Modern Vacuum and Overpressure Generation for Pneumatic Conveying ( Bernd Jack )

Modern vacuum and overpressure generation Modern vacuum and overpressure generation for pneumatic conveying Bernd Jack Dr.-Ing. K. Busch GmbH

Modern vacuum and overpressure generation Contents 1. General 3 3.6 Screw compressors 16 2. Fields of application 4 4. Comparison of characteristic 17 curves 3. Description of vacuum 5 and overpressure generators 5. Technological differences 18 3.1 Side channel blowers 5 6. Economic efficiency 21 3.2 Rotary vane vacuum pumps 6 7. Control: Larger customer benefit due 25 3.3 Liquid ring vacuum pumps 9 to variable drives 3.4 Roots blowers 10 3.5 Mink rotary claw vacuum pumps/compressors

Modern vacuum and overpressure generation 1. General length and diameter and the kind of conveyance (for example dense and lean The operating costs of a pneumatic con- phase pneumatic conveying). veying system mainly depend on the By means of these parameters the dif- correct selection of the vacuum or ferential pressure (∆p) can be deter- pressure generator. mined. Both in pressure and vacuum By choosing modern pressure generation conveying the differential pressure is the equipment an economic and environ- main influencing factor in selecting the mentally-friendly operation of conveying compressor. systems is possible. For economic analysis the following To select the most effective compressor it factors must be considered: primary is necessary to know the actual operating costs, energy consumption, costs de- conditions of the conveying system. pending on the compressor (e.g. throttle These parameters are of vital importance control, valves, control), operating time when selecting the most appropriate type (e.g. continuous operation, batch opera- of compressor. tion) and maintenance costs. Important factors influencing the selec- tion are the type of material being con- veyed and its characteristics, e.g. its p = pressure vacuum pump max. vacuum V = volume m = mass R = gas constant T = temperature p • V = m • R • T atmosphere ∆p, ∆V Figure 1: Principal design vacuum conveying

Modern vacuum and overpressure generation 2. Fields of application pressors in pressure operation offer high pressure reserves and therefore can be The following figure shows the vacuum used best for long or dense phase pumps and compressors mainly used in conveying. vacuum and overpressure conveying. Nowadays side channel blowers, con- Mink rotary claw vacuum pumps and ventional Roots blowers and Mink rotary compressors compress absolutely oil-free claw vacuum pumps/compressors can thus making them ideal for gas be used for both vacuum and over- circulation conveying. pressure conveying, with Mink rotary claw vacuum pumps/compressors being Additionally there are further types of used in nearly all applications in vacuum generators such as liquid ring pneumatic conveying. vacuum pumps, multi-stage side channel blowers, ejectors etc. which were not Side channel blowers can be used best successful in pneumatic conveying be- for short conveying and for vacuum cause of their characteristics or ineffi- conveying because only small differential ciency. pressures are required. Rotary vane vacuum pumps and Mink rotary claw vacuum pumps in vacuum operation respectively screw com- Side channel blowers Screw compressors Roots blowers Rotary vane vacuum pumps Mink rotary claw vacuum pumps Mink rotary claw compressors 0 100 200 300 400 500 600 700 800 900 1000 0,2 0,4 0,6 0,8 1,2 1,4 1,6 1,8 2,2 2,4 2,6 2,8 0 1 2 3 vacuum (mbar) overpressure (bar) Figure 2: Application areas of various vacuum and overpressure generators

Modern vacuum and overpressure generation 3. Description of the vacuum and In the side channel the gas forms a whirl overpressure generators shaped like an eight. The unidirectional wheel together with the aluminium 3.1 Side channel blowers housing forms the side channel, in which the gas is compressed and then dischar- There are two different types of side ged through the pressure-side silencer. channel blowers: single- and two-stage By the rotation of the impeller the gas is side channel blowers. transported through the side channel. Conventionally driven side channel blo- Depending on the back pressure the gas wers can achieve differential pressures of gets impulse energy by the impeller up to ±300 mbar. vanes. In frequency controlled operation side Compression takes place if gas pene- channel blowers can be used for vacuum trates into the impeller. conveying up to -400 mbar for over- The side channel between suction and pressure and up to +500 mbar for pressure connection is separated by the overpressure conveying. so-called interruptor which is a narrow- ing of the side channel to the width of In contrast to vacuum pumps/compres- the impeller. Part of the circulating gas sors side channel blowers work accor- stream is transported to the pressure ding to the impulse principle: impeller connection. and side channel form an annular wor- king chamber. Figure 3: Sectional drawing of a side channel blower

Modern vacuum and overpressure generation The other part of the circulating gas is Due to these effects the efficiency of side tranported to the suction side in the channel blowers does not exceed 50%. spaces between the impeller vanes side. Low primary costs and almost no main- This residual volume causes a flow within tenance costs make side channel blowers the side channel even if the inlet or outlet ideal for short and vacuum conveying is closed converting motor power into because only small differential pressures potential and kinetic energy of the gas. are required. Therefore a closed inlet or outlet of the Side channel blowers have no stable blower can cause overheating of the characteristic curves. Therefore they do blower. For protection against too high not compress isochorously, e.g. not at temperatures (especially of the bearings) equal volume. For multi-stage operation it is necessary to fit a vacuum or pressure more power is required than for other relief valve. compression principles. 3.2 Rotary vane vacuum pumps In an excentrically installed rotor driven by an electric motor so called vanes slide Currently oil lubricated rotary vane in slots in the rotor and are pushed vacuum pumps and side channel blowers towards the wall of the cylinder by the are the most important resp. most centrifugal force of the rotation. frequently used vacuum generators in The gas to be sucked of enters through pneumatic conveying. the suction flange in the sickel-shaped These pumps distinguish themselves by space between rotor and cylinder. the high differential pressure (almost 1,0 The pumping effect results from the in- bar) and an almost constant charac- creasing sickel-shaped space between teristic curve concerning the relevant rotor and cylinder when the pump ro- working area (in suction conveying). tates. If this space is as large as possible These outstanding qualities offer a high the following vane covers the suction slot power reserve in critical conveying forming a space in which the sucked gas processes or with critical conveying is trapped. materials resulting in a high planning At the same time new gas is sucked in by reliability. the following chamber while the closed Also in continuous operation rotary vane chamber decreasing with further rotation vacuum pumps guarantee a stable vol- compresses the trapped gas until the ume flow for the entire pressure range exhaust valve is opened at a pressure of from atmospheric pressure to ultimate about 1200 mbar and the gas is pressure, e.g. these pumps do not need discharged through the separator. to be protected by a pressure relieve The exhaust valve is covered with oil for valve (for example, by an infiltrated air better sealing. valve). The principle of operation of a rotary vane vacuum pump is shown in figure 4.

Modern vacuum and overpressure generation Inlet screen Exhaust Non-return valve filter Rotor Vane Cylinder Oil filter Figure 4: Sectional drawing of an oil lubricated rotary vane vacuum pump The oil and the air are then discharged The circulating oil also serves as a coo- into the bottom part of the oil mist se- ling, sealing and lubricating medium. parator and then separated from rough The automatic oil circulation is based on oil drips by gravity and the demister. the differential pressure between the oil The gas now only contaminated by oil mist separator, where considerable mist is conveyed through oil mist sepa- overpressure exists caused by the filter rators for fine separation; the filter is resistance, and the inlet flange of the covered with permeable fibres collecting pump. the residual oil that drains off. No separate oil pump is required. The pump is equipped with a suction-side The gas, if it is air, is discharged directly valve that prevents the flow of air or con- into the atmosphere or can be trans- veyed gas back into the vacuum chamber ported through a pipeline away from when the pump is switched off to avoid the pump. pump oil being drawn into the suction Oil lubricated rotary vane vacuum line by the differential pressure. pumps operate nearly free of wear since The inlet of the valve is equipped with a the vanes always slide on an oil film and screen to prevent the pump from being are of robust design. polluted

Modern vacuum and overpressure generation No separate oil pump is required. Dry running rotary vane vacuum The pump is equipped with a suction-side pumps valve that prevents the flow of air or con- veyed gas back into the vacuum chamber Basically there are two types of rotary when the pump is switched off to avoid vane vacuum pumps: pump oil being drawn into the suction – oil lubricated vacuum pumps line by the differential pressure. – dry running vacuum pumps The inlet of the valve is equipped with a The vanes of dry running vacuum pumps screen to prevent the pump from being are made of special carbon. These vanes polluted. are self-lubricating. These vacuum pumps need no oil as operating medium. Additionally conventional rotary vane The more the sealing gaps increase due vacuum pumps used for pneumatic to permanent abrasion of the vanes the conveying are equipped with a fine filter more the suction capacity decreases. (filtration efficiency 5 µ) at the suction This effect is aggravated more or less side which is to protect the pump against drastically if particles are conveyed. admittance of solid particles (e.g. product To avoid a total vacuum pump failure abrasion). caused by broken vanes it is necessary to check them for wearout regularly and Oil lubrication of rotary vane vacuum replace them as required. pumps requires maintenance work, like Therefore it is not recommended to use for example: dry running rotary vane vacuum pumps • check oil level at the oil sight glass in pneumatic conveying. at regular intervals • change oil every 2000 to 5000 operating hours depending on application; at least change oil once a year • change oil and replace oil filter • replace exhaust filter once a year depending on saturation or conta- mination. A so-called filter pressure gauge is used to check the condition of the exhaust filters; the filter pressure gauge is in- stalled in the drilling through which oil is filled in. The exhaust filters need to be replaced if the filter pressure is ≥0,6 bar.

Modern vacuum and overpressure generation 3.3 Liquid ring vacuum pumps The vacuum pump can be operated with water recirculation cooling, semi-open or Liquid ring vacuum pumps normally closed loop cooling circuit. operate with water as operating Depending on the efficiency and design medium. the fluid used as operating medium has An excentrically installed impeller rotates the following functions: in the cylindrical pump casing partly filled • energy transmission from the impeller with liquid (usually water). to the medium to be compressed By the rotational movement of the im- • sealing of the cells in radial direction peller and the resulting centrifugal force • sealing of the space between impeller the liquid within the cylinder forms the and casing parts so-called liquid ring. • cooling and lubrication of the shaft Gas is conveyed in the spaces between seals the single blades and the liquid ring. Depending on the application the ring As a result of the excentrical installation liquid has the following functions: of the impeller the spaces enlarge and • absorption of heat caused by the process gas is sucked in through the compression and friction suction slot. • absorption of heat caused by Further rotation reduces the spaces, the condensation and reaction gas is compressed and discharged • absorption of gaseous contaminants through the pressure slot. • absorption of particles Casing Suction slot Discharge slot Impeller Liquid ring Figure 5: Sectional drawing of a liquid ring vacuum pump

Modern vacuum and overpressure generation Advantages of liquid ring vacuum • Danger of calcifying pumps: Should water evaporate from the • simple and cost-effective design ring liquid, the dissolved salts settle • 100% oil free on the casing surface. • condensation of vapours is The deposits reduce the suction possible capacity of the vacuum pump which • good material resistance can result in a failure of the unit (alternative materials possible) depending on the thickness of the deposit. Disadvantages: • high supply or disposal costs in • Operation strongly depends on continuous operation – temperature of the operating liquid These costs possibly will be increased – density of the operating liquid dramatically by environmental – solubility of gases in the operating process gas in the operating fluid. liquid • „possible “danger of silting up” in – temperature of the gases to be circulation operation with solid sucked off particles. – condensation effect • Danger of cavitation 3.4 Roots blowers Damage or even destruction the surfaces by implosions caused by the Conventional two- and three-lobe Roots condensation of vapour bubbles. blowers work according to the well tried Roots principle. Two parallel rotors with identical profiles rotate in opposite directions within a casing. As they rotate gas is drawn into the space between each rotor and the casing where it is trapped, transported and discharged by the rotation. The gas is compressed isochorously, e.g. at equal volume. If a chamber containing gas under suction pressure reaches the pressure inlet the chamber is filled with returning gas from the pressure side and therefore compresses to atmospheric pressure.

Modern vacuum and overpressure generation There is no actual internal compression Disadvantages are their high acquisition and no mechanical contact between costs and costs for noise reduction. rotors and cylinder. In pneumatic conveying Roots blowers These blowers are contact-free (and no are normally equipped with the following oil is needed as sealing fluid) and can be accessories: used for differential pressures of up to 0,6 bar in vacuum operation and up to • inlet filter 1,0 bar(g) in pressure operation. • vacuum or pressure relief valve as In pneumatic conveying Roots blowers protection against overload can only be used in applications in • suction-side non-return valve which high volume flows at low diffe- • inlet/outlet silencer rential pressures are required. In this small range of applications Roots blowers are distinguished by their small power consumption. Roots lobe Cylinder Internal compression Figure 6: Sectional drawing of a Roots blower

Modern vacuum and overpressure generation 3.5 Mink rotary claw vacuum Further savings are achieved by minimum pumps/compressors maintenance work and considerably lower energy consumption thus making Mink rotary claw vacuum pumps are Mink compressors the most economical available with suction capacities from alternative in pressure conveying up to 2 3 3 60 m /h to 500 m /h for vacuum bar(g). conveying up to -750 mbar (continuous working pressure). Mink rotary claw Two rotary claws rotate in opposite di- compressors can be operated in rections within a cylinder. There is neither overpressure conveying up to 2 bar. contact between claws and casing nor By connecting several single Mink between the two claws. vacuum pumps or compressors in parallel bigger volume flows can be generated. Mink vacuum pumps or compressors operate according to the well tried rotary claw principle. They compress absolutely oil and contact free and hence free of wear. Housing Claw Claw Figure 7: Sectional drawing of a Mink rotary claw vacuum pump

Modern vacuum and overpressure generation Internal compression Figure 8: Compression process of a Mink rotary claw vacuum pump

Modern vacuum and overpressure generation The remaining gaps at the virtual point of Bearing/Sealing contact are manfactured so precisely that they serve as sealing (labyrinth) because There is an atmospherically ventilated of air turbulence within the gap. inter-space between compression Therefore the compression chamber is chamber and gear side. divided into two parts: This inter-space: • an enlarging chamber on the suction • causes thermal separation between the side drawing in air compression chamber and bearings. • a decreasing chamber on the discharge The bearings are not affected by the side compressing and discharging air compression heat resulting in long at the same time. service life. The exact rotation of both claws is • prevents pressure fluctuations on the controlled by a synchronized drive. shaft seal rings by atmospheric However, in contrast to Roots blowers, ventilation and therefore excessive or Mink rotary claw vacuum pumps work premature wearout. with internal compression. • prevents both the penetration of As a result of the special profile of the product gas into the bearing or claws the conveying medium is pre- storage chamber respectively and the compressed internally in rotational conveying of oil particles into the direction in front of the claws before compression chamber. Compared to reaching the pressure side. At the same conventional Roots blowers the com- time gas is drawn in behind the claws in pression is guaranteed to be actually rotational direction for the next cycle. oil free. Mink rotary claw vacuum pumps and compressors are driven by three-phase The bearings are robust roller bearings: motors that are conventionally used for • the fixed side (gear side) has angular frequency controlled operation and are ball bearings equipped with PTC thermistors. • the free bearing side has cylindrical In frequency controlled operation the roller bearings pump adapts optimally to the process by The shaft sealing between the compres- speed control. This results in a further sion chamber and the atmospheric inter- saving in energy or increase of conveying space is affected by labyrinth seals. power by about 20%. Towards gear and storage chamber radial Mink vacuum pumps and compressors shaft seal rings are used. are air cooled. A separate additional electric fan guarantees a constant cooling, also with variable pump speeds.

Modern vacuum and overpressure generation Standard equipment for pneumatic With the appropriate combination (pa- conveying: rallel operation) of several individual units every conveying capacity can be realized. In pneumatic conveying Mink vacuum pumps/compressors are equipped with Such modular solutions have the the following accessories: following advantages: • simple power adaptation by series a) Suction conveying: switch-on or switch-off • suction air filter • in case of power control by frequency • vacuum relief valve as protection converter only a smaller converter against overload (adapted to an individual unit) is re- • Suction-side non-return valve quired • exhaust gas silencer • availability of a stand-by machine b) Pressure conveying: guaranteed at any time • inlet silencer, in combination • cost-effective due to standardized with suction air filter modular design • pressure relief valve as protection • cost-effective silencing measures against overload possible • pressure-side vessel for pulsation reduction • pressure-side non-return valve Figure 9: System solutions for higher conveying capacities

Modern vacuum and overpressure generation 3.6 Screw compressors Compared to Mink rotary claw pumps the synchronism of both screw rotors is Single-stage air cooled screw compressors guaranteed by a synchronisation drive. are used for volume flows from approx. Furthermore, integrated transmission 200 to 15000 m3/h. These compressors gears allow variable power levels. achieve pressures of more than 10 bar. Roller bearings are used. Drive shaft and They should only be used where main rotor are equipped with multiple pressures of >2 bar (abs.) are actually bearings. required. Screw compressors are expensive and Oil lubricated screw compressors are their power consumption is very high. additionally equipped with an oil pump, Furthermore the oil required for com- oil cooler and an oil mist separator. pression must be extracted from the compressed air. Figure 10: Screw compressor with integrated gear stage

Modern vacuum and overpressure generation In pneumatic conveying screw compressors are equipped with the following accessories: • Inlet air filter • Safety valve • Non-return valve • Pressure silencer • Silencing hood 4. Comparison of the characteristic The volume flows are stated in % of the curves nominal suction or conveying capacity of the individual types. The following diagrams show the suction or pressure characteristics of the indivi- dual types of vacuum generator depen- ding on the corresponding differential pressure. Vacuum generator 100 Rotary vane 90 vacuum pump, oil lubricated 80 Mink rotary claw vacuum pump ] % 70 [ y t i 60 c a p a 50 c n Rotary vane Roots blower o 40 i vacuum pump, Side channel t c dry running blower, u S 30 single-stage 20 Side channel blower, two-stage 10 0 -1000 -900 -800 -700 -600 -500 -400 -300 -200 -100 0 Vacuum [hPa (mbar)] Figure 11a: Comparison of characteristic curves stability of various vacuum generators

Modern vacuum and overpressure generation Compressor 100 90 ] Screw compressor % 80 [ Mink y t rotary claw compressor i Roots blower c a p 70 a c e g r 60 a Side channel blower, h c two-stage s i d l 50 a u t c A 40 30 0 0,5 1 1,5 2 2,5 Operating pressure [bar(g)] Figure 11b: Comparison of characteristic curves stability of various pressure generators 5. Technological differences constant until there is an opening towards this chamber between point 2 There are technological differences bet- and point 3. The gas increases in ween the various vacuum and over- pressure at the same volume. Between pressure generators presented that in- point 3 and four it reaches discharge fluence power characteristic and effi- pressure and is ejected. ciency of the corresponding unit. The region between points 1, 2, 3 and 4 “Internal compression” is the decisive in the pV-diagram (on the next page) advantage of the design principle Mink describes the compression work to be rotary claw compressor over the conven- done. tional Roots blower working according to the Roots principle. This is described in The functional principle of the Mink the following pV-diagram: rotary claw vacuum pumps and com- pressors on the other hand result in the The region between point 1 and point 2 following process: in the pV-diagram shows the filling of the Between points 1 and 2 in the pV- respective pumping chamber at suction diagram the chamber within the com- pressure. In case of the Roots blower (left pressor is filled with gas at suction diagram, next page) the pressure remains pressure.

Modern vacuum and overpressure generation savings p p 4 3 4 3’ 3 1 2 1 2 V V savings without internal savings with internal compression compression (Mink rotary claw vacuum pump) (Roots blower) Figure 12: Advantage due to internal compression shown in the pV-diagram However, before the gas volume reaches The pV-diagram shows that the region the pressure side, there is internal com- and therefore the compression work to pression of the gas in front of the claws. be done is considerably smaller for The pressure simultaneously increases compressors with internal compression to the decrease of the volume. than for blowers without internal com- No gas flows back into the compression pression. Therefore the same compres- chamber from the pressure side. During sion work can be done more the rotation the pressure mouth in the economically. compression chamber is opened as a result of the specific design of the claw and the gas is discharged on the pressure side. The region between points 2 and 3‘ describes the compression process, the region between points 3‘ and 4 describes the discharge process.

Modern vacuum and overpressure generation When compressing gas the temperature used to realize higher discharge pres- is increased both in the case of the Roots sures at constant power consumption. blower and the Mink rotary claw vacuum The claw principle achieves a discharge blower. The temperature increase is pro- pressure of 2 bar(g) and a vacuum of portional to the energy added. 80%. If a compressor with internal compression requires lower energy for compressing this results in lower temperature increase. This lower temperature increase can be 16 14 ] W k 12 [ n Mink rotary o 10 claw vacuum pump i t p m 8 u s n o 6 c r e Roots blower w 4 o P 2 0 0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 Operating pressure [bar(g)] Figure 13: Comparison power requirement Roots blower/Mink rotary claw vacuum pump This diagram shows the power consump- At small overpressures Roots pumps are tion of a Roots blower compared with a more economical. The point of intersec- Mink vacuum pump. tion is about 0,8 bar(g). It is obvious that The effect of the internal compression to the power consumption of the Roots power data is obvious. pump increases up to about 1 bar since At small overpressures the Roots blower Roots blower have no internal compres- is better than the Mink vacuum pump sion. with internal compression. Mink compressors with economical The reason is the fact that internal com- internal compression can compress up to pression creates internal overpressure 2 bar(g). that is blown out at the pressure gap.

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