CA2899275A1

CA2899275A1 - Heat exchanger having a compact design

Info

Publication number: CA2899275A1
Application number: CA2899275A
Authority: CA
Inventors: Andrew R. Hodsdon; Christopher J. Holiday; Bertram L. Walters
Original assignee: Laars Heating Systems Co
Current assignee: Laars Heating Systems Co
Priority date: 2013-01-25
Filing date: 2014-01-23
Publication date: 2014-07-31
Also published as: WO2014116805A3; EP2948726A2; US20150323265A1; EP2948726A4; WO2014116805A2

Abstract

Disclosed is a heat exchanger having an economizer configured as a ring of tubes in a periphery of the heat exchanger. The heat exchanger includes a cylindrical flue collector and a manifold at either end of the cylindrical flue collector. The manifold has a plurality of chambers. The manifold can be made of steel or plastic and governs fluid flow rate and direction within a ring of tubes. At least two rings of heat exchanging tubes, an outer ring and an inner ring, are within the cylindrical flue collector. The rings of tubes are concentric with respect to each other.

Description

HEAT EXCHANGER HAVING A COMPACT DESIGN
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to and claims the benefit of U.S. Provisional Application No. 61/756,784 entitiled "HEAT EXCHANGER HAVING A COMPACT DESIGN"
filed on January 25, 2013, the contents of which are incorporated herein by reference.
FIELD OF THE INVENTION
The present inventive subject matter generally relates to heat exchanging devices including heat exchanging tubes.
BACKGROUND OF THE INVENTION
Heat exchangers, are devices for transferring heat from one medium to another, typically from one fluid to another or to a surrounding environment, without allowing the fluids to mix. Some examples are: automobile radiators; air conditioners, and steam and hot water radiators, which are used to produce or remove heat. In order to prevent mixing of the fluids, or liquids, a barrier is provided between the two liquids or media. Many different heat exchanger barrier designs are used. In a "plate and frame"
design, which is very compact, two liquid streams pass on opposing sides of one or more plates. The total heat transfer surface may be increased by increasing the area of plates and the number of plates. In a "tube and shell" design, one stream of liquid flow passes through tube(s) and the other through the remaining space inside a shell that surrounds the tubes.
Though improvements to such heat exchangers have been made over the years, there remains a need for further improvements that increase efficiency, improve performance, reduce cost, and/or reduce the size of heat exchangers.
SUMMARY
A first embodiment of the present inventive subject matter is a heat exchanger having an economizer configured as a ring of tubes in a periphery of the heat exchanger. This embodiment includes a cylindrical flue collector and a manifold at either end of the cylindrical flue collector. In most embodiments, the outermost ring of tubes is an economizer; however, it is conceivable that the innermost ring of tubes would be an economizer. The manifold has a plurality of chambers. The manifold can be made of steel or plastic and governs fluid flow rate and direction within a ring of tubes. At least two rings of heat exchanging tubes, an outer ring and an inner ring, are

2 within the cylindrical flue collector. The rings of tubes are concentric with each other, although this is not required.
Fluid flows within the tubes from the outer ring of tubes, which is an upstream ring, to the inner ring of tubes, which is the downstream ring. The outer ring of tubes preheats fluid within the tubes to a predetermined temperature range and the inner further heats the fluid to a temperature higher than the predetermined temperature range.
Additional features of this embodiment include multi-dimensional heat exchange tubes. The heat exchanger tubes have a main section that has a circular cross-section and have end sections that have a flattened cross-section. A tube sheet secures each of the tubes within the heat exchanger. The tube sheet has holes that engage respective tubes. The tube sheet can act as a barrier between the manifold and a tube chamber within the cylindrical flue collector. The tubes can be welded directly into the manifold. The tubes can be finned or finless. If the tubes, are finless, the ring of tubes should be equipped with baffles to help enhance the surface area for heat transfer. In this embodiment, the baffles would be positioned in gaps between the tubes.
Material used for the fins is preferably 439 stainless steel; however, the material can also be a hybrid of stainless steel and titanium.
A second embodiment of the inventive heat exchanger includes a plurality of heat exchange tubes. Each heat exchange tube has a main section with a circular cross-section, and an end section with a non-circular cross-section. The end sections of the heat exchange tubes are positioned proximal to one another in an arc. A
dimension of the end sections extending along the arc (i.e., tangential to the arc) is smaller than a dimension of the end sections extending transverse to the arc, thereby providing a first gap between the end sections of the heat exchange tubes that is larger than a second gap between the main sections of the heat exchange tubes. The flattened cross-section of the tube can be oval, rectangular, or any other shape having a dimension tangential to the arc that is shorter than the dimension that is perpendicular to the arc. A
transition zone exists between the main section and the end section. The tubes are welded to a tube sheet at the transition zone.
A third embodiment of the heat exchanger includes a plurality of heat exchange tubes. Each heat exchange tube has a main section with a circular cross-section and an end section with a non-circular cross-section. The end sections of the heat exchange tubes are positioned proximal to one another in an arc. And a distance of a center of

3 each tube from an adjoining tube center is approximately one and a half times or less than the tube diameter. In this or in any other embodiment, the tubes have microfins for enhancing heat transfer between flue gas and water contained within each tube.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a perspective view of a heat exchanger;
Fig. 2 shows the heat exchanger of Fig. 1 with the outer shell removed;
Fig. 3 shows a cut-away view of a manifold of the heat exchanger of Fig. 1;
Fig. 4 shows a view of a plurality of heat exchange tubes coupled to a tube sheet;
Fig. 5 shows a perspective view of the tube sheet of Fig. 4;
Fig. 6 shows a perspective view of a tube of the present heat exchanger;
Fig. 7 shows a perspective view of an end of the tube of Fig. 6; and Fig. 8 shows a perspective view of a header and heat exchange tubes having attached baffles.
DETAILED DESCRIPTION
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown.
Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.
One common component of heat exchangers is an economizer, which preheats fluid that is intended to be heated. An economizer lowers the difference between the temperature of the flue gases and the temperature of the fluid exiting the economizer to reduce the work necessary to heat water to a target temperature.
Tubular type heat exchangers such as used in economizers or superheaters in heat recovery steam generators usually utilize pairs of upper and lower headers which are connected together by multiple vertically-oriented tubes, so that hot gases such as derived from a gas turbine exha st can flow across the tubes to heat fluid flowing vertically in the tubes. Such heat exchangers having various tube configurations are known; however, such heat exchanger designs utilizing pairs of upper and lower headers have typically been undesirably expensive, so that improved configurations and designs for such heat exchangers have been sought.

4 The use of an economizer in a heat exchanger naturally causes the heat exchanger to require additional material and, therefore, additional space. The overall design of a heat exchanger has to account for the additional equipment required for the heat exchanger. This problem is exacerbated by the necessity to have access space to weld heat exchanger tubes in place. Therefore, it would be extremely beneficial to have either a compact economizer design or a compact heat exchanger tube arrangement or both to limit the amount of space and material consumed by the entire heat exchanger.
Fig. 1 is an embodiment of the inventive heat exchanger 2. The heat exchanger 2 includes a shell 4, a water inlet 6 and a water outlet 8. An upper header 10a is located on an upper end of the heat exchanger 2 and a lower header 10b is located on a lower end of the heat exchanger 2. The upper and lower headers 10a and 10b are for directing the fluid flow of the heat exchangers throughout The shell 4 acts as a flue collector and provides an alternate means for exhaust of flue gas. Condensate and flue gas is expelled via a port 12 at the bottom of the shell 4. The shell 4 is preferably made from stainless steel; however, it can be made from any other material capable of withstanding plastic deformation from varying thermodynamic stresses and resistance to acidic condensate. The shell 4 shown in Fig.
1 is round. However, the shell of the inventive heat exchanger is not required to be round, it can be any shape capable of housing a plurality of heat exchanger tubes consistent with the tube descriptions provided below.
Inside the shell 4 is a plurality rings of vertical tubes. As shown in Fig. 2.
Two rings 14 and 16 of vertical tubes are placed within the shell 4. The rings of tubes are essentially concentric with the shell 4; however, as mentioned above, it is not necessary that these rings be concentric with the shell or even with each other.
The outer ring of tubes 14 is an economizer ring tubes. The economizer ring takes advantage of waste heat coming from flue exhaust to preheat water flowing through the tubes to a temperature slightly below a target temperature.
Therefore, the outer ring of tubes 14 is the upstream ring. Fluid flows into the outer ring of tubes 14 from the inlet 6 to be preheated. It circulates through tube groups until it is finally discharged into the inner ring of tubes to be heated to a target temperature.
It is conceivable that the innermost ring of tubes would be an economizer. For example, if flue exhaust is configured to travel along the interior of the shell 4, the economizer should be configured as the inner ring to improve heat exchanger efficiency.

5 The inner ring of tubes 16 is configured in closer contact with a heat source to finish heating the water up to the target temperature. As a result of the preheating from flowing through the tube groups of the outer ring of tubes 14, the final heating is not as laborious and the heat exchanger is thus more efficient. Like the outer ring of 5 tubes 14, the inner ring of tubes 16 is divided into tube groups. Thus, heat makes multiple passes along the tube groups before it finally passes through the heat exchanger outlet 8 and made available for use.
The upper header 10a (also herein referred to as a "manifold") is shown in Fig.
3. The header is what ultimate determines flow direction 11a and llb of each tube group. The header is typically made of steel; however, it is possible to use plastic or any other material cap able of withstanding temperature variations resulting from the heat exchanger's operation. The header governs fluid flow rate and direction within a ring of tubes. The header is compartmentalized with a plurality of waterways 18a and with a transition waterway 18b. Water flows into the chamber through the inlet

6 and is directed by waterways 18a of the water into the outer ring of tubes. As shown in Fig.
3, one of the waterways 18a connect to a tube group having six tubes. However, it is not required that the tube groups be a minimum or maximum of six tubes.
Rather, any number of tubes is possible as long as the most efficient heat transfer is achieved.
Fluid is directed back and forth along the outer ring of tubes 14 until it reaches transition waterway 18b. At that point, fluid is transitioned to the inner ring of tubes 16. As shown in Fig. 3, the inner ring of tubes is intimately positioned next to flue 20 for greater heat transfer.
With reference to Fig. 3 and Fig. 4, a tube sheet 22 is provided at an upper end and a lower end of the rings of tubes 14 and 16. The ends (or at most an end portion) of the tubes within the rings of tubes 14 and 16 engage with through holes 20 within the tube sheet 18. The tube sheet acts as a barrier between the headers 10a and 10b and a tube chamber within the shell 4. Thus, a heat exchange tube chamber is formed by the bounds of opposing tube sheets (a second tube sheet 26 is shown in Fig.
2) and the shell 4. The holes 24 shown in the tube sheet 22 of Fig. 4 are oval or oblong;
however, this shape is not a requirement. The holes 22 can be round or any other shape capable of receiving the tubes of the rings 14 and 16 and a corresponding weld.
The interaction of the rings of tubes 14 and 16 with the tube sheet 22 is shown in Fig. 5. Each ring of tubes 14 and 16 is made of a plurality of individual tubes 26 that have a variable cross section. Each heat exchange tube 28 has a main section with a circular cross-section, and an end section with a non-circular cross-section.
The end sections of the heat exchange tubes are positioned proximal to one another in an arc (which ultimately forms a circle). It is preferable that a dimension of the end sections extending along the arc (i.e., tangential to the arc) is smaller than a dimension of the end sections extending transverse to the arc, thereby providing a first gap between the end sections of the heat exchange tubes that is larger than a second gap between the main sections of the heat exchange tubes. That way space is provide between adjacent tubes 28 to access the tube sheet 22 to provide a weld.
The tube 28 is shown generally in Fig. 6 and includes a main body 30 and end sections 32 having a flattened cross-section. The flattened end 32 is shown is being relatively oval. However, the flattened end 32 can have a rectangular or a circular cross section having a size that is smaller than that of the main body cross section. In is notable that, with end sections 32 having cross sections smaller than that of main body 30, heat transfer can be enhanced as the flowrate through the tube exit is choked or at least hindered, which causes the fluid within the tube to be in contact with the heated surfaces of the main body 30 of the tube for a greater amount of time than if the tube outlet were the same size of the main body 30. The effect should be that the shape of the tube end 32 has a dimension tangential to the arc that is shorter than the dimension that is perpendicular to the arc.
As shown in Fig. 7, the tube end 32 includes the flattened section 34 and a transition zone 36. The transition zone 36 is that part of the tube that has a variable cross-section between the main body 30 and the tube end 32. Each of the tubes is welded to the tube sheet 22 at a point just above the transition zone 36 and below the flattened section 34. The flattened tubes allow for welding access so that the tubes can be welded to the tube sheet 22. The result is that the distance between tubes on the side of the tube sheet having the flattened ends 34 is no less than it would have been if the tubes were not flattened. On the other side of the tube sheet 22, however, the tubes are allowed to be much closer together thereby redirecting the overall size of the heat exchanger or providing the addition of more tubes. Generally, a distance of a center of each tube from an adjoining tube center is approximately one and a half times or less than the tube diameter. One advantage to bringing the tubes closer together is that flue gas velocity over the fin tips is increased, which increases heat transfer on the inner row of tubes.

7 The tubes are preferably 439 Stainless Steel, which helps to avoid fouling.
Alternatively, the tubes can be a hybrid of 439 stainless steel and titanium or any other material that helps to avoid fouling (with or without 439 stainless steel).
The tubes of the heat exchanger can be supplied with fins (not shown) to increase heat transfer. Due to the close proximity of one tube relative to an adjacent tube, the tube should be microfins A higher fin (non-microfin) is possible.
However, the microfin allows the tubes to be placed closer together, which thereby increases the velocity of the gas flowing around the pipes. Since the microfin is a much smaller size that a higher fin, its use results in a substantial material cost savings. As an alternative to microfins, baffles can be used to direct the flow of hot gas over the tubes.
As shown in Fig. 8, baffles are positioned in gaps between the tubes. For example, the baffles 38 are added around the tubes of the economizer ring 16. Further a baffle strap 40 can be added to the space between the tubes to help aid the fluid flow along the pipe material used for the fins is preferably 439 stainless steel; however, the material can also be a hybrid of stainless steel and titanium.
The benefits of the economizer of the present heat exchanger are many. The configuration of the economizer in combination with the inner ring of tubes provides the ability to meet these benefits. The circumferential spacing between the centerlines of the inner row of tubes is designed to provide sufficient flue gas velocity across the fin tips to provide adequate heat transfer. This is controlled in part by the oval or elliptical tube end. The residual heat not absorbed by the inner tube row is absorbed by the outer tube row. The tubes of the outer row also have oval tube ends to control the tube centerline spacing. The absorption of heat in the outer row of tubes causes flue gas condensation. The result is a boiler with a thermal efficiency greater than 90%.
Because the tube ends are welded into the tube sheet, a minimum of one and a half times the tube diameter spacing is between the tube centerlines in order to allow sufficient room to deposit the welding filler metal. When using a micro fin tube, a spacing of one and a half times the tube diameter may be too large of a gap between tubes. In order to bring the tubes closer while maintaining the one and a half times the diameter spacing, the tube end is formed into an oval. The minor radius (rM) of the oval is about one fourth the size of the tube diameter. The tube is oriented so that the centerline of the tube oval is in line with any line starting from the center of the round tube sheet to any tangent point on the tube sheet's outer diameter. The tubes are spaced at 1.5 x (2 x rM) and adequate space is provided to deposit weld filler material.

8 The heat exchanger utilizes baffles on the outside of the outer row of tubes to increase the velocity of the flue products, resulting in increased heat transfer into the fins, through the tube and into the water. The clearance between the fin tips and baffle is 0". Further, pliable insulation is added to the hot face of the baffles to assure that the baffle contour closely follows the fin profile, minimizing any gaps caused by inconsistent fin height or non-straight tubes. The baffle is also insulated on the cold side to prevent excessive heat loss through the baffle face directly into the exhaust products of combustion. The baffles are held in in place by circumferential bands around the outside of the tubes in multiple locations.
It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the principal and scope of the invention as expressed in the appended claims.
While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.

Claims

What is claimed is:

1. A heat exchanger comprising:
a cylindrical flue collector;
a manifold at either end of the cylindrical flue collector, the manifold having a plurality of chambers; and an inner ring of heat exchanging tubes and an outer ring of heat exchanging tubes within the cylindrical flue collector, wherein the rings of tubes are concentric with each other, sets of heat exchanging tubes being in fluid communication a respective chamber, the sets of heat exchanging tubes having alternating fluid flow direction and configured within the rings of tubes so that fluid flows through all of the sets of tubes and flows around the entire circumfry of each set before outletting from the ring of tubes;
wherein the outer ring is an upstream ring of tubes configured to preheat water flowing therethrough; and wherein the inner ring is downstream of the outer ring and configured to transfer additional heat to water flowing therethrough.

2. The heat exchanger as recited in claim 1 wherein the rings of tubes comprise tubes having a main section with a circular cross-section and an end section with a flattened cross-section.

3. The heat exchanger as recited in claim 2 further comprising a tube sheet for securing each of the tubes within the heat exchanger.

4. The heat exchanger as recited in claim 3 wherein the tube sheet is a barrier between the manifold and a tube chamber within the cylindrical flue collector.

5. The heat exchanger as recited in claim 4 wherein the tubes are welded directly into the manifold.

6. The heat exchanger as recited in claim 1 wherein the outer ring of tubes is an economizer.

7. The heat exchanger as recited in claim 1 wherein tubes comprise fins.

8. The heat exchanger as recited in claim 7 wherein the fins are formed from stainless steel.

9. The heat exchanger as recited in claim 8 wherein the fins are formed from a hybrid of stainless steel and titanium.

10.The heat exchanger as recited in claim 1 wherein the manifold is formed from plastic and governs fluid flow rate and direction within a ring of tubes.

11.The heat exchanger as 'recited in claim 1 further comprising baffles for enhancing heat transfer between gas within the cylindrical flue collector and fluid within the tubes.

12.The heat exchanger as recited in claim 1 further comprising fins integrated with the tubes, wherein the baffles are coupled with respect to an outer ring of tubes to increase flue gas flow along the tubes and fins.

13.The heat exchanger as recited in claim 12 wherein the baffles are positioned in gaps between tubes of the outer ring of tubes.

14.A heat exchanger configured to heat water, the heat exchanger comprising:
a plurality of heat exchange tubes, each heat exchange tube having a main section with a circular cross-section, an end section with a non-circular cross-section, and a tube sheet having a plurality of through holes;
wherein end sections of the heat exchange tubes are received respective through holes of the tube sheet and are proximal to one another in an arc; and wherein a dimension of the end sections extending along the arc is smaller than a dimension of the end sections extending transverse to the arc, thereby providing a first gap between the end sections of the heat exchange tubes that is larger than a second gap between the main sections of the heat exchange tubes.

15.The heat exchanger as recited in claim 14 wherein the flattened cross-section is oval.

16.The heat exchanger as recited in claim 14 wherein the flattened cross-section is rectangular.

17.The heat exchanger as recited in claim 14 further comprising a transition zone between the main section and the end section.

18.The heat exchanger as recited in claim 17 wherein the tube is welded to the tube sheet at an end of the end section that is in contact with the transition zone.

19. The heat exchanger as recited in claim 14 wherein a distance between centerlines of adjacent tubes is one and a half times a diameter of the main section.

20.A heat exchanger configured to heat water, the heat exchanger comprising:
a plurality of heat exchange tubes, each heat exchange tube having a main section with a circular cross-section, and an end section with a non-circular cross-section, wherein end sections of the heat exchange tubes are positioned proximal to one another along an arc; and wherein a distance of a center of each tube from an adjoining tube center is approximately one and a half times or less than the tube diameter.

21.The heat exchanger as recited in claim 20 wherein the tube has a flattened cross-section.

22.The heat exchanger as recited in claim 20 wherein the flattened cross-section is rectangular.

23.The heat exchanger as recited in claim 20 wherein the flattened cross-section is oval.

24.The heat exchanger as recited in claim 20 further comprising tube fins.

25.The heat exchanger as recited in claim 24 wherein the tube fins are microfins.