given top operating temperature. To increase plant 
capacity from 1 to 10 mgd, the heat transfer 
surface area must be increased tenfold to maintain 
the same heat transfer rate (assuming a linear 
extrapolation). 
Going to SO or 100 mgd would then require 
still further size increases. It is clear this would 
result in undue requirements for plant size and 
corresponding increases in material costs. In other 
words, how can a plant be scaled up so its new size 
need not be increased in direct proportion to its 
new capacity. 
Many substantial technical factors other than 
heat transfer rates must be considered, such as 
scale control and construction materials. They are 
discussed in greater detail below. 
a. Materials of Construction The single most 
important capital cost item in a multistage flash 
distillation system is the condenser tubing (as 
much as 20 to 30 per cent of cost). The condenser 
tubing’s longevity is, of course, very important, 
and much more must be learned about the 
materials exposed to hot concentrated brine. 
The diversity of opinion regarding tube material 
is demonstrated by a recent group of conceptual 
designs requested by the Office of Saline Water. 
Three contractors specified titanium, three chose 
aluminum-brass, four selected 90-10 copper-nickel, 
and two chose 70-30 copper-nickel, while three 
used some combination of these materials. The 
70-30 copper-nickel has had somewhat longer 
exposure to hot brine in actual operations than the 
90-10 combination. However, the latter indicates a 
higher life expectancy, which could result in lower 
overall costs. The 150 mgd Bolsa Island facility 
would have required about 15,000 miles of 
copper-nickel tubing. 
b. Heat Transfer Rates Basic heat transfer in a 
multistage flash plant is from condensing steam 
through a tube wall to the circulating brine. An 
increase in the heat transfer coefficients would 
reduce the tubing area and result in reduced costs. 
Resistances to heat flow from condensing steam 
to circulating brine are found in the brine’s film 
resistance, tube wall resistance, outside or con- 
densing steam film resistance, and in a fouling 
factor that includes the resistance due to scale or 
dirt on the tube or to non-condensable gases in the 
system. 
VI-210 
The least understood of the various compo- 
nents in the overall heat transfer coefficient is the 
fouling factor, a strong function of the system’s 
cleanliness and the degree of deaeration and 
decarbonation of the feed sea water. A research 
program is needed to isolate the fouling factor 
experimentally and to study its dependence on 
system variables. 
c. Scale Control Formation of calcium carbon- 
ate, magnesium hydroxide, and calcium sulfate 
scales is a major problem in sea water distillation. 
Formation of the first two compounds can be 
prevented by injecting acid into the circulating 
brine stream and through subsequent deaeration. 
However, this does not prevent calcium sulfate 
deposition. 
The accepted scale control technique in multi- 
stage flash distillation is to inject about 120 ppm 
of sulfuric acid into the sea water feed followed by 
deaeration. Calcium sulfate scaling is prevented 
simply by operating the plant at temperatures and 
concentrations at which the solubility product of 
calcium sulfate is not exceeded. 
The addition of sulfuric acid normally adds 
about three to four cents per 1,000 gallons to 
water. Accordingly, it might be economical to 
manufacture sulfuric acid at the plant site, partic- 
ualarly in the case of very large plants. 
4. Brine Disposal 
Disposal costs of the brine from desalination 
processes must be considered; they may be as 
much as one-third of desalting expense for inland 
sites. It may be necessary to dispose of the brine in 
man-made evaporation ponds. Membranes have 
been developed to curb pond leakage and prevent 
contaminating aquifers or other underground 
water sources. 
Concerning sea disposal, primary effort will be 
to determine the ecological and other effects of 
contaminants resulting from the plants themselves: 
copper from heat exchanger tubes; iron from 
water boxes, evaporator shells, and piping; and 
trace elements from several sources. ° ° 
5. Inland Brackish Water Resources 
Although unlimited sea water exists, many 
areas where additional fresh water supplies are 
5SSenate Hearings, February 1968, op. cit., p. 24. 
