Abstract:
A fuel-fired heating appliance has multiple premix type fuel burners horizontally disposed in a row in its combustion chamber and operable in a staged manner. The burners are upwardly spaced apart from a rigid fiberboard insulation panel structure extending along the bottom interior side of the combustion chamber. Sandwiched between and contacting the bottom sides of the burners and the top side of the fiberboard panel is a blanket of resilient ceramic fiber insulation material which functions to (1) prevent uncombusted fuel from firing burners from being circulated under non-firing burners, (2) increase the operating temperatures of bottom sides of the burners during firing thereof to lessen thermal stresses in the firing burners, (3) resiliently permit differential thermal expansion of the burners, and (4) reduce harmonic resonance of the burners, and associated operational noise of the appliance, during firing of the burners.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to heating apparatus and, in an illustrated embodiment thereof, more particularly relates to a fuel-fired heating apparatus having a specially designed staged fuel burner system incorporated therein.  
         [0002]     Multiple burner combustion systems are commonly utilized in a variety of fuel-fired heating appliances such as water heaters, pool heaters and boilers. In this type of combustion system, a plurality of mutually spaced apart, parallel, horizontally oriented tubular burners are disposed in a combustion chamber portion of the heating appliance. Each burner is typically of the “premix” type in which, during firing thereof, fuel from a source thereof, and fan-supplied primary combustion air are flowed through the tube to form therein a fuel/air mixture which is discharged through burner outlet openings and combusted without the use of secondary combustion air.  
         [0003]     Many multiple burner combustion systems of this type are operated in a “staged” manner in which for low heating loads only some of the burners are fired, with the remaining burners remaining in an unfired state. When the heating load increases, some or all of the remaining burners are also fired to correspondingly increase the heating capability of the appliance. Conventional multiple burner combustion systems of this general type have associated therewith a variety of well known problems, limitations and disadvantages.  
         [0004]     For example, when only some of the burners are being fired, uncombusted fuel being discharged from the firing burners tends to undesirably circulate under the unfired burners, and then is discharged from the heating apparatus, resulting in poor overall fuel combustion which is undesirable from an environmental emission standpoint. Further, during firing of a given burner of this type, a substantial temperature differential exists between the (hotter) top side of the burner and its (cooler) bottom side. This temperature difference causes differential longitudinal expansion of the burner during firing, thereby subjecting the burner to a substantial amount of thermal stress during its operation. Additionally, under certain operating conditions, the burners may harmonically resonate—a condition which undesirably increases the operating noise level of the appliance.  
         [0005]     As can readily be seen from the foregoing, it would be desirable to provide in a fuel-fired heating appliance a multiple burner combustion system in which the above mentioned burner-related problems, limitations and disadvantages are eliminated or at least substantially reduced.  
       SUMMARY OF THE INVENTION  
       [0006]     In carrying out principles of the present invention, in accordance with an illustrated embodiment thereof, fuel-fired heating apparatus is provided which is representatively a boiler and has a combustion chamber with a bottom interior side portion. A heat exchanger structure horizontally extends through the combustion chamber and is adapted to receive a through-flow of a fluid, such as water, to be heated. A plurality of tubular fuel burners, representatively premix-type gas burners, longitudinally extend horizontally through the combustion chamber in a laterally spaced apart mutually parallel orientation, the burners being positioned below the heat exchanger structure and having bottom side portions spaced upwardly apart from and facing the bottom side portion of the combustion chamber.  
         [0007]     The fuel-fired heating apparatus also has control apparatus operative to provide for staged firing of the fuel burners to heat fluid flowing through the heat exchanger structure. A resilient insulation structure is sandwiched between and contacts the bottom interior side portion of the combustion chamber and the bottom side portions of the fuel burners. Preferably, but not by way of limitation, the bottom interior side portion of the combustion chamber is of a relatively rigid insulation material, representatively a fiberboard material, and the resilient insulation structure is representatively a ceramic fiber insulation blanket structure.  
         [0008]     In the representatively illustrated embodiment of the fuel-fired heating apparatus, the resilient insulation structure sandwiched between and contacting the bottom sides of the burners and the underlying bottom interior side portion of the combustion chamber provides several desirable functions.  
         [0009]     First, during low stage firing of the heating apparatus when only some of the burners are being fired, it substantially blocks migration of uncombusted fuel from the firing burners to the non-firing burners via any portion of the vertical space between the bottom sides of the burners and the bottom interior side portion of the combustion chamber. This lessens the pollution emission level of the heating apparatus.  
         [0010]     Second, the sandwiched resilient insulation structure reduces the thermal stress in the burners during firing thereof by elevating the operating temperature of the bottom sides of the burners, which reduces the temperature difference across the burner and reduces differential expansion.  
         [0011]     Third, the resilient insulation structure allows differential expansion of the burners without impeding it, thus avoiding additional mechanical stresses in the burner. Specifically, when any of the burners thermally expands in a lateral direction it simply compresses the underlying resilient insulation structure. When the burner later cools and returns to its original lateral dimension, the resilient insulation structure simply expands to its original thickness dimension under the burner to remain engaged therewith.  
         [0012]     Fourth, the resilient engagement of the burners by the underlying resilient insulation structure acts as a damper to any harmonic vibrations created in the burners during firing thereof. This, in turn, desirably reduces any operational noise of the fuel-fired heating apparatus.  
         [0013]     From at least thermal stress and operational noise reduction standpoints, the disclosed placement of a resilient insulation material in engagement with only a side portion of a fuel burner could also be used to advantage in applications where only a single burner is employed in a fuel-fired heating apparatus. Moreover, principles of the present invention could also be utilized to advantage with burners positioned in non-horizontal orientations and positioned in other locations within a combustion chamber portion of a fuel-fired heating appliance.  
         [0014]     It should be noted that while in a preferred embodiment of the present invention a resilient, insulative material is sandwiched between and resiliently contacts the burners and a facing surface portion of the combustion chamber, and is operative to block the flow of uncombusted fuel between the burners and the facing combustion chamber surface portion, the invention could provide some of the above-described advantages without incorporating therein all of the structural and operational features present in the preferred invention embodiment.  
         [0015]     For example, if a non-resilient insulation material were to be used, the sandwiched structure would still desirably increase the temperature of a side of the firing burner it contacts. As another example, if a resilient, non-insulative sandwiched structure were to be used, burner operational noise that may occur would still be reduced, and the thermal expansion of the burners during firing thereof would still be resiliently resisted. Also, even if the sandwiched structure did not substantially block the flow of uncombusted fuel between the burners and the facing combustion chamber surface, the sandwiched structure could still provide some or all of the other benefits that it does in the preferred embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic, partially phantomed side elevational view of a fuel-fired heating apparatus having incorporated therein a staged fuel burner system incorporating principles of the present invention;  
         [0017]      FIG. 2  is a perspective view of a longitudinal portion of one of the fuel burners removed from the heating apparatus;  
         [0018]      FIG. 3  is an enlarged scale schematic cross-sectional view through a lower portion of the heating apparatus; and  
         [0019]      FIG. 4  is an enlarged scale schematic cross-sectional view through the heating apparatus taken along line  4 - 4  of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION  
       [0020]     Referring initially to  FIGS. 1 -3 , the present invention provides fuel-fired heating apparatus  10  which is representatively a boiler. However, principles of the present invention are not limited to boilers, and could alternatively be incorporated to advantage in a variety of other types of fuel-fired heating appliances such as, for example, water heaters and pool heaters.  
         [0021]     As illustrated in  FIGS. 1 and 3 , the fuel-fired heating apparatus  10  has a metal housing  12  within which a combustion chamber  14  is disposed, the combustion chamber  14  being operatively communicated at its top side  16  with a suitable flue pipe  18 . Extending across a top interior portion of the combustion chamber  14  is a heat exchanger structure  20  through which a fluid to be heated, such as water  22 , may be suitably flowed. The interior of the combustion chamber  14  is lined with panels, such as bottom panel  24  and side panels  26 , of a relatively rigid insulation material, representatively a fiberboard insulation material (see  FIG. 3 ). A fuel burner system  28  is operatively associated with the combustion chamber  14  and is used to heat the water  22  flowing through the heat exchanger structure  20 .  
         [0022]     The fuel burner system  28  includes a plurality of tubular metal fuel burners  30  (representatively six premix-type gas burners  30   a - 30   f ) which longitudinally extend horizontally through the combustion chamber  14  in a mutually spaced, parallel relationship, and suitable controls  32  permitting a staged firing of the burners  30 —for example, firing only the burners  30   a - 30   c  under low fire conditions or firing all or the burners  30   a - 30   f  under high fire conditions. Other firing stage combinations may be alternatively utilized if desired, and there may be a greater or lesser number of burners incorporated in the burner system  28 . Both the premix-type burners  30  and the associated burner controls  32  may be of a suitable conventional construction well known to those of ordinary skill in this particular art. As best illustrated in  FIG. 3 , the burners  30  are spaced downwardly apart from the heat exchanger structure  20 , and are also spaced upwardly apart a smaller distance from the top side of the bottom relatively rigid insulation structure  24  within the interior of the combustion chamber  14 .  
         [0023]     Burners  30  have open inlet end portions (not shown) which are suitably anchored to a vertical wall portion of the combustion chamber  14 , and have closed outlet ends  34  (see, for example, the representative burner  30 a in  FIG. 2 ) and top and bottom body side portions  36 , 38 . Formed in the top side  36  of each burner  30  are fuel/air mixture discharge openings such as slots  40  or outlet holes  42  (or both as representatively shown in  FIG. 2 ). During firing of the heating apparatus  10  a fan portion  44  thereof draws combustion air  46  inwardly through a conduit  48  and forces the air  46  (via a non-illustrated air plenum) into the inlet ends of all of the burners  30   a - 30   f . Alternately, the blowers can be staged to operate only when the respectively staged burners are operating. At the same time, the controls  32  operate to force fuel  50  (representatively natural gas) from a source  52  thereof inwardly through the inlet ends of the burners  30  which are being fired (for example the burners  30   a - 30   c ).  
         [0024]     In each of the firing burners (such as burner  30   a  partially illustrated in  FIG. 2 ) this creates an interior flow  54  therethrough of premixed fuel and air which upwardly exits the top side discharge openings  40  and  42  and is suitably ignited to create burner flames  56  within the combustion chamber  14 . The non-firing burners  30  (if any) have only air traversing their interiors. Combustion gases  58  from the burner flames  56  pass upwardly and exteriorly across the heat exchanger structure  20  to heat water  22  being flowed therethrough, and are then discharged upwardly through the flue pipe  18  as shown in  FIG. 1 .  
         [0025]     With reference now to  FIGS. 3 and 4 , in accordance with a key aspect of the present invention, the pollutant discharge level of the heating apparatus  10  is diminished by sandwiching a resilient insulation structure, representatively a blanket  60  of ceramic fiber insulation material, between the bottom sides  38  of the burners  30   a - 30   f  and the top side of the relatively rigid bottom interior insulation structure  24  within the interior of the combustion chamber  14 . The ceramic fiber insulation blanket  60  contacts both the bottom sides  38  of the burners  30   a - 30   f  and the top side of the relatively rigid bottom insulation structure  24 , and representatively extends along essentially the entire top side of the bottom insulation structure  24 .  
         [0026]     The sandwiched ceramic fiber insulation blanket  60  serves several functions in the representatively illustrated fuel-fired heating apparatus  10 . First, it basically “plugs” the space within the interior of the combustion chamber  14  between the bottom sides  38  of the burners  30   a - 30   f  and the top side of the underlying relatively rigid bottom insulation structure  24 . During low fire operation of the heating apparatus  10  in which, by way of non-limiting illustration and example, only the burners  30   a - 30   c  are being fired, this prevents migration of uncombusted fuel from the firing burners  30   a - 30   c  to the non-firing burners  30   d - 30   f  via any portion of the vertical space between the bottom sides  38  of the burners  30   a - 30   f  and the top side of the underlying relatively rigid bottom interior insulation structure  24  within the combustion chamber  14 . This substantially lessens the amount of unburned fuel discharged through the flue pipe  18  during low fire operation of the heating apparatus  10 , thereby reducing its pollutant emissions.  
         [0027]     Second, during firing of any of the burners  30  there exists a substantial temperature differential between its (hotter) top side  36  and its (cooler) bottom side  38 . This top-to-bottom temperature differential causes the burner (such as the burner  30   a  depicted in  FIG. 2 ) to thermally expand, as indicated by the double-ended arrow  62  in  FIG. 4 ., thereby subjecting the burner to substantial thermal stress during its firing. However, because a bottom side portion  38  of each burner  30  is contacted by the ceramic fiber insulation blanket  60 , during firing of the burner the operating temperature of its bottom side portion  38  is increased, thereby reducing the top-to-bottom temperature differential of the firing burner and correspondingly reducing the differential expansion of the burner. This, in turn, desirably reduces the thermal stress imposed on the burner during firing thereof.  
         [0028]     Third, this reduction of thermal stress on each firing burner  30  is achieved without substantially impeding the differential longitudinal expansion (i.e., bowing down) of the burner. When a firing burner  30  expands during firing thereof, as indicated by the double-ended arrow  62  in  FIG. 4 , the ceramic fiber insulation blanket  60  is simply resiliently compressed between the bottom side  38  of the burner  30  and the underlying relatively rigid insulation structure  24  in a manner moving the top side  64  of the blanket  60  downwardly from its solid line position in  FIG. 4  to its dotted line position therein. When the firing of the burner  30  ceases, and the burner  30  cools, the lateral dimension of the burner diminishes and the top side  64  of the ceramic fiber blanket  60  resiliently returns to its solid line burner-engaging position shown in  FIG. 4 .  
         [0029]     Fourth, the resilient engagement of the ceramic fiber insulation blanket  60  with the bottom sides  38  of the burners  30   a - 30   f  acts as a damper to any harmonic vibrations created in the burners during firing thereof. This, in turn, desirably reduces the operational noise of the fuel-fired heating apparatus  10 .  
         [0030]     AS previously mentioned, a greater or lesser number of burners  30  could be utilized in the fuel burner system  28  if desired, and the multiple burners could be grouped for staging purposes in a variety of different manners. Additionally, the disclosed placement of a resilient insulation material in engagement with only a side portion of a fuel burner could also be used to advantage (at least from thermal stress and operational noise reduction standpoints) in applications where only a single burner is employed in a fuel-fired heating apparatus. Further, while the interior of the combustion chamber is representatively lined with a relatively rigid fiberboard insulation material, and the sandwiched insulation material is illustratively a ceramic fiber insulation blanket structure, a variety of other types of resiliently compressible and relatively rigid insulation materials could be alternatively utilized if desired without departing from principles of the present invention. Moreover, principles of the present invention could also be utilized with burners positioned in non-horizontal orientations and positioned in other locations within a combustion chamber.  
         [0031]     The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.