Mechvacuumpump

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Las Positas College Vacuum Technology 60A & 60B

Page 87 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

Chapter 6: Mechanical Vacuum Pumps

In this chapter we will review the principles of operation of several commonly usedmechanical vacuum pumps, provide information on the performance and typicalapplications, and describe appropriate preventative maintenance techniques. Thischapter 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 tocreate 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 vacuumtechnology; they are simple in design, require little maintenance, are relativelyinexpensive, and can operate for long periods of time without failure. Several

mechanical vacuum pumps that we are aware of have operated continuously for fifteenyears with only occasional oil changes! The range of pumping speeds for commerciallyavailable 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, androtary 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 abrief 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 slidingvane 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 toremove gases from the system, and exhaust them to the atmosphere. In most designs amotor driven rotor spins inside a cylindrical stator of larger diameter. The ratio of theexhaust pressure (atmospheric) to the base pressure (lowest pressure obtained at the





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, itscompression ratio is:

7 6 0 Torr

0.0 1 5 Torr = 5 1,0 00

Another more common way to state this is to say that the pump has a compression ratioof 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, andapproaches zero at the pump's base pressure. Most manufacturers of mechanical

vacuum pumps will include in their product literature information on the pump'sperformance including a pump speed curve.

1000100101.1.01.1

1

10

100

Pressure [Torr]

P u m p

S p e e

d [ L i t e r s

/ s e c

]

Rotary Vane Mechanical Vacuum Pumps

Rotary vane pumps typically have an electric motor driven rotor (either belt or directlydriven) 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 drypumps. Centripetal force acts upon the vanes in the spinning rotor so as to force themagainst the inner sealing surface of the stator. In some mechanical pumps springs areused to augment this action. Rotary vane pumps may be of the single or double stagedesign. Single stage pumps are simpler, having only one rotor and stator, and are lessexpensive. 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 isconnected 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 moreexpensive than single stage pumps.





12

In the figure above are simplified drawings of a single stage oil sealed rotary vanemechanical pump (left) and a two stage, or compound pump of the same type. In thecompound 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 thatstage. It should be noted that supply of very little or no oil to the first stage of acompound pump in order to achieve even lower pressures can, in practice, lead tosevere difficulties in the reliable operation of a compound pump.The oil in an oil sealed pump serves three important functions: A) providing avacuum seal at the pump exhaust, B) as a lubricant and C) provides cooling for the pump.

1 2

3 4





In this figure, and on the following page sequences in a single pump cycle of a rotaryvane pump are shown. Note how the rotor vanes work with the stator to createincreasing and decreasing volumes on each stroke.

7

5 6

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 thedirect-drive models spin at 1500 to 1725 RPM. Most failures in rotary vane pumps canbe attributed to poor oil maintenance. O'Hanlon states that 95% of all mechanical pumpproblems can be resolved by flushing the pump and changing the oil. Because of theclose 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 ina decrease in pump performance. For this reason, precautions should be taken tominimize intake of particulates. Several manufacturers produce small screens and filtersthat 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 thecompression ratio of the pump?6.3 What are the three functions of the oil in a mechanical vacuum pump?





Rotary Piston Mechanical Vacuum Pumps

Rotary piston (or rotary plunger)mechanical pumps like that to theleft also operate on the principle of

positive displacement of gas. Oneach cycle the rotating eccentricpiston and the sliding valve worktogether to suck gas into the stator,compress it, and expel the gas toatmosphere. As with rotary vanepumps, rotary piston type pumpsmay be single stage or compound.Rotational speed is typically 600 to800 RPM.

Dimensional tolerances between the stator and piston in pumps of this design areusually 0.003 to 0.004". Because of this, piston pumps are more tolerant of particulatecontamination that rotary vane pumps. Higher viscosity oil is used in rotary piston

pumps due to the larger dimensional tolerances. Large rotary piston pumps are oftenwater cooled to increase pump life and performance.

Mechanical Vacuum Pump FluidsSelecting the appropriate pump fluid is as important as choosing the right pump. Intoday's vacuum technology, many processes are not compatible with typicalhydrocarbon pump oil. For example, if you're running a process in which a significantamount of oxygen is used, a synthetic pump oil that is much less reactive with oxygen isthe 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 aprocess 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 tomechanical pump oil vapor. Fluorocarbon pump fluids may be chosen for an applicationsuch as this due to their low chemical reactivity. Under certain circumstances, you maywish to operate a mechanical pump with fluid of higher viscosity. For this purpose, theclearances between moving parts may need to be increased. Pumps that are modifiedfor special service should be permanently labeled to let future users know of themodifications and application.





One last word on mechanical vacuum pump fluids-research the characteristics of a fluidcarefully before using it. Many of the current commercially available fluids will notoperate well when mixed with one another! For a good review of mechanical pumpfluids, see O'Hanlon's A User's Guide to Vacuum Technology , page 163.

Dry Mechanical Vacuum PumpsIn recent years, the concern over mechanical pump fluids (from both safety and vacuumsystem contamination standpoints) has become a great concern. Vacuum pumpmanufacturers have responded by developing and marketing oil-free mechanicalroughing pumps. These pumps have, for some applications, very appealingcharacteristics, 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 eliminatethe possibility of backstreaming pump oil into your vacuum vessel. In addition, drypumps 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 andgases.Disadvantages of dry mechanical vacuum pumps include their initial high cost (as muchas 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, NewYork, NY, 1980.

Practical Vacuum Techniques, Batzer, T.H., and Brunner, W.F., Robert E.Krieger Publishing Company, New York, NY, 1974.

Vacuum Technology, Roth, A., North-Holland Publishing Company, New York,NY, 1978.

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 appropriatereference information. What is the advertised pump speed? What is the base pressurelisted? 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





plug bent or missing? Check the pump fluid. Is the fluid transparent or milky; is the fluidlevel correct? If the pump is a belt-driven model, is the belt tensioned correctly, and isthe belt worn? Is the safety cover in good condition? Locate the gas ballast, inlet andexhaust ports. Is everything as expected? Once you have carefully inspected the pumpand 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 followingexercises using guidelines presented in the section called "How to Use ThisManual"}

Laboratory Exercise 6.2:Mechanical Pump Ultimate Base Pressure.

The two operational characteristics that define the performance of a mechanicalvacuum 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 theseresults with the manufacturer's specifications.

Procedure :A. Measurement of ultimate pressure . Place a valve on the inlet of the mechanicalpump. Devise a manifold so that a thermocouple gauge (or pirani gauge) can beinstalled somewhere near the pump inlet. Close the valve, and turn the mechanicalpump on. Observe the pump's behavior. Once you're certain the pump is operatingproperly, open the valve, and allow the pump to base out (achieve its ultimatepressure). This may take 15 to 20 minutes. Record the ultimate pressure. How doesyour reading compare with the manufacturer's specification? If there is a discrepancy,what do you attribute it to?

TC1

A schematic of the experimentalset-up for part A of Exercise II isshown to the left.

B. Measurement of Pump-down Curve:





50 - 100 Liter

Vacuum Vessel TC1

TC2

Attach a suitable vacuum vesselhaving a volume of from 50 to 100liters to the manifold assemblyused in part A. Place a secondthermocouple gauge on a port of the vacuum vessel; connect allrequired read-outs to the vacuumgauges.

Before beginning this procedure the vacuum pump should be running, and basepressure should be read at gauge TC1, the valve to the vacuum vessel should beclosed, and the vessel at atmospheric pressure. In the next step, the pressure as readat the vacuum vessel (TC2) will be recorded as a function of time. We suggest takingpressure readings every 30 seconds for the first five minutes, then recording pressure atone minute intervals until base pressure is achieved in the vacuum vessel. The table toplot your data is on the following page. This data will allow you to plot vessel pressureas a function of time on semi-logarithmic graph paper. Label your graph with allpertinent pump data.

Now vent your system to atmosphere, and leave it open for one minute. Repeatprocedure 6.2-B. Plot the data collected for this second pump down measurement asyou did for the first measurement, and compare the results. Is there a noticeabledifference between the two curves? Would you expect a difference? What would youattribute 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 isto 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 vacuumsystem}.

Data Table 6.2-B.1Time Press. Time Press. Time Press. Time Press.




5 Rights Reserved, Biltoft, Benapfl, and Swain Fall 2002

Data Table 6.2-B.2Time 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 freeair displacement at STP (standard temperature and pressure). As pressure decreasesfrom 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 slightlyuntil a pressure of about 1 Torr is attained. Below this pressure, the decrease inpumping speed becomes more rapid, depending upon the type of mechanical vacuumpump, and falls to zero at the ultimate pressure.

We can determine the speed of a pump by measuring either pumping speedunder constant volume or constant pressure conditions. The constant volume techniqueis generally used in the pressure range between atmospheric and one Torr. In thismethod, 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 theequation:

Sp = 2.3 Vt2

− t1

Log1 0

P1

P2

V = volume of vessel [liters]t1= time at pressure P 1 [seconds]

t2= time to reach pressure P 2 frompressure P 1 [seconds]

In contrast to the constant volume method, the measurement of pumping speed atconstant pressure is typically performed in the pressure range between one Torr andthe mechanical pump's ultimate pressure. To determine pumping speed by the constantpressure method, a measured amount of gas (Q) is admitted to the vacuum systembeing pumped to establish a constant pressure P. Pumping speed is then obtained fromthe equation:

S = QP

S = pump speed [liters/sec]Q = mass flow rate [Torr-liters/sec]P = pressure [Torr]





Laboratory Procedures:

6.3-A. Pumping Speed by constant volume method:For this exercise, you will need a functioning rotary mechanical pump, a vacuumchamber, 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 andthe mechanical pump using the minimumamount of connecting line to reduceconductance losses. Begin this exercisewith all valves closed and the vessel atatmospheric pressure. Start themechanical pump, and after it haswarmed up, open the valve to the vacuumvessel and

Record the time required to achieve a pressure of 100 Torr as read with the pressuregauge mounted on the vessel. Repeat this measurement until you are confident in theconsistency of your readings. Now record the time required to pump from 100 Torr to 10Torr, exactly as was done before. And finally, record the time required to pump from 10Torr 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]measurement 1measurement 2measurement 3measurement 4measurement 5Average of measurements:Time from 100 Torr to 10 Torr: measurement 1

measurement 2measurement 3measurement 4measurement 5Average of measurements:Time from 10 Torr to 1 Torr: measurement 1measurement 2





measurement 3measurement 4measurement 5Average of measurements:Time from 1 Torr to 0.1 Torr:

measurement 1measurement 2measurement 3measurement 4measurement 5Average of measurements:

From the data in Table 6.3-A.1 you will be able to calculate pumping speeds for severalpressure ranges using the equation:

Sp = 2.3 Vt2

− t1

Log1 0

P1P

2

Table 6.3-A.2 Calculation of Speed at Constant Volume for Vessel #1Pressure[Torr]

Range Average[Torr]†

Pressure Pumping Speed[Torr-L/s]

760 to 100100 to 1010 to 11 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 eachof the four pressure regimes in Table 6.3-A.2.

Following your splendid success in this measurement, replace the vacuum vessel inyour system with another vessel of significantly different volume. Repeat themeasurements performed and plot the data. How do the speed vs. average pressurecurves 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]





measurement 1measurement 2measurement 3measurement 4measurement 5

Average of measurements:Time from 100 Torr to 10 Torr: measurement 1measurement 2measurement 3measurement 4measurement 5Average of measurements:Time from 10 Torr to 1 Torr: measurement 1

measurement 2measurement 3measurement 4measurement 5Average of measurements:Time from 1 Torr to 0.1 Torr: measurement 1measurement 2measurement 3measurement 4measurement 5Average of measurements:





Table 6.3-A.4 Calculation of Pumping Speed at Constant volume for Vessel #2Pressure[Torr]

Range Average[Torr]†

Pressure Pumping Speed[Torr-L/s]

760 to 100100 to 10

10 to 11 to 0.1

Discussion:Is it possible to make your plots more representative by using shorter timeincrements? What are the drawbacks (if any) for this idea?

How do the speeds that you have calculated compare to those listed by themanufacturer for this pressure range?

Is there any significant difference in speeds calculated for the two vacuumvessels 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 vacuumvalve, a variable leak valve, an atmosphere valve, a vacuum vessel, a flow indicator anda pressure gauge capable of reading pressures from one Torr to about one millitorr.

Vacuum Vessel TC1

TC2

atmospherevalve

pipette

Install the pump valve at the pump inlet. Place the pressure gauge on the vacuumvessel, and install the variable leak valve on the chamber also. The flow meter must beplumbed to the inlet of the leak valve and the atmosphere valve must be plumbed to theflow meter. Confused? Follow the diagram and have a lab instructor check your setupbefore 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 atmosphericpressure; the leak valve is closed. Start the mechanical pump, and allow it a fewminutes to warm up to operating temperature. Open the valve between the pump andvessel, and allow the pressure to be reduced to a stable base pressure (~20 mTorr).





Once a stable base pressure is achieved, with the atmosphere valve open, slowly openthe calibrated leak valve until you notice a slight rise in vessel pressure. Observe thispressure (~100mTorr might be a good initial value) for a little time to insure that thesystem is stable at this pressure. Close the atmosphere valve, and observe air beingdrawn into the vessel through the flow meter. Fluid will rise in the volumetric burette toreplace air being pumped out of the system by the mechanical pump. We now knowthat the air being leaked into the chamber is at atmospheric pressure, we know thevolume being leaked in per unit time, and we know the pressure inside the vacuumchamber. We are now prepared to calculate the rate at which the vacuum pump isremoving air from the chamber using the equation:

S = QP

S = pump speed [Liters/sec]Q = mass flow rate [Torr-Liters/Sec]P = pressure in vacuum vessel [Torr]

where:

Q =VA

× P A

t

VA = atmospheric volume [liters]P A = atmospheric pressure [Torr]t = time to leak in V A [seconds]

Table 6.3 B.1 VesselPress[Torr]

AtmosphericVolume [liter]

Time for VA

[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 allpertinent data regarding the experiment.

Discussion:

How do the speeds you have calculated compare with those listed in the vacuumpump manufacturer's literature?

What would be the effect of using a vessel having twice the volume on thepumping speed?

How do the speeds obtained using the constant pressure method compare withthose you found using the constant volume method?

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