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

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2.3.7 Temperature Considerations

107

Rg.8.

Temperature rise factor.

10''

10'

10-^h

10"

i- R=2 < 2300 RPM^ '!!l^--^sr^'*!r"'^

! 11800 RpM-rr^:^-':::'-^—'

,^^'' ^^^..^^^^^'^ ^-2300 RPM

i y^ ^—1800 RPM

TYP TEMPERATURE RISE FACTOR

(BASED ON AIR) VS INLET PRESSURE.

BLOWER HANDLING AIR OR DIATOMIC GASES. _

.. 1 1 1 i 1 1 1 ;

10"

lO'-

10'

10^

10-^

INLET PRESSURE (TORR)

Empirical data used to modify the theoretical temperature rise.

rise,

Ta, must be derated as follows:

la corrected = Ta - 0.66(rin - ITQ)

(10)

^in = degrees C inlet temperature

Because thermal expansion is time dependent, in pumpdown situations it may

be possible to exceed the maximum allowable temperature rise provided that the

time from blower start pressure to the safe operating pressure does not exceed

a certain limit. The blower manufacturer should be consulted for this limit. As a

word of caution, if time between pumpdowns is very short and the cycle is repeti-

tive,

it would be best not to exceed the continuous allowable limit.

2.3.7.2 Maximum Allowable Discharge Temperature

The maximum allowable discharge temperature is based on the limits established

by materials of construction such as seals, lubricants, etc. The manufacturer's rec-

ommendations should be followed.

2.3.7.3 Maximum Allowable Compression Ratio

The curve in Figure 9 depicts a typical range of allowable pressure ratios versus

inlet pressure based on the allowable temperature rise and average temperature

108

Chapter 2.3: Vacuum Blowers

Fig.

Maximum allowable compression ratio.

10''

10 10 10

INLET PRESSURE (TORR)

Typical values based on "average" temperature rise factor.

rise factors.

It can be

used

for a

first approximation

establish blower-backing

pump relationships subject

to a

more accurate analysis.

2.3.7.4 Discharge Gas Cooling

Coolers that employ water-cooled surfaces in close proximity to the impellers

the

discharge port may

used

cool

the

interstage gas that surges into

the

pumped

gas pockets. The resulting cooling effect allows

higher compression ratio.

2.3.8

FLOW

AND

COMPRESSION RATIO CONTROL MECHANISMS

Several arrangements

are

available

keep power requirements

low and the

heat

of compression within allowable limits.

2.3.8.1 Sequenced Start Systems

In sequenced start systems,

the

backing pumps

are

started first,

and

blowers

are

brought

line (starting with

the

first upstream blower)

safe operating pres-

2.3.8 Flow and Compression Ratio Control Mechanisms 109

sures by pressure switches or timers. In some variations, with several stages, two

or more blowers may be switched into parallel at low pressure to provide more

pumping speed.

2.3.8.2 Inlet Throttling

In inlet throttling systems, the blower and backing pump run at full speed contin-

uously. A throttling valve at blower inlet controls blower inlet pressure to main-

tain an allowable temperature rise or, if power limited, the differential pressure. In

the former case, over-temperature switches are still required. The valve is par-

tially open at high pressure and may be fully open at low pressure.

23.B.3 Blower Rotating Speed Control

Blower rotating speed control systems maintain allowable compression ratios by

gradually increasing blower rotating speed as the inlet pressure decreases. Fol-

lowing are some typical speed control systems.

Variable-speed AC or DC motors and drives.

Hydraulic or magnetic slip-type coupling between the blower and drive motor.

2.3.8.4

Bypass

Valves

In a bypass valve type of system (Figure 10), an integral bypass valve with spring

or deadweight loaded poppet or external bypass throttling valve, feeds a portion

of the blower discharge gas back to the blower inlet. The valve is adjusted to pro-

vide a fixed safe differential pressure across the blower. When the throughputs of

the blower and backing pump are equal, the valve closes. The valve closes at pres-

sure P = (P2 - PijIiPilPx) - 1. Figure 10 illustrates a stock blower with integral

bypass. The blower and backing pump run at full speed from atmosphere to ul-

timate pressure. Blower power required is a function of the pressure difference

selected.

Inlet-throttling, blower rotating speed control, and bypass valve systems can

produce higher pumping speeds from atmosphere to safe operating pressure than

a sequenced start arrangement; see Figure

11.

Assuming that the systems are ar-

ranged to provide the same safe differential pressure, the gain in pumping speed

would be the same for all three systems.

Fig. 10.

Blower with integral bypass valve.

-BYPASS

VALVE

GAS

PRESS

TEMP

MVHR

DISCHARGE

GAS

PRESS

TEMP

MVHR

Safe differential pressure established by spring selection.

Fig.

11.

Typical

blower-backing pump speed performance with compression ratio

control.

3000

I >—,

{/)

2500

2000

1500

1000

500

"1 ^n r-

1 n ^

10 ' 10 10

PRESSURE

(TORR)

Both pumps run from atmosphere. Shaded area indicates increase of pumping speed (using

ratio control).

23.8 Flow and Compression Ratio Control Mechanisms

111

2.3.8.5 Cool Gas Feedback Blowers

In cool gas feedback blowers, the blower is arranged to permit the admission of

cool gas into the trapped volume of the blower where it mixes with and reduces

the temperature of the discharge gas. These blowers are generally used at pres-

sures of 200 torr and higher but they have also been used at lower pressures. The

advantage of these blowers is that at higher pressues a compression ratio of 4:1

may be maintained, compared to 2.2:1 for uncooled blowers. This feature may

make it possible to eliminate one blower stage when used alone to discharge to at-

mosphere or as backing pumps in chain blower systems. The penalty paid is high

power consumption, as discussed in Section 2.3.6.

Atmospheric air may be used for feedback or if special gases are being pumped

that must be preserved, a portion of the discharge gas may be fed back through a

cooler (see Figure 12).

Fig. 12.

COOLGAS FEEDBACK

BACKING PUMP

-ATMOSPHERIC AIR

BLOWER

BACKING PUMP

(WHEN REQUIRED)

Cool gas feedback systems: (a) Pumped gas recirculation system, (b) Atmospheric air cooled.

112 Chapter 2.3: Vacuum Blowers

2.3.9

LIQUID-SEALED BLOWERS

Blowers can be arranged to use water or other fluids (such as oil) injected into

the inlet port to act as a cooling and sealing medium. With water injection, oper-

ating pressure with a single-stage blower is approximately 100 torr and with a

two-stage blower, 50 torr. Pressure limit is generally based on the water vapor

pressure at a given temperature. Lower pressures are attainable with lower-vapor-

pressure fluids. These blowers compete with liquid ring pumps. They generally

use less water and power than liquid ring pumps, but their initial cost may be

higher. As with liquid ring pumps, consideration must be given to the cost of dis-

posal of contaminated liquids and the cost, when required, of a closed loop sys-

tem to cool and recover expensive liquids or to conserve water.

2.3.10

SELECTED SYSTEM ARRANGEMENTS

The broad range of blower and backing pump types available provides the sys-

tem designer with an opportunity to optimize systems for application require-

ments. Following are some interesting examples of systems developed for spe-

cific applications.

2.3.10.1 Multistage (chain blower) Systems

Multistage systems are generally employed when large volumes must be pumped

down in a relatively short time. The multiple stages are used to maintain the al-

lowable compression ratio across each stage when starting from atmosphere. An

example is a 3-stage chain blower system (Figure 13) built to evacuate a 1308-m'^

chamber from 760 torr to 0.01 torr in two hours with a 20 torr L/S air leak. Be-

ginning with the last stage, each stage is sequentially started with a few seconds

time delay between stages to reduce initial power surge. With an approximately

2:1 staging ratio between the three stages, the interstage relief valves maintained

a maximum allowable differential pressure of 400 torr. Excess air was discharged

through the valves until the throughput of each backing stage equaled that of

the preceding stage. The interstage heat exchangers removed the heat of compres-

sion, maintained an approximately constant compression ratio, and limited the in-

let temperatures to the downstream stages. With this type of system, the first-

stage pumping speed of 10,270 mVhr was essentially maintained from atmosphere

2.3.10 Selected System Arrangements

113

Fig. 13.

Multistage blower pumping system.

V__^

^""^ HX HX

-INTERSTAGE-

RLF VALVES

SILENCER

EKl

BLO-1

760 TO 2 TORR 10,272 M^HR

BLO-2

5.246 M^/HR

MECH PUMPS

3.056 M^/HR

2 TORR TO BLANKOFF / 15,518 M^/HR

Three-stage, series-parallel operation. Pumpdown of 1,308 m^ space chamber from 760 to

0.01 torr in 100 minutes with 20 torr L/s air leak. Blowers are parallel at 2 torr to handle air

leak at 0.01 torr. Courtesy: Stokes Vacuum Inc.

down to 2 torr. At 2 torr the first and second stages were valved in parallel to pro-

vide 15,518 mVhr to handle the air leak, which at 0.01 torr equaled 7200 mVhr.

Actual time to 0.01 torr was 100 minutes.

Peak power requirements for stages 1 and 2 were 159 kw and 75 kw respec-

tively. Mean average power over a 20-minute period was approximately 60% of

peak. Motors selected had ratings of 100 kw and 56 kw. Time at overload was ap-

proximately 3 minutes. High-temperature insulation was used for the windings,

and special over-temperature protection was provided by sensors buried in the

windings.

2.3.10.2 Slush Hydrogen Pumping System

The slush hydrogen pumping system (Figure 14) was designed to pump

10,200 m-Vhr of pure hydrogen at 52 torr from a slush hydrogen source at

14° Kelvin. Through heat gain, the temperature at the inlet was approximately

249° Kelvin. To handle the cold gas, a three-stage liquid-sealed blower sys-

tem was selected, using ethylene glycol for the sealing liquid. The glycol was

circulated through a closed-loop system, where it was heated to 40°C to main-

tain the blowers at a reasonable operating temperature. Liquid sealing increased

blower efficiency, permitting the use of smaller blowers and direct discharge to

atmosphere.

114

Chapter

2.3:

Vacuum Blowers

Fig. 14.

Slush hydrogen pumping system.

STAGE 1

15,300

MVHR

•*•

STAGE 2

10.200 M^/HR

56 KW

224 KW

START AT-

75 TORR

PERFORMANCE SPECS-

STAGE 3

5.100 MVHR

CLOSE AT

400 TORR

• 10,200 M^/HR AT 52 TORR.

• PUMP 249X H2 CONTINOUS

IN RANGE FROM 40 TO 60 TORR.

• 25 TO 760 TORR TOTAL

PRESSURE RANGE.

(—1 AIR/GLYCOL

U SEPARATOR

DISCHARGE

CHECK VALVE

SILENCER

Heated glycol injected into the suction ports of the blowers for sealing and to maintain blow-

ers at reasonable operating temperature. Courtesy: Stokes Vacuum Inc.

At start of pumpdown, stages 2 and 3 were in operation with a portion of the

discharge from stage 2 bypassed around stage 3 through a pressure-sensitive

check valve directly to the air-giycol separator and the silencer. At approxi-

mately 400 torr, the check valve closed, and the discharge of stage 2 was handled

entirely by the third stage. At 75 torr, the first stage was started and hydrogen flow

commenced. Pressure decreased to 52 torr as pumping progressed.

2.3.10.3 Product Dehydration System

The product dehydration system (Figure 15) was designed to hold 6 torr with

11.4 kg/hr air plus 100 kg/hr water vapor. A blower-condenser-mechanical pump

system was employed to provide the net effective pumping speed of 18,500 m^/hr.

The blower was used to increase the pressure at which the condensers would op-

erate—

this case, from 6 to 18 torr. Two condensers, each with receivers, were

required to handle the entire water load, with a chiller furnishing 10°C cooling

water for the condensers. An automatically operated pressure control valve lo-

2.3.10 Selected System Arrangements

115

Rg.l5.

Product dehydration system.

6 TORR

18 TORR

z::

IQ-C

WATER

CHILLER

30 KW

23.700

MVHR

rPERFORMANCE SPECS-

• 11.3

KG/HR

AIR

•100

KG/HR

H2O

VAPOR

AT 27'C 8c 6

TORR

PRESSURE

CONTROL

VALVE

EXHAUST

I 1 VALVL /-->^

^ I ri O

wc^

hH I

22 KW

a I—I c^FT AT

wcv

SET AT

82*C

1,240

MVHR

13*C

High water vapor load (100 kg/hr) plus small air load (11.3 kg/hr) to be pumped at 6 torr. The

backing pump handles the air load plus partial pressure of water vapor at 10°C condenser tem-

perature. Courtesy: Stokes Vacuum Inc.

cated between the condensers and the mechanical pump maintained a pressure of

18 torr in the condensers to prevent reflux of the condensed vapor. Most of the

water was removed by the condensers, and the backing pump was small because

it was only required to handle the air load plus the small water vapor load corre-

sponding to the partial pressure of the water vapor in the condenser. The backing

pump was operated at 82°C.

The condensing system was designed to operate for eight hours. With the addi-

tion of automatic valving to drain the receivers while operating, the system could

have been made to operate on a continuous basis.

CHAPTER

2.4

Vapor Jet Pumps

[Diffusion Pumps]

Marsbed Hablanian [Retired)

Varian Associates

Many high-vacuum pumping systems include a vapor jet pump and a mechanical

pump.

Mechanical vacuum pumps are used to remove about 99.99% of the air from

the chamber, reducing the pressure in the vacuum system to the correct operating

range. This is commonly termed rough pumping or roughing. After suitable flow

and pressure conditions are reached (usually

10 ~^

to 10"^ torr), the vapor jet

pump can take over. The mechanical pump is now used to maintain proper dis-

charge pressure conditions for the vapor jet pump at the foreline connection. This

is called backing or forepumping.

Vapor jet pumps are normally used when constant high speeds for all gases

are desired for long periods of time without attention. They are essentially vapor

ejectors specialized for high-vacuum applications. In the past, the gas vapor

dif-

fusion aspects and the vapor condensation have been overemphasized, giving the

original name for the pump.

The basic design form was stabilized approximately 70 years ago. Modern

pumps direct the vapor stream at high velocity in the pumping direction. Pumped

gas entrainment into this stream, in principle, is not very different from steam

ejectors or other vapor pumps or compressors. The original pumping fluid was

mercury. Oil-like substances were first used in 1928.

The following discussion is devoted mainly to oil vapor jet pumps.

1Q98

by Academic Press

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116