Abstract:
A system is provided for extracting heat from a flue gas after it is expelled from a cooking appliance, such as a commercial deep fryer. A flue extension is provided between the flue of the cooking appliance and an exhaust hood positioned above the cooking appliance. Within the flue extension is disposed a heat exchanger assembly, which includes a coil that forms part of a closed fluid loop including a second heat exchanger that is disposed within a fluid medium that is to be heated, such as water. A mount supports the flue extension, and allows movement of the flue extension between an operating configuration and a stowed configuration. The flue extension is positioned closer to the flue of the cooking appliance when in the operating configuration compared to the stowed configuration. Mounting the flue extension is achieved without modifying the cooking appliance or the exhaust hood.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to the field of waste heat recovery. More particularly, the present invention relates to a method and system for extracting heat from flue gas that is exiting from a cooking appliance, such as for instance a commercial deep fryer or gas oven. 
       CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0002]    This application claims priority to Canadian Patent Application No. 2911796, filed Nov. 12, 2015, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the restaurant industry, and in other settings in which commercial cooking appliances are used, it is required to vent waste heat to the outside via an exhaust hood that is positioned above the cooking appliance. Unfortunately, this practice results in a very inefficient use of energy in the kitchen. For instance, the flue gas from a commercial deep fryer may be exhausted at a temperature in the range 700-1100° F. At the same time, additional energy is consumed for space heating and to heat water for other applications in the kitchen, such as dish washing, hand cleaning, etc. 
         [0004]    Various attempts have been made to partially recover the waste heat that is produced by commercial cooking appliances, to be used for other purposes such as space heating and/or production of hot water. In one approach a heat exchanger is located within the exhaust hood above the appliance for extracting heat from the air as it is being vented to the outside. This approach is not entirely satisfactory because grease and other particulates tend to build up rather quickly on the heat-exchanger surfaces, thereby insulating the surfaces and reducing the efficiency of the system over time. Further, the temperature of the air that enters the exhaust hood is much lower than the temperature of the air immediately after it exits the flue of the cooking appliance, and as such less heat is available to be extracted. In another approach a heat exchanger is introduced directly into the flue of the cooking appliance. This approach is also not entirely satisfactory because the heat exchanger is subjected to extremely high temperatures, which necessitates a very robust design and may cause solder joints etc. to fail relatively quickly. Additionally, modifying the cooking appliance to accept the heat exchanger within the flue may violate safety code regulations and/or void manufacturer warranties. 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with an aspect of at least one embodiment, there is provided a system for extracting heat from a flue gas exiting from a cooking appliance, the system comprising: a flue extension having an open first end and an open second end opposite the first end, the first end aligned with but spaced apart from a flue of the cooking appliance for receiving a flue gas at a first temperature as it exits via the flue of the cooking appliance, and the second end facing but spaced apart from an exhaust hood for discharging the flue gas at a second temperature to the exhaust hood, the second temperature being lower than the first temperature; a mount for supporting the flue extension within a space between the flue of the cooking appliance and the exhaust hood, the mount being other than secured to either the cooking appliance or the exhaust hood; and a heat exchanger assembly disposed within the flue extension and comprising a heat exchanger tube for circulating a volume of a thermal fluid, the heat exchanger tube comprising a plurality of substantially straight tube sections that are arranged substantially normal to a flow direction of the flue gas through the flue extension. 
         [0006]    In accordance with an aspect of at least one embodiment, there is provided a system for extracting heat from a flue gas exiting from a cooking appliance, comprising: a flue extension having an open first end, an open second end that is opposite the first end, and a central passageway extending between the first and second ends; a first heat exchanger assembly disposed within the central passageway of the flue extension; a mount for supporting the flue extension and the first heat exchanger within a space between a flue of the cooking appliance and an exhaust hood disposed above the cooking appliance, the mount being operable between: a first configuration in which the first end of the flue extension is aligned with but spaced apart from the flue of the cooking appliance by a first distance, and a second configuration in which the first end of the flue extension is spaced apart from the flue of the cooking appliance by a second distance that is greater than the first distance; a storage tank for containing a medium to be heated; a second heat exchanger disposed within the storage tank, the second heat exchanger being in fluid communication with the first heat exchanger; and a pump for circulating a thermal fluid within a closed circuit that includes the first heat exchanger and the second heat exchanger. 
         [0007]    In accordance with an aspect of at least one embodiment, there is provided a method for extracting heat from a flue gas exiting a cooking appliance, comprising: using a mount, supporting a flue extension between a flue of the cooking apparatus and an exhaust hood disposed above the cooking apparatus, such that an open first end of the flue extension is aligned with but spaced apart from the flue of the cooking apparatus by a first distance, and wherein the mount is other than secured to either the cooking appliance or the exhaust hood; providing a heat exchanger assembly within the flue extension, the heat exchanger assembly including a heat exchanger coil; receiving the flue gas via the open first end of the heat exchanger, the flue gas being constrained by the flue extension to flow around the heat exchanger coil and out through an open second end of the flue extension along a direction generally toward the exhaust hood; and while the flue gas is being constrained to flow around the heat exchanger coil, circulating a heat transfer fluid within the heat exchanger coil, whereby heat is transferred from the flue gas to the thermal fluid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The instant invention will now be described by way of example only, and with reference to the attached drawings, wherein similar reference numerals denote similar elements throughout the several views, and in which: 
           [0009]      FIG. 1  is a block diagram showing the components of a system according to an embodiment. 
           [0010]      FIG. 2  is a schematic diagram showing a system according to an embodiment. 
           [0011]      FIG. 3  is a diagram showing the components of a sensing and control sub-system. 
           [0012]      FIG. 4  is a perspective view of a flue extension secured to a mount. 
           [0013]      FIG. 5  is a side view showing the flue extension supported by the mount when the mount is in a first (operating) configuration. 
           [0014]      FIG. 6  is a side view showing the flue extension supported by the mount when the mount is in a second (stowed) configuration. 
           [0015]      FIG. 7  shows the flue extension supported by the mount in the first (operating) configuration, relative to an exhaust hood and cooking appliance. 
           [0016]      FIG. 8  shows the flue extension supported by the mount in the second (stowed) configuration, relative to an exhaust hood and cooking appliance. 
           [0017]      FIG. 9  is a perspective view of a heat exchanger assembly that is suitable for use in a system according to an embodiment. 
           [0018]      FIG. 10  is a side view of the heat exchanger assembly of  FIG. 9 . 
           [0019]      FIG. 11  is a perspective view of an alternative heat exchanger configuration. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The following description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the embodiments disclosed, but is to be accorded the widest scope consistent with the principles and features disclosed herein. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The phrase “heat exchanger assembly” is used interchangeably with the term “coil” throughout the description and in the claims. 
         [0021]      FIG. 1  is a simplified block diagram showing the components of a system  100  according to an embodiment. A heat exchanger assembly  102  is disposed within a flue extension  104 , which is supported by a not illustrated mount within a space  106  between a cooking appliance  108  and an exhaust hood  110 . The heat exchanger assembly  102  is in fluid communication with a second heat exchanger assembly  112 , which is disposed within a tank  114  containing a medium that is to be heated. For instance, tubing  116  and  118  are connected between the heat exchanger assembly  102  and the second heat exchanger assembly  112  for circulating a thermal fluid therebetween. For simplicity, various elements including sensors, controllers, manifolds, valves and pumps etc., which are described below with reference to  FIG. 2 , have been omitted from  FIG. 1 . The dashed lines in  FIG. 1  represent data communication paths for receiving feedback signals and for providing control signals, and are intended to be illustrative in nature. In this example, both the medium that is to be heated and the thermal fluid are water. Optionally, the thermal fluid is another fluid, such as for instance a water/propylene glycol mixture, e.g. 60% propylene glycol and 40% water. Optionally, the configuration of the second heat exchanger assembly  112  differs from the configuration of the heat exchanger assembly  102 . 
         [0022]      FIG. 2  is a more detailed schematic diagram of system  100 . Controller  120  receives feedback signals from temperature sensors  200  and from flow sensors  202 . In the embodiment that is shown in  FIG. 2 , temperature sensors  200  are located: (1) in the tubing  118  before the inlet to the second heat exchanger assembly  112 ; (2) in the tubing  116  after the outlet from the second heat exchanger assembly  112 ; (3) in the cold water supply line before the inlet to the storage tank  114 ; and (4) in the hot water supply line after the outlet from the storage tank  114 . A fifth temperature sensor  200  is disposed within the flue extension  104 . The fifth temperature sensor is used to detect a temperature increase in the flue extension  104 , which causes the system to switch on and begin sensing other parameters, etc. For instance, a relay is responsive to an output signal from the fifth temperature sensor for switching on and off a pump  206 . Additionally, flow meters  202  are located in tubing  116  after the outlet from the second heat exchanger assembly  112  and in the cold water supply line before the inlet into the storage tank  114 . Based on the signals that are received from the temperature sensors  200  and the flow meters  202 , the controller  120  provides control signals for controlling various components of the system, such as for instance valves  204 , pumps  206 , etc. Optionally, the temperature sensors  200  and/or flow meters  202  and/or valves  204  and/or pumps  206  are disposed within the system  100  at other than the locations described above. Optionally, more or fewer temperature sensors  200  and/or flow meters  202  and/or valves  204  and/or pumps  206  are utilized in the system  100 . 
         [0023]      FIG. 3  is a simplified schematic diagram showing the components of a sensing and control sub-system of the system  100 . Controller  120  includes a plurality of analog inputs (numbered 2, 3, 5-7, 9-12 in  FIG. 3 ), which receive signals that are generated by various sensing devices in response to sensed system parameters. As discussed supra with reference to  FIG. 2 , in this specific and non-limiting example, the controller  120  receives signals from two flow meters  202  and from five temperature sensors  200 . A display device  300 , such as for instance an LCD screen, is provided for displaying at least some of the data that are read from the temperature sensors  200  and/or flow meters  202 . Optionally, the display device  300  displays calculated values relating to the operation of the system  100 , such as for instance the amount of money saved due to use of the system  100 , the reduction in CO 2  emissions achieved, the amount of natural gas saved, etc. 
         [0024]    Referring now to  FIG. 4 , shown is a perspective view of a mount  400  supporting flue extension  104 . The mount  400  includes four wall-mounting brackets  404  (three are visible in  FIG. 4 ), for securing the mount  400  to a support surface such as a wall (not shown in  FIG. 4 ). For instance, the mount  400  is secured to the mounting surface using four ¼″ bolts (not shown in  FIG. 4 ). The mount  400  includes left and right single link pivoting assemblies  406 , which support generally vertical movement of the flue extension  104  between a first (operating) configuration in which the lower end of the flue extension  104  is aligned with but spaced apart from the flue of the cooking appliance (not shown in  FIG. 4 ) by a first distance, and a second (stowed) configuration in which the lower end of the flue extension  104  is spaced apart from the flue of the cooking appliance by a second distance that is greater than the first distance. For instance, the flue extension  104  is supported in the first configuration during operation, in order to maximize heat transfer from the flue gas to the thermal fluid that is circulating within the heat exchanger assembly  102  (not shown in  FIG. 4 ). On the other hand, the flue extension  104  is supported in the second configuration during cleaning and/or maintenance of the heat exchanger assembly  102  and/or cooking appliance and/or surrounding spaces. The single link pivoting assemblies  406  are pivotally attached to left and right frame-members  408 , each of which is secured to a respective pair of the wall-mounting brackets  404 . A handle  410  extends between the left and right single link pivoting assemblies  406 , and may be grasped by a user when moving the flue extension  104  between the first and second configurations. 
         [0025]    Of course, the material that is used to fabricate the flue extension  104  and the mount  400  must meet hygienic kitchen standards, must not corrode, and must be able to withstand the operating temperature of the cooking appliance  108 . By way of a specific and non-limiting example, “403 stainless steel” is used. 
         [0026]      FIG. 5  is a side view showing the flue extension  104  supported by the mount  400  in the first configuration, and  FIG. 6  is a side view showing the flue extension  104  supported by the mount  400  in the second configuration. Additionally,  FIGS. 7 and 8  show the flue extension  104  installed adjacent to a surface  700  via the mount  400 . For improved clarity, the tubing  116  and  118  have been omitted from  FIGS. 7 and 8 . As is apparent, the mount  400  is supported entirely by the surface  700 , which in this example is a wall that is situated behind the cooking appliance  108 . None of the system components, including the mount  400 , the flue extension  104 , or the heat exchanger assembly  102 , comes into contact with the cooking appliance  108  or with the exhaust hood  110 . Accordingly, installing the system does not involve making any modifications to the cooking appliance  108  or to the exhaust hood  110 . 
         [0027]    Referring now to  FIG. 9 , shown is a perspective view of a heat exchanger assembly  102  that is suitable for use in the system  100  of  FIG. 1 .  FIG. 10  shows a side view of the heat exchanger  102 . In this specific and non-limiting example, the heat exchanger assembly  102  comprises tubing  900  that is fabricated from 13 mm (outside diameter) seamless 310s stainless steel, with a wall thickness of 1.0 mm, and with fins  902  that are made of 304 stainless steel. The tubing  900  is formed into an elongated, double helical coil configuration, having a height H, and a length L that is greater a width W. The values of H, L and W are limited by inter alia the physical size of the space  106  between the cooking appliance  108  and the exhaust hood  110 . In a typical application, the length L of the coiled assembly is in the range 203-381 mm, and more preferably in the range 292-381 mm. Of course, these values are examples only and assume a length-wise restriction imposed by a commercial cooking appliance  108  having a flue width of 425 mm. In the specific and non-limiting example that is shown in  FIGS. 9 and 10 , the length L is approximately 304 mm, the width W is approximately 127 mm, and the height H is approximately 304 mm. The uncoiled length of the tubing  900  is approximately 6705 mm. Additionally, tube-end connectors  904  and  906  are provided at the opposite ends of the tube  900 , for connecting the heat exchanger assembly  102  to the tubing  116  and  118 . In practice, the tubing  116  and  118  may be standard flexible tubing, which may be routed or plumbed throughout the building and then transitioned to copper piping in the kitchen and finally to high temperature stainless steel flex lines for the final connection to inlets and outlets of the heat exchanger assembly  102 . 
         [0028]    In general, maximizing the outside diameter of the heat exchanger tube  900  and maximizing the number of turns of the coil, while also minimizing the wall thickness of the heat exchanger tubes  900 , results in greater heat transfer to the thermal fluid and accordingly produces a higher thermal fluid outlet temperature. The spacing between adjacent turns of the coil is limited by the maximum allowable flue gas flow rate reduction. That is to say, packing the turns of the double helical coil too closely together causes a flow restriction, and the flue gas venting may become inadequate. In the heat exchanger assembly  102  that is shown in  FIGS. 9 and 10 , the center of the double helical coil is substantially open and thereby ensures adequate flow through the structure. 
         [0029]    Alternatively, a heat exchanger assembly having a different shape and/or a different configuration may be used in the system  100 . For instance a common “trombone-shaped” coil  1100 , as shown in  FIG. 11 , may be used in place of the heat exchanger assembly  102  that is shown in  FIGS. 9 and 10 . Further alternatively, finless tubing or tubing that is fabricated from a different material etc. is used. 
         [0030]    During operation the flue extension  104  is aligned above the flue  702  of the cooking appliance  108 , as is shown in  FIG. 7 . An optional gasket  704  (shown using dashed lines), which is fabricated from a heat resistant material, is disposed between the flue extension  104  and the flue  702 . Pump  206  (not shown in  FIG. 7 ) circulates a thermal fluid through the tubing  900  of the not illustrated heat exchanger assembly  102 . Flue gas is vented from the cooking appliance  108  via flue  702  and enters into the flue extension  104 , optionally being guided by the gasket  704  when present. The flue gas flows around the finned tubing  900  of the heat exchanger assembly  102 , which results in transfer of energy (heat) from the flue gas to the thermal fluid that is being circulated within the heat exchanger assembly  102 . More particularly, the finned heat exchanger tubing  900  comprises a plurality of substantially straight tube sections that are arranged substantially normal to a flow direction of the flue gas through the flue extension. Pump  206  then causes the heated thermal fluid to flow out of the heat exchanger assembly  102  and into the second heat exchanger assembly  112 , which is disposed within storage tank  114 . Water stored within storage tank  114  absorbs energy (heat) from the thermal fluid, thereby raising the temperature of the stored water and lowering the temperature of the thermal fluid. The cooled thermal fluid exits the second heat exchanger assembly  112  and is then returned to the heat exchanger assembly  102 , and the cycle repeats. 
         [0031]    When maintenance or cleaning is required, a user simply grasps the handle  910  and pivots the flue extension  104  (including the optional gasket  704 ) upward and away from the flue  702  of cooking appliance  108 . The flue  702  and/or surrounding surfaces etc., may then be cleaned, moved and/or maintained. 
         [0032]    Providing a flue extension  104  in alignment with the flue  702  of the cooking appliance  108 , and disposing the heat exchanger assembly  102  within the flue extension  104 , avoids the need modify either the flue  702  or the exhaust hood  110 . Further, the volume and pressure of the flue gas flow through the flue  702  and exhaust hood is substantially unaffected by the presence of the heat exchanger assembly  102  within the flue extension  104 . Further still, the heat exchanger assembly  102  and flue extension  104  can be rapidly moved out of the way to provide access to the flue  702  when required, including in emergency situations. The flue extension  104  channels the flue gas upward and toward the exhaust hood, limiting the amount of heat that escapes into the environment around the cooking appliance. Additionally, the heat exchanger assembly  102  removes some of the heat from the flue gas, thereby lowering the temperature of the flue gas and further limiting the amount of heat that escapes into the environment around the cooking appliance. 
         [0033]    Optionally, a different type of mount is used, such as for instance a “drawer-slider” type mount that support horizontal movement of the flue extension and heat exchanger assembly. Such a mount is appropriate when the vertical dimension of the space  106  between the cooking appliance  108  and exhaust hood  110  is limited. Alternatively, the mount is stationary and the flue extension  104  is secured to the mount using quick-release fasteners, or the flue extension  104  simply “snaps” into place on the mount, such that the flue extension  104  and heat exchanger assembly  102  may be removed from the mount for cleaning/maintenance purposes. Further alternatively, the flue extension  104  and heat exchanger assembly  102  may be suspended from the ceiling or from another support surface above the cooking appliance, such as for instance using cables or chains. 
         [0034]    While several inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the invent of embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure. 
         [0035]    All definitions, as defined and used herein, should be understood to control over dictionary definitions, and/or ordinary meanings of the defined terms. The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. 
         [0036]    Multiple elements listed with “and/or” should be construed in the same fashion, i.e., “one or more” of the elements so conjoined. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, a reference to “A and/or B”, when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A only (optionally including elements other than B); in another embodiment, to B only (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. 
         [0037]    As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law. 
         [0038]    As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. 
         [0039]    It should also be understood that, unless clearly indicated to the contrary, in any methods claimed herein that include more than one step or act, the order of the steps or acts of the method is not necessarily limited to the order in which the steps or acts of the method are recited. 
         [0040]    Numerical ranges include the end-point values that define the ranges. For instance, “between X and Y” includes both X and Y, as well as all temperature values between X and Y. 
         [0041]    In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively. 
         [0042]    The foregoing description of methods and embodiments of the invention has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise steps and/or forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention and all equivalents be defined by the claims appended hereto.