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How to Size Gas/Steam Motive Eductors for Evacuating Gases

SG and HG Eductor NozzlesUsing the Eductor Models SG and HG

The times given in the ML, MLE evacuation charts below are the time in minutes to evacuate 1 Ft3 to the pressure (vacuum) listed, from atmospheric pressure.

  • Step 1 Before beginning to do the actual sizing, convert all pressures and flows to the units used in the sizing table.
  • Step 2 Divide the desired time to evacuate the volume by the number ofFt3 to be evacuated.
  • Desired evacuation Time/Ft3 to be Evacuated = Desired Time per Ft3
  • Step 3 Locate the value equal to or less than the available flowing pressure for your system in the Motive Pressure (Pm) column. If your available system motive pressure is between the listed pressures, you may interpolate between them. Choosing a lower pressure will yield a more conservative estimate.
  • Step 4 Locate the Suction Pressure (Ps) row that corresponds to the suction pressure that you need. If your desired suction pressure is between the listed pressures, you can interpolate between them. This will result in a more accurate sizing. At the intersection of the row you have located and the column from Step 3 is the time per Ft3 for a 1-1/2" unit.
  • Step 5 Choose a S.F. that when divided into the time/Ft3 from Step 4 will yield a time/Ft3 equal or lower than the Desired time/Ft3 from Step 2.
  • Step 6 To determine the time required, multiply the Ft3 of the suction vessel by the time per Ft3 determined from the table in Step 5. This will give the time required to evacuate the vessel under actual conditions.
  • Step 7 To determine the motive water required, refer to the same column in the row labeled Motive Flow (Qm). Multiply this number times the Sizing Factor (S.P.) for the unit selected. The result will be the GPM of motive water required to operate the selected unit.

Note: Eductors for evacuations are sized at atmospheric outlet pressure. For liquid motives, these values calculated can be used with higher outlet pressures provided they do not exceed 5% of the motive pressure or 5 PSIG, which ever is lower. If higher outlet pressure is being used, consult with us.

Adjusting Formulas for Priming: Step 1 Multiply the Ft3 to be evacuated by 2, then go to Step 1 under Evacuation and proceed through the procedure described above.

Show TableLiquid Motive Evacuating Gases in Times Tables (click to show/hide table)


Gas Jet PumpExample:
Required Time to Evacuate Vessel 4 Minutes
Motive Pressure, (Pm) 60 PSIG
Size of Vessel to be Evacuated 45 Ft3
Outlet Pressure, (Po) 1 PSIG
Desired Final Suction Pressure, (Ps) 10 In Hg Abs

  • Step 1 All units are in pressures and flows used in charts.
  • Step 2 4 minute desired evacuation time/45 Ft3 = .089 minute/Ft~
  • Step 3 Motive Pressure, Pm is 60 PSIG.
  • Step 4 Go to the ML and MLE table to find an MLE operated at 60 PSIG and pulling down to 10 In Hg Abs will evacuate each Ft3 in 0.14 minutes.
  • Step 5 The S.P. for a 2" MLE gives .14/1.82 = .077 minutes per Ft3, the first number below .089, the desired time from Step 2.
  • Step 6 The actual time to evacuate will be 45 Ft3 x .077 minutes =3.47 minutes.
  • Step 7 The motive flow (Qm) required will be 26.2 gal. x 1.82 S.P. = 47.7 GPM. Will the outlet pressure effect the unit? 60 x .05 = 3 PSIG. Therefore, as the outlet pressure (Po) does not exceed 5% of the motive pressure or 5 PSIG it should have little effect on the performance of the unit.