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2. VACUUM PUMPS

In reality a hole leading to an ideal vacuum does not exist. Real vacuum pumps arrive to some ultimate

limit pressure where their pumping speed goes to zero. Let us consider in the place of an ideal vacuum

a hole connecting to a volume with the pressure p`. The number of gas particles colliding with the holeper unit time (f'A):

( )1A5 0

2c 1 1f = p-p' A

c m T (1)

The above formula contains a forward stream and a backward gas stream. Writing (1) in a different

way we get:

1

A

5 0

2cp' 1 pf =A 1-

p c m T

(2)

The factorp'

A 1-p

characterizes the pump efficiency.

when p>>p` (at high pressures) the pumping speed S=A

when p=p` (close to the limiting pressure) S=0, i.e., p`=p

For an optimally designed pump one requires the maximum A and the minimum p`.

Vacuum pumps are characterized by the pumping speed S, [m3/h] or [l/s], (1 m3/h = 0.28 l/s) and by the

ultimate pressure p. Observe that S=S(p) pump characteristics. S~A, p. depends on the total back-stream and the materials used to build and lubricate the pump.

Insert example of pump S(p) characteristics mech.-Roots-diff.

Zoology of vacuum pumps

1. Volume pumps - based on the concept of reducing the pressure by increasing the volume.

2. Pumps based on speeding gas particle into the exit by means of:

stream of liquid (aspirator pump)

stream of vapor (diffusion pump)

moving surfaces (turbo-molecular pump)

electric field (ion pump)

3. Pumps based on gas sorption

zeolit sorption pump

cryogenic-pump

Ti-sublimation pump

4. Mixed action.

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2.1. Volume pumps

Old designs were in the form of a piston pump.

Modern design a rotary pump. In a single cycle

of the pump operation the number of particles in

the pumped volume will be reduced by:

c

c 0

V

V -V,

where Vc is the volume of the pump cylinder, Vois the pumped volume. All volume pumps have a

dead volume Vs which determines the ultimate

pressure: The amount of gas in the dead volume

Qs=pVs=760 Vss

c

Vp [Tr] =760

V

The pumping volume is lubricated with vacuumoil, which means that in reality:

s

c

Vp =760 +p (oil v.p.)

V

To improve p one should increase the pumping

volume and decrease the dead volume.

The best way is to reduces the back

pressure well below 760 Tr e.g., by

multistage pumping.

Multistage pumping.=: An example the three stage pump:

2.1.1. Rotary oil pumps (Rotary vane pumps)

The most commonly used pumps for all basic vacuum applications from atmospheric pressure down to

10-3 Torr. They are used as backing pumps for other high-

vacuum gas transfer pumps such as turbo-molecular and

diffusion pumps.

Principle of operation:

Gas enters the inlet port and is trapped between the rotor vanes

and the pump body. The eccentrically mounted rotor

compresses the gas and sweeps it toward the discharge port.

When gas pressure exceeds atmospheric pressure, the exhaust

valve opens and gas is expelled. Oil is used as lubricant and

sealant.

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The geometrical speed of pumping Sg=V N, where V is the volume between the rotor and the stator and

N is the speed of rotation. The geometrical speed is limited by the conductivity of the pump inlet and

inside channels.

1

1g

G

P

S SS

G

=+

The main contribution to1 GP

is the size of holes leading to the cylinder (limited by the slide

width). p is determined by the design, quality of machining and oil vapour pressure.

Typically:

one stage 10-1 - 10-3[Tr] two stage down to 10-5 [Tr]

Cooling -forced air or water.

Venting - necessary in the old types to prevent the

back flow of oil.

choosing the proper size and type of a pump.

hermetical pumps

stability to chemicals.

Gas ballast valve (gas ballast prevents vapours from condensing in the pump by letting in

frees air during compression).

Exhaust purifier - recycling the pumped gases. e.g. He in cryogenics systems

Vibrations - standard solutions: bellows, shock absorbers (avoid squeezing T-bellow design)

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2.1.2. Dry pumps

Membrane pumps

Roots pumps

The rotary lobe pump (Roots) is excellent for moving large quantities of gas at higher pressure than

possible with the rotary oil or piston pumps. Roots pumps are in use in molecular beam experiment and

IC fabrication lines that need high pumping speed at 0.1 to 10 Tr.

The lobes are similar to the figure eight in cross section (see Fig 2d). They mash with each other and

counter-rotate to continuously transfer gas from one side of the pump to the other. The compression

ratio of the pump is small and varies with the molecular weight of the gas. Light gases easily escape

back into the vacuum vessel around the edges of the rotors, creating a beck stream, so rotary or piston

pumps must back roots pumps. Typically pump speed: N 1500-3000 rpm, gap between the rotors: 1-0.15 mm.

Geometrical pumping speed S0 (N volume ) creates the forward stream

+ 0I =S p

The backward stream is constituted by gas flow through a clearance between the stator and rotor

surfaces. If Gs is the clearance conductivity then:

I G p ps = ( )1The effective pumping:

p + - 0 1I =I -I =S p-G(p -p)

Pumping speed at the pump inlet:

p s 10

0

I G pS = = S 1- ( -1)

p S p

To maximize the speed, maximize s 0G S -( high N, low leaks high volume).

1 s1

0 0

s

p G p = p(S=0) = p

S S1+G

when 0 sS G>>

Typically 0sG S 10-2Roots pumps needs backing pump.

Roots pump characteristics Insert Fig. 2e

Problems:

Oil leaks - multiple gasketsInsert (Fig 2h)

Closed sealed pumps - Teflon seals

Vibration

2.2. Velocity pumps

2.2.1. Molecular drag and hybrid pumps

Principle of operation:Insert Figs:

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I G p ps = ( )1

p p k L v

hpab =1 2

kp is a coefficient, h chamber height,

Lab chamber length, v linear speed of the

rotor.

To increase Lab a spiral rotor is used (Fig:

3b). p 10-6[Tr.], S 100[l/s], (pback 10-

3)

Maglev pumps p~10-9 Tr, S~10l.s @ 10-7 Tr

The drag pump has a smooth, high-speed rotor, shaped like an inverted bowl, that spins between two

closely spaced, cylindrical walls. The walls have helical grooves facing the rotor. The rotor reaches atangential velocity that approaches the average velocity of gas molecules. The pumping action is

induced by momentum transfer from the rotor to the gas molecules in the direction of the exhaust port.

The spiral grooves are designed to assist gas flow in the right direction

A molecular drag pump may reach a compression ratio of 109 for N2, 104 for He, and 103 for H2,

while discharging into a fore-line pressure of 10 to 40 Torr.

These pumps accept continues inlet pressure below 0.1 Torr and are used where low pumping speed

(less than 10 L/s) and modest ultimate pressure (no lower than 10-6 Torr) are demanded.

The hybrid pump combines several stages of turbo pumps with drag pumps. The result is a pump with a

higher pumping speed that backs into a high fore-line pressure.

2.2.2. Turbomolecular pumps

Turbomolecular pumps known also as turbo pumps; their application cover

all processes and vacuum system in the 10-4 - 10-10 Tr. pressure range.

Turbo pump resembles a jet engine. Several very high-speed rotor, each with

multiple blade shaped with angled leading edges, impart momentum to gas

molecule in the direction of the next rotor down the stack. The compression

ratio across the pump for a meditate molecular weight gas may exceed 108.

Turbo pumps have advantages over diffusion pumps.

Correctly operated they do not back-stream oil into

the vacuum system at any time and can be started and

stopped in a few minuets. The last feature means a

turbo pump can be directly connected to the chamber

without a high vacuum valve. This saves money and

improves pumping conductance. But turbo pump can

be noisy and they induce vibration. The turbo pumps

are also expensive and the compression ratio for

hydrogen and helium are low.

Insert figures 3 (c-e)

p ~5 10-11 Tr, N~105 rpm, S~10000 l/s

Drives and bearings ceramics and Maglev

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Venting at 50% of the normal pump speed to avoid oil contamination

Purging with dry nitrogen

Baking

Needs electronics controllers

High price

1 High vacuum connection flange.

2 Stator pack.

3 Venting connection flange.

4 For-vacuum connection flange.

5 Splinter guard.

6 Rotor.

7 Pump casing.

8 Bearing.

9 Motor.

10 Bearing.

2.2.4. Booster pumps (ejector pumps)

p 10-4[Tr]

S up to15000 [l/s]Oil volume 5[l]Heating power

5kW

2.2.3. Diffusion pumps

There are numerous applications for diffusion pumps, ranging from high gas volume found in

molecular beam investigation and large-scale vacuum furnace processing to ultra-high vacuum level

needed for surface physics research and space simulation chambers. These pumps operate from 10 -4

Tr. to5*10-11 Tr. The latter pressure range is not easily obtained with a diffusion pump and requires an

exceptionally good LN

2

trap and additional pumping from a Titanium sublimation pump.

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Diffusion pumps operate by boiling a fluid, often hydrocarbon oil, and angling the dense vapour stream

in a downward conical direction back into the pump boiler. Gas molecules from the system that enter

the oil curtain are pushed toward the boiler by momentum transfer from the large fluid molecules.

Oil traps are designed so that there will be no optical connection between the pump and the vacuum

chamber, in that way it is possible to reduce the back-stream of oil vapour with no influence on the

conductivity. Fig 4a-b.

Diffusion pumps Principle of operation see Fig: 4.

Vapour disk clean surface for gas particles

I+=kvpn(x)

I--=D dn(x)/dx

Ipump=I+ - I-- ; p =p(S=0); kvpn(x)= D dn(x)/dx

pkv-D

1p =p e LThe limiting pressure is lower when vapour speed and pumping length is higher and D is smaller. D

decreases with increasing vapour density, molecular mass of vapour particles and molecular mass of

gas particles.

Typically: p p13

10