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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 |
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| 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 |
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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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