Chapter 6: Mechanical Vacuum Pumps

Transcription

1 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

2 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 Torr = 51,000 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 Pump Speed [Liters/sec] 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

3 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 Page 89 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

4 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 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

5 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 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

6 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. The advantages of dry pumps (usually of the rotary vane design) are that they eliminate the possibility of backstreaming pump oil into your vacuum vessel. In addition, dry pumps may be used to safely pump large percentages of oxygen without fear of explosion. Dry pumps are also well suited for pumping of certain corrosive vapors and gases. Disadvantages of dry mechanical vacuum pumps include their initial high cost (as much as 5 times the cost of a oil-sealed pump of equal capacity), excessive noise, and higher ultimate pressure. For Further Reading: Rotary oil sealed mechanical vacuum pumps- A User's Guide to Vacuum Technology, O'Hanlon, J., Wiley-Interscience, New York, NY, Practical Vacuum Techniques, Batzer, T.H., and Brunner, W.F., Robert E. Krieger Publishing Company, New York, NY, Vacuum Technology, Roth, A., North-Holland Publishing Company, New York, NY, Laboratory Exercise 6.1: Mechanical Pump Identification and Inspection. Identify the mechanical vacuum pump you have selected for the next three exercises: A. Pump Identification: Who is the manufacturer? What is the pump model number? Locate the manufacturer's literature from the bookcase, and find the appropriate reference information. What is the advertised pump speed? What is the base pressure listed? Is the pump of single stage or compound design? What is the rotational speed? What is the suggested volume of pump fluid? B. Physical Inspection of Mechanical Pump: Inspect the pump for signs of wear or misuse. Check electrical cables for cracks in insulation. Are the prongs of the electrical Page 92 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

7 plug bent or missing? Check the pump fluid. Is the fluid transparent or milky; is the fluid level correct? If the pump is a belt-driven model, is the belt tensioned correctly, and is the belt worn? Is the safety cover in good condition? Locate the gas ballast, inlet and exhaust ports. Is everything as expected? Once you have carefully inspected the pump and corrected any problems, cap off the pump inlet and operate the pump briefly. Record your observations. {Please prepare a written laboratory report on this and each of the following exercises using guidelines presented in the section called "How to Use This Manual"} Laboratory Exercise 6.2: Mechanical Pump Ultimate Base Pressure. The two operational characteristics that define the performance of a mechanical vacuum pump are: 1) the ultimate (or base) pressure, and 2) the pumping speed. In this exercise, you will determine the base pressure of your pump, and compare these results with the manufacturer's specifications. Procedure: A. Measurement of ultimate pressure. Place a valve on the inlet of the mechanical pump. Devise a manifold so that a thermocouple gauge (or pirani gauge) can be installed somewhere near the pump inlet. Close the valve, and turn the mechanical pump on. Observe the pump's behavior. Once you're certain the pump is operating properly, open the valve, and allow the pump to base out (achieve its ultimate pressure). This may take 15 to 20 minutes. Record the ultimate pressure. How does your reading compare with the manufacturer's specification? If there is a discrepancy, what do you attribute it to? TC1 A schematic of the experimental set-up for part A of Exercise II is shown to the left. B. Measurement of Pump-down Curve: Page 93 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

8 Liter Vacuum Vessel TC2 TC1 Attach a suitable vacuum vessel having a volume of from 50 to 100 liters to the manifold assembly used in part A. Place a second thermocouple gauge on a port of the vacuum vessel; connect all required read-outs to the vacuum gauges. Before beginning this procedure the vacuum pump should be running, and base pressure should be read at gauge TC1, the valve to the vacuum vessel should be closed, and the vessel at atmospheric pressure. In the next step, the pressure as read at the vacuum vessel (TC2) will be recorded as a function of time. We suggest taking pressure readings every 30 seconds for the first five minutes, then recording pressure at one minute intervals until base pressure is achieved in the vacuum vessel. The table to plot your data is on the following page. This data will allow you to plot vessel pressure as a function of time on semi-logarithmic graph paper. Label your graph with all pertinent pump data. Now vent your system to atmosphere, and leave it open for one minute. Repeat procedure 6.2-B. Plot the data collected for this second pump down measurement as you did for the first measurement, and compare the results. Is there a noticeable difference between the two curves? Would you expect a difference? What would you attribute this behavior to? The table to plot your data is on the following page. Remember the first (and easiest) way to test the integrity of a vacuum system is to check its ultimate pressure, and the time required to reach this pressure. {Hint: after characterizing the pump down behavior of your clean, dry and empty vacuum system, plot the data as time vs. pressure and file that information away for future reference. Your curve becomes an excellent tool for gauging the performance of your vacuum system}. Data Table 6.2-B.1 Time Press. Time Press. Time Press. Time Press. Page 94 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

9 Data Table 6.2-B.2 Time Press. Time Press. Time Press. Time Press. Laboratory Exercise 6.3: Measurement of Pumping Speed The manufacturer's listed pumping speed for any given pump is usually the free air displacement at STP (standard temperature and pressure). As pressure decreases from atmospheric, there will be a reduction in the amount of gas pumped per unit time (mass flow rate). The pumping speed (volumetric flow rate) will decrease only slightly until a pressure of about 1 Torr is attained. Below this pressure, the decrease in pumping speed becomes more rapid, depending upon the type of mechanical vacuum pump, and falls to zero at the ultimate pressure. We can determine the speed of a pump by measuring either pumping speed under constant volume or constant pressure conditions. The constant volume technique is generally used in the pressure range between atmospheric and one Torr. In this method, you will measure the time required to reduce the pressure in a vessel a specified amount. The pump speed in that pressure range is then calculated using the equation: S p =2.3 V P Log 1 t t P 2 V = volume of vessel [liters] t 1 = time at pressure P 1 [seconds] t 2 = time to reach pressure P 2 from pressure P 1 [seconds] In contrast to the constant volume method, the measurement of pumping speed at constant pressure is typically performed in the pressure range between one Torr and the mechanical pump's ultimate pressure. To determine pumping speed by the constant pressure method, a measured amount of gas (Q) is admitted to the vacuum system being pumped to establish a constant pressure P. Pumping speed is then obtained from the equation: S = pump speed [liters/sec] Q = mass flow rate [Torr-liters/sec] P = pressure [Torr] S= Q P Page 95 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

10 Laboratory Procedures: 6.3-A. Pumping Speed by constant volume method: For this exercise, you will need a functioning rotary mechanical pump, a vacuum chamber, a valve, and at least one vacuum gauge capable of reading from atmospheric pressure to about one Torr. Vacuum Vessel TC1 Install the valve between the chamber and the mechanical pump using the minimum amount of connecting line to reduce conductance losses. Begin this exercise with all valves closed and the vessel at atmospheric pressure. Start the mechanical pump, and after it has warmed up, open the valve to the vacuum vessel and Record the time required to achieve a pressure of 100 Torr as read with the pressure gauge mounted on the vessel. Repeat this measurement until you are confident in the consistency of your readings. Now record the time required to pump from 100 Torr to 10 Torr, exactly as was done before. And finally, record the time required to pump from 10 Torr to 1 Torr. Table to record your data is on the following page. Table 6.3-A.1 Data from pumping speed measurement at constant volume. Mechanical pump data: Vacuum vessel size & volume: Time from 760 Torr to 100 Torr: Time [seconds] Time from 100 Torr to 10 Torr: Time from 10 Torr to 1 Torr: Page 96 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

11 Time from 1 Torr to 0.1 Torr: From the data in Table 6.3-A.1 you will be able to calculate pumping speeds for several pressure ranges using the equation: S p =2.3 V P Log 1 t t P 2 Table 6.3-A.2 Calculation of Speed at Constant Volume for Vessel #1 Pressure [Torr] Range Average [Torr] Pressure Pumping Speed [Torr-L/s] 760 to to to 1 1 to 0.1 {Note: the average pressure is defined as (P 1 + P 2 )/2} Now plot the calculated pumping speed as a function of the average pressure for each of the four pressure regimes in Table 6.3-A.2. Following your splendid success in this measurement, replace the vacuum vessel in your system with another vessel of significantly different volume. Repeat the measurements performed and plot the data. How do the speed vs. average pressure curves compare? Is the behavior as you would expect? Why or why not? {Another data table is provided on the following page.} Table 6.3-A.3 Data from Pumping Speed Measurement at constant volume. Mechanical pump data Vacuum vessel size & volume: Time from 760 Torr to 100 Torr: Time [seconds] Page 97 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

12 Time from 100 Torr to 10 Torr: Time from 10 Torr to 1 Torr: Time from 1 Torr to 0.1 Torr: Page 98 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

13 Table 6.3-A.4 Calculation of Pumping Speed at Constant volume for Vessel #2 Pressure [Torr] Range Average [Torr] Pressure Pumping Speed [Torr-L/s] 760 to to to 1 1 to 0.1 Discussion: Is it possible to make your plots more representative by using shorter time increments? What are the drawbacks (if any) for this idea? How do the speeds that you have calculated compare to those listed by the manufacturer for this pressure range? Is there any significant difference in speeds calculated for the two vacuum vessels of differing volumes? 6.3 B: Measurement of pumping speed by the constant pressure method. For this portion of the exercise, you will need a mechanical vacuum pump, a vacuum valve, a variable leak valve, an atmosphere valve, a vacuum vessel, a flow indicator and a pressure gauge capable of reading pressures from one Torr to about one millitorr. TC2 atmosphere valve pipette Vacuum Vessel TC1 Install the pump valve at the pump inlet. Place the pressure gauge on the vacuum vessel, and install the variable leak valve on the chamber also. The flow meter must be plumbed to the inlet of the leak valve and the atmosphere valve must be plumbed to the flow meter. Confused? Follow the diagram and have a lab instructor check your setup before you begin. Initial conditions should be something like this: mechanical vacuum pump is off, the valve between the vessel and pump is closed; the vessel is at atmospheric pressure; the leak valve is closed. Start the mechanical pump, and allow it a few minutes to warm up to operating temperature. Open the valve between the pump and vessel, and allow the pressure to be reduced to a stable base pressure (~20 mtorr). Page 99 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

14 Once a stable base pressure is achieved, with the atmosphere valve open, slowly open the calibrated leak valve until you notice a slight rise in vessel pressure. Observe this pressure (~100mTorr might be a good initial value) for a little time to insure that the system is stable at this pressure. Close the atmosphere valve, and observe air being drawn into the vessel through the flow meter. Fluid will rise in the volumetric burette to replace air being pumped out of the system by the mechanical pump. We now know that the air being leaked into the chamber is at atmospheric pressure, we know the volume being leaked in per unit time, and we know the pressure inside the vacuum chamber. We are now prepared to calculate the rate at which the vacuum pump is removing air from the chamber using the equation: S= Q P where: Q = V A P A t S = pump speed [Liters/sec] Q = mass flow rate [Torr-Liters/Sec] P = pressure in vacuum vessel [Torr] V A = atmospheric volume [liters] P A = atmospheric pressure [Torr] t = time to leak in V A [seconds] Table 6.3 B.1 Vessel Press [Torr] Atmospheric Volume [liter] Time for V A [seconds] Q [Torrliters/sec] S P [Liters/ sec] Repeat the procedure for various pressure values between one millitorr and one Torr. Try to get at least five stable readings. Plot your calculated data as pump speed (S P ) vs. pressure. Be sure to include all pertinent data regarding the experiment. Discussion: How do the speeds you have calculated compare with those listed in the vacuum pump manufacturer's literature? What would be the effect of using a vessel having twice the volume on the pumping speed? How do the speeds obtained using the constant pressure method compare with those you found using the constant volume method? Page 100 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002