How to Move Molecules in Vacuum Systems - Part One: Mechanical and Booster Pumps
November 7, 2007
In order to create a vacuum within a closed container, or vessel, we need to remove the molecules of air and other gases that reside inside by means of a pump. The vacuum vessel and pumps (mechanical, booster, diffusion, holding) together with the associated piping manifolds valves, gages and traps comprise a typical vacuum system. Let’s learn more.
Mechanical PumpsTo reach the various vacuum levels, different vacuum pumping systems are required. The foundation of any of these systems is the positive displacement mechanical, or roughing, pump. The roughing pump – so called because it is used to produce a “rough” vacuum – is used in the initial pumpdown from atmospheric pressure to around 2 x 10-2 torr, depending on the type of pump.
The internal components of the mechanical pump (Fig.2) help us understand its operation. Basically, it is an eccentric cylinder driven about an axis by an electric motor. During operation, the rotor turns with the shaft, which causes the piston to sweep the volume between it and the stator. The piston does not turn in this case, but the vane-like extension on the piston (called the slide, or slide valve) moves up and down in an oscillating seal (called the slide pin or slide-valve pin).
At the start of a rotation, the ported slide valve is open. As the rotation occurs, the slide valve closes, trapping a given volume of gas. This volume is compressed as the revolution continues. Near the end of the revolution, the pressure is above atmospheric, and the gas discharges through a spring-loaded poppet valve. On the completion of the revolution, the slide valve opens, and another increment of gas is admitted.
A vacuum pump will remove a number of molecules with each rotation. How many molecules will depend largely on the actual pump displacement, rotational speed and vacuum-system pressure. Each time molecules are removed, the remaining molecules spread out in the vacuum chamber to occupy the available volume. This repeats (molecules are removed by the pump) the pressure reduces and there are less and less molecules to expand into the pump inlet with each rotation.
Mechanical pumps can be single or dual stage. A single-stage design will achieve a pressure of about 1 x10-2 torr, while a dual-stage pump is capable of reaching pressures around 1 x 10-3 torr. A two-stage, or compound, pump has two pumping chambers connected in series. The exhaust of the first stage is coupled to the inlet of the second stage.
Lower pressure, less molecules and more speed in the same volume results in less pumping efficiency. Why mechanical pumps start off with high efficiency and fall of at these pressure ranges can be explained as follows. Consider one cubic foot of volume at atmospheric pressure (760 torr). If we were to put this volume of gas in a container that was twice as large, the pressure would be exactly half, or 380 torr. If we double the volume, we halve the pressure. Thus, doubling the volume again to 4 cubic feet results in a pressure of 190 torr. So, to evacuate a chamber to 1 x 10-3 torr theoretically requires that we remove a volume of 760,000 cubic feet. In everyday operation, a mechanical roughing pump will have great difficulty achieving this ultimate pressure (lowest attainable pressure) since its efficiency begins to fall off at 1 x10-1 to 8 x 10-2 torr.
An alternative to “wet” mechanical pumps – those that use mechanical pump oil – are the so-called “dry” mechanical pumps. These pumps are used in applications where pumping efficiency and process contamination concerns are important issues. They have positive environmental impact due to reduced oil consumption and minimal disposal issues, and they operate with less noise and vibration.
Dry pumps operate on the compressor principle. As the two rotors rotate, gas is drawn in through an inlet slot aligned with the cavity in one of the rotors. Further rotation closes the inlet while the lobes, or claws, compress the trapped volume of gas until the cavity in the second rotor exposes the outlet or exhaust slot. A small volume of gas remains trapped and is carried over into the next pumping cycle. These designs produce high compression ratios and operate at high efficiency.
Booster PumpsEnter the booster pump, or blower, a different type of mechanical pump that is placed in series with the roughing pump. It is designed to “cut in” at around 700 torr and provide higher speeds in the pressure range of 100 torr to 1 x10-3 torr. In this intermediate pressure range, the roughing pump is losing efficiency while the diffusion (vapor) pump is just starting to gain efficiency.
The operation of the booster (Fig.3) is as follows. Two impellers are mounted on parallel shafts and rotate in opposite directions. They are geared together so that the correct relative position of each impeller to the other can be maintained. The impellers do not touch each other, and no sealing fluid is used. Any back leakage is small compared to the total speed of the pump in its useful range.
During operation, gas from the inlet side is trapped between the impeller and housing. No compression takes place as this gas is moved from the inlet to the discharge port. When the leading lobe of the impeller passes the discharge port, gas from the discharge area (at higher pressure) enters, but it is swept away by the trailing lobe.
Mechanical booster pumps have a useful compression ratio of 10:1. Therefore, they must be backed by a mechanical roughing pump in order to reach their maximum efficiency. The mechanical booster pump is highly efficient in reducing the time required to evacuate a large or “gassy” system to the operating pressure at which the diffusion pump is efficient.
Part two of this article will cover diffusion pumps and offer troubleshooting tips for all types of vacuum pumping systems. IH