The evolution of vacuum science which started in the 17th Century, has mirrored many other scientific achievements, including the the in-development of the Gas Laws and the discovery of the electron. Nevertheless, the world of vacuums still continues to excite and galvanise engineers and scientists. Indeed, ground breaking developments continue to push the boundaries of this fascinating subject.
Vacuum Physics – Basic Terms
PRESSURE UNITS
What is the unit of Vacuum Pressure?
Below is an overview of the major pressure units and conversion of pressure units:
VACUUM RANGES
It is common in vacuum science to sub-divide pressure ranges into five individual regimes:
- Rough (or Low) vacuum (R): Atmospheric to 1 mbar
- Medium (or Fine) vacuum (MV): 1 to 10–3 mbar
- High vacuum (HV): 10–3 to 10–7 mbar
- Ultra-high vacuum (UHV): 10–7 to 10–12 mbar
- Extreme High Vacuum (XHV): greater than 10-12 mbar.
These divisions are somewhat arbitrary, with various engineering disciplines using their own definitions, ie chemists frequently refer to their spectrum of greatest interest (100 to 1 mbar), as an “intermediate vacuum”, whilst some engineers may refer to a vacuum as “low pressure” or “negative pressure”.
FLOW TYPES
Vacuum technology is usually associated with three types of flow: viscous or continuum flow; molecular flow; and a transitional range between these two known as Knudsen flow.
Viscous (or continuum) flow is found in the rough vacuum range and is determined by the close interaction of molecules. There are three sub-divisions of viscous flow: “turbulent flow” (if vortex motion appears in the streaming process); “Poiseuille flow” where layers slew over each other (which is frequently the case in vacuums); and “choked flow” which occurs when venting vacuum vessels, or where there are leaks.
Molecular flow prevails in the high and ultra high vaccum (uhv) ranges when molecules can move freely, without any mutual interference. Molecular flow is present where a molecule’s mean free path ƛ defined as the mean distance travelled by molecules between collisions) is much larger than the diameter of the pipe.
Knudsen flow is the transitional range between viscous and molecular flow. It is prevalent in the medium vacuum range where a molecule’s free path length is similar to the diameter of the pipe.
In viscous flow, the preferential movement of gas molecules will be identical to the macroscopic direction of gas flow, since the particles are densely packed and will collide with one another far more frequently than with the boundary walls. However, in molecular flow, particles impacting with the walls, predominate.In rough vacuums, the collision of gas particles frequently occurs, whereas in the high and ultra-high vacuums, impact of the gas particles with the container walls predominates.
CONDUCTANCE
All fixtures between intake of pump system and chamber will lead to a reduction of pumping speed. The pV flow through any desired piping element, i.e. pipe or hose, valves, nozzles, openings in a wall between two vessels, etc., is indicated with
Here Δp = (p1 – p2) is the differential between the pressures at the inlet and outlet ends of the piping element. The proportionality factor C is designated as the conductance value or simply “conductance”. In the molecular flow range, C is a constant which is independent of pressure; in the transitional and viscous flow range it is, by contrast, dependent on pressure. As a consequence, the calculation of C for the piping elements must be carried out separately for the individual pressure ranges.
Above equation is often referred to as the “Ohm’s law for vacuum technology”, in which qpV corresponds to current, Δp the voltage and C the electrical conductance value. Analogous to Ohm’s law in the science of electricity, the resistance to flow has been introduced as the reciprocal value to the conductance value:
The equation thus can then be re-written as:
If components are connected in parallel, the following applies:
For components connected in series the following applies:
Vacuum Generation
PUMP PARAMETERS
Pumping speed
The volume flow rate (qV) or pumping speed (S) is the (net) volumetric flow rate or volume of gas discharged per unit time (m3/s, l/s, cfm, m3/h…). This is measured at the pump inlet and depends upon gas species, vapour etc.
Pump throughput
The pumping capacity (throughput) for a pump is equal either to the mass flow through the pump intake port:
or to the pV flow through the pump’s intake port:
It is normally specified in mbar · l · s–1. Here p is the pressure on the intake side of the pump. If p and V are constant at the intake side of the pump, the throughput of this pump can be expressed with the simple equation
where S is the pumping speed of the pump at intake pressure of p.
The throughput value is important in determining the size of the backing pump in relationship to the size of a high vacuum pump with which it is connected in series in order to ensure that the backing pump will be able to “take off” the gas moved by the high vacuum pump.
Ultimate pressure
Ultimate pressure pult is the lowest pressure of a blank-flanged vacuum pump under defined conditions without gas inlet. At ultimate pressure, the usable pumping speed will be zero. It is a theoretical value.
The lowest pressure which can be achieved in a vacuum vessel will be determined by
- Pumping speed
- Vapor pressure of lubricants
- Degassing of solved gases in lubricants
- Desorption of gases from internal surfaces of vessel
- Leak tightness of system itself
- Diffusion of gas though vacuum wall or seals
- Compression of vacuum pump system
Compression ratio
The compression ratio CR (or k) is the ratio of the exhaust/outlet pressure (pout) to the inlet pressure (pin)
The maximum compression k0 (Outlet pressure / Inlet pressure) assumes no flow conditions (zero pumping speed) and is a theoretical value.