In the world of vacuum systems, scroll pumps hold a valuable place as one of the few pumps that are traditionally employed in low (i.e. 1000 mbar to 1 mbar) and medium (i.e. 1 mbar to 10-3 mbar) systems, and yet are now also frequently being employed as fore (or backing) pumps in high and ultra-high (i.e. 10-3 to 10-12 mbar) vacuum systems.
Part of their attraction is that, being a “dry” pump, there is no risk of contamination, although to counter this significant characteristic, it must be acknowledged that they are intolerant to particles and, in addition, there is a potential of particle generation due to tip seal wear.
working principles of scroll pumps
The main working parts of a scroll pump consist of two co-wound spiral-shaped scrolls contained within a vacuum-housing, with an exit valve at the centre of the scroll assembly. One spiral is fixed whilst the other (the “orbiter”) moves eccentrically without rotating, against the other. Gas enters the (outside) open end of the spirals and, as one of the spirals orbits, gas is trapped between the scrolls and transported towards the centre as the void occupied by the gas is “squeezed and transported” between the two spirals.
As this finite “slug” of gas moves towards the centre, the volume it occupied decreases and, as such, this captive gas is continuously compressed until, at the centre of the housing, it is expelled at pressure via a non-return valve. Although none of the moving parts contained within the chamber require lubrication, the PTFE tip seals are subject to wear and will need periodic changing.
The performance characteristics of scroll pumps vary depending on a number of factors, including the unit size, speed of operation, ambient temperature and characteristics of the gas/vapour being pumped. The following (typical) listings provide some of the most significant performance characteristics:
- ultimate pressure approximately 10-2 mbar;
- pumping speed between 3 and 60 m3/hr;
- leak tightness of less than 10-6l.s-1
- noise (measured 1m from the end of the pump) is less than 55.4 +/- 2.5 db(A);
- and suck-back protection is provided by an exhaust valve
Furthermore, scroll pumps are available with frequency converters designed so that their performance throughout the world and, in addition, three-phase electronic-free variants of scroll pumps are available for use in radiation and high-magnetic field environments.
Applications for scroll pumps
Scroll pumps are but one of several oil-free/dry fore pumps (the others being diaphragm, multi-stage Roots and screw pumps) which are employed as fore pumps in high vacuum and ultra-high vacuum systems. However, as always, final selection comes down to their advantages and disadvantages and limitations, as well as what the system is being employed to achieve.
Scroll pumps are used in a diverse range of applications, but are most frequently employed to pump and compress gases and vapours when clean, dry vacuum pumping is essential. They are also used for non-corrosive/non-hazardous applications, surface analysis and sample preparation. Furthermore, scroll pumps which are often used for analytical purposes (e.g. leak detection or with electron microscopes), are frequently used as fore pumps for turbomolecular systems, where a clean final product is required.
Advantages, disadvantages and limitations
In common with every other type of vacuum pump, scroll pumps have a number of advantages and — remarkably few though, not insignificant — disadvantages, as well as limitations which need to be considered before a definitive decision is made to employ them in a vacuum system. However, perhaps their most significant advantages are that although they have a relatively high initial cost, their operating costs are low because they do not require oil (which also makes them environmentally friendly).
Advantages of Scroll Pumps
- High speeding pumps
- Good vapour handling capacities
- Good ultimate pressure
- Low noise level and vibrations (< 55 dB (A))
- Low power consumption
- Pump bearings (in some models) are isolated from vacuum space (so there is no contamination risk)
- Simple, compact design and low weight
- Small pumps are suitable as fore pumps for low gas throughputs
- Low operating costs (although initial costs are high)
- Some models are fitted with an automatic non-return exhaust valve
- Pumping characteristics are (almost) independent of gas type
Disadvantages of Scroll Pumps
- Tip seals have a low tolerance to introduced debris
- With increasing tip seal wear, pumping speed and ultimate vacuum performance will be reduced
- Some models (without an exhaust valve) require an automatic inlet non-return valve
- Tip seal wear generates particles
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