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Size Steam/Gas Motive for Exhausting Gases |
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| Using the Models SG and
HG |
The JRG/JT models SG and HG are general purpose
eductors used to pump gas with a gas as the motive stream. They
can be used to pull both liquid and gas suction loads. In this
case, we will deal with gas suction stream applications. The
model SG is generally used with higher pressure motive flows
in the ranges of 60 to 120 PSIG. It is possible to use lower
pressures with these units if the outlet pressures are strictly
controlled. The model HG is designed to be operated with motive
pressures of 20 through 80 PSIG. Because of the increased motive
flow rate in this unit, it generally will discharge against higher
pressures. In general, the SG will pull a deeper vacuum
on the suction connection than the model HG. If it is possible
to meet the discharge conditions with the model SG, this unit
is generally more efficient. Both SG and HG models can be used
to evacuate, exhaust, or prime a given area.
If the suction gas is not air at 70° F convert the suction
gas to Dry Air Equivalent. If the suction units are not in
Lb/Hr, refer to the back cover to convert from other units
to Lb/Hr. If your units are not found here, go to a conversion
table in a book of chemical standards. |
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| Instructions on Sizing Steam or Gas Motive
Eductors for Exhausting Gases |
| Step 1 |
| Before beginning any application, convert all pressure
and flow units into the units used in the sizing table. (If sizing
is done on a regular basis with other units, request special sizing
tables.) |
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| Step 2 |
| Find the motive pressure (Pm) column on the
table that is equal to the motive pressure available at the eductor
under flowing conditions. |
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| Step 3 |
| Locate the desired suction pressure (Ps) from
the left-hand side of the performance table. Follow this across
the table until it intersects the pressure (Pm) column selected
in Step 2. This is the suction flow (Qs) of the 1-1/2" unit
for model SG, HG. |
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| Step 4 |
| Determine the outlet pressure (Po) of the eductor,
and then determine which eductor(s) will meet these outlet conditions.
If the SG will meet the outlet conditions, choose this unit to
minimize motive consumption. |
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| Step 5 |
| Take the desired suction flow for the eductor divided
by the actual flow for a 1-1/2" unit found in Step 4. The
result will be the desired S.F. for the eductor. Choose a standard
eductor that has a Tabulated S.F. that meets or exceeds the desired
S.F. |
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| Step 6 |
| Determine the motive flow (Qm) required, by going
to the performance charts. Then multiply the number found there
by the S.F. of the unit selected in Step 5. |
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| Example: |
| Desired Suction
Flow (Qs) |
17 SCFM |
| Desired Suction Pressure(Ps) |
7 In Hg Vac |
| Motive Gas Media |
Steam |
| Motive Pressure (Pm) |
80 PSIG |
| Outlet Pressure (Po) |
5 PSIG |
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| Step 1 |
Convert the desired conditions into the units
used in the sizing table. See the conversion factors on back
cover of this manual. Convert 7 In Hg Vac to In Hg Abs: 29.92
In Hg Abs - 7 In Hg Vac = 22.92 In Hg Abs |
| All other units are in the terms used in the sizing
table. |
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| Step 2 |
| Locate the columns for both SG and HG models with
80 PSIG motive pressure. |
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| Step 3 |
| Locate desired suction pressure, (Ps) of 25 In Hg
Absolute on the left hand side of the table. Follow this line across
the table until it intersects the column for 80 PSIG operating
pressure. In this case, this is at a suction flow of 47 SCFM
for the model SG 1-1/2" unit. |
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| Step 4 |
| While determining that both models will work against
the outlet pressure (Po), the model SG will require less motive
resource. Choose the model SG unit. |
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| Step 5 |
Determine the Sizing Factor for the unit. 17
SCFM Desired/47 SCFM Actual = 0.362 Desired S.F. |
| If the desired flow is the absolute minimum flow,
as in this case, choose the next larger S.F. This would be the
model SG 1-1/4", with a S.F. of 0.61. If a slightly lower
suction flow would be acceptable, choose the model SG 1". |
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| Step 6 |
Calculate the actual performance of the unit selected.
For the SG 1-1/4" unit: Steam Motive Flow = 278 Lb/Hr Steam
x 0.61 =170 Lb/Hr Suction Flow = 47 SCFM x 0.61= 28.7 SCFM. For
the model SG 1" unit: Motive Flow = 278 Lb/Hr Steam x 0.34
= 94.5 Lb/Hr Suction Flow 47 SCFM x 0.34 =16.0 SCFM. Choose the
unit the will best meet your application conditions. |
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