W aler, Waler Everywhere . . .
79
Solar stills which also rely on surface evaporation of fresh water
from salt have an obvious advantage in terms of energy. Unfortun—
ately they lose heavily in comparison With the compactness of a
diffusion still. A solar unit for a 120—acre farm would cover a third
of this area. The solar still consists essentially of a large transparent
dome covering a trough of salt water. The heat of the sun evaporates
the water and the vapour rises from the surface. The vapour
condenses on the underside of the dome and runs down to a collect—
ing channel. To improve the absorption of the sun’s energy, the
bottom of the salt—water trough is usually painted black.
Unfortunately, even under ideal conditions solar stills produce
fresh water very slowly—about one pint of fresh water can be obtained each day per square foot of energy absorbing surface. A study
completed by the Battelle Memorial Institute in the United States
in 1965 shows that solar stills can compete with other forms of
distillation only where small amounts of fresh water are needed.
Nevertheless water could be provided more cheaply than by
conventional ash or vapour compression processes in certain areas
of the world providing that the demand was less than 50,000 g.p.d.
Freezing. Long ago the Eskimos learned that the rst ice which forms
when sea—water freezes contains very little if any salt. The Russian
emigrant Alexander Zarchin who designed the freezing plant at
Eilat in Israel is reputed to have watched Russian peasants living in
coastal villages skimming the ice from the surface of shallow bowls
Which they had lled with salt water. Ice forms on the surface of a
solution containing 35,000 ppm. salt at —2-2°C. As the tempera—
ture falls even lower, more ice forms. Salt does not start to be
deposited from the brine until —21—1°C, although other impurities
may be released from solution at
Fortunately both tempera—
tures are below the normal Working ranges of present processes——
direct and secondary refrigerant freezing.
In the direct freezing method, the sea—water is cooled by feeding
it into a vacuum chamber. On entering this chamber some of the
water ashes and cools the remainder. Approximately half of the
water entering the chamber freezes. The mixture of ice and brine
is then pumped to the bottom of a separating column where the ice
crystals oat to the surface. This collection of ice forms a core inside
the column. As the core is pushed upwards by ice collecting beneath
it and the pressure of the brine, it is washed free of salt. Finally the
79
Solar stills which also rely on surface evaporation of fresh water
from salt have an obvious advantage in terms of energy. Unfortun—
ately they lose heavily in comparison With the compactness of a
diffusion still. A solar unit for a 120—acre farm would cover a third
of this area. The solar still consists essentially of a large transparent
dome covering a trough of salt water. The heat of the sun evaporates
the water and the vapour rises from the surface. The vapour
condenses on the underside of the dome and runs down to a collect—
ing channel. To improve the absorption of the sun’s energy, the
bottom of the salt—water trough is usually painted black.
Unfortunately, even under ideal conditions solar stills produce
fresh water very slowly—about one pint of fresh water can be obtained each day per square foot of energy absorbing surface. A study
completed by the Battelle Memorial Institute in the United States
in 1965 shows that solar stills can compete with other forms of
distillation only where small amounts of fresh water are needed.
Nevertheless water could be provided more cheaply than by
conventional ash or vapour compression processes in certain areas
of the world providing that the demand was less than 50,000 g.p.d.
Freezing. Long ago the Eskimos learned that the rst ice which forms
when sea—water freezes contains very little if any salt. The Russian
emigrant Alexander Zarchin who designed the freezing plant at
Eilat in Israel is reputed to have watched Russian peasants living in
coastal villages skimming the ice from the surface of shallow bowls
Which they had lled with salt water. Ice forms on the surface of a
solution containing 35,000 ppm. salt at —2-2°C. As the tempera—
ture falls even lower, more ice forms. Salt does not start to be
deposited from the brine until —21—1°C, although other impurities
may be released from solution at
Fortunately both tempera—
tures are below the normal Working ranges of present processes——
direct and secondary refrigerant freezing.
In the direct freezing method, the sea—water is cooled by feeding
it into a vacuum chamber. On entering this chamber some of the
water ashes and cools the remainder. Approximately half of the
water entering the chamber freezes. The mixture of ice and brine
is then pumped to the bottom of a separating column where the ice
crystals oat to the surface. This collection of ice forms a core inside
the column. As the core is pushed upwards by ice collecting beneath
it and the pressure of the brine, it is washed free of salt. Finally the