TM 5-813-8
Given:
T = 1 minute
TB = 15 minutes
T2 = 4.083 minutes
(1- T1/T2 ) x 100
= SDI
TB
The formula for the SDI is as follows:
SDI = (1 - 1 minute/4.083 minutes) 100/15 minutes = 5.03 silt density index
A-8. Concentration factor. If the percentage of the feed stream that is removed as product water (i.e., percent recovery)
is known, the concentration of the brine can be calculated for the purpose of estimating scaling concentrations as follows:
Symbols:
Percent Rec. = 100 x the fraction of the feed water that becomes product water
Given:
30 percent of the feed water becomes product
At 30 percent recovery, 70 percent of water contains all the original salt, ∴
1/0.7 x original concentration = final concentration
1
1-Percent Rec. = Concentration factor
100
1
1-30 = 1.429
100
A-9. Reverse osmosis membrane requirement (manufacturers will supply the necessary number of permeators).
The following equations can be used with membrane manufacturer's water flux constants to calculate a water flux or, by
rearrangement of the equation, a membrane area requirement can be calculated.
Symbols:
Qw = Water flow
Kw = Water flux constant for the membrane
A = Area of membrane
∆P = The pressure drop across the membrane (not including head losses to or from the membrane)
Aπ = The osmotic pressure difference across the membrane
10 pounds per square inch
Usually about:
1,000 milligrams per liter of total dissolved solids
Given:
-7
Kw = 2.53 x 10 gallons/pound-hour
The membrane area, A = 1,000 square feet
The applied pressure, P = 900 pounds/square inch
The osmotic-pressure difference, ∆π = 350 pounds/square inch.
Qw = KwA (∆P - ∆π)
-7
Qw = 2.53 x 10 gallons/pound-hour x 1,000 square feet x 144 square feet x (900 pounds/square inch - 350
pounds/square inch)
Qw = 20 gallons/hour
A-43
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