Environmental Engineering Reference
In-Depth Information
Figure 2. A simplified schematic of the solar desalination plant.
The heat collecting water leaving the collector bank flows into the top of the heat
accumulator which has a total capacity of 300 m
3
. The heat accumulator is of the thermally
stratified liquid type where, by virtue of density variation between the top and bottom layers,
the higher temperature water is located in the upper region of the accumulator tank while the
lower temperature water occupies the lower region. The lower temperature water is drawn
from the tank bottom and pumped through the collectors by the heat collecting pump which
has a capacity of 80m
3
/hr at 26m discharge head. The heat collecting water is drawn from the
top of the accumulator tank by the heating water circulating pump and is forced to flow into
the heating tubes of the first effect of the MED evaporator. This evaporator is designed for a
maximum distillate production of 120m
3
. By transferring heat to the cooler brine flowing on
the outside of the tubes, the heating water is cooled down and is then discharged into the
accumulator.
The MED evaporator has 18 effects stacked one on top of the other with the highest
temperature effect (No. 1) located at the top of the stack and the lowest temperature effect
(No. 18) located at the bottom. The 18 effects are actually arranged in a double-stack
configuration where effects 1, 3, 5,….17 are in one stack and effects 2, 4, 6…18 in the
second. The double-stack arrangement is incorporated into one evaporator vessel as will be
shown in detail later.
In addition to the 18 effects, the evaporator has a final condenser designed to condense
the vapor generated in the bottom (last) stage (No. 18). Heat input supplied to the first effect
by the heating water is repeatedly used by evaporating a portion of the brine flowing into each
effect. The evaporator operates under vacuum that is effected by a positive displacement
pump connected to the final condenser. The absolute pressure to be maintained in the final
condenser is designed to be 50 mmHg. The pressure to be maintained in each effect varies
from slightly below atmospheric in the first effect to about 50 mmHg in the 18
th
effect.
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