CHAPTER 10

Mechanical Pumps

In this chapter we review the operation of five low vacuum pumps (the rotary vane and piston, scroll, screw and diaphragm pump), and two medium vacuum pumps (the lobe or Roots pump and the claw pump). Vane and piston pumps are widely used for backing high vacuum pumps and for initial chamber evacuation. Lobe blowers are used with piston or vane pumps to rough large systems, to back large pumps, and to pump large quantities of gas in plasma processing systems. Screw, scroll, and diaphragm pumps are used in oil-fiee applications, and the claw pump is particularly well suited for pumping particulate-laden atmospheres.

10.1 ROTARY VANE PUMP

The rotary piston pump and the rotary vane pump are two oil-sealed pumps commercially available for pumping gas in the pressure range of 1-10' Pa. Of the two, the rotary vane is the most commonly used in small to medium-sized vacuum systems. Rotary vane pumps of 10-200 m3/h displacement are used for rough pumping and for backing diffision or turbomolecular pumps.

In a rotary vane pump (Fig. lO.l), gas enters the suction chamber (A) and is compressed by the rotor (3) and vane (5) in region B and expelled to the atmosphere through the discharge valve (8) and the fluid above the valve. An airtight seal is made by one or more spring or centrifugally loaded vanes and the closely spaced sealing surfaces (10). The vanes and the surfaces between the rotor and housing are sealed by the low vapor- pressure fluid, which also serves to lubricate the pump and fill the volume above the discharge valve. Pumps that use a speed-reduction pulley operate in the 400-600 rpm range, whereas direct drive pumps operate at speeds of 1500-1725 rpm. The fluid temperature is considerably higher in the direct drive pumps than in the low-speed pumps, typically 80°C and 60"C, respectively. These values will vary with the viscosity of the fluid and the quantity of air being pumped.

183

A User’s Guide to Vacuum Technology, 3rd Edition. John F. O’Hanlon Copyright 0 2003 John Wiley & Sons, Inc.

ISBN: 0-471-27052-0

184 MECHANICAL PUMPS

Fig. 10.1 Sectional view of the Pfeiffer DUO-35, 35 m3h double-stage, rotary vane pump: (1) intake, (2) filter, (3) rotor, (4) spring, (5) vane, (6) gas ballast valve, (7) filter, (8) discharge valve, (9) exhaust, (10) sealing surface. Reprinted with permission from A. Pfeiffer Vakuumtechnik, GmbH, Wetzlar, Germany.

Single-stage pumps consist of one rotor and stator block (Fig. 10.1). If a second stage is added as shown schematically in Fig. 10.2, by connecting the exhaust of the first stage to the intake of the second, lower pressures may be reached. The ultimate pressure at the inlet of the second stage is lower than at the inlet to the first because the fluid circulating in the second stage is rather isolated fiom that circulating in the second stage. The fluid in the second stage contains less gas than the fluid in the reservoir. Physically, the second pumping stage is located adjacent to the first and on the same shaft. The pumping speed characteristics of single-stage and two- stage rotary vane pumps are shown in Fig. 10.3. The fiee-air displacement and the ultimate pressure are two measures of the performance of roughing pumps. 'The fiee air displacement is the volume of air displaced per unit time by .the pump at atmospheric pressure with no pressure differential. For

10.1 ROTARY VANE PUMP 185

Fig. 10. from Vacuum Technology, Leybold-Heraeus, GmbH, Kbln, Germany.

Schematic section through a two-stage rotary pump. ReprinteL with permission

the two pumps whose pumping speed curves are shown in Fig. 10.3, this has the value of 30 m3/h (17.7 c h ) at a pressure of lo5 Pa (1 atm). At the ultimate pressure of the blanked-off pump, the net speed (forward flow - back flow) drops to zero because of dissolved gas in the fluid, leakage around the seals, and trapped gas in the volume below the valve. Rotary vane pumps have ultimate pressures in the 3~10-~-1 Pa range; the lowest ultimate pressures are achieved with two-stage pumps. The single- and two-stage pumps characterized in Fig. 10.3 have ultimate pressures of 1.4 and 1.5~10-* Pa, respectively. These ultimate pressures are obtained with a new pump using clean, low vapor pressure fluid. As the fluid becomes contaminated and the parts wear, the ultimate pressure will increase.

Pressure (Torr) lo4 lo9 lo-* 10-l loo lo1 lo2 lo3

-

inlet Pressure (Pa)

Fig. 10.3 Pumping speed curves for the Pfeiffer UNO 30A and DUO 30A rotary vane pumps. Reprinted with permission from A. Pfeiffer Vakuumtechnik, GmbH, Wetzlar, Germany.

186 MECHANICAL PUMPS

Kendall [ 11 demonstrated the effect of dissolved gas in the fluid on the ultimate pressure of a two-stage rotary vane pump. He showed a rotary vane pump could reach an ultimate pressure of 4 ~ 1 0 - ~ Pa when the fluid reservoir was exhausted by another pump. Figure 10.4 shows the effect of prolonged outgassing and the effect of admitting COz to the fluid reservoir after degassing the fluid.

When large amounts of water, acetone, or other condensable vapors are being pumped, condensation occurs during the compression stage after the vapor has been isolated from the intake valve. As the vapor is compressed, it reaches its condensation pressure, condenses, and contaminates the fluid before the exhaust valve opens. Condensation causes a reduction in the number of molecules in the vapor phase and delays or even prevents the opening of the exhaust valve. If condensation is not prevented, the pump will become contaminated, the ultimate pressure will increase, and gum deposits will form on the moving parts. Some compounds will eventually cause the pump to seize. To avoid condensation and its resulting problems, gas is admitted through the ballast valve. The open valve allows ballast, usually room air, to enter the chamber during the compression stage; the trapped volume is isolated from the intake and exhaust valves. This inflow of gas, which can be as much as 10% of the pump displacement, is

i’

M/z

Fig. 10.4 Relative abundance of gases at pump ultimate: Top: After prolonged outgassing of oil and with zero pressure at the exhaust port. Helium pressurization applied intermittently to provide lubricating oil pressure (P = 7 ~ 1 0 - ~ Torr, 9.3~10” Pa). Bottom: After oil had been exposed to carbon dioxide and then exhaust pressure reduced to zero (P = 3~10-’ Torr, 4 ~ 1 0 ~ Pa). Reprinted with permission from J. Vuc. Sci Technol., 21, p, 886, B. R. F. Kendall. Copyright 1982, The American Vacuum Society.

10.1 ROTARY VANE PUMP 187

controlled by valve 6 (Fig. 10.1). The added gas causes the discharge valve to open before it reaches the condensation partial pressure of the vapor. In this manner the vapor is swept out of the pump and no condensation occurs. The ultimate pressure of a gas-ballast pump is not very low with the ballast valve open. Figure 10.3 shows the effect of full gas ballast on the performance of a single- and double-stage pump.

Gas ballast can be used to differentiate contaminated fluid from a leak. If the inlet pressure drops when the ballast valve is opened, but drifts upward slowly after the valve is closed, the fluid is contaminated with a high vapor pressure impurity. Additional gas ballast details are covered in reference [2], and Van Atta [3] describes alternative methods for pumping large amounts of water.

10.2 ROTARY PISTON PUMP

Rotary piston pumps are used as roughing pumps on large systems alone or in combination with lobe blowers. They are manufactured in sizes ranging from 30-to-1500 m3h. A piston pump is a rugged and mechanically simple pump. There are no spring-loaded vanes to stick in a piston pump; all parts are mechanically coupled to a shaft that can be powered by a large motor.

Fig. 10.5 shows a sectional view of a rotary piston pump. As the keyed shaft rotates the eccentric (1) and piston (2), gas is drawn into the space A. After one revolution, this volume of gas has been isolated from the inlet, while the piston is closest to the hinge box. During the next revolution the isolated volume of gas (B) is compressed and vented to the exhaust through the poppet valve when its pressure exceeds that of the valve spring. Like the vane pump, the piston pump is manufactured in single and compound or multistage types.

The clearance between the piston and housing is typically 0.1 mm, but is three or four times larger near the hinge box. Because the clearance between moving and fixed parts is greater in a piston than in a vane pump, the piston pump is tolerant of particulate contamination. A lubricating fluid is used to seal and lubricate the spaces between fixed and moving parts. As in a vane pump, the fluid must have low vapor pressure and good lubricating ability. A rather viscous fluid is used in the piston pump.

The rotational speed of the piston pump is typically 400-600 rpm, although some run as slow as 300 rpm and others as fast as 1200 rpm. The maximum rotational speed of a piston pump is limited by vibration from the eccentric. Small piston pumps are air cooled in the same manner as rotary vane pumps. Large pumps are water-cooled.

The pumping speed curves for a 5 l-m3h single-stage rotary piston pump are shown in Fig. 10.6 with and without gas ballast. The shaft power is also

188 MECHANICAL PUMPS

Fig. 10.5 Sectional view of a Stokes 212H, 255-m3/h rotary piston pump: ( 1 ) eccentric, (2) piston, (3) shaft, (4) gas ballast, (5) cooling water inlet, (6) optional exhaust, (7) motor, (8) exhaust, (9) oil mist separator, (10) poppet valve, (11) inlet, (12) hinge bar, (13) casing, (14) cooling water outlet. Reprinted with permission from Stokes Division, Pennwalt Corp., Philadelphia, PA.

given and it is seen to peak at a pressure of 4 ~ 1 0 ~ Pa and is independent of ballast. At lower pressures operation of the pump with full gas ballast requires more than twice the shaft power as without the use of gas ballast. The ultimate pressure of the piston pump shown in Fig. 10.6 is 1 Pa.

10.2 ROTARY PISTON PUMP

II

189

105 9

0 Gas Ballast

- 11.5 a L

---A?.-- A 1.0 n __/--- 3 TJ

m

I I I

I I , I I 0 t

10' 103 105 2 01

lo-' Inlet Pressure (Pa)

Fig. 10.6 Pumping speed and shaft power for the Stokes 146H, 51-m3/h rotary piston pump. Reprinted with permission from Stokes Division, Pennwalt Corp., Philadelphia, PA.

10.3 LOBE PUMP

The positive displacement blower, or lobe blower is used in series with a rotary fluid-sealed pump to achieve higher speed and lower ultimate pressure in the medium vacuum region than can be obtained with a rotary mechanical pump alone. Lobe blowers, also known as Roots pumps, consist of two figure-eight-shaped rotors mounted on parallel shafts. The rotors have substantial clearances between themselves and the housing- typically about 0.2 mm. They rotate in synchronism in opposite directions at speeds of 3000 to 3500 rpm. These speeds are possible because a fluid is not used to seal the gaps between the rotors and the pump housing. A sectional view of a single-stage lobe blower is given in Fig. 10.7.

Fig. 10.7 Section through a single-stage lobe blower: (1) inlet, (2) rotors, (3) housing, (4) pump chamber (swept volume), (5) outlet. Reprinted with permission from Vacuum Technologv, Leybold-Heraeus GmbH, KOln, Germany.

190 MECHANICAL PUMPS

The compression ratio, or ratio of outlet pressure to inlet pressure, is pressure dependent and usually has a maximum near 100 Pa. At higher pressures the compression ratio should, theoretically, remain constant. In practice it decreases. Outgassing and the roughness of the rotor surfaces contribute to compression loss at low pressures. Each time the rotor surface faces the high-pressure side it sorbs gas. Some of this gas is released when the rotor faces the low-pressure side. The compression ratio K,,,, for air for a single-stage Lobe blower of 500-m3k displacement is shown in Fig. 10.8. It has a maximum compression ratio of 44. Large pumps tend to have a larger compression ratio than small pumps, because they have a smaller ratio of gap spacing to pump volume. The compression ratio for a light gas such as helium is about 15-20'30 smaller than the ratio for air. The Compression ratio Koma is a static quantity and is measured under conditions of zero flow. The inlet side of the pump is sealed and a pressure gauge is attached. The outlet side is connected to a roughing pump and the system is evacuated. Gas is admitted to the backing line that connects the blower to the roughing pump. The backing pressure Pb, is measured at the blower outlet, and the pressure P, is measured at the inlet. The compression ratio is given by PdP,.

Lobe blowers generate considerable heat, when pumping gas at high pressures. Heat causes the rotors to expand; if unchecked, rotor expansion could destroy the pump. To avoid overheating, a maximum pressure difference between the inlet and outlet of a lobe blower is specified. This maximum pressure difference is typically 1000 Pa, but that value may be exceeded for a short time without harm to the pump. Lobe blowers are connected as compression or transport pumps.

1 10 1 o2 I o3 I o4 Backing Pressure (Pa)

Fig. 10.8 Dependence of the air compression ratio KO- of the Leybold WS500 lobe pump on the backing pressure. Values for helium are about 20% smaller. Reprinted with permission fiom Vacuum Technology, Leybold-Heraeus GmbH, K6ln, Germany.

10.3 LOBE PUMP 191

In compression pumping, the common method, a lobe pump is placed in series with a rotary pump whose rated speed is 2-10 times smaller than its own speed. When pumping is initiated at atmospheric pressure, a bypass line around the lobe pump is opened, or the pump is allowed to free wheel. All the pumping is done by the rotary pump until the backing pressure is below the manufacturer's recommended pressure difference, at which time the lobe blower is activated and the bypass valve is closed. Some lobe blowers have this bypass feature built into the pump housing; others are allowed to free-wheel until a pressure sensor activates a clutch between motor and blower. The net speed of a lobe blower of 500-m3/h capacity backed by a 100-m3/h rotary piston pump is shown in Fig. 10.9. The speed curve for the mechanical pump alone is shown for comparison. Such lobe blower-rotary pump combinations are often used when speeds of 170 m3k or greater are required because the combination costs less than a rotary pump of similar capacity.

The second method, transport pumping, uses a lobe blower in series with a rotary pump of the same displacement. Figure 10.9 shows the pumping speed of a 60-m3/h lobe blower backed by a 60-m3/h rotary vane pump. The pumping speed of the rotary vane pump is shown for comparison. Both pumps are started simultaneously at atmospheric pressure because the critical pressure drop will never be exceeded.

Pressure (Pa)

Fig. 10.9 Lobe blower-rotary pump combinations. Transport mode: (A) Leybold RUTA 60 lobe blower and S60 rotary vane; (B) S60 only. Compression mode: (C) Leybold WSSOO lobe blower and DKlOO rotary piston pump: (D) DKlOO only. Reprinted with permission from Vacuum Technology, Leybold-Heraeus GmbH, Keln, Germany.

192 MECHANICAL PUMPS

Detailed calculations of the effective pumping speed of the lobe blower have been carried out by Van Atta [3 3. Here we give approximate formulas for the inlet pressure Pi, and the inlet speed Si [2].

b ( l O . l )

b (10.2)

SD is the pump displacement, or speed of the lobe blower at atmospheric pressure. With these approximate equations, the pumping speed curve for the rotary pump, the compression ratio Komm, and the lobe blower displacement, a curve of the speed of the lobe pump versus inlet pressure can be calculated. The point on the upper end of the line marked X in Fig. 10.9 is the result of applying (10.1) and (10.2). In this example the inlet pressure PI, and the inlet speed S, were calculated for a backing pressure of 20 Pa. At 20 Pa, KO- = 30 (Fig. 10.8), and s b = 90 m3/h (Fig. 10.94. With (10. I), (10.2), and SD = 500 m3k, we get PI = 4.3 Paand S, = 422 m3/h.

Lobe pumps are also used to back large diffusion or turbomolecular pumps. For example, a 35-in.-diameter diffusion pump used to evacuate a 2-m3 chamber is backed by a series combination of a 1300-m3/h lobe blower and a 1 70-m3h rotary piston pump.

Fig. 10.10 Compression sequence for 1 revolution of a claw stage: (a)-(f) show the intake sequence with the inlet port open from (bHd). The compression and exhaust sequence is shown in (a')-(f ') with the exhaust port open from (c')-(d'). The carryover volume, which limits the maximum obtainable compression, is shown in (f '). Reprinted with permission from J. Vuc. Sci. Technol. A, 9, p. 2048, A. P. Troup and N. M. T. Dennis. Copyright 1991, AVS-The Science and Technology Society.

10.4 CLAW PUMP 193

10.4 CLAW PUMP

The lobe or Roots design is one of many shapes into which a counter- rotating pair may be machined for vacuum pumping. The Northey hook and claw compressor is one design that has proven useful for particular applications. It has two irregularly shaped lobes or “claws” that counter- rotate in the same manner as a lobe blower. However, a lobe blower does not compress gas inside the pump-rather it transports the gas from the inlet to the foreline, where compression takes place. The claw pump does compress gas inside the pump, as described in the rotational sequence illustrated in Fig. 10.10. Each complete revolution of the claw pair compresses a volume of gas. Claw pumps are more efficient at compressing gas at high pressures than Roots pumps [5] and have at least equal compression ratios at low pressure [6]. The maximum compression is limited by the volume of gas depicted in illustration (f) of Fig. 10.10, which returns to the inlet at the end of the cycle. Claw and Roots stages are combined to form an efficient multistage pump that can effectively pump corrosive and abrasive gas [7]. One design uses a Roots inlet stage followed by three claw stages before exhausting to atmosphere; its pumping speed curve is depicted in Fig. 10.1 1 exhausting to atmosphere. Both Roots and claw pumps require cooling and gas purging [8]. Pumps with combined Roots-claw stages have ultimate pressures in the range 0.5-1 Pa (4-7 mTorr) when exhausting directly to atmosphere. They are used to provide oil-free backing for turbomolecular pumps in corrosive or

Inlet Pressure (Torr)

1 00 10’ 102 103 104 105 Inlet Pressure (Pa)

Fig. 10.11 Pumping speed curve for a four-stage booster pump exhausting directly to atmosphere. This pump contains a lobe inlet stage followed by three claw stages. Reprinted with permission from BOC Edwards, 301 Ballardvale Street, Wilmington, MA 01887.

194 MECHANICAL PUMPS

abrasive etching and deposition systems. Because of their shape, claw stages efficiently remove deposited particles. As a result, maintenance intervals can be increased when using these pumps in chemically aggressive process environments.

10.5 SCROLL PUMP

The scroll pump is a relatively simple compressor consisting of two surfaces, one fixed and one orbiting. Note that the movable plate does not rotate, but rather it orbits. Mirror-image spiral grooves are cut in two facing, stator plates; in turn these plates mesh within a plate containing sets of complementary spiral ridges. Figure 10.12 illustrates cross-sectional and plan views of an orbiting scroll pump. Gas enters the chambers at the periphery, and is forced around in a spiral helical path until it reaches the exit port located at the pump center. The pump inlet is located at the periphexy, which must be sealed with a bellows. The ultimate pressure of a scroll pump is -1 Pa (lo-’ Torr), see Fi . 10.13. These pumps are manufactured in small sizes of order 15-40 m ih. They find application for backing turbomolecular drag pumps that have in-built molecular drag stages, and they find other applications where small oil-free pumps are needed. The polymer seal that prevents gas flow between the closely spaced edges of the orbiting seal plate and the stator will generate wear particles. One can

5

Fig. 10.12 Anest-Iwata orbiting scroll pump mechanism. Right: Plan view of orbiting scroll. Left: Cross-sectional view of stator and orbiting rotor. Reprinted with permission from Synergy Vacuum, P. 0. Box 2084 Montrose, CO 81402.

10.5 SCROLL PUMP 195

Inlet Pressure (Torr)

loo 1 0' lo2 lo3 1 o4 1 o5 Inlet Pressure (Pa)

Fig. 10.13 Pumping speed of a model ESDP12 orbiting scroll pump, reprinted with permission from BOC Edwards, 301 Ballardvale Street, Wilmington, MA 01887.

trap these in a point-of-use 0.02-pm-diameter particle filter mounted on the pump inlet, and prevent them fiom migrating to the process chamber, where they can contaminate the process.

10.6 SCREW PUMP

The screw pump is another compressor with a long history of use in fields other than vacuum technology. Recently screw pumps have been manufactured for oil-fiee applications, and for pumping abrasive and corrosive gases. They are used for backing magnetic levitated turbomolecular pumps on reactive ion etching and low-pressure chemical vapor deposition systems. The basic design of a screw pump is illustrated in Fig. 10.14. The two uniquely shaped screws, located inside a closely fitting stator, are counter rotated at equal rotational velocities. Gas enters at the end of the rotating pair, and is transported along screws in a trapped region defined by the contact of the screws and the wall. It is expelled into the exhaust plenum at the right-hand end of the screw pair. The rotors may be coated for use in pumping corrosive gases. In some designs, asymmetrical screws rotate at different velocities, provided that the total number of turns is limited. Pumping speed curves for one family of pumps is shown in Fig. 10.15. The ultimate pressure of a screw pump is of order 0.7 Pa ( 5 ~ 1 0 ' ~ Torr) with speeds as high as 2500 m3/h. Screw pumps are rugged and reliable.

196 MECHANICAL PUMPS

Fig. 10.14 Cross-sectional view of a screw vacuum pump. (1) Inlet, (2) discharge, (3) cooling jacket, (4) screws, (5) gear case, (6) stator housing, (7) drive gears, (8) shaft bearings, (9) shaft seals, (10) motor shaft seal. Reprinted with permission from Busch Semiconductor Vacuum Group, 18430 Sutter Blvd., Morgan Hill, CA 95037.

10.7 DIAPHRAGM PUMP

Diaphragm pumps, in the form of bellows, are one of the oldest known pumps. They consist of a small chamber designed to have minimal dead space containing a flexible diaphragm connected to a piston, such as illustrated in Fig. 10.16. The eccentric shaft moves the piston-diaphragm

Inlet Pressure (Tort-)

lo-' 1 10 102 1 0 3 104 105

Inlet Pressure (Pa)

Fig. 10.15 Pumping speed curves for a family of screw pumps. Reprinted with permission from Busch Semiconductor Vacuum Group, 18430 Sutter Blvd., Morgan Hill, CA 95037.

10.7 DIAPHRAGM PUMP 197

Fig. 10.16 Neuberger USA, Two Black Forest Road, Trenton, NJ, 08691.

Diaphragm pumping mechanism. Reprinted with permission from KNF

Inlet Pressure (Torr) 10-1 1 00 10' 1 02 103 -

I I ( , , , I ' ' , ' . ' , ' 1 ' , I , , & , , ,',...I , , . # I , , , I .'/ I I I / I - 2.0- m' E 'D 1.5-

v, 1.0-

r

- -4 st. He

Q a, Q

W C Q E 0.5 - 3 L

......... 4 st. N,

.--.-.. 3st.He ------ 3 st. N,

---..I 2 st. N,

.- ---2 st. He

, 1 . 1 , , . ~ I I I , ..,'.' 8 I I I

1 02 103 104 105 0 ~ ' ' ' ' ~ " ' ' ' " ' I ' . ."'"'. ' ' " " '

Inlet Pressure (Pa)

Fig. 10.17 Pumping speed curves for four-stage, three-stage, and two-stage diaphragm pumps for nitrogen and helium. Reprinted ffom Vacuum, 47, F. J. Eckle, P. Bickert, R. Lachenmann and B. Wortmann, 799-801, copyright 1996, with permission from Elsevier.

198 MECHANICAL PUMPS

assembly in rocking, swinging manner so as to draw gas in through the left-hand inlet valve, and a half-stroke later expel the gas through the right- hand exit valve. The pumps are oil-free and may be constructed from chemically inert materials. Up to four stages can be combined to provide ultimate pressures of, respectively, 7000-20,000 Pa (50-1 50 Torr), 700- 1400 Pa (5-10 Torr), 200-400 Pa (1.5-3 Torr), and 50-75 Pa (0.4-0.6 Torr). Sizes do not exceed several m3/h. Figure 10.17 illustrates the nitrogen and helium pumping speeds for small two-, three-, and four-stage diaphragm pumps. Diaphragm pumps can be used for medium vacuum chemically aggressive applications and can be used as oil-free backing pumps for turbomolecular-drag pumps.

10.8 MECHANICAL PUMP OPERATION

There are several common rules for operating rotary mechanical pumps. The exhaust should be vented outside the building. Most pumps are supplied with an oil mist separator, but it does not adequately remove all the vapors. Most laboratories and plant safety rules require the use of an outside vent. The vent hose should not run vertically from the exhaust connection, because water or other vapors, which have condensed on cold pipe walls, will flow into the pump exit and contaminate the fluid. A satisfactory solution to this problem is the addition of a sump at the exhaust connection to collect the vapors before they can flow into the pump. A vane pump must also be vented at the time it is stopped to prevent fluid from being forced back into the vacuum system by external air pressure. Venting is done automatically in most pumps, and it can be achieved in others by the addition of a vent valve above the inlet port. The fluid level in mechanical pumps should be checked frequently, especially those that are used on systems regularly cycled to atmosphere. Small rotary pumps of ca acity less than 30 m3/h have fluid consumption rates (cm3/h) of about 10' -10 PS where P is the inlet pressure and S is the inlet speed in m3/h [4]. Larger pumps will use more fluid. The fluid should be changed when the pump performance deteriorates or when it becomes discolored or contaminated with particles. Poor fluid maintenance is the major cause of mechanical pump failure. Flushing the pump and changing the fluid can solve ninety-five percent of all pump problems. A discussion of mechanical pump fluids is given in Chapter 13. Vapor pressure and kinematic viscosity of mechanical pump fluids are given in Appendixes F.l and F.3.

! -5

REFERENCES

REFERENCES

199

1. €3. R. F. Kendall, J. Vac. Sci. Technol., 21,886 (1982). 2. Leybold-Heraeus Publication HU152, Leybold-Heraeus, GmbH, K6ln, Germany. 3. C. M. Van Atta, Vacuum Science and Engineering, McGraw-Hill, New York, 1965,

Chapter 5. 4. Reference 2, p. H-B 61. 5. A. P. Troup and N. T. M. Dennis, J. Vac. Sci. Technol. A , 9,2048 (1991). 6. H. Wycliffe, J. Vac. Sci. Technol. A, 5,2608 (1987). 7. P. Bachmann and M. Kuhn, Vacuum, 41,1827 (1990). 8. M. Hablanian, Vacuum, 41, 1814 (1990).

PROBLEMS

10.1

10.2

10.3

10.4

10.5

10.6

10.7

10.8

10.9

t A simple mechanical piston has a displacement of 1 L. It is connected to a chamber of 10 L. If pumping commences at 1 atmosphere, what is the pressure in Pa after four complete strokes?

Define free air displacement. Give a formula for the free air displacement SD, in terms of rotational speed n and swept volume V, of (a) a piston pump and (b) a two-stage vane pump.

Rank the following pumps in order of increasing physical size for equal displacements: (a) Rotary vane pump, two-vane belt drive; (b) rotary piston pump; (c) rotary vane pump, two-vane direct drive.

t What are three functions of the fluid in a rotary vane or piston pump?

Plot throughput versus pressure for the single-stage and two-stage mechanical pumps whose no-ballast pumping speed curves are given inFig. 10.3.

What is the staging ratio of a lobe-piston pump set? What performance differences would you expect between a set with a staging ratio of 1O:l and one of 2:1, provided that the two sets use the same mechanical pump?

t What will happen if the mechanical pump exhaust hose runs vertically in a direct line from the pump to a connection in the exhaust plenum?

t What is the most important step in ensuring long, trouble free operation of a rotary pump?

The piston pump whose speed characteristic is shown in Fig. 10.9d is connected to a chamber by a 400-cm length of 4-cm-diameter

200 MECHANICAL PUMPS

pipe. When the pressure at the inlet of the pump is 1 Pa, what is the pressure in the chamber?

10.10 f What is the hazard in discharging mechanical pump exhaust fbmes into the work area? What will happen if the exhaust hose forms a loop like a sink trap, before being connected to the exhaust plenum?