Environmental Engineering Reference
In-Depth Information
effective height of 7.6m. The heat collecting water from the collector bank is introduced at
the top of tank No. 1. The heat collecting water to the collector bank is taken from the bottom
of tank No. 3. Heating water to the evaporator is drawn from the top of tank No. 1 and returns
to the bottom of tank No. 3. The water is therefore stratified in such a way that the top water
layers of tank No. 1 are always at the highest temperature and the bottom layer of tank No. 3
at the lowest temperature.
Figure 6. Heat accumulator subsystem.
The heat accumulator tanks have enough capacity to be able to provide the required
thermal load (for the evaporator) for about 16 hours after sunset provided that the tanks are
fully charged just before sunset. This feature makes it possible to operate the desalination unit
during night time. Only during extended overcast or hazy days when sandstorms prevail we
expect plant shut-down to occur due to insufficient energy collection.
3.1.3. MED Evaporator Subsystem
A horizontal tube, thin film multi-effect distiller (MED) is used for desalination of
seawater. The distiller is manufactured by Sasakura Engineering Co., Ltd. It was chosen
because of its capability to accommodate large load fluctuations and its small consumption of
electrical power. The maximum capacity of the distiller is 120 m
3
/day.
A flow diagram of the MED desalination process is shown in figure 7. Preheated
feedwater is sprayed into the top of the first effect and descends down the evaporator stack,
flowing as a thin film over the tube bundle in each effect. The feedwater flashes and thereby
cooled by several degrees as it passes from one effect to the next. It is rejected at the bottom
of the plant as cool, concentrated brine. In the top effect, heating water from the accumulator
is used to partially evaporate the thin seawater film on the outside of the tubes. The generated
vapor passes through demisters to the inside of the tubes in the second effect where it
condenses to form part of the product.
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