Method for Manufacturing Tube and Fin Heat Exchanger with Reduced Tube Diameter and Optimized Fin Produced Thereby

Abstract

An improved method for manufacturing tube and fin heat exchangers that, according to a preferred embodiment, includes a process for increasing the stiffness and rigidity of heat exchanger fins. Stiffer fins have a greater tendency to maintain proper alignment within a stack of fins, which aids in lacing long stacks of fins with small (e.g., 5 mm) diameter tubing. Preferably, fin stiffness is increased by forming a plurality of longitudinal ribs within the fin during the fin stamping process. More preferably still, two ribs for each longitudinal row of collared holes are provided. The preferred embodiment also includes a slotted heat exchanger fin that is dimensioned and arranged for optimized thermodynamic performance when used with small diameter tubing, thus reducing the space required for a given heat exchanger system.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon provisional application 61/061,498 filed on Jun. 13, 2008, the priority of which is claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to tube and fin heat exchangers, and in particular, to a novel fin design for tube and fin heat exchangers.

2. Description of the Prior Art

As illustrated in FIG. 1, a typical tube and fin heat exchanger (10) consists of a stack of generally planar metallic fins (12) sandwiched between a top end plate (14) and a bottom end plate (16). The terms “top” and “bottom” used for designating heat exchanger end plates are derived based on the heat exchanger orientation during expansion in a vertical hairpin expander press and are not necessarily indicative of the heat exchanger orientation in any particular installation.

The fins (12) have a number of collared holes (18) formed therethrough, and the top and bottom end plates (14, 16) have corresponding holes (20) formed therethrough. When the fins (12) and end plates (14, 16) are stacked, the holes (18, 20) are in axial alignment for receiving a number of U-shaped hairpin tubes (“hairpins”) (22) through the stack. Hairpins (22) are formed by bending lengths of small tubes, typically copper, aluminum, steel or titanium, 180 degrees around a small diameter mandrel. The hairpin tubes (22) are fed, or laced, through the loosely-stacked assembly of fins from the bottom end plate (16) so that the open ends (26) of the hairpin tubes (22) extend beyond the top end plate (14). The top end plate (14) is slipped over the open ends (26) of the hairpins (22), and the hairpins (22) are mechanically expanded from within to create a tight fit with the fins (12). Finally, return bend fittings (24) are soldered or brazed to the open ends (26) of the hairpin tubes (22) to create a serpentine fluid circuit through the stack of fins (12).

It is advantageous to use hairpin tubing of very small diameter in order to maximize heat transfer area within a given heat exchanger size and geometry. Smaller tubes increase the overall heat transfer area and heat transfer coefficient at the refrigerant side of the heat exchanger, which significantly enhances system efficiency. In addition, smaller tubing diameter reduces the air flow wake effect behind the heat tube, which reduces the pressure loss due to the presence of the tube facing the incoming air. Lower pressure loss at the air side reduces the fan motor power requirement and increases the fin area to further improve the system heat transfer efficiency. Additionally, the larger the tube diameter, the thicker the tube wall thickness must be in order to withstand a given pressure differential. Therefore, smaller tube diameters allow thinner tube walls for a given refrigerant pressure, which reduces material costs.

According to the present state of the art, the heating, ventilation, and air conditioning (“HVAC”) industry typically manufactures tube and fin heat exchangers using hairpin tubes with diameters ranging between 7.0 mm and 9.5 mm (⅜ inch). Although the industry desires to manufacture heat exchanger coils of smaller diameter, manufacturing techniques of prior art have restricted such coils to short lengths, with the result that small diameter coils have had limited commercial success. The source of the problem is that when the hairpin tubing becomes too small, the lacing process becomes exceedingly difficult, prohibiting commercially viable manufacturing of any but the shortest heat exchangers. For example, heat exchangers six or more feet in length are readily manufactured using ⅜ inch copper tubing. However, when 5 mm copper tubing is used, it has not been commercially feasible to lace a heat exchanger longer than about 36 inches because of the “Chinese handcuff” effect of the large number of fins. It is desirable, therefore, to provide a manufacturing process that produces a stiffer heat exchanger fin produced to ease the lacing process of small diameter, e.g., 5 mm or smaller, coils.

The prior art tube-fin exchanger, characterized by 7 mm to ⅜ inch tubing, generally employ fins with a fin width between 19 mm and 22 mm, and a transverse tube pitch ranging between 19 mm and 25.4 mm. Fins of these prior art fin dimensions do not deliver optimized performance for smaller diameter, e.g., 5 mm, tubes. It is also desirable, therefore, to provide a heat exchanger fin that has enhanced thermodynamic performance optimized for small diameter tubing, which results in heat exchanger systems that occupies less space.

3. Identification of the Objects of the Invention

A primary object of the invention is to provide a manufacturing process for producing stiffer fins to promote the lacing of tube and fin heat exchangers of large size with 5 mm or smaller tubing.

Another object of the invention is to provide a heat exchanger manufacturing process in which heat exchanger fins having a plurality of longitudinal ribs are utilized to enhance the lacing process.

Another object of the invention is to provide a heat exchanger fin that is designed and arranged for use with 5 mm or smaller tubing to maximize thermodynamic heat transfer.

Another object of the invention is to provide a heat exchanger fin that promotes condensation flow from the fin.

SUMMARY OF THE INVENTION

The objects above as well as other features of the invention are realized in an improved method for manufacturing tube and fin heat exchangers that, according to a preferred embodiment, includes a process for increasing the stiffness and rigidity of heat exchanger fins. Stiffer fins have a greater tendency to maintain proper alignment within a stack of fins, which aids in lacing long stacks of fins with small (e.g., 5 mm) diameter tubing. Preferably, fin stiffness is increased by forming a plurality of longitudinal ribs within the fin during the fin stamping process. More preferably still, two ribs for each longitudinal row of collared holes are provided.

The preferred embodiment of the invention also includes a slotted heat exchanger fin that is dimensioned and arranged for optimized thermodynamic performance when used with small diameter tubing, thus reducing the space required for a given heat exchanger system.

The fin preferably includes slits with ends having a 30 degree incident angle with respect to the airflow, which helps to re-direct the airflow from the tube passing through the collared hole to avoid the wake region behind the tube and provides for a more effective air mixture in parallel slits. The angled slit ends also create turbulence at the area of the fin that has largest distance to neighboring tubes, which enhances the heat transfer over that area.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail hereinafter on the basis of the embodiments represented in the accompanying figures, in which:

FIG. 1 is a perspective exploded diagram of a typical tube and fin heat exchanger of prior art;

FIG. 2 is a perspective view of a portion of a heat exchanger fin arranged for a single longitudinal row of 5 mm hairpin tubes according to a first embodiment of the invention, showing a preferred slot pattern, which is repeated between pairs of collared holes, and a pair of longitudinal ribs formed in the fin, which bounds the collared holes;

FIG. 3 is a top view of the portion of the single hairpin row heat exchanger fin of FIG. 2;

FIG. 4 is a perspective view of a portion of a heat exchanger fin arranged for two longitudinal rows of 5 mm hairpin tubes according to a second embodiment of the invention, showing a preferred slot pattern, which is repeated between pairs of collared holes, and two pairs of longitudinal ribs formed in the fin, which bounds the two longitudinal rows of collared holes;

FIG. 5 is a top view of the portion of the heat exchanger fin of FIG. 4;

FIG. 6 is a bottom view of the portion of the heat exchanger fin of FIG. 4;

FIG. 7 is an enlarged cross section view of the heat exchanger fin of FIG. 4 taken along lines 7-7 of FIG. 5, shown with the collared holes in hidden line to reveal the detail of the raised slots;

FIG. 8 is a left side view (with the front of the fin defined by the incident air flow) of the portion of the heat exchanger fin of FIG. 4;

FIG. 9 is an enlarged cross section view of a longitudinal rib of the portion of heat exchanger fin of FIG. 4 taken along lines 9-9 of FIG. 5;

FIG. 10 is a top view of a portion of the heat exchanger fin of FIG. 4 showing the detail and preferred dimensions of pattern of raised slots for optimizing thermodynamic performance with 5 mm hairpin tubes; and

FIG. 11 is an enlarged cross section view of a raised vane taken along lines 11-11 of FIG. 11.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

FIGS. 2-12 illustrate a fin 12′ dimensioned for small tubing, e.g. 5 mm outer diameter or less, optimized for use with a condenser or evaporator of a conventional air conditioner. FIGS. 2 and 3 illustrate a heat exchanger fin 12′ according to a first embodiment of the invention that is characterized by a single longitudinal row of collared holes 18′ for use in a single-row coil assembly. FIGS. 4-8 illustrate a heat exchanger fin 12′ according to a second embodiment of the invention that contains two longitudinal rows of collared holes 18′ for use in a double-row coil assembly. However, fins 12′ may be arranged for three, four, five, and six or more rows of coils according to the invention. The leading and trailing edges of fin 12′ preferably have corrugated edges.

Referring primarily to FIGS. 2-6, according to the preferred embodiment of the invention, a 5 mm or smaller tube and fin heat exchanger manufacturing process includes a novel and unobvious processing step in forming the heat exchanger fins. As with heat exchanger fins 12 of prior art, fins 12′ are formed by a stamping process in a fin press, such as those produced by Burr Oak Tool, Inc. of Sturgis, Mich. Fin stock is delivered to a press in a roll of sheet metal. Various metals, heat treatments, and thicknesses may be used, but aluminum is the general industry selection. Fin stock is paid out from an uncoiler, lubricated, then fed through a press, where a die draws, details, punches collared holes, and cuts fins to a desired length and width. Stamping generally occurs in several stages.

However, in the preferred manufacturing process, the fin press includes a die that forms two longitudinal ribs 100 into fin 12′ for each longitudinal row of collared holes 18′. The purpose of the lengthwise strengthening ribs 100 is to aid in the fabrication of the coil assembly. Stiffer fins have a greater tendency to maintain proper alignment on a lacing table within a stack of fins, which aids in lacing long stacks of fins with small (e.g., 5 mm) diameter tubing.

Each longitudinal row of collared holes 18′ is disposed between its own pair of longitudinal ribs 100. For a single row coil arrangement, fin 12′ has two ribs 100 (FIGS. 2-3), and for a double-row coil arrangement, fin 12′ has four ribs 100 (FIGS. 4-6). Thus, between adjacent rows of longitudinal collared holes 18′, there are two longitudinal ribs 100. In a preferred embodiment of the invention, the height h_r (FIG. 9) of rib 100 above surface 103 of fin 12′ is between 0.05 and 0.25 mm. More preferably, h_r is about 0.125 mm.

Ribs 100 are also beneficial in the removal of condensate that forms on the fin during the refrigerant evaporation process. Ribs 100 function to provide a path for condensate to follow between tubing rows in multiple-coil arrangements. In single row coil arrangements, ribs 100 provide flow paths for condensate on both the leading and trailing edges of the fin (with respect to the airflow over the fin). Ribs 100 promote the draining of condensate from the fin 12′, thus minimizing the potential for condensate carry-over, i.e., condensate blowing off of the fin and becoming entrained in the stream of air flowing across the fins 12′.

Heat exchanger capacity and efficiency are determined by both fin area and tube area. An optimized heat exchanger must properly balance the utilization of fin and tube area to create the best heat transfer between the refrigerant side and the air side in a cost-effective manner. The combination of smaller diameter tubes, e.g., 5 mm or smaller, with fins 12′ according to the preferred embodiment of the invention provides optimal heat transfer efficiency and cost-effectiveness.

As best shown by the perspective views of FIGS. 2 and 4, a plurality of slits 110 are disposed at spaces between the collared holes 18′ within a given longitudinal row. Each slit 110 forms a projecting or raised ribbon-like segment or vane 112, which is parallel to fin surface 103 and is connected at its two longitudinal ends 113 to the surface 103 of fin 12′. Segment 112 defines an open portion 114 between the raised vane 112 and the fin surface 103 that separates the incoming air flow. The slit depth dimension d_v (along the direction of airflow) (FIGS. 8, 9) is optimized to reduce the boundary layer development on the segment 112, which improves heat transfer ability. Preferably, d_v ranges between 0.5 and 1.5 mm. More preferably, d_v equals about 1.0 mm. The interval depth d_i (FIG. 8) of the fin between adjacent vanes 112 is also preferably equal to the vane depth d_v.

Referring to FIG. 10, slits 110 are arranged in an ‘X’-shaped pattern 105, with each pattern 105 of slits 110 repeating between each pair of collared holes 18′ within a given longitudinal row. In pattern 105, according to the preferred embodiment of the invention, the slits 110 are ideally grouped by five longitudinal rows 120, 122, 124, 126, 128, respectively. The leading two rows (on the basis of the direction of air flow) 120, 122, and the trailing two rows 126, 128 each preferably employ two slits 110, for which the connecting ends 113 are preferably formed at an angle α between 15 and 45 degrees with respect to the normal direction of airflow (airflow being assumed to be perpendicular to the longitudinal direction of the fin). Ideally, α is 30 degrees. The center row 124 preferably employs a single slit 110 with ends 113 formed parallel to the incident airflow. By the nature of the tube and fin heat exchanger, the center portion of fin 12′ that has largest distance to neighboring tubes has the lowest heat transfer efficiency. Pattern 105 is designed to guide the airflow to create more turbulence, which enhances the heat transfer over the area. The angled ends 113 of the slits 110 in first, second, fourth and fifth rows 120, 122, 126, 128 create vortices and corresponding turbulence.

Referring to FIG. 5, fin 12′ also provides an optimized and balanced tube distance and fin width for 5 mm tubing. Prior art tube-fin exchangers arranged for 7 mm to ⅜ inch diameter tubing have fin widths typically ranging between 19 mm and 22 mm and transverse tube pitches ranging between 19 mm and 25.4 mm. These prior art fins 12 do not deliver optimized performance for the smaller tube size, which results in a larger space for the heat exchanger system than is necessary using the fins 12′ according to the preferred embodiment of the invention. Fin 12′, on the other hand, has a reduced fin width dimension p_w (i.e., the distance from center to center between two adjacent collared holes 18′ within a single longitudinal row) between 12 and 18 mm and a transverse tube pitch dimension p_t (i.e., the perpendicular distance between the centerline of two adjacent longitudinal rows of collared holes 18′) between 10 and 15 mm to give optimized heat transfer capacity and efficiency with minimal use of fin and heat tube material, which results in a space efficient product. More preferably, p_w is 16 mm and p_t is 13.86 mm.

Referring to FIG. 9, the height h_v from the top surface of vane 112′ to the top surface 103 of fin 12′ preferably ranges from 0.25 to 0.75 mm. More preferably still, h_v is about 0.5 mm.

The Abstract of the disclosure is written solely for providing the United States Patent and Trademark Office and the public at large with a way by which to determine quickly from a cursory reading the nature and gist of the technical disclosure, and it represents solely a preferred embodiment and is not indicative of the nature of the invention as a whole.

While some embodiments of the invention have been illustrated in detail, the invention is not limited to the embodiments shown; modifications and adaptations of the above embodiment may occur to those skilled in the art. Such modifications and adaptations are in the spirit and scope of the invention as set forth herein:

Claims

1. A fin (12′) for a tube and fin heat exchanger (10) comprising:

a generally planar metallic sheet;

a first plurality of apertures (18′) formed through said sheet and defining a first longitudinal row of apertures (18′); and

first and second longitudinal ribs (100) formed in said sheet, said first longitudinal row of apertures (18′) disposed between said first and second longitudinal ribs (100);

said first and second ribs (100) each having a top surface projecting beyond the upper surface (103) defined by said sheet.

2. The fin (12′) of claim 1 further comprising:

a second plurality of apertures (18′) formed through said sheet and defining a second longitudinal row of apertures (18′); and

third and fourth longitudinal ribs (100) formed in said sheet, said second longitudinal row of apertures (18′) disposed between said third and fourth longitudinal ribs (100), said second and third longitudinal ribs (100) disposed between said first and second longitudinal lows of apertures (18′).

3. The fin (12′) of claim 1 further comprising:

a plurality of raised vanes (112) forming a generally ‘X’-shaped pattern (105) in said sheet between first and second apertures (18′) of said first plurality of apertures (18′);

each of said plurality of raised vanes (112) formed between first and second longitudinal slits (110) in said sheet such that an opening (114) is defined between said raised vane (112) and the surface (103) of said sheet.

4. The fin (12′) of claim 3 wherein:

said plurality of raised vanes (112) are arranged in first, second, third, fourth and fifth rows (120, 122, 124, 126, 128) parallel to said first longitudinal rib (100).

5. The fin (12′) of claim 3 wherein:

at least one of said plurality of raised vanes (112) is attached to the sheet at a first end (113);

said first end (113) is oriented at an angle (α) from an imaginary line that is perpendicular to said first longitudinal rib (100); and

said angle (α) is between 15 and 45 degrees.

6. The fin (12′) of claim 5 wherein:

said at least one of said plurality of raised vanes (112) is attached to the sheet at a second end (113);

said first and second ends (113) are oriented at said angle (α) from an imaginary line that is perpendicular to said first longitudinal rib (100); and

said angle (α) is between 25 and 35 degrees.

7. The fin (12′) of claim 1 further comprising:

nine raised vanes (112) forming a generally ‘X’-shaped pattern (105) in said sheet between first and second apertures (18′) of said first plurality of apertures (18′);

each of said nine raised vanes (112) formed between first and second longitudinal slits (110) in said sheet such that an opening (114) is defined between said raised vane (112) and the surface (103) of said sheet;

wherein first and second vanes (112) of said nine raised vanes (112) are disposed in a first row (120) of vanes (112) that is parallel to said first rib (100), third and fourth vanes (112) of said nine raised vanes (112) are disposed in a second row (122) of vanes (112) that is parallel to said first rib (100), a fifth vane (112) of said nine raised vanes (112) is disposed in a third row (124) of vanes (112) that is parallel to said first rib (100), said second row (122) of vanes (112) is disposed adjacent to and between said first and third rows (120, 124) of vanes (112), sixth and seventh vanes (112) of said nine raised vanes (112) are disposed in a fourth row (126) of vanes (112) that is parallel to said first rib (100), said third row (124) of vanes (112) is disposed adjacent to and between said second and fourth rows (122, 126) of vanes (112), eighth and ninth vanes (112) of said nine raised vanes (112) are disposed in a fifth row (128) of vanes (112) that is parallel to said first rib (100), and said fourth row (126) of vanes (112) is disposed adjacent to and between said third and fifth rows (124, 128) of vanes (112).

8. The fin (12′) of claim 3 wherein:

each of said plurality of raised vanes (112) has a depth dimension (dv) from said first slit (110) to said second slit (110) between 0.5 and 1.5 millimeters.

9. The fin (12′) of claim 3 wherein.

each of said plurality of raised vanes (112) has a height dimension (hv) from said upper surface (103) of said sheet to said top surface of said vane (112) between 0.25 and 0.75 millimeters.

10. The fin (12′) of claim 1 wherein:

each of said ribs (100) has a height dimension (hr) from said upper surface (103) of said sheet to the top surface of said rib (100) between 0.05 and 0.25 millimeters.

11. The fin (12′) of claim 1 wherein:

the longitudinal distance (pw) between the centers of two adjacent apertures (18′) of said first plurality of apertures (18′) in said first longitudinal row of apertures (18′) is between 12 and 18 millimeters.

12. The fin (12′) of claim 2 wherein:

the perpendicular distance (pt) between the center of said first longitudinal row of apertures (18′) and the center of the second longitudinal row of apertures (18′) is between 10 and 15 millimeters.

13. A fin (12′) for a tube and fin heat exchanger (10) comprising:

a generally planar metallic sheet;

a first plurality of apertures (18′) formed through said sheet and defining a first longitudinal row of apertures (18′); and

a plurality of raised vanes (112) forming a generally ‘X’-shaped pattern (105) in said sheet between first and second apertures (18′) of said first plurality of apertures (18′);

each of said plurality of raised vanes (112) formed between first and second parallel slits (110) in said sheet such that an opening (114) is defined between said raised vane (112) and the surface (103) of said sheet.

14. The fin (12′) of claim 13 wherein:

first and second vanes (112) of said plurality of raised vanes (112) are disposed in a first row (120) of vanes (112) that is parallel to said first longitudinal row of apertures (18′);

third and fourth vanes (112) of said plurality of raised vanes (112) are disposed in a second row (122) of vanes (112) that is parallel to said first longitudinal row of apertures (18′);

a fifth vane (112) of said plurality of raised vanes (112) is disposed in a third row (124) of vanes (112) that is parallel to said first longitudinal row of apertures (18′);

sixth and seventh vanes (112) of said plurality of raised vanes (112) are disposed in a fourth row (126) of vanes (112) that is parallel to said first longitudinal row of apertures (18′);

eighth and ninth vanes (112) of said plurality of raised vanes (112) are disposed in a fifth row (128) of vanes (112) that is parallel to said first longitudinal row of apertures (18′);

said second row (122) of vanes (112) is disposed adjacent to and between said first and third rows (120, 122) of vanes (112);

said third row (124) of vanes (112) is disposed adjacent to and between said second and fourth rows (122, 126) of vanes (112); and

said fourth row (126) of vanes (112) is disposed adjacent to and between said third and fifth rows (124, 128) of vanes (112).

15. The fin (12′) of claim 14 wherein:

said first, second, third, fourth, sixth, seventh. eight, and ninth vanes (112) of said plurality of raised vanes (112) each have first and second distal ends (113) connected to said sheet, each of said distal ends (113) being oriented at an angle (α) between 15 and 45 degrees from an imaginary line that is perpendicular to said first longitudinal row of apertures (18′).

16. A tube and fin heat exchanger (10) comprising:

a plurality of fins (12′) arranged in a stack, each of said plurality of fins (12′) characterized by a generally planar metallic sheet, a first plurality of apertures (18′) formed through said sheet and defining a first longitudinal row of apertures (18′), and first and second longitudinal ribs (100) formed in said sheet, said first longitudinal row of apertures (18′) disposed between said first and second longitudinal ribs (100), said first and second ribs (100) each having a top surface projecting beyond the upper surface (103) defined by said sheet; and

a tube (22) received through said stack and in physical contact with each of said plurality of fins (12′).

17. A tube and fin heat exchanger (10) comprising:

a plurality of fins (12′) arranged in a stack, each of said plurality of fins (12′) characterized by a generally planar metallic sheet, a first plurality of apertures (18′) formed through said sheet and defining a first longitudinal row of apertures (18′), and a plurality of raised vanes (112) forming a generally ‘X’-shaped pattern (105) in said sheet between first and second apertures (18′) of said first plurality of apertures (18′), each of said plurality of raised vanes (112) formed between first and second parallel slits (110) in said sheet such that an opening (114) is defined between said raised vane (112) and the surface (103) of said sheet; and

a tube (22) received through said stack and in physical contact with each of said plurality of fins (12′).