Mechanical Pumps for Vacuum Processing

Transcription

1 Mechanical Pumps for Vacuum Processing Vibert, Phil. Chemical Engineering ; New York Vol. 111, Iss. 11, (Oct 2004): ProQuest 文档链接 Like water, power, and compressed air, vacuum is a standard utility in many chemical process plants. Commonly used, for instance, to remove gases or vapors that otherwise would interfere with a reaction, vacuum enhances reaction efficiency and yield and the recovery of essential compounds. Under vacuum, the boiling point of liquids is reduced, which is useful for the processing of temperature-sensitive materials and the separation of liquids. Heat transfer through liquids is more efficient without the presence of air bubbles, while solid end-products produced under vacuum from a liquid phase are more homogeneous, and are virtually free of voids caused by unwanted gas bubbles. Of the primary positive-displacement pumps, oil-sealed rotary piston and vane pumps are most similar in range in terms of pumping capacity and ultimate pressures (vacuum levels). Liquid ring pumps employ one or two multiblade impellers concentrically mounted to the drive shaft. Headnote Liquid ring and dry pumps are best-suited for applications in the chemical process industries (ProQuest:... denotes formulae omitted.) Like water, power, and compressed air, vacuum is a standard utility in many chemical process plants. Commonly used, for instance, to remove gases or vapors that otherwise would interfere with a reaction, vacuum enhances reaction efficiency and yield and the recovery of essential compounds. Under vacuum, the boiling point of liquids is reduced, which is useful for the processing of temperature-sensitive materials and the separation of liquids. Heat transfer through liquids is more efficient without the presence of air bubbles, while solid end-products produced under vacuum from a liquid phase are more homogeneous, and are virtually free of voids caused by unwanted gas bubbles. Also, aggressive compounds that must be contained can be better and more safely handled and transferred using vacuum. In particular, vacuum is used to: * Remove air and its constituents, such as oxygen and water vapor, which, if they are combined with a process constituent, could alter a chemical reaction * Remove excess reactants or unwanted byproducts that can compromise efficiency and yield * Reduce the boiling point for distillation of mixtures * Dry solute material by removing the solvent * Create a pressure differential for initiating transport of material from one section to another or through filtration media Vacuum is generated by vacuum pumps. The spectrum of vacuum pumps is large, and it can include multiple stages of pumps in combination to provide systems that either operate at lower pressures or accommodate larger pumping capacities. The main building block for any vacuum system is a primary-stage or atmospheric-stage vacuum pump, which exhausts directly to the atmosphere. Primary pumping devices are categorized based on the method by which they pump gas: SEARCH.PROQUEST.COM PDF Page 1 of 10

2 * Mechanically trap gas and transport it from suction to discharge. Positivedisplacement pumps are the best example of this method of operation * Transfer momentum through a motive fluid. Steam or vapor ejectors and air ejectors employ this method of operation * Capture gas on extended surfaces using porous media at cryogenic temperatures. Sorption pumps work on this principle The first two categories are most widely used the chemical process industries (CPI). Steam ejectors, long considered the workhorses of vacuum processing, are easy to use and operate (CE, March 1999, pp ). However, concerns about energy consumption and environmental pollution associated with them have slowed the demand for these types of pumps. Preferable for a growing number of applications are hybrid systems that incorporate a steam-ejector stage backed by a mechanical pump stage - for instance, a steam ejector stage/interstage condenser/liquid-ring pump stage, all in series - or systems consisting entirely of mechanical pumps. PUMP MECHANICS AND OPERATION A pump used in CPI applications should have the capability to: * Process various solvent vapors without harm * Avoid pollution of the process and the environment * Keep waste generation to a minimum * Resist corrosion * Handle flammable gases or vapors * Ingest some liquid without harm Oil-sealed pumps Of the primary positive-displacement pumps, oil-sealed rotary piston and vane pumps are most similar in range in terms of pumping capacity and ultimate pressures (vacuum levels). Both rely on oil for three main functions: * Sealing the internal clearances between rotary components and housing to reduce gas slippage * Transferring the heat of gas compression * Lubricating the rotary internals This dependency upon the oil for internal lubrication is a limiting factor in the use of these type pumps in the CPI. The integrity of the oil must be maintained to avoid internal damage that could cause contaminant buildup. Particulates in the oil must be filtered out ahead of the pump, and water or solvent vapors must be either knocked out ahead of the pump by precondensers, or prevented from condensing within the pump by gas ballasting (air stripping) or oil distillation. Rotary piston. The rotary piston pump is extremely robust and stands up well to adverse process conditions. Typically constructed of cast iron, this pump has four basic rotary components: a shaft, eccentric cam, piston, and slide pin. The cam is eccentrically mounted to the shaft and the piston is concentric to the cam. This arrangement allows the piston to ride on the cam and rotate around the periphery of the pump housing (cylinder), forming a void between the piston and housing that alternates from maximum to zero at top dead center. The positioning of the eccentric cam on the shaft results in an imbalance. To compensate, two or more such stages of rotary components are placed on the shaft, with each 180 deg out of phase with the stage next to it. These stages are normally arranged in parallel for single-stage pumps or in series for compound pumps (Figure 1). Single-stage rotary piston pumps can provide pressures down to mm Hg abs and capacities to 850 cfm; compound pumps, mm Hg abs and 200 cfm. The geometric positioning of the piston-cam-shaft assembly prevents the piston from touching the cylinder wall, allowing a constant clearance to be maintained at all times. Oil is used to seal this gap between the piston and cylinder, provide lubrication and transfer heat throughout the rest of the pump. Rotary vane. The advantage of the rotary vane pump is its inherently well-balanced design. Its disadvantage is the sensitivity of its vanes to sticking or breaking from deterioration of the lubricant or ingestion of process liquid. SEARCH.PROQUEST.COM PDF Page 2 of 10

3 This pump employs a rotor concentric with the shaft, with slots for acceptance of two or more vanes, providing the inherently balanced design. The rotor-shaft assemblage is mounted eccentrically in the stator cylinder to provide the necessary crescent-shaped volume for expansion and compression, with the critical dimension being the rotorto-stator clearance between the suction and discharge porting at top dead center. The clearance must be kept as small as possible to reduce gas slippage where the pressure differential is greatest. The use of multiple vanes in the rotor allows the pumping volume to be swept the same number of times in one shaft rotation, providing a compact design. The vanes can be spring-loaded, or more commonly, can rely on centrifugal force to make contact with the cylinder and seal off the gas pocket. Contact pressure between the vane and housing is high, resulting in significant frictional effects that increase internal localized temperatures, even in the presence of oil, which provides lubrication, sealing and cooling. Various designs of vane pumps are available, typically with capacities to 1,000 cfm. Some are capable only of ultimate pressures from 15 to 0.5 mm Hg absolute for rough industrial applications, while others are capable of ultimate pressures from 0.1 to mm Hg abs for use in applications with higher vacuum requirements. A vane pump that uses a oncethrough oiling system is an alternative to those that recirculate oil. In the oncethrough approach, oil flow is metered out in amounts just sufficient to seal and lubricate the vanes to the housing. Instead of being recycled, the oil is collected for disposal. The advantage of this design is that any contamination of the oil by the process vapor is passed out of the pump and not allowed to build up and cause additional problems. This design also avoids any increases in the operating pressure due to the vapor pressure of residual process vapors. The drawback is the need for waste oil disposal, which is an environmental and cost issue. Liquid ring pumps Liquid ring pumps employ one or two multiblade impellers concentrically mounted to the drive shaft. The impeller shaft assembly is eccentrically mounted in the pump housing, in such a way that at top dead center the clearance between impeller blade tip and housing is at a minimum, and at bottom dead center the clearance is at a maximum, resulting in a relatively large void (Figure 2). The sealant liquid, typically water (but see further discussion below), is used to seal between the impeller blade tips and housing. The sealant liquid is thrown by the impeller against the inside walls of the pump housing, where it forms a rotating ring of liquid. At top dead center, the ring of sealant liquid completely fills the voids between the blades of the impeller. Because of the eccentric position of the impeller with respect to the housing as it rotates around the ring of liquid, the sealant is peeled away from the spaces between the blades, creating voids where gas can enter and be trapped. At bottom dead center, voids between the impeller blades and ring liquid are at a maximum, while the blade tips remain immersed in the liquid ring for sealing. As the impeller continues to rotate back up from bottom dead center to top dead center, the sealant begins to refill the volume between the blades, creating an essentially isothermal compression of the gas trapped within. Inlet-outlet porting are positioned either in an endplate or a port cone positioned at the axial end of the impeller. In the endplate arrangement, two ports for entering and exiting of gas normally straddle an imaginary line connecting top dead center to bottom dead center, but the ports (normally triangular) can vary in shape and exact location depending upon the vendor. With this kind of arrangement, the sealant liquid acts as a liquid piston, alternately creating expansion and compression for the gas trapped in the spaces between the impeller blades. Because ring energy is derived from the impeller rotational speed, the minimum number of rotations per minute (rpm) that a given liquid ring pump can operate can be estimated. Since pressure = force/area,... (1) where A = unit area in in.2 G = gravitational acceleration of ft/s2 at sea level p = sealant liquid density in (lb/in.3) Ap = gas differential pressure across the pump for single-stage pumps and the pressure differential for two-stage SEARCH.PROQUEST.COM PDF Page 3 of 10

4 pumps in (lb/in.2) P = ring pressure due to the impeller in (lb/in.2 A) V = ha is the volume of sealant in (in.3) h = impeller blade height or sealant ring thickness in inches R = effective impeller radius in ft rpm = rotations per minute Then, for a unit surface area, A, where the ring is sealing against a maximum differential pressure, Ap, for the gas being pumped across each impeller stage, the minimum ring pressure, P must be greater than Ap then... (2) As shown, ring pressure for sealing is dependent on both the impeller rpm and radius, as well as the density of the sealant liquid. Liquid ring pumps are available as single-stage (one or two impellers in parallel) or two-stage (two impellers in series). Using 60 F sealant water, single-stage pumps are capable of achieving 100 mm Hg abs, while twostage or compound pumps can achieve 30 mm Hg abs. Pumping capacities up to and over 20,000 cfm are available. Liquid ring pumps are well designed for the CPI. They do not require internal lubrication of the impellers, which do not contact the housing. The sealant liquid, used for both sealing and cooling, can be any liquid that is compatible with the process and falls within the following range of physical properties: * Specific gravity 0.5< S.G< 1.5 * Specific heat 0.35< S.H. <1, relative to that of water * Viscosity 1 cp <v <32 cp * Vapor pressure Vp sealant at operating temperature <Vp water at 60 F Low-viscosity oils, glycols, and many process solvents, such as toluene, xylene, methanol, ethanol, propanol, butanol and ethylbenzene, can be used as sealants. These sealants can be recirculated in a full recovery system that includes a gas-liquid separator tank and a heat exchanger for cooling. Even higher-vapor-pressure liquids can be used if a low-temperature coolant is used in the heat exchanger to reduce the sealant temperature. This recovery system allows process materials to be collected in the pump and either returned to the process or collected for disposal, while minimizing contamination of other liquids or the environment. Liquid ring pumps offer many advantages, among them: * Simplicity of operation (such a pump is essentially a pinwheel on bearings) with minimal moving parts, and no lubricating liquid in the vacuum chamber to be contaminated * Large choice of sealant liquids * Accommodation of both condensable vapors and noncondensables, while operating as both a vacuum pump and condenser * Ability to handle small liquid streams along with the gas flows from the process or precondensers * Wide choice of materials of construction, with all-ferrous, allbronze, and all-stainless steel being the most common The major disadvantage of the liquid ring pump is its power consumption, compared with that for other types of mechanical pumps. While frictional power due to seals, bearings, and drag on rotational elements represents 30-40% of total peak power consumption in rotary vane and piston pumps, it accounts for 50-75% of total power consumed in liquid ring pumps. The power consumed by liquid ring pumps in pumping gas can be determined from the isothermal compression of gas across each stage: GHP = (144/33, QmP-iDlniPjPx) (3) where GHP = work done on gas (hp) Pi = inlet pressure (psia) D = displacement (cfm) SEARCH.PROQUEST.COM PDF Page 4 of 10

5 P2 = discharge pressure (psia) The low-pressure performance of single-stage liquid ring pumps is normally limited by gas slippage from discharge back to suction, while in two-stage pumps, a combination of slippage, sealant vapor pressure and gas solubility of the sealant limits the ultimate pressure. When a low-viscosity oil is used as sealant in a two-stage liquid ring pump, ultimate pressures of 2-5 mm Hg abs are routinely achieved. Here the limitation is not the vapor pressure of the sealant, which is likely to be less than 10~4 mm Hg abs at 100 F, but the air solubility in the oil and slippage between stages. All liquid ring pumps must cope with cavitation when running at low inlet pressures. Cavitation is the rapid formation and collapse of vapor bubbles within the sealant liquid, which can remove minute amounts of metal from surfaces. If cavitation is allowed to continue over long periods of time, serious damage can be done to the liquid ring pump. Tiny voids within the sealing liquid can be created by the pump's impeller. When the ring is exposed to the suction port at low pressures, some of the sealant liquid can vaporize to fill the void with a small vapor bubble, which travels around from suction to discharge, causing vapor bubbles to collapse. When the bubbles collapse on a metal surface, the shock force can tear small amounts of metal away. The amount of cavitation can be affected by the sealant liquid, sealant temperature, impeller rpm, blade angle, and inlet pressure. For a given pump and sealant liquid, cavitation can normally be suppressed by bleeding air into the pump inlet to raise its total pressure above the vapor pressure of the sealant at operating temperatures. Dry pumps Dry vacuum pumps do not use any liquid in the pumping chamber. In the 1980s, semiconductor fabricators realized the potential of dry pumps as an alternative to the oil-sealed pumps that were used to provide pressures of mm Hg abs for chemical vapor deposition and etching of wafers. In semiconductor manufacture, oilsealed pumps require lubrication with inert, fluorine-based fluids for protection from the corrosive gases and harsh conditions of the fabrication operation. In addition to the expense of the lubricants are the costs of associated materials and maintenance. With a dry vacuum pump, not only are lubricants eliminated; buildup of process gases within the pump and waste disposal are also reduced. But eliminating a liquid within the pumping chamber also eliminates a method of sealing between the pump clearances, a heat-transfer material for temperature control, as well as a flush medium for cleaning the pump internals of process material. So, the challenges of providing dry-runing pumps were large. Early dry pumps consisted of several pumping stages in series, with either rotary lobes or hook-and-claw internals that did not make any contact with the housing and used timing gears to synchronize the two parallel rotor shafts. While this configuration eliminated the need for a lubricant within the pumping chamber, the lack of a seal medium meant that the internal clearances had to be kept tight to reduce gas slippage. The tight clearances made some of the dry pumps sensitive to buildup of process particulate. The evolution of these designs saw the introduction of various inert gas purges to flush process material through the pump or act as a diluent for flammable or corrosive gases, or help to control internal temperatures. The eventual success of dry vacuum pumps in the semiconductor industry has inspired pumpmakers to introduce these pumps into other segments of the CPI, where the benefits of a dry pumping chamber can lower operating costs and justify the cost of the generally higher-priced pump. For CPI use, considerations for handling liquid slugs or higher vapor loads from the process need to be weighed. Various types of dry pumps are currently available, including scroll, diaphragm, rotary vane, rotary lobe, hookandclaw, and rotary I screw. However, the rotary lobe, hook-andclaw, and rotary screw pumps are the ones that dominate the CPI sector, particularly in larger-size pump applications. Scroll pump. The scroll pump uses a rotating plate shaped into a spiral (involute curve), which moves within a second stationary plate, shaped as a similar spiral. This rotating motion of one spiral within another creates crescent-shaped trapped volumes, within which the gas moves from the outside of the spiral to the center, where the gas is exhausted through a valve. Multiple stages can be used to provide lower pressures, down to 0.01 mm Hg abs, with pumping capacities limited to less than 50 cfm. Because its tortuous spiral gas path can act as a trap for SEARCH.PROQUEST.COM PDF Page 5 of 10

6 particulates within the pump, this type of pump is limited to clean gas applications. Diaphragm pump. The diaphragm pump uses a rotating eccentric pistonplunger to move an elastomeric diaphragm back and forth within a small cavity, resulting in a rapid reduction and expansion of volume to provide pumping action for gases. Chemical-resistant diaphragms are available in polytetrafluoroethylene (PTFE), but the relatively small size of this type of pump precludes its use in production-scale operations and relegates it to laboratory applications. Ultimate pressures vary, with the lowest being about 1 torr. Dry rotary vane. While the dry rotary vane pump is available with self-lubricating carbon vanes, the increased gas slippage compared to that of the oil-lubricated vane pump limits the ultimate pressure of the dry vane to about 75 mm Hg abs. These pumps offer capacities up to approximately 400 cfm. As with most dry pumps, which lack a liquid heat-transfer medium in the pumping chamber, it operates at elevated internal temperatures. Air cooling is used. Due to this pump's sensitivity to particulates, inlet filtration is normally recommended. Rotary lobe. In use for more than 50 years, the rotary lobe pump is typically used as a mechanical booster (Figure 2). Traditionally it has been used in series with an oil-sealed piston or vane pump to amplify or boost pumping capacity at low pressures, or to extend low-pressure capability. Today, it continues to be used in this capacity, as well as in combination with other types of dry pumps that function as the primary or atmospheric-stage pump. The rotary lobe pump consists of two symmetrical two-lobe (figure-eight) rotors, each mounted on a separate shaft in parallel, which rotate in opposite direction to each other at high rotational speeds without making any contact or using any sealing liquid. This pump uses timing gears to synchronize the rotation of the lobes to provide constant clearance between the two. Internal clearances are kept to a minimum - as tight as in. - to reduce the back slippage of gas and still allow for thermal expansion of the rotors. No internal compression of gas occurs. The booster traps a pocket of gas and transports it from low pressure to high pressure. It is the discharge pressure conditions at the booster produced by its backing pump that causes the pressure ratio. Typically, the rotary two-lobe is not an effective pumping device for pressures greater than 100 torr, due to its increased power consumption. A pressure switch is often used to energize the blower only at lower pressures, or a bypass circuit with valve, either internal or external, is used to limit the pressure differential between suction and discharge, limiting the power requirements and exhaust gas temperature, while running at higher inlet pressures (>100 torr). The booster does not enhance the backing pump capac- ity until its bypass valve starts to close at inlet pressures below 100 torr. The greatest use of the rotary lobe booster is as a separate pumping stage, connected by piping to another stage that discharges to the atmosphere. This separate atmospheric stage can be provided by either another type of dry pump, thus forming an all-dry vacuum pumping system, or by the more-conventional wet pumps. Some dry pumps are manufactured utilizing the rotary lobe design within a single housing that can discharge to atmosphere. One such design starts out like a booster, with two counterrotating shafts in parallel. The design diverges, however, with a rotary lobe rotor mounted on each shaft as the high vacuum stage, in series with two or more different-design rotors all mounted in series on the same drive shaft. Each stage is separated by endplates with porting within the same housing. Another such design consists of two counter-rotating shafts, each driving three or more three-lobe rotors in series within the same housing. This design requires interstage cooling by recirculating a portion of the discharged gas from one stage through a heat exchanger before injecting it back into a point midway between suction and discharge. The three-lobe design rotor makes this possible without excessive slippage. The problem with this design is the tortuous gas path through a complicated cooling circuit, where process materials can accumulate or precipitate out. Some modifications to this design have included replacing the external interstage gas coolers with an internal cooling-jacket design. This configuration allows a portion of the discharged gas to be passed through a peripheral passage that is sandwiched between the pumping chamber and cooling jacket. This design allows the gas to be cooled before being injected back into the pumping chamber and reduces the external complications of SEARCH.PROQUEST.COM PDF Page 6 of 10

7 accessories, however, it still leaves a tortuous path for the gas where process material can accumulate. A complete rotary-lobe dry pump that can operates from atmosphere to less than 0.1 torr is possible if multistaging is used to reduce the differential pressure across each stage and its corresponding gas slip. Hook and claw. The hook-and-claw pump makes use of the Northey rotor design developed in the 1930s and first used on compressors. This geometrical shape allows for a greater compression ratio to be taken across the rotors at higher pressures. Two claw rotors rotate in opposite directions of rotation without touching, using timing gears to synchronize the rotation; two complete rotations are required to pass through the inlet, compression, and discharge cycle. The gas enters through an inlet port after it has been uncovered and fills the void space between the rotors and pump housing. On the next rotation, that same trapped sample of gas is compressed and discharged as the discharge port opens. Hook-and-claw rotors perform two functions: one is to trap, transport and compress gas through the pumps, and the other is to automatically open and close the suction and discharge ports like a valve by covering or exposing the porting to the gas stream at the appropriate times. A minimum of three stages in series is required to achieve ultimate pressures comparable to those of an oilsealed mechanical pump. Some designs use a mixture of hook-and-claw rotor stages in series with rotary lobe stages, while others use soley hookand-claw stages. Gas purges are used to avoid particulate buildup. Discharge gas temperature is controlled by controlling the flow of cooling water. Rotary screw. The rotary screw pump makes use of two long helical rotors in parallel, which rotate in opposite directions without touching (Figure 3). Helical timing gears are used to synchronize the rotation. Gas flow moves axially along the screw without any internal compression from suction to discharge. Pockets of gas are trapped within the convolutions of the rotors and the casing, and transported to the discharge. Compression occurs at the discharge port, where the trapped gas must be discharged against atmospheric pressure. Each convolution of the rotor acts similarly to a stage in series with the one behind it. A minimum of at least three convoluted gas pockets in the rotor are required to achieve acceptable vacuum levels. Mechanical face seals or lip seals are used to separate the pumping chamber from the bearings and gears. The first generation of rotary screw pumps use rotors with a constant pitch (number of convolutions per unit length). The second generation of screw rotors utilize a variable pitch design, which essentially consists of two individual short rotors, each with a different pitch, connected in series. The gas at the inlet is first transported by the lower pitch (fewer convolutions per unit length) portion of the rotors and then by the higher pitch portion, which results in internal compression of the trapped gas. The work for gas compression (as measured by the area under a PV diagram) using a variable pitch rotor is less than that for the same task accomplished with a constant pitch rotor. Because less energy is required, the motor size of the variable pitch rotor is smaller and the discharge gas temperature is lower. The third generation uses a continuous variable-pitch design rotor where the trapped gas is continuously compressed from inlet to discharge for greater efficiency with the lowest energy requirement and lowest discharge gas temperature. The rotary screw pump is unique in that it uses a singe stage (no interstage walls) rather than the multistage design of the other dry pumps, which are separated by endplates and seals. Because of this design, its gas flow path is simple, short and straight without any volumes in which material can accumulate. The symmetrical helical design lends itself to a wellbalanced rotor capable of high rotational speeds. Various protective coatings such as PTFE or PFA (a copolymer of tetrafluoroethylene and perfluoroalkoxy resin), or composites of PTFE and nickel, are available for wetted internals to provide corrosion resistance to aggressive process streams. Even with these coatings, it is advisable to avoid condensing the process corrosives within the pump through the use of inert gas purges and elevated gas temperature control. Pumps sizes range from 50 to 1500 cfm with ultimate pressures of 0.2 to <0.01 torr. Dry service in the CPI Of this selection of dry pumps, three types are recommended for generalpurpose use in the CPI: the rotary lobe, SEARCH.PROQUEST.COM PDF Page 7 of 10

8 hook-and-claw, and rotary screw pumps. These three dry pumps share the following features: * Rugged rotor design. Whether rotary lobe, hook-and-claw, or rotary screw all of the rotors are constructed of sturdy cast iron, or ductile iron construction without any flimsy rotating components. * Noncontact design. Timing gears are oil lubricated in a sealed-off end chamber to synchronize the rotors for proper phasing and noncontacting operation * High rotational speed. Operation at high speeds reduces the ratio of gas slip to displacement, increases net pumping capacity and reduces ultimate pressure. To accomplish this, rotors are well balanced * Multiple staging. Multiple staging provides inlet pressures below 1 mm Hg absolute while discharging to atmosphere (Figure 4). Being a separate stage, the rotary lobe booster is connected to another separate stage of dry pump that discharges to atmosphere. The rotary lobe and hookand-claw pumps use multiple stages within one housing, with each stage sealed off from the other with endplates, except for the porting that directs the gas along a tortuous path. The rotary screw uses the pockets formed by the convolutions in the helical rotors as separate stages to transport gas along a straight path before discharging LIQUID RING VS DRY PUMPS When all capabilities are considered, liquid ring and dry pumps offer the most advantages for the CPI. Both of these type pumps have bearings sealed off from the pumping chamber and do not require any internal lubrication because the rotors do not contact the housing; therefore, any solvent vapor that condenses within the pump will not compromise lubrica- tion. Both employ a coolant system that prevents the coolant from contacting the process and causing contamination, and both use mechanical shaft seals for containment. Dry pumps are free of any liquid within the pumping chamber, so that any process carryover is not contaminated and can be returned to the process. Also the lack of any sealing liquid means that the dry pump poses no danger to contaminating the process on system upsets. Although water is the most commonly used sealant in liquid ring pumps, in many applications, the process fluid can instead serve as the sealant liquid, provided the vapor pressure is compatible with the operating pressure. In other situations, a compatible sealant liquid can be found that meets the pump sealant requirements and will not be a problem for the process. The solvent liquid is recirculated in a full recovery system that includes a gasliquid separator tank at discharge and a water-cooled heat exchanger (normally shell-and-tube) in the recirculating line for cooling. For handling corrosive vapors, dry pump manufacturers recommend passing the vapors through the pump without condensing, by maintaining an elevated temperature at discharge through control of the cooling water flow, as well as auto start-stop and seal inert gas purges. Some also offer protective coatings. Makers of liquid ring pumps normally offer all-ferrous and all Type 316 stainless steel construction as standard options, with some also offering construction in Alloy 20 or Hastelloy. Dry pumps can handle many flammable vapors, if the maximum gas temperature is controlled below the autoignition temperature through coolant usage. An inert gas cooling stream is added during compression, or an inert gas stream is added as a diluent to avoid an explosive mixture while limiting the introduction of any oxygen into the system through air leakage. In some cases, detonation arresters may be used. Liquid ring pumps normally operate at low temperatures, well below the auto-ignition temperature of the materials, and gas compression occurs in a wetted environment where sparking or combustion is less likely to occur. Normally, the sealant liquid can be selected with this factor in mind. In many cases, water may be the preferred sealant. Inert gas or recycled gas from discharge is used to prevent cavitation while avoiding the introduction of air. The liquid ring pump is the pump best equipped for handling liquid ingestion. In fact, in many applications the condensate from a precondenser is run directly into the liquid ring pump or a liquid spray is used as a contact condenser directly upstream of the pump's suction. Some dry pumps can handle small amounts of liquid with the rotary screw pump being able to handle the most without hydraulically locking. In summary, both the liquid ring and rotary screw dry pumps offer advantages to the CPI. * SEARCH.PROQUEST.COM PDF Page 8 of 10

9 Edited by Deborah Hairston References References 1. Van Atta, C.M., Vacuum Science and Engineering, McGraw-Hill, New York, N.Y., Vibert, P.D., Dry versus Oil Sealed Vacuum Pumps for Vacuum Coaters, Soc. of Vacuum Coaters 41st Annual Technical Conference Proceedings, ISSN , pp. 7-8, Vibert, P.D., Mechanical Booster Vacuum Pumps, Society of Vacuum Coaters 42nd Annual Technical Conference Proceedings, ISSN , pp , AuthorAffiliation Phil Vibert Tuthill Vacuum &Blower Systems AuthorAffiliation Author Phil Vibert is a senior engineer for Tuthill Vacuum &Blower Systems (4840 West Kearney Street, Springfield, MO 65801; Phone: ; pvibert@tuthill.com). His career with the company, including Kinney Vacuum, spans more than 33 years. Involved in the design, application, operation, and troubleshooting of all types of vacuum pumps and systems, he has sized, selected, and designed thousands of vacuum systems for the chemical process industries. Vibert, who holds a B.S. in physics from Northeastern University (Boston, Mass.), has authored several papers and technical publications in the field of vacuum pumps and systems. : Electron tubes; Chemical process industries; Heat transfer; Pumps : United States--US : 9190: United States; 8640: Chemical industry : Mechanical Pumps for Vacuum Processing : Vibert, Phil : Chemical Engineering; New York : 111 : 11 : : 8 : 2004 : Oct 2004 SEARCH.PROQUEST.COM PDF Page 9 of 10

10 : Feature Report : Access Intelligence LLC : New York /: United States, New York : Chemistry, Engineering--Chemical Engineering ISSN: : Trade Journals : English : Feature : Photographs Diagrams Tables References ProQuest ID: URL: ?accountid=7093 : Copyright Access Intelligence LLC Oct 2004 : : Business Premium Collection; SciTech Premium Collection Find TSU 2018 ProQuest LLC 条款与条件联系 ProQuest SEARCH.PROQUEST.COM PDF Page 10 of 10