FRESH WATER FOR ARID LANDS—JENKINS 289 
separation of water from salt by freezing—is approaching the point 
of commercial possibility, but it still requires much further develop- 
ment. And, as part of the distillation group, small low-temperature 
stills using the heat of the sun have been extensively developed ex- 
perimentally on a small-scale basis in the United States and else- 
where, particularly in several Mediterranean countries and Australia. 
THERMAL AND MECHANICAL DISTILLATION 
Improvement of conventional distillation processes, both to reduce 
their high investment cost by increasing the rate of heat transfer and 
to reduce the energy cost by diminishing the heat losses, engaged ex- 
tensive study from the first. The earliest projects of the Office of 
Saline Water included studies of very large vapor-compression systems 
[10] and the possibility of reducing their costs, as well as large-scale 
multiple-effect evaporation systems separately and in combination with 
power-generation cycles. 
Although there are several different types of distillation equip- 
ments and cycles [11], all are presently subject to the same general 
limitations—scale deposition and corrosion. Scale-forming constitu- 
ents, principally calcium carbonate, calcium sulphate, and magnesium 
hydroxide, are precipitated out of solution as evaporator tempera- 
tures rise above 160° F. The scale fouls heat transfer surfaces 
and impedes fluid circulation. In addition, brines become more cor- 
rosive, necessitating use of expensive alloy as temperatures approach 
or exceed the normal boiling point of water, 212° F. 
Therefore, a series of research and development studies has been 
in progress in the fields of heat transfer, scale prevention, and less 
expensive corrosion-resistant materials of construction. Some of these 
studies are sponsored by the Office of Saline Water and other Govern- 
ment agencies, while others are carried out by independent manufac- 
turers in the United States and elsewhere and by agencies of other 
governments, such as the British Admiralty. In most instances, each 
of the principal investigations, which are described below, are directed 
toward a particular evaporator type. However, much of the knowl- 
edge and techniques gained from one series of experiments, where 
successful, will be adaptable to the general distillation field. 
Formation of scale deposits in the equipment is a serious problem 
in practically all distillation processes, and under certain conditions 
in electrodialysis. A fundamental investigation of the basic factors 
affecting scale deposition is being conducted at the University of 
Michigan. 
An attractive distillation process using long-tube vertical evapora- 
tors of the kind employed in the salt industry has been proposed 
by W. L. Badger of Ann Arbor, Mich. [12]. In this cycle sea water 
