Patent Application
20170131046
The present disclosure relates generally to a heat exchanger and, more particularly, to a heat exchanger having foul-resistant characteristics.
Heat exchangers are available in many different configurations (e.g., corrugated-plate configurations, shell-and-tube configurations, tube-and-fin configurations, etc) and are used to transfer thermal energy between two fluids without direct contact between the fluids. In particular, a primary fluid (e.g., air or exhaust) is typically directed through a fluid passage of the heat exchanger, while a secondary fluid (e.g., air, water, glycol, etc.) is brought into external contact with walls of the passage. In this manner, heat may be transferred between the two fluids via the walls.
In some applications, one or both of the fluids circulated through the heat exchanger could have particles (e.g., unburned hydrocarbons, such as fuel or oil) entrained in the fluids. As the passage walls are exposed to the particles, the walls can become coated with the particles (i.e., fouled). This may be particularly true when the walls have a rough surface texture that readily accepts deposition of the particles. When the particles cling to the passage walls, a thermal conductivity of the walls is reduced.
An exemplary heat exchanger is described in U.S. Patent Publication No. 2014/0165558 of Birgler et al. that published on Jun. 19, 2014 (“the '558 publication”). In particular, the '558 publication discloses an exhaust gas heat exchanger having a stainless steel housing, a stainless steel tube disposed therein, and stainless steel coils and fins connected to the tube. Surfaces of the heat exchanger exposed to exhaust gas are provided with a protective coating fabricated from ceramic. The ceramic coating is intended to protect the stainless steel from corrosion.
Although the heat exchanger of the '558 publication may help to reduce the likelihood of corrosion in some applications, it may be excessive for other applications, expensive, and ineffective against fouling. Specifically, in some situations, it may be too expensive and/or difficult to completely shield an entire length of the heat exchanger. Further, because the coefficient of thermal expansion for ceramic materials and stainless steel may be different, coating the entire heat exchanger with the ceramic material may expose the ceramic material to a large range of temperatures, which could result in cracking and flaking of the material during uneven expansion. When the ceramic material cracks and/or flakes, it may lose its effectiveness. In addition, although the ceramic material used in the '558 publication may help protect the heat exchanger against corrosion, it may do little to protect the heat exchanger from fouling. That is, the anti-corrosive ceramic material used in the '558 publication may have a surface roughness that facilitates or even promotes particle deposition.
The disclosed heat exchanger is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.
In one aspect, the present disclosure is directed to a heat exchanger. The heat exchanger may include an inlet, an outlet, and at least one passage fluidly connecting the inlet and the outlet. The at least one passage may include a wall configured to transfer heat between a first fluid inside the at least one passage and a second fluid outside the at least one passage. The heat exchanger may also have a plurality of heat conducting features disposed along a length of the at least one passage, and a foul-resistant coating applied to only a subset of the plurality of heat conducting features.
In another aspect, the present disclosure is directed to another heat exchanger, This heat exchanger may include an inlet, an outlet, and at least one passage fluidly connecting the inlet and the outlet, the at least one passage may have a wall configured to transfer heat between a recirculated exhaust gas located at one side of the wall and a coolant located at an opposing side of the wall. The heat exchanger may also include a plurality of stainless steel fins operatively disposed along a length of the at least one passage, and a foul-resistant ceramic coating applied to only a downstream portion of the stainless steel fins. The foul-resistant ceramic coating may be fabricated from one of a boron-nitride super-enhanced graphite or a silicon carbide. The foul-resistant coating may have a surface finish that is smoother than a surface finish of the plurality of stainless steel fins.
In yet another aspect, the present disclosure is directed to a fluid handling system. The fluid handling system may include a supply of air, a supply of recirculated exhaust gas, and an inlet manifold configured to communicate with the engine. The fluid handling system may also include a heat exchanger configured to cool the recirculated exhaust gas and to direct the cooled recirculated exhaust gas to mix with the air entering the inlet manifold. The heat exchanger may have an inlet in communication with the supply of recirculated exhaust gas, an outlet in communication with the inlet manifold, and at least one passage fluidly connecting the inlet and the outlet. The at least one passage may have a wall configured to transfer heat between a recirculated exhaust gas located at one side of the wall and a coolant located at an opposing side of the wall. The heat exchanger may also have a plurality of stainless steel fins operatively disposed along a length of the at least one passage, and a foul-resistant ceramic coating applied to only a portion of the stainless steel fins. The foul-resistant ceramic coating may be fabricated from one of a boron-nitride super-enhanced graphite or a silicon carbide, and have a surface finish that is smoother than a surface finish of the plurality of stainless steel fins.
Induction circuit 16 may include, among other things, one or more compressors 22, and at least one cooler 24 located to chill air compressed by compressor(s) 22 before the air enters combustion chambers 14 of engine 10. Each compressor 22 may embody a fixed-geometry compressor, a variable geometry compressor, or another type of compressor configured to receive air and compress the air to a desired pressure level. In the disclosed exemplary embodiment, induction system 16 has two compressors 22 disposed in series with each other and connected to combustion chambers 14 via a passage 2.6 and an intake manifold 28, it is contemplated that compressors 22 could alternatively be disposed in parallel with each other, if desired. Cooler 24 may be disposed within passage 26 at a location downstream of and/or between compressors 22 and upstream of intake manifold 28. It is also contemplated that compressors 22 and/or cooler 24 could be omitted in a naturally aspirated embodiment, if desired.
Exhaust circuit 18 may include, among other things, at least one turbine 30 driven by the exhaust from engine 10 to rotate compressors 22 of induction system 16. Each turbine 30 may embody a fixed geometry turbine, a variable geometry turbine, or another type of turbine known in the art. In the disclosed exemplary embodiment, exhaust circuit 18 has two turbines 30 disposed in series with each other and connected to combustion chambers 14 via a passage 32 and an exhaust manifold 34. It is contemplated that turbines 30 could alternatively be disposed in parallel with each other, if desired. Turbines 30 may be configured to receive exhaust and convert potential energy in the exhaust to a mechanical rotation of the connected compressors 22. After exiting turbines 30, the exhaust may be discharged to the atmosphere and/or to recirculation circuit 20 via a passage 36. One or more exhaust treatment devices (not shown), for example a hydrocarbon closer, a diesel oxidation catalyst (DOC), a diesel particulate filter (DPF), a selective reduction catalyst (SCR), an ammonia absorber (AMOx), an attenuation device, and/or any other treatment device known in the art may be disposed within passage 36, as desired.
Recirculation circuit 20 may be configured to selectively divert exhaust from exhaust circuit 18 (e.g., from a location downstream of turbines 30) into induction circuit 16 (e.g., to a location upstream of compressors 22), In particular, recirculation circuit 20 may include a passage 38 that extends between passage 36 of exhaust circuit 18 and passage 26 of induction circuit 16. A cooler 40 may be located within passage 38 and configured to chill exhaust passing therethrough. The recirculation of exhaust may help to lower an overall temperature of the associated combustion process, thereby lowering a production of NOx and/or other regulated exhaust constituents.
Coolers 24 and 40 may each be configured to cool a primary fluid flowing therethrough. The primary fluid passing through cooler 24 may be air or an air/exhaust mixture, while the primary fluid passing through cooler 40 may be only exhaust. In one example, coolers 24 and 40 are gas-to-gas type exchangers, wherein heat from the primary fluids is transferred to gaseous secondary fluids (e.g., to air). In another example, one or both of coolers 24, 40 are gas-to-liquid type exchangers, wherein heat from the primary fluids is transferred to liquid secondary fluids (e.g., to water, glycol, a water/glycol mixture, etc).
Coolers 24 and 40 each be any type of heat exchanger known in the art. For example, one or both of coolers 24 and 40 may embody a corrugated-plate type heat exchanger, tube-and-fin type heat exchanger, or another common type of heat exchanger. As shown in
Features 48 may be hollow and filled with the primary fluid or the secondary fluid, or solid, as desired. Features 48 may be integrally formed with passage 46 or joined to passage 46 (e.g., via deformation, crimping, welding, fastening, press-fitting, bracketing, etc) after formation. Features 48, as well as walls of passage 46 may be fabricated from a non-corrosive, thermally conductive material, such as stainless steel.
The primary fluids passing through one or both of coolers 24 and 40 may have particles entrained therein. These particles can include, among other things, unburned hydrocarbons, such as fuel or oil. Due to the relatively rough surface texture and/or nature of stainless steel, the particles may adhere to the surfaces of passage 46 and/or features 48. When this happens, a thermal conductivity of coolers 24 and 40 may be reduced. In order to inhibit the particles from sticking to the exposed surfaces of coolers 24, and 40, a portion of these surfaces (e.g., a subset 48a of features 48) may be coated with a material that has a smoother, less sticky surface texture than stainless steel. This material may be a ceramic that is spray-deposited onto walls 46 and/or features 48. In the disclosed embodiment, a layer 50 having a thickness t about 1/50- 1/100th of a thickness T of features 48 is deposited. Layer 50 may be fabricated from one of a boron-nitride super-enhanced graphite or a silicon carbide material. For the purposes of this disclosure, the term “about” may be defined as “within engineering tolerances.”
As the primary fluids pass through coolers 24 and 40 (particularly cooler 40), the primary fluids may cool, For example, the primary fluid may enter cooler 40 at about 500-600° C., and exit cooler 40 at about 80-120° C. In order for the ceramic coating applied to cooler 40 to maintain its integrity and thereby provide for longevity of cooler 40, the ceramic coating should have a coefficient of thermal expansion about the same as a coefficient of thermal expansion for stainless steel. In this way, as the two materials are exposed to the same temperatures, both materials will expand at about the same rate. These substantially identical expansions may help to reduce cracking or flaking of the ceramic material.
In the disclosed exemplary embodiment, the ceramic material and the stainless steel of passage 46 and features 48 may have substantially equal coefficients of thermal expansion when the materials are exposed to a temperature in the range of about 100-180° C., Accordingly, the ceramic material may only be applied to the portion of cooler 40 (and, in some cases also cooler 24) that is regularly exposed to this temperature range. In the disclosed embodiment, this includes only a downstream or final 20-50% of passage 46 and features 48 within cooler 40. That is, the upstream half (i.e., the half closest to inlet 42) of cooler 40 may not include layer 50. The amount of cooler 24 that may be coated with layer 50 may be larger, as the primary fluid passing through cooler 24 may have a lower temperature than the primary fluid passing through cooler 40.
The disclosed fluid handling system may be implemented in any cooling or heating application, where one or more of the fluids that flow through the system have the potential to foul coolers of the system. In particular, the disclosed fluid handling system may provide for extended heat exchanger component life in a simple and inexpensive package by providing foul-resistant characteristics to the associated heat exchangers. The operation of fluid handling system 12 will now be explained, with respect to
Atmospheric air may be drawn into induction circuit 16 to compressors 22, where it may be pressurized to a. predetermined level before entering combustion chambers 14 of engine 10. Fuel may be mixed with the pressurized air before or after entering combustion chambers 14. This fuel-air mixture may then be combusted by engine 10 to produce mechanical work and an exhaust flow containing gaseous compounds and solid particulate matter. The exhaust flow may be directed from engine 10 to turbines 30, where the expansion of hot gasses may cause turbines 30 to rotate and drive compressors 22 to compress the inlet air. After exiting turbines 30, the flow of exhaust may be divided into two flows, including a main flow directed to the atmosphere and a bypass flow that is diverted to induction circuit 16.
As the bypass flow of exhaust passes through recirculation circuit 20, it may encounter cooler 40 and be chilled before being drawn back into induction circuit 16 by compressors 22. In particular, the exhaust may transfer heat through walls of passage 46 and through features 48 to a cooler secondary fluid, thereby providing for a temperature drop in the exhaust. The recirculated exhaust flow may then be mixed with the air entering combustion chambers 14. The exhaust gas directed to combustion chambers 14 may reduce the concentration of oxygen therein, which in turn may lower the maximum combustion temperature within engine 10. The lowered maximum combustion temperature may slow the chemical reaction of the combustion process, thereby decreasing the formation of nitrous oxides. In this manner, the gaseous pollution produced by engine 10 may be reduced.
As the recirculated exhaust flows first through cooler 40 and then through cooler 24, particles in the gas may collect on the exposed surfaces of passages 46 and features 48. That is, as the exhaust travels along the length of passages 46, from the inlets 42 to outlets 44, the particles may be attracted to and adhere to the stainless steel surfaces of passages 46 and features 48. If unaccounted for, this fouling may create an insulative layer that reduces or inhibits thermal transfer between the primary and secondary fluids. However, the portions of coolers 24 and/or 40 that are coated with the disclosed ceramic material may have a smoother exposed surface that does not attract or promote adhesion of the particles. Accordingly, these portions of coolers 24 and 40 may acquire less of the particles, allowing for a greater rate of heat transfer. It should be noted that, in some embodiments, layer 50 may itself reduce a heat-transfer capacity of features 48. However, the reduction caused by layer 50 may be much less than a reduction otherwise caused by the deposition of particles on features 48. For example, layer 50 may result in a loss of thermal conductivity by an amount less than about 0-10%.
As described above, in order to provide for longevity of layer 50, the ceramic material may be deposited only on surfaces within coolers 24 and 40 that are exposed to a desired operating range of temperatures. These surfaces may he found on only a subset of features 48 (e.g., on only the final 20-50% of features 48) included within coolers 24 and 40. This operating range of temperatures may be a range that allows for the deposited ceramic material to expand at a rate similar to a rate of expansion experienced by the underlying stainless steel of coolers 24 and 40.
Because only portions of the disclosed coolers may be coated with the ceramic material, a cost of the coolers may be low. In addition, because the ceramic material may be applied only in areas having the desired operating temperature range, the material may be less likely to fail. This may enhance an effectiveness and longevity of the coolers. And the increased effectiveness of coolers 24 and 40 may allow for coolers 24 and/or 40 to be smaller and even less expensive.
It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed heat exchanger. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed heat exchanger. For example, although
