EP1223400B1 - Tube for heat exchanger and process for making same - Google Patents

Abstract

A metallic heat exchanger tube, especially for vaporizing liquids comprising pure materials and mixtures on the outside of the tube, comprises integral ribs (3) on the outside of the pipe, which have a foot (13) that extends radially from the pipe wall (18). Grooves between the ribs have cut-outs in the base. The cut-outs are in the form of cut-back secondary grooves (7).

Description

The invention relates to a metallic heat exchanger tube, in particular for the evaporation of liquids from pure substances or mixtures on the tube outside, according to the preamble of claim 1.

Evaporation occurs in many areas of refrigeration and air conditioning technology as well as in process and energy technology. Tube heat exchangers are frequently used in industry in which liquids of pure substances or mixtures on the outside of the tube evaporate, cooling a brine or water on the inside of the tube. Such apparatuses are referred to as flooded evaporators.

By intensifying the heat transfer on the outside of the pipe and the inside of the pipe, the size of the evaporator can be greatly reduced. As a result, the production costs of such apparatuses decrease. In addition, the necessary filling quantity of refrigerant, which can account for a not inconsiderable share of the total investment costs in today's predominantly used chlorine-free safety refrigerants, is decreasing. In the case of toxic or flammable refrigerants, the danger potential can be reduced by reducing the filling quantity. The standard high performance pipes today are about three times more efficient than smooth pipes of the same diameter.

State of the art

The present invention relates to structured pipes in which the heat transfer coefficient is intensified on the pipe outside. As a result, the majority of the heat transfer resistance is often shifted to the inside, the heat transfer coefficient on the inside usually also needs to be intensified. An increase in the heat transfer on the inside of the pipe usually results in an increase in the pipe-side pressure drop.

Heat exchanger tubes for shell and tube heat exchangers usually have at least one structured area and smooth end pieces and possibly smooth spacers. The smooth end or intermediate pieces limit the structured areas. So that the tube can be easily installed in the shell and tube heat exchanger, the outer diameter of the structured areas must not be greater than the outer diameter of the smooth end and intermediate pieces.

To increase the heat transfer during evaporation, the process of bubbling is intensified. It is known that the formation of bubbles begins at nucleation sites. These germinal sites are usually small gas or steam inclusions. Such nucleation sites can already be produced by roughening the surface. When the growing bubble reaches a certain size, it detaches from the surface. If in the course of bladder detachment the germinal site is flooded by inflowing liquid, the gas or vapor inclusion may be displaced by liquid. In this case, the germinal site is inactivated. This can be avoided by a suitable design of the germinal sites. For this purpose, it is necessary that the opening of the nucleus is smaller than that under the Opening cavity.

It is state of the art to produce such structures based on integrally rolled finned tubes. Integrally rolled finned tubes are understood to mean finned tubes in which the fins have been formed from the wall material of a smooth tube. In this case, various methods are known with which the channels located between adjacent ribs are closed in such a way that connections between the channel and the surroundings remain in the form of pores or slots. Since the opening of the pores or slots is smaller than the width of the channels, the channels are suitably shaped cavities that promote formation and stabilization of nucleation sites. In particular, such substantially closed channels are formed by bending or flipping the rib (US 3,696,861, US 5,054,548), splitting and upsetting the fin (DE 2,758,526, US 4,577,381), and notching and upsetting the rib (US 4,660,630, EP 0,713,072, US 4,216,826).

The most powerful, commercially available finned tube finned tubes have on the tube exterior a ribbed structure having a rib density of 55 to 60 ribs per inch (US 5,669,441, US 5,697,430, DE 197 57 526). This corresponds to a rib pitch of about 0.45 to 0.40 mm. In principle, it is possible to improve the performance of such pipes by an even higher fin density or smaller fin pitch, as this increases the bubble nuclei density. A smaller rib division inevitably requires equally finer tools. However, finer tools are subject to a higher risk of breakage and faster wear. The currently available tools enable the safe production of finned tubes with rib densities of up to 60 ribs per 2.54 cm (inch). Further, with decreasing rib pitch reduces the production rate of the tubes and consequently the manufacturing cost higher.

It is known that performance enhanced evaporation structures can be created with consistent fin density on the pipe outside by patterning the bottom of the groove between the ribs. In EP 0.222.100 it is proposed to provide the bottom of the groove by means of a notch disc with indentations. The indentations on the groove bottom can have a V-, trapezoidal or semicircular cross section and represent additional bubble nucleation sites. However, the performance increases achievable by such structures, particularly in the area of small heating surface loads, no longer meet the requirements of the market. Furthermore, the indentations represent a weakening of the core wall of the tube and lead to a reduction of the mechanical stability of the tube.

Task:

It is a Leistungsgesteigertes heat exchanger tube for the evaporation of liquids on the outside of the tube with the same tube-side heat transfer and pressure drop and the same production costs are produced. The mechanical stability of the tube should not be adversely affected.

Brief description of the invention

The object is achieved according to the invention in a heat exchanger tube of the type mentioned, in which recesses are arranged in the region of the groove bottom of the helically extending primary grooves between the ribs, that the recesses are formed in the form of undercut secondary grooves.

• in einer Schnittebene ein nicht abgeschlossenes Gebiet X zu finden ist,
• dieses Gebiet X mittels einer Strecke AB abgeschlossen werden kann,
• eine Strecke PQ mit P, Q ∈ Rand von X gefunden wird, so daß PQ parallel zu AB und die Länge von PQ größer ist als die Länge von AB.

An undercut groove (see Fig. 1) is present when

• in a sectional plane an incomplete area X can be found,
• this area X can be completed by means of a route AB,
• a distance PQ with P, Q ∈ edge of X is found, so that PQ parallel to AB and the length of PQ is greater than the length of AB.

An undercut secondary groove offers significantly better conditions for the formation and stabilization of bubble nuclei than the simple indentations proposed in EP 0.222.100. The location of the undercut secondary grooves in the vicinity of the primary groove bottom is particularly favorable for the evaporation process, since at the groove bottom, the Wandübertemperatur is greatest and therefore there is the highest driving temperature difference for the bubble formation available.

Claims 2 to 14 relate to preferred embodiments of the heat exchanger tube according to the invention.

According to the invention, after the ribs have been formed by suitable additional tools, material is displaced from the region of the rib flanks toward the groove bottom, so that incomplete cavities are formed there, which represent the desired undercut secondary grooves. The cavities extend from the primary groove bottom to the fin tip, extending to at most 45% of the fin height H, typically up to 20% of the fin height H. The rib height H is thereby measured from the deepest point of the groove bottom, which was formed by the largest rolling disk, to the fin tip of the completely shaped finned tube.

The invention further provides, according to claims 15 to 20, various processes for the production of the heat exchanger tube according to the invention.

Detailed description:

The invention will be explained in more detail with reference to the following exemplary embodiments:

Fig.1:

die Prinzipskizze einer hinterschnittenen Nut;

Fig.2:

schematisch die Herstellung eines erfindungsgemäßen Wärmeaustauscherrohres mit hinterschnittenen Sekundärnuten, die auf der Rohraußenseite mit im wesentlichen konstanten Querschnitt schraubenlinienförmig umlaufen;

Fig.3:

eine Teilansicht eines erfindungsgemäßen Wärmeaustauscherrohres mit hinterschnittenen Sekundärnuten, die mit im wesentlichen konstanten Querschnitt schraubenlinienförmig umlaufen;

Fig.4:

schematisch die Herstellung eines erfindungsgemäßen Wärmeaustauscherrohres mit schraubenlinienförmig verlaufenden, hinterschnittenen Sekundärnuten, deren Querschnitt in regelmäßigen Abständen variiert ist;

Fig.5:

eine Teilansicht eines erfindungsgemäßen Wärmeaustauscherrohres mit schraubenlinienförmig verlaufenden, hinterschnittenen Sekundärnuten, deren Querschnitt in regelmäßigen Abständen variiert ist;

Fig.6:

schematisch die Herstellung eines erfindungsgemäßen Wärmeaustauscherrohres mit hinterschnittenen Sekundärnuten, die im wesentlichen quer zur Richtung der Primärnuten verlaufen;

Fig.7:

eine Teilansicht eines erfindungsgemäßen Wärmeaustauscherrohres mit hinterschnittenen Sekundärnuten, die im wesentlichen quer zur Richtung der Primärnuten verlaufen;

Fig.8:

das Foto einer erfindungsgemäßen hinterschnittenen Sekundärnut am Nutengrund, die mit im wesentlichen konstanten Querschnitt schraubenlinienförmig umläuft;

Fig.9:

ein Diagramm, das den Leistungsvorteil durch die hinterschnittenen Sekundärnut am Nutengrund dokumentiert.

It shows:

Fig.1:

the schematic diagram of an undercut groove;

Figure 2:

schematically the production of a heat exchanger tube according to the invention with undercut secondary grooves which rotate on the outside of the tube with a substantially constant cross-section helically;

Figure 3:

a partial view of a heat exchanger tube according to the invention with undercut secondary grooves which rotate helically with a substantially constant cross section;

Figure 4:

schematically the production of a heat exchanger tube according to the invention with helically extending, undercut secondary grooves whose cross section is varied at regular intervals;

Figure 5:

a partial view of a heat exchanger tube according to the invention with helically extending, undercut secondary grooves whose cross section is varied at regular intervals;

Figure 6:

schematically the preparation of a heat exchanger tube according to the invention with undercut secondary grooves which extend substantially transversely to the direction of the primary grooves;

Figure 7:

a partial view of a heat exchanger tube according to the invention with undercut secondary grooves extending substantially transverse to the direction of the primary grooves;

Figure 8:

the photograph of an undercut secondary groove according to the invention on the groove base, which rotates helically with a substantially constant cross section;

Figure 9:

a diagram that documents the performance advantage of the undercut secondary groove at the bottom of the groove.

The integrally rolled finned tube 1 according to FIGS. 2 to 7 has helical circumferential ribs 3 on the tube outside, between which a primary groove 4 is formed. Material of the rib flanks 5 is suitably displaced, so that in the region of the groove bottom 6 not completely closed cavities 7 are formed, which constitute the undercut secondary grooves according to the invention. Material of the rib tips 8 is displaced such that the rib gaps are closed to form channels 9 to pores 26.

The production of the finned tube according to the invention takes place by means of a rolling process (cf., US Pat. Nos. 1,865,575 / 3,327,512) by means of the devices shown in FIGS.

A device is used which consists of n = 3 or 4 tool holders 10 into each of which a rolling tool 11 is integrated. The tool holder 10 are each arranged offset by 360 ° / n on the circumference of the finned tube. The tool holder 10 are radially deliverable. They are in turn arranged in a stationary (not shown) rolling head.

The smooth tube 2, which enters the device in the direction of the arrow, is set in rotation by the driven rolling tools 11 arranged on the circumference, the axes of the rolling tools 11 running obliquely to the tube axis. The rolling tools 11 consist in a conventional manner of several juxtaposed rolling disks 12 whose diameter increases in the direction of the arrow. The centrally arranged rolling tools 11 form the helically extending ribs 3 from the tube wall of the smooth tube 2, wherein in the forming zone, the tube wall is supported by a rolling mandrel 27. The rolling mandrel 27 can be profiled. The distance between the centers of two adjacent ribs measured along the tube axis is referred to as the rib pitch T. The rolling discs are profiled on their outer periphery so that the shaped ribs 3 have a substantially trapezoidal cross-section. Only in the transition region 13 between rib edge 5 and groove bottom 6, the rib deviates from the ideal trapezoidal shape. This transition region 13 is commonly referred to as Rippenfuß. The radius formed there is required to allow an unobstructed flow of material during the rib formation.

After the shaping of the essentially trapezoidal ribs 3 by the rolling tool 11, the undercut secondary grooves 7 according to the invention are produced in the region of the base 6 of the primary grooves 4. Three different embodiments can be used here:

Embodiment 1:

After the last disc of the rolling tool 11 is a cylindrical disc 14 is engaged, whose diameter is smaller than the diameter of the largest rolling disc (Fig. 2). The thickness D of this cylindrical disk 14 is slightly larger than the width B of the primary groove 4 formed by the rolling disks 12, in which case the width B of the primary groove 4 is measured at the point at which the rib edge 5 merges into the radius region of the rib foot 13. Typically, the thickness D of the cylindrical disc is 50% to 80% of the rib pitch T. The cylindrical disc 14 displaces material from the rib flank 5 towards the groove bottom 6. The displaced material is displaced by the appropriate choice of the tool geometry such that it forms over the groove bottom 6 material projections 15 and thus directly on the groove base 6, a not completely closed cavity 7 is formed (Fig. 3). This cavity 7 extends in the circumferential direction with a nearly constant cross-section. The cavity 7 constitutes an undercut secondary groove according to the invention.

It may prove expedient to provide the disc 14 on its lateral surface along its circumference with a completely or partially concave profile, so as to favor the displacement of the material of the rib flank 5.

Since the diameter of the cylindrical disc 14 is smaller than the diameter of the largest rolling disc of the rolling tool 11, the deepest point of the primary Nutengrunds 6 is not processed by the cylindrical disc 14. The pipe wall 18 is therefore not weakened in the formation of the undercut secondary grooves 7.

Embodiment 2:

This embodiment is an extension of embodiment 1: After the cylindrical disc 14 is in the second embodiment, a gear-like notching disc 16 is engaged, the diameter of which is larger than the diameter of the cylindrical disc 14, but at most as large as the diameter of the largest rolled disc of the rolling tool 11 (Fig. 4). The formed by the cylindrical disc 14, extending in the circumferential direction with constant cross-section cavity is divided by the notch disc 16 by circumferentially regularly arranged indentations 17. Thus, circumferential, undercut secondary grooves 7, whose cross-section is varied at regular intervals, arise in the circumferential direction (FIG. 5). The notching disc 16 may be straight or obliquely toothed.

Since the diameter of the gear-like notching disk 16 is not larger than the diameter of the largest rolling disk of the rolling tool 11, the deepest point of the primary groove bottom 6 is not further deepened by the gear-like notching disk 16. The pipe wall 18 is therefore not weakened in the formation of the undercut secondary grooves 7 according to Embodiment 2.

Embodiment 3:

After the last disc of the rolling tool 11 is a gear-like notching disc 19 is engaged, wherein the diameter of the notching disc 19 is at most as large as the diameter of the largest rolling disc (Fig. 6). The thickness D 'of the notching disc 19 is slightly larger than the width B of the primary discs 4 formed by the rolling discs 12, in which case the width B of the primary groove 4 is measured at the point at which the rib edge 5 merges into the radius range of the rib foot 13. Typically, the thickness D 'of this notch disc 50% to 80% of the rib pitch T. The notching disc 19 may be straight or obliquely toothed. The notching disc 19 displaces material from the region of the rib flanks 5 and from the radius region at the rib base 13 and leaves there spaced from each other indentations 20. The displaced material is preferably displaced in the unprocessed area between the indentations 20, so that there pronounced dams 21 am Groove base 6 arise, which extend transversely to the primary grooves 4 between the ribs 3. The now following roll plate 22 of constant diameter deforms the upper portions of these dams 21 in the direction of the tube circumference, so that between the deformed upper portions 23 of the dams 21 and the groove bottom 6 small cavities 7 between two adjacent dams 21 are formed (Fig. 7). These cavities 7 represent the undercut secondary grooves according to the invention. The diameter of the roll-over disc 22 must be selected to be smaller than the diameter of the base notch disc 19.

Since the diameter of the gear-type notching disc 19 is not larger than the diameter of the largest rolling disk of the rolling tool 11, the deepest point of the primary groove bottom 6 is not further deepened by the gear-like notching disk 19. The pipe wall 18 is therefore not weakened in the formation of the undercut secondary grooves 7 according to Embodiment 3.

After the undercut secondary grooves 7 have been produced on the groove bottom 6, the rib tips 8 are notched by means of a gear-type notching disc 24. This is shown in Figures 2/4/6. Subsequently, the upsetting of the notched rib tips by one or more compression rollers 25. The ribs 3 are thus given a substantially T-shaped cross-section, and the grooves 9 between the ribs 3 are closed except for pores 26 (see Figures 3/5/7) ,

The fin height H is measured on the finished finned tube 1 from the lowest point of the groove bottom 6 to the fin tip of the fully formed finned tube.

The undercut secondary grooves 7 according to the invention at the base 6 of the primary grooves 4 extend from the groove bottom 6 to the rib tip, extending to a maximum of 45% of the rib height H, typically up to 20% of the rib height H.

8 shows the photograph of an undercut secondary groove 7 according to the invention on the groove bottom 6. The sectional plane is perpendicular to the circumferential direction of the tube. Here, an example according to Embodiment 1 is shown. The apparent asymmetry of the structure is due to unavoidable tolerances in tooling and pre-material dimensions. The projections 15 are made of material which has been displaced from the rib flanks 5 to the groove bottom 6.

FIG. 9 shows in comparison the performance of two structured tubes on evaporation of the refrigerant R-134a on the outside of the tube, one of the tubes having undercut secondary grooves being made on the groove bottom. Shown is the external heat transfer coefficient over the Heizflächenbelastung. The saturation temperature is 14.5 ° C. It can be seen that a performance advantage is achieved by the undercut secondary grooves at the bottom of the groove, which is about 30% for small Heizflächenbelastungen, with large Heizflächenbelastungen about 20%.

Structures with undercut secondary grooves at the bottom of the groove are also proposed in EP 0,522,985. However, the structure is located on the inside of a tube. To the mechanical stability of such pipes, especially during To ensure expansion of the tubes, the secondary grooves must be designed as flat as possible. This is achieved by the acute-angled geometry of the secondary grooves described in EP 0,522,985. In the case of tube-side evaporation of refrigerants, a higher pressure usually prevails in the pipe than on the pipe outside. Under internal pressure loading is due to the notch effect of the acute-angled edges of the secondary grooves increased mechanical stress on the wall of the tube. This must be compensated by a thicker pipe wall. However, this safety margin in the pipe wall leads to an increased use of materials and thus to increased costs.

In the design of the undercut secondary grooves 7 proposed here in the region of the primary groove bottom 6 on the outside of ribbed tubes, however, no weakening of the tube wall 18 takes place, since exclusively the material from the region of the rib flanks 5 and possibly from the radius region 13 forms the secondary grooves 7 above the groove bottom 6 is used.

Claims (20)

1. Metal heat exchanger tube, in particular for evaporating liquids from pure substances or mixtures on the outside of the tube, with integral fins (3) which are shaped on the outside of the tube and the base (13) of which projects substantially radially from the tube wall (18), wherein the fins (3) have a substantially T-shaped cross section, and wherein recesses are disposed in the region of the groove bottom (6) of the primary grooves (4) extending between the fins (3), characterised in that the recesses are in the form of undercut secondary grooves (7).
2. Metal heat exchanger tube according to Claim 1,
   characterised in
   that the fins (3) and the primary grooves (4) extend in a helical manner.
3. Metal heat exchanger tube according to Claim 1,
   characterised in
   that the fins (3) and the primary grooves (4) extend in an annular manner.
4. Metal heat exchanger tube according to Claim 1,
   characterised in
   that the fins (3) and the primary grooves (4) extend in the axial direction.
5. Metal heat exchanger tube according to Claim 2, 3 or 4,
   characterised in
   that the undercut secondary grooves (7) extend with a substantially constant cross section in the direction of the primary grooves (4).
6. Metal heat exchanger tube according to Claim 2, 3 or 4,
   characterised in
   that the cross section of the undercut secondary grooves (7) extending in the direction of the primary grooves (4) is varied at regular spacings.
7. Metal heat exchanger tube according to Claim 2, 3 or 4,
   characterised in
   that the undercut secondary grooves (7) extend substantially transversely to the direction of the primary grooves (4).
8. Metal heat exchanger tube according to one or more of Claims 1 to 7, characterised in
   that the undercut secondary grooves (7) extend over up to 45 % of the fin height H at most.
9. Metal heat exchanger tube according to Claim 8,
   characterised in
   that the undercut secondary grooves (7) extend over up to 20 % of the fin height H at most.
10. Metal heat exchanger tube according to one or more of Claims 1 to 9, characterised in
    that the fins (3) are of a uniform height H.
11. Metal heat exchanger tube according to one or more of Claims 1 to 9, characterised in
    that the fins tips (8) are notched.
12. Metal heat exchanger tube according to one or more of Claims 1 to 11, characterised in
    that it has plain ends and/or plain intermediate regions.
13. Metal heat exchanger tube according to one or more of Claims 1 to 12, characterised in
    that it is formed as a seamless tube.
14. Metal heat exchanger tube according to one or more of Claims 1 to 12, characterised in
    that it is formed as a longitudinally welded tube.
15. Process for making a heat exchanger tube according to Claim 2, in which the following process steps are carried out:

    a) helically extending fins (3) are rolled out at the outer surface of a bare tube (2) in that the fin material is obtained by displacing material outwards out of the tube wall by means of a rolling operation and the resulting finned tube (1) is rotated and/or advanced according to the resulting fins (3) through the rolling forces, wherein the fins (3) are shaped out of the otherwise unshaped bare tube (2) with an increasing height,

    b) the bare tube (2) is supported by a rolling mandrel (27) lying therein,

    c) after the fins (3) have been shaped out, material is displaced through radial pressure from the fin flanks (5) and/or out of the transition region (13) at the fin base to the groove bottom (6) while forming the undercut secondary grooves (7),

    d) the fin tips (8) are upset through further radial pressure by means of at least one upsetting roll (25) into a substantially T-shaped cross section.
16. Process according to Claim 15 for making a heat exchanger tube according to Claim 5, characterised in
    that the radial pressure in the process step c) is produced by means of a cylindrical disc (14), the diameter of which is smaller than the diameter of the largest rolling disc (12) and the thickness D of which is at least 50 % and at most 80 % of the fin pitch T.
17. Process according to Claim 16 for making a heat exchanger tube according to Claim 6, characterised in that the process step c) is followed by the process step (d),
    in which the groove bottom (6) is shaped in places through further radial pressure by means of a gearwheel-like notching disc (16), the diameter of which is larger than the diameter of the cylindrical disc (14), although at most as large as the diameter of the largest rolling disc (12), such
    that regularly spaced indentations (17) result in the circumferential direction.
18. Process according to Claim 15 for making a heat exchanger tube according to Claim 7, characterised in
    that the radial pressure in the process step c') is produced by means of a gearwheel-like notching disc (19), the diameter of which is smaller than the diameter of the largest rolling disc (12), whereby spaced indentations (17) result, and
    that the process step d') follows,
    in which the undercut secondary grooves (7) are produced through further radial pressure by means of a cylindrical lapping disc (22).
19. Process according to Claim 17 or 18, characterised in that a straight-toothed or helically toothed notching disc (16, 19) is used in each case.
20. Process according to any one of Claims 16 to 19 for making a heat exchanger tube according to Claim 11, characterised in that in a further process step e) the fin tips (8) are notched through radial pressure by means of a gearwheel-like notching disc (24).

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