Where is vacuum used

Of course, since the applications and markets are so diverse, each application requires a different vacuum solution and poses different challenges. With this considered, an understanding of the underlying principles that govern vacuum science and the hardware available is essential for all those in the industry.

This includes the fundamentals of vacuum and how they impact the mechanism, choice and use of vacuum pumps, as well as the classification of pump technologies and pump selection. Download it for free by clicking here. Privacy Policy. This insulating effect of vacuum leads to a number of practical applications. Perhaps best known is the venerable dewar flask or thermos bottle shown in Figure 3 that is commonly used to keep things hot or cold. This easily leads to other applications such as transfer lines for liquefied gases that are surrounded by an annular tube with a vacuum between the tubes to prevent heat from reaching the liquid and causing vaporization.

As with mechanical effects from pressure differentials, thermal conductivity can be used as a pressure gauging technique where a wire is heated by the application of constant power and the heat loss of the wire is indicated by its temperature which is either measured directly thermocouple gauge or by its resistance pirani gauge. The same effects as found with thermal conductivity can also be applied to sound or electrical insulation with some specific complex differences.

Virtually any material will vaporize if you get it hot enough, but vacuum processes tend to be mostly concerned with the vaporization of fluids. A fluid that has any volatility at all at room temperature will vaporize sooner or later at atmospheric pressure. In terms of a practical process, sooner will usually be better than later, so means to increase the vaporization rate can be important. Since vaporization is really molecules of the liquid leaving the surface and not returning as liquid, increasing the rate of vaporization will mean increasing the number of molecules leaving in a given time.

At atmospheric pressure, the rate of loss will be relatively slow because of the high number of molecules directly above the surface. This means that a vaporizing molecule will probably immediately impact a gas molecule, lose its energy, and return to the liquid state. If, however, the liquid is within a chamber that has been evacuated to some extent, fewer molecules will be above the surface. This means that a vaporizing molecule will have a lesser chance of impacting a gas molecule because there are fewer molecules to hit, bigger spaces between molecules, and fewer molecules impacting the liquid surface.

A practical example would be the difference in the boiling point of water between valley and mountaintop. When a liquid boils, it has reached a critical point where the heat being added to the liquid is instantly translated into vaporization so the temperature of the liquid will not change.

As altitude is increased, the pressure is reduced so there are fewer molecules the inhibit vaporization and less energy is required for the vaporizing molecules to overcome the collisional losses from the ambient gas molecules.

Figure 4 shows the difference in the boiling point of water at various altitudes. A practical vacuum process would be vacuum distillation where it is necessary to separate two liquids with different vapor pressures. Flowing a film of the liquid mixture into an evacuated container at a fixed temperature would force or allow the most volatile liquid to vaporize at a low temperature because fewer molecules would be available to inhibit vaporization than would be present at atmospheric pressure.

Hence, fast distillation for a practical process. An example of this kind of process would be the distillation of mechanical pump oil where it is necessary to remove the high vapor pressure volatile components before it can be used in a vacuum pump. Chemical effects come into play in most cases where the chemical reactivity and properties of the gases will either help or inhibit a process. This often concerns not only the particular gases in question, but also their respective concentrations.

Any container, chamber, or plumbing will have been exposed to atmospheric air at some time during its history. Before any of these are used to contain or transfer pure process gases, they need to be evacuated to minimize the detrimental effects of the gases inside before the pure gas is introduced.

If this were not done, it would be much like pouring a purified chemical solution into a dirty beaker. The degree of purity of the gas required would dictate the ultimate vacuum that was necessary since the population of the residual gases would all be considered as contaminants.

For example, an oxygen pressure of 10 -3 torr would result in a contamination level of 1 PPM if the container was backfilled to atmospheric pressure with pure gas. In this system, a container with urethane in it would be placed in the vacuum chamber and the acrylic lid replaced on top.

The see-through lid lets the operator watch the process and control how fast it degasses. Degassing resin, varnish and urethane can result in a lot of frothing.

As the gas bubbles in the liquid move to the top of the liquid, expand and burst they release the trapped gas into the chamber where it is evacuated by the vacuum pump. To reduce the frothing the rate of pressure drop is controlled by a valve such as a ball valve in the vacuum line outside the chamber.

After the first high rate of frothing has diminished the operator can visually see when there is no more gas escaping from the product. In larger systems with gallons of liquid to degas, a stirring mechanism is often added to the chamber to slowly move liquid from the bottom of the chamber to the top. The gas bubbles will escape from the liquid easier when they are near the top of the liquid with less weight of liquid pressing on them. The chamber shown in Fig. To prevent arcing the special oil used in electrical transformers is degassed to remove entrained air and absorbed moisture before it is used to fill a transformer case.

The oil runs over the plates in a thin film which improves the efficiency of the degassing. Varnishes and resins are used for potting and also for insulating items such as electric motor and transformer windings. The liquid is degassed and then used in a two chamber unit called an impregnation system Fig. The windings are placed in a vacuum chamber and placed under vacuum. Molecules of air are trapped between the layers of copper wire and it takes some time for this air to escape from the small spaces and be pumped away.

Once the air is removed the liquid is introduced to the vacuum chamber from a storage vessel. With the air molecules removed the liquid is able to penetrate every small space between the windings and create a layer of insulation. A positive pressure is applied to the top of the liquid to help force the liquid into the tiny voids. Many metal castings often have voids in them when they solidify.

These castings are often impregnated with liquid resin to seal any porosity and imperfections. Once the casting is impregnated it is heated to cure the resin. The process is similar to VPI described above and is especially important in the automotive engine industry to seal aluminum casting.

If you commute to work by car and your drive is an hour or so, you may well take a coffee or another hot or cold beverage in your own personal container to drink on the way. Often this type of container is a vacuum insulated cup with a lid. A step up from that, if you work on a job site for example, would be a vessel that holds several cups of liquid for all day use. Air at atmospheric pressure, Torr or mbar, contains about 25 million, million, million molecules in each cubic centimeter of volume Fig.

This volume can be visualized as being similar to that of a sugar cube or a die. These molecules generally move in straight lines until they either collide with each other or with a surface. They move at a speed of about miles per hour. At higher pressure such as atmospheric pressure, they are more likely to collide with another molecule and will move off in another direction until the next collision.

Each molecule will travel only about 2. If the molecules are inside a chamber at atmospheric pressure, some molecules will collide with the inner surface of the chamber and any work holders or fixtures. When that occurs, the molecule typically resides on the surface for a fraction of a second and then releases and moves away from the surface in a completely random direction.

There will be different results if the chamber wall is either heated or cooled. If a molecule hits a cooler surface it will lose some of its heat to that surface and may have less energy available to release off the surface. In some cases, it may adsorb onto the surface if it loses most of its energy. On the other hand, if the surface is warmer it will gain energy and will release with increased energy.

So, as these gas molecules are moving randomly around the vacuum chamber they can be constantly gaining or losing energy, depending on whether they collide with a hotter or cooler molecule of the surface. If we think about the last statement, we can conclude that if there are fewer molecules in the closed vessel, there will be less heat transmitted due to fewer molecular collisions.

That leads us to consider molecular density. How many molecules are there in unit volume at lower pressures than atmospheric conditions?

A few examples are shown in Fig. These guys are very tiny. At a typical vacuum furnace pressure of between 10 -5 and 10 -6 Torr there are still about 33 thousand million to thousand million molecules of gas in each cc of volume. Even in outer space 10 Torr , and we know from the science shows that there are gas clouds out there, physicists estimate that there are around 4 molecules of gas in every cubic meter of volume.

As the density of the molecules is reduced as the pressure drops, the distance between the individual molecules increase. There are fewer molecules in the volume so they must move a longer distance before colliding with another one.

This distance will vary, so the physicists have another term, mean free path, to average the distances for comparison. Mean Free Path is the average distance a gas molecule will move before it collides with another molecule, at a certain pressure.

It is another part of the wonders of science that have slowly been discovered over the last couple of hundred years or so. See Fig. At higher pressure,s the Mean Free Path is very short and the molecules collide frequently.

The MFP is 2. As the pressure is reduced to 1 Torr the MFP is has increased to about 2 thousandths of an inch, a movement that we can relate to in engineering. As the pressure reduces to the level that a combination of an oil-sealed rotary piston vacuum pump and Roots booster can reach 0. If those molecules are in a part of the system larger than about 6 inches diameter, they are still moving in a viscous flow. Meaning they are more likely to collide with another molecule than with the vessel wall.

If however, the molecules are in the holding pump piping usually a 25mm line they will be moving in molecular flow; meaning they are more likely to collide with the inner wall of the piping than with another molecule. As the pressure drops to about 10 -5 Torr the MFP has lengthened to about 16 feet. For most average sized vacuum furnaces that means that molecules of gas inside the vacuum chamber are much more likely to collide with an interior surface of the chamber than with another gas molecule.

In between are many different shapes and sizes of vacuum insulated containers, some called Dewars, and also installed systems where connection pipelines have to be vacuum-insulated as well. Typical insulated flasks for hot and cold liquids look like the one shown in Fig. Although Sir James Dewar invented what is known as the Dewar flask, he never patented it.

Two Germans discovered its use for commercial purposes and registered the name Thermos in The Thermos company still exists today although they have many competitors. Vacuum insulation is used because it works well and as long as there are no leaks, the item will last for many years.

It is mainly used where the temperature difference between the outside and the inside of the vessel is large and many scientific and industrial applications involve cryogenic liquids.

One of the most popular cryogenic liquids, liquefied gases, is liquid nitrogen. It is usually written as LN 2. Safety precautions must be taken when handling any cryogenic liquids. They will burn skin and damage your eyes. Gloves and a face mask are required if transferring any cryogenic liquid into or out of an open Dewar flask or other suitable vessels.

Obviously, glass containers are fragile, and the glass used is quite thin, so care must be taken not to drop or bang them which could break the glass. The silvered surface reflects heat and is often a coating of evaporated aluminum. Another vacuum process! The laboratory-sized Dewars, Fig. These can be carried by hand or on a small cart. The small Dewars shown are typically purchased by the end user and filled in-house from a larger container such as the larger cylinder shown.

The larger cylinder would only be handled by a qualified technician due to its weight and value. These vessels are evacuated to the 10 -5 Torr region using either turbo or diffusion pump systems, and then left to pump for another 48 hours Ref. Gas flow through the insulation layers is slow and it takes a while to reach the vacuum level required throughout the whole interspace. Although the large gas companies such as Air Products, Linde and Air Liquide produce these cryogenic liquids, they have mostly let the distribution of these smaller volumes be carried out by companies such as AirGas and other smaller local distributors.

They have a regular schedule of deliveries so that labs and hospitals are never without product. The large gas companies look after the large volume storage tanks that have to be refilled by trailer loads of product, Fig. These vessels are very heavy, and the inner vessel requires substantial supports between it and the outer vessel to provide strength, support, and the correct spacing. The annular vacuum volume is filled with layers of aluminized material and also Perlite.

Perlite is a material similar to natural glass. When heated it expands to between 4 and 20 times its original volume with lots of voids in its structure. It is a good insulator and resists crushing. Literature suggests that Perlite can settle, especially in a vessel such as a road tanker that is subject to vibration over a long period of time.

If too much product is lost by boiling off, this is an additional cost to the gas company and the trailer may have to be serviced.

In this area would also be a vacuum gauge head on a gauge head connection, to allow the interspace pressure to be checked as needed. The vacuum gauge typically used for this is a battery operated thermocouple gauge. This gauge actually reads the temperature of a hot filament in the gauge sensor, but instead of reading it in degrees of heat, it reads it as an equivalent vacuum reading.

The hot filament changes temperature as the vacuum level changes. When the pressure drops in the sensor there are fewer molecules of gas to take heat away from the filament, so it gets hotter. This change can be calibrated to indicate the pressure. When you see these large tank trailers on the highway please allow them plenty of space. They carry a precious and valuable cargo.

To increase the Mean Free Path to a useful dimension. The article printed back in January this year talked about solid, liquid and gas states of matter. The following is a short excerpt from that article. In air at atmospheric pressure and room temperature, the actual space occupied by atoms and molecules is about 0.

The equivalent for solid copper is about 74 percent or close to three quarters. In air, the molecules are in constant random movement, typically in a straight line, and the interatomic forces have little effect due to the space between the molecules.

The moving molecules will constantly collide with other molecules and then move away in a different direction. These collisions occur about 10,,, times per second at atmospheric pressure.

Atmospheric pressure is always the starting point of any vacuum process, and we know that we can reduce that pressure in a closed vacuum chamber by using one or more vacuum pumps to reach whatever lower pressure vacuum is required for the process. In the excerpt above it states that molecules in the chamber are constantly colliding with other molecules and changing direction. Molecules of gas that collide with the inner chamber wall, or the surface of any fitment, work holder or product in that chamber will reside on that surface for a fraction of a second and then release off the surface in a completely random direction.

We usually show the mean free path at a certain pressure because that is how we measure the vacuum level using some type of vacuum gauge Fig. Density is related to the pressure but is rarely used as an indicator for vacuum processes. From Fig. Molecules of gas are being evacuated by the pumps, the density of the gas is being reduced and the molecules have to move further before they collide with another molecule.