|What is Pressure?
Imagine a closed container with air, or some other gas, inside. Gases are composed of molecules that you can imagine as round elastic balls. Molecules move in straight lines until they collide with neighboring molecules or the container wall. Molecules of gas hitting the wall impose a force on the wall. The amount of this impact force per area of the container inner walls is called pressure. Since gases are considered fluids, the “pressure” exerted by these collisions applies to all surfaces of the container no matter what their orientation.
At room temperature and normal atmospheric pressure, one cubic foot of air contains approximately 7×1023 molecules moving in random directions and at speeds of around 1,000 miles per hour. The momentum exchange imparted to the walls is equal to a force of 14.7 pounds for every square inch of wall area. This atmospheric pressure can be expressed in a number of different units, but is commonly expressed in terms of the weight of a column of mercury of unit cross section and 29.92 inches (760 mm) high. Thus, one standard atmosphere equals 29.92 in. Hg, but to avoid the anomaly of equating apparently different units, a term, torr, has been postulated. One standard atmosphere = 29.92 in. or 760 torr (1 torr =1 mm Hg).
The Table below shows conversion factors for the most common units of pressure:
From To Mult by:
What is Vacuum?
It is important to note that vacuum is NOT a “sucking” process. A molecule is only removed from a chamber when it enters the pump via random collisions. It is a common mistake to think a vacuum pump sucks gas from a chamber. There is no such force as suction. If gas molecules in one “section” of an enclosed volume are removed, then molecules from the remaining volume, in their normal random, high-speed flight, collide and bounce off walls until they fill the total space at a lower pressure. Expressing it differently, until a molecule, propelled by random collisions, enters the pumping mechanism of a pump, it cannot be removed from the chamber. The pump does not reach out, grab a molecule from the chamber, and suck it in. Grasping that basic fact makes all other aspects of vacuum easy to understand.
2. Air avoidance applications where it is merely desired to avoid some undesirable physical or chemical property of one or more of the constituents of air such as friction, convection currents, heat conduction, radiation absorption, or oxidation. (e.g., vacuum melting of reactive metals such as titanium)
3. High purity environments where any foreign material at all is an impurity. Gases dissolve in liquids and solids in amounts proportional to their pressure and contaminants settle on surfaces are a rate that is dependent upon the molecular density of the gas above the surface. Vacuum chambers can be used to disturb this equilibrium condition that exists at normal room conditions, such as the removal of occluded or dissolved gas or volatile liquid from the bulk of material (e.g., degassing of oils, freeze-drying) or desorption of gas from surfaces (e.g., the cleanup of microwave tubes and linear accelerators during manufacture)
4. Atomic and molecular beam applications. As the distance that a molecular or atomic particle can travel is directly dependent upon the space between the stray molecules in its surroundings, beams of these particles will move in an increasingly unimpeded fashion as the ambient pressure is lowered and the mean free path increases. Thus, a vacuum chamber is used to extend the distance that a particle must travel before it collides with another, thereby helping the particles in a process to move without collision between source and target (e.g., in vacuum coating, particle accelerators, television picture tubes)
5. To reduce the number of molecular impacts per second, thus reducing chances of contamination of surfaces prepared in vacuum (e.g., in clean-surface studies and preparation of pure, thin films).
6. Thermodynamic applications where the temperature at which a chemical or physical process proceeds depends upon the absolute pressure of the system.
|Vacuum Terminology||Mean Free Path –
Reduction in pressure results in a lower density of gas molecules. Given a certain average velocity for each constituent molecule of air at a given temperature (at room temperature this is about 1673 km/hr) an average molecule will travel a certain distance before it interacts (collides) with another at any given pressure. This average distance between collisions is the mean free path. At 1 Torr in air this distance is about 0.005 cm, a value that scales directly with pressure. Thus the mean free path would be 5 cm at 0.001 Torr and 50 meters at 1×10-6 Torr. The lengthening of mean free path at low pressures is a key enabler for devices such as vacuum tubes and particle accelerators as well as for processes such as vacuum coating where microscopic particles such as electrons, ions or molecules must traverse considerable distances with minimal interference.
Liquids and solids are characterized by their density or specific gravity, which answers the question — what does one cubic centimeter volume of this liquid/solid weigh in grams? The units, in this case, are gm/cc. But gases fill the volume that contains them and a density measurement wouldn’t mean much. However, there is an analog to density called number density — how many molecules are contained in one cubic centimeter volume. This term allows us to describe a ‘quantity’ of any gas without knowing: its composition, the molecular weight of the components, or the mass of the molecules. It is known that any gas under the conditions of atmospheric pressure and zero degrees Celsius has the same number density. It does not matter if it’s pure nitrogen, pure oxygen, pure argon, pure hydrogen or a mixture of all four, if it is at atmospheric pressure and 0°C, the number density is 2.5 x 1019 cm-3. The huge number density at atmospheric pressure and the high velocities of the gas molecules means there are many, many collisions every second. Expressed another way, even with its high speed a molecule can’t travel far before hitting another. The distance the average molecule travels before colliding with another is called the Mean Free Path (MFP).
Particle Flux –
Outgassing and Vapor Pressure –
Base Pressure –
Working Pressure –
Ultimate Vacuum or Pressure –
Applications of Vacuum
At lower pressures down to about 10-4 torr, many metallurgical processes such as melting, casting, sintering, heat treatment, and brazing can derive benefit. Chemical processes such as vacuum distillation and freeze-drying also need this range of vacuum. Freeze-drying is used extensively in the pharmaceutical industry to prepare vaccines and antibiotics and to store skin and blood plasma. The food industry freeze-dries coffee mainly, although most foods can be stored without refrigeration after freeze-drying, and the technique is receiving widespread acceptance.
The pressure range down to about 10-6 torr is used for cryogenic (low-temperature) and electrical insulation. It is used in the production of lamps; television picture tubes, X-ray tubes; decorative, optical, and electrical thin-film coatings; and mass spectrometer leak detectors.
In thin-film coating, a metal or compound is evaporated under high vacuum from a source onto a base material or substrate. The base material is generally plastic for decorative coatings; glass for optical coatings; and glass ceramic, or silica for electrical coatings. Thickness of the film can vary from about 1/4 wavelength of visible light to 0.001 inches or more. In the optical field, antireflection coatings are deposited on lenses for cameras, telescopes, eyeglasses, and other optical devices, considerably reducing the amount of light reflected by the lenses and thus giving a brighter transmitted image.
To achieve vacuum high enough for thin-film coating and for other industrial uses requiring pressures down to 10-6 torr, a pumping system consisting of an oil-sealed rotary pump and a diffusion pump is used. The oil-sealed rotary pump (sometimes referred to as forepump) “roughs” the chamber down to a pressure of about 0.1 torr, after which the roughing valve is closed. The fore valve and high-vacuum baffle valve are then opened so that the chamber is evacuated by the diffusion pump and rotary pump in series.
Typical of the research equipment using vacuum down to about 10-6 torr are the electron microscope, analytical mass spectrometer, particle accelerator, and large space simulation equipment. Particle accelerators range from small van de Graaff machines to large proton synchrotrons.
In space simulation, large units that simulate space around a complete vehicle require a vacuum of 10-6 torr or below. Such vessels incorporate a complete shroud at liquid nitrogen temperature and a port through which high-intensity light can be beamed to simulate the sun’s radiation. In the pressure region down to and below 10-9 torr, research applications include electrical insulation, thermonuclear energy conversion experiments, microwave tubes, field ion microscopes, field emission microscopes, storage rings for particle accelerators, specialized space simulator experiments, and clean-surface studies. In many experiments it is not only necessary to reach such pressures of 10-9 torr but to reduce the hydrocarbons in the residual gases to an absolute minimum. Even small traces of hydrocarbons can render the results unreliable. To achieve a vacuum of this order the vacuum vessel and the equipment inside must be cleared of residual gas (degassed) to the greatest extent possible. A common solution is to bake the whole apparatus for a number of hours at about 350oC while maintaining a vacuum in the 10-5 torr region. Baking at this temperature requires the use of all-metal sealing rings. To eliminate hydrocarbons, the unit is pumped down to about 10-3 torr using sorption pumps; and from there, sputter ion pumps and titanium sublimation pumps complete the task down to 10-9 torr or below.
|Vacuum Equipment||Oil-sealed Rotary Pump –
Capacities are available from 1/2 to 1,000 cubic feet per minute, operating from atmospheric pressure down to as low as 2 x 10-2 torr for single-stage pumps and less than 5 x 10-3 torr for two-stage pumps. The pumps develop their full speed in the range from atmosphere to about one torr. The speed then decreases to zero at their ultimate pressures. Two of the most common designs are useful for pumping both liquids and gases. One is a two-bladed pump in which the rotor is eccentric to the stator, forming a crescent-shaped volume swept by the blades through the outlet valve. The second, a rotary piston pump, similar to a single blade, is part of the sleeve fitting around the rotor. The blade is hollow and acts as an inlet valve, closing off the pump from the system when the rotor is at top center.
Ultimate pressures attainable are limited by leakage between the high and low-pressure sides of the pump (due mainly to carry over of gases and vapors dissolved in the sealing oil that flash off when exposed to the low inlet pressure) and decomposition of the oil exposed to high temperature spots generated by friction.
Gas ballasting helps to prolong pump life because it removes the chief source of pump contamination, condensable vapors. The gas ballast is a vented exhaust that admits a small amount of air at atmospheric pressure to the compression side of the pump, thus permitting most condensable vapors to pass through the pump without condensing.
Typical applications of this pump are in food packaging, high-speed centrifuges, and ultraviolet spectrometers. It is also widely used as a forepump or a roughing pump, or both, for most of the other pumps described.
Mechanical Booster –
Molecular Pump –
Vapor Diffusion Pump –
Sputter Ion Pump –