modern vacuum and overpressure generation for...

Modern vacuum and overpressure generation

Modern vacuum and

overpressure generation

for

pneumatic conveying

Bernd Jack

Dr.-Ing. K. Busch GmbH


Contents

1. General 3

2. Fields of application 4

3. Description of vacuum 5 and overpressure generators

3.1 Side channel blowers 5

3.2 Rotary vane vacuum pumps 6

3.3 Liquid ring vacuum pumps 9

3.4 Roots blowers 10

3.5 Mink rotary claw vacuum pumps/compressors 12

3.6 Screw compressors 16

4. Comparison of characteristic 17 curves

5. Technological differences 18

6. Economic efficiency 21

7. Control: Larger customer benefit due 25 to variable drives


Figure 1: Principal design vacuum conveying

1. General

The operating costs of a pneumatic con-veying system mainly depend on thecorrect selection of the vacuum orpressure generator.By choosing modern pressure generationequipment an economic and environ-mentally-friendly operation of conveyingsystems is possible.To select the most effective compressor itis necessary to know the actual operatingconditions of the conveying system.These parameters are of vital importancewhen selecting the most appropriate typeof compressor.Important factors influencing the selec-tion are the type of material being con-veyed and its characteristics, e.g. its

length and diameter and the kind ofconveyance (for example dense and leanphase pneumatic conveying).By means of these parameters the dif-ferential pressure (∆p) can be deter-mined. Both in pressure and vacuumconveying the differential pressure is themain influencing factor in selecting thecompressor.For economic analysis the followingfactors must be considered: primarycosts, energy consumption, costs de-pending on the compressor (e.g. throttlecontrol, valves, control), operating time(e.g. continuous operation, batch opera-tion) and maintenance costs.

3

vacuum pumpmax. vacuum

p • V = m • R • T

∆p, ∆V atmosphere

p = pressureV = volumem = massR = gas constantT = temperature


2. Fields of application

The following figure shows the vacuumpumps and compressors mainly used invacuum and overpressure conveying.Nowadays side channel blowers, con-ventional Roots blowers and Mink rotaryclaw vacuum pumps/compressors canbe used for both vacuum and over-pressure conveying, with Mink rotaryclaw vacuum pumps/compressors beingused in nearly all applications inpneumatic conveying.

Side channel blowers can be used bestfor short conveying and for vacuumconveying because only small differentialpressures are required.Rotary vane vacuum pumps and Minkrotary claw vacuum pumps in vacuumoperation respectively screw com-

pressors in pressure operation offer highpressure reserves and therefore can beused best for long or dense phaseconveying.

Mink rotary claw vacuum pumps andcompressors compress absolutely oil-freethus making them ideal for gascirculation conveying.

Additionally there are further types ofvacuum generators such as liquid ringvacuum pumps, multi-stage side channelblowers, ejectors etc. which were notsuccessful in pneumatic conveying be-cause of their characteristics or ineffi-ciency.

Figure 2: Application areas of various vacuum and overpressure generators

4

0 100 200 400 500 600 700 900800 0,6 0,81

1,23

1000 0,2 0,40

2,82,62,42,22

1,81,61,4300

Side channel blowers

Mink rotary claw vacuum pumps Mink rotary claw compressors

Screw compressors

Rotary vane vacuum pumps

Roots blowers

vacuum (mbar) overpressure (bar)


3. Description of the vacuum andoverpressure generators

3.1 Side channel blowers

There are two different types of sidechannel blowers: single- and two-stageside channel blowers.Conventionally driven side channel blo-wers can achieve differential pressures ofup to ±300 mbar.In frequency controlled operation sidechannel blowers can be used for vacuumconveying up to -400 mbar for over-pressure and up to +500 mbar foroverpressure conveying.

In contrast to vacuum pumps/compres-sors side channel blowers work accor-ding to the impulse principle: impellerand side channel form an annular wor-king chamber.

In the side channel the gas forms a whirlshaped like an eight. The unidirectionalwheel together with the aluminiumhousing forms the side channel, in whichthe gas is compressed and then dischar-ged through the pressure-side silencer.By the rotation of the impeller the gas istransported through the side channel.Depending on the back pressure the gasgets impulse energy by the impellervanes.Compression takes place if gas pene-trates into the impeller.The side channel between suction andpressure connection is separated by theso-called interruptor which is a narrow-ing of the side channel to the width ofthe impeller. Part of the circulating gasstream is transported to the pressureconnection.

Figure 3: Sectional drawing of a side channel blower

5


The other part of the circulating gas istranported to the suction side in thespaces between the impeller vanes side.This residual volume causes a flow withinthe side channel even if the inlet or outletis closed converting motor power intopotential and kinetic energy of the gas.Therefore a closed inlet or outlet of theblower can cause overheating of theblower. For protection against too hightemperatures (especially of the bearings)it is necessary to fit a vacuum or pressurerelief valve.

Due to these effects the efficiency of sidechannel blowers does not exceed 50%.Low primary costs and almost no main-tenance costs make side channel blowersideal for short and vacuum conveyingbecause only small differential pressuresare required.Side channel blowers have no stablecharacteristic curves. Therefore they donot compress isochorously, e.g. not atequal volume. For multi-stage operationmore power is required than for othercompression principles.

3.2 Rotary vane vacuum pumps

Currently oil lubricated rotary vanevacuum pumps and side channel blowersare the most important resp. mostfrequently used vacuum generators inpneumatic conveying.These pumps distinguish themselves bythe high differential pressure (almost 1,0bar) and an almost constant charac-teristic curve concerning the relevantworking area (in suction conveying).These outstanding qualities offer a highpower reserve in critical conveyingprocesses or with critical conveyingmaterials resulting in a high planningreliability.Also in continuous operation rotary vanevacuum pumps guarantee a stable vol-ume flow for the entire pressure rangefrom atmospheric pressure to ultimatepressure, e.g. these pumps do not needto be protected by a pressure relievevalve (for example, by an infiltrated airvalve).The principle of operation of a rotaryvane vacuum pump is shown in figure 4.

In an excentrically installed rotor drivenby an electric motor so called vanes slidein slots in the rotor and are pushedtowards the wall of the cylinder by thecentrifugal force of the rotation.The gas to be sucked of enters throughthe suction flange in the sickel-shapedspace between rotor and cylinder.The pumping effect results from the in-creasing sickel-shaped space betweenrotor and cylinder when the pump ro-tates. If this space is as large as possiblethe following vane covers the suction slotforming a space in which the sucked gasis trapped.At the same time new gas is sucked in bythe following chamber while the closedchamber decreasing with further rotationcompresses the trapped gas until theexhaust valve is opened at a pressure ofabout 1200 mbar and the gas isdischarged through the separator.The exhaust valve is covered with oil forbetter sealing.

6


Figure 4: Sectional drawing of an oil lubricated rotary vane vacuum pump

The oil and the air are then dischargedinto the bottom part of the oil mist se-parator and then separated from roughoil drips by gravity and the demister.The gas now only contaminated by oilmist is conveyed through oil mist sepa-rators for fine separation; the filter iscovered with permeable fibres collectingthe residual oil that drains off.

The gas, if it is air, is discharged directlyinto the atmosphere or can be trans-ported through a pipeline away fromthe pump.Oil lubricated rotary vane vacuumpumps operate nearly free of wear sincethe vanes always slide on an oil film andare of robust design.

The circulating oil also serves as a coo-ling, sealing and lubricating medium.The automatic oil circulation is based onthe differential pressure between the oilmist separator, where considerableoverpressure exists caused by the filterresistance, and the inlet flange of thepump.No separate oil pump is required.The pump is equipped with a suction-sidevalve that prevents the flow of air or con-veyed gas back into the vacuum chamberwhen the pump is switched off to avoidpump oil being drawn into the suctionline by the differential pressure.The inlet of the valve is equipped with ascreen to prevent the pump from beingpolluted.

7

Cylinder

Exhaustfilter

Oil filter

Inlet screen

RotorVane

Non-return valve


No separate oil pump is required.The pump is equipped with a suction-sidevalve that prevents the flow of air or con-veyed gas back into the vacuum chamberwhen the pump is switched off to avoidpump oil being drawn into the suctionline by the differential pressure.The inlet of the valve is equipped with ascreen to prevent the pump from beingpolluted.

Additionally conventional rotary vanevacuum pumps used for pneumaticconveying are equipped with a fine filter(filtration efficiency 5 µ) at the suctionside which is to protect the pump againstadmittance of solid particles (e.g. productabrasion).

Oil lubrication of rotary vane vacuumpumps requires maintenance work, likefor example: • check oil level at the oil sight glass 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 usedto check the condition of the exhaustfilters; the filter pressure gauge is in-stalled in the drilling through which oil isfilled in. The exhaust filters need to bereplaced if the filter pressure is ≥0,6 bar.

Dry running rotary vane vacuumpumps

Basically there are two types of rotaryvane vacuum pumps:- oil lubricated vacuum pumps- dry running vacuum pumpsThe vanes of dry running vacuum pumpsare made of special carbon. These vanesare self-lubricating. These vacuum pumpsneed no oil as operating medium.The more the sealing gaps increase dueto permanent abrasion of the vanes themore the suction capacity decreases.This effect is aggravated more or lessdrastically if particles are conveyed.To avoid a total vacuum pump failurecaused by broken vanes it is necessary tocheck them for wearout regularly andreplace them as required.Therefore it is not recommended to usedry running rotary vane vacuum pumpsin pneumatic conveying.

8


Figure 5: Sectional drawing of a liquid ring vacuum pump

Casing

Impeller

Liquid ring

Suction slotDischarge slot

3.3 Liquid ring vacuum pumps

Liquid ring vacuum pumps normallyoperate with water as operatingmedium.An excentrically installed impeller rotatesin the cylindrical pump casing partly filledwith liquid (usually water).By the rotational movement of the im-peller and the resulting centrifugal forcethe liquid within the cylinder forms theso-called liquid ring.Gas is conveyed in the spaces betweenthe single blades and the liquid ring.As a result of the excentrical installationof the impeller the spaces enlarge andthe process gas is sucked in through thesuction slot.Further rotation reduces the spaces, thegas is compressed and dischargedthrough the pressure slot.

The vacuum pump can be operated withwater recirculation cooling, semi-open orclosed loop cooling circuit.Depending on the efficiency and designthe fluid used as operating medium hasthe following functions:• energy transmission from the impeller to the medium to be compressed• sealing of the cells in radial direction• sealing of the space between impeller and casing parts• cooling and lubrication of the shaft sealsDepending on the application the ringliquid has the following functions:• absorption of heat caused by compression and friction• absorption of heat caused by condensation and reaction• absorption of gaseous contaminants• absorption of particles

9


Advantages of liquid ring vacuumpumps:• simple and cost-effective design• 100% oil free• condensation of vapours is possible• good material resistance (alternative materials possible)

Disadvantages:• Operation strongly depends on - temperature of the operating liquid - density of the operating liquid - solubility of gases in the operating liquid - temperature of the gases to be sucked off - condensation effect

• Danger of cavitation Damage or even destruction the surfaces by implosions caused by the condensation of vapour bubbles.

3.4 Roots blowers

Conventional two- and three-lobe Rootsblowers work according to the well triedRoots principle.Two parallel rotors with identical profilesrotate in opposite directions within acasing.As they rotate gas is drawn into thespace between each rotor and the casingwhere it is trapped, transported anddischarged by the rotation. The gas iscompressed isochorously, e.g. at equalvolume. If a chamber containing gasunder suction pressure reaches thepressure inlet the chamber is filled withreturning gas from the pressure side andtherefore compresses to atmosphericpressure.

• Danger of calcifying Should water evaporate from the ring liquid, the dissolved salts settle on the casing surface. The deposits reduce the suction capacity of the vacuum pump which can result in a failure of the unit depending on the thickness of the deposit.• high supply or disposal costs in continuous operation These costs possibly will be increased dramatically by environmental process gas in the operating fluid.• „possible “danger of silting up” in circulation operation with solid particles.

10


There is no actual internal compressionand no mechanical contact betweenrotors and cylinder.These blowers are contact-free (and nooil is needed as sealing fluid) and can beused for differential pressures of up to0,6 bar in vacuum operation and up to1,0 bar(g) in pressure operation.In pneumatic conveying Roots blowerscan only be used in applications inwhich high volume flows at low diffe-rential pressures are required. In thissmall range of applications Rootsblowers are distinguished by their smallpower consumption.

Disadvantages are their high acquisitioncosts and costs for noise reduction.In pneumatic conveying Roots blowersare normally equipped with the followingaccessories:

• inlet filter• vacuum or pressure relief valve as protection against overload• suction-side non-return valve• inlet/outlet silencer

Figure 6: Sectional drawing of a Roots blower

11

Roots lobe

Cylinder

Internalcompression


3.5 Mink rotary claw vacuumpumps/compressors

Mink rotary claw vacuum pumps areavailable with suction capacities from60 m3/h to 500 m3/h for vacuumconveying up to -750 mbar (continuousworking pressure). Mink rotary clawcompressors can be operated inoverpressure conveying up to 2 bar.By connecting several single Minkvacuum pumps or compressors inparallel bigger volume flows can begenerated.Mink vacuum pumps or compressorsoperate according to the well triedrotary claw principle. They compressabsolutely oil and contact free andhence free of wear.

Further savings are achieved by minimummaintenance work and considerablylower energy consumption thus makingMink compressors the most economicalalternative in pressure conveying up to 2bar(g).

Two rotary claws rotate in opposite di-rections within a cylinder. There is neithercontact between claws and casing norbetween the two claws.

Figure 7: Sectional drawing of a Mink rotary claw vacuum pump

12

Housing

Claw

Claw


Internalcompression

Figure 8: Compression process of a Mink rotary claw vacuum pump

13


Bearing/Sealing

There is an atmospherically ventilatedinter-space between compressionchamber and gear side.This inter-space:• causes thermal separation between the compression chamber and bearings. The bearings are not affected by the compression heat resulting in long service life.• prevents pressure fluctuations on the shaft seal rings by atmospheric ventilation and therefore excessive or premature wearout.• prevents both the penetration of product gas into the bearing or storage chamber respectively and the conveying of oil particles into the compression chamber. Compared to conventional Roots blowers the compression is guaranteed to be actually oil free.

The bearings are robust roller bearings:• the fixed side (gear side) has angular ball bearings• the free bearing side has cylindrical roller bearingsThe shaft sealing between the compres-sion chamber and the atmospheric inter-space is affected by labyrinth seals.Towards gear and storage chamber radialshaft seal rings are used.

The remaining gaps at the virtual point ofcontact are manfactured so precisely thatthey serve as sealing (labyrinth) becauseof air turbulence within the gap.Therefore the compression chamber isdivided into two parts:• an enlarging chamber on the suction side drawing in air• a decreasing chamber on the discharge side compressing and discharging air at the same time.The exact rotation of both claws iscontrolled by a synchronized drive.However, in contrast to Roots blowers,Mink rotary claw vacuum pumps workwith internal compression.As a result of the special profile of theclaws the conveying medium is pre-compressed internally in rotationaldirection in front of the claws beforereaching the pressure side. At the sametime gas is drawn in behind the claws inrotational direction for the next cycle.Mink rotary claw vacuum pumps andcompressors are driven by three-phasemotors that are conventionally used forfrequency controlled operation and areequipped with PTC thermistors.In frequency controlled operation thepump adapts optimally to the process byspeed control. This results in a furthersaving in energy or increase of conveyingpower by about 20%.Mink vacuum pumps and compressorsare air cooled. A separate additionalelectric fan guarantees a constant cooling,also with variable pump speeds.

14


Standard equipment for pneumaticconveying:

In pneumatic conveying Mink vacuumpumps/compressors are equipped withthe following accessories:

a) Suction conveying:• suction air filter• vacuum relief valve as protection

against overload• Suction-side non-return valve• exhaust gas silencerb) Pressure conveying:• inlet silencer, in combination

with suction air filter• pressure relief valve as protection

against overload• pressure-side vessel for pulsation

reduction• pressure-side non-return valve

With the appropriate combination (pa-rallel operation) of several individual unitsevery conveying capacity can be realized.

Such modular solutions have thefollowing advantages:• simple power adaptation by series

switch-on or switch-off• in case of power control by frequency

converter only a smaller converter(adapted to an individual unit) is re-quired

• availability of a stand-by machineguaranteed at any time

• cost-effective due to standardizedmodular design

• cost-effective silencing measurespossible

Figure 9: System solutions for higher conveying capacities15


3.6 Screw compressors

Single-stage air cooled screw compressorsare used for volume flows from approx.200 to 15000 m3/h. These compressorsachieve pressures of more than 10 bar.They should only be used wherepressures of >2 bar (abs.) are actuallyrequired.Screw compressors are expensive andtheir power consumption is very high.Furthermore the oil required for com-pression must be extracted from thecompressed air.

Compared to Mink rotary claw pumpsthe synchronism of both screw rotors isguaranteed by a synchronisation drive.Furthermore, integrated transmissiongears allow variable power levels.Roller bearings are used. Drive shaft andmain rotor are equipped with multiplebearings.

Oil lubricated screw compressors areadditionally equipped with an oil pump,oil cooler and an oil mist separator.

Figure 10: Screw compressor with integrated gear stage

16


Figure 11a: Comparison of characteristic curves stability of various vacuum generators

In pneumatic conveying screwcompressors are equipped with thefollowing accessories:• Inlet air filter• Safety valve• Non-return valve• Pressure silencer• Silencing hood

4. Comparison of the characteristic curves

The following diagrams show the suctionor pressure characteristics of the indivi-dual types of vacuum generator depen-ding on the corresponding differentialpressure.

The volume flows are stated in % of thenominal suction or conveying capacity ofthe individual types.

17

-100-400-500-800-9000

10

Vacuum [hPa (mbar)]

-700 -600 -300 -200 0

20

30

40

70

60

Suct

ion

capa

city

[%

]

80

50

90

100

-1000

Mink rotary clawvacuum pump

Rotary vanevacuum pump,oil lubricated

Roots blowerRotary vanevacuum pump,dry running

Side channel blower,two-stage

Side channelblower,single-stage

Vacuum generator


Compressor

Figure 11b: Comparison of characteristic curves stability of various pressure generators

5. Technological differences

There are technological differences bet-ween the various vacuum and over-pressure generators presented that in-fluence power characteristic and effi-ciency of the corresponding unit.“Internal compression” is the decisiveadvantage of the design principle Minkrotary claw compressor over the conven-tional Roots blower working according tothe Roots principle. This is described inthe following pV-diagram:

The region between point 1 and point 2in the pV-diagram shows the filling of therespective pumping chamber at suctionpressure. In case of the Roots blower (leftdiagram, next page) the pressure remains

constant until there is an openingtowards this chamber between point 2and point 3. The gas increases inpressure at the same volume. Betweenpoint 3 and four it reaches dischargepressure and is ejected.The region between points 1, 2, 3 and 4in the pV-diagram (on the next page)describes the compression work to bedone.

The functional principle of the Minkrotary claw vacuum pumps and com-pressors on the other hand result in thefollowing process:Between points 1 and 2 in the pV-diagram the chamber within the com-pressor is filled with gas at suctionpressure.

18

0 2,521,510,5

30

40

50

60

70

80

90

Operating pressure [bar(g)]

Act

ual d

isch

arge

cap

acit

y [%

]

100

Side channel blower,two-stage

Roots blower

Screw compressor

Minkrotary claw compressor


However, before the gas volume reachesthe pressure side, there is internal com-pression of the gas in front of the claws.The pressure simultaneously increasesto the decrease of the volume.No gas flows back into the compressionchamber from the pressure side. Duringthe rotation the pressure mouth in thecompression chamber is opened as aresult of the specific design of the clawand the gas is discharged on the pressureside.The region between points 2 and 3‘describes the compression process, theregion between points 3‘ and 4 describesthe discharge process.

The pV-diagram shows that the regionand therefore the compression work tobe done is considerably smaller forcompressors with internal compressionthan for blowers without internal com-pression. Therefore the same compres-sion work can be done moreeconomically.

Figure 12: Advantage due to internal compression shown in the pV-diagram

19

p

V

1 2

34p

V

1 2

34

savings

savings without internalcompression(Roots blower)

savings with internal compression(Mink rotary claw vacuum pump)

3’


When compressing gas the temperatureis increased both in the case of the Rootsblower and the Mink rotary claw vacuumblower. The temperature increase is pro-portional to the energy added.If a compressor with internal compressionrequires lower energy for compressingthis results in lower temperature increase.This lower temperature increase can be

Figure 13: Comparison power requirement Roots blower/Mink rotary claw vacuum pump

This diagram shows the power consump-tion of a Roots blower compared with aMink vacuum pump.The effect of the internal compression topower data is obvious.At small overpressures the Roots bloweris better than the Mink vacuum pumpwith internal compression.The reason is the fact that internal com-pression creates internal overpressurethat is blown out at the pressure gap.

At small overpressures Roots pumps aremore economical. The point of intersec-tion is about 0,8 bar(g). It is obvious thatthe power consumption of the Rootspump increases up to about 1 bar sinceRoots blower have no internal compres-sion.Mink compressors with economicalinternal compression can compress up to2 bar(g).

used to realize higher discharge pres-sures at constant power consumption.The claw principle achieves a dischargepressure of 2 bar(g) and a vacuum of80%.

20

0 1,81,21,00,40,20

2

4

6

8

14

12

Operating pressure [bar(g)]

Pow

er c

onsu

mpt

ion

[kW

]

16

10

0,6 0,8 1,4 1,6 2,0

Roots blower

Mink rotaryclaw vacuum pump


6. Economic efficiency analysis ofvarious vacuum generators byanalysis of overall expenses

The efficiency of a pneumatic conveyingsystem also depends on the operatingcosts of the respective vacuum/pressuregenerator.The correct selection of vacuum or pres-sure generators is of vital importance.This especially applies to complex systemssince they cause a considerable part ofthe operating costs of the system.These operating costs, for example,depend on:• Reliability• Process compatible suction or

pressure characteristics• Good coefficient of efficiency• Low maintenance work• Price• Potentially good controlability of

volume• Smaller pipeline diameters

In this chapter only vacuum generatorsalready successfully used in pneumaticconveying for many years are comparedwith each other.The pumps are compared with respecteddense phase pneumatic conveying.This type of conveying is characterizedby higher differential pressure, comparedto lean phase conveying, for example.Mink rotary claw vacuum pumps, liquidring vacuum pumps with integratedsealing fluid circulation, four-stage sidechannel blowers and the already suc-cessfully used oil lubricated rotary vanevacuum pumps are compared with eachother.The following comparison shows thepower requirement of these vacuumgenerators (with equal suction capacities)in relation to the corresponding workingpressure.

Figure 14: Power requirement of various vacuum generators

21

0 100 10009008007006005004003002001

2

3

4

5

6

7

Mink rotary claw vacuum pump

Roots blowers

Rotary vane vacuum pump, oil lubricated

Liquid ringvacuum pump

Inlet pressure [mbar(abs.)]

Po

wer

co

nsu

mp

tio

n [

kW]

Modern vacuum and overpressure generationSu

ctio

n ca

paci

ty [

m3 / k

W]

The comparison of the power require-ment of various vacuum generators ata fixed operating point shows that Minkrotary claw vacuum pumps have thelowest energy consumption due to“internal compression”and oil andcontact free operation.

This is illustrated in the following figureas “specific suction capacity” to de-termine the suction capacity per kW.

Figure 15: Comparison specific suction capacity of diffrent vacuum generators

22

70

60

50

40

30

20

10

0

Roots blower Rotary vanevacuum pump,oil lubricated




The annual energy costs or savings usingMink rotary claw vacuum pumps andcompressors are shown in the followingfigure.

The calculation of these costs is based on7000 operating hours and 0,08 Euro perkW/h.

The following diagram shows the totalcosts in Euro refering to 8000 operatinghours/year.The acquisition costs have been deter-mined on the basis of actually publishedlist prices and refer to a linear deprecia-tion over 5 years with an interest of 7%.Energy costs were assumed to be Euro0,08 per kW/h.The maintenance costs only include ma-terial and work costs. Both additionalcosts for disposal of removed parts etc.and downtimes of the machines are notconsidered.

Energy costs are the biggest part of thetotal operating costs. Therefore energycosts are the most decisive factor inselecting the most effective vacuumgenerator.However, experience shows that main-tenance work to be planned and re-sulting downtimes of the machine willcause bigger problems.The argument that machines are main-tenance-free becomes more and moreimportant for customers.

23

3500

3000

2000

1500

1000

500

0

Euro/7000 h

savingswith Mink

Side channel blower,four stage




2500

Figure 16: Comparison of energy costs per year

Cos

ts [

Euro

]


Figure 17: Costs distribution

Figure 18: Total costs

24

3500

3000

2500

2000

1500

1000

500

0

Energy costs

Costs of maintenance

Roots blower Rotary vanevacuum pump,oil lubricated



Depreciation

6000

Depreciation Costs of energyCosts of maintenance

5000

4000

6000

2000

1000

0




Roots blower

Expe

nses

[Eu

ro/a

]Ex

pens

es [

Euro

/a]


Figure 19: Wear caused bynon-optimum conveying speed

7. Control: Larger customerbenefit due to variable drives

Optimum operating costs mainly dependon energy costs; therefore it is interestingwhether it is possible to optimize thisimportant cost factor by speed control.Basically the conveying speed at thematerial feed station is decisive for thedesign of pneumatic conveying systems.There is a pressure loss during conveyingas the gas expands.The pressure loss depends on the densityof the material to be conveyed, thelength of the pipes, the material (con-sistency of the materials) etc. The morethe pressure decreases (during convey-ing) the more the suction volume for thevacuum generators increases.Since many parameters collude an exactcalculation of these processes is notpossible. As a result the greater part ofsuch systems is usually oversized whichmay cause excessive conveying speedsleading to pipeline or product wearout.However, “controls” with flaps or infil-trated air are used to avoid unnecessarywearout and for careful conveying.Of course, this kind of control (by flapsor infiltrated air valves) is a waste ofenergy; adaptation to required powerdata by speed control is more econo-mical.

Not every type of vacuum pump orcompressor can be used with a speedcontrol, however.There are restrictions like, for example,minimum speed (centrifugal force),maximum speed (heating/wear) andrestrictions concerning efficiency.Mink rotary claw vacuum pumps andcompressors are ideal for speed control,since their power characteristic behaveslinearly over the entire speed range asshown in the following diagram.

25


Operating pressure500 mbar(abs.)

Figure 20: Suction capacity in case of frequency controlled Mink rotary claw vacuum pump

26

0 4500300025001000500

0

50

100

150

200

250

1500 2000 3500 4000

Mink MM 1140 AV

As an option Mink rotary lobe vacuumpumps can be equipped with anintegrated frequency drive.This drive makes it possible to adjust the

suction capacity almost linearly andthereby set the conveying speed to anoptimal level.

Due to the exact adjustment of theprocess, Mink vacuum pumps can beoperated very economically. There is noneed for throttle valves and furthermoresubstantial energy savings are possible asonly the amount of energy is consumedwhich is actually needed for the requiredsuction capacity.In the most simple cases, the required airflow, i. e. the conveying speed in thepipe, is set manually with the integratedpotentiometer. This leads to animprovement of process conditionsduring start-up. There is also thepossibility of external control by SPS.

Therefore, inputs with either 0-10 Vor 4-20 mA are provided.

Some additional features:• status output (motor on/off)• speed output (with analogue

output 0-10 V)• start /stop of the drive by voltage

signal from the SPS control• selection of 3 fixed points of reference

(preset references which are activatedthrough the binary inputs)

Suct

ion

capa

city

[m

3 /h]

Motor speed min-1


27

Due to the resulting capacity reserve theuse of a smaller pump size is possibleunder certain circumstances which leadsto substantial savings in costs.

Use of Mink rotary claw vacuumpumps with frequency controlleddrive

Above picture clarifies the range ofapplication of a Mink vacuum pump withfrequency controlled drive.When operated at a speed of up to3800 min-1 the suction capacity can beincreased significantly in comparison tomachines with a conventional drive.

Figure 21: Mink rotary claw vacuum pump with frequency controlled drive

0 100 200 300 400 500 600 700 800 900 1000

0

20

40

80

60

100

140

120

160

200

180

suct

ion

cap

acit

y

Min

k M

M 1

10

0 A

V 3

kW

suct

ion

capa

city

Min

k M

M 1

140

AV

4 k

Wm3/h

S

hPa (mbar)p


28

Above picture shows a claw vacuumpump of the Mink series with integratedfrequency drive. The compact and in-tegrated design of both motor and fre-quency inverter ensures full electromag-netic compatibility.

Compared to solutions with an externalfrequency inverter, there is no need foradditional measures such as throttlevalves or special shielding.

Figure 23: Mink rotary claw vacuum pump with frequency controlled drive

modern vacuum and overpressure generation for...

Documents