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Chapter 6: Mechanical Vacuum Pumps Las Positas College Vacuum Technology 60A & 60B Chapter 6: Mechanical Vacuum Pumps In this chapter we will review the principles of operation of several commonly used mechanical vacuum pumps, provide information on the performance and typical applications, and describe appropriate preventative maintenance techniques. This chapter also includes several laboratory procedures that are designed to aid in your understanding of mechanical vacuum pumps. Positive gas displacement pumps of one type or another have been used since 1640! Almost all of the very early pumps used liquid mercury within glass tubes and vessels to create a vacuum. For an excellent review of this early technology, read the History of Vacuum Science and Technology, edited by T.E. Madley and W.C Brown, published for the American Vacuum Society by the American Institute of Physics. Modern mechanical pumps may well be considered the workhorses of vacuum technology; they are simple in design, require little maintenance, are relatively inexpensive, and can operate for long periods of time without failure. Several mechanical vacuum pumps that we are aware of have operated continuously for fifteen years with only occasional oil changes! The range of pumping speeds for commercially available pumps runs from about 0.5 liters per second to over 300 liters per second. Mechanical vacuum pumps fall into two basic categories: reciprocating pumps, and rotary pumps. Further distinctions for mechanical pumps include: the number of stages (single stage or compound), the use of oil in a pump (pumps may be oil sealed or "dry"), and the means of driving the mechanics of a pump (direct drive or belt drive). Below is a brief outline of the types of modern mechanical vacuum pumps. + Mechanical positive displacement pumps + Reciprocating positive displacement pumps - Diaphragm pump - Piston pump + Rotary positive displacement Pumps - Liquid ring pump + Sliding vane pump - multiple vane rotary pump - Rotary piston pump - Rotary plunger pump - Roots pump For this laboratory, we will concentrate on two oil sealed mechanical pumps: the sliding vane rotary pump, and the rotary piston pump. Theory of Operation Mechanical vacuum pumps work by the process of positive gas displacement, that is, during operation the pump periodically creates increasing and decreasing volumes to remove gases from the system, and exhaust them to the atmosphere. In most designs a motor driven rotor spins inside a cylindrical stator of larger diameter. The ratio of the exhaust pressure (atmospheric) to the base pressure (lowest pressure obtained at the Page 87 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 Las Positas College Vacuum Technology 60A & 60B vacuum pump inlet) is referred to as the Compression Ratio of the pump. For example, if a mechanical vacuum pump obtains a base pressure of 15 mTorr, its compression ratio is: 760 Torr = 51,000 0.015 Torr Another more common way to state this is to say that the pump has a compression ratio of 51,000:1. At pressures above 1 Torr, rotary mechanical pumps have a fairly constant pumping speed. The pumping speed decreases rapidly below this pressure, and approaches zero at the pump's base pressure. Most manufacturers of mechanical vacuum pumps will include in their product literature information on the pump's performance including a pump speed curve. 100 10 1 Pump Speed [Liters/sec] .1 .01 .1 1 10 100 1000 Pressure [Torr] Rotary Vane Mechanical Vacuum Pumps Rotary vane pumps typically have an electric motor driven rotor (either belt or directly driven) which has one to three sliding vanes that maintain close contact with the inner wall of the cylindrical stator. The vanes are metal in oil sealed pumps, and carbon in dry pumps. Centripetal force acts upon the vanes in the spinning rotor so as to force them against the inner sealing surface of the stator. In some mechanical pumps springs are used to augment this action. Rotary vane pumps may be of the single or double stage design. Single stage pumps are simpler, having only one rotor and stator, and are less expensive. The base pressure one can expect from a good single stage mechanical pump is about 20 mTorr. In a two stage design, the exhaust port of the first stage is connected to the inlet port of the second stage which exhausts to atmospheric pressure. Two stage pumps may attain a base pressure of one to two millitorr, but are more expensive than single stage pumps. Page 88 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 Las Positas College Vacuum Technology 60A & 60B 2 1 In the figure above are simplified drawings of a single stage oil sealed rotary vane mechanical pump (left) and a two stage, or compound pump of the same type. In the compound design the high vacuum side of the pump (stage labeled 1) operates at a lower pressure due to the lack of exposure to high partial pressures of oxygen in that stage. It should be noted that supply of very little or no oil to the first stage of a compound pump in order to achieve even lower pressures can, in practice, lead to severe difficulties in the reliable operation of a compound pump. The oil in an oil sealed pump serves three important functions: A) providing a vacuum seal at the pump exhaust, B) as a lubricant and C) provides cooling for the pump. 1 2 34 Page 89 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 Las Positas College Vacuum Technology 60A & 60B In this figure, and on the following page sequences in a single pump cycle of a rotary vane pump are shown. Note how the rotor vanes work with the stator to create increasing and decreasing volumes on each stroke. 5 6 7 8 Also note how the gas discharge valve opens and closes on each cycle. Belt driven rotary vane pumps typically operate at about 400 to 600 RPM, while the direct-drive models spin at 1500 to 1725 RPM. Most failures in rotary vane pumps can be attributed to poor oil maintenance. O'Hanlon states that 95% of all mechanical pump problems can be resolved by flushing the pump and changing the oil. Because of the close tolerances between the rotor vanes and the stator, solid particulate matter entering the pump is likely to cause scoring of the vacuum sealing surfaces, resulting in a decrease in pump performance. For this reason, precautions should be taken to minimize intake of particulates. Several manufacturers produce small screens and filters that fit on the inlet of a pump to accomplish this. Sample Problems: 6.1 What is the principle by which positive displacement pumps operate? 6.2 If a mechanical pump achieves a base pressure of 30 mTorr, what is the compression ratio of the pump? 6.3 What are the three functions of the oil in a mechanical vacuum pump? Page 90 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 Las Positas College Vacuum Technology 60A & 60B Rotary Piston Mechanical Vacuum Pumps Rotary piston (or rotary plunger) mechanical pumps like that to the left also operate on the principle of positive displacement of gas. On each cycle the rotating eccentric piston and the sliding valve work together to suck gas into the stator, compress it, and expel the gas to atmosphere. As with rotary vane pumps, rotary piston type pumps may be single stage or compound. Rotational speed is typically 600 to 800 RPM. Dimensional tolerances between the stator and piston in pumps of this design are usually 0.003 to 0.004". Because of this, piston pumps are more tolerant of particulate contamination that rotary vane pumps. Higher viscosity oil is used in rotary piston pumps due to the larger dimensional tolerances. Large rotary piston pumps are often water cooled to increase pump life and performance. Mechanical Vacuum Pump Fluids Selecting the appropriate pump fluid is as important as choosing the right pump. In today's vacuum technology, many processes are not compatible with typical hydrocarbon pump oil. For example, if you're running a process in which a significant amount of oxygen is used, a synthetic pump oil that is much less reactive with oxygen is the preferred choice. In this example, if hydrocarbon oil is chosen, the potential for creating an explosive mixture of oxygen and hot pump oil vapor exists. Likewise, if a process involving the use of corrosive gases is being run, you should think about the chemical reactivity of the process gases being pumped that will be exposed to mechanical pump oil vapor. Fluorocarbon pump fluids may be chosen for an application such as this due to their low chemical reactivity. Under certain circumstances, you may wish to operate a mechanical pump with fluid of higher viscosity. For this purpose, the clearances between moving parts may need to be increased. Pumps that are modified for special service should be permanently labeled to let future users know of the modifications and application. Page 91 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002 Las Positas College Vacuum Technology 60A & 60B One last word on mechanical vacuum pump fluids-research the characteristics of a fluid carefully before using it. Many of the current commercially available fluids will not operate well when mixed with one another! For a good review of mechanical pump fluids, see O'Hanlon's A User's Guide to Vacuum Technology, page 163. Dry Mechanical Vacuum Pumps In recent years, the concern over mechanical pump fluids (from both safety and vacuum system contamination standpoints) has become a great concern. Vacuum pump manufacturers have responded by developing and marketing oil-free mechanical roughing pumps. These pumps have, for some applications, very appealing characteristics, but there are a few drawbacks of which to be aware.
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