EP0591094A1 - Internally ribbed heat transfer tube - Google Patents

Abstract

The heat transfer tube (10) has internal ribs (14) to enhance the heat transfer performance of the tube (10). The ribs (14) may be substantially triangular or substantially rectangular in cross section but have specific ranges of values for critical parameters of the ribs. In a preferred embodiment, the ribs (14) are equally distributed around the inner circumference of the tube (10) and the tube (10) is made of a copper or copper alloy.

Description

Background of the Invention
• This invention relates generally to tubes used in heat exchangers for transferring heat between a fluid inside the tube and a fluid outside the tube. More particularly, the invention relates to a heat exchanger tube having ribs on its internal surface to improve the heat transfer performance of the tube. Such a tube is adaptable to use in heat exchangers of air conditioning, refrigeration and similar systems.
• Designers of heat transfer tubes have long recognized that the heat transfer performance of a tube having surface enhancements is superior to a smooth walled tube. A wide variety of surface enhancements have been applied to both internal and external tube surfaces including ribs, fins, coatings and inserts, to name just a few. Common to nearly all enhancement designs is an attempt to increase the heat transfer area of the tube. Most designs also attempt to encourage turbulence in the fluid flowing through or over the tube in order to promote fluid mixing and break up the boundary layer at the surface of the tube.
• In fluid-to-air heat exchangers used in air conditioning and refrigeration systems and in liquid cooled engine radiators, for example, the tubing is frequently enhanced on its external surface by the use of plate fins. In heat exchanger tubing of a nominal outside diameter of 20 mm (3/4 inch) or less having an internal surface enhancement, the enhancement is usually a pattern of helical ribs placed on the internal surface.
• Helical rib enhancements to internal tube surfaces increase the heat transfer performance of the tube in two ways. First, working the tube wall to form the ribs increases the heat transfer area of the tube. Second, the ribs promote turbulence in the fluid flowing inside the tube. The turbulence minimizes the thickness of the heat transfer inhibiting laminar boundary layer on the inner surface of the tube.
• If the above were the only operative factors in optimizing the performance of an internally enhanced heat transfer tube, the task would be simple. One would simply crowd the greatest number of the highest possible ribs per unit length of tube. In a helical rib enhancement, this would mean using the largest possible rib helix angle.
• It is not, however, that easy. Increases in fin density and fin height result in increases in the fluid flow resistance of the tube. Higher flow resistance results in increased pressure loss through the tube and requires increased pumping power to produce a given flow rate through the tube. Increases in rib density and height also require increases in the material content of the tube wall. This is in order to provide the material necessary to form the ribs and also to maintain a minimum effective wall thickness and thus insure adequate burst strength in the tube.
Summary of the Invention
• The object of the present invention is to achieve high heat transfer efficiency in an internally enhanced heat transfer tube while at the same time assuring adequate tube burst strength with a minimum of wall material and acceptable fluid flow characteristics through the tube.
• The invention achieves this objective in a heat transfer tube having helical ribs on its internal surface. The ranges of rib helix angles, rib height and rib density have been selected to optimize heat transfer efficiency as well as provide adequate strength and fluid flow.
• The tube can be made out of any suitable material by any forming process but is particularly adaptable to being manufactured from a copper or copper alloy in a process in which the internal enhancement is first roll embossed on to one surface of a metal strip, then roll forming the strip into a tubular shape and then seam welding the roll formed strip into a tube. If the tube is manufactured using this method, then the helical groove will not be continuous around the entire inner circumference of the tube. There will be a region in the vicinity of the seam weld that does not contain the enhancement. The effect of this unenhanced region on tube performance is negligible.
Brief Description of the Drawings
• The accompanying drawings form a part of the specification. Throughout the drawings, like reference numbers identify like elements.
• FIG. 1 is a sectioned pictorial view of the heat transfer tube of the present invention.
• FIG. 2 is a cross sectioned elevation view of the heat transfer tube of the present invention.
• FIG. 3 is an illustrative cross sectioned view of a portion of the wall of the heat transfer tube of the present invention.
• FIGS. 4 and 5 are cross sectioned views of portions of the walls of two embodiments of the heat transfer tube of the present invention.
• FIG. 6 is an isometric view of a portion of an embossed strip used to manufacture the heat transfer tube of the present invention.
• FIG. 7 is a cross sectioned view of a portion of the wall of the heat transfer tube of the present invention showing the unenhanced weld zone of the tube when it is manufactured from roll formed and seam welded strip.
Description of the Preferred Embodiments
• FIG. 1, in a sectioned pictorial view, depicts a heat transfer tube in which the present invention is embodied. In FIG. 1, heat transfer tube 10 has surface enhancement 11 extending over substantially all of its internal surface.
• FIG. 2 depicts heat transfer tube 10 in a cross sectioned elevation view. Only a single rib 14 of axial surface enhancement 11 (FIG. 1) is shown for clarity, but in the enhancement of the present invention, a plurality of ribs 14, all parallel to each other, extend out from wall 13 of tube 14. Rib 14 is inclined at angle α from tube longitudinal axis a _T . Tube 10 has internal diameter, as measured from the internal surface of the tube between ribs, D _i .
• FIG. 3 is an illustrative cross sectioned view of a portion of wall 13 of heat transfer tube 10 of the present invention depicting details of the ribbed enhancements on the internal surface of the tube. Extending inward (toward the center of tube 10) are a plurality of ribs 14. Each of ribs 14 has base width W _R and height H _R . Fin apex angle β is the angle formed at the intersection of the planes of first and second sides 17 and 18 of ribs 14 when the planes are extended. Formed between ribs 14 are a plurality of grooves 15.
• The ribs shown in FIG. 3 are trapezoidal in cross section. Because of the dimensions of the ribs, the working characteristics of the metal used in the tube and the manufacturing processes employed to form the ribs, it would be difficult, if not impossible, to produce sharp edged trapezoidal ridges. During roll embossing of the ribs on to strip feedstock, for example, metal does not completely fill the grooves of the embossing roller and the ribs that are formed have cross sections more like those depicted in FIGS. 4 and 5. Depending on the apex angle, the rib cross section may be either substantially triangular, as shown in FIG. 4, or substantially rectangular, as shown in FIG. 5. There is an element of subjectivity in defining whether a rib has a triangular or a rectangular cross section at the margin, but a rib having an apex angle of less than 15° can rather arbitrarily be considered to have a rectangular cross section while an apex angle of more than 15° will produce a rib having a triangular cross section.
• It has been determined that, for optimum heat transfer performance consistent with acceptable fluid flow resistance, a tube embodying the present invention and having a nominal outside diameter of 20 mm (3/4 inch) or less should have an internal enhancement of ribs with the following parameters:
• a. the angle between the ribs and the longitudinal axis of the tube should be between 30 and 45 degrees, or $$\text{30° ≦ α ≦ 45° ;}$$
  
• b. the ratio of the rib height to the inner diameter of the tube should be between 0.018 and 0.03, or $${\text{0.0.18 ≦ H}}_{\text{R}}{\text{/ D}}_{\text{i}}\text{≦ 0.03;}$$
  
• c. the rib apex angle should be between 0 degrees (that is, the rib being substantially rectangular) and 60 degrees, or $$\text{0° ≦ β ≦ 60° ;}$$
  
• d. the width of the rib base, where the ribs join the inner wall of the tube, should be between 0.127 and 0.254 mm (0.005 and 0.01 inch), or $${\text{0.127 mm (0.005 inch) ≦ W}}_{\text{R}}\text{≦ 0.254 mm (0.01 inch); and}$$
  
• e. the number of ribs per unit length of the inner circumference of the tube (πD_i.) should be between 10 and 24 ribs per cm (25 and 60 per inch), or $${\text{10/cm (25/inch) ≦ Ribs/length of πD}}_{\text{i}}\text{≦ 24/cm (60/inch).}$$
  
• The ribs and grooves of the present invention may be formed on the interior surface of the heat transfer tube by any suitable process, such as drawing, mandrel insertion, machining or the like. In the manufacture of seam welded tubing using modern automated high speed processes, an effective method is to apply the enhancement pattern by roll embossing on one surface of a metal strip before the strip is roll formed into a circular cross section and seam welded into a tube. FIG. 6 shows metal strip 20 having enhancement pattern 21 embossed into one of its surfaces. Along each edge of strip 20 is left an unenhanced region 22 that will become the weld zone in the finished tube.
• The ribs should be distributed at equal intervals around the inner circumference of the tube, but minor variations in spacing are acceptable. If the tube is manufactured by roll embossing, roll forming and seam welding, it is likely that there will be a region along the line of the weld in the finished tube that either lacks the enhancement configuration that is present around the remainder of the tube inner circumference , due to the nature of the manufacturing process, or has a different enhancement configuration. FIG. 7 shows a cross sectioned view of a portion of wall 13 of tube 10. Weld zone 31 in this embodiment does not contain ribs. This region of different configuration will not adversely affect the thermal or fluid flow performance of the tube in any significant way.
• Because of their heat transfer characteristics, workability and relatively good cost and corrosion resistance properties, excellent materials from which to fabricate tubing embodying the present invention are copper or a suitable copper alloy.

Claims (6)

1. An improved heat transfer tube (10) having
      an inner surface,
      an inner diameter (D_i),
      a longitudinal axis (a_T) and
      a plurality of ribs and grooves formed on said inner surface,
      each of said ribs having
         a base,
         two opposite sides,
         an apex angle (β) between said sides,
         a height (H_R) and
         an angle of inclination (α) with respect to said longitudinal axis,
   in which the improvement comprises:
      said angle of inclination being between 30 and 45 degrees;
      the ratio of said rib height to said tube inner diameter being between 0.018 and 0.03;
      the width (W_R) of said base being between 0.127 and 0.254 mm (0.005 and 0.01 inch);
      said apex angle being between 0 and 60 degrees; and
      the number of ribs per unit length of said tube's inner circumference (πD_i) being between 10 and 24 per cm (26 and 60 per inch).
2. The heat transfer tube of claim 1 in which said ribs are substantially triangular in cross section, the apex angle between said rib opposite sides being between 15 and 40 degrees.
3. The heat transfer tube of claim 1 in which said ribs are substantially rectangular in cross section, the apex angle between said rib opposite sides being less than 15 degrees.
4. The heat transfer tube of claim 1 in which said ribs are disposed at substantially equal intervals around said heat transfer tube internal surface.
5. The heat transfer tube of claim 1 in which said heat transfer tube is fabricated of a material substantially comprised of copper.
6. The heat transfer tube of claim 1 in which said heat transfer tube is fabricated of a material substantially comprised of a copper alloy.

Images