Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02703317 2010-05-06
1931
SHELL AND TUBE HEAT EXCHANGERS
FIELD OF THE INVENTION
This invention relates to shell and tube heat exchangers, more specifically to
exchangers operating in service where a standard counter flow shell and tube
heat exchanger
would not be able to meet the required process conditions without experiencing
dew point
corrosion, more specifically to exchangers using a combination of counter flow
and parallel
flow throughout the exchanger to reduce the potential for dew point corrosion
while being
able to maintain a high thermal efficiency with only a minimal increase in
effective area, and
most specifically to gas to gas heat exchangers used to cool hot gases
containing sulphur
trioxide and/or acid vapour or heating cold gases containing sulphur dioxide
and/or entrained
acid mist.
BACKGROUND OF THE INVENTION
The invention relates to heat exchangers operating in potentially corrosive or
high
fouling conditions where said corrosion or fouling rates are highly dependent
upon the tube
wall temperature throughout the exchanger. Shell and tube exchangers are
generally the
preferred layout of exchangers when fouling is expected.
Dew point corrosion is a well known phenomenon in heat exchangers dealing with
a
condensable corrosive vapour. When the shell or tube wall temperature on the
corrosive side
of the heat exchanger falls below said corrosive vapour's dew point, there is
a potential for
corrosion. This can lead to fouling which causes a decrease in performance, an
increase in
pressure drop, and premature failure of the heat exchanger.
In sulphuric acid manufacturing, both gas streams in a heat exchanger are
often
potentially corrosive. The hot gas stream typically contains sulphur trioxide
(SO3) and acid
vapour which will rapidly corrode carbon steel if the walls drop below the
acid vapour dew
point temperature. The cold gas stream may be composed of various gas streams,
including
but not limited to ambient air, dried air, dried air containing sulphur
dioxide (SO2) gas, or
SO2 gas. The cold gas stream may also contain entrained acid mist from the
upstream
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process. This entrained acid mist can rapidly corrode the heat exchanger when
it comes into
direct contact with the tube walls, especially when said tube walls are
directly impacted with
droplets of entrained acid mist within the cold inlet gas.
In hydrocarbon power plants, it is beneficial to recover as much heat as
possible from
tail gas before releasing it to atmosphere. This is done through the use of
preheat exchangers
which transfers residual heat from combustion gas to preheat the combustion
air or other
fluids. The waste gas contains, amongst other gases, moisture and small
quantities of
S03 which combine to form sulphuric acid. Therefore, these preheat exchangers
often
experience dew point corrosion issues similar to those in the production of
sulphuric acid.
Counter-flow exchangers are widely preferred due to their high Log Mean
Temperature Differential (LMTD). These exchangers transfer heat from a hot
fluid to a cold
fluid, with the hot fluid flowing longitudinally along the effective length of
the exchanger a
cold fluid flowing in the opposite direction along the longitudinal length of
the exchanger, the
two fluids separated by a barrier or barriers, generally with said barrier
being a tube wall or
tube walls. Counter flow exchangers have the highest LMTD of the standard heat
exchanger
arrangements and therefore require less effective heat transfer area to attain
an equivalent
heat duty to other heat exchanger designs with equivalent process
requirements.
Counter flow exchangers have traditionally been designed to prevent dew point
corrosion by limiting the exchanger's heat duty. Limiting the heat duty can
prevent the
minimum tube wall temperature from falling beneath the dew point but may
prevent the
exchanger from being able to meet its process requirements. An alternative to
limiting the
effectiveness of the exchanger is to increase the gas inlet temperature,
although for process
reasons this may not be desirable or feasible.
Various prior art exchangers have attempted to overcome the difficulties with
corrosion associated with the standard counter flow design while maintaining a
high LMTD.
Many of these will be familiar to a person skilled in the art and are
discussed in standard
sources of heat exchanger literature.
Corrosion resistant materials of construction are commonly used when there is
a
potential for dew point corrosion. Use of these materials reduces the effects
of corrosion but
does not prevent the formation of dew within the exchanger. The capital cost
of the
exchanger can increase significantly by using corrosion resistant materials
depending on the
materials required, and yet the material may still experience a considerable
amount of
corrosion and fouling.
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Some counter flow exchangers contain a separate sacrificial tube bank on the
cold
side. These exchangers are designed so that any expected acid mist and dew
point corrosion
is contained to the sacrificial region. The use of a sacrificial tube bank
does not inhibit
corrosion and instead attempts to limit its long term effects by having a
separate replaceable
section.
Parallel flow exchangers are able to maintain more consistent tube wall
temperatures
than counter flow exchangers for equivalent inlet and outlet conditions. These
exchangers
transfer heat from a hot fluid to a cold fluid, with the hot fluid flowing
longitudinally along
the effective length of the equipment and exchanging heat indirectly with a
cold fluid flowing
in a generally parallel direction to the hot fluid, the two fluids separated
by a barrier or
barriers, generally with said barrier being a tube wall or tube walls. The
temperatures of the
two streams approach asymptotically and converge towards a common temperature
which in
turn limits the maximum heat duty and exit temperatures. A parallel flow
exchanger will
always have a lower LMTD than a comparable counter flow exchanger. There is
also a
potential for significant thermal differential stresses in the inlet region
where the temperature
difference between the hot and cold gases is greatest. Because of these
traits, pure parallel
flow exchangers are not preferable when there is a requirement for high heat
duties or where
there is a large inlet temperature differential.
The herein disclosed invention offers a novel improvement over the prior art
through
a unique combination of counter flow and parallel flow sections with other
additional
features, whose design and use will become apparent after a full review of
this disclosure.
SUMMARY OF THE INVENTION
The present invention provides a shell and tube heat exchangers utilizing an
improved flow combination of parallel flow and counter flow to retain a high
LMTD and
increase the minimum tube wall temperature in comparison to a counter flow
heat exchanger
operating under identical process conditions. This exchanger is particularly
well suited for
the prevention of dew point corrosion and damage from entrained acid mist.
The exchanger generally comprises two main sections with one section having a
generally parallel flow arrangement and the other having a generally counter
flow
arrangement between the two fluids. Partially cooled hot gas transfers heat in
a generally
parallel flow manner to a cold gas through the tube walls in the colder of the
two sections,
while hot gas transfers heat in a generally counter flow manner to a partially
heated cold gas
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through the tube walls in the hotter of the two sections. These two sections
may be separated
by a transition section where the flows alternate between shell side and tube
side. This
transition is particularly beneficial in plants that have unsteady process
conditions. Dividing
the exchanger into two sections allows for full control over the thermal
design of the system,
the inclusion of a partial gas by-pass or intermediate gas addition, and
easier repairs if
necessary. By alternating the shell side and tube side gas streams the overall
difference in
thermal growth between the shell and tubes is reduced, which in turn reduces
the stresses
caused by differential thermal growth and reduces fatigue stresses from
thermal cycling.
Accordingly, in one aspect the invention provides a process for exchanging
heat in a
shell and tube gas-to-gas heat exchanger between a plurality of gases,
said process comprising
passing a cold first gas in parallel flow to a second hot gas to provide a
warmer first
gas; and
passing said warmer first gas in counter-current flow to a hot third gas to
provide a
cooler said third gas.
Preferably, said second hot gas comprises said cooler said third gas.
In alternative embodiments, the invention as hereinabove defined provides a
process
comprising passing said cold first gas as a shell-side gas, and said warmer
said first gas as a
tube-side gas or a shell-side gas.
In further embodiments, the invention as hereinabove defined provides a
process
comprising passing said cold first gas as a tube-side gas and said warmer said
first gas as a
tube-side gas or a shell-side gas.
In yet further embodiments, the invention as hereinabove defined provides a
process
comprising removing a portion of said warmer said first gas from said heat
exchanger.
In still yet further embodiments, the invention as hereinabove defined
provides a
process comprising removing a portion of said cooler said third gas from said
heat exchanger.
In yet further embodiments, the invention as hereinabove defined provides a
process
comprising feeding a portion of said hot third gas in admixture with said
cooler said third gas
in parallel flow to said cold first gas.
In yet further embodiments, the invention as hereinabove defined provides a
process
comprising feeding a portion of said cold first gas in admixture with said
warmer said first
gas in counter-current flow to said hot third gas.
In yet further embodiments, the invention as hereinabove defined provides a
process
comprising said hot first gas comprises sulphur trioxide.
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In yet further embodiments, the invention as hereinabove defined provides a
process
comprising hot first gas further comprises entrained corrosive liquid
droplets.
In yet further embodiments, the invention as hereinabove defined provides a
process
wherein said hot first gas comprises entrained corrosive liquid droplets.
In yet further embodiments, the invention as hereinabove defined provides a
process
wherein said cold first gas comprises entrained corrosive liquid droplets.
In further embodiments, the invention as hereinabove defined provides a
process
wherein said second cold gas comprises air.
In a further aspect, the invention provides a process for the manufacture of
sulphuric
acid by the contact process comprising a process as hereinabove defined.
In a further aspect, the invention provides a process for exchanging heat in a
shell and
tube gas-to-gas heat exchanger between a first hot gas and a cold second gas
as hereinabove
defined in hydrocarbon power generation plants.
In a further aspect, the invention provides a gas-to-gas heat exchanger
comprising
a shell and tube first section comprising
means for receiving a cold first gas;
means for receiving a hot second gas; and
means for passing said cold first gas in parallel flow to said hot second gas
to provide
a warmer said first gas;
a shell and tube second section comprising
means for receiving a hot third gas;
means for receiving said warmer said first gas; and
means for passing said hot third gas in counter-current flow to said warmer
said first
gas to provide a cooler said third gas.
In further embodiments, the invention provides a heat exchanger as hereinabove
defined wherein said means for receiving said hot second gas comprises means
for receiving
said cooler said third gas.
In yet further embodiments, the invention provides a heat exchanger as
hereinabove
defined wherein said cooler said third gas constitutes said hot second gas.
Having the cold inlet gas flow in parallel to partially cooled hot gas
maintains a
higher LMTD throughout the exchanger and reduces thermally induced
differential stresses
when compared to a standard parallel flow design. Additional uses of this
exchanger design
including alternating the hot and cold gas streams to prevent against high
temperature
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corrosion will become apparent to a person skilled in the art of exchanger
design and
operation following a review of this disclosure.
The parallel flow section maintains a more consistent tube wall temperature
than the
counter flow section, which allows for further heat to be transferred between
the two gas
streams while maintaining the tube wall temperature above the dew point.
Maintaining the
tube wall temperature above the dew point prevents corrosive vapours present
in the hot gas
stream from condensing. This in turn allows for the use of standard materials
of construction
as opposed to corrosion resistant materials, thus reducing the capital cost of
the exchanger
while simultaneously extending its expected life.
In the counter flow section the two gases flow in a generally opposite
direction to
maximize heat transfer efficiency. By partially heating the cold gas in the
parallel flow
section, the counter flow section can be designed so that the tube wall
temperatures remain
above the dew point temperature of the potentially corrosive vapours. In a
heat exchanger
designed according to this invention, the coldest temperature within the
counter-flow section
will always be greater than the hottest tube wall temperature within the
parallel section.
Therefore, dew point corrosion is inhibited in the counter flow section if it
is also inhibited in
the parallel flow section.
Some heat exchangers of prior art have an initial parallel flow section, with
the hot
inlet gas flowing first in parallel with the cold inlet gas, and the remaining
heat exchange
occurring in a counter flow fashion. This design maintains an unnecessarily
high tube wall
temperature and LMTD within the parallel flow section. This limits the LMTD of
the
counter flow section to less than that of the parallel flow section, therefore
requiring the
exchanger to have a larger effective area in order to reach the required heat
duty than an
exchanger designed according to the invention. Finally, the lowest tube wall
temperature
within this prior art exchanger does not occur within the parallel flow
section and instead
occurs at the cold end of the counter flow section. Therefore, even if no dew
point corrosion
occurs within the parallel flow section, it is still possible to have dew
point corrosion within
the counter flow section. These shortcomings are overcome by the disclosed
invention.
An exchanger designed according to the invention may be designed to have the
gases
alternate sides of the tube wall during the transition between the parallel
flow and counter
flow sections. Doing so maintains the average shell wall temperature of the
exchanger closer
to the average tube wall temperature, thus reducing differential thermal
growth. Dividing the
tubes into two separate sections allows for the differential growth between
the shell and tubes
to be absorbed in stages, which reduces the forces on the tube sheets. The
combined
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reduction in differential growth and thermally induced stresses from
alternating the shell side
and tube side flows can be substantial in exchangers with a high temperature
differential
between their hot and cold gas streams, especially in plants with fluctuating
process
conditions; however, this is not required to realize the corrosion resistance
benefits and
relatively high heat duty capabilities of the exchanger design.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be better understood, preferred embodiments
will now
be described by way of example only with reference to the accompanying
drawings wherein:
Fig. 1 represents a diagrammatic cross-sectional picture of a heat exchanger
according
to the invention;
Fig. 2 represents a diagrammatic cross-sectional picture of an alternate
arrangement of
a heat exchanger according to the invention;
Fig. 3 represents a diagrammatic cross-sectional picture of an alternate
arrangement of
a heat exchanger according to the invention which utilizes two separate hot
gas streams;
Fig. 4 represents a diagrammatic cross-sectional picture of an alternate
arrangement of
a heat exchanger according to the invention which utilizes an alternate inlet
vestibule design;
Fig. 5 represents a plot of temperatures across the length of a heat exchanger
according to the invention; and wherein the same numerals denote like parts.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Fig. 1 shows a typical arrangement of a heat exchanger according to the
invention
comprised of two heat exchange sections, being parallel-flow section A and
counter-flow
section B.
Parallel flow section A is comprised of parallel flow shell 12, contained
within which
is parallel flow shell side inlet vestibule 14, parallel flow shell side 16,
and parallel flow shell
side outlet vestibule 18 where through said parallel flow shell side 16 there
passes parallel
flow section tubes 20 connecting parallel flow tube side inlet vestibule 22
and parallel flow
tube side outlet vestibule 24.
Counter flow section B is comprised of counter flow shell 26, contained within
which
is counter flow shell side inlet vestibule 28, counter flow shell side 30, and
counter flow shell
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side outlet vestibule 32 where through said counter flow shell side 30 there
passes counter
flow section tubes 34 connecting counter flow tube side inlet vestibule 36 and
counter flow
tube side outlet vestibule 38.
Cold gas 40 enters the exchanger through parallel flow shell side inlet 42
into parallel
flow shell side inlet vestibule 14 passing through parallel flow shell side 16
into parallel flow
shell side outlet vestibule 18 followed by parallel flow to counter flow
transition duct 44 into
counter flow tube side inlet vestibule 36 as partially heated cold gas 46
passing through
counter flow section tubes 34 into counter flow tube side outlet vestibule 38
before exiting
the exchanger as heated cold gas 48 through counter flow tube side outlet 50.
Hot gas 52 enters the exchanger through counter flow shell side inlet 54 into
counter
flow shell side inlet vestibule 28 passing through Section B counter flow
shell side 30 into
counter flow shell side outlet vestibule 32 followed by counter flow to
parallel flow transition
upper duct 56 into parallel flow tube side upper inlet vestibule 22 as
partially cooled hot gas
58 passing through parallel flow section tubes 20 into parallel flow tube side
outlet vestibule
24 before exiting the exchanger as cooled hot gas 60 through parallel flow
tube side outlet 62.
Disk baffles 64 and donut baffles 66 located throughout parallel flow shell
side 16 and
counter flow shell side 30 direct the shell side fluid flow across the tubes
to increase the heat
transfer rate between the fluids. Alternate baffle arrangements including, but
not limited to,
segmental baffles, double segmental baffles or an absence of baffles may also
be used;
however, disk and donut baffles combined with an axisymmetric donut tube
layout is
preferred for its uniformity of heat transfer rates and thermal growth between
tubes.
Separating parallel flow section A and counter flow section B while
alternating the
shell-side and tube-side gas flows reduces the difference in thermal growth
between the
combined growth of parallel flow shell 12 and counter flow shell 26 and the
combined
growth of parallel flow tubes 20 and counter flow tubes 34. Thus, thermal
cycling loads and
fatigue stresses are reduced on an exchanger according to the invention.
Cold gas 40 may contain entrained liquid droplets as it enters the exchanger
through
parallel flow shell side inlet 42 which can rapidly corrode the exchanger.
Parallel flow shell
side inlet vestibule 14 is designed such that droplets impinge on vestibule
inner wall 68
where they accumulate harmlessly and can be drained through liquid drain 70.
Parallel flow
shell side inlet vestibule 14 reduces the potential for and severity of
corrosion as well as the
amount of fouling on the exterior of parallel flow section tubes 20 due to the
previously
mentioned entrained liquid droplets when compared to allowing cold gas 40 to
directly enter
parallel flow shell side 16 of the exchanger.
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The coldest tube wall temperature in the exchanger occurs within parallel flow
section
A and, thus, this section is designed to maintain a tube wall temperature
above the dew point
of the corrosive liquids. A parallel flow exchanger has a higher minimum tube
wall
temperature than a counter flow exchanger with identical inlet and outlet
conditions;
therefore, parallel flow section A allows for additional heat transfer while
maintaining the
tube wall temperature above the dew point when compared to a standard counter
flow
exchanger. The coldest tube wall temperature within counter flow section B
occurs at
counter flow cold tube sheet 72 at the intersection of partially heated cold
gas 46 and partially
cooled hot gas 58. The hottest tube wall temperature in the exchanger is found
at counter
flow hot tube sheet 74 where hot gas 52 and heated cold gas 48 intersect.
The overall length of parallel section A and counter flow section B, along
with the
relative number of disk baffles 64 and donut baffles 66 within each section
can be varied to
modify the relative heat duties of each section. This can be used during
design to alter the
heat duty of the exchanger while maintaining control over the minimum tube
wall
temperatures. The number and diameter of the parallel flow section tubes 20
and counter
flow section tubes 34 can be varied to further alter the heat duty of each
section.
Fig. 2 shows an alternate arrangement of a heat exchanger according to the
invention.
Parallel flow section A is comprised of parallel flow shell 12, contained
within which
is parallel flow shell side inlet vestibule 14, parallel flow shell side 16
and parallel flow shell
side outlet vestibule 18 where through said parallel flow shell side 16 there
passes parallel
flow section tubes 20 connecting counter flow section tubes 34 and parallel
flow tube side
outlet vestibule 24. Counter flow section B is comprised of counter flow shell
26, contained
within which is counter flow shell side inlet vestibule 28, counter flow shell
side 30 and
counter flow shell side outlet vestibule 32 where through said counter flow
shell side 30 there
passes counter flow section tubes 34 connecting counter flow tube side inlet
vestibule 36 and
parallel flow section tubes 20. Cold gas 40 enters the exchanger through
parallel flow shell
side inlet 42 into parallel flow shell side inlet vestibule 14 passing through
parallel flow shell
side 16 into parallel flow shell side outlet vestibule 18 followed by parallel
flow to counter
flow transition duct 44 into counter flow shell side inlet vestibule 28 as
partially heated cold
gas 46 passing through counter flow section shell side 30 into counter flow
shell side outlet
vestibule 32 before exiting the exchanger as heated cold gas 48 through
counter flow shell
side outlet 74. Hot gas 52 enters the exchanger through counter flow tube side
inlet 76 into
counter flow tube side inlet vestibule 36 passing through counter flow section
tubes 34
continuing into parallel flow section tubes 20 as partially cooled hot gas 58
continuing into
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parallel flow tube side outlet vestibule 24 before exiting the exchanger as
cooled hot gas 60
through parallel flow tube side outlet 62. In this arrangement, parallel flow
section tubes 20
are a continuation of counter flow section tubes 34. Disk baffles 64 and donut
baffles 66
located throughout parallel flow shell side 16 and counter flow shell side 30
direct the shell
side fluid flow across the tubes to increase the heat transfer rate between
the fluids.
An exchanger arrangement as shown in Fig. 2 has an identical temperature
profile to
an exchanger arrangement as shown in Fig. 1 when the thickness and heat
resistance of the
tubes are negligible. The arrangement shown in Fig. 2 maintains the shell-side
and tube side
flows on their respective sides throughout the length of the exchanger, which
reduces the
capital cost and the initial overall pressure drop of the exchanger. It is
most preferred to have
an identical number and diameter of tubes in parallel flow section A and
counter flow section
B as the tubes run the entire length of the exchanger. This limits the overall
flexibility of the
initial design of the exchanger in comparison to an arrangement as shown in
Fig. 1. The
differential thermal growth between the shell and tubes of the exchanger in
Fig. 2 is on a
similar scale to that of standard counter flow exchanger. It is also not
possible to replace only
the parallel flow section of the exchanger arrangement shown in Fig. 2 in
contrast the
arrangement shown in Fig. 1. Therefore, an exchanger arrangement as shown in
Fig. 2 is
better suited for steady operating conditions, while an exchanger arrangement
as shown in
Fig. 1 is better suited for use in unsteady operating conditions.
Fig. 3 shows an alternate arrangement of a heat exchanger according to the
invention
wherein two hot gases are used in series to warm a single cold gas. Parallel
flow section A
comprises of parallel flow shell 12, contained within which is parallel flow
shell side inlet
vestibule 14, parallel flow shell side 16, and parallel flow shell side outlet
vestibule 18 where
through said parallel flow shell side 16 there passes parallel flow section
tubes 20 connecting
parallel flow tube side inlet vestibule 22 and parallel flow tube side outlet
vestibule 24.
Counter flow section B is comprised of counter flow shell 26, contained within
which is
counter flow shell side inlet vestibule 28, counter flow shell side 30, and
counter flow shell
side outlet vestibule 32 where through said counter flow shell side 30 there
passes counter
flow section tubes 34 connecting counter flow tube side inlet vestibule 36 and
counter flow
tube side outlet vestibule 38. Cold gas 40 enters the exchanger through
parallel flow shell
side inlet 42 into parallel flow shell side inlet vestibule 14 passing through
parallel flow shell
side 16 into parallel flow shell side outlet vestibule 18 followed by parallel
flow to counter
flow transition duct 44 into counter flow tube side inlet vestibule 36 as
partially heated cold
gas 46 passing through counter flow section tubes 34 into counter flow tube
side outlet
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vestibule 38 before exiting the exchanger as double heated cold gas 78 through
counter flow
tube side outlet 50. Hot gas 52 enters the exchanger through counter flow
shell side inlet 54
into counter flow shell side inlet vestibule 28 passing through counter flow
shell side 30 into
counter flow shell side outlet vestibule 32 before exiting the exchanger as
counter flow
cooled hot gas 80 through counter flow shell side outlet 74. Second hot gas 82
enters the
exchanger through parallel flow tube side inlet 84 into parallel flow tube
side inlet vestibule
22 passing through parallel flow section tubes 20 into parallel flow tube side
outlet vestibule
24 before exiting the exchanger as parallel flow cooled hot gas 86 through
parallel flow tube
side outlet 62. Disk baffles 64 and donut baffles 66 located throughout
parallel flow shell
side 16 and counter flow shell side 30 direct the shell side fluid flow across
the tubes to
increase the heat transfer rate between the fluids.
The separation between parallel flow section A and counter flow section B
allows for
the intermediate removal of counter flow cooled hot gas 80 and addition of
second hot gas
82. In a similar manner two cold gas streams could be used to cool a single
hot gas.
Modifying the gas flow rates of hot gas 52 and second hot gas 82 alters the
heat duty of the
exchanger in each section independently. Other benefits will be apparent to a
person skilled
in the art of heat exchanger design or fabrication.
Fig. 4 shows an alternate arrangement of an exchanger similar to that shown in
Fig. 2.
In this arrangement, cold gas 40 enters the exchanger through alternate
parallel flow shell
side inlet 88 into alternate parallel flow shell side inlet vestibule 90
passing through parallel
flow shell side 16 into parallel flow shell side outlet vestibule 18 followed
by alternate
parallel flow to counter flow transition duct 92 into counter flow shell side
inlet vestibule 28
as partially heated cold gas 46 passing through counter flow section shell
side 30 into counter
flow shell side outlet vestibule 32 before exiting the exchanger as heated
cold gas 48 through
counter flow shell side outlet 74. Hot gas 52 follows an identical flow path
to that described
in Fig. 2 and exits the exchanger as cooled hot gas 60. Alternate parallel
flow shell side inlet
vestibule 90 provides improved mist elimination capabilities in comparison to
parallel flow
shell side inlet vestibule 14 as previously shown in Figs. 1 through 3.
Numerous similar
alternate variations are apparent to a person skilled in the art of heat
exchanger design or
fabrication.
Fig. 5 shows a temperature profile for an exchanger designed according to the
invention as shown in Fig. 1, Fig. 2 or Fig. 4. This temperature profile will
be identical for
an exchanger as shown in Fig. 1, Fig. 2 or Fig. 4 when the tube wall thickness
and resistance
are negligible. The temperature profile for an exchanger as shown in Fig. 3
will also be
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identical provided that additionally said second hot gas 82 is composed of
counter flow
cooled hot gas 80. The process conditions used are arbitrary but
representative of those
found in a cold reheat exchanger in a sulphuric acid plant. This process has a
minimum
allowable tube wall temperature 94 of 300 F for the prevention of dew point
corrosion. Cold
gas 40 enters the exchanger at a temperature of 165 F and is heated in
parallel flow section A
to partially heated cold gas 46 at a temperature of 224 F. Following parallel
flow section A
partially heated cold gas 46 is heated in counter flow section B to heated
cold gas 48 at a
temperature of 680 F and exits the exchanger. Hot gas 52 enters the exchanger
at a
temperature of 860 F. It is cooled in counter flow section B to partially
cooled hot gas 58 at a
temperature of 435 F. Following counter flow section B partially cooled hot
gas 58 is cooled
in parallel flow section A to cooled hot gas 60 at a temperature of 380 F and
exits the
exchanger. The minimum tube wall temperature 96 within the exchanger is 300 F.
This is
equivalent to minimum allowable tube wall temperature 94 of 300 F for the
prevention of
dew point corrosion. The relative heat duties of parallel flow section A and
counter flow
section B can be adjusted to optimize the exchanger for its desired service.
Increasing the
relative heat duty to parallel flow section A will increase the minimum tube
wall temperature,
while increasing the relative heat duty to counter flow section B will
increase the overall
LMTD of the exchanger which in turn decreases the required effective area to
meet the
exchanger's heat duty. A prior art counter flow exchanger operating under
equivalent process
conditions would have a minimum tube wall temperature of approximately 272 F,
which is
less than minimum allowable tube wall temperature 94. It is, therefore,
expected that
condensation would form within the prior art exchanger, causing dew point
corrosion.
Although this disclosure has described and illustrated certain preferred
embodiments
of the invention, it is to be understood that the invention is not restricted
to those particular
embodiments. Rather, the invention includes all embodiments which are
functional or
mechanical equivalence of the specific embodiments and features that have been
described
and illustrated.
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