Fundamentals of Vacuum Technology

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There is an interesting consequence of the distribution of molecular speeds in a gas. Due to the bell-shaped curve of the speed distribution, there are significant numbers of molecules with a speed much faster about 6 times faster than the average. The escape velocity from earth for any moving object including gas molecules is So the fastest hydrogen atoms, those in the tail of the distribution, are energetic enough to overcome the grip of gravity. Hydrogen will, therefore, escape into space, as will helium. In the article above, we talked about the Kinetic Theory of Gases and how it can be used to calculate gas properties.

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Now we turn our attention to a discussion about temperature and kinetic energy, pressure and kinetic energy, and types of flow in vacuum systems. Again, we will focus on the basics, using fundamental comparisons to explain the concepts significant to industrial vacuum systems. The relevance of Temperature to the Kinetic Theory of Gases. Based on an atomic understanding of the world we live in, the Kinetic Theory reveals that gas properties are highly dependent on the speed of their molecules, which determines their kinetic energy, and therefore the gas pressure.

When considering the effects of the Kinetic Theory, it is also important to understand the influence of temperature. Specifically, the speed of the molecules in a gas is dependent on its temperature the higher the temperature the faster the gas molecules move. Another way to think of it is that the temperature of a gas is a measure of the average kinetic energy of that gas. According to Kinetic Theory, a gas consists of a large number of tiny molecules, all in constant random motion, elastically colliding with each other and the vessel that contains them.

Pressure is the net result of the impact force of those collisions against the vessel wall. The speed of the molecules during this motion is not random at all, but follows a bell curve, with a predictable distribution about the average. The molecular speed is dependent on the weight of the molecules and their temperature, or inherent heat energy. So for a given gas, the heat energy contained in its molecules determines their energy level and therefore their speed. In order to fully appreciate the influence of temperature on a gas, it is useful to understand degrees Kelvin more correctly defined simply as Kelvin , which is an absolute temperature scale degrees above so-called absolute zero where all motion stops.

This is a true measurement of kinetic energy. This is borne out when the volume of a gas at several different temperatures is measured and plotted Fig. When the graph is then extended to 0 K, the volume goes to zero. Keep in mind the gas molecules do not individually have zero volume, but the space between molecules approaches zero. Although absolute zero has never been attained, temperatures as low as a billionth of a kelvin have been achieved on small samples. With an understanding of absolute temperature, we can discuss the effects of temperature on gas volume and pressure.

It can be seen that for a given gas, as its temperature is increased, the average velocity of its molecules increases in proportion to the square root of its absolute temperature. Take nitrogen for example Fig. Also note on figure 2 that at lower temperatures, the curves are narrower and taller. At higher temperatures, there is a wider distribution of energy levels and corresponding molecular velocities among the population of molecules. At lower temperatures, there is less variability between the velocities of different molecules and so the curve is narrower at its base. When the absolute temperature of a gas is understood to be a measure of its kinetic energy, the implications of the Kinetic Theory of Gases in regard to temperature become clear.

Recall that under the Kinetic Theory of Gases , the pressure exerted by a gas is the sum total of the force exerted by all the physical impacts between the gas molecules and the vessel or vacuum chamber, or vacuum piping, etc. Since the velocity of the molecules is directly related to the square of temperature, the pressure of the gas is therefore also directly related to temperature, by the square root of its absolute temperature. The effect of temperature on pressure as described by kinetic theory can be illustrated by a simple experiment using a party balloon and liquid nitrogen Fig.

As a result of the decrease in molecular speed, two things happen; a the collisions between molecules become less frequent, allowing the space between them to decrease, and b the force with which the molecules collide with their container the balloon decreases, reducing the pressure the air exerts on the balloon. This causes the balloon to shrink.

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The reduced volume of the air is proportional to its absolute temperature. The manner in which a gas flows in a vacuum system is dependent on the gas pressure. At rough vacuum pressures above approximately 1 mbar. At these pressures, the molecules are relatively close together, and their collisions more frequent.

Therefore flow is governed by interaction between molecules. As a result, the entire volume of gas, or group of molecules can be made to move in an ordered motion, that is flow Fig. This ordered motion is superimposed, or added to, the normal random motion of the individual molecules.

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The gas at these pressures can be thought of as having a viscosity, or stickiness, that permits their ordered motion due to the internal friction between molecules. Therefore the preferred speed and direction of molecule flow will be the same as for the macroscopic gas flow. Molecular flow, on the other hand, is seen at pressures below.

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  • At these pressures, intermolecular collisions are much less frequent due to the fact that there is so much space between gas molecules Fig. Molecules move freely without any mutual interference, and therefore there is no ordered group flow possible. The molecules individually move in a straight line without colliding, until they strike the wall of the vessel or pipe containing them.

    Molecular flow is present where the mean free path length average distance a molecule must travel before colliding with another molecule is much larger than the diameter of the pipe the gas is flowing through and therefore the molecules are free to travel until they collide with the pipe walls. As a consequence, a gas particle can move in any arbitrary direction in a high vacuum and macroscopic, continuum flow is no longer possible.

    In the transitional range between continuum flow and molecular flow, Knudsen flow prevails. 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.

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    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.

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    • 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.

      Fundamentals of Vacuum Technology Fundamentals of Vacuum Technology
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