has the advantage that it shows where leaks actually occur. However, helium concentrations of 5ppm in ambient/atmospheric air mean that it is difficult to differentiate between a background signal and a very low leak rate.
The helium “vacuum” test is usually employed on units subjected to HV and UHV applications. In simple terms, the unit is placed inside a vessel, and pressurised with helium. Gas within the vessel is then subjected to a mass spectrometer test, and any helium detected will indicate a leak. The major disadvantage—though not the only one—is that the unit needs to be placed within a vessel of a suitable size. Alternatively, the vessel is evacuated by the leak detector and helium used to ‘sniff’ externally.
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