CA2029547C - Compact, high efficiency heat exchanger for a fuel-fired forced air heating furnace - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A compact, high efficiency heat exchanger for a fuel-fired forced air furnace has horizontally spaced apart inlet and outlet manifold structures which are innerconnected by a horizontally spaced series of vertically serpentined, relatively small diameter flow transfer tubes.
Larger diameter inlet flow tubes are positioned beneath the balance of the heat exchanger, extend parallel to the transfer tubes, and have upturned discharge ends connected to the underside of the inlet manifold. The heat exchanger is configured so that its total vertically facing peripheral surface area is considerably larger than its total horizontally facing peripheral surface area, thereby significantly reducing undesirable outward heat loss through the vertically extending furnace housing side walls upon burner shut off and increasing the overall efficiency rating of the furnace. To reduce the manufacturing cost of the heat exchanger its components are assembled using a weldless fabrication process which includes swedging the tubes to the manifolds and forming each manifold from two sections which are edge rolled and crimped together.

Description

r . C

JRK/dlb d6/28/go COMPACT, HIGH EFFICIEI~CY HEAT EXCHANGER
FOR A FUEL-FIRED ~ORCED AIR HEATING FURNACE

BACKGRO~ND ~F THE INVENTION
The present invention relates generally to heat exchangers for fuel-~ired, forced air heating furnaces, and more particularly relates to compact, hiBh efficiency heat exchangerq for such furnaces, and associated fabrication techniques for constructing the heat exchangers.
The National Appliance Energy Conservation Act of 1987 require~ that all forced air furnaces manufactured after January 1, 1992, and having heating capacities between 45,000 ~tuh and 400,000 Btuh, must have a minimum heating efficiency of 78~ based upon Department of Energy test procedures. For two primary reaqonq, each relating to conventional heat exchanger de~ign, the majority of furnace~
currently being manufactured do not meet this 78% minimum efficiency requirement.
First, until recently, mo~ furnace efficiencies ~5 were rated based upon "indoor ratings", meaning that the heat loQses through the furnace hou~ing wall~ to the surroundin~ space were ignored, the impllcit a~sumption being that the furnace was installed in an area within the conditioned space (such as a furnace clo3et or the like) qo that the heat transferred outwardly through the furnace hou~ing ultimately functioned to heat the conditioned qpace.
Under the new efficiency rating scheme, however, furnace efficiencies will be penalized for heat tran~ferred B ` ~

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~ ~ 2 9 5 ~
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JRK/dlb 06~28/90 outwardly through the furnace housing to the surrounding space on the asqumption that the furnace will be installed in an unheated area, such a3 an attic, even if the furnace will ultimately be installed within the conditioned space.
Gas-fired residential furnaces are typically provided with "clamshell" type heat exchangers through which the burner combustion products are flowed, and exteriorly across which the furnace qupply air is forced on its way to the conditioned space served by the furnace. The conventional clamshell heat exchanger is positioned within the furnace housing and is normally constructed from two relatively large metal stampings edge-welded together to form the heat exchanger body through which the burner combustion products are flowed. In the typical upflow furnace, the clamshell heat exchanger body has a large e~panse of vertically disposed side surface area which extend3 parallel to adjacent vertical qide wall portions of the furnace housing. In a similar fashion, in hsrizontal ; flow furnaces the clamshell heat exchanger body has a large expanqe of horizontally dispo~ed side surface area which extend3 parallel to the adjacent horizontally extending side wall portion of the furnace housing.
Due to the large surface area of clamshell heat exchangers, and its orientation within the furnace housing, there is a correspondingly large (and unde~irable) outward heat transfer from the heat exchanger through the furnace housing which represents a los~ of available heat when the furnace i9 install0d in an unheated space. This potential heat transfer from the heat exchanger through the furnace 30I housing side wall3 to the adjacent 3pace correspondingly diminishes the efficiency rating of the particular furnace, 2029~7 ~ a JRK/dlb 06/28/go under the new efficiency rating formula, even when the furnace is not installed in an unheated space.
The second heat exchanger-related factor which undesirably reduces the overall heating efficiency rating of a furnace of this general type arises from the fact the the typical clamshell heat exchanger has a relatively low internal pressure drop. ~ccordingly, during an ?loff cycle"
of the furnace, this ~1009e~ heat exchanger design permits residual heat in the heat exchanger to rather rapidly escape through the exhaust vent system (due to the natural buoyancy of the hot combustion gas within the heat exchanger) instead of being more efficiently transferred to the heating supply air which continues to be forced across the heat exchanger for shor~ periods after burner shutoff. Stated in another manner, in the typical clam hell type heat exchanger the retention time therein for combust-ion products after burner shut off is quite low, thereby significantly reducing the combustion product heat which could be usefully transferred to the continuing supply air flow being forced externally across the heat exchanger.
In addition to these heating efficiency problem3, conventional clamshell type heat exchangers have a long "dwell period~l (upon cold start ' up) during which condensation i~ formed on their interior surfaces and remains until the hot burner combustion products flowed ; internally through the heat exchanger e~aporates such ', , condensation. This dwell period, of cour~e, i9 repeated each time the furnace is cycled. 8ecause of these lengthy dwell periods (resulting from the large metal ma~s of the clamshell heat exchanger which must be re-heated each time the burners are energized), inte'rnal corrosion in clamshell 20295~7 JRK/dlb 06/2s/go heat exchangers tends to be undesirably accelerated.
Theqe and other problem~, limitations and di~advantages oommonly as~ociated with clam~hell heat exchangers have been substantially lessened by the compact, high efficiency configurational design incorporated in the heat exchanger illuqtrated and deqcribed in my copending Canadian application serial no. 2,003,802 ~iled November 24, 1989. Briefly, that heat exchanger comprises horizontally spaced apart inlet and outlet manifolds interconnected by horizontally spaced apart, vertically serpentined, relatively small diameter flow transfer tubes. A plurality of larger diameter primary inlet tubes extend horizontally beneath the manifolds and have upturned discharge end portions connected to the underside of the inlet manifold.
With the heat exchanger operatively installed in an upflow furnace, the inlet oY a draft inducer fan is connected to the outlet manifold and burner flames are flowed into the open inlet end~ of the primary inlet tubes.
Operation of the draft inducer fan drawq hot burner combuqtion productq sequentially through the primary inlet tubes, the inlet manifold, the ~erpentined flow transfer tube3, and the outlet ~anifold for discharge by the fan to a suitable vent stack.
A3 originally envisioned, the compact heat l exchanger illustrated and described in U.S. Patent 4,974,579 was to be fabricated utilizing a generally conventional welding proce~ to join the sections of each of its manifoldq, and to secure the primary inlet tube~ and the flow tranQfer tubes to the manifolds. In subqequent further development of the heat exchanger, however, it ha~ become de~irable to even further reduce it~

2029~7 ~3 JRK/dlb 06/28/go overall construction cost by essentially eliminating the need to form weld joints therein. It is accordingly an object of the present invention to provide a compact furnace heat exchanger which is similar in configuration and operation to the heat exchanger just described, but which is assembled essentially without using a welding process to join or form its components.
SUMMARY OF THE INV~NTION
The present invention provides a compact, high efficiency heat exchanger which may be operatively positioned in the supply plenum housing portion of an induced draft, fuel-fired forced air heating furnace and is operative to reduce heat outflow from the heat exchanger through the housing side walls 9 and thereby increase the overall heating efficiency rating of the furnace. When operatively disposed within the supply air plenum of the furnace, the heat exchanger has a first total peripheral surface area facing parallel to the direction of blower-produced air flow through the supply air plenum and externally across the heat exchanger, and a second total peripheral surface area which outwardly faces a side wall section of the housing in a direction transverse to the air flow acros~ the heat exchanger.
Importantly, the first peripheral ~urface area of the heat exchanger is sub~tantially greater than its second peripheral surface area. Accordingly, the radiant heat emanating from the heat exchanger toward the hou inB ~ide wall section is substantially les than its radiant heat directed parallel to the air flow. In this manner, the available heat from the heat exchanger is more efficiently apportioned to the supply air J thereby reducing outward heat RHEE B7557CIP 2 0 2 ~ 5 4 7 JRK/dlb 06/28/go loss through the furnace housing.
In a preferred embodiment thereof, the heat exchanger of the present invention is generally similar in configuration to the compact heat exohanger illustrated and described in my copending Canadian application serial no.

2,003,802 ~iled Novemb~r 24, 1989, and includes: an inlet manifold, an outlet manifold spaced apart from the inlet manifold in a direction transverse to the supply air flow; a plurality of relatively large diameter, generally L-shaped inlet tubes positioned upstream of the inlet and outlet manifolds and having discharge portions connected to th~ inlet manifold; and a series of relatively small diameter flow transfer tubes each connected at its opposite endq to the inlet and outlet manifolds, the small diameter flow transfer tubes being serpentined in the direction of supply air flow externally acro~s the heat exchanger~
During operation of the furnace in which the heat exchanger of the present invention is operatively installed, a draft inducer fan operatively connected to the heat exchanger outlet manifold draws burner flame~ sequentially through the larger diameter inlet tubes, the inlet manifold, the serpentined flow tran fer tubes, and the outlet manifold, and then di~oharges the combustion products into a suitable vent stack.
The serpentined, small diameter flow transfer tubes of the heat exchanger function to create a ~ubstantial resistance to burner combu~tion product flow through the heat exchanger, and impart turbulence to the combustion product throughflow r to thereby improve the thermal efficiency of the heat exchanger.
According to an important feature o~ the present ., . ~

2029547 c JRK/dlb 06/28/so invention, the compact heat exchanger is assembled using an essentially weldless fabrication process in which the combustion tubes are swedged to the manifolds.
Ad~itionally, each of the manifolds is defined by two sections, each of which has a peripheral edge portion. At each manifold, one of these two peripheral edge sections is folded around the other peripheral edge section and crimped therewith to form a weldless, essentially air tight joint extending around the manifold. Additionally7 in a preferred embodiment of the compact heat exchanger, the outlet manifold is provided with a diqcharge conduit portion which is Qwedged to a support plate portion of the heat exchanger.
The inlet end of each of the primary inlet tube i~ also swedged to the support plate.
BRIEF DESCRIP?ION OF THE DRAWINGS
FIG. 1 is a perspective view of a compact heat exchanger, for a fuel-fired air heating furnace, which embodieq principles of the present invention and is assembled using a weldles~ fabrication technique;
FIG. 2 is an enlarged qcale right side elevational view ot` the heat exchanger;
FIG. 3 is an enlarged scale partial cross-~ectional view of the dashed circle area ~'A~ in Fig. 2; and FIG. 4 is an enlarged scale partial crosq sectional view of the dashed circle area "B" in ~ig. 2.
D~TAILED DESCRIPTION
Illustrated in Figq. 1 and 2 i~ a compaot, high efficiency heat exchanger lO which embodies principles of the present invention and is ~imilar in configurat$on and operation to the heat exchanger illu~trated and de~cribed in my U.s. Patent 4,974,579-B

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202q5~7 JRK/dlb 06/28/go Like its counterpart in that patent, the heat ex¢hanger 10 may be operatively installed in the ~upply plenum housing portion of an upflow, fuel-fired forced air heating furnace to heat the supply air 12 flowing upwardly through the supply plenum, exteriorly traverqing the heat exchanger 10, and being delivered to a conditioned space. As ~ubsequently described in greater detail herein, the heat exchanger 10 is assembled using an essentially weldless fabrication technique which materially reduces the overall construction costs associated with the heat exchanger.
Heat exchanger 10 include~ a center or support plate structure 14, an outlet manifold 16 positioned rightwardly adjacent the support plate 14, an inlat manifold 18 spaced rightwardly and horizontally apart from the outlet manifold, a plurality of relatively large diameter, generally L-shaped primary inlet tubes 20 poqitioned beneath the manifolds 16 and 18 and interconnected at their opposite ends to the support plate 14 and the underside of the manifold 18, and a horizontally spaced qerie~ of vertically serpentined, relatively small diameter flow transfer tubeq 22 connected at their opposite endq to the outlet manifold 16 and the inlet manifold 18.
The outlet manifold 16 has a leftwardly projecting discharge conduit 24 which is secured to the ~upport plate structure 14 and may be connected to a draft inducer fan (not 3hown) a~sociated with the furnace in wh~ch the heat exchanger 10 is operatively installed. During operation of the furnace and its associated draft inducer fan, hot burner combustion products 26 are sequentially flowed in~o the open inlet ends 20a of tubeq 20, through the tubeQ 20 into the ....

202q547 JRK/dlb o~/2s/so inlet manifold 18, through the qmaller diameter tube~ 22 into the outlet manifold 16, and into the draft inducer fan, through the discharge conduit 24, f'or delivery to an external exhaust stack.
In a manner similar to that described in my copending Canadian application serial no. 2,003,802 filed November 24, 1989, the heat exchanger lO has a vertically facing total peripheral surface area, and a horizontally facing total peripheral surface area which is substantially less than the vertically facing total peripheral surface area. Accordingly, the radiant heat emanating from the heat exchanger lO toward the vertical sids wall section of the furnace in which it iq installed is ~ubqtantially les~ than its radiant heat directed parallel to the flow of the supply air 12. In thi~
manner, the available heat from the heat exchanger 10 is more efficiently apportioned to the supply air 1Z, thereby materially reducing outward heat loss through the furnace hou~ing. The serpentined, small diameter flow transfer tubes 22 of the heat exchanger 10 function to create a substantial resiYtance to burner combustion product flow through the heat exchanger, and impart turbulence to the combustion product throughflow, to thereby improve the thermal efficiency of the heat exchanger.
A~ mentioned above, the heat exchanger 10 assembled using a weldle~ fabrication process which will now be described with initial reference to Figs. 2 and 3.
The outlet housing 16 has a hollow fir~t qection 28 with a rear wall 30 and an open left or front end bordered by a peripheral flange 32, and a second section defined by a plate member 34 to which the di~charge conduit 24 is ~ecured in a manner subsequently de~cribed. In constructing the $,,~

JRK/dlb 06/28/go outlet housing 16, a peripheral edge portion 34a Of the plate member 34 is folded rearwardly over the flange 32, and a crimp 36 (Fig. 3) is formed around the periphery of the housing section peripheral portions 32 and 34 to form a weldless, essentially air tight joint between the two sections of the housing 16.
The inlet housing 18 is formed fro~ hollow front and rear sections 38 and 40 (Fig. 2) having facing peripheral edge port~ons that, as viewed in Fig. 2, diagonally slope downwardly and rightwardly. In a manner similar to the folding and crimping of the peripheral edge portions 32 and 34a Of the outlet manifold 16, one of these peripheral edge portions 38a~ 40a is folded over the other one, and a peripheral crimp is then formed in the interlocked edge portions to form a weldless, essentially air tight diagonal joint around the manifold 18.
Referring now to Fig. 4, each of the outlet ends 22a Of the small diameter flow transfer tubes 22 is operatively secured to a lower end portion of the rear wall 30 of outlet manifold 16 by a weldless swedge joint 42. In forming each of the swedge joints 42, the tube outlet end 22a is inserted inwardly through a circular opening 44 formed through the rearwall 30 and circumscribed by an inturned circular flange 46. A generally conventional cylindrical swedging tool 48, having radially expandable portions 50 and 52, is inserted into the inlet end 22a f the tube 22. A tapered pin member 54 i~ then driven rightwardly into the hollow center of the tool 48 to radially expand its portions 50 and 52 as indicated by the arrows 54. The radially outward movement of the swedging tool portions 50, 52 correspondingly forms annular radial JRK/dlb 06/28/go bulges 56 and 58 in the outlet end of tube 22, the bulge 56 being positioned inwardly of the flange 46, and the bulge 58 being ~`ormed at She outer ~ide surface of the rear wall 30 of the outlet manifold 16. These bulgeq 56, 58 axially lock the tube 22 to the housing 16 and form a weldless, e~entially air tight seal at the juncture between tube 22 and the manifold 16. After the swedge joint 42 is formed, the pin 54 may be removed from the 3wedging tool 48 to permit retraction of its portions 50, 52 and removal of the tool 4a from the tube 22.
Similar swedge joints 42a-42e are respectively formed between the di~charge conduit 24 and the support plate structure 14; the di3charge conduit 24 and the outlet hou~ing plate member 34; the inlet ends of the tubes 22 and a top portion of the front side wall o~ inlet housing section 38; the tubes 20 and the bottom wall of the inlet hou~ing section 38; and ~he inlet end~ o~ the tubes 20 and the ~upport plate ~tructure 14. It will be appreciated that, at each of the manifolds 16 and 18, the tubing swedge joints are formed prior to the folding and crimping together of the manifold section~.
It should also be noted that the diagonal orientation of the folded and crimped ~oint line on inlet manifold 18 faeilitate3 acceq~ to the interior of manifold se4tion 38 for the swedging tool 48.
From the foregoing it can readily be ~een that the heat exchanger 10 provide~ the configurational and operational adYantage~ of the compact heat exchanger illustrated and described in my copending Canadian application serial no. 2,003,802, while the weldless assembly technique of the present invention facilitates a substantial reduction in ~ 11 202~

JRK/dlb 06/28/go its overall construction costO
The Yoregoing detailed description is to be elearly understood as being given by way of illustration and example only, the ~pirit and scope of the present invention being limited solely by the appended claims.
What is claimed is:

Claims (9)

1. A single heat exchanger for providing essentially the entire combustion products-to-supply air heat exchange in a fuel-fired, forced air furnace having a housing portion through which supply air is forced generally parallel to a side wall section of the housing portion, said heat exchanger being assembled using an essentially weldless fabrication process and comprising:
an inlet manifold;
an outlet manifold spaced apart in a first direction from said inlet manifold and being connectable to the inlet of a draft inducer fan operative to draw hot combustion products through said heat exchanger, each of said inlet and outlet manifolds having two sections, each of the two sections having a peripheral edge portion, one of said peripheral edge portions being folded over the other of said peripheral edge portions, and crimped therewith, to form a weldless, essentially air tight joint around the manifold;
at least one relatively large diameter primary inlet tube adapted to receive hot combustion products form a source thereof and flow the received combustion products into said inlet manifold, each of said at least one primary inlet tube having a discharge portion connected to said inlet manifold and projecting outwardly therefrom in a second direction transverse to said first direction, and an inlet portion extending from an outer end portion of the discharge portion, in said first direction, toward said outlet manifold; anda series of relatively small diameter flow transfer tubes each connected at its opposite ends to said inlet manifold and said outlet manifold, said flow transfer tubes being operative to flow hot combustion products from said inlet manifold to said outlet manifold and configured to create a substantial internal flow resistance in said heat exchanger, said heat exchanger being operatively positionable within said housing portion in a manner such that said first direction of said heat exchanger extends generally transversely to said side all section, said heat exchanger having a first total peripheral surface area facing in said second direction, and a second total peripheral surface area facing generally perpendicularly to said second direction, said first total peripheral surface area being substantially greater than said second total peripheral surface area, whereby, when said single heat exchanger is operatively installed within said housing portion, the radiant heat transferred from said single heat exchanger to supply air flowing through said housing portion is substantially greater than the radiant heat transferred from said single heat exchanger to said side wall section of the furnace, thereby materially increasing the heating efficiency rating of the furnace.

2. The heat exchanger of claim 1 wherein:
said flow transfer tubes are serpentined in said second direction.

3. The heat exchanger of claim 1 wherein:
said inlet manifold has at least one opening therein which receives a discharge end portion of said at least one primary inlet tube, and at least opening therein which receives an inlet end portion of said at least one flow transfer tube, said outlet manifold has at least one opening therein which receives a discharge end portion of said at least one flow transfer tube, and said primary inlet and flow transfer tubes are swedged to said manifolds to form weldless, essentially air tight connection joints therewith.

4. The heat exchanger of claim 1 wherein:
said weldless, essentially air tight joint around said inlet manifold is disposed within a plane extending generally diagonally relative to said first and second directions.

5. A single heat exchanger for providing essentially the entire combustion products-to-supply air heat exchange in a fuel-fired, forced air furnace having a housing portion through which supply air is forced generally parallel to a side wall section of the housing portion, said heat exchanger being assembled using an essentially weldless fabrication process and comprising:
an inlet manifold;
an outlet manifold spaced apart in a first direction form said inlet manifold and being connectable to the inlet of a draft inducer fan operative to draw hot combustion products through said heat exchanger;
at least one relatively large diameter primary inlet tube adapted to receive hot combustion products from a source thereof and flow the received combustion products into said inlet manifold, each of said at least one primary inlet tube having a discharge portion received in a corresponding opening in said inlet manifold and projecting outwardly therefrom in a second direction transverse to said first direction, and an inlet portion extending from an outer end portion of the discharge portion, in said first direction, toward said outlet manifold, each primary inlet tube being swedged to said inlet manifold to form a weldless, essentially air tight connection joint therewith;
and a series of relatively small diameter flow transfer tubes each received at its opposite ends in corresponding openings in said inlet manifold and said outlet manifold, said flow transfer tube being operative to flow hot combustion products from said inlet manifold to said outlet manifold an configured to create a substantial internal flow resistance in said heat exchanger, said flow transfer tubes being swedged to said inlet and outlet manifolds to form weldless, essentially air tight connection joints therewith, said heat exchanger being operatively positionable within said housing portion in a manner such that said first direction of said heat exchanger extends generally transversely to said side wall section, said heat exchanger having a first total peripheral surface area facing in said second direction, and a second total peripheral surface area facing generally perpendicularly to said second direction, said first total peripheral surface area being substantially greater than said second total peripheral surface area, whereby, when said single heat exchanger is operatively installed within said housing portion, the radiant heat transferred from said single heat exchanger supply air flowing through said housing portion is substantially greater than the radiant heat transferred form said single heat exchanger to said side wall section of the furnace, thereby materially increasing the heating efficiency rating of the furnace.

6. The heat exchanger of claim 5 wherein:
said flow transfer tubes are serpentined in said second direction.

7. A single heat exchanger for providing essentially the entire combustion products-to-supply air heat exchange in a fuel-fired, forced air furnace having a housing portion through which supply air is forced generally parallel to a side wall section of the housing portion, said heat exchanger comprising:
a support plate structure having first and second opposite sides;
an inlet manifold positioned on said second side of said support plate structure and having an outlet conduit swedgingly connected at its opposite ends to said support plate structure and said outlet manifold, said outlet conduit being connectable to the inlet of a draft inducer fan operative to draw hot combustion products through said heat exchanger;
each of said inlet and outlet manifolds having two sections, each of the two sections having a peripheral edge portion one of said peripheral edge portions being folded over the other of said peripheral edge portions, and crimped therewith, to form a weldless, essentially air tight joint around the manifold;
at least one relatively large diameter primary inlet tube adapted to receive hot combustion products form a source thereof and flow the received combustion products into said inlet manifold, each primary inlet tube being swedgingly interconnected between said support plate structure and said inlet manifold and having a discharge portion projecting outwardly from said inlet manifold in a second direction transverse to said first direction, and an inlet portion extending from an outer end of the discharge portion in said first direction, to said support plate structure;
a series of relatively small diameter flow transfer tubes swedgingly connected at their opposite ends to said inlet manifold and said outlet manifold, said flow transfer tubes being operative to flow hot combustion products from said inlet manifold to said outlet manifold and configured to create a substantial internal flow resistance in said heat exchanger;
said heat exchanger having a first total peripheral surface area facing in said second direction, and a second total peripheral surface area facing generally perpendicularly to said second direction, said first total peripheral surface area being substantially greater than said second total peripheral surface area.

8. The heat exchanger of claim 7 wherein:
said flow transfer tubes are serpentined in said second direction.

9. The heat exchanger of claim 7 wherein:
said weldless, essentially air tight joint around said inlet manifold is disposed within a plane extending generally diagonally relative to said first and second directions.