Operation and design of claw vacuum pumps

The Ministry of Science and Higher Education of the Russian Federation Kazan National Research Technological University


A. Raykov, S. Salikeev, A. Burmistrov


            OPERATION AND DESIGN OF
            CLAW VACUUM PUMPS


Monograph




Kazan
KNRTU Press
2022

         UDC 621.521




Published by the decision of the Editorial Review Board of the Kazan National Research Technological University

Reviewers:
Doctor of Physical and Mathematical Sciences P. Osipov PhD in Engineering E. Kapustin








         Raykov A.
         Operation and design of claw vacuum pumps : monograph / A. Raykov, S. Salikeev, A. Burmistrov; The Ministry of Education and Science of the Russian Federation, Kazan National Research Technological University. - Kazan : KNRTU Press, 2022. - 168 p.

         ISBN 978-5-7882-3161-7

       The monograph focuses on the design variations of oil-free vacuum pumps and their pumping characteristics. It describes computational procedures of working processes for volume displacement pumps and the mathematical model of working process for an oil-free claw vacuum pump. The monograph describes the experimental study of pump characteristics and presents indicator and temperature diagrams. The gas flow in the pump inlet and outlet ducts is studied both mathematically and experimentally. The influence of the pump configurations on the pumping characteristics is analyzed.
       The monograph is designed for students majoring in Vacuum Engineering for the courses “Rotor machines” and “Dynamic machines”.
       The monograph was developed at the Department of Vacuum Engineering.

UDC 621.521

ISBN 978-5-7882-3161-7    © A. Raykov, S. Salikeev, A. Burmistrov, 2022
© Kazan National Research Technological University, 2022

CONTENT




LIST OF REFERENCE AND TERMS....................................5

INTRODUCTION ................................................. 7

Chapter 1. OIL-FREE VACUUM PUMPS..............................11
  1.1. ROOTS VACUUM PUMP..................................... 15
  1.2. CLAW VACUUM PUMP ..................................... 20
  1.3. SCREW VACUUM PUMPS ................................... 34
  1.4. SCROLL VACUUM PUMPS .................................. 38
  1.5. SUMMARY .............................................. 42

Chapter 2. EXPERIMENTAL STUDY OF CVP .........................44
  2.1. SUBJECT OF RESEARCH DESCRIPTION....................... 44
  2.2. GAUGES SELECTION FOR READING INDICATOR DIAGRAMS ...... 47
    2.2.1. Strain-gauge method ...............................48
    2.2.2. Piezoelectrical method ............................51
    2.2.3. Capacitive method .................................55
    2.2.4. Resonant method ...................................56
    2.2.5. Inductive method ..................................57
  2.3. THE EXPERIMENTAL TEST UNIT DESCRIPTION................ 62
  2.4. TEST OPERATION PROCEDURE ............................. 66
  2.5. MEASUREMENT DATA PROCESSING .......................... 72
  2.6. RESULTS AND DISCUSSION................................ 77
  2.7. FLOW RATE FACTORS INDICATION FOR CVP INLET AND OUTLET
  DUCTS ..................................................... 80
    2.7.1. Test unit and measuring technique .................81
    2.7.2. Data processing ...................................85

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Chapter 3. MATHEMATICAL SIMULATION OF CVP WORKING PROCESS    96
  3.1. STATE-OF-THE-ART ON FUNDAMENTAL AND EXPERIMENTAL
  RESEARCH OF CVP AND OTHER STRAIGHT-TOOTHED PUMPS .............. 96
    3.1.1. Simulation of the working process of non-contact vacuum pumps .................................................99
    3.1.2. Gas flow simulation in complexly-shaped channels .... 101
  3.2. FUNDAMENTALS AND BASIC ASSUMPTIONS .......................105
  3.3. MATHEMATICAL MODEL OF CVP WORKING PROCESS ................106
  3.4. PUMPING SPEED COMPUTATION ................................108
  3.5. THE WORKING CAVITY GEOMETRY...............................109
  3.6. SUCTION AND DISCHARGE PORTS CONSTRUCTION .................119
  3.7. GEOMETRICS OF THE PUMP UNDER STUDY ..................121
  3.8. DESCRIPTION OF CALCULATION ALGORITHM OF GAS LEAKAGE
  THROUGH ROTOR MECHANISM CLEARANCES .......................127
    3.8.1. Dependency of channels geometry features vs rotors rotation angle ......................................... 127
    3.8.2. Calculation procedure of leakage via slot channels .. 133
    3.8.3. Leakage integration by gas stream directions ........ 135
    3.8.4. Thermal deformations allowance ...................... 136

Chapter 4. MATHEMATICAL SIMULATION RESULTS
AND ANALYSIS OF GEOMERTICS INLUENCE ON WORKING PROCESS..........................................................138
  4.1. COMPARING EXPERIMENTAL AND THE CALCULATED DATA ......138
  4.2. ANALYSIS OF GEOMETRICS INFLUENCE VS WORKING PROCESS .143
    4.2.1. Clearances of rotor mechanism ....................... 143
    4.2.2. Discharge port length ............................... 145
    4.2.3. Rotor tooth thickness ............................... 151
    4.2.4. Center-to-center spacing and the casing bore radius . 153

CONCLUSION...................................................... 157

BIBLIOGRAPHY ................................................... 158


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LIST OF REFERENCE AND TERMS


CVP - claw vacuum pump
Roots - Roots vacuum pump
CFD - computational fluid dynamics
TDMA - Tridiagonal matrix algorithm
SIMPLE -Semi-Implicit Method for Pressure-Linked Equations LSODE - Livermore Solver for Ordinary Differential Equations VMAX - maximum volume of the working cavity VMIN - minimum volume of the working cavity POUT - outlet pressure PIN - inlet pressure
VSUC - suction cavity volume
VCOMP - compression-discharge cavity volume
VDEAD - carried-over (dead) volume
L - channel depth (rotor length)
R1, R2 - radiuses of channels curvature
RG - specific gas constant
T1 - channel inlet temperature
dL - elementary contour work of medium
dLM - elementary migration work of medium
dQM - elementary migration heat dQHEAT - external added heat n - rotors rotation speed j - rotors rotation angle w - angular velocity dU - internal energy change
dmIN, dmOUT - mass change by gas flowing in and gas flowing out from the considered volume, correspondingly
hIN, hOUT - enthalpies of gas flowing in and gas flowing out MIN, MOUT - gas in and gas out per second k - adiabatic exponent cp - specific thermal capacity at constant pressure U - vector velocity field r - gas density ^ - mass flow factor
lCH - length of channel connecting the considered volume with a gauge

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Ga - actual mass flow rate via inlet (outlet) duct
Gd - mass flow rate through the equivalent circular diaphragm
DSin - measuring data confidence limit F - cross-sectional area of channel dRR, dRC, dE - inter-rotor, radial and end clearances of rotor mechanism, correspondingly
U - channel conductance
l - channel length in flow direction
MEXP - experimental mass flow rate l - volumetric efficiency SD - displacement volume SIN - pumping speed R - rotor radius
A - center-to-center spacing h - dynamic viscosity rate

INTRODUCTION



       To adopt a policy of innovative development of Russia presupposes expansion and introduction of emerging technologies to manufacturing processes. The most of existing and promising techniques are implementable only in vacuum conditions. It would not be an exaggeration to say that none of priority trends of Russia is possible without vacuum [1-3]. Nanoelectronics, nanoengineering, functional nanomaterials and high clean substances, nanomaterials for power industry, space and biotechnology, construction and composite nanomaterials, nanotechnologies for safety systems - this is far from a complete list of vacuum applications. For microelectronics, medicine, food industry, metallurgy, petrochemical industry, nuclear power, energy saving technologies, production heatproof coatings for float glass, and other areas, vacuum has long become an integral part of manufacturing process.
       It should be noted that industrial and research applications of vacuum encompass an extremely wide operating pressures range (of the order of over 15). To ensure development of at least medium vacuum rather than high and ultrahigh vacuum by a pump of one type is impossible, and there are no pumping facilities capable to operate equally well in such a broad range of pressures. In the paper [4] it is noted that physical mechanisms of a specific pump assembly operation at atmospheric pressure, in medium and high vacuum, are normally different. So, provided that a vacuum developing facility is efficient at the whole pressures range, its overall application would not be successful due to its weight, dimensions and cost. These are the reasons of a great variety of pumps produced in the industrial scale.
       Evidently, one of the challenges to be solved when engineering a vacuum system is selection of pump facilities. It would be a mistake for a designer to focus only on pumping speed and ultimate pressure. A properly constructed system takes into account a great number of different aspects for every specific case. These include: operating pressure range, characteristics of the pumped medium (first of all, aggressivity, toxicity, the presence of solid inclusions), operating pressure time-on, durability of continuous performance, cost, dimensions, noise and vibration level, maintenance cost, specific power consumption, reliability. And this is a far more complete list by any means. For example, choosing a roughing vacuum pump or its preferable producer, it is reasonable to pay attention to other facets and operating features such as availability of gas ballast device, inlet pressure at open gas ballast, possibility of vacuum conservation at shutdown of a pump; corrosion

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resistance, period of initial pumping parameters recovery after pumping specific gases, etc.
       When designing a vacuum system one should think over the question if a pumping system must provide developing oil-free vacuum. If yes, how can it be achieved?
       As it is known, for many years the most demanded basic logic to obtain high vacuum there has been a diffusion pump unit with a liquid nitrogen trap mounted on the inlet. This scheme thereby was used, particularly in Russia it is being used rather often, even in the processes that are super critical in relation to the presence of oil vapours in residual medium, e. g. in the processes of coating thin films.
       Afterwards, for obtaining oil-free high and ultrahigh vacuum, turbo-molecular, electrophysical and cryogenic pumps were in use. Due to the principle of their operation, these pumping units provided practically ideally “pure” medium. Surprisingly, but for many years as roughing vacuum pumps there have been applied oil-sealed vacuum pumps, of rotary or slide valve types. This was determined by specific nature of parameters to be provided by roughing vacuum pumps or a fore-vacuum pump. First of all, it refers to the required residual rarefaction. For high vacuum oil-free pumps to operate efficiently the pressure of about 1 Pa is required. Such pressure is easily achieved by a two-stage oil-sealed vacuum pump. A quite different thing is producing oil-free vacuum of said pressure level. Diversity of mechanical pumps capable to develop oil-free vacuum may seem impressive. The best known pumps in question are positive displacement, membrane, screw, scroll, axial-flow, centrifugal, vane, Roots and claw vacuum pumps. However, conventionally “dry” membrane, axial-flow and centrifugal, as well as oil-free positive displacement machines “fall short” of this pressure. Actually the only oil-free rough pumping machines nearly till the end of the XX century were cryosorption pumps with all their disadvantages in operation.
       The situation changed radically when at the turn of the XXI century almost simultaneously the production of three types of mechanical oil-free vacuum pumps (scroll, claw and screw pumps) was developed. The trend analysis of vacuum equipment market reveals the growth of oil-free facilities of vacuum production compared to the remainder [5-6].
       The present work considers mechanical oil-free pumps for producing low and medium vacuum. These machines include sliding vane rotary, membrane, scroll, screw and Roots pumps. The principle of operation of oil-free rotary vane pumps is the same as that of oil-sealed rotary vane pumps, except that they operate without lubricants. For this reason, they have a considerably lower operation life, compression ratio and rotation speed. The main

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disadvantages of membrane pumps are a significantly limited useful life of the membrane and low pumping speed. Besides, their capabilities to reduce ultimate pressure are quite restricted (about 104 Pa for a single-stage pump and 103 Pa for a two-stage pump [7]).
      For oil-free low and medium vacuum production, non-contact pumps, namely scroll pumps, are the most promising. Compared to other positive displacement pumps, these possess the lowest suction loss which is determined by low gas flow velocity at suction and large opening of a suction cavity. Just this, as well as a total absence of dead space and low gas heating at suction, determine high volumetric efficiency [8]. The main drawbacks of these pumps are stringent requirements to the pumped gas purity and relatively low pumping speed. Besides, complexity of scroll profile manufacturing creates obstacles to the spread of scroll pumps.
      Screw pumps possess the best mass-dimensional parameters, complete balance and sustainable performance during a long-term useful life. However, a large scale use of these pumps is hampered by manufacture complexity of a three-dimensional scroll profile of rotors, especially with a variable pitch providing small clearance for engagement and consequently high pressure ratio [9, 10]. There is also of note the significant heating of gases when crossing the pump, which is not always allowed.
      The most introduced among oil-free pumps are Roots vacuum pumps, mainly due to their high pumping speed [11]. But because of low pressure ratio, there is the need for using them with fore-vacuum pumps. Conventional use for this purpose of oil-sealed vacuum pumps, e. g. rotary vane or slide valve pumps, means that such unit is not “dry” any more. Thus, as forevacuum units, there should be used oil-free structures. The most appropriate for this aim is a claw vacuum pump which has a greater pressure ratio compared to a Roots pump. This pump design makes it applicable as the main pumping facility and a fore-vacuum pump when used in combination with Roots pumping unit thus enabling to combine high pumping speed and a wide operating range [12].
      A claw vacuum pump (CVP) is a non-contact oil-free Roots-type mechanical rotary volume displacement pump where gas transfer is realized by means of periodic variation of the enclosed volume of the cavity formed between rotors teeth, the casing bore and end covers [13, 14]. In foreign literature this machine is named “Claw”. This pump along with screw and scroll pumps has complete internal compression which is required for operation with exhaust to atmosphere, and hence, higher adiabatic efficiency compared to external compression pumps (of Roots type) and pumps with partial

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internal compression. In spite of somewhat lower compression ratio compared to screw and scroll vacuum pumps, this machine is being widely used abroad. However, computational procedures of their pumping characteristics are hardly to be found in the literature. Experimental studies reduce to presenting the dependence of pumping speed vs pressure, generally for one rotation speed. Nowadays in Russian and the Commonwealth of Independent States, pumps of this type are not manufactured.
       This monograph presents the results of experimental and fundamental study on the working process of a claw vacuum pump, development of mathematical model of the pumping process, and provides guidelines for enhanced efficiency of the pump operation based on the data obtained.
       The paper contains introduction, four chapters and bibliography.
       The first chapter comprises the review of design variations of claw vacuum pumps. Methods and measuring instruments of quickly-varying pressures in low vacuum are considered. Selection of measuring instruments is grounded. The current status concerning fundamental and experimental research of CVP is analyzed. Methods of mathematical simulation of working processes of compressors and vacuum pumps are studied. Methods of solution problems of computational fluid dynamics are provided.
       The main part of the second chapter presents experimental research of pumping characteristics and pressure-volume diagrams of CVP. Experimental test units and methodology for obtaining indicator diagrams and CVP pumping speed are described. The results of experiments at different inlet pressures, rotors rotation speeds and temperature modes were presented. On a specially constructed test unit, experimental study on resistance of CVP inlet and outlet ducts was carried out, and gas mass flow factors were obtained by computation. Measurement error was calculated.
       The third chapter deals with mathematical model of CVP. Basic assumption of the mathematical model is defined. Computation of rotors profile and working cavities volumes of CVP with symmetrical and asymmetrical rotors is conducted. In the mathematical model, inlet and outlet ducts resistances are accounted for.
       The fourth chapter highlights comparison of experimental and the calculated indicator diagrams and pumping characteristics of the pump. Numerical computation of CVP pumping characteristics was performed with regard for variations of basic geometrics of rotor mechanism: amount of clearances, position of a discharge port edge, rotor tooth thickness, dependence of center-to-center spacing vs the casing bore diameter. Basing on the research results, guidelines for the pump design development are presented.

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Chapter 1. OIL-FREE VACUUM PUMPS



       By now, there is a great number of design variations of vacuum pumps applied for producing low and medium oil-free vacuum: positive displacement, membrane, screw, scroll, axial-flow, centrifugal, vane, Roots-type pumps, Roots pumps with partial internal compression, claw pumps.
       At first sight, contact-type pumps seem preferable, due to their ability to produce the pressure ratio up to 10⁸ for a two-stage embodiment of such pumps and up to 10⁵ for a single-stage embodiment, with operation life of several years. However, these parameters are provided only for pumps with oil seal and decrease noticeably when trying to use such structures without oil.
       Non-contact rotary pumps, due to the presence of minimum clearance in rotary mechanism, possess high rotation speeds and high specific pumping characteristics allowing one to pump aggressive, explosive and precious gases, vapour-gas condensable mixtures, and media with solid inclusions.
       Diagrams of non-contact rotary machines [15-17] are presented in fig.1.1. They differ by the profile of rotors, their quantity and the history of gas pressure rise. In the industrial scale, Roots, screw, scroll and claw pumps are produced.
       All these types of non-contact machines are difficult-to-make, since they require high accuracy of manufacturing parts and units included in their construction, and high production standards. This determines the greater cost of oil-free pumps compared to oil-sealed vacuum pumps. Fig. 1.2 shows the dependency of specific cost of pumping speed for oil-sealed vacuum pumps and oil-free pumps of medium vacuum vs pumping speed; the data presented in [4] is a result of the performance analysis of more than 64 pumps of key producers of pumping facilities [18-28]: Varian Vacuum Technologies (USA), Edwards (England), Pfeiffer Vacuum (Germany), Busch (Germany), Oerlikon Leybold Vacuum (Germany), Tuthill Vacuum (USA), An-est Iwata (Japan), Adixen (France).
       It can be seen that at comparable pumping speed, the cost for producing oil-free vacuum is by a factor of 3-10 higher than that of the vacuum provided by oil-sealed vacuum pumps. As one should expect, for all the pumps, cost per unit of pumping speed decreases with the pumping speed increase. The difference thereby is more pronounced for oil-free pumps and can achieve as much as 4 times.

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Figure 1.1. Diagrams of some rotary non-contact pumps

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