TM 5-813-8
A-10. Reverse osmosis membrane salt flux (manufacturers will meet desired water quality). The following
calculations can be used with membrane manufacturer's specific ion flux constants to estimate the concentrations of
particular ions in the product water.
Symbols:
Qs
= The salt flow
= The salt flux constant for the ion under consideration across the membrane
Ks
used
A
= The area of membrane
Cion feed/brine average
= One half the sum of the feed concentration and the reject brine concentration of
the ion under consideration (Mg+ + in this example)
Cion product
= The concentration in the product of the ion under consideration (Mg+ + in this
example)
Given:
-4
The salt flux constant for this membrane Ks = 2.8 x 10 gallons per square foot-hour
The membrane area A, = 1,000 square feet
The feed water contains 204.4 grains per gallon of magnesium Mg ++.
The product water contains 0.3 grains per gallon of magnesium Mg+ +.
Qs = KsA (Cion feed/brine average Cion product)
-4
Qs = (2.8 x 10 gallons per square foot-hour)
(1,000 square feet)
(204.4 grains per gallon Mg+ + -0.3 grains per gallon Mg++)
= 57 grains Mg+ + per hour
A-11. Second-law limiting thermal efficiency. A perfect conversion of heat into work is not possible. There is,
however, a limiting efficiency of conversion of heat energy to mechanical or electrical energy. No heat machine can
operate at this best efficiency, but it does allow a rough estimate of the value of lower grades of steam.
Symbols:
TH =
Temperature in degrees Rankine of steam source. Higher temperature in degrees
Rankine (degrees Rankine equal degrees Fahrenheit plus 460 degrees)
TL =
Temperature in degrees Rankine of the cooling sink. Lower or discharge
temperature in degrees Fahrenheit
emax =
Maximum possible second law efficiency (maximum percent of the heat available
that can be converted theoretically into work)
emax =
(TH - TL)/TH X 100
Given:
The steam temperature drop available is 264 degrees Fahrenheit to 68 degrees Fahrenheit.
(See problem A-2.)
emax = (724 degrees Rankine - 528 degrees Rankine)/724 degrees Rankine x 100
emax = 27 percent
A-44
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