EP2101136A2 - Vaporiser pipe with optimised undercut on groove base - Google Patents

Abstract

The pipe i.e. integrally rolled ribbed type pipe (1), has ribs (2) running helically on a pipe exterior. Primary grooves (6) are found between the adjacent ribs. An undercut secondary groove (8) is arranged in an area of a groove base (7) of the primary grooves. A pair of oppositely lying material edges (9) extends continuously along the primary grooves. A cross section of the secondary groove is varied in space intervals without influencing a shape of the ribs. A distance is formed between the edges such that the distance is varied in spaced intervals to form local cavities (10).

Description

The invention relates to a metallic heat exchanger tube with on the outside of the tube helically encircling, integrally molded ribs according to the preamble of claim 1.

Such metallic heat exchanger tubes are used in particular for the evaporation of liquids from pure substances or mixtures on the outside of the tube.

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

By intensifying the heat transfer on the pipe outside and inside 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 the chlorine-free safety refrigerants that are predominantly used today, is decreasing. In the case of toxic or flammable refrigerants, the risk potential can also be reduced by reducing the filling quantity. The standard high-performance pipes are about four times more efficient than smooth pipes of the same diameter.

It is state of the art to produce such efficient pipes based on integrally rolled finned tubes. Integrally rolled finned tubes are understood to mean finned tubes in which the fins are formed from the wall material of a smooth tube. Here, 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 environment remain in the form of pores or slits. In particular, such substantially closed channels are formed by bending or flipping the ribs (FIG. US 3,696,861 ; US 5,054,548 ; US 7,178,361 B2 ), by splitting and upsetting the ribs ( DE 2 758 526 C2 ; US 4,577,381 ) and by notching and upsetting the ribs ( US 4,660,630 ; EP 0 713 072 B1 ; US 4,216,826 ) generated.

The most powerful commercially available finned tube finned tubes have on the tube exterior a fin structure with a fin density of 55 to 60 fins per inch ( US 5,669,441 ; US 5,697,430 ; DE 197 57 526 C1 ). 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 means of an even higher fin density or smaller fin pitch, since 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 inch. Further, as the rib pitch decreases, the production rate of the tubes becomes smaller, and hence the manufacturing cost becomes higher.

Furthermore, it is known that performance-enhanced evaporation structures can be produced at the same rib density on the outside of the tube by introducing additional structural elements in the region of the groove bottom between the ribs. Since the temperature of the rib is higher in the area of the groove base than in the area of the fin tip, structural elements for intensifying the formation of bubbles in this area are particularly effective.

Examples of this are in EP 0 222 100 B1 ; US 5,186,252 ; JP 04039596A and US 2007/0151715 A1 to find. These inventions have in common that the structural elements have no undercut shape at the groove bottom, which is why they do not sufficiently intensify the formation of bubbles. In EP 1 223 400 B1 It is proposed to produce at the groove bottom between the ribs undercut secondary grooves which extend continuously along the primary groove. The cross section of these secondary grooves can remain constant or varied at regular intervals.

The invention has for its object to provide a performance-enhanced heat exchanger tube for the evaporation of liquids on the outside of the tube with the same tube-side heat transfer and pressure drop.

The invention is represented by the features of claim 1. The other dependent claims relate to advantageous embodiments and further developments of the invention.

The invention includes a metallic heat exchanger tube with on the outside of the tube helically encircling, integrally formed and continuously formed ribs, the fin foot protrudes substantially radially from the tube wall, as well as located between each adjacent ribs primary grooves. At least one undercut secondary groove is arranged in the region of the groove bottom of the primary grooves. This secondary groove is limited to the primary groove by a pair of opposing material projections formed from material of respectively adjacent rib feet. These material projections extend continuously along the primary groove. The cross section of the secondary groove is varied at regular intervals without affecting the shape of the ribs. There is a gap between the opposed material projections, this distance being varied at regular intervals, whereby local cavities are formed.

The invention is based on the consideration that to increase the heat transfer during evaporation of the process of bubbling is intensified. The formation of bubbles begins at germinal sites. These germinal sites are usually small gas or steam inclusions. When the growing bubble reaches a certain size, it detaches from the surface. If the germinal site is flooded with fluid in the course of bladder detachment, the germinal site is deactivated. The surface must therefore be designed in such a way that when the bubble is detached, a small bubble remains, which then serves as a germinal point for a new bubble formation cycle. This is achieved by applying cavities with openings on the surface. The opening of the cavity tapers in relation to the cavity located below the opening. Through the opening of the exchange of liquid and steam.

In the present invention, a connection between the primary and secondary groove is realized by the distance between the opposite material projections, so that the exchange of liquid and vapor between the primary groove and the secondary groove is made possible. The particular advantage of the invention is that the effect of the undercut secondary groove on the formation of bubbles is particularly great when the distance between opposing material projections according to the invention is varied at regular intervals. As a result, the exchange of liquid and vapor is controlled specifically and prevents the flooding of the bubble nucleation site in the cavity. The location of the cavities in the vicinity of the primary groove base is particularly favorable for the evaporation process, since at the groove bottom, the heat overtemperature is greatest and therefore there is the highest driving temperature difference for the bubble formation available.

In a particularly preferred embodiment of the invention, the distance between the opposite material protrusions can assume the value zero at regular intervals. As a result, the secondary groove is closed in certain areas relative to the primary groove. In these areas, the opposite material projections touch, without that it comes to a material conclusion. In this case, the bubbles in turn escape through the cavities which are opened into the center of the primary groove, and the liquid preferably flows from the side into the cavity near the closed regions of the secondary groove. In this case, the escaping bubble is not hindered by the inflowing liquid working medium and can expand undisturbed in the primary groove. The respective flow zones for the liquid and the steam are spatially separated from each other. In addition, even in the closed region of the secondary groove, a small channel is left between the cavities, which, however, has no connection to the primary groove. Nevertheless, for example, pressure differences between the mutually adjacent cavities can be compensated via these channels.

Preferably, in the regions in which the distance between the opposing material projections assumes the value zero, the secondary groove may be substantially pressed. In this embodiment, there is no connection of the cavities with one another more over the subsections of the secondary groove.

In a preferred embodiment of the invention, the maximum distance between the opposite material projections 0.03 mm to 0.1 mm. In addition, advantageously, the maximum distance between the opposite material projections 0.06 mm to 0.09 mm.

In a preferred embodiment, the length of the areas in the circumferential direction, in which the distance of the opposite material projections does not assume the value zero, be between 0.2 mm and 0.5 mm. As a result, an optimal coordination of the successive cavities and intermediate areas is achieved.

In a further advantageous embodiment of the invention, the rib tips may be deformed such that they cover the primary grooves in the radial direction and partially close and thus a helically encircling, partially closed Form cavity. The rib tips may have, for example, a substantially T-shaped cross section with pore-like recesses through which the vapor bubbles can escape.

The publication EP 1 223 400 B1 For the embodiment of further preferred and advantageous combinations with the solution according to the invention, this description is included in full in its entirety.

Embodiments of the invention will be explained in more detail with reference to the schematic drawings.

Show:

• Fig. 1 a partial view of the outside of a pipe section according to the invention,
• Fig. 2 a front view of the pipe section according to Fig. 1 .
• Fig. 3 a partial view of the outside of a pipe section according to the invention with partially closed secondary groove,
• Fig. 4 a front view of the pipe section according to Fig. 3 .
• Fig. 5 a partial view of the outside of a pipe section according to the invention with sectionally compressed secondary groove between the cavities, and
• Fig. 6 a front view of the pipe section according to Fig. 5 ,

Corresponding parts are provided in all figures with the same reference numerals.

Fig. 1 shows a view of the outside of a pipe section according to the invention. The integrally rolled finned tube 1 has, on the outside of the tube, helical circumferential ribs 2, between which a primary groove 6 is formed. The ribs 2 extend continuously without interruption along a helix line on the tube outside. The ribbed foot 3 projects essentially radially from the tube wall 5. A finned tube 1 is proposed, in which an undercut secondary groove extends in the region of the groove bottom 7, which extends between two respective adjacent ribs 2 located primary grooves 6 8 is arranged. This secondary groove 8 is limited to the primary groove 6 through a pair of opposed, formed of material respectively adjacent rib feet 3 shaped material protrusions 9. These material projections 9 extend continuously along the primary groove 6, wherein a distance S is formed between opposite material projections 9, which is varied at regular intervals. With variation of the cross section of the secondary groove 8, the shape of the ribs 2 is not affected. Due to the change in cross-section in conjunction with the variation of the distance S, cavities 10 form locally, which favor bubble nucleation in particular.

By the distance S between the opposite material projections 9, a connection between the primary groove 6 and the secondary groove 8 is formed so that the exchange of liquid and vapor between the primary groove 6 and the secondary groove 8 is made possible. In areas having a small distance S between the material projections 9, liquid preferably passes from the primary groove 6 into the secondary groove 8. The liquid evaporates within the secondary groove 8. The resulting vapor preferably exits at the points from the secondary groove 8 These vapor bubbles emerging there form nucleating sites for the further evaporation of liquid in the primary groove 6. For the further evaporation of liquid in the primary groove 6, it is advantageous that the Ribs 2 continuously on the tube outside along the primary groove 6 extend. By the targeted variation of the opening width of the secondary groove 8, the exchange of liquid and vapor between the primary groove 6 and secondary groove 8 is controlled by liquid supply and vapor discharge take place in separate areas. This advantageous feature, tubes of the prior art, for example, the after EP 1 223 400 B1 are made, not on, since here, although the cross-sectional shape of the secondary groove 8 is varied, but not their opening width and thus no preferred areas exist respectively for liquid supply and steam outlet. The extent of the secondary groove 8 in the radial direction is measured from the groove bottom 7 in the areas with a large distance between the material projections 9 maximum 15% of the height H of the ribs 2. The fin height H is measured on the finished finned tube 1 from the lowest point of the groove bottom 7 to the fin tip 4 of the fully formed finned tube.

Fig. 2 shows a front view of the pipe section according to Fig. 1 , In this partial view, the ribs 2 which extend helically on the outside of the pipe run into the plane of the drawing. Between the ribs 2, the primary groove 6 is formed. The ribbed foot 3 projects essentially radially from the tube wall 5. In the region of the groove bottom 7, which extends between each two adjacent ribs 2 located primary grooves 6, the undercut secondary groove 8 is formed. This secondary groove 8 is separated from the primary groove 6 by the opposing material projections 9.

These material projections 9 extend continuously along the primary groove 6 perpendicular to the plane of the drawing, wherein a distance S is formed between opposite material projections 9, which is varied at regular intervals. In different planes, S assumes the minimum value S _min in the region between the cavities 10 and the value S _max at the highest point of a cavity 10. By virtue of this change in cross-section, cavities 10 having an opening width are formed locally, which favor bubble nucleation in particular.

Fig. 3 shows a view of the outside of a pipe section 1 according to the invention with partially closed secondary groove 8. Here, the secondary groove 8 is completely closed at regular intervals to the primary groove 6 out. This corresponds to the case that in certain areas, the distance between the material projections 9 is reduced to zero. The secondary groove 8 then has only in the respective intermediate areas openings to the primary groove 6 down, whereby the width of these openings is reduced at the respective edges.

Fig. 4 shows a front view of the pipe section according to Fig. 3 , The material projections 9 in turn extend continuously along the primary groove 6 perpendicular to the plane of the drawing with a distance S between the opposed material projections 9, which is varied at regular intervals. While in the region of a cavity at the highest point of the value S _max to Fig. 2 remains unchanged, S assumes between the cavities 10 to the minimum value S _min = 0. In these areas, the opposing material projections 9 touch, without resulting in a material bond. The bubbles in turn escape through the cavities 10 which are opened into the center of the primary groove 6. Liquid flows into the cavity at the edges of the openings. In the closed region of the secondary groove 8, a small channel is maintained between the cavities 10, which has no connection to the primary groove 6. However, for example, pressure differences between the mutually adjacent cavities 10 can be compensated via these channels. The length L of the regions in which the secondary groove is not closed is advantageously between 0.2 mm and 0.5 mm.

Fig. 5 shows a partial view of the outside of a pipe section according to the invention with fully closed secondary groove between the cavities. As shown, it also proves to be advantageous in the areas in which the distance between the material projections 9 is reduced to the value zero, the material projections 9 to deform so far that they are displaced to the bottom of the secondary 8 and thus the Secondary groove 8 is pressed in this area. As a result, cavities 10 which are located in the intermediate regions and which are completely limited in their circumference are produced as undercut cavities at the bottom of the primary groove 6. These cavities 10 act as extremely effective nucleation sites, since in these structures the subsequent flow of liquid can be very controlled and even particularly small bubbles can not be displaced. The bubbles in turn escape through the cavities 10 which are opened into the center of the primary groove 6. Liquid flows into the cavity at the edges of the openings. The length L of the regions in which the secondary groove is not closed is advantageously between 0.2 mm and 0.5 mm.

Fig. 6 shows a front view of the pipe section according to Fig. 5 , As shown, it is once again clarified how in the regions in which the distance between the material projections 9 is reduced to the value zero, the material projections 9 are deformed. These are displaced to the bottom of the secondary groove 8, whereby the secondary groove 8 is pressed in this area.

The distance S between the opposing material projections 9 varies between 0 mm and 0.1 mm. In the ranges in which this distance assumes its maximum value S _max , this value is typically between 0.03 mm and 0.1 mm, preferably between 0.06 mm and 0.09 mm.

In addition to the formation of the undercut secondary grooves 8 on the groove bottom 7 of the primary grooves 6, the rib tips are expediently deformed as a distal region 4 of the ribs 2 such that they partially close the primary grooves 6 in the radial direction and thus form a partially closed cavity. The connection between the primary groove 6 and the environment is configured in the form of pores 11 or slots, so that vapor bubbles can escape from the primary groove 6. The deformation of the rib tips 4 is done with methods that can be found in the prior art. The primary grooves 6 then represent even undercut grooves.

By combining the cavities 10 according to the invention with a primary groove 6 which is closed except for pores 11 or slots, a structure is obtained which is further characterized in that it has a very high efficiency in the evaporation of liquids over a very wide range of operating conditions. In particular, when the heat flow density or the driving temperature difference is varied, the heat transfer coefficient of the structure at a high level remains almost constant.

The solution according to the invention relates to structured tubes in which the heat transfer coefficient is increased on the tube outside. In order not to the majority of the heat transfer resistance on the inside too Relocate the heat transfer coefficient on the inside can also be intensified by a suitable internal structuring.

The 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.

LIST OF REFERENCE NUMBERS

1

metallic heat exchanger tube, finned tube

2

ribs

3

fin base

4

Rib tips, distal areas of the ribs

5

pipe wall

6

primary groove

7

groove base

8th

secondary groove

9

Material advantage

10

cavity

11

pore

S

Distance between opposite material projections

S _max

maximum distance between opposite material projections

S _min

minimum distance between opposing material protrusions

L

Length of the areas in the circumferential direction, in which the distance S is not equal to zero

Claims (7)

Metallic heat exchanger tube (1) with on the outside of the tube helically encircling, integrally formed and continuously formed ribs (2), the ribbed foot (3) projects substantially radially from the tube wall (5), and with between each adjacent ribs (2) located primary grooves (6), wherein in the region of the groove bottom (7) of the primary grooves (6) at least one undercut secondary groove (8) is arranged, this secondary groove (8) to the primary groove (6) through a pair of opposing, each of material adjacent rib feet ( 3) shaped material projections (9) is limited, these material projections (9) extend continuously along the primary groove (6), the cross section of the secondary groove (8) is varied at regular intervals, without affecting the shape of the ribs (2), and between opposite material projections (9) a distance (S), characterized in that this distance (S) in regelmä is varied strength intervals, which local cavities (10) are formed. Metallic heat exchanger tube according to claim 1, characterized in that the distance (S) between the opposing material projections (9) assumes the value zero at regular intervals. Metallic heat exchanger tube according to claim 2, characterized in that in the areas in which the distance between the opposing material projections (9) assumes the value zero, the secondary groove (8) is substantially closed. Metallic heat exchanger tube according to one of claims 1 to 3, characterized in that the maximum distance (S _max ) between the opposing material projections (9) is 0.03 mm to 0.1 mm. Metallic heat exchanger tube according to claim 4, characterized in that the maximum distance (S _max ) between the opposing material projections (9) is 0.06 mm to 0.09 mm. Metallic heat exchanger tube according to one of Claims 2 to 5, characterized in that the length (L) measured in the direction of rotation of the regions in which the distance (S) of the opposing material projections (9) does not become zero lies between 0.2 mm and 0.5 mm. Metallic heat exchanger tube according to one of claims 1 to 6, characterized in that the rib tips (4) are deformed such that they cover the primary grooves (6) in the radial direction and partially close, thus forming a helically encircling, partially enclosed cavity.

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