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Double-wall, vented heat exchangerRelated Patent Categories: Heat Exchange, With Leakage CollectorDouble-wall, vented heat exchanger description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070169916, Double-wall, vented heat exchanger. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention is directed to a plate heat exchanger, and more specifically to a plate heat exchanger having double-wall, vented construction for venting a fluid resulting from leakage occurring within the plate heat exchanger along a predetermined leak path to a predetermined region along the exterior of the plate heat exchanger. BACKGROUND OF THE INVENTION [0002] Heat exchangers are traditionally used to heat or cool potable or process critical fluids using non-potable fluids while providing a physical, mechanical boundary to prevent contact between the respective fluid streams. Heat exchangers, as with all mechanical devices, have their respective, unique finite operating timeframes at the end of which the devices fail for one or more reasons. A typical failure mode for heat exchangers is a boundary breach that either allows one or both fluids: 1) to escape to the outside environment or atmosphere (external leak) or 2) to mix with one another without escaping to the outside environment (internal leak). With heat exchangers used in potable, sanitary, or critical fluid applications, an internal leak that allows the two fluids to mix can have catastrophic results, such as illness or poisoning in the case of potable and sanitary applications, or chemical fire or explosion in the case of critical fluid process applications. Internal leaks are generally not noticed immediately, whereas external leaks are usually visually evident. [0003] To avoid this possible situation of having an unseen (internal) leak, it is desirable to provide a fully vented, double-wall boundary that exhausts the leaking fluid to the outside environment or atmosphere in lieu of having the respective fluids mix inside the heat exchanger while the heat exchanger continues to operate. The manufacturing processes required to manufacture heat exchangers having double-wall construction are more intensive, thus double-wall heat exchangers are generally more expensive than conventional, single-wall heat exchangers. As a result, in unregulated competitive markets, many single-wall heat exchangers are used in applications that truly require fully vented, double-wall construction to provide an adequate level of safety. In an effort to increase the general public safety, various agencies and governmental bodies have implemented construction codes that require the use of a "double-wall" heat exchanger or "double-wall, vented" heat exchanger for potable fluid applications. [0004] For example, UNDERWRITERS LABORATORIES INC..RTM. or UL.RTM., both registered trademarks of Underwriters Laboratories, Inc. of Chicago, Ill., has established certification requirements to include double-wall vented construction for all refrigerant to potable water heat exchangers. Examples include water coolers/fountains and refrigerant desuperheaters. Also, IAPMO.RTM., a registered trademark of the International Association of Plumbing and Mechanical Officials of Ontario, Canada has established plumbing code certification requirements to include double-wall heat exchangers for potable water heating, or at minimum, double separation or two heat exchangers. Examples of double separation heat exchangers include domestic hydronic or steam boilers used to heat domestic hot water. In addition, various U.S. states and municipalities and other countries require double-wall or double separation of the respective fluids when used in potable water applications. [0005] A double-wall heat exchanger is one in which the heat transfer surface separating the two fluids is comprised of two separate surface layers, rather than one. Thus, if the first surface layer fails to provide a fluid tight barrier, the second layer should remain intact, causing an amount of fluid that is in contact with the failed surface layer to flow between the surface layers, preferably to a location where the leaking fluid can be detected externally of the heat exchanger so that the heat exchanger can be removed from service. The double-wall construction is intended to be a safety feature to prevent cross-contamination of the fluids. [0006] A further definition of this leak detection process is a "leak path" between the two surface layers. This leak path is either due to an inherent spacing between the metal-to-metal surfaces of the two layers, or an intentionally formed spacing between the two layers, or sufficient porosity between the two layers. In any event, the fluid from the failed surface layer will follow the "leak path" to an external location for detection of the leaking fluid. [0007] The first commercially available double-wall heat exchangers to incorporate these operational features were tubular designs. For example, U.S. Pat. No. 4,210,199 issued to Doucette et al. is directed to double-wall tubes utilizing two tubes, one tube disposed inside the other tube. The diameter of the outer tube is intermittently reduced, or swaged upon the inner tube, thereby creating the double-wall construction, the contact between the walls being provided for heat transfer and a vent path between the fluids. Other similar tubular designs have also been used. These methods yield a product that has a relatively accurately controlled vent-path and contact area dimensions. [0008] When the above-mentioned double-wall tube is inserted into a heat exchanger, a "double tube sheet" method is used, in which an outer tube and an inner tube is each inserted through adjacent tube sheets. A tube sheet is a typical component of tubular heat exchangers through which the tubes are inserted and subsequently sealed by means of mechanical rolling, hydraulic expansion, or welding/brazing. This method provides a double seal between the inner and outer tubes and a leak path for the tube joints, resulting in a fully-vented double-wall heat exchanger. These tubular designs suffer from at least two main drawbacks: 1) high fabrication cost, and 2) large physical size relative to conventional, single-wall designs. [0009] Double-wall plate-type heat exchangers are a later development. The double-wall plate-type heat exchanger is fabricated by simultaneously forming two thin-wall strips of material in the same tool, such that both strips are formed together, substantially identically, resulting in a pair of heat exchanger plates that serve as a double layer. The double-layer plate pairs or sets are then stacked to form fluid flow passages between the plate pairs and are separated from each other plate pair via elastomer gaskets disposed along the periphery of the plate pairs. The entire assembly is then compressed and held together via long, threaded bolts that are positioned along the heat exchanger periphery. [0010] A primary limitation of this construction is that the contact area between the two formed plates and the flow area in the vent path (i.e., leak path) are difficult to control because the tensile properties of the plates of the respective plate pairs cause a "spring back" effect that occurs after the forming process. This spring back effect prevents the strips from completely nesting together, or forming a substantially conformal contact therebetween. The vent path flow area can be extremely small when the vent path is adjacent to locations that have contact between the substantially conformal adjacent plates, such that elevated levels of fluid pressure can be required to force a flow of a leaking fluid through the vent path flow area. These locations of conformal surface contact between the adjacent plates typically have a relatively high heat transfer coefficient. Other locations between the adjacent plates that are not in conformal surface contact typically have a lower heat transfer coefficient because of the additional thermal resistance caused by the spacing between the plate surfaces. The leak path flow area adjacent to these locations are typically relatively large and thereby offer a relatively lower flow resistance for the leaking fluid. If the designed vent path gap is increased to minimize the amount of fluid pressure necessary to force fluid flow between the plates, then the coefficient of heat transfer for the double-wall plate is significantly decreased. This decrease in heat transfer coefficient is due to the increased thermal resistance associated with the increased spacing between the plates, the relationship between decreased heat transfer coefficient and increased spacing between the plates being a substantially linear relationship. The reduction of fluid leakage pressure versus the reduced heat transfer coefficient resulting from increased plate spacing is a primary dilemma of plate heat exchangers in double-wall applications. [0011] An additionally important consideration in the design of double-wall plate heat exchangers involves the regions surrounding the port areas where the two fluids are separated by a port seal. To fully meet the intent of the aforementioned building codes requiring double-wall construction, the port areas must be fully-vented to the outside environment via a double-port-seal system. In gasketed-type plate heat exchangers, various gasketing methods are used to create double port seal structures that allow a leak to be revealed externally to the heat exchanger if the first port seal fails. U.S. Pat. No. 4,976,313 issued to Dahlgren provides an example of this technology. However, gasketed-type plate heat exchanger designs suffer from three primary drawbacks: 1) high fabrication cost, 2) gasket life that is shorter than that of all-metal construction types, and 3) lower pressure-bearing capabilities than the tubular-type exchangers. [0012] Brazed-plate heat exchangers are the latest entry into the double-wall heat exchanger market. Brazed-plate type heat exchangers are similar in construction to the gasketed-type plate heat exchangers in that they are constructed using plates fabricated by simultaneously forming two thin-wall strips of material in the same tool, such that both are formed together, substantially identically, as a double layer. These double-plate pairs or sets are then stacked to form the fluid passages. However, instead of utilizing gaskets and long bolted fasteners to provide the sealing mechanism, brazed-plate heat exchangers utilize thin sheets of braze material such that the plate pairs braze together in a brazing furnace or other heating device. U.S. Pat. No. 5,291,945 issued to Blomgren et al. is directed to a double-wall brazed plate heat exchanger. [0013] A critically important manufacturing concern in the manufacture of double-wall, brazed-plate heat exchangers is in preventing the braze material from flowing into the vent path between adjacent plates via capillary action and thereby blocking the flow path for the leaking fluid to escape the heat exchanger. The aforementioned U.S. Pat. No. 5,291,945 addresses this problem at the periphery of the plate pairs only. The solution offered by the heat exchanger construction of this patent is to provide a sufficiently large spacing between the peripheral edges of the double plate pairs such that both capillary and gravity forces prevent the braze metal from wicking to the small interspace gaps between the respective double plate sets, with the heat exchanger plates being brazed in a particular orientation to take advantage of the gravity forces. [0014] Another drawback of the heat exchanger construction of U.S. Pat. No. 5,291,945 is its lack of pressure-bearing capability. This lack of pressure-bearing capability is a result of the peripheries of the respective double plate pairs not being brazed, such that the only use of braze material to hold the plate pairs together is limited to the port areas. To increase the pressure-bearing capability of the unit such that it can withstand the requirements of relatively low-pressure systems, a mechanical reinforcement system consisting of a threaded rod, two washers, and two nuts is added to each port structure. [0015] In summary, U.S. Pat. No. 5,291,945 design has several drawbacks including: [0016] i) port holes not having vented, double-seals needed to meet building code requirements; [0017] ii) port areas requiring structural re-enforcement; [0018] iii) braze material foil not being stamped at the same time as the plates; [0019] iv) lacking provisions to prevent or encourage braze material wicking into the vent path; [0020] v) lacking a predetermined location for the external leak to occur; and [0021] vi) low design working pressure. [0022] Others, such as SWEP International AB, of Sweden and WTT (Wilchwitz Thermo-Technik) of Germany, also manufacture double-wall, vented brazed-plate heat exchangers. They each separate the double-plate pairs after the forming process and apply to a portion of the periphery of the plate surfaces a coating, such as an oxide, that repels the capillary flow of the braze material, and then rebuild the double-plate pairs prior to assembling the heat exchanger for brazing. This step prevents the entire periphery of the plates from being filled with braze material, which would completely close the leak path from venting to the environment, and provides an amount of structural reinforcement to the heat exchanger. [0023] However, all of the known double-wall brazed-plate heat exchanger constructions have a common drawback in that no construction has provided a double seal around the ports or a separate vent path around the ports. An inherent assumption behind all known constructions of brazed-plate heat exchangers is that if a port joint fails (e.g., braze material etches away) the vent area opens up and allows the leak to be vented between the plates. This assumed venting of a leak in the port area cannot be guaranteed, and thus presents a major obstacle in meeting the intent of the building and plumbing codes and requirements that require double-wall separation. [0024] In summary, brazed-plate heat exchangers have the inadequacies of: a high vent path pressure drop, no specific vent path for plate leaks to vent to the external environment, and no double seal and vent path around the ports. Further, since the vent path is not sufficiently channeled, leak detection can be elusive. Coupled with the state of the art construction methods, low heat transfer efficiency is a problem, as well as low working pressure containment, due to the design. Thus, brazed-plate heat exchangers, when constructed in double-wall configurations, need specific features and attributes to meet safety requirements and a significant improvement over the state of the art. SUMMARY OF THE INVENTION [0025] The present invention relates to a plate heat exchanger including a plurality of nested pairs of plates, each plate of the plurality of pairs of plates having opposed surfaces and perimeter flanges and having substantially similar surface profiles. Each plate pair forms a substantially conformal fit between contacting surfaces when pressed together, opposed surfaces of each plate pair providing a portion of at least one flow path for each of at least two fluids. Facing surfaces and perimeter flanges of adjacent plate pairs of the plurality of plate pairs provide a flow path boundary for two fluids of the at least two fluids. Opposed surfaces of at least one plate pair of each pair of adjacent plate pairs provides a flow path boundary for two fluids of the at least two fluids. The at least one plate pair has a high thermal conductivity and provides a portion of the flow path boundary for two fluids of the at least two fluids, thereby providing thermal communication between the two fluids on the opposed surfaces of the plate. An inlet and outlet for each fluid of the at least two fluids is provided, the inlet and outlet for each fluid being in fluid communication with each flow path for said fluid. A predetermined vent path is formed in at least one of the facing surfaces of each plate pair capable of venting each fluid exterior of the perimeter flanges. [0026] The present invention further relates to a method for making plates for a plate heat exchanger, the steps include providing a plurality of nested pairs of plates, each plate of the plurality of pairs of plates having opposed surfaces and perimeter flanges and having substantially similar surface profiles. Each plate pair forms a substantially conformal fit between contacting surfaces when pressed together, opposed surfaces of each plate pair providing a portion of at least one flow path for each of at least two fluids. Facing surfaces and perimeter flanges of adjacent plate pairs of the plurality of plate pairs provide a flow path boundary for two fluids of the at least two fluids. Opposed surfaces of at least one plate pair of each pair of adjacent plate pairs provide a flow path boundary for two fluids of the at least two fluids, the at least one plate pair having a high thermal conductivity and providing a portion of the flow path boundary for two fluids of the at least two fluids, thereby providing thermal communication between the two fluids on the opposed surfaces of the plate. Each plate of plurality of plates includes the step of forming a plurality of apertures in the plate, at least two of the apertures having an embossed region surrounding the apertures, each embossed region defining a path for venting fluids of the at least two fluids leaking between nested plate pairs along aligned apertures of the plurality of apertures. The method further includes the step of forming at least one primary vent path in the plate, the at least one primary vent path in fluid communication with the at least two embossed regions for venting the at least two fluids exterior of the perimeter flanges. The method further includes the step of selectively applying a surface treatment to at least one surface and perimeter flanges of at least one plate, the at least one surface corresponding to a contacting surface of a plate pair. [0027] The present invention still further relates to a plurality of nested pairs of plates, each plate of the plurality of pairs of plates having opposed surfaces and perimeter flanges and having substantially similar surface profiles. Each plate pair forms a substantially conformal fit between contacting surfaces when pressed together, opposed surfaces of each plate pair providing a portion of at least one flow path for each of at least two fluids. Facing surfaces and perimeter flanges of adjacent plate pairs of the plurality of plate pairs provide a flow path boundary for two fluids of the at least two fluids. Opposed surfaces of at least one plate pair of each pair of adjacent plate pairs provide a flow path boundary for two fluids of the at least two fluids. The at least one plate pair has a high thermal conductivity and provides a portion of the flow path boundary for two fluids of the at least two fluids, thereby providing thermal communication between the two fluids on the opposed surfaces of the plate. An inlet and outlet for each fluid of the at least two fluids is provided, the inlet and outlet for each fluid being in fluid communication with each flow path for said fluid. A predetermined vent path is formed in at least one of the facing surfaces of each plate pair capable of venting each fluid exterior of the perimeter flanges. Continue reading about Double-wall, vented heat exchanger... 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