Hoffman D.M., Singh B., Thomas J.H. (Eds). Handbook of Vacuum Science and Technology

Подождите немного. Документ загружается.

2.4.1 Basic Pumping Mechanism

117

2A.\

BASIC PUMPING MECHANISM

The typical vapor jet pump consists of a vertical, usually cylindrical, body fitted

with a flanged inlet for attachment to the system to be evacuated. The bottom of

the cylinder is closed, forming the boiler, which is fitted with a heater. The upper

two-thirds of the body is surrounded by cooling coils. An outlet duct (or foreline)

is provided at the side of the lower pump body for discharging the pumped gases

and vapors to the mechanical forepump. The sectional view in Figure

illustrates

a schematic arrangement of a single-stage vapor jet pump.

A jet-forming structure is located within the pump body. This consists of a con-

centric cylinder partially capped and fitted with flared ends to form jets through

which the pump fluid vapors can emerge at high velocity. There are no moving

mechanical parts.

In operation, the working fluid in the boiler of the pump is heated by means

of the electric element clamped to the lower body, and a vapor stream is created.

This vapor rises in the jet structure and is emitted through the annular nozzle in a

downward and outward direction against the water-cooled wall of the pump body.

Flg.l.

COOLAN

Schematic arrangement of a single-stage vapor jet pump. Open circles = vapor jet molecules;

closed circles = gas molecules.

118 Chapter 2.4: Vapor Jet Pumps [Diffusion Pumps]

Fig.

COLO CAP

ORIFICES

BAFFLE

DEGASSING

Multistage

vapor

jet pump.

Gas molecules arriving at the pump inlet are entrained in the stream of working

fluid vapor and are given a downward momentum. The vapor stream normally

flows at supersonic velocities. The gas vapor mixture travels downward toward

the foreline. When the oil vapor constituents of such a jet stream strike the water-

cooled wall of the pump body, they are condensed and returned to the boiler in

liquid form. The entrained gas molecules continue their path toward the exit,

where they are removed by the mechanical forepump.

The condensed oil vapors return to the boiler. Heat is once more added and

the oil revaporized to maintain the vapor flow to

the

jet assembly and the continu-

ity of

the

pumping mechanism. A sectional view of

typical multistage vapor jet

pump is shown in Figure 2.

The pumping action in a

vapor

jet pump results from collisions between vapor

and gas molecules. It is more difficult for gas molecules to cross the vapor stream

in the counterflow direction. Thus a pressure (or molecular density) difference is

created across the vapor stream. The pressure ratio created by the vapor stream

can be approximately expressed by

P2lPx

= cxpipVL/D)

where p is vapor density, Vits velocity,

and

L the width of

the

stream. D is related

to the diffusion coefficient and depends on molecular weight (M) and diameters

2.4.1 Basic Pumping Mechanism

119

(cr) of the vapor and gas [1].

8(2J

1/2

RT-

M, +M2VV^i +^2

M1M2

where subscript 1 refers to the pumped gas and 2 to the pumping fluid, R is the

gas constant and Tis the temperature. From this, you can see that the pressure ra-

tio is much lower for lighter gases.

The interaction of pumping vapor and the pumped gas can be illustrated experi-

mentally by finding the density distribution of both species in the pumping re-

gion. The gas density distribution obtained by traversing the pumping region with

an ionization gauge probe is shown in Figure 3 and the distribution of vapor ar-

Fig.3.

Gas density distribution obtained by traversing the pumping region with an ionization gauge

probe.

120

Chapter 2.4: Vapor jet Pumps [Diffusion Pumps)

Fig.

8 ^

.01 I I 10 (00

FLOW RATE.cm3/hr

Distribution of vapor arriving at the pump wall.

riving at the pump wall in Figure 4. The pattern of penetration of pumped air into

the vapor jet, its relative absence in the core of

the

jet near the nozzle exit, and the

gradual compression of the gas can be deduced from Figure 3. The repetition of

the process in the second stage is also evident.

2.4.1.1 Number of Stages

The number of pumping stages or vapor nozzles depends on particular perfor-

mance specifications. A single-stage pump would have conflicting requirements of

high pumping speed and high compression ratio. Normally, the first stage at the

inlet has high pumping speed and low compression ratio, and the last (discharge)

stage vice versa. Small pumps often have three stages and large ones five or even

six. The initial stages have annular nozzles; the discharge stage sometimes has a

circular nozzle and is called an ejector. There are no principal differences in such

variations of geometry, although certain advantages are gained by one or the other

choice. Sometimes, to obtain certain performance effects two vapor jet pumps can

be used connected in series. This has an effect of increasing the number of com-

pression stages, and it allows the use of different pumping fluids in the two pumps.

2.4.1.2 Vapor and Gas Interaction

As noted before, in regard to their basic pumping action, vapor jet pumps or high-

vacuum vapor pumps are related to oil or steam ejectors. The potential energy of

elevated pressure inside the jet assembly (boiler pressure) is converted to the ki-

netic energy of high-velocity vapor stream or jet after it passes through a nozzle.

The gas is pumped by the jet by momentum transfer in the direction of pumping.

2.4.1 Basic Pumping Meciianism

121

Because the pumping fluids used in vapor jet pumps are easily condensable at

room temperature, a multistage nozzle-condenser system can be fitted in a com-

pact space.

2A.13 Operating Range

The pressure range for the use of vapor jet pumps is between

~ ^^

and

~ ^

torr.

At the high-pressure limit, the steady-state pressure (at the pump outlet) normally

should not exceed about 1 X 10~^ torr. Without the assistance of cryogenic

pumping and v^ithout baking, the lowest inlet pressures conveniently achieved are

near 10"^ torr. With the aid of cryogenic pumping, liquid-nitrogen-cooled traps,

for example, inlet pressures below 1 X 10"^^ torr range can be obtained. Gener-

ally, baking and subsequent cooling of pumped chambers and inlet ducts are re-

quired in order to extend the pressure range, since they can reduce outgassing as

well as produce sorption pumping effects.

2.4.1.4 Basic Performance

The pumping performance of a vapor jet pump is usually displayed in the form of

a plot of pumping speed versus inlet pressure, as shown in Figure 5. The graph

consists of four distinct sections. To the left, the speed is seen to decrease near the

limit of obtainable vacuum. The constant speed section results from the constant

Fig.

I CONSTANT

INLET PRESSURE

Pumping performance of a pump displayed in the form of a plot of pumping speed vs. inlet

pressure.

122

Chapter 2.4: Vapor Jet Pumps [Diffusion Pumps)

gas arrival rate at molecular flow conditions and a constant capture efficiency of

the vapor

jets.

The part marked "overload" is a constant throughput section that

indicates that the maximum mass flow capacity of the pump has been reached. In

the last section, at the right, the performance is highly influenced by the size of

the mechanical backing pump. For steady-state operation, the pump should not be

operated in the overload region.

The performance of vapor jet pumps is fundamentally similar to any other

pump, compressor, blower, ejector, and similar devices. The essential elements of

flow and pressure relationships are analogous.

2.4.2

PUMPING SPEED

2.4.2.1 System Speed and Pump Speed

The pumping speed measured by the AVS Recommended Practice^ refers to the

inlet plane of the pump. Figure 6. Ducts connecting a pump to a chamber, baffles

and traps impede flow, resulting in a pressure difference or pressure drop. Under

molecular flow conditions, it is common for a baffle or a trap to have a conduc-

tance numerically equivalent to the speed of the pump. Thus, the net speed at the

pumping port in the chamber can be easily half or a third of the pump speed.

Rg.6.

TEST .

DOME^

Recommended standard test setup.

2.4.2 Pumping Speed

123

In steady-state

flow,

throughput remains constant so that the maximum through-

put capacity of the pump is not affected by baffles, orifices, etc. The lower net

speed at the chamber results, of course, in higher pressure for any given gas load.

Because of

outgassing,

the gas load in high-vacuum systems is never

zero,

and the

ultimate pressure in the vacuum chamber, therefore, is always higher than the ul-

timate pressure of the pump

itself.

2.4.2.2 Speed Efficiency-Capture Probability

The speed is determined by the American Vacuum Society Practice as

5 = QI{P -

PoX

where Q is the flow rate (throughput) and

the ultimate pressure prior to the ex-

periment. Figure 6 shows the standard recommended test setup.

The pumping speed of vapor jet pumps is nearly proportional to the inlet area,

but the larger pumps are somewhat more efficient as shown in Figure 7. The en-

trance geometries of small and large pumps are not strictly similar. Thus, it is

possible that the largest pumps can be constructed with a speed efficiency of

50%

referred to the inlet plane rather than the conical surface, where pumping action

occurs. This can become an important consideration in systems where the desired

pumping speed is so high that there is simply not enough wall space available for

attaching additional pumps.

Rg.7.

\ */ir

\ "^

-X""

, — .

. _)i

X 1 '

• 2 1

• 3

0 25 50 75 100

PUMP DIAMETER, cm

Efficiency of large pumps (item 2—bulged body pumps).

125

124 Chapter 2.4: Vapor Jet Pumps [Diffusion Pumps]

Fig.

10"^

10'' 10 1

INLET PRESSURE. TORR

Throughput vs. inlet pressure graph.

2.4.2.3 Speed and Throughput

Pumping speed versus the inlet pressure relationship has caused some miscon-

ceptions about the pressure range in which vapor jet pumps can be used. Thus, it

is often assumed that the pumps are unstable at system pressures above

"^ ton.

Such judgments should not be made without regard to the pump size and system

gas load.

It is much simpler to see this relationship by looking at a throughput versus

inlet pressure graph. Figure 8. Any combination of throughput and pressure in-

cluded in the outlined region can be chosen for operation, provided that the

pumped gas admittance is restricted (throttled) at inlet pressures above approxi-

mately 1 X 10~^ torr. The pressure stability region can be extended even to 10"^

torr (dashed diagonal line in Figure 8) if the pumping speed is reduced. It also de-

pends to some extent on the size of the backing pump (as indicated in Figure 5).

2.4.2.4 Size Effects

Vapor jet pumps are made from 5 cm (2 in) to 120 cm (48 in) inlet flange sizes.

An obvious difference between the smaller and larger pumps is the distance that

the pumping fluid (oil vapor) must travel from the nozzle to the condensing sur-

face or pump wall.

The boiler pressure in small and large pumps is approximately the same be-

2.4.2 Pumping Speed

125

cause we are limited in maximum temperature of the oil to avoid thermal break-

down. Therefore, the vapor density at the nozzle exit is nearly the same for all

pumps. However, the vapor expands in both axial and radial directions and we

may assume that the density is inversely proportional to the square of the distance

from the nozzle. Thus, near the pump wall the jet is rare enough to be less effec-

tive in pumping gas molecules at higher pressures.

2.4.2.5 Speed for Various Gases

The pumping speed must be considered in relation to the partial pressure of each

gas species. When pumping speed is measured, the values obtained near the ulti-

mate pressure of the measurement system become meaningless. The total pres-

sure values cannot be used for obtaining speed due to the uncertainties of gas

composition and the condition of

the

gauge. The composite picture should look as

shown in Figure 9. Each gas has its separate speed and, what is more important,

separate ultimate pressure. The normally measured

"blank-off"

is due to pump

fluid vapor, cracked fractions, and perhaps water vapor remaining in the system.

Fig.

PUMPING FLUID CRACKED

PRODUCTS

PUMPING FLUID VAPOR

^-.

I I L,

I0-"

10-3 IQ-^ 10-5

INLET PRESSURE-TORR

Pumping speed for various gases that may be present in a test chamber.

10-

IQ-

126

Chapter 2.4: Vapor Jet Pumps [Diffusion Pumps)

Rg.lO.

100

1 1 1

HYDROGEN

•

HELIUM

•

AIR

1 1 1

I I I I I I I

A . A

• ^ ^

-.*

1 1 1 1

1 1—1 1

•

P-A

-^f"^*

• A

"" V

PUMP

32-90000

FLUID DC-709

POWER

20 KW

OAUOE 507,57MA

CORRECTED

FOR OAS

' 1 1

1 1

K)""

10" 10 ' 10'

INLET PRESSURE, TORR

Typical set of

results

for helium and hydrogen in a large pump. Pump Varian

HS 32-50,000:

fluid DC-705: power

20 kw; gauge Varian

507, 5 ma, corrected for gas.

Although the arrival rate, expressed

liters per second, should be inversely

proportional

to the

square root

molecular weight,

the

lighter gases

are not

pumped with the same efficiency as air (or nitrogen) and pumping speed values

for different gases do not peak at the same power setting. When actual values are

necessary, separate measurements must be made. Usually the gases present in vac-

uum systems, such as water vapor, carbon monoxide and dioxide, nitrogen, and

argon are pumped

approximately the same speed. Helium speed is about 20%

and hydrogen speed about 30% higher than that

air. The impedance of baffles

and traps

lower for lighter gases compared

air. Thus, the net system speed

values

for

lighter gases are relatively higher than those obtained

for

the same

baffle with air. Figure 10 shows a typical set of results for helium and hydrogen in

a large pump. Note that the maximum throughput

for

hydrogen

nearly three

times higher than for air, corresponding to the square root of molecular weight ra-

tio.

Thus, maximum throughput for argon is 15% lower compared to air.