| There are two major types of applications using eductors
to pump gases. These are evacuation and exhausting.
Exhausting
This application involves removing gases at a continuous rate
from an area while maintaining the pressure at a stable level.
These applications often involve removing gases or fumes that
are continuously recurring. An example would be removing smoke
from a welding or machining area. This process also could be
used for injecting oxygen into a liquid stream. If the gases
being removed have undesirable characteristics, it is possible
in some cases to neutralize them by using a reactive motive fluid.
Evacuation
This application involves pulling gases from a defined volume
by pumping the tank down from a starting pressure to a final
lower pressure. It is generally sized by determining the amount
of time it takes to reduce the pressure in the vessel to the
desired final pressure. Examples of this application would be
reducing the pressure in a reaction vessel to purge it of detrimental
gases, or removing steam from a vessel before opening it to increase
operator safety. A variation of this type of application is the
use of eductors to prime piping or a system with liquid. Priming
evacuations may be used to bring the level of liquid up to pump
level to avoid the pump being started dry or to establish a siphon.
JRG/JT eductor models for liquids pumping gases are: ML, MLE;
for gases pumping gases: SG, HG. These models can be used for
both exhausting and evacuation. See the following specifications
tables for operating parameters.
Other models of eductors are available for specific applications.
In some cases, these specialty eductors can pump a suction volume
up to 50 times the motive volume. Most JRG/JT eductors are available
in sizes from 1/2" through 3" in a wide variety of
materials. Expedited deliveries are possible if needed. Units
are available in sizes up to 12".
Principles of Operation for Pumping Gases
Eductors operate on the basic principles of flow dynamics. This
involves taking a high pressure motive stream and accelerating
it through a tapered nozzle to increase the velocity of the fluid.
Gas Motives are compressible fluids and are put through a converging-diverging
nozzle. The gas can exceed the speed of sound. This fluid is
then carried on through a secondary chamber where the friction
between the molecules of it and a secondary gas (generally referred
to as the suction fluid) causes this secondary gas to be pumped.
These fluids are intimately mixed together and discharged from
the eductor. There are three connections common to all eductors.
This connection is where the power for the eductor is generated,
by increasing the velocity of the motive fluid. The JRG/JT nozzle
in this section is manufactured to take advantage of the physical
properties of the motive fluid. Eductors with liquid motives
use a converging nozzle, as liquids are not generally compressible.
Eductors with gas motives utilize converging-diverging nozzles
to achieve maximum benefit from the compressibility of the gas.
All JRG/JT nozzles for eductors have smooth flow paths. Flow
paths with rough surfaces cause eductors to operate less efficiently.
Converging/mixing tube is manufactured to specified finish and
concentricity, assuring performance vastly superior to that of
competitive units.
SUCTION Connection
This connection of the eductor is where the pumping action of
the eductor takes place. The motive fluid passes through the
suction chamber, entraining the suction gas as it passes. The
friction between the fluids at the interface of the motive fluid
causes the chamber to be evacuated as the gas in the chamber
is removed. This allows the pressure in the suction vessel to
push additional flow into the suction connection of the eductor.
The high velocity of the motive stream in this section of the
eductor directs the combined fluid toward the discharge section
of the eductor.
OUTLET Connection
As the motive fluid entrains the suction gas, part of the kinetic
energy of the motive fluid is imparted to the suction gas. This
allows the resulting mixture to discharge at an intermediate
pressure. The percentage of the motive pressure that will be
recovered is dependent upon the percentage of motive flow to
suction flow and the amount of vacuum at the suction connection.
How to Size Liquid Motive Eductors for Exhausting
Gases
Exhausting is a continuous operation of removing gases at a
constant suction pressure. The units are sized based on a desired
flow rate of gas through the eductor.
Step 1 Before beginning to do the actual sizing, convert all
pressure and flow to the units used in the sizing table. (If
sizing is done regularly with other units, request a special
sizing table from your representative.)
Step 2 First find the value that is equal to or greater than
your system back pressure in the Outlet Pressure (Po) column.
After locating the correct value on the sizing table, use this
section of the table to size the unit.
Step 3 Locate the pressure that is equal to or higher than your
desired suction pressure (Ps) for the outlet pressure found in
Step 2. If the
pressure is between two values on the sizing table, you can
interpolate between them. If you don't wish to interpolate, the
higher pressure will give a conservative estimate.
Step 4 In the row for suction pressure (Ps), locate the column
where the motive pressure (Pm) is equal to your motive pressure
in the flowing condition. If the pressure is between two levels
on the sizing table, you can interpolate between the values.
If you don't wish to interpolate, the lower pressure will give
a conservative estimate.
Step 5 Divide your desired flow (Qs) rate by the flow rate of
each model. This will give you a desired Sizing Factor (S.E)
for each model. Pick a unit with a S.E that is equal to or larger
than the one calculated above.
Generally, the model with a larger suction capacity will be
the more efficient unit. To calculate the actual gas flow, multiply
the S.F. of the selected model by the suction flow (Qs) of the
model in the table.
Step 6 Calculate the amount of water required to operate the
eductor by multiplying the GPM in the Motive Flow (Qm) row by
the S.F. of the unit selected.
Note 1: In many cases, the capacity of the MLE and ML units
are close. Both should be sized to find the proper unit for the
application.
Note 2: When using liquids to pump gases, the eductor acts as
a volume displacing device. Therefore, the weight of the gas
has only minimal effect on the performance of the eductor.
|
Outlet
Pressure,
|
suction
Pressure, Ps
|
Motive Pm (PSIG)
20 40 60 80 100 . 140 200 250
|
|
Po (PSIG)
|
(In Hg Abs)
|
ML
|
MILE
|
ML
|
MILE
|
ML
|
MILE
|
ML
|
MILE
|
ML
|
MILE
|
ML
|
MILE
|
ML
|
MILE
|
ML
|
MILE
|
| |
30
|
8.4
|
7.3
|
10.5
|
9.4
|
13.6
|
12.6
|
14.7
|
13.6
|
18.9
|
14.7
|
19.9
|
18.9
|
22.0
|
19.9
|
30.8
|
21.9
|
| |
25
|
1.7
|
3.1
|
3.7
|
5.8
|
6.0
|
8.4
|
8.9
|
10.7
|
10.4
|
12.0
|
13.2
|
15.0
|
17.5
|
18.1
|
24.5
|
19.9
|
| |
20
|
0.9
|
1.7
|
2.0
|
3.7
|
3.5
|
5.9
|
5.2
|
8.1
|
6.5
|
9.2
|
9.7
|
11.8
|
13.6
|
14.7
|
19.0
|
16.2
|
| |
15
|
0.5
|
0.8
|
1.2
|
2.5
|
1.9
|
3.9
|
1.9
|
5.8
|
3.9
|
6.6
|
5.5
|
8.8
|
9.2
|
11.0
|
12.9
|
12.1
|
| |
10
|
|
|
0.9
|
1.3
|
0.9
|
2.7
|
1.4
|
3.8
|
2.1
|
4.1
|
2.7
|
6.2
|
5.4
|
7.3
|
7.6
|
8.0
|
| |
5
|
|
|
0.3
|
0.6
|
0.4
|
1.6
|
0.7
|
2.1
|
0.8
|
2.3
|
1.2
|
3.2
|
2.1
|
3.6
|
2.9
|
4.0
|
| |
3
|
|
|
.6
|
5.
|
.9
|
8.4
|
5.7
|
10.5
|
10.5
|
1 .6
|
13.6
|
16.
|
1 .
|
1 .9
|
7.9
|
20.8
|
| |
25
|
|
|
'1.8
|
3.7
|
2.9
|
6.2
|
4.0
|
8.4
|
6.4
|
10.5
|
8.9
|
14.7
|
14.9
|
17.7
|
20.9
|
19.5
|
| |
20
|
|
|
1.3
|
2.5
|
2.2
|
4.6
|
2.9
|
6.6
|
4.2
|
8.0
|
5.8
|
11.3
|
10.8
|
14.4
|
15.1
|
15.8
|
|
5
|
15
|
|
|
0.8
|
1.6
|
1.4
|
3.2
|
2.0
|
5.1
|
2.6
|
6.2
|
3.6
|
8.4
|
6.8
|
10.8
|
9.5
|
11.9
|
| |
10
|
|
|
0.5
|
|
0.8
|
2.1
|
1.2
|
3.1
|
1.4
|
4.0
|
2.1
|
5.8
|
3.8
|
7.2
|
5.3
|
7.9
|
| |
5
|
|
|
|
|
0.3
|
1.2
|
0.5
|
1.8
|
0.6
|
2.2
|
1.0
|
3.1
|
1.5
|
3.6
|
2.1
|
4.0
|
| |
30
|
|
|
|
|
3.1
|
.2
|
4.1
|
8.
|
6.0
|
11.5
|
|
15.7
|
16.
|
18.8
|
23.5
|
0.7
|
| |
25
|
|
|
|
|
2.3
|
4.9
|
3.1
|
7.0
|
4.3
|
9.4
|
6.4
|
13.5
|
11.2
|
17.6
|
15.7
|
19.4
|
| |
20
|
|
|
|
|
1.6
|
3.6
|
2.3
|
5.5
|
3.2
|
7.5
|
4.6
|
10.7
|
7.7
|
13.9
|
10.8
|
15.3
|
|
10
|
15
|
|
|
|
|
1.1
|
2.7
|
1.6
|
4.0
|
2.3
|
5.6
|
3.1
|
7.8
|
4.9
|
10.6
|
6.9
|
11.7
|
| |
10
|
|
|
|
|
0.7
|
1.5
|
1.0
|
2.8
|
1.3
|
3.8
|
1.8
|
5.6
|
2.9
|
7.0
|
4.1
|
7.7
|
| |
5
|
|
|
|
|
0.3
|
|
0.5
|
1.5
|
0.6
|
2.2
|
0.8
|
3.0
|
1.2
|
3.6
|
1.7
|
4.0
|
| |
30
|
|
|
|
|
|
|
3.3
|
7.4
|
4.6
|
9.
|
6.5
|
14.7
|
11.5
|
1 .9
|
16.1
|
20.8
|
| |
25
|
|
|
|
|
|
|
2.6
|
6.0
|
3.5
|
8.1
|
5.2
|
12.3
|
9.0
|
16.7
|
12.6
|
18.4
|
| |
20
|
|
|
|
|
|
|
2.0
|
4.8
|
2.7
|
6.7
|
4.0
|
10.4
|
6.7
|
13.5
|
9.4
|
14.9
|
|
15
|
15
|
|
|
|
|
|
|
1.4
|
3.7
|
1.9
|
5.2
|
2.7
|
7.6
|
4.5
|
10.4
|
6.3
|
11.4
|
| |
10
|
|
|
|
|
|
|
0.9
|
2.5
|
1.2
|
3.6
|
1.6
|
5.5
|
2.5
|
6.9
|
3.5
|
7.6
|
| |
5
|
|
|
|
|
|
|
0.4
|
1.0
|
|
0.6
|
2.1
|
0.8
|
3.0
|
0.9
|
3.6
|
1.3
|
4.0
|
| |
30
|
|
|
|
|
|
|
3.0
|
|
|
.8
|
8.
|
5.8
|
1 .
|
8.9
|
17.8
|
12.5
|
19.6
|
| |
25
|
|
|
|
|
|
|
2.3
|
|
3.1
|
7.5
|
4.6
|
11.7
|
7.2
|
16.4
|
10.1
|
18.0
|
| |
20
|
|
|
|
|
|
|
1.6
|
|
2.4
|
6.3
|
3.5
|
9.6
|
5.4
|
13.2
|
7.6
|
14.5
|
|
20
|
15
|
|
|
|
|
|
|
1.1
|
|
1.6
|
4.9
|
2.5
|
7.5
|
3.7
|
10.1
|
5.2
|
11.1
|
| |
10
|
|
|
|
|
|
|
0.7
|
|
1.1
|
3.4
|
1.4
|
5.5
|
2.3
|
6.9
|
3.2
|
7.6
|
| |
5
|
|
|
|
|
|
|
0.2
|
|
0.6
|
1.9
|
0.8
|
3.0
|
0.9
|
3.6
|
1.3
|
4.0
|
|
Motive Flow Qm (GPM)
|
18.8
|
18.0
|
25.4
|
24.2
|
30.4
|
28.2
|
34.7
|
31.7
|
38.5
|
34.7
|
45.2
|
40 -
|
53.2
|
45.9
|
159.2
|
1 50.3
|
|
