Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE: DESALINATION PROCESS/EQUIPMENT II
FIELD OF THE INVENTION
This invention relates to the desalination of salt
water. More particularly it provides an apparatus and method for
producing fresh water from salt water using solar radiation.
BACKGROUND TO THE INVENTION
In many parts of the world there is an extreme shortage
of drinking water. Fresh water which can be used by man is only
a minute fraction of the total amount of water present as sea
water. It is becoming important to find inexpensive ways to
provide fresh water for drinking and agriculture for the
increasing population of the world.
Nearly 65% of the population of the world live within
60 km radius from an ocean. Many countries that experience a
shortage of drinking water have an abundant amount of sunshine
which could be converted to heat.
Currently about two-thirds of the world's sea water
desalination is done by distillation. Reverse osmosis technology
("RO") is becoming more popular because it is currently more
economical than distillation. Worldwide, there are 10,000 sea
water desalination plants, producing about 20 Mm3/d of fresh
water. Recently, more efficient and cheaper membranes have become
available to allow treatment of sea water which has high salt
content (3.5 to 4.6 wt%). Mechanical vapour compression (MVC) is
another method becoming popular.
In these processes one of the major costs is in the use
of high pressure pumps to achieve the high pressure (-6 MPa)
necessary to overcome the osmotic pressure of sea water. This
drives about 50% of the water through the membrane as permeate,
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leaving a waste brine stream that is sent back to the sea. Better
economies are achieved by recovering some of the pressure energy
from the waste brine.
The total dissolved solid (TDS) content of water from
RO is around 300 mg/L.
Though this meets drinking water
standards, it often does not suffice for industrial use. Further
treatment may be necessary. The osmotic pressure is dependent on
temperature and the salt content and this will affect the RO
operation.
In some situations, multistage flash (NSF) evaporation
method may be better suited. In this method the pressure of the
water is suddenly reduced below its equilibrium vapor pressure
causing "explosive" boiling. This pressure reduction is achieved
by introducing the sea water into a chamber through an orifice.
An external heat source is used.
In yet another method of multi-effect distillation
(MED), preheated sea water is sprayed onto the heat transfer area
of a single-effect evaporator and the resulting vapour is
transferred to the second and further stages by operating at
progressively lower temperatures. Generally, power requirements
are higher for distillation methods than for RO.
RO, MVC,
thermocompression, MET) and NSF require about 22, 38, 8, 8 and 16
kWh/1000 US gal, respectively. NSF, MED and thermocompression
also require steam 11b/7-12 lb of water. Improvements are made
to lower steam usage through serrated-profile tubes in the
evaporator.
Use of solar energy for water distillation is well
known, particularly in arid, coastal regions. In simple terms,
sea water is evaporated by solar energy and then condensed to
provide the fresh water. Since solar energy is free, the cost of
the desalination process mainly depends on the cost of the
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materials used for construction of the desalinator. Since salt
water is extremely corrosive, the right choice of materials is
very important.
Techniques of increasing the rate of evaporation by
increasing the surface area of evaporation are well known and many
inventions, such as U. S . Patents 6,001,222; 2,213,894 and
3,801,474 exploit this idea.
The current invention provides a simple method that is
extremely low energy consuming and that does not require
specialized materials or high capital investments. When properly
designed, it would produce very little waste and it would require
little maintenance.
The invention in its general form will first be
described, and then its implementation in terms of specific
embodiments will be detailed with reference to the drawings
following hereafter. These embodiments are intended to
demonstrate the principle of the invention, and the manner of its
implementation. The invention in its broadest and more specific
forms will then be further described, and defined, in each of the
individual claims which conclude this Specification.
SUMMARY OF THE INVENTION
In one aspect of the present invention, there is provided a desalination
system comprising: (1) a
vapour-containing desalinator container with a heat receiving surface for
receiving heat from an
exterior source and delivering heat into the interior of the container; (2) a
saltwater source for the
supply of saltwater; (3) an evaporator surface comprising a displaceable
hydrophilic water
support, positioned to convey a thin film of salt water from the supply of
salt water to the interior
of the container whereby the salt water will receive heat from the heat
receiving surface within
the desalination container and to thereupon emit water vapour; (4) a
condensing surface
positioned within the container, proximate the evaporator surface, to receive
the water vapour
.and upon which the water vapour may condense and produce fresh water; and (5)
fresh water
collection means to collect fresh water condensing on the condensing surface,
wherein water
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vapour emitted from the hydrophilic water support by exposure to the heat
within the container
passes to the condensing surface to be condensed into freshwater.
According to the present invention, in one aspect an evaporator surface is
provided for the water
portion of salt water to evaporate when heated with solar energy or any other
form of heat. A
cooler, condensing surface is provided proximately to the evaporator surface
for the vapour to
condense and produce fresh water. Preferably, the vapour passes to the
condensing surface
primarily by diffusion. In order for the water to be evaporated more readily
it is spread thinly
over the evaporator surface and simultaneously heated. This encourages the
water component to
evaporate readily, provided that the vapour pressure in the
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evaporating environment can be maintained below the equilibrium
value at the temperature of the water present therein.
According to one variant of the invention the salt water
is preferably carried by a hydrophilic fibrous support, such as
cotton, in order for it to evaporate more readily. Woven cotton
spreads water very well due to the wicking effect caused by the
surface tension of water.
Though cotton can soak-up a large
amount of water, it can also release it very easily when heated
to relatively low temperatures in air whose relative humidity is
less than 100% at these temperatures. In our daily lives we do
this when we dry our clothes in an electric drier or in the sun.
When drying in the sun, it takes a short time when the humidity
of the air is low and a longer time when the humidity is high.
Although cotton is a preferred support for evaporating
water, any hydrophilic fiber or surface may be employed, such a
sisal, hemp, fine wire mesh and other similar materials.
According to a further variant of the invention, a layer
of cotton, preferably woven, is positioned inside a flat plate-
type solar heat collector presented at a suitable angle to capture
solar energy. The vapour produced by the heat absorbed is then
condensed on an adjacent cooler surface and collected as fresh
water.
Salt water may be dripped into a woven cotton evaporator
layer while the cotton is held taut within the interior of a
desalinator container and heat is applied through the collector
surface to the cotton layer. It is preferable to hold the cotton
taut to minimize local accumulation of water and avoid dripping.
Water vapour is produced due to heat that is provided. At the
same time water is condensed on a cooler condensing surface
located adjacent to the hydrophilic cotton evaporator surface,
across a gap within the desalinator container. The evaporator
support, eg. cotton, may or may not touch the heat collection
surface, so long as it is exposed to heat.
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The solar collector side of the desalinator may be made
according to the current knowledge on solar energy collectors
including the use of a simple glass plate. A stainless steel
sheet may be alternately employed as the heat collection surface,
with the evaporator layer preferably placed in intimate contact
with such surface.
An advantage of using a hydrophilic fiber such as cotton
for the evaporator is that the temperature rise in the desalinator
container will be limited by the absorption of heat through
vaporization of water. This will reduce the cooling load that
must be accommodated by the collection surface.
There are various methods available to increase the
capture of energy through the use of black bodies, including
cotton dyed black and the use of a glass painted black. The angle
of the energy capture surface and with it the cotton layer can be
easily adjusted from vertical to almost horizontal to maximize the
capture of the heat energy.
At controlled flow rates of water through the evaporator
layer, cotton of a density suitable for desalination, will hold
the salt water sufficiently well so that no dripping from the
cotton onto the condensing side of the desalinator occurs even
when the heat capture side is almost horizontal.
The vapour condensing surface can be cooled by one of
many means available. Cold salt water may be made to flow over
external portion of the condensing surface via a cotton layer
placed on the outside of the desalinator container. In this case
the cotton is used advantageously to do even more for the
desalinator. The layer of cotton placed on the external side of
the condensing surface of the desalinator can carry a significant
amount of water to effectively cool the surface for vapour
condensation to occur on the inside surface.
Alternately, a
jacket may be provided on the external side for cooling water to
flow directly against the cooled wall of the desalinator on the
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opposite side to the condensing surface. As a further alternative
a water pump may used to spray cooling water on the cooled wall
surface.
Hence, a large number of choices is available for
cooling the condensing side of the desalinator.
The condensing surface may be made of a number of
materials that can be cooled easily including glass or stainless
steel. Since the condensed water that comes in contact with the
cooling surface is fresh water free of salt, problems related to
corrosion are reduced on this side of the desalinator. If the
external cooling is not carried out with salt water then a large
choice of materials such as aluminum and plain steel is available
for construction of this surface.
Even without external cooling, the condensing surface
will be cooler since heat is applied to the evaporator cotton even
if some of that heat is absorbed by the water evaporating from it.
Therefore, in some cases active external cooling may not have to
be provided at all for the vapour to condense.
The escape of vapour from the desalinator container's
interior should be minimized by at least closing its upper end,
and preferably by closing the lower end as well. The objective
is to contain convective air flow. Thus the narrow ends of the
desalinator are preferably covered to avoid water vapour escaping
the desalinator. These covers may be made of a suitable material
such as wood, plastic or metal.
The fresh water produced from the condensing vapour can
be removed by a number of methods that make sure that a minimum
amount of vapour within the desalinator is allowed to escape.
Condensing water may be allowed to drip from the desalinator
container through a narrow slot. The condensing water may also
be removed from the desalinator using a short length of cotton
fabric wedged between the condensing surface and the cover plate
to serve as a wick. The bottom edge of this cotton layer may then
be cut to form one or more V-notches so that the condensed water
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rolling down the cotton will drip from the V-notches in an
uniform, directed fashion into collection containers. Within the
desalinator, this wicking layer of cotton need only extend a few
millimetres into the interior of the desalinator container so that
the area of the available cooling surface is not reduced.
The flow of sea water through the desalinator may be
controlled depending on the type of operation desired.
For
example, the flow may be sufficiently small such that no brine
exits the bottom of the desalinator. In this case, the water is
completely evaporated leaving salt to accumulate on the evaporator
surface. On the other hand, a very small flow of water may be
allowed to exit the evaporator in which case all the cotton inside
the evaporator remains wet at the temperature of evaporation. In
any case, the flow of water through the evaporator is preferably
kept low so that the water is evaporated efficiently without
wasting the injected heat through the exiting of heated water from
the desalinator.
Periodically the cotton will have to be removed from the
desalinator and the accumulated salt removed by washing the cotton
with sea water, or by simply shaking it to let the salt fall off
the cotton.
In this invention the fresh water production rate
depends on the rate of flow of water through the evaporator cotton
and the rate of heat capture through the heat collector surface.
As mentioned above, wastage of heat can be avoided if excess water
is not flowed through the evaporator cotton. Excess water flowing
this way will carry the heat out of the desalinator rendering the
efficiency lower than achievable.
As a further variant of the invention, the woven
evaporator cotton may be in the form of a circulating band that
passes adjacent to the heat collecting surface. Similarly the
condensing surface may be cooled by a circulating band of moist
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cotton fabric. Preferably the rotations of these two bands are
in opposite directions.
As another variant, the evaporating cotton may be fixed
to a rotating wheel that carries moisture up from a reservoir and
passes it closely to the heat collecting surface.
A similar
cooling wheel of cotton may optionally be used to carry cooling
water up to the external side of the condensing surface.
A further variant of the desalinator consists of a
chamber with a sloping side to receive the sun energy, a
horizontal surface to hold the salt water, and a cooler, nearly
vertical side to condense the water vapour. In its simplest form,
the salt water carried on the bottom floor of the desalinator in
a shallow pond inside the desalinator is heated by the sun rays.
A glass or any other suitable plastic material that would allow
the sun's rays to pass through with minimal absorption and
reflection is used on the sloping, solar energy capture side.
The condensation surface may be vertical or sloped and
cooled by water or air either internally or on its exterior
surface. The salt water (e.g. sea water) being fed to the pond may
also be used to cool the condensation side by first passing it
through a cooling tank formed on the outside of the condensation
surface.
In this way the heat of condensation can also be
recovered, rendering the process more efficient.
A continuous stream of sea water may flow through the
desalinator instead of a batch type operation.
The condensed water rolling down the cooler surfaces may
be collected by a number of methods known in the art.
For
example, troughs may be positioned to run along the base of the
condensing walls to collect the condensing water and then carry
water outside into fresh water collection containers.
Alternately, wicks may be placed with one end inside, at the base
of the condensing surface, and the other end placed outside the
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desalinator to deliver the condensing water to a receiving
container
To promote the evaporation of water from the salt water
pond suitable hydrophilic, wettable materials such as black
cotton or a fine wire mesh may be placed on a form in the shape
of a rotating cylinder positioned within the desalinator chamber.
This cylinder may be rotated by a simple low speed motor,
optionally powered by solar energy. The rotating cylinder should
be placed in such a way that the lower part of it is immersed in
the sea water pond. As the cylinder is rotated, the cotton picks
up the water from the pond and the sun rays, streaming through the
glass, heats the water carried on the cylinder and evaporates it.
The water vapour then condenses on cooler surfaces within the
desalinator and water droplets that roll down these surfaces are
collected. The diameter and length of the cylinder should be as
large as possible to occupy the volume of the desalinator so that
the evaporation surface is maximized. The direction of rotation
should be such that the water picked up from the pond is heated
the best way possible.
This may mean that the direction of
rotation will be clockwise.
The cylinder may be made by any convenient way. Thin
strips of wood or a suitable metal (e.g. stainless steel) attached
to circular end plates to form a frame may be used. The cotton
can then be wrapped around this frame to complete the cylinder.
Another method is to form a cylinder using a metal screen with
suitable end plates and use this as the frame around which the
cotton is wrapped. Of course the end plates should be amenable
to passing the shaft of the motor through.
The evaporation surface need not be limited to cotton.
Any suitable material on which water would form a film on may be
used. For example, fine mesh screens (woven, expanded) or thin
sheets of metal punched with holes are suitable. The materials
should be selected so that they are compatible with salt water.
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Examples of suitable screening are 10 to 400 mesh stainless
screen, woven or expanded. The small openings present in these
screens are ideal for water to be picked up as a thin film because
of surface tension properties. Also, the screen may be corrugated
so that the available surface area can be further increased quite
significantly. The depth and pitch of the corrugations can be
adjusted to alter the available area for evaporation.
The foregoing summarizes the principal features of the
invention and some of its optional aspects. The invention maybe
further understood by the description of the preferred
embodiments, in conjunction with the drawings, which now follow.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a side view of the desalinator showing the
stationery cotton layer used for evaporation and the condensing
surface;
Figure 2 is a side view showing the desalinator with
circulating wetted cotton used for both evaporation and for
cooling; and
Figure 3 is a side view showing desalination using a
wheel of cotton for evaporation.
Figure 4 is a face view of Figure 3.
Figure 5 is a pictorial depiction of an alternate
embodiment wherein condensation occurs outside of the desalinator
container.
Figure 6 is a schematic pictorial depiction of a
desalinator using a rotating cylinder as the evaporator.
DESCRIPTIONOFTHEPREFERREDEMBODIMENT
In the installation of Figure 1 a heat collector surface 1 is made of a glass
plate or metal is
provided to serve as a heat receiving surface. A cotton layer 2 is positioned
adjacent the inner side
of this heat collector surface 1 to serve as a hydrophilic salt water support
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inside surface of the glass may be coated with black paint to increase
absorption of the heat
energy. Alternately, the cotton layer 2 may be dyed black. The cotton 2 is
preferably in intimate
contact with this heat collecting surface 1 so that the heat is transferred
efficiently to the salt water
3 being introduced therein from a salt water source.
The evaporating water 4 (released by the sun's rays 42) in vapour state is
condensed on the
condensing surface 6 located on the other internal side of the desalinator
container 5 to provide
condensed fresh water 7. This condensing surface 6 can be maintained at a
cooler temperature
simply by circulating cool salt water 8 through a cotton-drenched cooling
layer 9 placed against its
exterior surface. If sea water is used for this cooling process, it may be
necessary to use glass or
stainless steel on this side so that rusting-related problems are avoided.
As a schematic demonstration, a receiving container 11 with three compartments
11 a, 1 1 b, and
11c is positioned to respectively catch the escaping brine 10, the condensed
water 7, and the
cooling water 8.
A significant amount of the heat is recovered from the condensing water vapour
4 developed
within the desalinator. This recovered heat is present within the cooling
water 8 present on the
cooling layer 9 placed on the exterior side of the condensing surface 6. This
partially heated water
8 can be diverted as feed water for the evaporator side of the desalinator,
taking advantage of the
heat of condensation that it has absorbed.
The invention outlined above using cotton overcomes number of deficiencies of
the solar
distillation apparatus markets as the Rosendahl system. In the Rosendahl
system, sea water is
evaporated by flowing it over a surface made of a special material ("soakage
filler") and then
condensed on the
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inside surface of the same glass plate-covering that lets the heat
waves in. Since heat is received through this glass covering, the
condensation process is made inefficient.
In the present
invention the vapour 4 is condensed on a significantly cooler
surface condensating surface 6 since it is on the other side of
the desalination container from cotton-evaporator layer 2.
Allowing the vapour 4 to condense on the "non-heated"
side of the desalinator container 5 in the current invention is
more efficient since the condensed water 7 is not heated
unnecessarily by the heat entering through the heat collector
surface 1.
According to the invention, the ability of the cotton
2 to spread the water 3 on its surface and hold a significant
amount avoids the need for specialized materials for evaporation.
The fact that the water 3 can be held by cotton 2 without dripping
enables the provision of a condensing surface 6 that is located
below the evaporating surface.
Another method of desalination using the above idea is
shown in Figure 2. In this apparatus an endless evaporator cotton
band 20 is circulated through the desalinator container 5 and a
salt water reservoir 21 so that the salt water 22 picked up in the
reservoir 21 by the cotton band 20 is evaporated fully or
substantially within the desalinator container 5. The use of
cotton to transport the water 22 into the desalinator in this
fashion renders it possible for the water 22 to be heated
conveniently by solar energy compared to other known arrangements.
Here again, the current knowledge available to capture the sun
energy can be used to its fullest capacity.
If the cotton is mostly dry as it exits the desalinator,
provisions can be made to remove the salt residue on it by means
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of continuous vibration or scraping applied to the moving belt.
In this case, salt can be a by-product of the process.
A similar endless cotton band 23 may be used to cool the
condensing surface.
Preferably this band 23 rotates in the
opposite direction to band 20.
In case the heat collector is made of a material that
is prone to corrosion, then the circulating cotton band 20 may be
made to pass adjacent to the interior side of the heating surface
1 without coming into contact with the actual heat collector
surface 1.
If it is required to squeeze excess water out of
cotton band 20 to avoid dripping within the desalinator, wiper
blades 24 made of rubber materials that are resistant to salt
water may be positioned at the entrance to the desalinator
container 5.
If a need arises to place a separating layer between the
circulating, heated, evaporator cotton band 20 and the
condensation side, Teflon coated screens (not shown) may be used.
Aluminum or stainless mesh screens may be coated with Teflon so
that salt water 22 will not come in contact with the condensing
surface 6. These screens placed between the evaporator cotton
band 20 and the condensing surface 6 can be perforated so that the
vapour 4 can flow freely through the screen without any
resistance.
Since the only power requirement for this variant of the
desalinator is that required to circulate the cotton belts 20,23,
use of a low hp motor 25 with a minimal overall power requirement
is possible.
For a given heat collector with a specific area and heat
absorption characteristics, the rate of fresh water 7 production
in this apparatus can be controlled by adjusting the evaporator
cotton belt 20 recirculation rate. The rate of circulation of
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cotton belt 20 may be programmed within a controller 26 to be a
function of the heat available for absorption which would depend
on the time of the day (when solar energy is used for
evaporation). In this manner, the temperature rise within the
belt 20 can be adjusted to the maximum level possible.
The use of recirculating cotton 20 allows flexibility
to produce very little waste brine 10 from the process since the
rate of circulation can be adjusted to evaporate all most all the
salt water 22 entering the desalinator container 5. The salt
residue on the cotton belt 20 can be removed by a simple scraping
or by vibration, or it can be redissolved in the sea water 22 in
the pond or reservoir 21.
Another configuration for the desalinator is shown in
Figure 3. This is similar to dehumidification wheels used for
building humidity control. An evaporator wheel 30 carrying a
layer of cotton, is almost half immersed in a sea water pond 31
rotates slowly, thereby picking up the water 32 from the pond and
losing it in the top section where solar energy is used to
evaporate the water 32. As the side view shows, the top half or
so of the wheel 30 is contained within the desalinator chamber 5,
adjacent a heat collecting surface 1 and adjacent a condensing
surface 6. The condensing surface 6 can be cooled by one of many
methods including another rotating cotton-covered cooling wheel
33 located in intimate contact with the external side of the
condensation surface 6.
A pump (not shown) may alternately be used instead to
simply spray the pond water over the external side of the
condensing surface 6 to maintain a lower temperature.
The structure supporting the evaporator and cooling
wheels 30, 33 may be made of plastic materials. Seals 34 between
the rotating wheels 30,33 and the desalinator chamber 5 may be
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maintained with rubber wiper blade-type seals to provide maximum
containment of water vapour 4.
In another configuration of the desalinator shown in
Figure 5, multiple layers of cotton 40 may be used to evaporate
the water in a common chamber 41 heated by solar energy 42 or
other forms of heat. Sea water 43 can be gently sprayed on the
layers of cotton 40 placed in the chamber 41 at suitable angles
to capture the heat. The vapour 44 produced may be transferred
continuously by applying a slight vacuum provided by a vacuum pump
45 into a separate cooling chamber 46 where it is condensed to
form fresh water 47.
Example 1
A test was carried out using the set-up shown in Figure
1 with the evaporator cotton vertical and in contact with the heat
collector surface.
A 250 w infra red heat lamp was used to
simulate solar power. It was placed 50 mm away from the heat
collection surface of the desalinator. Approximately 144 mL/h
of water were dripped onto the top end of the evaporator cotton
and allowed to flow down the cotton. Sufficient water was made
to flow over the condensation side cooling cotton to maintain it
in a moist state.
Salt-free condensed water soaked the small piece of
cotton at the bottom on the fresh water condensation surface of
the desalinator and dripped from it into a container within 15 min
of starting the experiment.
In a 90 min long test, approximately 45 mL of fresh
water was collected. The density of this fresh water was found
to be 0.999 g/mL showing that there was no salt in the water. The
material of the heat collection surface was stainless steel foil
and the cotton layer was 100 mm wide by 300 mm long with a
thickness of 0.3 mm. The gap between the cotton layer and the
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condensing surface was around 20mm. The salt water used for the
test had a density of 1.033 g/mL which translates into 4.1% salt
in water and the brine collected had a density of 1.083 g/mL which
represents about 11.5% salt in water. The temperature of vapour
within the desalinator was within the range of 40 to 50'C and this
temperature was reached within 5 min of starting the experiment.
The exterior ambient temperature was 200C.
Based on theoretical assessments, it is believed that a
heat collector surface of 150 mm by 300 mm dimensions exposed to
a solar flux of 256 cal/s/m2 can produce 50 grams of water per
hour as a maximum. The gap between the evaporator cotton layer
and the condensing surface should be as short as possible. A gap
of lOmm is believed to be practical if provision is made to
minimize sagging of the cotton layer. In larger units a 20mm or
even a 30-40mm gap may be suitable.
Figure 6 shows another form of the desalinator. In this
arrangement a chamber 48, shown with panels cut out for a clear
view of the inner components, includes a glass panel 49 through
which solar energy will pass. This energy is absorbed by water
present on the rotating hydrophilic screen 50 which is partially
immersed in a salt water pond 51. As the warm screen 50 rotates
by mean of the drive motor 52 evaporation takes place. The vapour
condenses on the surface 6 of a tank of cool water 53 and is
collected as fresh water 7 in the fresh water tank 54.
The attached Table presents some of the results obtained from
an experimental rotating cylinder-type desalinator. A desalinator
chamber, approximately 24 cm wide (glass side) by 30 cm long (side
walls) by 30 cm high (condensation side), was assembled using wood
pieces, thin aluminum sheets, Silicone sealant, 28-mesh stainless
screen, a glass plate, a 12 V dc motor and photovoltaic cells.
A 250 W infrared heat lamp was used to simulate the sun. A wooden
frame was first built and sheets of aluminum cut and folded in
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appropriate dimensions were placed inside the wooden frame to form
the vessel shape as shown in Figure 6. The seams were sealed with
Silicone sealant.
An 11-cm diameter cylinder was fabricated out of the 28-
mesh stainless steel screen with aluminum end plates. A threaded
rod was passed through the end plates and the screen was secured
in place with bolts and nuts. The threaded rod was also passed
through two holes on the opposite sidewalls of the desalinator
chamber with one end connected to the motor to mount the shaft
for rotation. The height of the shaft above the pond water was
such that the lower part of the screen cylinder was immersed in
the saltwater in the pond.
After placing a known volume of a salt solution
(prepared from tap water and cooking salt) in the pond, the sheet
of glass was secured on the desalinator making sure that a good
vapour seal was formed between the glass plate and the desalinator
vessel. The motor was switched on to rotate the screen cylinder,
and the number of batteries employed was adjusted to control the
speed of rotation.
Since this work was done inside the home
during winter/spring months, batteries were used instead of the
solar cells to power The motor.
The cooling water tank in the backside of the
desalinator was filled with tap water. An infrared heat lamp was
turned on after setting a timer for 5 hours. Temperatures at
various locations within the desalinator were measured and
recorded periodically. An immersed copper coil carrying cooling
water was used to maintain the temperature of the water in the
cooling water tank (at the back of the desalinator condensation
surface) below 27 C.
The water from the saltwater pond evaporated and
condensed on the inside walls of the desalinator. Condensation
was heavy on the surface of the back wall, which was cooled by the
cooling water in the tank. The condensing water rolled down the
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condensing surface walls and soaked cotton wicking strips that had
been placed at the base of the condensing surface to drain the
water out into a collection container.
After a period of 5 hours, the heat lamp and the motor
were turned off and the amount of water collected was measured.
When the desalinator had cooled down to ambient temperature, any
water present on the inside walls and the floor of the desalinator
was syringed out and its volume measured. The total volumes and
densities of the fresh 'water collected and the remaining saltwater
inside the desalinator were measured and recorded.
The attached Table shows the performance, using various
combinations of parameters, of a rotating cylinder desalinator.
This Table indicates that the presence of a rotating cylinder of
black cotton or plain 28 mesh screen or corrugated 28 mesh screen
increased the amount of water evaporated and recovered as fresh
water quite significantly.
Based on the foregoing, an efficient, low cost system
may be provided to produce fresh water from salt water.
CONCLUSION
The foregoing has constituted a description of specific
embodiments showing how the invention may be applied and put into
use. These embodiments are only exemplary. The invention in its
broadest, and more
specific aspects, is further described and
defined in the claims which now follow.
These claims, and the language used therein, are to be
understood in terms of the variants of the invention which have
been described. They are not to be restricted to such variants,
but are to be read as
covering the full scope of the invention
as is implicit within the invention and the disclosure that has
been provided herein.
18
:
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ITable : Results from desalinator experiments with and without screen
rotators.
I Exp#18 Exp# 3
Exp#17 Exp#20 Exp#22
__________________________________________________ _A.
Screen (stainless screen) None 28 mesh 28 mesh
corrugated 28 mesh corrugated Black Cotton
, Speed of motor ______________ rpm 0 1.4
2.9 5 5
i --,
Distance of lamp from glass cm 12.7 12.7
12.7 12.7 12.7
i
,
Test duration h 5 5
5 5 5
i
IVolume of salt water started with mL 800 800
800 800 800
Volume of brine at the end of experiment -mL 648 616.8
556.8 = 502.2 573.4
_
Volume of fresh water collected mL 124.2 140.6
214.4 242.4 170.9
Density of salt water started with g/mL t034 1.00
1.033- 1.03 1.032 0
Density of fresh water collected Ig/mL 0.9991 0.9981
0.999 0.999.: 0 999
I
. Temperature of vapour within desalinator 1
1 75T 68-6- I
88 '
821 81 0iwv
0.
i Temperature of salt water in pond C 55 5-5.
52 53 co .4
0.
Temperature of screen surface C - -
53 55 53
0,
µ Temperature of water in the cooling tank C
25 to 29 18-20 _____ 25 to 29 25 24,
_
0
0
0
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1-.
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