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

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CHAPTER

2.3

Vacuum Blowers

Jay Richman

Consultant, Stokes Vacuum Inc.

2.X1

INTRODUCTION

Roots-type blower systems comprised

blowers

and

backing pumps

are

almost

universally used

vacuum systems where large

gas

loads

are

pumped

in the in-

termediate vacuum range and/or volumes must

evacuated

in a

relatively short

time.

They

are

used

primary pumps

intermediate vacuum levels,

for

rough

pumping

to the

vacuum level where high-vacuum pumps

can be

effectively

em-

ployed,

and,

where required,

backing

for

high-vacuum pumps.

some sys-

tems,

the

blower system provides

all

three functions. This section

intended

provide the user with

understanding

the pumping principles, operating char-

acteristics,

and

limitations

blower systems

assist

proper application

and

problem diagnostics.

2.3.2

EQUIPMENT DESCRIPTION

Roots-type blowers (mechanical boosters)

are

positive displacement machines

that employ two identical counterrotating impellers, configured

to run

with close

clearances between

the

impellers

and

between

the

impellers

and the

enclosing

casing.

The

impellers

may

have

two or

three lobes with cycloidal, involute,

ISBN 0-12-325065-7 Copyright

1998

Academic Press

$25.00

All

rights

reproduction

in any

form reserved.

98 Chapter 2.3: Vacuum Blowers

Fig.

TIMING

TYPICAL VACUUM BLOWER

CONSTRUCTION

Vacuum seals may be lip or face type. Labyrinth and shaft seals can be replaced by four face

seals to isolate the pumping chamber from the bearings and gears.

compound curve flank geometry. Figure

illustrates the typical construction of a

two-lobe blower. The impellers are mounted on bearings, are well balanced, and

normally run at rotational speeds in the range of 1800 to 3600 rpm. Lip or face

type seals are used for the atmospheric shaft seals. For critical applications, blow-

ers are available with hermetically sealed direct drive motors to replace the shaft

seals.

In one version of these motors, the rotor is mounted directly on the drive

shaft and isolated from the stator by a hermetic enclosure (Figure 2).

Pumping action is obtained by the impellers trapping pockets of gas at inlet

pressure between the impellers and casing, and transferring the gas to a higher-

pressure discharge port (Figure 3). The blowers are valveless, run without sealing

fluids (except in special cases), and close clearances are required to maintain

pumping efficiency by minimizing backflow. Clearance between impellers is main-

tained by minimum backlash timing gears, and clearances between impeller ends

and casing end plates are controlled by accurate positioning of the shafts in the

bearings.

2.3.2 Equipment Description

Fig.

Sealess blower drive.

STATOR

Rotor isolated from the stator

and

atmosphere by hermetically sealed, thin-walled shell. Blower

has a bypass valve to facilitate starting from atmosphere. Courtesy: Stokes Vacuum Inc.

Fig.

SUCTION

GAS

PRESS

TEMP

s-i

MVHR

Blower pumping principle.

DISCHARGE

GAS

PRESS

TEMP

MVHR

Two lobe impellers discharge four trapped volumes (per rotation of drive shaft) to higher-

pressure region.

100

Fig.

Chapter 2.3: Vacuum Blowers

Typical blower-backing pump performance.

1200

800

600

400

200

~^—r -^~r

CUT IN

PRESSURE

I FOREPUMP #1

Pumping speed curves for the same blower backed by two different-capacity backing pumps.

Blower systems consist of one or more blowers in series with dry-running or

liquid-sealed mechanical vacuum pumps that discharge to atmosphere. The blow-

ers provide high pumping capacities at low pressure and extend the operating

range of the backing pumps by a decade or more. Staging ratios (relative dis-

placements of blower and backing pump) range from 2 to 10 or more, depending

on operating pressure. The blowers also provide vapor compression that reduces

the amount of vapor backstreaming from liquid-sealed backing pumps.

Blank-off pressures of two-stage blower-mechanical pump systems range from

1 X

""^

to 1 X 10"^ torr depending to a large extent on backing pump blank-

off. Capacities range from 170 m^/hr to 30,000 m^/hr (and higher). Figure 4 il-

lustrates typical pumping speed relationship to pressure level of two systems that

employ the same blower with different capacity backing pumps.

2.3.3

BLOWER OPERATING PRINCIPLE

As noted, pumping action is obtained by the transfer of low-pressure pockets of

gas to a higher-pressure regime. At the instant that the pocket is exposed to the

blower exit port, high-pressure gas diffuses into the displacement volume to equal-

2.3.4 Blower Pumping Efficiency 101

ize pressure. Recompression is complete when the impeller has rotated 90 de-

grees and interstage volume between the blower and backing pump has been re-

duced to its original size. This action takes place four times per revolution of the

drive shaft when two lobe impellers are employed. Volumetric displacement (max-

imum theoretical pumping speed), m^/hr, is obtained by multiplying the four vol-

umes displaced by the rotational speed per hour (rph).

2.X4

BLOWER

PUMPING

EFFICIENCY

As noted, the blower has no valves and depends on close clearances to minimize

backflow from discharge to inlet pressure. The amount of gas that flows back

through the clearances reduces the net effective pumping speed. Efficiency is

measured by the ratio of the actual volume of gas pumped to the gross displace-

ment or theoretical maximum pumping speed.

E = ^ (1)

£=1-^

(2)

where E = blower pumping efficiency

= blower displacement, mVhr

SfB ^ total backflow, m Vhr

S^ = net pumping speed, m Vhr

P2 = interstage pressure, ton-

Pi = blower suction pressure, ton-

Total backflow,

5FB,

(mVhr) is comprised of flow through clearances, canyback

of gas on impeller surfaces, and reexpansion of minor volumes trapped between

impellers.

Efficiency is influenced by the following factors.

2.3.4.1 Pressure Level and Compression Ratio

Impeller clearances vary with blower size but typically are on the order of

0.25

mm.

At high pressures, with a compression ratio of

(or higher), backflow is

sonic and turbulent and is a function of upstream pressure. As suction pressure

decreases, flow may become viscous and backflow will be a product of the differ-

ential pressure times clearance conductance. With a further reduction in pressure.

102

Chapter 2.3: Vacuum Blowers

flow becomes transitional and finally molecular; flow resistance increases, con-

ductance backflow becomes nearly constant and compression ratio increases. The

results of this phenomenon can be observed in Figure 4, where, at pressures be-

low 10 millitorr the pumping speed of the blower is essentially the same despite

the difference in pumping speeds of the backing pumps.

2.3.4.2 Gas Species

Conductance through clearances varies not only with the pressure regime but also

with the type of

gas

being pumped. In the turbulent and viscuous flow ranges, flow

is inversely proportional to gas viscosity. As an example, the viscosity of air =

180 and hydrogen = 87 micropoise, and conductance and backflow for hydrogen

would be twice that for air. In the molecular flow range, conductance is propor-

tional to the square root of the ratio of the molecular weights, and conductance

backflow for hydrogen would be 3.79 times that for air. The effect of the in-

creased backflow (and compression ratio) on pumping efficiency when pumping

air and hydrogen at two different compression ratios is illustrated in Figure 5.

Rg.5.

Blower pumping speed efficiency curves.

100

5 80

S ^°

o 60

o 50

d) 30

i 20

10 10 10 10 10 10 10 10

INLET PRESSURE (TORR)

Effect of staging ratio (R) and gas species (air and hydrogen) on pumping speed efficiency.

2.3.4.3 Rotational Speed

Theoretical pumping speed increases with rpm, but clearances and conductance

backflow remain constant for a given compression ratio. As RPM increases, effi-

ciency increases because backflow becomes smaller relative to displacement.

2.3.5 Blower Pumping Speed Calculations

103

2J.4.4 Gas Carryback

Gases adsorbed on impeller surfaces and trapped in minor impeller volumes are

carried from the high-pressure interstage and released at low inlet pressure. The

effect of carryback at pressures above 100 mtorr are minimal. Polishing the im-

peller surfaces can significantly reduce carryback.

2.3.5

BLOWER PUMPING SPEED CALCULATIONS

The net effective pumping speed of the first-stage blower in a blower system is

a function of blower efficiency and the efficiency of the backing pumps. In stan-

dard two-stage systems, the pumping speed of the system is usually published by

the manufacturer. For nonstandard, two-stage, and multistage systems, the pub-

lished pumping speed of the backing pump is the starting point for calculations.

The pumping speed of

the

blower adjacent to the backing pump is calculated, and

in multistage systems this progresses until the speed of the first-stage blower is

determined.

The information usually supplied by the blower manufacturer for purposes

of pumping speed calculations is a zero-flow maximum compression ratio

(KQ)

curve. Figure 6 illustrates typical compression ratio curves at three rotational

Fig.

Typical

zero flow compression ratios.

'—r

2764 RPM

2215 RPM

^"1

10 10 10 10 10

P2 (TORR)

10 10 10

Data taken with three blower rotational speeds. Backflow (slip) decreases as a proportion of

displacement with increased rotational speed.

104 Chapter 2.3: Vacuum Blowers

speeds. These compression ratio curves are obtained by blanking off the blower

inlet (zero flow) and, with the blower and backing pump running, admitting air

or other gas into the interstage volume between blower and backing pump. The

interstage and resulting blower suction pressures are measured, and a curve is

plotted of

(compression ratio ^2/^1) ^s a function of the interstage pressure

P2.

Following Equation (2) and letting Pj/Pi = K

E=l-

KS^^/S^

(3)

When flow at the inlet is zero, backflow is balanced by pumping capacity, net

pumping speed is zero, efficiency is zero, and

Ko = 5D/5FB (4)

The general equation for blower efficiency is derived from Equations (3) and (4).

E=l~ K/KQ (5)

Relating the throughput of the backing pump to blower pumping speed:

S2P2

= {ES]))Pi (ignoring temperature effects)

K=(ESi^)/S2 and K = EK^) (6)

(5D/'^2

—

^D ~ theoretical compression ratio)

Combining Equations (5) and (6),

E = —^^- (7)

Sample pumping speed calculation:

= blower displacement = 600 m-^/hr

^2 = backing pump pumping speed at

= 200 m^/hr

P2 = 1 torr

= 44 From Figure 6 at P2 and 1800 rpm, for air

KD = SD/S2 = 600/200 = 3 | 5, = £5D = 0.94 * 600 = 563 mVhr

Ko 44 , 52P2 200*1

E = '— = = 0.94

P^=-^-^

= = 3.6*10~4orr

ATo

+ ^D 44 + 3 ' ^ 5i 563

2.3.6

POWER REQUIREMENTS

Power requirements in blower pumping systems are somewhat unusual. Many

systems run at low operating pressure for most of their operating cycle. Time at

2.3.6 Power Requirements

105

Fig.

Power comparison for single- and two-stage blower systems.

1 STAGE SYSTEM

PV=150,000

(TORR)

200

(TORR)

100

STAGE

2 STAGE SYSTEM

PV TOTAL=100,000

100 X

500

50.000

STAGE

PV=50

1000=50.000

1000

V, (INLET)

1 STAGE BLOWER SYSTEM

Staging ratio of 2 assumed for this illustration.

500 1000

V, (INLET)

2 STAGE BLOWER SYSTEM

high pressure may be relatively short, and the large motor required to handle the

high gas loads may be underutilized. Also, large motors running at low loads can

cause power factor problems. To reduce initial and operating costs, systems have

been designed with motors that have been much overloaded at peak loads. These

have performed satisfactorily, but to prevent equipment damage safe time at over-

load must

carefully predetermined. High-temperature windings and thermal

overload protection must be used.

Power savings can be obtained by the use

two blowers in series or a blower-

backing pump combination

obtain

given pressure rise. Contributing

this

phenomenon is the pressure-volume relationship of Roots-type blowers.

The rotary positive blower follows

"square card" diagram. That is,

pres-

sure rise is plotted against the constant displacement volume, the resulting curve

throughout one complete cycle would

rectangle under ideal conditions.

Figure

with

single blower, and

pressure rise

200 torr,

PV =

150,000.

If two stages are used with

assumed compression ratio

2, PV would equal

100,000. Power consumption is a direct function of

PV,

and the savings would be

significant. The cost of using two blowers

series may outweigh the power sav-

ings,

but such systems have been built for factors other than economy and power

savings have been achieved.

The power required by a blower is the sum

the power required for gas com-

pression plus the power required to overcome the friction of bearings, seals, gears,

etc.

The values for friction losses must be obtained from the blower manufacturer.

Power for compression is calculated as follows:

/ny

3.7 * 10-5 ^ 5^ ^ PdiPilPi)

1)]

PilPx^r, P\[r-\]=P2-P^

and

inv

3.7 * 10-5 ^ ^^ ^ (p^

p^)

(8)

106 Chapter 2.3: Vacuum Blowers

Note that power for compression is dependent on blower displacement and delta

P but independent of gas species. In the absence of specific information, approxi-

mately 20% may be added to the calculated power for the power to overcome fric-

tion but it would be best to contact the manufacturer for this value. In addition

to these power requirements, each blower has lower and (as might be expected)

upper power limits. The lower limit is established by the power required to over-

come friction and impeller inertia when accelerating the impellers from a stand-

ing start to operating speed with no gas load. The upper limit is dictated by the

mechanical strength of the operating components: impellers, gears, shafts, bear-

ings,

etc. These data must also be obtained from the manufacturer.

2.3.7

TEMPERATURE CONSIDERATIONS

2.3.7.1 Maximum Allowable Temperature Rise

A maximum allowable continuous gas temperature rise is established by the

blower manufacturers to prevent seizure from excessive differential expansion be-

tween the impellers and casing. The maximum rise allowed varies with blower

size and manufacturer but a fair average may be 100°C, based on a maximum in-

let temperature of 37°C. The differential expansion occurs because both the cas-

ing and impellers absorb significant amounts of the heat of compression, but the

impeller is partially insulated by vacuum and expands at a greater rate than the

casing. Gas temperature rise may be calculated as follows:

= degrees Kelvin discharge temperature

Ti = degrees Kelvin inlet temperature

k = c^lc^ =1.4 air

F = temperature rise factor

The temperature rise factor, F, is obtained empirically and accounts for heat

loss by radiation and convection. Figure 8 illustrates typical (varies with the

blower) factors related to inlet pressure, compression ratio, and rotational speed.

Note that as compression ratio decreases and rotational speed increases, the fac-

tor increases. These trends increase efficiency, which tends to decrease the theo-

retical temperature rise. However, the increased efficiency also results in less

thermal losses, hence an increase in the factor.

When inlet temperatures are above 37°C, the maximum allowable temperature