Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND APPARATUS FOR THE REGASIFICATION OF
LNG ONBOARD A CARRIER
Field of the Invention
The invention relates to the transportation and regasification of liquefied
natural gas
(LNG).
Background of the Invention
Natural gas typically is transported from the location where it is produced to
the location
where it is consumed by a pipeline. However, large quantities of natural gas
may be produced in
a country in which production by far exceeds demand. Without an effective way
to transport the
natural gas to a location where there is a commercial demand, the gas may be
burned as it is
produced, which is wasteful.
Liquefaction of the natural gas facilitates storage and transportation of the
natural gas.
Liquefied natural gas ("LNG") takes up only about 1/600 of the volume that the
same amount of
natural gas does in its gaseous state. LNG is produced by cooling natural gas
below its boiling
point (-259 F at ambient pressures). LNG may be stored in cryogenic
containers either at or
slightly above atmospheric pressure. By raising the temperature of the LNG, it
may be
converted back to its gaseous form.
The growing demand for natural gas has stimulated the transportation of LNG by
special
tanker ships. Natural gas produced in remote locations, such as Algeria,
Borneo, or Indonesia,
may be liquefied and shipped overseas in this manner to Europe, Japan, or the
United States.
Typically, the natural gas is gathered through one or more pipelines to a land-
based liquefaction
facility. The LNG is then loaded onto a tanker equipped with cryogenic
compartments (such a
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tanker may be referred to as an LNG carrier or "LNGC") by pumping it through a
relatively
short pipeline. After the LNGC reaches the destination port, the LNG is
offloaded by cryogenic
pump to a land-based regasification facility, where it may be stored in a
liquid state or regasified.
To regasify the LNG, the temperature is raised until it exceeds the LNG
boiling point, causing
the LNG to return to a gaseous state. The resulting natural gas then may be
distributed through a
pipeline system to various locations where it is consumed.
For safety, ecological, and/or aesthetic considerations, it has been proposed
that
regasification of the LNG take place offshore. A regasification facility may
be constructed on a
fixed platform located offshore, or on a floating barge or other vessel that
is moored offshore.
The LNGC may be either docked or moored next to the offshore regasification
platform or
vessel, so that LNG may then be offloaded by conventional means for either
storage or
regasification. After regasification, the natural gas may be transferred to an
onshore pipeline
distribution system.
It also has been proposed that regasification take place onboard the LNGC.
This has
certain advantages, in that the regasification facility travels with the LNGC.
This can make it
easier to accommodate natural gas demands that are more seasonal or otherwise
vary from
location to location. Because the regasification facility travels with the
LNGC, it is not
necessary to provide a separate LNG storage and regasification facility,
either onshore or
offshore, at each location at which LNG may be delivered. Instead, the LNGC
fitted with
regasification facilities may be moored offshore and connected to a pipeline
distribution system
through a connection located on an offshore buoy or platform.
When the regasification facility is located onboard the LNGC, the source of
the heat used
to regasify the LNG may be transferred by use of an intermediate fluid that
has been heated by a
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boiler located on the LNGC. The heated fluid may then be passed through a heat
exchanger that
is in contact with the LNG.
It also has been proposed that the heat source be seawater in the vicinity of
the LNGC.
As the temperature of the seawater is higher than the boiling point of the LNG
and the minimum
pipeline distribution temperature, it may be pumped through a heat exchanger
to warm and
regasify the LNG. However, as the LNG is warmed, regasified, and superheated,
the seawater is
chilled as a result of the heat transfer between the two fluids. Care must be
taken to avoid
cooling the seawater below its freezing point. This requires that the flow
rates of the LNG being
warmed and the seawater being used to warm the LNG be carefully controlled.
Proper balancing
of the flow rates is affected by the ambient temperature of the seawater, as
well as the desired
rate of gasification of the LNG. Ambient temperature of the seawater can be
affected by the
location where the LNGC is to be moored, the time of year when delivery
occurs, the depth of
the water, and even the manner in which the chilled seawater from warming the
LNG is
discharged. Moreover, the manner in which the chilled seawater is discharged
may be affected
by environmental considerations, e.g., trying to avoid an undesirable
environmental impact such
as ambient water temperature depression in the vicinity of the chilled
seawater discharge.
Environmental concerns can affect the rate at which the LNG may be heated,
and, therefore, the
volume of LNG that can be gasified in a given period of time with
regasification equipment on
board the LNGC.
Summary of Invention
In one aspect, the present invention relates to an LNGC that has a
regasification system
that includes an onboard vaporizer for vaporizing the LNG, a primary source of
heat, and one or
more secondary sources of heat to the LNG and vaporizer.
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Brief Description of Drawings
Figure 1 is a schematic of a prior art keel cooler system.
Figure 2 is a schematic of a submerged heat exchanger used as a source of heat
for the
vaporizer.
Figure 3 is a schematic of an alternative dual heat source system.
Figure 4A is a partial cross-section of the LNGC at approximately mid-ship,
showing the
heat exchanger stored on deck.
Figure 4B is a partial cross-section of the LNGC at approximately mid-ship,
showing the
heat exchanger lowered into the water.
Figure 5 is a partial cross-section of an alternative preferred embodiment of
the LNGC,
showing the ship hull integrally moored on a buoy, and two heat exchangers
attached to the
mooring buoy and fluidly connected to the LNGC after it moors to the buoy.
Detailed Description
Various improvements can be made to the manner in which LNG is regasified
onboard an
LNGC. Specifically, there are other sources of heat, components for heat
transfer, and
combinations of heat sources, that can be used to provide additional
flexibility with respect to the
locations and the environmental impact of the onboard LNGC regasification.
Devices commonly referred to as "keel coolers" have been used in the past to
provide a
source of cooling for marine equipment, such as propulsion engine coolers and
air conditioning.
As shown in FIG. 1, the keel cooler 2 is a submerged heat exchanger that
typically is located on
or near the bottom of the ship's hull 1, and uses ocean water as a "heat sink"
for the heat
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generated by onboard equipment (such as marine air conditioning units 3) that
requires cooling
capacity.
The keel cooler 2 operates by either using one or more pods (not shown) that
are either
built into the lower part of the hull 1 or attached to the exterior of the
hull 1 as a heat exchanger
that cools an intermediate fluid (such as fresh water or a glycol) that is
circulated by pump 1
through the pod. This intermediate fluid is then pumped to one or more
locations on the ship to
absorb excess heat. Such keel coolers are available commercially from
manufacturers such as
R.W. Fernstrum & Co. (Menominee, MI) and Duramax Marine LLC (Hiram, OH).
Among the advantages of such a system, as compared to a system that brings in
and
subsequently discharges seawater used as a cooling fluid, is the reduced
sinking hazard and
corrosion hazard that is associated with the circulation of the seawater to
various locations
onboard the ship. Only the exterior of the keel cooler pod 2 is exposed to the
seawater, fresh
water, or another relatively non-corrosive fluid that is circulated through
the remainder of what
amounts to a closed system. Pumps, piping, valves, and other components in the
closed loop
system do not need to be manufactured from more exotic materials that would be
resistant to sea
water corrosion. Keel coolers 2 also obviate the need for filtering the
seawater, as may be
required in a system that passes seawater into the interior of the shipboard
machinery
components.
As shown in FIG. 2, in one preferred embodiment of the invention, one or more
primary
sources of heat, which are preferably submerged heat exchangers 21, are
employed - not to
provide cooling capacity, but instead to provide heating capacity for the
closed loop circulating
fluid, which in turn is used to regasify the LNG.
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In the preferred embodiment, the heat exchangers 21, instead of being mounted
in the
ship hull 1 like a traditional keel cooler, are separate heat exchangers 21
that are lowered into the
water after the LNG vessel reaches its offshore discharge facility or
terminal. In the most
preferred embodiment, two heat exchangers 21 are used, each of which is about
20 feet by 20
feet by 40 feet, and collectively meet the heating needs of the LNGC. Each of
these heat
exchangers 21 has the capacity of about 100 conventional keel coolers. The
heat exchangers 21
are connected to the LNGC by suitable piping 66, which may be flexible or
rigid. Referring to
FIGS. 4A and 4B, the heat exchangers 21 are preferably stored on deck when not
in use (see
FIG.4A), and may be stored under a cover, in a shed, or in some other
structure (not shown). At
time of use, the heat exchangers 21 are lowered by mechanical equipment 64,
such as, but not
limited to, a winch system or elevator system, which equipment is well known
to those skilled in
the art (see FIG. 4B). After lowering the heat exchangers 21 into the water,
rigid attachment of
the heat exchangers 21 to the ship is preferred where there is concern that
the heat exchangers 21
might bump against the ship.
In another preferred embodiment, the heat exchangers 21 are permanently
submerged
installations at the offshore discharge terminal. For example, the submerged
heat exchanger
system 21 may be mounted to the buoy 68 that is used to moor the LNGC. Either
of these
alternative heat exchanger 21 configurations (FIG. 4B, 5) is connected to the
LNGC so as to
allow the intermediate fluid to be circulated through the submerged heat
exchangers 21.
When the heat exchangers 21 are attached to a mooring buoy 68, an LNGC turret
recess
78 mates with the buoy 68, permitting the LNGC to rotate around the buoy 68.
The heat
exchangers 21 are connected by lines 74 to the ship hull 1, and thereby
fluidly connected to the
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vaporizer 23 and to any secondary sources of heat 26. A gas pipe riser 72
connects the LNGC
and a pipeline distribution system for offloading the regasified LNG.
In another embodiment of the invention, one or more submerged heat exchanger
units 21
are located at any suitable location below the waterline of the hull 1, and
are mounted directly
within the hull 1 of the LNGC. Alternatively, the heat exchangers 21 may be
partially, rather
than fully, submerged.
An intermediate fluid, such as glycol, propane or fresh water, is circulated
by a pump 22
through the vaporizer 23 and the submerged heat exchangers 21. Other
intermediate fluids
having suitable characteristics, such as acceptable heat capacity and boiling
points, also may be
used and are commonly known in the industry. LNG is passed into the vaporizer
23 through line
24 where it is regasified and exits through line 25.
The submerged heat exchangers 21 enable heat transfer from the surrounding
seawater to
the circulated intermediate fluid without the intake or pumping of seawater
into the LNGC, as
mentioned above. The size and surface area of the heat exchangers 21 may vary
widely,
depending upon the volume of LNG cargo being regasified for delivery and the
temperature
ranges of the water in which the LNGC makes delivery of natural gas.
For example, if the temperature of the circulated intermediate fluid is
approximately 45
F upon return to the submerged heat exchangers 21 and the seawater temperature
is about 59 F,
the temperature differential between the two is about 14 F. This is a
relatively modest
temperature differential, and, accordingly, the heat exchangers 21 will
require a larger surface
area to accommodate the heat transfer needs of the present invention, as
compared to the typical
keel coolers described above, which were designed for the rejection of a few
million BTUs per
hour.
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In one preferred embodiment, two submerged heat exchangers 21, collectively
designed
to absorb approximately 62 million BTUs per hour and having approximately
450,000 square
feet of surface area are used. These heat exchangers 21 are about 20 feet by
20 feet by 40 feet
and preferably contain bundles of tubes that are exposed to permit water to
pass over them, while
intermediate fluid circulates inside the tubes. This quantity of surface area
may be arranged in a
variety of configurations, however, including, in the preferred embodiment,
multiple tube
bundles arranged similarly to those in conventional keel coolers 2. The heat
exchanger 21 of the
present invention may also be a shell and tube heat exchanger, a bent-tube
fixed-tube-sheet
exchanger, spiral tube exchanger, plate-type exchanger, or other heat
exchangers commonly
known by those skilled in the art that meet the temperature, volumetric and
heat absorption
requirements for the LNG to be regasified.
The vaporizer 23 preferably is a shell and tube vaporizer, and such a
vaporizer 23 is
schematically depicted in FIG. 2. This type of vaporizer 23 is well known to
the industry, and is
similar to several dozen water heated shell and tube vaporizers in service at
land-based
regasification facilities. Other types of vaporizers that may be used include,
but are not limited
to, intermediate fluid vaporizers and submerged combustion vaporizers. In
alternative shipboard
applications, where seawater may be one of the heating mediums or may contact
the equipment,
the vaporizer 23 is preferably made of a proprietary AL-6XN super-austenitic
stainless steel
(ASTM A-240, B688, UNS N08367) for wetted surfaces in contact with seawater
and type 316L
stainless steel for all other surfaces of the vaporizer 23. A variety of
materials may be used for
the vaporizer, including but not limited to titanium alloys and compounds.
In the preferred embodiment, a shell and tube vaporizer 23 is used that
produces about
100 million standard cubic feet per day ("mmscf/d") of LNG with a molecular
weight of about
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16.9. For example, when operating the LNGC in seawater with a temperature of
about 59 F and
an intermediate fluid temperature of about 45 F, the vaporizer 23 will
require a heated water
flow of about 2,000 cubic meters per hour. The resulting heat transfer of
approximately 62
million BTUs per hour is preferably achieved using a single tube bundle of
about forty foot long
tubes, preferably about 3/4 inch in diameter. Special design features are
incorporated in the
vaporizer 23 to assure uniform distribution of LNG in the tubes, to
accommodate the differential
thermal contraction between the tubes and the shell, to preclude freezing of
the heating water
medium, and to accommodate the added loads from shipboard accelerations. In
the most
preferred embodiment, parallel installation of 100 mmscf/d capacity vaporizers
23 are arranged
to achieve the total required output capacity for the regasification vessel.
Suppliers of these types
of vaporizers 23 in the U.S. include Chicago Power and Process, Inc. and
Manning and Lewis,
Inc.
In the preferred embodiment of the invention, the circulating pumps 22 for the
intermediate fluid are conventional single stage centrifugal pumps 22 driven
by synchronous
speed electrical motors. Single stage centrifugal pumps 22 are frequently used
for water/fluid
pumping in maritime and industrial applications, and are well known to those
skilled in the art.
The capacity of the circulating pumps 22 is selected based upon the quantity
of vaporizers 23
installed and the degree of redundancy desired.
For example, to accommodate about a 500 million standard cubic feet per day
("mmscf/d") design capacity, a shipboard installation of six vaporizers 23,
each with a capacity
of about 100 mmscf/d each, is made, providing a redundant vaporizer. The
required total heating
water circulation for this system is about 10,000 cubic meters per hour at the
design point, and
about 12,000 cubic meters per hour at the peak rating. Taking shipboard space
limitations into
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consideration, three puinps 22, each with a 5,000 cubic meter per hour
capacity are used and
provide a fully redundant unit at the design point circulation requirements of
10,000 cubic meters
per hour. If five vaporizers are used, then only two pumps are required. These
pumps 22 have a
total dynamic head of approximately 30 meters, and the power requirement for
each pump 22 is
approximately 950 kW (kilowatts). The suction and discharge piping for each
pump 22 is
preferably 650 mm diameter piping, but piping of other dimensions may be used.
The materials used for the pumps 22 and associated piping preferably can
withstand the
corrosive effects of seawater, and a variety of materials are available. In
the preferred
embodiment, the pump casings are made of nickel aluminum bronze alloy and the
impellers have
Monel*pump shafts. Monel*is a highly corrosive resistant nickel based alloy
containing
approximately 60 - 70% nickel, 22 - 35% copper, and small quantities of iron,
manganese,
silicon and carbon.
While the preferred embodiment of the invention is drawn to a single stage
centrifugal
pump 22, a number of types of pumps 22 that meet the required flow rates may
be used and are
available from pump suppliers. In altemative embodiments, the pumps 22 may be
smooth flow
and pulsating flow pumps, velocity-head or positive-displacement pumps, screw
pumps, rotary
pumps, vane pumps, gear pumps, radial-plunger pumps, swash-plate pumps,
plunger pumps and
piston pumps, or other pumps that meet the discharge head and flow rate
requirements of the
intermediate fluid. Drives for the pumps may be hydraulic motors, diesel
engines, DC motors, or
other prime movers with requisite speed and power characteristics.
A submerged or partially submerged heat exchanger system 21 may be used as
either the
only source of heat for regasification of the LNG, or, in an alternative
embodiment of the
invention as shown in F'IG. 3, may be used in conjunction with one or more
secondary sources
*Denotes Registered Trade-mark.
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of heat. In the event that the capacity of the submerged or partially
submerged heat exchanger
system 21, or the local sea water temperature, are not sufficient to provide
the amount of heat
required for the desired level of regasification operations, this embodiment
of the invention
provides operational advantages.
In one preferred altemative embodiment, the intermediate fluid is circulated
by pump 22
through steam heater 26, vaporizer 23, and one or more submerged or partially
submerged heat
exchangers 21. In the most preferred embodiment of the invention, the heat
exchanger 21 is
submerged. Steam froni a boiler or other source enters the steam heater 26
through line 31 and
exits as condensate through line 32. Valves 41, 42, and 43 permit the
isolation of steam heater
26 and the opening of bypass line 51, which allows the operation of the
vaporizer 23 with the
steam heater 26 removed from the circuit. Altematively, valves 44, 45, and 46
permit the
isolation of the submerged heat exchanger 21 and the opening of bypass line
52, which allows
operation of the vaporizer 23 with the submerged heat exchanger 21 removed
from the circuit.
The valves used are conventional gate or butterfly valves for isolation
purposes and are
constructed of materials suitable for the circulated fluid. In the case of
seawater, butterfly valves
are preferably made of cast steel or ductile iron construction with a
resilient liner material, such
as neoprene or viton. Gate valves are preferably made of bronze construction
with stainless steel
*
or Monel trim.
The steam heater 26 preferably is a conventional shell and tube heat exchanger
fitted with
a drain cooler to enable the heating of the circulated water, and may provide
either all or a
portion of the heat required for the LNG regasification. The steam heater 26
is preferably
provided with desuperheated steam at approximately 10 bars of pressure and
about 360 F
*Denotes Registered Trade-mark.
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temperature. The steam is condensed and sub-cooled in the steam heater 26 and
drain cooler and
returned to the vessel's steam plant at approximately 160 F.
In another alternative embodiment, the heating water medium in the steam
heater 26 and
drain cooler is seawater. A 90-10 copper nickel alloy is preferably used for
all wetted surfaces in
contact with the heating water medium. Shell side components in contact with
steam and
condensate are preferably carbon steel.
For the shipboard installation described above, three steam heaters 26 with
drain coolers
are used, each preferably providing 50% of the overall required capacity. Each
steam heater 26
with a drain cooler has the capacity for a heating water flow of about 5,000
cubic meters per
hour and a steam flow of about 50,000 kilograms per hour. Suitable steam heat
exchangers 26
are similar to steam surface condensers used in many shipboard, industrial and
utility
applications, and are available from heat exchanger manufacturers worldwide.
The addition of a seawater inlet 61 and a seawater outlet 62 for a flow
through seawater
system, permit seawater to be used as either a direct source of heat for the
vaporizer 23 or as an
additional source of heat to be used in conjunction with the steam heater 26,
instead of the
submerged heat exchangers 21. This is shown in FIG. 3 by the dashed lines.
Alternatively, the submerged or partially submerged heat exchanger system 21
may be
used as the secondary source of heat, while another source of heat is used as
the primary source
of heat for regasification operations. Examples of another such source of heat
would include
steam from a boiler, or a flow-through seawater system in which seawater is
introduced as a
source of heat from the ocean (or other body of water in which the LNGC is
located) and
discharged back into the ocean after being used to heat either the LNG or an
intermediate fluid
that subsequently is used to heat the LNG. Other sources of heat could include
a submerged
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combustion vaporizer or solar energy. Having a secondary or alternative source
of heat in
addition to the primary source of heat, whether or not either of the sources
is a submerged heat
exchanger system, also is considered advantageous.
The use of a primary source of heat coupled with the availability of at least
one secondary
source of heat provides additional flexibility in the manner in which the LNG
may be heated for
regasification purposes. The primary source of heat may be used without
requiring that source of
heat to be scaled up to accommodate all ambient circumstances under which the
regasification
may take place. Instead, the secondary source of heat may be used only in
those circumstances
in which an additional source of heat is required.
The availability of a secondary source of heat that is based on an entirely
different
principal than the primary source of heat also guarantees the availability of
at least some heat
energy in the event of a failure of the primary heat source. While the
regasification capacity may
be substantially reduced in the event of a failure of the primary source of
heat, the secondary
source of heat would provide at least a partial regasification capability that
could be used while
the primary source of heat is either repaired or the reason for the failure
otherwise corrected.
In one embodiment of such a system, the primary source of heat may be steam
from a
boiler, and the secondary source a submerged heat exchanger system.
Alternatively, the primary
source of heat may be steam from a boiler, and the secondary source may be the
use of an open,
flow-through seawater system. Other combinations of sources of heat also may
be used
depending on availability, economics, or other considerations. Other potential
heat sources
include the use of hot water heating boilers, or submerged combustion heat
exchangers, each of
which are commercially available products.
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In another embodiment of the system, the LNGC may be equipped with a primary
heat
source, and made ready for the addition of a secondary heat source by
including piping and other
items that otherwise could require substantial modification of the ship to
accommodate. For
example, the LNGC could be equipped to use steam from a boiler as the primary
source of heat,
but also be equipped with suitable piping and locations for pumps or other
equipment to facilitate
the later installation of a submerged heat exchanger system or a flow-through
seawater system
without requiring major structural modification of the ship itself. While this
may increase the
initial expense of constructing the LNGC or reduce the capacity of the LNGC
slightly, it would
be economically preferable to undergoing a major structural modification of
the ship at a later
date.
The preferred method of this invention is an improved process for regasifying
LNG while
onboard an LNG carrier. The LNGC, fitted with regasification facilities as
described above, may
be moored offshore and connected to a pipeline distribution system through a
connection located
on an offshore buoy or platform, for example. Once this connection is made, an
intermediate
fluid, such as glycol or fresh water, is circulated by pump 22 through the
submerged or partially
submerged heat exchanger or heat exchangers 21 and the vaporizer 23. Other
intermediate fluids
having suitable characteristics, such as acceptable heat capacity and boiling
points also may be
used as described above.
The heat exchanger 21 is preferably fully submerged and enables heat transfer
from the
surrounding seawater to the circulated intermediate fluid due to the
temperature differential
between the two. The intermediate fluid thereafter circulates to the vaporizer
23, which
preferably is a shell and tube vaporizer 23. In the preferred embodiment, the
intermediate fluid
passes through parallel vaporizers 23 to increase the output capacity of the
LNGC. LNG is
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passed into the vaporizer 23 through line 24, where it is regasified and exits
through line 25.
From line 25, the LNG passes into a pipeline distribution system attached to
the platform or buoy
where the LNGC is moored.
In the most preferred method of the invention, the intermediate fluid is
circulated through
submerged heat exchangers 21 that are mounted in one or more structures
connected to the
LNGC by suitable piping and lowered into the water after the LNGC moors at an
offshore buoy
or terminal. In yet another alternative method of the invention, the submerged
heat exchangers
21 are mounted to the buoy 68 or other offshore structure to which the LNGC is
moored, and
connected to the ship after docking.
In another preferred method of the invention, one or more secondary sources of
heat are
provided for regasification of the LNG. In one embodiment, the intermediate
fluid is circulated
by pump 22 through steam heater 26, vaporizer 23, and one or more submerged or
partially
submerged heat exchangers 21. Steam from a boiler or other source enters steam
heater 26
through line 31 and exits as condensate through line 32. Valves 41, 42 and 43
permit operation
of the vaporizer 23 with or without the steam heater 26. In addition, the
vaporizer 23 may be
operated solely with use of the secondary sources of heat such as the steam
heater 26. Valves 44,
45, and 46 permit isolation of these submerged heat exchangers 21, so that the
vaporizer 23 may
operate without them.
In another method of the invention, a flow through seawater system, with an
inlet 61 and
an outlet 62, permit seawater to be used as a direct source of heat for the
vaporizer 23 or as an
additional source of heat to be used in conjunction with the steam heater 26,
instead of the
submerged heat exchanger 21. Of course, the submerged or partially submerged
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system 21 may be used as a secondary source of heat, while one of the other
described sources of
heat is used as the primary source of heat. Examples of this are described
above.
Various exemplary embodiments of the invention have been shown and described
above.
However, the invention is not so limited. Rather, the invention shall be
considered limited only
by the scope of the appended claims.
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