U.S. patent application number 10/818794 was filed with the patent office on 2005-10-06 for multi-pass heat exchanger.
Invention is credited to Reinke, Michael J., Valensa, Jeroen.
Application Number | 20050217834 10/818794 |
Document ID | / |
Family ID | 34967219 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050217834 |
Kind Code |
A1 |
Valensa, Jeroen ; et
al. |
October 6, 2005 |
Multi-pass heat exchanger
Abstract
A multi-pass heat exchanger is provided. The multi-pass heat
exchanger may be used in a fuel processing subsystem for cooling a
reformate flow or in any other application where uniform outlet
temperatures are desired. The multi-pass heat exchanger includes a
plurality of tube run groups having a first pass, an intermediate
pass and a final pass. The respective locations of the passes
relative to each other provide more uniform outlet temperatures
than conventional automotive-style, parallel flow, single-pass heat
exchangers.
Inventors: |
Valensa, Jeroen; (New
Berlin, WI) ; Reinke, Michael J.; (Franklin,
WI) |
Correspondence
Address: |
WOOD, PHILLIPS, KATZ, CLARK & MORTIMER
500 W. MADISON STREET
SUITE 3800
CHICAGO
IL
60661
US
|
Family ID: |
34967219 |
Appl. No.: |
10/818794 |
Filed: |
April 6, 2004 |
Current U.S.
Class: |
165/150 ;
165/152 |
Current CPC
Class: |
F28F 1/126 20130101;
H01M 8/04074 20130101; Y02E 60/50 20130101; F28D 1/0478
20130101 |
Class at
Publication: |
165/150 ;
165/152 |
International
Class: |
F28D 001/047 |
Claims
1. A heat exchanger for transferring heat between a first fluid
flow and a second fluid flow, the heat exchanger comprising: an
inlet manifold; an outlet manifold; a plurality of aligned and
spaced tube run groups extending between the inlet manifold and the
outlet manifold to direct the first fluid flow through the heat
exchanger, each tube run group having three parallel tube runs; the
tube runs of each tube run group including a first tube run coupled
to the inlet manifold to direct the first fluid in a first
direction, an intermediate tube run coupled to the first tube run
to receive the first fluid therefrom and direct the first fluid in
a second direction opposite the first direction, and a final tube
run coupled to the intermediate tube run to receive the first fluid
therefrom and direct the first fluid in the first direction to the
outlet manifold, the intermediate tube run located adjacent the
first tube run and the final tube run; for each adjacent pair of
tube run groups, the final tube run of one of the tube run groups
of the pair being located adjacent the first tube run of the other
tube run group of the pair; and a plurality of fins extending
between the adjacent tube runs of the tube groups to direct the
second fluid flow between the adjacent tube runs of the tube groups
from an inlet face to an outlet face of the heat exchanger.
2. The heat exchanger of claim 1 wherein each tube run group
comprises a single tube that defines the three tube runs.
3. The heat exchanger of claim 2 wherein each single tube is
arranged in a generally serpentine configuration.
4. The heat exchanger of claim 1 wherein the tube runs and fins
have sufficient effectiveness under normal operating conditions to
partially vaporize the first fluid flow where the first fluid flow
enters the heat exchanger inlet manifold as a single-phase fluid
and exits the heat exchanger outlet manifold as a two-phase
fluid.
5. The heat exchanger of claim 1 wherein the tube runs are
flattened tubes.
6. The heat exchanger of claim 1 wherein the fins are serpentine
fins.
7. The heat exchanger of claim 1 wherein the tube runs are
constructed of aluminum.
8. A heat exchanger for transferring heat between a first fluid
flow and a second fluid flow, the heat exchanger comprising: an
inlet manifold; an outlet manifold; a plurality of aligned and
spaced tubes extending between the inlet manifold and the outlet
manifold to direct the first fluid flow through the heat exchanger,
each tube having three parallel tube runs; the parallel tube runs
of each tube including a first tube run coupled to the inlet
manifold to direct the first fluid flow in a first direction, an
intermediate tube run coupled to the first tube run to direct the
first fluid flow in a second direction opposite the first
direction, and a final tube run coupled to the intermediate tube
run and the outlet manifold to direct the first fluid flow in the
first direction, the intermediate tube run located adjacent the
first tube run and the final tube run; for each adjacent pair of
tubes, the final tube run of one of the tubes of the pair being
located adjacent the first tube run of the other tube of the pair;
and a plurality of fins extending between the adjacent tube runs of
the tubes to direct the second fluid flow between the adjacent runs
of the tubes from an inlet face to an outlet face of the heat
exchanger.
9. The heat exchanger of claim 8 wherein the tube runs and fins
have sufficient effectiveness under normal operating conditions to
partially vaporize the first fluid flow where the first fluid flow
enters the heat exchanger inlet manifold as a single-phase fluid
and exits the heat exchanger outlet manifold as a two-phase
fluid.
10. The heat exchanger of claim 8 wherein the tubes are flattened
tubes.
11. The heat exchanger of claim 8 wherein the fins are serpentine
fins.
12. The heat exchanger of claim 8 wherein the tubes are constructed
of aluminum.
13. The heat exchanger of claim 8 wherein each of the tubes is
arranged in a generally serpentine configuration.
14. A method of transferring heat from a first fluid to a second
fluid in a heat exchanger, the heat exchanger including a planar
inlet face to receive the second fluid, a planar outlet face
opposite the inlet face to exhaust the second fluid, a first fluid
inlet on a first side of the faces, and a first fluid outlet on a
second side of the faces opposite the first side, the method
comprising the steps of: flowing the first fluid from the inlet to
the outlet via a plurality of aligned multi-pass flow paths; each
of the flow paths including a first pass extending between the
first and second sides in a first flow direction transverse to the
inlet and outlet faces, an intermediate pass extending between the
first and second sides in a flow direction opposite the first
direction transverse to the inlet and outlet faces, and a final
pass extending between the first and second sides in the first flow
direction, the passes being parallel to each other; each of the
intermediate passes running between the first and final passes of
the associated flow path, for each adjacent pair of flow paths; the
final pass of one of the flow paths of the pair running adjacent
the first pass of the other flow path of the pair; flowing the
second fluid from the first face to the second face via flow paths
between and transverse to the passes; and transferring heat between
the first and second fluids as the first and second fluids flow
through the respective flow paths.
15. The method of claim 14 wherein the first fluid comprises
water.
16. The method of claim 14 further comprising the step of
transferring sufficient heat to only partially vaporize the first
fluid.
Description
FIELD OF THE INVENTION
[0001] This invention relates to heat exchangers, and in more
particular applications, to multi-path heat exchangers wherein at
least one fluid requires a relatively uniform temperature as it
exits the heat exchanger, such as heat exchangers for a reformate
flow in a fuel processing subsystem for a fuel cell system.
BACKGROUND OF THE INVENTION
[0002] In many proton exchange membrane (PEM) fuel cell systems, a
fuel such as methane or a similar hydrocarbon fuel is converted
into a hydrogen-rich stream for the anode side of the fuel cell. In
many systems, humidified natural gas (methane) and air are
chemically converted to a hydrogen-rich stream known as reformate
by a fuel processing subsystem of the fuel cell system. This
conversion takes place in a reformer where the hydrogen is
catalytically released from the hydrocarbon fuel. A common type of
reformer is an Auto-Thermal Reformer (ATR), which uses air and
steam as oxidizing reactants. As the hydrogen is liberated, a
substantial amount of carbon monoxide (CO) is created which must be
reduced to a low level (typically less than 10 ppm) to prevent
poisoning of the PEM membrane.
[0003] The catalytic reforming process consists of an oxygenolysis
reaction with an associated water-gas shift
[CH.sub.4+H.sub.2OCO+3H.sub.2- , CO+H.sub.2OCO.sub.2+H.sub.2]
and/or a partial oxidation reaction
[CH.sub.4+1/2O.sub.2CO+2H.sub.2]. While the water-gas shift
reaction removes some of the CO from the reformats flow stream, the
overall reformate stream will always contain some level of CO, the
amount being dependent upon the temperature at which the reforming
process occurs. After the initial reactions, the CO level of the
reformate flow is well above the acceptable level for the PEM fuel
cell. To reduce the CO concentration to within acceptable levels,
several catalytic reactions will generally be used in the fuel
processing subsystem to remove CO in the reformate flow. Typical
reactions for reduction of CO in the reformate flow include the
aforementioned water-gas shift, as well as a selective oxidation
reaction over a precious metal catalyst (with a small amount of air
added to the reformate stream to provide oxygen). Generally,
several stages of CO cleanup are required to obtain a reformate
stream with an acceptable CO level. Each of the stages of CO
cleanup requires the reformate temperature be reduced to precise
temperature ranges so that the desired catalytic reactions will
occur and the loading amount of precious metal catalyst can be
minimized.
[0004] In this regard, liquid-cooled heat exchangers are frequently
employed to control the reformate temperature at each stage because
of their compact size when compared to gas-cooled heat exchangers.
Further, because liquid water entering the fuel processing
subsystem must be heated so that it can be converted to steam for
the reforming reactions, it is thermally efficient to use process
water as the liquid coolant for the heat exchangers to cool the
reformate flow prior to CO removal. However, such an approach can
be difficult to implement.
[0005] Specifically, it would be economical to leverage
automotive-style heat exchangers to be utilized as heat exchangers
for fuel processing subsystems. However, these heat exchangers can
have certain drawbacks. For example, in typical parallel flow
single-pass automotive-style heat exchangers, the flow that is
being cooled typically will have localized cool regions because of
the subcooled inlet side of the heat exchanger where the coolant or
refrigerant enters. Additionally, if the coolant is completely
vaporized prior to exiting the heat exchanger, the flow being
cooled will have localized hot regions. These phenomenon produce a
temperature gradient across the exhaust face of the heat exchanger
in the flow being cooled. Such temperature gradients can be
unacceptable in fuel processing subsystems, which typically require
a uniform temperature in the reformate flow exiting a heat
exchanger. The variance between the localized hot and cool regions
can have significant negative effects on the CO removal processes
within fuel processing subsystems such as decreased efficiency and
decreased life of the catalyst.
SUMMARY OF THE INVENTION
[0006] In accordance with one form of the invention, a heat
exchanger is provided for transferring heat between a first fluid
flow and a second fluid flow. The heat exchanger includes an inlet
manifold, an outlet manifold, and a plurality of aligned and spaced
tube run groups. The tube runs groups extend between the inlet
manifold and the outlet manifold to direct the first fluid flow
through the heat exchanger, each tube run group having three tube
runs. The tube runs of each group include a first tube run coupled
to the inlet manifold to direct the first fluid in a first
direction, an intermediate tube run coupled to the first tube run
to receive the first fluid therefrom and direct the first fluid in
a second direction opposite the first direction, and a final tube
run coupled to the intermediate tube run to receive the first fluid
therefrom and direct the first fluid in the first direction to the
outlet manifold, the intermediate tube run located adjacent the
first tube run and the final tube run. For each adjacent pair of
tube run groups, the final tube run of one of the tube run groups
of the pair being located adjacent the first tube run of the other
tube run group of the pair. The heat exchanger also includes a
plurality of fins extending between the adjacent tube runs of the
tube groups from an inlet face to an outlet face of the heat
exchanger.
[0007] In one form, the tube runs are flattened tubes.
[0008] In accordance with one form, the fins are serpentine
fins.
[0009] In one form, the tube runs are constructed of aluminum.
[0010] In a preferred form, each tube run group comprises a single
tube that defines the three tube runs.
[0011] According to one form, each single tube is arranged in a
generally serpentine configuration.
[0012] According to one form, a method is provided for transferring
heat from a first fluid to a second fluid in a heat exchanger.
[0013] In accordance with one form, the method includes the steps
of:
[0014] flowing the first fluid from the inlet to the outlet via a
plurality of aligned multi-pass flow paths;
[0015] each of the flow paths including a first pass extending
between the first and second sides in a first flow direction
transverse to the inlet and outlet faces, an intermediate pass
extending between the first and second sides in a flow direction
opposite the first direction transverse to the inlet and outlet
faces, and a final pass extending between the first and second
sides in the first flow direction, the passes being parallel to
each other;
[0016] each of the intermediate passes running between the first
and final passes of the associated flow path;
[0017] for each adjacent pair of flow paths, the final pass of one
of the flow paths of the pair running adjacent the first pass of
the other flow path of the pair;
[0018] flowing the second fluid from the first face to the second
face via flow paths between and transverse to the passes; and
[0019] transferring heat between the first and second fluids as the
first and second fluids flow through the respective flow paths.
[0020] In one form, the first fluid comprises water.
[0021] In a preferred form, the method further includes the step of
transferring sufficient heat to only partially vaporize the first
fluid.
[0022] Other objects, advantages, and features will become apparent
from a complete review of the entire specification, including the
appended claims and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a diagrammatic representation of a fuel processing
subsystem including heat exchangers embodying the present
invention;
[0024] FIG. 2 is a perspective view of a heat exchanger embodying
the present invention;
[0025] FIG. 3 is a somewhat diagrammatic representation of a
portion of the heat exchanger of FIG. 2; and
[0026] FIG. 4 is graph depicting a comparison of the temperature
profiles of a reformate flow flowing through a heat exchanger
embodying the present invention and a single-pass cross-flow type
heat exchanger.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] While the present invention is susceptible of embodiment in
many different forms, there are shown in the drawings and will be
described herein in detail specific embodiments thereof with the
understanding that the present disclosure is to be considered as an
exemplification of the principles of the invention and is not
intended to limit the invention to the specific embodiments
illustrated.
[0028] As seen in FIG. 1, a pair of heat exchangers 10 embodying
the present invention are provided for use in a fuel processing
subsystem, shown schematically at 12, for producing a reformate
flow 14 from a hydrocarbon flow 16 and for reducing a level of
carbon monoxide (CO) in the reformate flow 14 for use in a proton
exchange membrane fuel cell system (not shown). As used in the
specification and claims, the phrase fuel flow is meant to
encompass both the hydrocarbon flow 16 and the reformate flow 14.
While two of the heat exchangers 10 are shown, it should be
understood that the heat exchangers 10 do not depend on each other
and can operate independently. Additionally, any number of heat
exchangers 10 can be utilized as required by the fuel processing
subsystem 12. For example, some subsystems 12 may require a single
heat exchanger 10, while others may require three or more of the
heat exchangers 10. Each of the heat exchangers 10 provides an
advantageous coolant flow scheme that can allow for optimization of
the temperature distribution of the reformate flow 14 exiting the
heat exchanger 10.
[0029] In the illustrated embodiment, the fuel processing subsystem
12 includes a reformer 18. A commonly used method called
auto-thermal reforming may be used to produce the reformate flow 14
from the hydrocarbon flow 16 in the reformer 18. The reactions
consist of an oxygenolysis reaction, a partial oxidation, and a
water-gas shift [CH.sub.4+H.sub.2OCO+3H.sub.2,
CH.sub.4+1/2O.sub.2CO+2H.sub.2, CO+H.sub.2OCO.sub.2+H.sub.2]. For
these catalytic reactions to occur, the reactants must be brought
to an elevated temperature typically in excess of 500.degree. C. As
shown in the first reaction, a process water flow 20 is used in the
form of superheated steam 22 to partially elevate the temperatures
of the reactants entering the reformer 18. As in most fuel
processing subsystems for fuel cell systems, the necessary heat to
create the steam flow 22 must be added to the process water flow 20
from an external source such as a heater or, as shown in FIG. 1, by
burning a reformate, hydrogen, natural gas, or other hydrocarbon
containing combustible mixture, such as anode tail gas stream 24
and transferring heat in a heat exchanger 26 to create the steam
flow 22. In the illustrated embodiment, the process water flow 20
is supplied by a suitable pressurized water source 27.
[0030] As shown in the above mentioned reactions, CO is created in
the reforming process. The CO created must be removed before
entering a fuel cell because it is poisonous to the membrane,
limiting the fuel cell performance and lifetime. Additionally, the
amount of CO created in the reforming reactions is highly dependent
upon the reaction temperature. At higher temperatures, the
reactions yield more hydrogen gas useful in the fuel cell, but also
yield more poisonous CO. In order to eliminate the poisonous CO
from the reformate flow 14, CO elimination stages may be
utilized.
[0031] As illustrated in FIG. 1, after the hydrocarbon flow 16 is
used to produce the reformate flow 14 in the reformer 18, the
reformate flow 14 is flowed to at least one water-gas shift 28. The
water-gas shift 28 is utilized to further remove poisonous CO from
the reformate flow 14 and create more hydrogen gas for use in the
fuel cell system. The water-gas shift requires water as shown in
the water-gas shift reaction [CO+H.sub.2OCO.sub.2+H.sub.2].
Optionally, additional water (as indicated by the dotted lines in
FIG. 1) may be added at the water-gas shift 28 as required by the
fuel processing subsystem 12 to maintain the water-gas shift
reaction. The additional water may come from the process water flow
20, water source 27, or any other suitable water source.
Additionally, multiple water-gas shifts 28 and 29 may be utilized
to further reduce the amount of poisonous CO in the reformate flow
14.
[0032] Even after multiple water-gas shift units 28 and 29, the
reformate flow 14 still typically contains excessive amounts of
poisonous CO in the reformate flow 14. To eliminate more of the
poisonous CO, at least one hydrogen purification device or
subsystem, such as selective oxidizer 30 may be utilized. Selective
oxidation reactions typically require a small amount of air to be
added to the reformate flow 14 to provide oxygen as required by the
selective oxidation reaction [CO+1/2O.sub.2CO.sub.2]. Selective
oxidation reactions typically occur over a precious metal catalyst.
For the catalytic reaction to occur, the reformate flow 14 must be
reduced to a desired temperature range to optimize the efficiency
of the precious metal catalyst. Typically, selective oxidation
occurs in a temperature range of 130.degree. C. to 180.degree. C.
Highly efficient selective oxidation occurs over a much narrower
temperature range depending upon the catalyst. To minimize the
amount of catalyst required for the selective oxidation reaction,
it is preferred that the temperature to which the reformate is
cooled be precisely controlled. In the embodiment of FIG. 1,
multiple selective oxidizers 30 and 31 are utilized and operate at
different desired temperature ranges from each other to remove
poisonous CO, preferably to a level less than 10 ppm in the
reformate flow 14. Each of the heat exchangers 10 is used to cool
the reformate flow 14 to within the desired temperature range for
the respective selective oxidizers 30 and 31.
[0033] FIG. 2 illustrates a preferred embodiment for each of the
heat exchangers 10. Each of the heat exchangers 10 includes a
planar inlet face 50 to receive the reformate flow 14, a planar
outlet face 52 opposite the inlet face 50 to exhaust the reformate
flow 14, a fluid inlet 54 on a first side 56 of the faces 50, 52 to
receive a first fluid or coolant, such as a water flow 53 from the
source 27, and a fluid outlet 58 on a second side 60 of the faces
50, 52 opposite the first side 56 to exhaust the flow 53. Each heat
exchanger 10 further includes an inlet manifold 100 at the first
side 56 to receive and distribute the flow 53. The water flow 53 is
dispersed by the inlet manifold 100 into to a plurality of aligned
and spaced tube run groups 104 for directing the water flow 53
through a plurality of aligned multi-pass flow paths shown
schematically by the dashed and arrowed lines 106. The water flow
53 flows through the tube run groups 104 while in a heat exchange
relationship with a second fluid, which is the reformate flow 14 in
the illustrated embodiment, flowing through a plurality of
serpentine fins 108. In the illustrated embodiment, each tube run
group 104 is provided in the form of a flattened multi-port tube
110 that has been shaped into a serpentine configuration. From the
tube run groups 104, the water flow 53 flows to an outlet manifold
112 at the second side 60.
[0034] As best seen in FIG. 3, each tube run group 104 includes a
first tube run 120 directing the water flow 53 through a first pass
121 of the flow path 106, an intermediate tube run 122 directing
the water flow 53 through an intermediate pass 123 of the flow path
106 and being coupled to the first tube run 120, and a final tube
run 124 directing the water flow 53 through a final pass 125 of the
flow path 106 and being coupled to the intermediate tube run 122
and the outlet manifold 112. As shown in FIG. 2, for each tube run
group 104, the intermediate tube run 122 is located adjacent the
first tube run 120 and the final tube run 124. Additionally, for
each adjacent pair of tube run groups 104, the final run 124 of one
of the tube run groups 104 of the pair is located adjacent the
first run 120 of the other tube run group 104 of the pair. In the
illustrated embodiment, the tube runs 120, 122, and 124 of each of
the tubes 110 are coupled by 180.degree. bends 126 in the tube
110.
[0035] In each tube run group 104, the water flow 53 is directed by
the first run 120 in a first direction, indicated by arrow A,
through the first pass 121. The water flow 53 exits the first tube
run 120 and enters the bend 126 at the second side 60 prior to
entering the intermediate tube run 122. The water flow 53 is
directed by the intermediate tube run 122 in a second direction,
indicated by arrow B, substantially opposite the first direction A
through the intermediate pass 123 towards the first side 56. The
water flow 53 flows from the intermediate tube run 122 through the
bend 126 at the first side 56 into the final tube run 124. The
water flow 53 is directed by the final tube run 124 from the first
side 56 through the final pass 125 in the direction A to the outlet
manifold 112 at the second side 60.
[0036] While the water flow 53 is passing through the tube run
groups 104, the reformate flow 14 is passing through the fins 108.
The reformate flow 14 flows from the inlet face 50 through the fins
108 to the outlet face 52 in a direction as indicated by arrow C in
FIG. 2. It should be understood that the reformate flow 14 could
also flow in an opposite direction as indicated by arrow D. While
the fins 108 are shown as serpentine, it should be understood that
the fins 108 can be of any suitable type known in the art to
provide a sufficient heat exchange relationship between the
reformate flow 14 and the water flow 53.
[0037] It should be understood that the embodiment shown in FIGS. 2
and 3 is merely one form of the present invention. For example,
while each of the tube run groups 104 is shown in the form of a
single flattened multi-port tube 110 that has been shaped into a
serpentine form with two 180.degree. bends 126, other types and
arrangements of tubes may be desirable to provide the tube run
groups 104 depending on the particular requirements for each
application. By way of further example, while five tube runs groups
104 are shown, some applications may require more than or less than
the five groups 104.
[0038] The tube run groups 104 and/or tube runs 120, 122, and 124,
fins 108, inlet manifold 100, and outlet manifold 112 maybe
manufactured from any suitable material. For example in a preferred
embodiment, all of these components are manufactured from aluminum
and brazed together using suitable brazing techniques. Aluminum is
one preferable material because it is lightweight and has a high
thermal conductivity for heat transfer between fluids.
Additionally, aluminum is corrosion resistant and capable of
handling thermal stresses. Examples of other materials include
stainless steel, titanium, copper, and other materials suitable for
use in heat exchangers. It should be understood that all of the
components need not be made of the same materials. For example, the
tubes 110 and fins 108 may be made out of aluminum while the inlet
manifold 100 and outlet manifold 112 may be made of a material with
a lower thermal conductivity.
[0039] As previously discussed, the reformate flow 14 must be
cooled to specific temperature ranges prior to entering the
selective oxidizers 30 and 31. This is because the catalyst
utilized in the selective oxidizers 30 and 31 is optimized to
remove CO from the reformate flow 14 at specified temperature
ranges. If the reformate flow 14 is not within the specified
temperature range, CO will not be removed in sufficient quantities
for the fuel cell or will be removed inefficiently causing a
shortened life for the catalyst. Additionally, if water in the
reformate flow 14 were to condense, the condensed water could
deactivate the catalyst and/or shorten the catalyst life.
[0040] One option to make the reformate flow 14 temperature more
uniform in an automotive-style heat exchanger is by ensuring that
the water flow 53 exits the heat exchanger 10 at less than 100%
vapor quality. This avoids creating a superheated region within the
heat exchanger 10, and will prevent the high reformate temperatures
which are caused by the relatively low heat transfer coefficients
inherent in single-phase vapor flow within the tube run groups 104.
While this operation requires an additional heat exchanger 26 to
complete vaporization, such a heat exchanger would most likely be
necessary to superheat the steam prior to entering the reformer 18.
While this would eliminate one cause of the temperature
maldistribution, it would not eliminate the increased reformate
temperatures in the subcooled region near the inlet side 56
relative to the two-phase region across the remainder of the heat
exchanger 10.
[0041] More specifically, conventional automotive-style heat
exchangers such as parallel flow single-pass heat exchangers (not
shown), typically create a temperature maldistribution in the width
direction across the outlet face of the heat exchanger from the
coolant inlet side to the coolant outlet side, as shown in FIG. 4
by line E, which illustrates a model of the temperature
distribution of the reformate flow 14 across the reformate outlet
face of a conventional automotive- style, parallel flow,
single-pass heat exchanger (not shown) where 0% represents the
inlet side (corresponding to inlet side 56) and 100% represents the
outlet side (corresponding to outlet side 60). As illustrated, the
minimum temperature T.sub.E(min) of the reformate flow 14 is near
115.degree. C. at the inlet side 56 (due to the cold temperature of
the incoming water flow 53) and the maximum temperature
T.sub.E(max) of the reformate flow 14 is near 156.degree. C. at a
distance approximately 10% the distance from the inlet side 56 to
the outlet side 60 (due to the relatively poor heat transfer
coefficient inside the tubes for liquid water). After the maximum
temperature peaks, the water flow 53 in the tubes begins to
vaporize and the heat transfer coefficient increases thereby
cooling the reformate flow 14 to the outlet temperature T.sub.O.
For most applications, this temperature range is too broad for
optimal CO removal in selective oxidizers 30 and 31.
[0042] By multi-passing the water flow 53 through each of the tube
run groups 104 and by the arrangement of the tube runs 120, 122,
and 124 relative to each other, the heat exchanger 10 and method of
the present invention create a reformate flow 14 with a more
uniform temperature distribution when exiting the heat exchanger
10. As illustrated in FIG. 4, line F represents the temperature
distribution of the reformate flow 14 across the outlet face 52 of
the heat exchanger 10 in the width direction from the inlet side 56
to the outlet side 60. The minimum temperature T.sub.F(min) of the
reformate flow 14 is near 130.degree. C. at the inlet side 127 and
the maximum temperature T.sub.F(max) of the reformate flow 14 is
near 143.degree. C. at a distance approximately 30% the distance
from the inlet side 56 to the outlet side 60. Compared to line E,
line F represents a temperature range reduction of approximately
70%. Therefore, the reformate flow 14 exits the heat exchanger 10
with a much narrower and more uniform temperature distribution when
compared to a conventional automotive-style, parallel flow,
single-pass heat exchanger.
[0043] The water flow 53 is heated as it passes through the tube
runs 120, 122, and 124 and is preferably only partially vaporized
prior to exiting the tube 110 as a partially vaporized flow 202. It
is preferred for the water flow 53 to only be partially vaporized
to avoid a superheated region in the tube runs 120, 122, and 124
which would worsen the temperature distribution. Specifically, if
the water flow 53 were to be fully vaporized, it would become a
superheated steam flow. Superheated steam has a relatively low heat
transfer coefficient as a single-phase vapor flow in the tube 110
when compared to a two-phase liquid and vapor flow.
[0044] The heat exchanger 10 is especially effective at decreasing
the temperature maldistribution of the reformate flow 14 because of
the relationship between the first tube run 120, intermediate tube
run 122, and the final tube run 124 for adjacent tube run groups
104. To alleviate the localized cool regions associated with a
conventional automotive-style, parallel flow, single-pass
construction, multiple passes 121, 123, and 125 are utilized to
provide warmer water flow 53 near the inlet side 56 via the tube
runs 122 and 124. The inlet portion of the tube run 120 is
typically the coldest portion of each of the tube run groups 104,
so it is sandwiched between two warmer tube runs 122 and 124.
Specifically, for each adjacent pair of tube run groups 104, the
first tube run 120 of tube run group 104 is sandwiched between the
intermediate tube run 122 of tube run group 104 and the final tube
run 124 of the adjacent tube run group 104. The partially vaporized
flow 53 in the intermediate tube run 122 and the final tube run 124
are at a higher temperature than the temperature of the liquid
water flow 53 at the initial portion of the first tube run 120.
Therefore, the minimum temperature T.sub.F(min) created at the
first side 56 by locating the first tube run 120 between the
intermediate tube run 122 and the final tube run 124 is much closer
to the outlet temperature T.sub.O of the flow 53 at the second side
60 entering the manifold 112 because the flow 53 in the tube runs
122 and 124 increase the temperature of the fins 108 adjacent the
tube run 120. Additionally, besides increasing the minimum
temperature of the reformate flow 14, the maximum temperature is
decreased because the first tube run 120 is sandwiched between the
intermediate tube run 122 and the final tube run 124. The water 53
in the intermediate tube run 122 and the final tube run 124 is
partially vaporized and therefore these tube runs have a higher
heat transfer coefficient than the first tube run 120. Thus, only
one tube run 120 per group of three tube runs 120, 122, 124 has a
low heat transfer coefficient. The combination of the heat transfer
coefficients created by the adjacent rube runs 120, 122, 124
increases the heat transfer in the fins 108 near the inlet side 56
and causes the reformate flow 14 to spike at a maximum temperature
T.sub.F(max) that is approximately 13.degree. C. lower than the
maximum temperature T.sub.E(max) in a conventional
automotive-style, parallel flow, single-pass heat exchanger.
[0045] As discussed, the partially vaporized flow 202 can be
recycled to recapture the heat contained therein to other parts of
the fuel processing subsystem as illustrated in FIG. 1. The
partially vaporized flow 202 is combined with additional water from
the pressurized water source 27 and sent to the heat exchanger 26
to be transformed into steam flow 22. Steam flow 22 is utilized in
the reformer 18, thus recapturing heat from the reformate flow 14
at the heat exchangers 10 and returning it to the hydrocarbon flow
16 used to make reformate flow 14.
[0046] The heat exchangers and method of the present invention are
suitable for cooling a reformate flow to within a desired
temperature range while maintaining a narrower temperature
distribution across the width of the heat exchanger than for
conventional automotive-style, parallel flow, single-pass heat
exchangers. The narrower temperature distribution is essential to
optimizing CO removal in the fuel processing subsystem prior to
flowing the reformate flow to the fuel cell. Additionally, thermal
efficiency of the fuel processing subsystem 12 is improved by
utilizing process water to cool the reformate flow 14.
[0047] It should be understood that while the heat exchanger 10 has
been described herein in connection with a fuel processing
subsystem 12, heat exchangers made and operated according to the
present invention can prove useful in other types of systems where
a relatively uniform outlet temperature is desired across the exit
face of the heat exchanger. Accordingly, no limitation to use with
a fuel cell system or a fuel processing subsystem is intended
unless expressly stated in the claims.
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