Dry Vacuum Pumps
Run Clean with Dry Vacuum Vacuum Pumps Pumps The use of dry pumps is growing, replacing workhorse steam jets and liquid-ring pumps. Here is a comprehensive selection guide.
Jim Ryans and Joe Bays, Bays, Eastman Chemical Co.
T
he term “dry vacuum pump” is used to describe a positive-displacement vacuum pump that discharges continuously to atmospheric pressure and in which the swept volume is free of lubricants or sealing liquids. Dry vacuum pumps were originally introduced in the Japanese semiconductor industry in the mid1980s to address reliability problems associated with oil-sealed pumps and contamination caused by back-migration of vacuum pump oil. The success of these pumps revolutionized semiconductor processing. Dry vacuum pumps were introduced into the U.S. chemical process industries (CPI) in the late 1980s. In the next ten years, it is anticipated that they will completely displace oil-sealed pumps, and will make significant inroads into traditional markets for steam jets and liquid-ring vacuum pumps — workhorses for the high-throughput mainstream processing operations in the CPI. Dry pumps are compact and energy efficient, and do not contribute to air pollution, a problem with oil-sealed pumps, or water pollution, a problem with steam jets and water-sealed liquid ring pumps. Dry pumps are unique among CPI vacuum pumps, because they do not require a working fluid to produce vacuum, so nothing contacts the load being pumped. Solvents or products aspirated from the process can be dis-
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charged to an aftercondenser. Contamination is not a concern, and the condensate can be recycled directly to the process.
Rough vacuum Subatmospheric pressures can be divided into four regions: Rough vacuum 760 to 1 torr Medium vacuum 1 to 10-3 torr High vacuum 10-3 to 10-7 torr Ultrahigh vacuum 10-7 torr and below Rough vacuum is the region of greatest interest to the CPI, because it is where polymer reactors, vacuum distillation columns and vacuum dryers normally operate. Medium vacuum is used in molten metals degassing, molecular distillation and freeze drying. High and ultrahigh vacuum are used in the production of thin films, mass spectrometry, low-temperature research, surface-physics research, nuclear research and space simulation. Semiconductor applications span rough to ultrahigh vacuum, but the capital-intensive, precision-technology operations that so characterize the industry are high and ultrahigh vacuum operations. Semiconductor processing is characterized by corrosive gases (e.g., HCl), and the condensation/precipitation of hard solids (for example, AlCl3 and SiO2) from the process gas stream in the pump (1). These challenges guided the
b
a Figure 1. (a.) Three-stage Roots dry pump with interstage coolers (Courtesy of Stokes Vacuum) (b.) Three-stage Roots dry pump with water-cooled jacket (Courtesy of Stokes Vacuum (c.) Four-stage Roots/claw dry vacuum pump (Courtesy of BOC Edwards).
early development of dry vacuum pumps. The dry pumps that were developed for the semiconductor industry are medium vacuum pumps. Ultimate or c base pressure is typically 3–5 × 10-3 torr; dry pumps used as backing pumps for the turbomolecular pumps required for high and ultrahigh vacuum typically operate at 10-2 to 1.0 torr. In the beginning, building dry pumps for the CPI meant redesigning the semiconductor pumps for rough vacuum and considering a wider variety of applications. CPI dry pumps are rough vacuum pumps that typically operate at 0.1–100 torr. These pumps are designed to handle a wider variety of materials than the semiconductor pumps and to cope with liquid slugs and solids carried over from the process.
Principles of operation CPI dry pumps employ the operating principles of rotary-lobe Roots blowers, claw compressors or screw compressors (2). These three all have certain things in common. Tight clearances practically dictate cast iron or ductile iron construction. These pumps run hot and the potential for overheating is inherent in their design. Dissipating the heat of compression is a problem. Temperature control is required and is, increasingly, the key to engineering the next generation of dry pumps. Generally, temperature control is done by using a water jacket or injecting cooled process gas or nitrogen into the working volume of the pump. Occasionally, both methods are used. Rotary-lobe Roots blowers These dry pumps were developed from the rotary-lobe
Roots blower, a positive-displacement machine that normally operates as a dry compressor. Two interlocking rotors on two parallel shafts synchronized by timing gears and rotating in opposite directions trap and transport gases. Gears and bearings are oil-lubricated, but are external to the pump; the rotors run dry. Clearances between the rotors and between the rotors and the casing are generally 0.004–0.020 in. Back-leakage across these clearances reduces pump capacity, increasing as the pressure differential between intake and exhaust increases. Dry compression and noncontacting rotors mean that blowers can operate at high rotational speeds up to 4,000 rpm. These machines are, therefore, limited to use across relatively small pressure differentials, but since they can run at high speeds, they can be designed for high throughput. Roots blowers have limited application as process vacuum pumps discharging against high-pressure differentials to the atmosphere, but they are used extensively as vacuum boosters in the 0.001–50 torr range. Roots vacuum systems were developed in the 1950s as backing pumps with enough capacity to handle the discharge from diffusion pumps used in medium and high vacuum. Blowers were used to extend the operating range and to boost the capacity of rotary-piston pumps, thus, the convention of referring to blowers used in integrated vacuum pumping systems as vacuum boosters.
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Dry Vacuum Pumps
e r u s s e r P t e l n I . s v e r u s s e r P t e l t u O f o o i t a R
60 Claw-Type Mechanism
50 40 30
Roots Mechanism
20 10 10-3
10-2
10-1
100
101
102
Outlet Pressure, mbar
Figure 2. Maximum compression ratio (for air) vs. discharge pressure (1.0 mbar = 0.75 torr) (Courtesy of BOC Edwards).
Dry vacuum pump systems can be built by connecting Roots vacuum pumps in series. Interstage coolers prevent overheating in the initial stages of the train. In the final stage, gas recycled from an aftercooler is admitted to the working volume of the pump. The gas cools the pump and is transported, along with the process gas, to the discharge port. (Injection is in a location that does not significantly reduce the pump’s throughput.) Gas injection allows Roots blowers to achieve an ultimate pressure of about 100 torr when discharging to atmospheric pressure. Process constraints justify building such elaborate systems. Dry compressors are often required, for example, for pumping hydrogen, HCl vapor, helium-SF 6 test gases, and highly reactive mixtures of combustible gases. Many of the same concerns that drove the development of elaborate fivestage blower systems and the development of semiconductor dry pumps are now driving the development of dry vacuum pumps for the CPI.
Multistage Roots pumps The first commercially successful dry vacuum pump was introduced in Japan in 1984 and was based on the Roots blower with six stages in series (3). The six-stage machine was actually two three-stage machines operating in series. In each machine, the rotors for three Roots stages were mounted on two parallel drive shafts and were held in phase by timing gears. The first commercial dry vacuum pump for the CPI (introduced in 1987) was also based on the Roots principle with three stages in series. The three-stage pump shown in Figure 1a uses both interstage coolers and intercooling. The shell-and-tube heat exchangers in between the second and the third stages act as interstage condensers. This is the principal advantage, and the principal disadvantage of the design. The pump runs cool, making it nearly ideal when solvent recovery is re-
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quired and the process involves, for example, alcohols from the condenser train of a distillation column. The condensate is not corrosive, and solids fouling of the heat exchangers is not a concern. When the condensate is corrosive, corrosion will compromise performance. When solids are present, even soft polymers, the heat exchangers can foul. The three-stage Roots pump (Figure 1b), the latest version of the pump, has a water jacket surrounding the working volume. Interstage heat exchangers have been eliminated, resulting in a compact design. The provisions for intercooling in this pump are very sophisticated. The way the gas recirculates from the discharge of one stage to the working volume of the previous stage minimizes the temperature difference between the rotors and the casing. This addresses the major issue in protecting the pump from overheating — uneven thermal expansion that causes the rotors to come in contact with the casing.
Claw compressors The first Roots/claw dry vacuum pump (Figure 1c) was marketed in Japan in 1985. The first stage of the pump is the familiar Roots configuration. The second, third and fourth stages are intermeshing claws. Machines like the one shown in Figure 1c are remarkably successful in competing with dry pumps based strictly on the Roots principle. The Roots/claw pump is fundamentally more rugged. In the early 1990s, dry pumps based on intermeshing claws premiered in the U.S. The volumetric efficiency of the compressor is limited, as in all dry pumps, by backstreaming through the clearances between the rotors. The critical clearances are between circular profiles that can be machined to small tolerances. Since there is no relative movement between the profiles of the rotors, the gaps between them can be kept small; 0.005 in. is typical. The self-valving action of the claws means that continuous reworking of the gas in the swept volume, a problem with the Roots, is not a problem here. The valving action of the rotors limits the backflow of hot gas into the next compression cycle. Intercooling, used to cool Roots machines, is not required. Gas injection, used for screw compressors, is also not required. The rationale for combining Roots rotors with intermeshing claws and the order in which they are combined can be found by plotting the maximum compression ratio vs. outlet pressure (4), as presented in Figure 2. This figure shows that intermeshing claws are more efficient at higher pressures, and the the Roots, at lower pressures. CPI pumps are designed for operation across the range 0.1–760 torr. There are few applications in the range 0.01–0.1 torr. Figure 2 shows that the three-stage claw is more efficient across the range 0.2–760 torr. The Roots/claw machine is more efficient across the range 0.1–0.2 torr, but the difference is not significant. And the three-stage claw is fundamentally a simpler and more-rugged machine. Thus, in developing pumps for the CPI, the Roots/claw design was abandoned in favor of two- and three-stage claws.
Figure 3. (a.) Three-stage claw — Key: 1: Inlet; 2: Sealed high-vacuum bushings; 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 of Busch, Inc.).
Figure 3a illustrates two aspects of claw machines that are especially intriguing — the pump is vertical and, in the design shown, the second set of claws is reversed. Vertical mounting is advantageous in handling condensable vapors, or when the aspiration of a liquid slug or particulates from the process is possible. Liquid drains through from the suction to the discharge and out the bottom of the pump. Reversing the orientation of the rotors in the second stage so that the outlet of the first stage aligns with the inlet of the second allows particulates to fall straight through the pump and minimizes the area available for buildup of corrosive residues (5).
Screw compressors Screw compressors have been used as vacuum pumps since the mid-1950s, but these machines were not designed as vacuum pumps and were generally restricted to 100–760 torr. A screw compressor designed as a dry vacuum pump was introduced in the early 1990s (Figure 3b — the dashed circle in the figure indicates the pump’s inlet). Process vapors entering the pump are trapped between two constantpitch Archimedean screws and are conveyed from the suction side to the discharge. Operation is isochoric. Compression occurs in the final half-turn of the screw. Busch, Kinney Vacuum, Nash Engineering, Stokes, Rietschle Pumps, and Sterling SIHI currently market such vacuum pumps in
the U.S. They are not simply conventional screw compressors adapted for vacuum service. Ultimate pressure for these machines is less than 0.1 torr, and some are capable of compression ratios in excess of 1,000,000:1 and operation across 0.001–760 torr. To operate effectively as a vacuum pump, a screw compressor must have tight clearances or run at high speeds, typically 6,000–18,000 rpm (6). In developing vacuum pumps for the CPI, manufacturers looked at the problems associated with high-speed operation, and elected, initially, to design for 3,600 rpm. To preserve volumetric efficiency at lower speeds, it was necessary to design for tight clearances. Clearances between the rotors and between the rotors and the casing are very tight, frequently less than 0.004 in. Some tolerance is required to allow the rotors to “bed in.”
Figure 4. Vertical screw compressor designed as a vacuum pump (Courtesy of Sterling SIHI).
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Dry Vacuum Pumps
200 Jet 175 m f c a , d e e p S g n i p m u P
150 Dry Pump Liquid-Ring Pump
125 100 75 50 25 0 0.1
1
Performance curves for: • 3-stage Steam Jet • 2-stage Liquid Ring-Pump • Dry Vacuum Pump
10
100
1,000
Suction Pressure, torr
Figure 5. Performance curves for a three-stage steam jet, two-stage liquid-ring pump, and a dry vacuum pump.
Some manufacturers address this by coating the rotors and the casings with polytetrafluoroethylene (PTFE). This sacrificial coating is abraded as the rotors bed in and the running clearances for the pump are established (2). The pump shown in Figure 4 operates at 8,000 rpm. The rotors are stainless steel and a PTFE coating is not used. Vertical screws transport process vapors from the top inlet to the bottom discharge. Problems posed by high rotational speeds — contamination of the working volume by bearing lubricant, vacuum-tight sealing of shafts, and high noise levels associated with timing gears — have been addressed in the design. Cartridge-mounted bearings, used to support the shafts, are mounted inside the rotors. The resulting cantilever design addresses contamination of the working volume by bearing lubricant and vacuum-tight sealing of the shafts. The bearings are on the discharge side of the pump and mechanical shaft seals have been eliminated. Gear lubrication has also been eliminated. Two electronically synchronized motors drive the rotors. The timing gears are noncontacting. Isochoric operation of the rotors means that the temperature at the discharge end of vacuum screw compressors can exceed 300°C. High temperatures prevent the condensation of process vapors; this protects the pump from corrosion. High temperatures, however, reduce the life of seals 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-
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vent recovery is required, injected gas can drive up capital and operating costs for the recovery system. Research has been done by Japan (7) and U.S. manufacturers on reducing operating temperatures, and making pumps more energy-efficient and compact. The results call for changing the pitch, or profile, of the screw axially along its length. A change in the profile, for example, midway through the pump, shifts part of the work away from the discharge, creating a more energy-efficient machine that generates less heat. Changing the profile of the screw reportedly drops temperatures to 130–200°C (8). In addition, the reduced lead angle at the inlet gives the pump greater volumetric capacity, so it can achieve the same throughput with about one-third less horsepower and a smaller footprint (7, 8). Virtually every manufacturer of screw-compressor dry pumps has an aggressive program to redesign its pump line based on these findings.
Why specify a dry pump? The potential for eliminating process contamination is the main driving force for specifying dry vacuum pumps for fine-chemicals and current good manufacturing practices (cGMP) plants. It is anticipated that, in the future, environmental constraints and the incentive for solvent and product recovery will increasingly dictate the specification of these pumps for mainstream CPI applications. Process integration is also a factor, because dry pumps are so versatile. Process contamination Oil-sealed pumps in pharmaceutical, pharmaceutical intermediates and some food processing plants are coming under increased scrutiny from regulatory agencies. The potential for contamination of the process by pump oil was always an issue, but the real issue now is cleanliness; the potential for contamination of the process and for contamination associated with the use, handling, and disposal of pump oil. Dry pumps provide an ideal solution to the problem. Eliminating the oil eliminates the problem. Process contamination is also an issue when steam jets or water-sealed liquid-ring pumps are used in cGMP plants. When a single batch is worth $500,000, the potential for contamination, for example, as a result of backstreaming of steam from an unstable steam jet, is unacceptable if installing a dry pump can eliminate the risk. If the steam sys-
device, such as a stripping column, may be used to handle wastewater discharged from the entire plant. If a large control device is installed, condensate from steam jets and spent sealant from water-sealed liquidring pumps can be discharged to the control device. In this case, emissions reductions alone will seldom justify installation of dry pumps.
$70k Three-Stage Roots $60k Screw Compressor $50k t s o C e s a h c r u P
$40k Skid-Mounted Liquid-Ring
$30k
Three-Stage Claw
$20k
$10k
50
100
150
200
250
300
Free Air Displacement, cfm
Figure 6. Purchase costs for dry pumps vs. liquid-ring pumps.
tem at a cGMP plant is not a sanitary system approved for direct or incidental contact in cGMP applications, backstreaming of steam to the process will contaminate the product. Also, there are regulatory and legal issues. A process upset, triggered by backstreaming of steam, interrupts the processing cycle. The upset, especially if it is an aberration not provided for in a Drug Master Filing, may require that the plant scrap the batch. The manufacturer must establish that the upset had no impact on product quality and did not result in contamination. Dry pumps eliminate this potential for contamination.
Environmental constraints As environmental regulations place increasing restrictions on the discharge of contaminated working fluids, dry pumps are being considered for point source elimination of pollution from steam jets, liquid-ring pumps, and oil-sealed pumps. Steam jets and water-sealed liquid-ring pumps contribute to water pollution. Oil-sealed pumps contribute to air pollution, and the contaminated oil presents yet another waste disposal problem. Dry pumps do not contribute to the problem; dry pumps are part of the solution. The Clean Air Act Amendments of 1990 resulted in severe restrictions on discharging wastewater containing air pollutants 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 pump development. But, in many plants, vacuum system wastewater is a small part of the total wastewater problem. A control
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Solvent/product recovery In most applications involving dry pumps, solvent/product recovery is easy. The dry pump discharges to an aftercondenser. Contamination is not a concern, and the condensate from the aftercondenser can be recycled directly to the process. The success of dry pumps in solvent/product recovery follows the precedent established by solventsealed liquid-ring pumps. Liquid-ring pumps are a natural choice for vacuum distillation, vacuum drying and evaporator service, because the pump handles noncondensables saturated with process vapors. The “condensing effect,” inherent in the operation of the pump, means that vapors discharged to the liquid-ring pump may condense in it. If the pump is dedicated to a single process that uses a solvent with a sufficiently high boiling point, for example, xylene, the solvent can be used as the sealing liquid. Process vapors condense in the pump, and the condensate is recycled to the process (9). Solvent-sealed liquid-ring pumps have been used extensively in the CPI to replace water-sealed liquid-ring pumps, oil-sealed pumps, and single- and two-stage jets. There are, of course, limitations to this approach. The vacuum that can be achieved by a liquid-ring pump is limited by the vapor pressure of the sealing liquid. The lower limit for process applications is approximately 25 torr. (Operation at lower pressures, in the range 5–10 torr, is possible, but careful engineering is needed to ensure that reliability is not compromised.) Liquid inventory is also a problem. Changing to a new solvent contaminates the sealing liquid, and the potential for contamination may dictate changing out the sealing liquid at the end of each production campaign. Dry pumps eliminate both of these problems. Dry pumps offer similar performance and economics across the same operating range as solvent-sealed liquidring pumps, but with the additional benefit of lower ultimate pressures. Because of this, dry pumps are viable alternatives to three-, four-, and five-stage jets. There are no liquid inventory problems; replacing a solvent-sealed pump with a dry pump eliminates the liquid inventory. Dry pumps are frequently a better choice for general-purpose use, because changes in the process, product or solvent that might affect the performance of solvent-sealed pumps will usually have little, if any, effect on the performance of dry pumps.
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Dry Vacuum Pumps
Note: 1.0 torr = 133.3 Pa 0.60
1-Stage Pump 2-Stage Screw Compressor
0.50
provide vacuum across the entire range 1–760 torr. The same pump that is used to maintain 5 torr on a reactor can be used to maintain 50 torr on a dryer and 500 torr on a rotary vacuum filter.
Suction pressure and capacity Three-Stage Roots 0.40 The most important parameters affecting vacuum pump selection are the suction pressure 0.30 and capacity required for the Two-Stage Liquid-Ring process. Suction pressures and Three-Stage Claw 0.20 capacities for steam jets, liquidSingle-Stage ring pumps, dry pumps, and inLiquid-Ring tegrated systems are described 0.10 in the table. The information Multistage Steam Jets presented here can be used to eliminate pumps or pumping 2 4 6 8 10 20 40 60 80 100 200 400 600 systems that cannot meet process requirements. The ultimate Suction Pressure, torr pressures shown in the table are synonymous with the “blind” Figure 7. Adiabatic thermal efficiency of various pumps. suction pressures for the pumps or pumping systems; that is, the suction pressures at zero load. Process integration The lower limit for process applications is an approxProcess integration is an iterative approach to reduce imate limit established by technical considerations the complexity of the process flow diagram, and ultimately and economics. to reduce capital and operating costs for the plant. The Dry pumps span the range from 0.05–760 torr with caprincipal advantage of dry pumps in this context is versatilpacities in the range of 50–1,400 acfm. Steam jets can be ity. Dry pumps are often a cost-effective alternative to designed for throughputs in excess of 1 million acfm in a steam jets and liquid-ring pumps in batch operations besingle unit. Liquid-ring pumps are available with capacicause they are so versatile. The same pump that is used to ties up to 22,000 acfm. Dry pumps are limited to 1,400 pull vacuum on the reactor can be used to pull vacuum on acfm, but they have relatively flat operating curves. This downstream operations. gives them the advantages, compared to steam jets, of Dry pumps are anticipated to increasingly dominate faster pumpdown and better response to overloading. Dry process applications in fine-chemicals, pharmaceuticalpump makers are, however, moving away from building intermediates and pharmaceutical plants. These are genthe larger pumps, those with capacities in excess of 500 erally multipurpose facilities built around reactor bays. A acfm. There is simply no demand for them. They are exreactor bay consists of several stirred-tank reactors that pensive, and it makes more sense to couple vacuum can be configured to make different products. If the boosters to smaller pumps to boost the capacity of the product is heat-sensitive, the reactor and downstream smaller pumps than to build the larger machines. distillation column or evaporator will probably run under Purchase costs vacuum. If the product is a solid, downstream crystallization, filtration and drying operations will usually be Figure 6 can be used to estimate purchase costs for dry under vacuum. pumps and compare them with those of liquid-ring pumps. Figure 5 presents performance curves that are based on Costs for Roots pumps and claw compressors are based on actual equipment. Steam jets are used traditionally in finethree-stage machines. Purchase costs for screw compressors chemicals, pharmaceutical-intermediates and pharmaceutiare based both on machines that are mechanically and eleccal plants in the range 1–50 torr; liquid-ring pumps, for tronically simple and on smart pumps that are complex. Be25–500 torr. The performance curves in Figure 5 show why cause of differences in the level of complexity from one dry pumps are often a cost-effective alternative to steam jets pump to another, screw compressors are both the least and, and liquid-ring pumps in batch operations. A dry pump can paradoxically, the most expensive of the dry pumps. The y c n e i c i f f E l a m r e h T
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pressure, P2, divided by the actual brake horsepower (bhp) required (10): E = Theoretical adiabatic hp Suction Load
Actual hp
PLC N2
Water Outlet Temp.
This concept provides a convenient means to evaluate the energy costs of vacuum pumps at a specific vacuum level. The adiabatic horsepower required to compress w lb/h of dry, 70°F air from P1 to P2 may be calculated from:
TI
Gas Ballast
TI
Casing Temp.
(1)
Exhaust Temp. and Pressure
TI
Adiabatic hp = (w /20) [(P2 / P1)(0.286) – 1] (2) PI
The bhp actually required can be Water In found by performance testing and by converting motive steam usage for steam jets into an equivalent electriOil Level LI II Amp Meter or Pressure cal power requirement. Efficiencies calculated by this techSC M nique were used to generate the curves shown in Figure 7. MotiveVariable-Frequency Drive steam requirements for steam jets are based on 100-psig steam and were converted to an equivalent electrical requirement (1,000 Btu = 0.293 kWh). Calculated efficiencies for Figure 8. Instrumentation required for smart pump installations. multistage jets were based on condensing jets with surface condensers, upper limit for liquid-ring pumps is based on stainless-steel and 70°F cooling water. Mechanical pumps are assumed skid-mounted models with total sealant recirculation systo be electrically driven. The efficiencies for single-stage tems. These systems provide solvent/product recovery beneliquid-ring pumps were based on 70°F sealing water. The fits similar to a dry pump. The companies that market dry curve for two-stage liquid-ring pumps assumes a lowpumps realize that they are competing with such systems. vapor-pressure sealing liquid ( i.e., vapor pressure of < 1 The lower limit for screw compressor costs is, therefore, torr at 70°F). about equal to the upper limit for top-of-the-line skidThe curves represent approximations because there are mounted liquid-ring models. significant variations in the efficiencies of pumps from difDifferences in capital costs are seldom the determining ferent manufacturers. The motive steam requirement for a factor in an evaluation of alternatives. Operating costs, the steam jet is a function of steam pressure. The bhp for a mealready-mentioned environmental factors, and chanical pump depends on rpm, and larger pumps are gensolvent/product recovery will almost always be more imerally more efficient than smaller ones of the same type. portant. Also, purchase cost is only one component of capNevertheless, the efficiencies indicated in Figure 7 are genital cost. Purchase cost for a steam jet may be lower than erally representative of the efficiencies with which the that for a dry pump, but total installed costs may be higher pumps evaluated will handle noncondensable loads. The when factoring in the cost of adding boiler capacity, runfigure indicates that dry pumps are more efficient than ning steam lines, and installing steam separators, stream steam jets across practically the entire range 1–760 torr. Dry traps, and piping for condensers. pumps are more efficient than liquid-ring pumps across the range 1–50 torr, and this difference is significant for the Energy consumption range 1–20 torr. The adiabatic thermal efficiency, E , of a vacuum pump Higher thermal efficiency is not synonymous with lower may be defined as the adiabatic horsepower required to comenergy costs. The electrical equivalent of a pound of steam press a process gas from an initial pressure, P1, to a discharge will usually cost 3–6 times more than the steam due to
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Dry Vacuum Pumps
over-instrumentation and redundancy are the rule. The casing temperature transmitter, for example, interfaces Type Ultimate Lower Limit Single-Unit with the DCS and is hard-wired to a or Base Pressure for Process Capacity field-mounted thermal snap switch. Applications Range, ft3 /min When temperature control is crucial and there is an upper control limit Steam-jet ejectors 10–1,000,000 One-stage 50 torr 75 torr (UCL), two thermal snap switches may Two-stage 4 torr 10 torr be used. One is tied to the DCS and Three-stage 0.8 torr 1.5 torr acts to trip an alarm. If the uplink to the Four-stage 0.1 torr 0.25 torr DCS is lost, the second switch provides Five-stage 10 micron* 50 micron Six-stage 1 micron 3 micron redundancy. If the casing temperature exceeds the UCL, the second thermal Liquid-ring pumps 3–18,000 snap switch shuts down the pump. 60˚F water-sealed The configuration of the DCS is One-stage 50 torr 50 torr Two-stage 20 torr 25 torr crucial to the strategy for protecting Oil-sealed 1 torr 10 torr the pump. Both startup and shutdown Air ejector first stage 1 torr 10 torr are especially crucial: Startup — The DCS is configured Dry vacuum pumps Three-stage rotary-lobe 0.5 torr 1.5 torr 60 –240 to ensure that the pump has time to Three-stage claw 0.1 torr 0.3 torr 60 –270 come up to its operating temperature Screw compressor 50 micron † 0.1 torr 50–1,400 before it comes online. The pump is isolated by a block valve and allowed Integrated pumping systems Booster — liquid-ring pump 1 torr 5 torr 100 –15,000 to work against an inert gas or nitrogen Booster — rotary-lobe dry pump 25 micron 0.25 torr 100 –1,500 bleed until the heat of compression Booster — claw compressor 10 micron 0.1 torr 100 –2,500 brings it to operating temperature. This Booster — screw compressor < 0.1 micron 1 micron 100 –5,000 protects the pump from corrosion *1.0 micron = 0.001 torr caused by condensation of process va† The base pressure depends on the pump model. Fifty microns is an "averaged" value. The range, across pors and ensures that the vapors do not several vendor pump lines, is almost four orders of magnitude — 0.75 micron to 0.5 torr. “freeze out” as solids. Shutdown — The DCS activates a cleaning cycle prior to shutdown. This steam-cycle condensing losses and the more expensive hardensures that shutting down does not trap process vapors in ware required to generate electricity. Projects aimed at rethe pump. Condensation of vapors trapped in the pump placing steam jets with dry pumps to reduce energy costs could leave the pump full of corrosive liquid that could dammust, therefore, be reviewed carefully. Energy costs for dry age it during a prolonged shutdown. (The DCS is, of course, pumps, especially at operating pressures in the range 1–20 configured to allow manual intervention to shut down the torr, may be higher. pump immediately in an emergency.) During the cleaning cycle, the pump is isolated from the Smart pumps process and an inert gas or nitrogen bleed purges it of all Running clearances for dry pumps are typically 0.010 residual gases prior to shutdown. Such purging also disin. or less. Dry pumps must be protected to minimize melodges solids. The purge gas scours the rotors and the caschanical damage. Equipment manufacturers use the smart ing, and blows out debris. In demanding applications, the pump or intelligent pumping system concept to address DCS interrupts the production cycle and isolates the pump this issue. Microprocessors monitor and control the pump, when the motor amperage exceeds a UCL. The pump is aland support the interlocks and self-diagnostics required to lowed it to run, sometimes for extended periods, at near-atprotect the pump. The usual configuration is a vendor-supmospheric pressures to clear the debris. When the amps plied standalone programmable logic controller (PLC) that drop, the DCS brings the pump back online. interfaces with a distributed control system (DCS) or a Safety PLC housed within the operating system for a DCS. The instrumentation required for smart pump installaSafety related to the operation of dry pumps has retions includes, but is not limited to, the requirements deceived a lot of attention in the literature, because dry scribed in Figure 8. An array of sophisticated electronic pumps are new. The issues are not unique to dry pumps. sensors is required to monitor the pump and support the Safety must be addressed, for example, when flammable higher-level control functions. Variable-frequency drives, solvents such as acetone or gasoline are used as the sealing Capacity and operating range for steam jets, liquid-ring pumps, dry vacuum pumps, and integrated systems.
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liquids for liquid-ring pumps. Indeed, it may be argued that the there are more safety issues associated with solventsealed liquid-ring pumps than with dry pumps. Still, the safety issues associated with dry pumps must be understood to ensure safe operation (11). Safety is an issue in pumping flammable vapors and gases because of the potential for an explosion initiated, for example, by a spark caused by contact between the rotors and the casing. Dry pump manufacturers address safety in part by designing pumps that will contain an internal explosion. Flame propagation is still a consideration. Inerting with nitrogen or other inert gas prior to startup takes care of propagation back to the process during startup. When the process runs at < 75 torr, an explosion is not a consideration since the vapor/gas mixture in the void space in the pump and in the process is inert. Installing a flame arrestor in the vent line addresses the concern that an explosion might propagate from the pump discharge to the atmosphere. Autoignition is also a consideration (12) . Dry pumps run hot, with discharge temperatures for screw compressors sometimes reaching 350–400°C. To cope with this, the latest generation of dry pumps runs at lower temperatures and has precise temperature control. This is accomplished by designing the machines to be more energy-efficient, by
redesigning the rotors to avoid hot spots, and by applying state-of-the-art technology to the cooling system. Dry pumps are offered that are rated for T4 applications, that is, those in which internal temperatures must not exceed 135°C. It is, nevertheless, good practice to use caution in specifying dry pumps for any application with vapors with an autoignition temperature of less than 200°C. CEP
Short glossary Backing pump: The pump that produces the necessary discharge pressure for a vacuum pump incapable of discharging directly to atmospheric pressure. Booster: A pump that operates as part of a multistage system to boost the capacity of a pump that discharges directly to atmospheric pressure. Compression ratio: Discharge pressure divided by suction pressure. Discharge pressure: The absolute static pressure measured at the discharge of the pump, torr. Isochoric: Constant volume.
Literature Cited 1. 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 Harris Publisher, Crawley, West Sussex, U.K., website: www.modernvacuumpractice.com/, pp. 289–310. 3. Troup, A. P., and N. T. M. Dennis , “Six Years of Dry Pumping: A Review 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 System for Plasma Processes,” BOC Edwards Publication No. 12-A401-31895, BOC Edwards, Crawley, West Sussex, U.K. (1987). 5. Wycliffe, H., U.S. Patent No. 4,504,201 (1985) and U.K. Patent GB 2.088.957B. 6. Tadashi, S., and M. Nakamura , “Spiral Grooved Vacuum Pump Working 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 in Dry-Compressing Vacuum Pumps,” Chem.–Anlagen Verfahren, 29 (5), pp. 32–33 (1996).
Suction pressure: The absolute static pressure measured at the suction of the pump, torr. Torr: 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.
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.
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