HEAT EXCHANGER FOR A BOILER

Abstract

A heat exchanger for a boiler comprises a central chamber for receiving a combustion gas. A baffle divides the central chamber into a first section and a second section. The baffle also restricts the combustion gas from flowing directly from the first section of the central chamber to the second section of the central chamber. There is a plurality of spaced-apart flues disposed about a periphery of the central chamber. The flues allow for communication between the first section of the central chamber and the second section of the central chamber, in particular, the flues allow for combustion gas to flow from the first section of the central chamber to the second sections of the central chamber.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of provisional application 61/167,797 filed in the United States Patent and Trademark Office on Apr. 8, 2009, the disclosure of which is incorporated herein by reference and priority to which is claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to boilers and, in particular, to a heat exchanger for a boiler.

2. Description of the Related Art

Boilers are well known in the art and have numerous applications. For example, boilers may be used to provide hot water and heating for domestic or industrial use. Boilers may also be used to provide steam for use in process applications or locomotion of ships, trains, and other vehicles.

In fire-tube boilers, flue gases are channelled through flues surrounded by a fluid being heated. The boiler housing is a pressure vessel and contains the fluid which is typically water. Preferably numerous flues are provided to maximize the surface area on which heat may be transferred from the flue gases in the flues to the water being heated. U.S. Pat. No. 4,271,789 issued on Jun. 9, 1981 to Black discloses a boiler in which a plurality of flues extend from a combustion chamber into a boiler housing.

However, in the prior art, the use of numerous flues or fluid carrying tubes increases the number of component parts in the boiler. This may result in increased manufacturing costs, particularly costs associated with the assembly of the boiler, because of the increased need for welding and/or riveting. There is accordingly a need for an improved heat exchanger for a boiler.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved heat exchanger for a boiler.

In particular, it is an object of the present invention to provide a heat exchanger which has sufficient surface area to efficiently heat a fluid in a boiler and is formed, at least in part, from a single blank to reduce welding requirements and thereby reduce metallurgical disturbances. It is also an object of the present invention to provide a heat exchanger that is in full ASME Section IV design conformity.

There is accordingly provided an improved heat exchanger for a boiler. In one embodiment, the heat exchanger comprises a central chamber for receiving a combustion gas. A baffle divides the central chamber into a first section and a second section. The baffle also restricts the combustion gas from flowing directly from the first section of the central chamber to the second section of the central chamber. There is a plurality of spaced-apart flues disposed about a periphery of the central chamber. The flues allow for communication between the first section of the central chamber and the second section of the central chamber, in particular, the flues allow for combustion gas to flow from the first section of the central chamber to the second sections of the central chamber.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be more readily understood from the following description of an embodiment thereof given, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a first embodiment of an improved heat exchanger disposed within a boiler housing;

FIG. 2 is a perspective view of the heat exchanger of FIG. 1;

FIG. 3 is a section view taken along lines A-A of FIG. 2;

FIG. 4 is a perspective view of a section of a blank used to form the central chamber, flues and fins of the heat exchanger FIG. 1;

FIG. 5 is a front elevation view of the blank of FIG. 4;

FIG. 6 is a sectional view taken along lines B-B of FIG. 3;

FIG. 7 is a fragmentary, perspective view of a flue and fin of the heat exchanger of FIG. 1;

FIG. 8 is an elevation view of the flue and fin of FIG. 7;

FIG. 9 is a sectional view taken along line C-C of the flue channel of FIG. 8 showing a flue and fin as they would appear disposed therein;

FIG. 10 is a sectional view taken along line D-D of the flue channel of FIG. 8 showing a flue and fin as they would appear disposed therein;

FIG. 11 is a sectional view taken along line E-E of the flue channel of FIG. 8 showing a flue and fin as they would appear disposed therein;

FIG. 12 is a perspective view of a second embodiment of an improved heat exchanger;

FIG. 13 is a perspective view of a section of a stamped steel strip used to form the central chamber, flues and fins of the heat exchanger of FIG. 12;

FIG. 14 is a perspective view of a first side of a section of a blank used to form the central chamber, flues, and fins of the heat exchanger of FIG. 12;

FIG. 15 is a perspective view of a second side of a section of a blank used to form the flues and fins of the heat exchanger of FIG. 12;

FIG. 16 is a section view taken along lines F-F of FIG. 12 showing the outside of the heat exchanger;

FIG. 17 is another section view taken along lines F-F of FIG. 12 showing the inside of the heat exchanger.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings and first to FIG. 1, this shows a first embodiment of an improved heat exchanger 10 disposed within a boiler housing 11. As best shown in FIG. 2, the heat exchanger 10 comprises a central cylindrical chamber 12. The central chamber 12 is divided into two sections 16 and 18 by a baffle 20 which is best shown in FIG. 3. Referring back to FIG. 2, a plurality of flues, for example, flues 22a and 22b are spaced-apart about a periphery of the central chamber 12. Each of the flues 22a and 22b is in communication with both the first and second sections 16 and 18 of the central chamber 12 via a corresponding fin 24a and 24b, respectively. As shown for a first one of the flues 22a, each of the flues is provided with a cap or fitting 23 and 25 at each end thereof. The fittings 23 and 25 close off ends of the flue 22a. The caps or fittings 23 and 25 together with boiler housing 11 allow the heat exchanger 10 to be used in elevated pressure service, for example, domestic hot water supply at up to 150 psig. As further shown, for the first one of the flues 22a, each of the flues is also provides with a divider stay 21 which is generally co-planar with the baffle 20.

In this example, flue gases from a combustion chamber (not shown) enter the first section 16 of the central chamber 12 as indicated generally by arrow 100. The baffle 20 prevents the flue gases from flowing directly from the first section 16 of the central chamber 12 into the second section 18 of the central chamber 12. Instead the combustion gases must flow radially outward from the first section 16 of the central chamber 12, through the fins 24a and 24b, and into the flues 22a and 22b. The divider stays 21 prevents the combustion gases from flowing downward along the fins 24a and 24b. Accordingly, the combustion gases are shunted along the flues 22a and 22b, past the baffle 20, as indicated generally by arrow 102. After passing the baffle 20 the flue gases flow radially inward from the flues 22a and 22b, through the fins 24a and 24b, and into the second section 18 of the central chamber 12. The combustion gases flow out of the second section 18 of the central chamber 12 as indicated generally by arrow 104. This will be described in greater detail below.

In this example and as best shown in FIG. 3, the central chamber 12, the flues 22a and 22b, and the fins 24a and 24b have a unitary structure. Accordingly, a single blank 28, which is shown in FIGS. 4 and 5, may be used to form the central chamber 12, the flues 22a and 22b, and the fins 24a and 24b. The blank 28, in this example, is a corrugated stainless steel blank having alternating crests 30 and troughs 32. The ends of each of the troughs 32 are provided with a round 33. A wall 34 extends between adjacent crests 30 and troughs 32. There are a plurality of stays 21, 36, and 38 stamped into each of the walls 34. The crests 30 have generally planar apexes 31 and will form an inner wall 13 of the central chamber 12 which is shown in FIGS. 2 and 3. The grooves 32 have a generally rounded shape and will form the flues 22a and 22b which are shown in FIGS. 2 and 3. The walls 34 extending between adjacent crests 30 and troughs 32 will define the fins 24a and 24b which are shown in FIGS. 2 and 3.

Referring back to FIGS. 4 and 5, the blank 28 is formed into a desired structure, shown in FIG. 3, by applying clamps (not shown) about the rounded troughs 32. The clamps engage the walls 34 to form the fins 24a and 24b which allow communication between the central chamber 12 and flues 22a and 22b.

FIG. 6 is a sectional view taken across line B-B of FIG. 3. As shown for a first one of the fins 24a, an inner wall 34 each fin is provided with a plurality of channel stays 36 and 38. The channel stays 36 and 38 are spaced-apart and extend the length of the inner wall 34 of the fin 24a. The channel stays 36 and 38 are button-like function to maintain opposed inner walls of the fin 24a spaced-apart. This is best shown in FIG. 7 by slot-like openings 40 and 42 in the fin 24a. The openings 40 and 42 are on opposite sides of a divider stay 21. In this example, the channel stays 36 on a first side of the divider stay 21 differ in size from the channel stays 38 on a second side of the divider stay 21. This allows the spacings 40 and 42 to have different widths on different sides of the baffle 20. In other examples, the channel stays may be graduated in size from one end of the inner wall 34 of the fin 24a to the other. This will result in a longitudinally graduated spacing between the inner walls of the fin which may be desirable in some applications. Longitudinally graduated spacing accounts for the fact that the hot flue gases contract volumetrically as they cool, thus, requiring narrower channels as they progress though the heat exchanger.

Referring now to FIG. 8 an elevation view of the fin 24a and corresponding flue channel 22a is shown. FIGS. 9 to 11 are sectional views taken along lines C-C, D-D, and E-E of FIG. 8.

FIG. 9 shows a portion 46 of the fin 24a extending radially from the first section 16 of the central chamber 12. The width W₁ of the opening 40 in the fin 24a is sufficient to allow the combustion gases to readily flow from the first section 16 of the central chamber 12, through the fin 24a, and into the flue 22a. FIG. 10 shows a portion 48 of the fin 24a aligned with baffle 20. In this situation the inner walls 34 and 35 of the fin 24a abut to prevent flue gases from flowing around baffle 20 through the fin 24. Flue gases from the first section 16 of the central chamber must therefore flow along the flue 22a to bypass the baffle 20. FIG. 11 shows a portion 50 of the fin 24a as it appears when extending radially from the second section 18 of the central chamber 12. The width W₂ of the opening when the fin 24a extends radially from the second section of the central chamber 12 is less than the width W₁ of the fin 24a when it extends radially from the first section 16 of the central chamber 12. This results in the combustion gases tending to collect in the flue 22a instead of flowing into the second portion 18 of the central chamber 12. This is desirable because it increases the surface area of the heat exchanger 10 which is in contact with the combustion gases.

Furthermore, this difference in the width of the openings 40 and 42 in the fin 24a on opposite sides of the baffle 20 provides for a graduated decline in combustion gas temperature with a more even distribution of heat transfer. The lower temperature quench rate also reduces accretions of impurities in the combustion gases on heat transfer surfaces, and spreads out those which do occur. This results in reduced sensitivity to flue blockages and cleaning requirements. As additional combustion gases flow into the flue 22a and the pressure increases, the combustion gases will eventually flow into the second portion 18 of the central chamber 12 via the fin 24a. The combustion gases then flow out of the second section 18 of the central chamber 12 as exhaust gases.

A mill certified coil strip of stainless steel having a fixed width is used to construct the heat exchanger 10. The width of the strip will determine the height of the heat exchanger 10. The strip first is uncoiled allowing it to be stamped and folded to produce the blank 28 shown in FIGS. 4 and 5. The blank 28 is brought around and the side ends are welded together to form the central chamber 12 of the heat exchanger 10 shown in FIG. 2. This design allows for scaling since the diameter of the heat exchanger 10 is dependent on the length of the strip used. Alternatively, a plurality of blanks may be welded together to form the heat exchanger 10. The height of heat exchangers formed from the same coil strip of stainless steel will be constant.

In greater detail, and with reference to FIGS. 2 and 4, the stainless steel strip first passes through an in-line annealer to remove any work hardening induced through the manufacturing or coiling and uncoiling processes. From the annealer the strip is incrementally advanced through a stamping press that forms all the intricate features of the heat exchanger 10. All the turbulating and pressure reinforcing features of the heat exchanger 10 are formed in the blank 28 at this stage because tooling has easy access to the strip in flat form. In situations where a plurality of blanks 28 are welded together a die in the stamping press consists of two half sections and, in particular, a left half section for stamping a first blank and a right half section for stamping an adjacent blank. This allows a seam that connects adjacent blanks to occur on the outside of the folded blank making it easier to weld.

A second stamping process cuts rounds 33 into the sections of the blank 28 that form the ends of the flues 22a and 22b. The rounds 33 allow edges of the blank 28 to meet for butt welding as will be discussed below. The next step involves forming the stamped sections of the strip into a fan like structure to form the blank 28 shown in FIG. 4. The rounds 33 are drawn in to form the caps or fittings 23 and 25 found at each end of the flues 22a and 22b as shown in FIG. 2. These two steps may be separated to allow for simpler tooling. It may also be desirable to incorporate an intermediate step to anneal the rounds to minimize the force required to form the caps or fittings 23 and 25 and reduce spring back.

The blank 28 is cut to a desired length based on the desired diameter of the heat exchanger 10 being constructed. The blank 28 is subject to cleaning and passivation prior to applying clamps (not shown) to form the flues 22a and 22b, and the fins 24a and 24b. Removal of any tooling debris, lubricants, and dirt is important for consistent welding and corrosion resistance. It is desirable to clean and passivate the blank 28 prior to forming the flues and fins. Once the flues and fins are formed they are largely inaccessible for further tooling.

The last step involves clamping the blank to the point that the opposing faces of corresponding ones the divider stays 21 and channel stays 36 and 38 meet. The side edges of the blank 28 are rounded about the baffle 20 and welded together. Spring back from the removal of the clamps is relieved by localized annealing of the flue channels 22a and 22b while the heat exchanger 10 is still clamped. The complete heat exchanger 10 is encased in a cylindrical boiler housing having ends with means to allow for the induction and exhaustion of combustion gases and ancillary sensor mountings.

In operation, the heat exchanger 10 is disposed with a boiler housing (not shown) which contains a fluid being heated. Typically the fluid is water which pumped to flow in a direction opposite the direction in which the combustion gases flow through the heat exchanger 10. Combustion gases from a combustion chamber (not shown) flow into the first section 16 of the central chamber 12 as indicated by arrow 100 in FIG. 2. The baffle 20 prevents the combustion gases from flowing directly from the first section 16 of the central chamber 12 into the second section 18 of the central chamber 12. Instead the combustion gases must flow radially outward from the first section 16 of the central chamber 12, via the fins 24a and 24b, and into the corresponding flues 22a and 22b. The combustion gases then flow along the flues 22a and 22b, past the baffle 20, as indicated by arrow 102 in FIG. 2. This increases a surface area of the heat exchanger 10 on which heat may be transferred from the flue gases in the flues 22a and 22b to the water being heated. As flue gases continue to flow into the flues 22a and 22b, the pressure in flues will increase. This, in turn, will result in combustion gases flowing from the flues 22a and 22b into the second section 18 of the central chamber 12. The combustion gases then flow out of the second section 18 of the central chamber 12 as indicated generally by arrow 104 in FIG. 2.

The heat exchanger 10 offers the advantage that the central chamber 12, flues 22a and 22b, and fins 24a and 24b of the heat exchanger 10 may be formed from a single blank 28 shown in FIGS. 4 and 5. This may result in a decrease in manufacturing costs, particularly in costs associated with the assembly of the boiler, because of the reduced for welding and/or riveting.

Referring now to FIG. 12 this shows a second embodiment of an improved heat exchanger 110. In FIG. 12 like parts have been given like reference numerals as in

FIG. 2 with the additional prefix “1”, i.e. the central chamber is given reference numeral 12 in FIG. 2 and reference numeral 112 in FIG. 12, similarly the baffle is given reference numeral 20 in FIG. 2 and reference numeral 120 in FIG. 12.

The heat exchanger 110 shown in FIG. 12 is generally similar to the heat exchanger shown in FIG. 2 with the exception that the second embodiment of the heat exchanger 110 is provided with a plurality of divider stays 121a and 121b which extend radial from the first section 116 of the central chamber 112. As shown for a first one of the fins 124a, the second embodiment of the heat exchanger 110 is also provided with a plurality of protrusions 127a and 127b stamped into the fins. The divider stays and protrusions are best shown in FIGS. 12 to 17.

A lowermost one of the divider stays 121b is generally co-planar with the baffle 120. Having a plurality of divider stays 121a and 121b above the baffle 120 keeps the flow of combustion gases laminar and the pressure drop low. Combustion gases have the highest volumetric and linear flow rate in the first section 116 of the heat exchanger 110. Combustion gases also have the highest temperature and provide the highest heat flux in first section 116 of the heat exchanger 110. Maintaining laminar flow helps reduce heat flux and prevent overloading of the fluid being heated.

The protrusions 127a and 127b protrude inwardly from the inner wall 134 of the fins as best shown in FIG. 17. The protrusions 127a and 127b function as turbulators. By the time the flue gases pass the baffle 120 they will have cooled significantly and heat flux will be considerably lower. The turbulators enhance heat flux. In addition, the turbulators features help disperse any condensate allowing the flue gases direct contact with the surface of the heat exchanger 110.

It will be understood by a person skilled in the art that many of the details provided above are by way of example only, and are not intended to limit the scope of the invention, which is to be determined with reference to following claims.

Claims

1. A heat exchanger for a boiler, the heat exchanger comprising:

a central chamber for receiving a combustion gas;

a baffle dividing the central chamber into a first section and a second section, the baffle restricting the combustion gas from flowing directly from the first section of the central chamber to the second section of the central chamber; and

a plurality of spaced-apart flues disposed about a periphery of the central chamber, the flues allowing for communication between the first section of the central chamber and the second section of the central chamber, and the flues allowing for combustion gas to flow from the first section of the central chamber to the second sections of the central chamber.

2. The heat exchanger as claimed in claim 1 wherein a fin extends between the central chamber and a corresponding one of the flues allowing for communication between the central chamber and said flue.

3. The heat exchanger as claimed in claim 2 wherein a width of the fin is longitudinally graduated.

4. The heat exchanger as claimed in claim 2 wherein the fin is provided with turbulators to enhance heat flux.

5. The heat exchanger as claimed in claim 2 wherein the fin is provided with a plurality of divider stays extending radially from the central chamber to maintain laminar flow of the combustion gas.

6. The heat exchanger as claimed in claim 1 wherein the central chamber and flues have a unitary structure and are formed from a single blank.

7. The heat exchanger as claimed in claim 1 wherein at least a portion of the central chamber and the flues disposed about said portion of the central chamber are formed from a single blank.

8. A boiler comprising:

a unitary heat transfer structure formed from a single blank, the blank being stamped and formed from a strip pulled from a slit coil of stainless steel, and the blank being pulled around to form a central chamber and longitudinal seamed to form a plurality of spaced-apart flues disposed about a periphery of the central chamber, the flues being in communication with the central chamber;

a baffle dividing the central chamber into a first section and a second section, the baffle restricting combustion gas from flowing directly from the first section of the central chamber to the second section of the central chamber;

an outer cylindrical boiler housing surrounding the unitary heat transfer structure, the housing having upper and lower with provisions for receiving and exhausting combustion gases; and

wherein a fluid being heated flows outside the unitary heat transfer structure and combustion gases flow within the unitary heat transfer structure in a counter flow direction as compared to the fluid being heated.

9. The heat exchanger as claimed in claim 8 wherein the flues are longitudinally and laterally graduated.