INTRODUCTION TO VACUUM
PUMP CAPACITY RATINGS
INLT VOLUME CALCULATIONS
ALTITUDE EFFECTS ON LIQUIDRING VACUUM PUMPS
EFFECT OF SATURATED AIR SERVICE ON THE CAPACITY OF VACUUM PUMPS
EFFECT OF SERVICE WATER TREMPERATURE ON THE LIQUIDRING VACUUM PUMPS
LIQUIDRING VACUUM PUMP PRINCIPLE OF OPERATION
RADIAL BLOWER PRINCIPLE
SIDE CHANNEL PRINCIPLE
ROTARY VANE PRINCIPLE
TEMPERATURE CONVERSION TABLE
Introduction to Vacuum
atmosphere exerts a pressure upon us, known as the atmospheric pressure,
which can be measured in a number of ways. At sea level the standard pressure
is 14.7 psia or 29.92" Hg or 760 mm of Hg (Torr). Because the barometric
pressure varies, the above sea level pressures are used as a reference
The term "vacuum" is used to describe the zone of pressure below atmospheric
pressure. The most common standard to measure rough vacuum is inches of
mercury ("Hg), which can be measured in two different ways. One method
is as "Hg gauge" ("HgV), where the scale starts at 0 "Hg
(atmospheric pressure) and goes up to 29.92 "Hg, which is perfect
vacuum. The other way is to measure in "Hg absolute ("HgA),
which is a gauge with a reversed scale. In this case the scale on the
gauge reads 29.92" Hg at atmospheric pressure and 0 "Hg would
be perfect vacuum. Note that a perfect vacuum is not possible on earth,
no matter which vacuum pump is used.
To show the
relationship between "Hg gauge and "Hg absolute we can use the
26 "Hg gauge at sea level would be 29.92 - 26 = 3.92 "Hg absolute.
Because of the two different ways of measurement, the customer should
be asked if they mean "Gauge" or "absolute". It is
important to know which scale is used because the wrong assumption can
mean a large error.
When we operate
in the higher vacuum range (low absolute pressure) it is more common to
measure in Torr. 1 Torr = 1 mm Hg and is always absolute pressure. 25.4
mm = 1 ", therefore calculating the barometric pressure gives 29.92
X 25.4 = 760 Torr. An absolute pressure gauge reading in Torr reads 760
Torr at atmospheric pressure, which is zero vacuum and would read 0 Torr
at perfect vacuum.
table below shows the relationship between the different pressure measurements.
Hg) = 760 - ("HgV x 25.4)
= "HgA x 25.4
= psia x 51.7
= mbar x .75
= "wc x 1.868 "
Absolute = mm Hg / 25.4
= 29.92 - "HgV
= psia x 2
= mbar x 0.0295
= "wc x 0.0734
(gauge) = 29.92 - (mm Hg / 25.4)
= 29.92 - "HgA
= 29.92 - (psia x 2.036)
= 29.92 - (mbar x 0.0295)
= "wc x 0.0735
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Pump Capacity Ratings
for vacuum pumps is specified in a couple of different ways, depending
on the type of vacuum pump and manufacturer. It is important to know the
ACFM ("Actual Cubic Feet per Minute) inlet capacity at a specific
vacuum pumps are all rated in ACFM, the actual capacity at the different
vacuum levels as shown on the individual pump performance curves.
expressed in CFM or SCFM (Standard Cubic Feet per Minute) can be very
misleading because we have to take into consideration the volumetric efficiency
of the pump at a specific vacuum level (refer to example below).
pumps are generally rated in CFM of free air displacement, which is the
theoretical displacement at 0 "Hg vacuum.
of small rotary vane pumps, such as Gast, rate their pumps in SCFM at
different vacuum levels. In order to convert these values to ACFM refer
to the calculations on the following page.
pumps are rated by the theoretical displacement in CFM, known as piston
be able to compare capacities of different pumps we need to calculate
the actual capacity (ACFM) at different vacuum levels. To be able to do
this we need to know the volumetric efficiency of the pump at a
specific vacuum level (request this form from the manufacturer), which
can vary anywhere between 90% and 40%, depending on the pump design.
For example, if a specific pump has a displacement of 100 CFM and the
volumetric efficiency at 28 "Hg gauge is 80%, the actual pump capacity
at 28 "Hg would be 80 ACFM. These values can also be obtained from the
individual performance curves, if available.
Inlet Volume Calculations
a great deal of confusion about the terms SCFM and ACFM:
SCFM is measured at standard conditions (68 oF, 20 oC, 29.92 "Hg
or 14.7 psia).
ACFM is measured at actual inlet conditions.
Conversion from SCFM to ACFM and vice versa, is derived form the gas laws,
specifically Boyle's law.
states that the volume and pressure of a gas will change in inverse proportion
to one another. i.e. if the pressure in a system decreases (higher vacuum)
then the volume the gas occupies will increase proportionally according
to the following formula:
P1 V1 = P2 V2
The product of the initial pressure and volume equals the product of the
final pressure and volume. When using the above formula in calculations
the values must be in absolute terms ("Hg absolute or Torr).
1: Convert 20 SCFM of air to ACFM at a vacuum level of 25 "Hg
at sea level.
Solution: First convert 25 "Hg gauge to "Hg absolute:
P2 = 29.92 - 25 = 4.92 "HgA or 125 Torr
Use above formula to convert:
29.92 x 20 SCFM = 4.92 x V2 ACFM
V2 = (29.92/4.92) x 20 = 121.6 ACFM
2: The customer has a 200 ACFM pump installed which holds a vacuum
level of 22 "Hg and they want to increase the vacuum level to 26
Convert the vacuum levels quoted to absolute terms:
P1 = 29.92 - 22 = 7.92 "HgA
P2 = 29.92 - 26 = 3.92 "HgA
Use above formula to convert:
7.92 x 200 ACFM = 3.92 x V2 ACFM
V2 = (7.92/3.92) x 200 = 400 ACFM
in order to increase the vacuum level to 26 "Hg the customer would
have to double the pump capacity from 200 ACFM to 400 ACFM.
3: Show the effect of pressure loss in inlet piping and filters on
pump capacity. The customer requires a total capacity of 100 SCFM at a
vacuum level of 24 "Hg at sea level. Inlet line losses, including
inlet filters are 2 "Hg.
First convert to ACFM based on 24 "Hg:
P2 = 29.92 - 24 = 5.92 "HgA
29.92 x 100 SCFM = 5.92 x V2 ACFM
V2 = (29.92/5.92) x 100 = 505 ACFM
Without any line losses we would require a vacuum pump sized for 505 ACFM
at 24 "Hg. However, we must overcome these losses and it will increase
the required capacity substantially:
P2 = 29.92 - 24 - 2 = 3.92 "HgA
29.92 x 100 SCFM = 3.92 x V2 ACFM
V2 = (29.92/3.92) x 100 = 763 ACFM
This indicates that the pump capacity needs to be 33% higher to overcome
the 2 " pressure drop at 24 "Hg.
above calculations are based on constant temperatures. If the temperature
varies substantially from one condition to another, a correction needs
to be made. If this is the case, contact G.E.E. for more details.
below shows the effect of undersized inlet piping and dirty inlet filters
on the capacity of a pump at four different inlet pressure drops at a
vacuum range from 15 - 28 "Hg at sea level.
Level Capacity Loss At Inlet Pressure Drop
example below as well as the graph on the next page showing the expansion
factor (F), are computed assuming that the pump is evacuating a closed,
dry vessel. No leaks or presence of moisture have been considered.
The volume of a tank including connecting piping is 750 ft3. Initial
atmospheric pressure (760 mm HgA [Torr]). Vacuum level required is 24
"HgV (150 Torr). The amount of gas to be removed from the vessel can be
calculated by using the following formula:
Q = V x Ln(P1/P2) in which,
Q = total amount of air to be removed (ft3)
V = volume of reservoir plus connecting piping
P1 = Initial pressure
P2 = Required pressure
Ln(P1/P2) = Ln(760/150) = Ln 5.067 = 1.62
When using the log graph, locate the required vacuum level on the chart
(150 Torr) and read expansion factor (F) off the scale (1.62).
amount of air to be removed to reduce the pressure inside the vessel from
atmospheric pressure to a vacuum level of 24 "HgV:
750 x 1.62 = 1215 ft3
If evacuation is required in three minutes, the average pump capacity
from 760 to 150 Torr should be:
1215/3 = 405 ACFM
Therefore, select a pump with this capacity.
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Altitude Effects on Liquidring Vacuum Pumps
curves for vacuum pumps are always derived relative to atmospheric pressure
at sea level. When a vacuum pump operates at altitudes higher than sea
level, the atmospheric pressure decreases. The important thing to remember
when you encounter applications at altitude is to measure the vacuum level
relative to barometric pressure. Refer to the example below to see how
this affects the calculation of vacuum level. As a quick rule of thumb,
you can assume that for each increase of 1,000 feet of elevation, the
barometric pressure will decrease by 1 "Hg.
Example: The installation site is located in Denver, CO (elevation 5280 ft). A
capacity of 750 ACFM at 20 "Hg gauge is required for the application.
What is the equivalent vacuum level at sea level?
The following formula helps to calculate the equivalent vacuum level:
Pref = P1 x (29.92/P2) which
Pref = corrected vacuum level
P2 = barometric pressure at altitude
= 25.3 "HgA (chart)
P1 = P2 - required vacuum level at altitude
= 25.3 "HgA - 20 "Hg = 5.3 "HgA
Pref = 5.3 "Hg x (29.92/25.3) = 6.3 "HgA or 23.4 "HgV
Therefore, select the pump with a minimum capacity of 750 ACFM at 23.4 "HgV.
Effect of Saturated Air Service on the Capacity of Liquidring Vacuum Pumps
graphs illustrated below show the average condensing factors for vacuum
pumps in saturated air service. When handling air/water vapor mixtures,
the pump capacity will increase depending on the saturated air temperature
as well as the sealing liquid temperature as well as the sealing liquid
temperature entering the pump.
Consider a liquidring vacuum pump operating at 27 "HgV (75 Torr)
with a dry air capcity of 200 ACFM of dry air when using 59 0C seal water.
If the same pump handles saturated air at 86 0F and seal-water of 59 0F,
the actual pump capacity would be:
CFM (dryair) x condensing factor = Actual CFM
200 x 1.37 = 274 ACFM
Effect of Service ater Temperature on the Capacity of Liquidring Vacuum Pumps
of the sealing fluid (water) can have a dramatic effect on the capacity
of liquidring vacuum pump.
data are based on using 59 0F water as the sealing liquid for the vacuum
pressure of the service liquid has a direct influence on the vacuum pump
capacity. When the vapour pressure of the service liquid is less than
that of water at 59 0F, the pump capacity will increase and when the vapour
pressure of the service liquid is higher, the pump capacity will decrease.
below allow the user to select the correct pump for the application in
question, while taking capacity correction factors into account.
A high seal-water temperature (approx. 860F) can have a significant effect
on the capacity of the pump. For a single stage pump, the capacity correction
factor when operating at 75 Torr (27 "HgV), is 0.76. This means that
if the published capacity of the pump is 300 ACFM at 75 Torr, the pump
will have a corrected capacity of 228 ACFM with the higher seal water
Liquidring Vacuum Pump Principle of Operation
1: In a cylindrical housing, partially filled with sealing liquid,
a multi-blade impeller on a shaft is positioned eccentrically. Port plates
with inlet and discharge openings are positioned on either side of the
2: A liquid ring is created by the centrifugal force generated by
the rotating impeller. This force holds the liquid ring against the inner
wall of the pumping chamber. Since the impeller is located eccentric to
the pumping chamber, the
depth of entry of the blades into the liquid ring decreases and increases
as the impeller rotates. This creates increasing impeller cell volume
on the inlet port side, creating a vacuum. On the discharge port side,
the impeller cell volume decreases, as the blades move further into the
liquid ring, increasing the pressure, until discharge takes place through
the discharge port. A continuous flow of fresh sealing liquid is supplied
to the pump via the sealing liquid inlet.
Radial Blower Principle
operate using a dynamic principle (the velocity of air is changed into
pressure). This method has proved very successful for the generation of
both low vacuum and pressure combined with relatively large capacities.
Single stage and multi stage models, as well as those with direct or gear
box drive, produce vacuum up to 92 "H2O or pressure up to 120 "H2O.
Capacities range from 60 to 1770 cfm. Radial blowers are air-cooled and
non-contact operation means there is no wear, making the design virtually
As soon as
the impeller starts rotating, the air in the blade chamber (1) of the
impeller (2) is directed in a centrifugal outward movement and exits at
the edge of the impeller (3). Therefore, in the center (4) of the impeller,
suction is created causing air to flow in from the inlet port (5). At
the hub (6) this entering air is deflected from an axial into a radial
direction, and enters into the blade chambers (7).
Due to the high speed on the impeller circumference, the air flows outward
into the spiral housing (8). At this point the velocity is reduced with
part of the energy transferred into compression energy. Inside the spiral
housing an airstream is created. Its energy comes partly from its high
speed and partly from the produced over pressure. The airstream flows
through the outlet (9) of the spiral housing into the pressure line and
is transported to the workplace.
operation, a pipeline is connected to the suction port (5) of the blower.
When the suction opening is throttled, a vacuum is produced in the suction
line. With this method of operation, the impeller again produces a pressure
increase, but this time from vacuum to atmospheric pressure.
At a higher
motor speed, for example 60 Hz operation instead of 50 Hz, both capacity
and pressure difference are increased. The performance of the blower and
subsequently the required motor power are also increased considerably.
Side Channel Principle
channel principle is also based on the method of dynamic compression (transforming
flow energy into pressure energy). The
side channel design is suited for applications requiring both, pressure
and vacuum. Depending upon model size-single stage or two stage-vacuum
up to 20 "H2O in suction operation and pressure mode can be obtained.
The capacities range from 10 to 650 CFM. Non-contact operation means side
channel blowers are practically free of wear and maintenance. The pumping
medium is not contaminated from carbon dust or oil, as occurs with dry
running or oil lubricated rotary vane pumps.
working chamber (1) has a circular cross section (2). One half of this
cross section is formed by the impeller (3) with its radial blades (4)
on one side, while the other fixed half is formed in the housing (5).
The working chamber has an inlet port (6) and an outlet port (7) with
the impeller shown in the diagram rotating in an anti-clockwise direction.
Between the inlet and outlet ports is the rotor (8) filling
up the side channel. Air is trapped between the impeller pockets of the
rotating impeller and is then accelerated centrifugally. The air stream
is ducted by the centrifugal force into the side channel blower. The impeller
pockets then take it up and the air is then re-directed back into the
following pocket, which repeats the process. The air is accelerated and
compressed in the impeller several times. The more the blower is throttled,
either at the inlet or discharge, the greater the number of impeller re-entries
and hence increased compression.
One can compare
the movement of an air molecule within a side channel with a spring,
the pitch of which tighter the more the air is throttled. When you measure
the pressure at different points on the ring channel, you find that it
rises constantly from inlet (6) to outlet (7). The side channel principle
works as a vacuum pump when throttled on the suction side, and as a compressor
when throttled on the pressure side.
Rotary Vane Principle
increase by volume reduction is the principle behind rotary vane operation.
This static design offers excellent service in pressure, vacuum or a combination
of both. Depending upon size and design (i.e. oil lubricated or dry running)
vacuum up to 29.92 "Hg gauge with capacities ranging from 2 to 700
cfm and pressures up to 21.8 psig with capacities ranging from 2 to 350
cfm can be reached. When used as a combined unit, 23.6 "Hg gauge
vacuum and 11.6 psig pressure can be achieved simultaneously.
In a cylindrical
housing (1) a rotor (2) is positioned eccentrically so that it is on the
top (3) almost touching the cylinder. Rotor blades (5) are positioned
into numerous rotor slots (4). When the rotor starts turning, due to centrifugal
force the blades are thrown out and slide against the internal surface
of the cylinder. In this way a cell (6) is formed between two blades with
a volume which changes constantly during rotation. Air enters from the
inlet port into a cell until the rear blade reaches the inlet port (8).
At this point the cell (6) has achieved its maximum air volume. As the
cell then moves away from the port its volume becomes smaller and smaller,
the air is thus compressed and pressure rises. This continues until the
pressure in the cell (9) exceeds that in the pressure chamber (10) and
the air then exits through the outlet port (11). Some models are fitted
with exhaust valves (12) which stop the back flow of this discharged air
if the maximum pressure has been reached.
In a vacuum
pump the process is similar, but the cell (9) gives decreasing pressure,
and the chamber (10) is atmospheric pressure.
pumps the lower end of the inlet port(s) (7) for the vacuum is moved forward.
This provides the ability to fill the cell from a second inlet (14). To
avoid impairing the vacuum, this second inlet port is located about one
cell segment away from the main suction port (7). The ratio between vacuum
and pressure capacities can be influenced by the arrangement of the inlet
ports (7) and (14).
the rotary vane pumps, the Roots pumps are also static compression systems,
although the compression does not result from an internal volume reduction.
The single stage Roots vacuum pumps that G.E.E. use are designed for use
in pump sets in combination with a rotary vane backing vacuum pump. The
conveyed air is not discharged to atmosphere but piped into the inlet
port of the connected high-pressure stage (rotary vane pump). The achievable
capacities range from 300 to 2800 cfm at an ultimate vacuum of 29.92 "Hg
the rotary vane pump, air enters the inlet opening (1) into a conveying
cell formed by the two rolling pistons (2) in the housing. This is until
the cell is separated from the inlet by the following piston head (5).
The air in the cell is conveyed without reduction until it reaches the
outlet (6), the air with a higher absolute pressure flows from the pressure
chamber into the following cell, and must then be discharged. It is during
this stage of the conveying that external compression takes place.
rotary vane pumps and roots blowers, claw compressors and vacuum pumps
utilize a static compression principle of design. In contrasts to roots
blowers, compression works by internal volume contraction.
for pressure, vacuum or both combined, the claw principle provides very
favorable efficiency and operational characteristics. Depending on size,
vacuum up to 25.5 "Hg (gauge) and capacities between 59 and 353 cfm,
or pressure up to 32 psig at flow rates between 59 and 353 cfm can be
achieved. In combined pressure/vacuum operation simultaneous vacuum up
to 18.1 "Hg (gauge) and pressure up to 14.5 psig can be obtained.
claw pump consists of two rotors (1, 2). They turn in opposite
directions in a compressor housing (9) without friction and with very
tight clearances. They are synchronized via a precision gear. As the
claw moves over the suction connection (3) and the axial suction
channel inlet (4) the gas is sucked into the compressing chamber. Due
to the revolution of the rotors the gas is conveyed from the suction
side to the pressure side. There it is compressed by volume reduction
between the rotors until the lower rotor uncovers the discharge port
(5). This "internal compression" leads to high differential pressures
at efficiencies of more than 60 %. Afterwards, the compressed gas is
discharged via the pressure connection (8).
heat of compression, cooling air is sucked between the compressing housing
(9) and a silencing cover (10) and then it is laterally exhausted.
Temperature Conversion Table
the center column of the known temperature. If the known temperature is
in Fahrenheit, the Centigrade equivalent is in the left-hand column, and
= 0F - 32 x 5/9 or 0C = (0F - 32)/1.8
= 0C x 9/5 + 32 or 0F = 0C x 1.8 + 32
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