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How to Size Liquid Motive Eductors for Pumping
Liquids |
| Using Liquid Motives to Pump Liquid
Suction Fluids |
| To determine the correct eductor
for a specific application, follow the steps in this section, using
the performance tables provided to achieve your desired results.
(NOTE: All JRG/JT tables use the 1-1/2 inch unit as the standard,
and eductors are sized using a Sizing Factor (S.E) based on this
standard unit.) |
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| Step 1 |
| Find the suction lift* or head (Hs)
that is equal to or greater than your desired lift. If your lift
is between two of the lifts on the table, use an average of the
two. You can also or use the calculated result from the NPSH formula
found on page 5 of this manual. Using the NPSH number will correct
for temperature variations and friction losses, resulting in a
more accurate value. |
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| Step 2 |
| Find the outlet head** (Ho) equal
to or greater than your actual outlet head. It is important to
include friction losses into the desired outlet head. (Be certain
that friction losses in the outlet line are calculated using the
combined rate of both the motive and the suction flows.) It is
important that the outlet line from the eductor be as large or
larger than the outlet connection. |
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| Step 3 |
| Find the motive pressure (Pm)' Locate
the motive pressure from the table that is closest to or lower
than your actual motive pressure. The flow specified represents
the Tabulated Suction Flow for each of the different models of
eductors. To determine the size of eductor needed, first use the
following formula to determine the Desired Sizing Factor (S.F.)
Do this for each of the models. |
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| Desired
S.F. = |
Desired
Suction Flow |
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| Tabulated Suction Flow |
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If a standard unit is being used,
pick the size unit that has a Tabulated S.E equal to or greater
than the Desired S.F.
If an exact match is desired, consult your Jerguson/Jacoby-Tarbox representative. |
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| Step 4 |
| Calculate the amount of motive flow
used by multiplying the Q," and Qs found in the tables by the Tabulated
S.E obtained in Step 3. Do this for each of the models of eductors. |
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| Step 5 |
Select the unit from Steps 1-4
that best meets the motive and suction parameters of the specific
application. If a turndown ratio*** of greater than 35% is needed,
then choose two or more eductors that have the correct turndown
ratio and operate these units in parallel.
In some cases, the unit chosen will have the greatest suction
flow while consuming as little motive fluid as possible. This
is generally true for pumping applications. In other applications,
such as the dilution of chemicals, the motive flow should be
as high as possible: while the suction flow will be low. In
this case, the motive flow should be matched to the desired
motive flow and the suction port should be throttled to achieve
the desired dilution rate. As a general rule in dilution applications,
the HL is the best unit to choose. |
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| In all cases, the correct unit
is the one that matches your desired range of motive-to-suction
flows the closest. |
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| Correcting for Non-Water Fluid
Specifications |
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The performance specifications
for JRG/JT eductors are based on using waterwith a specific gravity
of 1.0 and a viscosity of 1 Centipoise. Fluids with differing
viscosities or specific gravities need to be corrected to water,
to obtain accurate performance estimates.
Viscosity is the measure of the internal resistance of a fluid
to flow. This should be taken into consideration in most pressure
drop and flow calculations within a given system. When used
with JRG/JT eductors, fluids with viscosities of less than
100 Cp. have a negligible effect. Viscosities of up to 500
Cp. can be used with only small corrections. For higher viscosities
(applications above 500 Cp.), we suggest that you work with
your trained representative or the applications personnel at
the factory. Eductors can be used with viscosities over 500
Cp. with calculated adjustments. The effects of viscosity on
the pressure drops in the line leading to the eductor must
be calculated separately.
Specific gravity is the measure of the weight per volume of
a liquid. The performance data for eductors is based on water
having a specific gravity of 1.0; other specific gravities
will require that adjustments be made to the performance table
value of the eductors. See the topics that follow for specifics
on how to make these adjustments. |
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| Motive Flow Adjustments |
| The motive flow is the amount of
liquid used to power the eductor. To adjust the value from the
performance table for specific gravity (Sg) of the motive fluid:
Multiply the motive flow in the performance chart by the square
root of(1/Sg). |
| Example: 50 GPM
Tabulated Flow adjusted for a Specific Gravity of 1.3: 50 x(1/1.3)
= 43.85 GPM Actual motive flow |
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| Suction Flow Adjustments |
| The best way to adjust for the specific
gravity or temperature effects of the suction fluid is to do the
calculation for NPSH (see page 5 of this manual). If you desire
a rough estimate of the specific gravity effect, multiply the suction
lift by the specific gravity of the liquid. If the liquid temperature
exceeds 100°F, you must use the NPSH calculation, or consult
your representative or the factory. |
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| Outlet Adjustments |
| The outlet pressure of the eductor
must be adjusted for the specific gravity of the outlet liquid,
particularly if the eductor is discharging to an elevated surface.
If the outlet is being measured or controlled by a pressure regulator
or valve, no adjustment is required. To calculate the actual outlet
pressure, multiply the feet of elevation by the specific gravity
of the outlet liquid. |
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