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Size Gas/Steam Motive Eductors for Evacuating |
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| Priming |
| Priming is simply a special type of evacuation
where the reduction of pressure in the vessel is used to draw liquid
into the vessel. A good rule of thumb for priming applications
is that they take twice as long as an evacuation for the same volume
and pressure. |
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- Step 1 - Before beginning to do
the actual sizing, convert all pressures and flows to the units
used in the SG, HG sizing table below. (If sizing is being
done on a regular basis with a variety of units, request a special
sizing
table from your representative.) A list of common conversion
factors can be found on the back cover of this brochure.
- Step 2 - After determining the
required suction pressure to be achieved, divide the desired
time to achieve this vacuum by the number of Ft' to be evacuated.
This will result in a time per Ft' to perform the evacuation.
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| Desired Evacuation Time |
| ------------------------------------- |
| Ft3 to be Evacuated |
| = Desired Time Per Ft3 |
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- Step 3 - Find the column from the
performance
table that corresponds to your motive pressure. Find the row
with the desired suction pressure on the left-hand side
of the performance table. By dropping down to this intersection,
the time for evacuating 1 Ft 3 in minutes is found for the
1-1/2" unit.
- Step 4 - Take the tabulated time per Ft3 found
in Step 3 and divide it by the desired minutes per Ft3 found
in Step 2. The result will be a Desired Sizing Factor (S.F.).
Go to the Table on Pg. 3 and pick the unit that will meet or
exceed the factor found above.
- Step 5 - Divide the tabulated time per Ft3 by the S.F. of the unit selected. Multiply this by the number
of Ft3 to be evacuated. The result will be the actual time required
to evacuate the vessel with the eductor selected.
- Step 6 - To determine the motive flow required,
go to the chart for the motive gas you are using. Then find the
motive pressure you are using under this column and locate the
model of unit you are using. Multiply this flow by the S.F. of
the unit you selected in Step 4.
- Notes: The models SG and HG
are sized using the same steps as the ML and MLE; the only
difference is the motive force is provided by steam or gas.
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| Example: |
| Area to be Evacuated |
35 Ft3 |
| Time to Evacuate |
12 Minutes |
| Desired Suction Pressure (Ps) |
15 In Hg Abs |
| Gas to be Removed |
Air |
| Motive Gas |
Steam |
| Motive Pressure (Pm) |
60 PSIG |
| Outlet Pressure (Po) |
Atmospheric Pressure |
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- Step 1 - No conversions are needed as all units are
the same as the units in the tables.
- Step 2 - 12 minutes desired time / 35 Ft3
= 0.343 minutes per Ft3.
- Step 3 - The model SG 1-1/2" unit will
evacuate 1 Ft' in 0.012 minutes, as the performance table for
60 PSIG reads.
- Step 4 - 0.012 minutes per FO actual/0.343
minutes per Ft3 required = 0.035 Desired Sizing Factor. In this
case, choose the SG 1/2" which exceeds the Desired S.F.
- Step 5 - The time per Ft3 for the 1-1/2" unit
0.012/0.12 S.F. = 0.100 minutes per Ft3 x 35 FO = 3.5 minutes
to evacuate the given volume to 15 In Hg Abs.
- Step 6 - The chart says the motive flow (Qm)
for an SG 1-1/2" unit
is 222 Lb Hr at 60 PSIG motive pressure x 0.12
S.F. = 26.6 Lb/Hr of motive steam required to operate the
unit.
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| Models SG, HG |
Steam Motive Evacuation Times |
1-1/2" Unit |
| |
Minutes Per Cubic Foot |
| Model SG Eductor |
Model HG Eductor |
| Suction Pressure, Ps (In Hg Abs) |
60 |
80 |
100 |
120 |
20 |
40 |
60 |
80 |
| 25 |
0.003 |
0.003 |
0.003 |
0.003 |
0.007 |
0.005 |
0.005 |
0.006 |
| 20 |
0.007 |
0.006 |
0.007 |
0.008 |
0.014 |
0.012 |
0.014 |
0.015 |
| 15 |
0.012 |
0.012 |
0.014 |
0.016 |
0.026 |
0.025 |
0.028 |
0.029 |
| 10 |
0.023 |
0.024 |
0.026 |
0.027 |
0.045 |
0.047 |
0.057 |
0.060 |
| Motive Flow, Qm |
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| Steam (Lb/Hr) |
222 |
278 |
335 |
392 |
294 |
378 |
499 |
626 |
| Air (SCFM) |
74 |
92 |
131 |
131 |
97 |
125 |
167 |
209 |
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