Patent Application
20050217834
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.
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.
The catalytic reforming process consists of an oxygenolysis reaction with an associated water-gas shift [CH4+H2OCO+3H2, CO+H2O
CO2+H2] and/or a partial oxidation reaction [CH4+½O2
CO+2H2]. 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.
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.
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.
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.
In one form, the tube runs are flattened tubes.
In accordance with one form, the fins are serpentine fins.
In one form, the tube runs are constructed of aluminum.
In a preferred form, each tube run group comprises a single tube that defines the three tube runs.
According to one form, each single tube is arranged in a generally serpentine configuration.
According to one form, a method is provided for transferring heat from a first fluid to a second fluid in a heat exchanger.
In accordance with one form, the method includes 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.
In one form, the first fluid comprises water.
In a preferred form, the method further includes the step of transferring sufficient heat to only partially vaporize the first fluid.
Other objects, advantages, and features will become apparent from a complete review of the entire specification, including the appended claims and drawings.
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.
As seen in
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 [CH4+H2OCO+3H2, CH4+½O2
CO+2H2, CO+H2O
CO2+H2]. For these catalytic reactions to occur, the reactants must be brought to an elevated temperature typically in excess of 500° 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
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.
As illustrated in CO2+H2]. Optionally, additional water (as indicated by the dotted lines in
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+½O2CO2]. 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° C. to 180° 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
As best seen in
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.
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
It should be understood that the embodiment shown in
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.
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.
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.
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
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
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.
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 TF(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 TO 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 TF(max) that is approximately 13° C. lower than the maximum temperature TE(max) in a conventional automotive-style, parallel flow, single-pass heat exchanger.
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
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.
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.
