How is vacuum created

You've got plenty of time to seal the bottle before it cools down. When it does cool down, the pressure will then drop to less than atmospheric pressure, so you'll have a partial vacuum. Probably the easiest way to create a vacuum at home is with a suction cup. If you press a suction cup flat against a wall and pull back, the inside of the cup will contain a vacuum. This is why the cup sticks to the wall. You can also create a vacuum inside of a syringe.

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 range both wall collisions and intermolecular collisions are influential in determining flow characteristics. Vacuum pumps that operate in the continuum viscous flow range such as roots blowers, screw pumps, claw pumps and rotary vane pumps function by moving the molecules as a group. They have the advantage of using the interactions between molecules the viscous quality of the gas to their advantage.

They create a suction to draw the volume of gas to the pump inlet, then push it through the pump mechanism, and expel it at atmospheric pressure.

As a result they can generate a high throughput and provide a quick drawdown in the roughing and low vacuum phases. Pumps used to create high and ultra-high vacuum include diffusion pumps, cryogenic pumps, and ion pumps and must operate in the molecular flow range. They therefore use different technology than roughing and low-vacuum pumps. Rather their mode of operation is to simply capture the molecules that randomly enter the pump inlet.

In order to understand the operation of a pump in the molecular flow range it is useful to imagine the molecules in a vacuum chamber as billiard balls on a pool table Fig. If the balls are all set in motion in random directions, such as after a break, but are allowed to keep bouncing off the bumpers without slowing down as gas molecules do after colliding with the chamber walls , some of them would begin to fall into the pocket. At first, while more balls are present, there is a high probability that a ball will fall into the pocket, and the balls are removed rather quickly.

As more of the balls fall into the pocket, there are fewer and fewer of them remaining, reducing the likelihood of a ball falling into the pocket. Eventually there are only a few balls left, and the probability of a ball falling into the pocket becomes very low. At this point, further ball removal is very unlikely and for practical purposes, no more balls will be removed in a reasonable amount of time. The equivalent of ultra-high vacuum has been attained.

Earlier in this article, we talked about the Kinetic Theory of Gases and in particular, we showed how the movement of atoms affects principles such as molecular density, mean free path, and molecular velocity and how they are used to analyze the macroscopic properties of gases such as pressure, temperature, and flow in a vacuum environment. Below we will discuss the related topic of gas flow, gas speed, conductance, diffusion, and effusion once again focusing on the fundamental concepts.

When designing and using vacuum systems, it is critical to be able to predict the time required to draw the pressure down to the desired level. Since this time is directly related to the speed of the gas flowing through the system, it is therefore important to understand what influences the speed of gas flow.

Of practical importance is the influence of pipe diameter, pipe bends, and devices such as filters and condensers. Flow phenomena are not easily understood unless the differing properties of the gas in the so-called continuum and molecular flow ranges are taken into account. At low flow rates in the continuum, or viscous range of gas flow occurring at pressures above roughly 1 mbar , the flow rate through a pipe or orifice is directly proportional to the pressure differential across the pipe or orifice.

Increasing the pressure further will not increase the flow rate Fig. This condition is referred to as choked flow. Choked flow occurs when the pressure differential across the orifice, or through the pipe, is such that the pressure on the ratio between the high-pressure side and the low-pressure side of the orifice reaches a certain value specific to that gas. For air this value is This is of critical importance when venting a vacuum chamber. When the vent is opened, air at atmospheric pressure flows into the chamber at no greater than sonic velocity, no matter how low the pressure is inside the chamber.

As venting continues and the chamber fills with gas, the pressure inside the chamber rises until it reaches P ATM x 0. After the pressure ratio rises above this point, the flow rate becomes proportional to the pressure difference across the vent. The takeaway is this — since the velocity of gas flow cannot be increased above sonic speed, the only way to increase the venting speed of a chamber is by using a larger vent.

Analogous to electrical systems, where the conductivity of a wire permits the flow of electrons through a circuit caused by the electrical potential, conductance in a vacuum system permits the flow of gas molecules through a vacuum system caused by the pressure differential generated by the pump. Conductance must be closely considered when selecting the vacuum pump and other components, to prevent reduced pumping rates and extended drawdown times. The capacity of the pump must be increased to accommodate the resistance or the inverse of the conductance.

Conductance has units of volumetric flow rate divided by pressure drop, expressed as liters per second or cubic feet per minute. The conductance between two points is defined as the gas flow rate flowing through a device divided by the resulting pressure drop.

Conductance is greatest in the viscous flow region Cv , smallest in the molecular flow region Cm , and in between in the transitional flow region Ct.

The flow resistance the reciprocal of conductance is greatest in the molecular flow range, at higher vacuum, and lowest in the viscous flow range, at lower vacuum. This may seem counterintuitive since gas is denser at higher pressures and less dense at higher vacuum. Why would a gas be easier to pump when in a denser state, and harder to pump while in a less dense state? The answer lies in the interaction of the gas molecules in the viscous flow vs molecular flow ranges.

At higher pressure, which is where viscous flow occurs, the gas molecules are relatively close together and move collectively as a group. Note that in Figure 8 the chamber pressure is greater than the vacuum pump inlet pressure. In the viscous flow range, collisions between molecules are frequent, since they are relatively close together, and when a pressure differential is exerted, the molecules move as a group. In the molecular range, on the other hand Fig.

The pump must rely on the random motion of the molecules to enter the pump inlet, at which time they are simply captured. The pump does not have the ability to draw the molecules toward it. A few essential facts about conductance worth reviewing:.

Conductance can be calculated for a system. Pipe conductance is commonly taken from charted values and is dependent on pipe diameter, pipe length, flow rate, and pressure. Conductance values for components such as valves, filters, and traps are published by their manufacturers and are based on empirical values. Conductance changes during the three modes of flow through the system: continuum flow, molecular flow and Knudsen flow the transition between the two.

Recall that continuum flow occurs at higher pressure low vacuum , and molecular occurs at lower pressure high vacuum. The desorption rate of the water vapor is shown in Figure 5. Since the desorption rate drops off slowly, we can easily see that the total amount of water vapor that needs to be desorbed will control the pumpdown. The only way to traverse this desorption rate controlled region is to wait long enough for the water vapor to desorb as shown in Figure 5 or to increase its desorption rate artificially through heat or UV energy.

As the pumpdown moved into molecular flow, our thinking needs to change in several ways in addition to the conductance and gas flow criteria discussed above. Additionally, we can see how a previously trivial source of pumpdown problems can suddenly become a controlling source of gas. As the amount of water vapor is finally reduced, the pressure can be lowered by continued pumping assuming that the pump is capable of achieving lower pressures and that there is enough pumping speed to do so.

In terms of the partial pressures of residual gas makeup, hydrogen will slowly become the dominant gas. Creating a vacuum within a chamber is a fairly complex process that does not allow a single thinking process to dominate across the whole pressure range. Instead, it goes through a series of processes that can be successfully dealt with by breaking up the thinking process into stages based upon the breakdown discussed here.

Although a single mistake can negate the expected performance; applying a completely thought through, in terms of expected performance, analysis can avoid mistakes that might occur. Normandale Community College This accessibility navigation can jump to down to content on the page. Skip to main content Skip to footer content. Following federal COVID guidelines, Minnesota State is now requiring all employees, students, and visitors including contractors and vendors to wear a face covering when indoors at Normandale facilities, regardless of vaccination status.

Coronavirus Information. Anatomy of a Pumpdown In general, a vacuum is created by starting with air at atmospheric pressure within a chamber of some sort. However, two things are happening. As more and more molecules are removed, the mean free path increases and that results in a slightly longer and longer time to achieve equilibrium in molecular population so that the apparent driving force of pressure is reduced, and 2.

Flow Regimes As a pumpdown from atmospheric pressure to high vacuum is accomplished, the behavior of the molecules in the chamber is affected and controlled by three separate flow regimes.

Viscous Flow Viscous flow, as discussed above, occurs when the mean free paths are short and molecule-to-molecule collisions are constant.

Transition Flow Transition flow is an extremely complex condition that occurs in a band between viscous and molecular flow. Molecular Flow Molecular flow begins to occur when the mean free path of the molecules that are left within the chamber is longer than the internal dimensions of the chamber.