June_1971 — Page 48

works will take HK$30 million.

Binnie and Partners are the consult- ing engineers for the project,and tend- ers for the supply of the plant - three or four units will make up the total of 20 mgd - will be invited international- ly early next year. Delivery will prob- ably take about 11⁄2 years.

A pilot desalting plant of 50,000 gmd is presently operating in the Castle Peak area to assess the per- formance under Hong Kong conditions of various construction materials. The site for the large scale plant has been selected because it is protected from the open sea by the nearby island of Lantao. Also, the wide channel be- tween the island and shore will provide adequate quantities of clean seawater as well as a sheltered anchorage for tankers delivering the fuel oil.

Mr. A.S. Robertson, Hong Kong's director of water supplies, said at a conference last month that although a conventional water system enjoyed long life and was relatively cheap to maintain, there were a number of com- pensating advantages for a desalting plant.

These included relative freedom to locate the plant near to centres of population or near existing aqueducts. And should plentiful rainfall occur in any year, the desalter need not be used, so that fuel costs could be kept to a minimum.

Desalting process

There are seawater desalting plants in operation in many parts of the world, such as the Channel Islands, Dutch East Indies and the United States. Probably the largest to date,

Number of separate units Output of each

Output of all units together

SEA WATER

000

CONDENSERS

Logo

PRODUCT WATER

FLASH 'VAPOURA

FLASH 'VAPOUR

FLASH VAPOUR

BRINE

114° C

116°C

118° C

HEATER

built by British contractors, is in lower than its predecessor in which the Kuwait.

same brine may flash, up to 40 times, each time giving up some salt-free

Firms in France, Britain, Japan and USA manufacture desalting plants to their own patented designs, but basi- cally the process is universal.

In desalting, brine is passed through steam heated tubes where its tempera- ture is raised to boiling point. This hot brine then flows into a large box shap- ed vessel (the evaporator) where it is permitted to boil into vapour. In this process which is termed 'flashing' the salt is left behind in the brine and the salt-free vapour is made to condense as fresh water on the cool surface of tubes from which it drips into collect- ing trays.

After the first 'flashing', both the brine and the fresh distillate are still hot enough to boil again though at a lower pressure and to enable this to be done the evaporator is divided into numerous chambers each at a pressure

Desalting Plant Data

Estimated cost of all plant and associated

Civil engineering

Estimated cost of continuous operation

for 12 months

Probable performance ratio

Temperature range

Heat source

Scale treatment by means of

Each unit (if of 10 million gal/day output)

will

recirculate brine within the

plant at the rate of

take in seawater at the rate of

2 to 4

5 to 10 million Imp/gal/day 20 million Imp/gal/day $190 million

$21 million

10-12 lb, distillate/1000 BTU 250°F to 100°F

Steam from oil fired boiler Acid

70 million Imp/gal/day 30 to 50 million Imp/gal/day

vapour.

The largest parts of the cost of pro- ducing fresh water by desalting, are the cost of the fuel consumed in heat- ing the brine, and in providing the cor- rosion resistant tubes for the salt free vapour to condense upon. Controlled flashing through a large number of stages enables these expensive tubes to be used to the best advantage.

The water which flows inside the tubes to keep them cool is in fact the same brine which is later heated and flashed into salt-free vapour. The heat given up to the cool tube surface when the vapour condenses is by this means largely recovered, the 'recovery' tech- nique being the principal means by which the cost of desalting seawater by distillation has been reduced to an extent sufficient for the process to be economic for potable water produc- tion.

The heating steam which provides the 'driving force' for the process does not come into contact with brine and after passing on its heat and con- densing, it can be returned to the boiler for re-use.

Mr. Robertson said that Hong Kong's desalting plant when complet- ed would be one of the largest in the world both in total output and as re- gards the size of individual evapora tors. This would enable the colony's water engineers to gain invaluable experience for the future in the latest techniques for solving what is now a world-wide problem – that of provid- ing sufficient fresh water to meet the needs of a modern and thriving indus- trial society.

Far East BUILDER, June 1971 Page 48

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