Environmental Engineering Reference
In-Depth Information
Seawater is used to condense the vapor generated in the 18
th
effect. Part of the discharged
warm seawater leaving the final condenser returns to the sea, while the other part constitutes
the evaporator feedwater. The feedwater flow rate amounts to 17.3 m
3
/hr; it flows through 17
preheaters before reaching the first effect, one preheater for each effect except the 18
th
effect.
These preheaters are designed to raise the feedwater temperature incrementally by flowing
from the bottom effect (No. 18) to the top effect (No.1).
3.1. Plant Description
3.1.1. The Solar Heat Collector Subsystem
The solar energy collecting system (SECS) has the function of collecting the solar energy
when it is available during the day using the collector bank and storing this energy in the heat
accumulator which supplies thermal energy to the evaporator with minimum fluctuations in
the supply temperature. This is desirable since steady state operation of the evaporator near its
optimum operating condition is highly recommendable.
The basic unit in the collector bank is the Sanyo evacuated tube solar collector which is
shown in isometric in figure 3. This is a flat plate-type collector that employs selective
coating absorber plates enclosed in glass tubes maintained under high vacuum of 10
-4
mm Hg.
Ten glass tubes with their absorber plates are incorporated in each collector. Along the
centerline of each glass tube is located a single copper tube which is attached to the middle of
the absorber plate. The heat collecting water flows through this center pipe and absorbs the
solar energy collected.
The ends of each glass tube are sealed to a special stainless steel end cap using a ceramic
glass material having a coefficient of thermal expansion approximately the same as that of the
glass tube. The difference in the thermal expansion between the copper tube and the glass
tube is taken up by bellows installed between the end cap and the copper tube.
Each collector consists of 10 individual tubes arranged in parallel. The heat collecting
water moves inside the center tubes in a parallel/series arrangement whereby in five of the
tubes the flow is in one direction and in the other five it is in the opposite direction.
Table 1. Specifications of a single collector
Item
Specification
Selective coating
Absorptivity α ≥ 0.91
Emissivity ε ≤ 0.12
1.75 m
2
Absorber area
External dimensions
2860 mm x 985 mm x 115 mm
Net weight
64 kg
Flow rate
700 - 1,800 lit/hr
Max. operating pressure
6 bar
Attached to one end of the center tubes is a header tube with an orifice located in the
middle of the header tube. The other ends are connected to return bends which are used to
connect pairs of center tubes in series. Several collectors (14 in number) are connected in
Search WWH ::
Custom Search