run clean with dry vacuum pumps

32 www.cepmagazine.org October 2001 CEP

Dry Vacuum Pumps

he term “dry vacuum pump” isused to describe a positive-dis-placement vacuum pump that dis-charges continuously to atmospher-

ic pressure and in which the swept volume isfree of lubricants or sealing liquids. Dry vacu-um pumps were originally introduced in theJapanese semiconductor industry in the mid-1980s to address reliability problems associatedwith oil-sealed pumps and contaminationcaused by back-migration of vacuum pump oil.The success of these pumps revolutionizedsemiconductor processing. Dry vacuum pumpswere introduced into the U.S. chemical processindustries (CPI) in the late 1980s. In the nextten years, it is anticipated that they will com-pletely displace oil-sealed pumps, and willmake significant inroads into traditional mar-kets for steam jets and liquid-ring vacuumpumps — workhorses for the high-throughputmainstream processing operations in the CPI.

Dry pumps are compact and energy efficient,and do not contribute to air pollution, a problemwith oil-sealed pumps, or water pollution, aproblem with steam jets and water-sealed liquidring pumps. Dry pumps are unique among CPIvacuum pumps, because they do not require aworking fluid to produce vacuum, so nothingcontacts the load being pumped. Solvents orproducts aspirated from the process can be dis-

charged to an aftercondenser. Contamination isnot a concern, and the condensate can be recy-cled directly to the process.

Rough vacuumSubatmospheric pressures can be divided into

four regions:Rough vacuum 760 to 1 torrMedium vacuum 1 to 10-3 torrHigh vacuum 10-3 to 10-7 torrUltrahigh vacuum 10-7 torr and belowRough vacuum is the region of greatest in-

terest to the CPI, because it is where polymerreactors, vacuum distillation columns and vacu-um dryers normally operate. Medium vacuum isused in molten metals degassing, molecular dis-tillation and freeze drying. High and ultrahighvacuum are used in the production of thin films,mass spectrometry, low-temperature research,surface-physics research, nuclear research andspace simulation.

Semiconductor applications span rough to ul-trahigh vacuum, but the capital-intensive, preci-sion-technology operations that so characterizethe industry are high and ultrahigh vacuum oper-ations. Semiconductor processing is character-ized by corrosive gases (e.g., HCl), and the con-densation/precipitation of hard solids (for exam-ple, AlCl3 and SiO2) from the process gas streamin the pump (1). These challenges guided the

The use of dry pumps is growing, replacingworkhorse steam jets and liquid-ring pumps.Here is a comprehensive selection guide.

Run Clean with Dry Vacuum Pumps

TJim Ryans and Joe Bays,Eastman Chemical Co.

CEP October 2001 www.cepmagazine.org 33

early development of dry vacuum pumps. The drypumps that were developed for the semiconductorindustry are medium vacuum pumps. Ultimate orbase pressure is typically 3–5 × 10-3 torr; drypumps used as backing pumps for the turbomolec-ular pumps required for high and ultrahigh vacu-um typically operate at 10-2 to 1.0 torr.

In the beginning, building dry pumps for the CPImeant redesigning the semiconductor pumps for roughvacuum and considering a wider variety of applications.CPI dry pumps are rough vacuum pumps that typicallyoperate at 0.1–100 torr. These pumps are designed to han-dle a wider variety of materials than the semiconductorpumps and to cope with liquid slugs and solids carriedover from the process.

Principles of operationCPI dry pumps employ the operating principles of ro-

tary-lobe Roots blowers, claw compressors or screw com-pressors (2). These three all have certain things in com-mon. Tight clearances practically dictate cast iron or duc-tile iron construction. These pumps run hot and the poten-tial for overheating is inherent in their design. Dissipatingthe heat of compression is a problem. Temperature controlis required and is, increasingly, the key to engineering thenext generation of dry pumps. Generally, temperature con-trol is done by using a water jacket or injecting cooled pro-cess gas or nitrogen into the working volume of the pump.Occasionally, both methods are used.

Rotary-lobe Roots blowersThese dry pumps were developed from the rotary-lobe

Roots blower, a positive-displacement machine that nor-mally operates as a dry compressor. Two interlocking ro-tors on two parallel shafts synchronized by timing gearsand rotating in opposite directions trap and transport gases.Gears and bearings are oil-lubricated, but are external tothe pump; the rotors run dry. Clearances between the rotorsand between the rotors and the casing are generally0.004–0.020 in. Back-leakage across these clearances re-duces pump capacity, increasing as the pressure differentialbetween intake and exhaust increases. Dry compressionand noncontacting rotors mean that blowers can operate athigh rotational speeds up to 4,000 rpm. These machinesare, therefore, limited to use across relatively small pres-sure differentials, but since they can run at high speeds,they can be designed for high throughput.

Roots blowers have limited application as process vacu-um pumps discharging against high-pressure differentialsto the atmosphere, but they are used extensively as vacuumboosters in the 0.001–50 torr range. Roots vacuum systemswere developed in the 1950s as backing pumps withenough capacity to handle the discharge from diffusionpumps used in medium and high vacuum. Blowers wereused to extend the operating range and to boost the capaci-ty of rotary-piston pumps, thus, the convention of referringto blowers used in integrated vacuum pumping systems asvacuum boosters.

a

c

b

■ Figure 1. (a.) Three-stage Roots dry pump with interstagecoolers (Courtesy of Stokes Vacuum) (b.) Three-stage Roots drypump with water-cooled jacket (Courtesy of Stokes Vacuum (c.) Four-stage Roots/claw dry vacuum pump (Courtesy of BOC Edwards).

Dry vacuum pump systems can be built by connectingRoots vacuum pumps in series. Interstage coolers preventoverheating in the initial stages of the train. In the finalstage, gas recycled from an aftercooler is admitted to theworking volume of the pump. The gas cools the pump and istransported, along with the process gas, to the dischargeport. (Injection is in a location that does not significantly re-duce the pump’s throughput.) Gas injection allows Rootsblowers to achieve an ultimate pressure of about 100 torrwhen discharging to atmospheric pressure.

Process constraints justify building such elaborate sys-tems. Dry compressors are often required, for example, forpumping hydrogen, HCl vapor, helium-SF6 test gases, andhighly reactive mixtures of combustible gases. Many of thesame concerns that drove the development of elaborate five-stage blower systems and the development of semiconductordry pumps are now driving the development of dry vacuumpumps for the CPI.

Multistage Roots pumpsThe first commercially successful dry vacuum pump was

introduced in Japan in 1984 and was based on the Rootsblower with six stages in series (3). The six-stage machinewas actually two three-stage machines operating in series. Ineach machine, the rotors for three Roots stages were mount-ed on two parallel drive shafts and were held in phase bytiming gears. The first commercial dry vacuum pump for theCPI (introduced in 1987) was also based on the Roots prin-ciple with three stages in series.

The three-stage pump shown in Figure 1a uses both inter-stage coolers and intercooling. The shell-and-tube heat ex-changers in between the second and the third stages act asinterstage condensers. This is the principal advantage, andthe principal disadvantage of the design. The pump runscool, making it nearly ideal when solvent recovery is re-

quired and the process involves, for example, alcohols fromthe condenser train of a distillation column. The condensateis not corrosive, and solids fouling of the heat exchangers isnot a concern. When the condensate is corrosive, corrosionwill compromise performance. When solids are present,even soft polymers, the heat exchangers can foul.

The three-stage Roots pump (Figure 1b), the latest ver-sion of the pump, has a water jacket surrounding the work-ing volume. Interstage heat exchangers have been eliminat-ed, resulting in a compact design. The provisions for inter-cooling in this pump are very sophisticated. The way the gasrecirculates from the discharge of one stage to the workingvolume of the previous stage minimizes the temperature dif-ference between the rotors and the casing. This addresses themajor issue in protecting the pump from overheating — un-even thermal expansion that causes the rotors to come incontact with the casing.

Claw compressorsThe first Roots/claw dry vacuum pump (Figure 1c) was

marketed in Japan in 1985. The first stage of the pump isthe familiar Roots configuration. The second, third andfourth stages are intermeshing claws. Machines like theone shown in Figure 1c are remarkably successful in com-peting with dry pumps based strictly on the Roots princi-ple. The Roots/claw pump is fundamentally more rugged.

In the early 1990s, dry pumps based on intermeshingclaws premiered in the U.S. The volumetric efficiency of thecompressor is limited, as in all dry pumps, by backstreamingthrough the clearances between the rotors. The critical clear-ances are between circular profiles that can be machined tosmall tolerances. Since there is no relative movement be-tween the profiles of the rotors, the gaps between them canbe kept small; 0.005 in. is typical. The self-valving action ofthe claws means that continuous reworking of the gas in theswept volume, a problem with the Roots, is not a problemhere. The valving action of the rotors limits the backflow ofhot gas into the next compression cycle. Intercooling, usedto cool Roots machines, is not required. Gas injection, usedfor screw compressors, is also not required.

The rationale for combining Roots rotors with intermesh-ing claws and the order in which they are combined can befound by plotting the maximum compression ratio vs. outletpressure (4), as presented in Figure 2. This figure shows thatintermeshing claws are more efficient at higher pressures,and the the Roots, at lower pressures. CPI pumps are de-signed for operation across the range 0.1–760 torr. There arefew applications in the range 0.01–0.1 torr. Figure 2 showsthat the three-stage claw is more efficient across the range0.2–760 torr. The Roots/claw machine is more efficientacross the range 0.1–0.2 torr, but the difference is not signif-icant. And the three-stage claw is fundamentally a simplerand more-rugged machine. Thus, in developing pumps forthe CPI, the Roots/claw design was abandoned in favor oftwo- and three-stage claws.

Dry Vacuum Pumps


10-3 10-2 10-1

Claw-TypeMechanism

RootsMechanism

100

Outlet Pressure, mbar

Ratio

of O

utle

t Pre

ssur

e vs

. Inl

et P

ress

ure

101 102

60

50

40

30

20

10

■ Figure 2. Maximum compression ratio (for air) vs. discharge pressure(1.0 mbar = 0.75 torr) (Courtesy of BOC Edwards).

Figure 3a illustrates two aspects of claw machines thatare especially intriguing — the pump is vertical and, in thedesign shown, the second set of claws is reversed. Verticalmounting is advantageous in handling condensable vapors,or when the aspiration of a liquid slug or particulates fromthe process is possible. Liquid drains through from the suc-tion to the discharge and out the bottom of the pump. Re-versing the orientation of the rotors in the second stage sothat the outlet of the first stage aligns with the inlet of thesecond allows particulates to fall straight through the pumpand minimizes the area available for buildup of corrosiveresidues (5).

Screw compressorsScrew compressors have been used as vacuum pumps

since the mid-1950s, but these machines were not designedas vacuum pumps and were generally restricted to 100–760torr. A screw compressor designed as a dry vacuum pumpwas introduced in the early 1990s (Figure 3b — the dashedcircle in the figure indicates the pump’s inlet). Process va-pors entering the pump are trapped between two constant-pitch Archimedean screws and are conveyed from the suc-tion side to the discharge. Operation is isochoric. Compres-sion occurs in the final half-turn of the screw. Busch, Kin-ney Vacuum, Nash Engineering, Stokes, Rietschle Pumps,and Sterling SIHI currently market such vacuum pumps in

the U.S. They are not simply conventional screw compres-sors adapted for vacuum service. Ultimate pressure forthese machines is less than 0.1 torr, and some are capableof compression ratios in excess of 1,000,000:1 and opera-tion across 0.001–760 torr.

To operate effectively as a vacuum pump, a screw com-pressor must have tight clearances or run at high speeds,typically 6,000–18,000 rpm (6). In developing vacuumpumps for the CPI, manufacturers looked at the problemsassociated with high-speed operation, and elected, initially,to design for 3,600 rpm. To preserve volumetric efficiencyat lower speeds, it was necessary to design for tight clear-ances. Clearances between the rotors and between the rotorsand the casing are very tight, frequently less than 0.004 in.Some tolerance is required to allow the rotors to “bed in.”


■ Figure 3. (a.) Three-stage claw — Key: 1: Inlet; 2: Sealed high-vacuumbushings; 3: Indirect cooling; 4: Modular construction; 5: Gearbox; 6: Torque limiter; 7: Outlet; 8: Reversed claw(Courtesy of BOC Edwards) (b.) Horizontal screw compressor designed as a vacuum pump (Courtesy ofBusch, Inc.).

■ Figure 4. Vertical screw compressor designed as a vacuum pump (Courtesy of Sterling SIHI).

Some manufacturers address this by coating the rotors andthe casings with polytetrafluoroethylene (PTFE). This sacri-ficial coating is abraded as the rotors bed in and the runningclearances for the pump are established (2).

The pump shown in Figure 4 operates at 8,000 rpm. Therotors are stainless steel and a PTFE coating is not used.Vertical screws transport process vapors from the top inletto the bottom discharge. Problems posed by high rotationalspeeds — contamination of the working volume by bearinglubricant, vacuum-tight sealing of shafts, and high noiselevels associated with timing gears — have been addressedin the design. Cartridge-mounted bearings, used to supportthe shafts, are mounted inside the rotors. The resulting can-tilever design addresses contamination of the working vol-ume by bearing lubricant and vacuum-tight sealing of theshafts. The bearings are on the discharge side of the pumpand mechanical shaft seals have been eliminated. Gear lu-brication has also been eliminated. Two electronically syn-chronized motors drive the rotors. The timing gears arenoncontacting.

Isochoric operation of the rotors means that the temper-ature at the discharge end of vacuum screw compressorscan exceed 300°C. High temperatures prevent the conden-sation of process vapors; this protects the pump from cor-rosion. High temperatures, however, reduce the life ofseals and bearings and can result in thermal degradation,polymerization or autoignition of process vapors. Gas in-jection has been used to solve the problem, but when sol-

vent recovery is required, injectedgas can drive up capital and operatingcosts for the recovery system.

Research has been done by Japan(7) and U.S. manufacturers on reduc-ing operating temperatures, and mak-ing pumps more energy-efficient andcompact. The results call for chang-ing the pitch, or profile, of the screwaxially along its length. A change inthe profile, for example, midwaythrough the pump, shifts part of thework away from the discharge, creat-ing a more energy-efficient machinethat generates less heat. Changingthe profile of the screw reportedlydrops temperatures to 130–200°C(8). In addition, the reduced leadangle at the inlet gives the pumpgreater volumetric capacity, so it canachieve the same throughput withabout one-third less horsepower anda smaller footprint (7, 8). Virtuallyevery manufacturer of screw-com-pressor dry pumps has an aggressiveprogram to redesign its pump linebased on these findings.

Why specify a dry pump?The potential for eliminating process contamination is

the main driving force for specifying dry vacuum pumpsfor fine-chemicals and current good manufacturing prac-tices (cGMP) plants. It is anticipated that, in the future,environmental constraints and the incentive for solventand product recovery will increasingly dictate the specifi-cation of these pumps for mainstream CPI applications.Process integration is also a factor, because dry pumpsare so versatile.

Process contaminationOil-sealed pumps in pharmaceutical, pharmaceutical in-

termediates and some food processing plants are comingunder increased scrutiny from regulatory agencies. The po-tential for contamination of the process by pump oil was al-ways an issue, but the real issue now is cleanliness; the po-tential for contamination of the process and for contamina-tion associated with the use, handling, and disposal of pumpoil. Dry pumps provide an ideal solution to the problem.Eliminating the oil eliminates the problem.

Process contamination is also an issue when steam jets orwater-sealed liquid-ring pumps are used in cGMP plants.When a single batch is worth $500,000, the potential forcontamination, for example, as a result of backstreaming ofsteam from an unstable steam jet, is unacceptable if in-stalling a dry pump can eliminate the risk. If the steam sys-

Dry Vacuum Pumps


0.1 1 10 100 1,000

Jet

Dry PumpLiquid-Ring

Pump

Performance curves for:• 3-stage Steam Jet• 2-stage Liquid Ring-Pump• Dry Vacuum Pump

Suction Pressure, torr

Pum

ping

Spe

ed, a

cfm

200

175

150

125

100

75

50

0

25

■ Figure 5. Performance curves for a three-stage steam jet, two-stage liquid-ring pump, and a dry vacuum pump.

tem at a cGMP plant is not a sanitary system approved fordirect or incidental contact in cGMP applications, back-streaming of steam to the process will contaminate the prod-uct. Also, there are regulatory and legal issues. A processupset, triggered by backstreaming of steam, interrupts theprocessing cycle. The upset, especially if it is an aberrationnot provided for in a Drug Master Filing, may require thatthe plant scrap the batch. The manufacturer must establishthat the upset had no impact on product quality and did notresult in contamination. Dry pumps eliminate this potentialfor contamination.

Environmental constraintsAs environmental regulations place increasing restric-

tions on the discharge of contaminated working fluids, drypumps are being considered for point source elimination ofpollution from steam jets, liquid-ring pumps, and oil-sealedpumps. Steam jets and water-sealed liquid-ring pumps con-tribute to water pollution. Oil-sealed pumps contribute to airpollution, and the contaminated oil presents yet anotherwaste disposal problem. Dry pumps do not contribute to theproblem; dry pumps are part of the solution.

The Clean Air Act Amendments of 1990 resulted in severerestrictions on discharging wastewater containing air pollu-tants to industrial sewers and wastewater-treatment plants.Dry pumps can eliminate wastewater emissions at the source,and this has been one of the driving forces behind dry pumpdevelopment. But, in many plants, vacuum system wastewa-ter is a small part of the total wastewater problem. A control

device, such as a stripping column, may be used tohandle wastewater discharged from the entire plant. Ifa large control device is installed, condensate fromsteam jets and spent sealant from water-sealed liquid-ring pumps can be discharged to the control device. Inthis case, emissions reductions alone will seldom justi-fy installation of dry pumps.

Solvent/product recoveryIn most applications involving dry pumps, sol-

vent/product recovery is easy. The dry pump dis-charges to an aftercondenser. Contamination is not aconcern, and the condensate from the aftercondensercan be recycled directly to the process.

The success of dry pumps in solvent/product re-covery follows the precedent established by solvent-sealed liquid-ring pumps. Liquid-ring pumps are anatural choice for vacuum distillation, vacuum dry-ing and evaporator service, because the pump han-dles noncondensables saturated with process vapors.The “condensing effect,” inherent in the operation ofthe pump, means that vapors discharged to the liq-uid-ring pump may condense in it. If the pump isdedicated to a single process that uses a solvent witha sufficiently high boiling point, for example, xy-lene, the solvent can be used as the sealing liquid.

Process vapors condense in the pump, and the condensateis recycled to the process (9).

Solvent-sealed liquid-ring pumps have been used ex-tensively in the CPI to replace water-sealed liquid-ringpumps, oil-sealed pumps, and single- and two-stage jets.There are, of course, limitations to this approach. Thevacuum that can be achieved by a liquid-ring pump islimited by the vapor pressure of the sealing liquid. Thelower limit for process applications is approximately 25torr. (Operation at lower pressures, in the range 5–10 torr,is possible, but careful engineering is needed to ensurethat reliability is not compromised.) Liquid inventory isalso a problem. Changing to a new solvent contaminatesthe sealing liquid, and the potential for contaminationmay dictate changing out the sealing liquid at the end ofeach production campaign. Dry pumps eliminate both ofthese problems.

Dry pumps offer similar performance and economicsacross the same operating range as solvent-sealed liquid-ring pumps, but with the additional benefit of lower ulti-mate pressures. Because of this, dry pumps are viable al-ternatives to three-, four-, and five-stage jets. There are noliquid inventory problems; replacing a solvent-sealedpump with a dry pump eliminates the liquid inventory.Dry pumps are frequently a better choice for general-pur-pose use, because changes in the process, product or sol-vent that might affect the performance of solvent-sealedpumps will usually have little, if any, effect on the perfor-mance of dry pumps.


50 100 150 200 250 300 350

Three-Stage Claw

Screw Compressor

Skid-MountedLiquid-Ring

Three-Stage Roots

Free Air Displacement, cfm

Purc

hase

Cos

t

$70k

$60k

$50k

$40k

$30k

$20k

$10k

■ Figure 6. Purchase costs for dry pumps vs. liquid-ring pumps.

Process integrationProcess integration is an iterative approach to reduce

the complexity of the process flow diagram, and ultimatelyto reduce capital and operating costs for the plant. Theprincipal advantage of dry pumps in this context is versatil-ity. Dry pumps are often a cost-effective alternative tosteam jets and liquid-ring pumps in batch operations be-cause they are so versatile. The same pump that is used topull vacuum on the reactor can be used to pull vacuum ondownstream operations.

Dry pumps are anticipated to increasingly dominateprocess applications in fine-chemicals, pharmaceutical-intermediates and pharmaceutical plants. These are gen-erally multipurpose facilities built around reactor bays. Areactor bay consists of several stirred-tank reactors thatcan be configured to make different products. If theproduct is heat-sensitive, the reactor and downstreamdistillation column or evaporator will probably run undervacuum. If the product is a solid, downstream crystal-lization, filtration and drying operations will usually beunder vacuum.

Figure 5 presents performance curves that are based onactual equipment. Steam jets are used traditionally in fine-chemicals, pharmaceutical-intermediates and pharmaceuti-cal plants in the range 1–50 torr; liquid-ring pumps, for25–500 torr. The performance curves in Figure 5 show whydry pumps are often a cost-effective alternative to steam jetsand liquid-ring pumps in batch operations. A dry pump can

provide vacuum across the entirerange 1–760 torr. The same pumpthat is used to maintain 5 torr on areactor can be used to maintain 50torr on a dryer and 500 torr on a ro-tary vacuum filter.

Suction pressure and capacity

The most important parame-ters affecting vacuum pump se-lection are the suction pressureand capacity required for theprocess. Suction pressures andcapacities for steam jets, liquid-ring pumps, dry pumps, and in-tegrated systems are describedin the table. The informationpresented here can be used toeliminate pumps or pumpingsystems that cannot meet pro-cess requirements. The ultimatepressures shown in the table aresynonymous with the “blind”suction pressures for the pumpsor pumping systems; that is, thesuction pressures at zero load.

The lower limit for process applications is an approx-imate limit established by technical considerationsand economics.

Dry pumps span the range from 0.05–760 torr with ca-pacities in the range of 50–1,400 acfm. Steam jets can bedesigned for throughputs in excess of 1 million acfm in asingle unit. Liquid-ring pumps are available with capaci-ties up to 22,000 acfm. Dry pumps are limited to 1,400acfm, but they have relatively flat operating curves. Thisgives them the advantages, compared to steam jets, offaster pumpdown and better response to overloading. Drypump makers are, however, moving away from buildingthe larger pumps, those with capacities in excess of 500acfm. There is simply no demand for them. They are ex-pensive, and it makes more sense to couple vacuumboosters to smaller pumps to boost the capacity of thesmaller pumps than to build the larger machines.

Purchase costsFigure 6 can be used to estimate purchase costs for dry

pumps and compare them with those of liquid-ring pumps.Costs for Roots pumps and claw compressors are based onthree-stage machines. Purchase costs for screw compressorsare based both on machines that are mechanically and elec-tronically simple and on smart pumps that are complex. Be-cause of differences in the level of complexity from onepump to another, screw compressors are both the least and,paradoxically, the most expensive of the dry pumps. The

Dry Vacuum Pumps


2 4 6 8 10 20 40 60 80 100 200 400 600

Three-Stage Claw

Three-Stage Roots

Multistage Steam Jets

Screw Compressor

Suction Pressure, torr

Ther

mal

Effi

cien

cy

0.60

Note: 1.0 torr = 133.3 Pa

0.50

0.40

0.30

0.20

0.10

1-Stage Pump2-Stage

Two-StageLiquid-Ring

Single-StageLiquid-Ring

■ Figure 7. Adiabatic thermal efficiency of various pumps.

upper limit for liquid-ring pumps is based on stainless-steelskid-mounted models with total sealant recirculation sys-tems. These systems provide solvent/product recovery bene-fits similar to a dry pump. The companies that market drypumps realize that they are competing with such systems.The lower limit for screw compressor costs is, therefore,about equal to the upper limit for top-of-the-line skid-mounted liquid-ring models.

Differences in capital costs are seldom the determiningfactor in an evaluation of alternatives. Operating costs, thealready-mentioned environmental factors, andsolvent/product recovery will almost always be more im-portant. Also, purchase cost is only one component of cap-ital cost. Purchase cost for a steam jet may be lower thanthat for a dry pump, but total installed costs may be higherwhen factoring in the cost of adding boiler capacity, run-ning steam lines, and installing steam separators, streamtraps, and piping for condensers.

Energy consumptionThe adiabatic thermal efficiency, E, of a vacuum pump

may be defined as the adiabatic horsepower required to com-press a process gas from an initial pressure, P1, to a discharge

pressure, P2, divided by the actualbrake horsepower (bhp) required (10):

E = Theoretical adiabatic hp (1)Actual hp

This concept provides a convenientmeans to evaluate the energy costs ofvacuum pumps at a specific vacuumlevel. The adiabatic horsepower re-quired to compress w lb/h of dry,70°F air from P1 to P2 may be calcu-lated from:

Adiabatic hp = (w/20) [(P2/P1)(0.286) – 1](2)

The bhp actually required can befound by performance testing and byconverting motive steam usage forsteam jets into an equivalent electri-cal power requirement.Efficiencies calculated by this tech-nique were used to generate thecurves shown in Figure 7. Motive-steam requirements for steam jets arebased on 100-psig steam and wereconverted to an equivalent electricalrequirement (1,000 Btu = 0.293kWh). Calculated efficiencies formultistage jets were based on con-densing jets with surface condensers,

and 70°F cooling water. Mechanical pumps are assumedto be electrically driven. The efficiencies for single-stageliquid-ring pumps were based on 70°F sealing water. Thecurve for two-stage liquid-ring pumps assumes a low-vapor-pressure sealing liquid (i.e., vapor pressure of < 1torr at 70°F).

The curves represent approximations because there aresignificant variations in the efficiencies of pumps from dif-ferent manufacturers. The motive steam requirement for asteam jet is a function of steam pressure. The bhp for a me-chanical pump depends on rpm, and larger pumps are gen-erally more efficient than smaller ones of the same type.Nevertheless, the efficiencies indicated in Figure 7 are gen-erally representative of the efficiencies with which thepumps evaluated will handle noncondensable loads. Thefigure indicates that dry pumps are more efficient thansteam jets across practically the entire range 1–760 torr. Drypumps are more efficient than liquid-ring pumps across therange 1–50 torr, and this difference is significant for therange 1–20 torr.

Higher thermal efficiency is not synonymous with lowerenergy costs. The electrical equivalent of a pound of steamwill usually cost 3–6 times more than the steam due to


LI II

TI

TI

SuctionLoad

Water OutletTemp.

CasingTemp.

N2

ExhaustTemp.

and PressureWater In

Oil Levelor Pressure Amp Meter

GasBallast

PLC

M

TI

SC

Variable-Frequency Drive

LI

PI

LI

■ Figure 8. Instrumentation required for smart pump installations.

steam-cycle condensing losses and the more expensive hard-ware required to generate electricity. Projects aimed at re-placing steam jets with dry pumps to reduce energy costsmust, therefore, be reviewed carefully. Energy costs for drypumps, especially at operating pressures in the range 1–20torr, may be higher.

Smart pumpsRunning clearances for dry pumps are typically 0.010

in. or less. Dry pumps must be protected to minimize me-chanical damage. Equipment manufacturers use the smartpump or intelligent pumping system concept to addressthis issue. Microprocessors monitor and control the pump,and support the interlocks and self-diagnostics required toprotect the pump. The usual configuration is a vendor-sup-plied standalone programmable logic controller (PLC) thatinterfaces with a distributed control system (DCS) or aPLC housed within the operating system for a DCS.

The instrumentation required for smart pump installa-tions includes, but is not limited to, the requirements de-scribed in Figure 8. An array of sophisticated electronicsensors is required to monitor the pump and support thehigher-level control functions. Variable-frequency drives,

over-instrumentation and redundancyare the rule. The casing temperaturetransmitter, for example, interfaceswith the DCS and is hard-wired to afield-mounted thermal snap switch.When temperature control is crucialand there is an upper control limit(UCL), two thermal snap switches maybe used. One is tied to the DCS andacts to trip an alarm. If the uplink to theDCS is lost, the second switch providesredundancy. If the casing temperatureexceeds the UCL, the second thermalsnap switch shuts down the pump.

The configuration of the DCS iscrucial to the strategy for protectingthe pump. Both startup and shutdownare especially crucial:

Startup — The DCS is configuredto ensure that the pump has time tocome up to its operating temperaturebefore it comes online. The pump isisolated by a block valve and allowedto work against an inert gas or nitrogenbleed until the heat of compressionbrings it to operating temperature. Thisprotects the pump from corrosioncaused by condensation of process va-pors and ensures that the vapors do not“freeze out” as solids.

Shutdown — The DCS activates acleaning cycle prior to shutdown. This

ensures that shutting down does not trap process vapors inthe pump. Condensation of vapors trapped in the pumpcould leave the pump full of corrosive liquid that could dam-age it during a prolonged shutdown. (The DCS is, of course,configured to allow manual intervention to shut down thepump immediately in an emergency.)

During the cleaning cycle, the pump is isolated from theprocess and an inert gas or nitrogen bleed purges it of allresidual gases prior to shutdown. Such purging also dis-lodges solids. The purge gas scours the rotors and the cas-ing, and blows out debris. In demanding applications, theDCS interrupts the production cycle and isolates the pumpwhen the motor amperage exceeds a UCL. The pump is al-lowed it to run, sometimes for extended periods, at near-at-mospheric pressures to clear the debris. When the ampsdrop, the DCS brings the pump back online.

SafetySafety related to the operation of dry pumps has re-

ceived a lot of attention in the literature, because drypumps are new. The issues are not unique to dry pumps.Safety must be addressed, for example, when flammablesolvents such as acetone or gasoline are used as the sealing

Dry Vacuum Pumps


Capacity and operating range for steam jets, liquid-ring pumps, dry vacuum pumps, and integrated systems.

Type Ultimate Lower Limit Single-Unitor Base Pressure for Process Capacity

Applications Range, ft3/min

Steam-jet ejectors 10–1,000,000 One-stage 50 torr 75 torr Two-stage 4 torr 10 torr Three-stage 0.8 torr 1.5 torr Four-stage 0.1 torr 0.25 torr Five-stage 10 micron* 50 micron Six-stage 1 micron 3 micron

Liquid-ring pumps 3–18,000 60˚F water-sealed One-stage 50 torr 50 torr Two-stage 20 torr 25 torr Oil-sealed 1 torr 10 torr Air ejector first stage 1 torr 10 torr Dry vacuum pumps Three-stage rotary-lobe 0.5 torr 1.5 torr 60–240 Three-stage claw 0.1 torr 0.3 torr 60–270 Screw compressor 50 micron† 0.1 torr 50–1,400

Integrated pumping systems Booster — liquid-ring pump 1 torr 5 torr 100–15,000 Booster — rotary-lobe dry pump 25 micron 0.25 torr 100–1,500 Booster — claw compressor 10 micron 0.1 torr 100–2,500

Booster — screw compressor < 0.1 micron 1 micron 100–5,000

*1.0 micron = 0.001 torr† The base pressure depends on the pump model. Fifty microns is an "averaged" value. The range, across several vendor pump lines, is almost four orders of magnitude — 0.75 micron to 0.5 torr.

liquids for liquid-ring pumps. Indeed, it may be argued thatthe there are more safety issues associated with solvent-sealed liquid-ring pumps than with dry pumps. Still, thesafety issues associated with dry pumps must be under-stood to ensure safe operation (11).

Safety is an issue in pumping flammable vapors andgases because of the potential for an explosion initiated, forexample, by a spark caused by contact between the rotorsand the casing. Dry pump manufacturers address safety inpart by designing pumps that will contain an internal explo-sion. Flame propagation is still a consideration. Inerting withnitrogen or other inert gas prior to startup takes care of prop-agation back to the process during startup. When the processruns at < 75 torr, an explosion is not a consideration sincethe vapor/gas mixture in the void space in the pump and inthe process is inert. Installing a flame arrestor in the vent lineaddresses the concern that an explosion might propagatefrom the pump discharge to the atmosphere.

Autoignition is also a consideration (12). Dry pumpsrun hot, with discharge temperatures for screw compres-sors sometimes reaching 350–400°C. To cope with this, thelatest generation of dry pumps runs at lower temperaturesand has precise temperature control. This is accomplishedby designing the machines to be more energy-efficient, by

redesigning the rotors to avoid hot spots, and by applyingstate-of-the-art technology to the cooling system. Drypumps are offered that are rated for T4 applications, that is,those in which internal temperatures must not exceed135°C. It is, nevertheless, good practice to use caution inspecifying dry pumps for any application with vapors withan autoignition temperature of less than 200°C. CEP


Literature Cited1. Lessard, P. A., “Dry Vacuum Pumps for Semiconductor Processes:

Guidelines for Primary Pump Selection,” J. Vac. Sci. Technol. A, 18(4), pp. 1777–1781 (Jul./Aug. 2000).

2. Harris, N. S., “Modern Vacuum Practice,” 2nd. ed., Nigel HarrisPublisher, Crawley, West Sussex, U.K., website: www.modernvacu-umpractice.com/, pp. 289–310.

3. Troup, A. P., and N. T. M. Dennis, “Six Years of Dry Pumping: AReview of Experience and Issues,” J. Vac. Sci. Technol. A, 9 (3), pp.2048–2052 (May/Jun. 1991).

4. May, P. L., and B. S. Emslie, “Oil Free Vacuum Pumping Systemfor Plasma Processes,” BOC Edwards Publication No. 12-A401-31-895, BOC Edwards, Crawley, West Sussex, U.K. (1987).

5. Wycliffe, H., U.S. Patent No. 4,504,201 (1985) and U.K. Patent GB2.088.957B.

6. Tadashi, S., and M. Nakamura, “Spiral Grooved Vacuum PumpWorking in High Pressure Ranges,” Vacuum, 43 (11), pp. 1097–1099(1992).

7. Akutsu, I., et al., “A Gradational Lead Screw Dry Vacuum Pump,”J. Vac. Sci. Technol. A, 18 (3), pp. 1045–1047 (May/Jun. 2000).

8. Crabb, C., “Vacuum Pumps Fill a Void,” Chem. Eng., 107 (2), pp.37–41 (Feb. 2000).

9. Bays, J., “Minimizing Wastes from Vacuum Pumping Systems,”Chem. Eng., 103 (20), pp. 124–130 (Oct. 1996).

10.Ryans, J. L., and D. L. Roper, “Process Vacuum System Design &Operation,” McGraw-Hill, New York, pp. 221–226 (1986).

11. Oliver, G., “Vacuum Explosions,” The Chem. Engr., Issue 619, pp.21–22 (Sept. 1996).

12.Fuessel, U., “Keep Explosion Risk Low — Gas Temperatures inDry-Compressing Vacuum Pumps,” Chem.–Anlagen Verfahren, 29(5), pp. 32–33 (1996).

JIM RYANS is an engineering associate with Eastman Chemical Co., in

Eastman’s Process Design group (P.O. Box 511, Kingsport, TN 37662-5054;

Phone: (423) 229-3486; Fax: (423) 224-0453; E-mail:

[email protected]). He has 28 years’ experience in the design,

development and operation of a variety of chemical processes and

equipment. Ryans coauthored “Process Vacuum System Design &

Operation” (McGraw-Hill, 1986), he wrote the section “Pressure

Measurement” in the 4th edition of Kirk-Othmer’s “Encyclopedia of

Chemical Technology” (John Wiley, 1996), and holds patents on the design

of vacuum systems for controlling pressure in PET reactors. He earned a BS

in mathematics from East Tennessee State Univ. and a BSChE from the

Univ. of Tennessee. He is a member of AIChE and the American Vacuum

Soc., and is a registered professional engineer in Tennessee.

JOE BAYS is a principal chemical engineer with Eastman Chemical Co. (P.O.

Box 511, Kingsport, TN 37662-5054; Phone: (423) 229-5854; Fax: (423)

224-7268; E-mail: [email protected]). He works in Eastman’s

Chemicals-from-Coal Facility, and has 13 years’ experience in the design,

development and operation of a variety of chemical processes and

equipment. Bays previously authored an article “Minimizing Wastes from

Vacuum Pumping Systems” on recovering process material in vacuum

systems. He earned a BSChE from Virginia Tech and an MSChE from the

Univ. of Tennessee. He is a member of AIChE and a registered professional

engineer in Tennessee.

Short glossary

BBaacckkiinngg ppuummpp:: The pump that produces the necessary

discharge pressure for a vacuum pump incapable of

discharging directly to atmospheric pressure.

BBoooosstteerr:: A pump that operates as part of a multistage

system to boost the capacity of a pump that discharges

directly to atmospheric pressure.

CCoommpprreessssiioonn rraattiioo:: Discharge pressure divided by

suction pressure.

DDiisscchhaarrggee pprreessssuurree:: The absolute static pressure

measured at the discharge of the pump, torr.

IIssoocchhoorriicc: Constant volume..

SSuuccttiioonn pprreessssuurree:: The absolute static pressure measured

at the suction of the pump, torr.

TToorrrr:: One millimeter of mercury absolute. 1 micron =

0.001 torr; 1 in. Hg absolute = 25.4 torr; 1 mbar = 0.750

torr; 133.3… Pascal = 1 torr.

run clean with dry vacuum pumps

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