TM 5-813-8
Rule 4 states that when sea water is used and internal combustion engines are being considered for power
production, vapor-compression distillation should be considered. Laboratory analyses of the sea water should include total
dissolved solids, bacterial count, and turbidity. An evaluation of possible waste heat recovery from the power generation
system should also be undertaken.
Assume the results from the laboratory analyses recommended in table 4-1, shown in figure A-9, are as follows (the
format for the Physical and Chemical Analysis of Water used in this manual is for illustration purposes only and users are
advised to develop their own format): The following data were extracted from the laboratory analysis and bacterial plate
counts:
Seawater
TDS 35,000 milligrams per liter
++
++
350 milligrams per liter as Ca
Ca
=
SO 4 2,650 milligrams per liter
pH 7.2
Bacterial count 10/100 milliliters (membrane filter technique)
Turbidity 15 nephelometric turbidity units
Assume that the following was obtained from analysis of the design of the internal combusion engine to be used for
power production.
Waste heat: Approximately 1.0 x 109 British thermal units per day between 264 degrees Fahrenheit and 68 degrees
Fahrenheit.
Calculating the cost of electricity from the power utility $.50/kilowatt hour versus the cost of waste heat steam which
6
can be produced at less than
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.03/1.0 x 10 British thermal units gives the following:6
6
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.50/kilowatt hour/
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.03/1.0 x 10 British thermal unit = 16.67 x 10 British thermal unit per kilowatt hourUse of this information and table 4-2 indicates that Rule 3 is applicable for this facility (see fig. A-10). Steam and
electricity costs are compared as a ratio to eliminate dollar value changes, but technology could easily change the 10.0 x
106 British thermal unit per kilowatt hour guideline. Rule 3 states that a comparison between reverse osmosis and
distillation/condensation systems should be made. Additional testing should include an alum jar test and 10-micron filter
plugging, as well as evaluation of the effects of ultraviolet (UV) sterilizations.
Assume that the results from the recommended testing indicate that the turbidity can be reduced by alum addition to
below 1 nephelometric turbidity unit, and a 10-micron filter will not plug rapidly on this treated water. Assume that a
computer analysis in addition to figure 3-2, TM 5-813-1, shows scaling is not a problem until the total dissolved solids is
concentrated about 1.5 times at temperatures below 212 degrees Fahrenheit.
Use of table 4-3 indicates that Rule 3 applies for this facility (see fig. A-11).
Rule 5 states that if temperature over 212 degrees Fahrenheit are economical for distillation at
this site, acid feed may be necessary to prevent scaling.
A distillation/condensation system with an associated vapor-compression system should be evaluated. Specifications
should be prepared. A drawing of a sea water distillation system capable of recompressing low-grade steam is shown in
figure A-12.
A-3. Calcium sulfate solubility product (Ca S04 XH2O) scale. As can be seen below, the calculation of a solubility limit
is moderately complex for a simple, pure solute at low concentration. Hand calculation is not practical in solutions
containing a few percent of several cationic and/or several anionic species. Most manufacturers have computer programs
that include the concentration polarization factors and any other surface-related factors that set the scaling limits for their
products. The following is a sample calculation of calcium ion sulfate ion, acid-base solubility for 64 degrees Fahrenheit.
To estimate solubilities for distillation condensation or other temperatures different from 64 degrees Fahrenheit, the
solubility product for calcium sulfate at temperatures up to 392 degrees Fahrenheit can be found in reference (4).
These calculations are suitable for initial design and process selection (see tables 4-2 and 4-3). The principle
purpose of these calculations is to indicate waters that are well below saturation and therefore do not present scaling
problems. A full computer evaluation of scaling potential should be performed by the manufacturer as part of the final bid.
A-12
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