EP3465057B1 - Heat exchanger tube - Google Patents

Description

The invention relates to a heat exchanger tube according to the preamble of claim 1.

Such metallic heat exchanger tubes are used in particular for evaporating 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. Tube bundle heat exchangers are often used in which liquids of pure substances or mixtures evaporate on the outside of the tube and brine or water cools down on the inside of the tube. Such apparatuses are referred to as flooded evaporators.

The size of the evaporator can be greatly reduced by intensifying the heat transfer on the outside and inside of the tube. This reduces the manufacturing costs of such devices. In addition, the amount of refrigerant required is reduced, which can make up a non-negligible proportion of the total system costs with the chlorine-free safety refrigerants that are predominantly used today. In the case of toxic or flammable refrigerants, the risk potential can also be reduced by reducing the filling quantity. Today's high-performance pipes are already about four times more efficient than smooth pipes of the same diameter.

It is state of the art to produce such high-performance tubes on the basis of integrally rolled finned tubes. Below are integrally rolled finned tubes understood ribbed tubes in which the ribs were formed from the wall material of a smooth tube. Various methods are known in this regard, 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 essentially closed channels are formed by bending or folding over the ribs ( U.S. 3,696,861 ; U.S. 5,054,548 ; U.S. 7,178,361 B2 ), by splitting and upsetting the ribs ( DE 2 758 526 C2 ; U.S. 4,577,381 ) and by notching and upsetting the ribs ( U.S. 4,660,630 ; EP 0 713 072 B1 ; U.S. 4,216,826 ) generated.

The highest performance commercially available finned tubes for flooded evaporators have a fin structure on the tube face with a fin density of 55 to 60 fins per inch ( U.S. 5,669,441 ; U.S. 5,697,430 ; DE 197 57 526 C1 ). This corresponds to a rib spacing of approx. 0.45 to 0.40 mm. In principle, it is possible to improve the performance of such tubes by an even higher rib density or smaller rib pitch, since this increases the bubble nucleation point density. A smaller rib pitch inevitably requires equally finer tools. However, finer tools are subject to a higher risk of breakage and faster wear. The tools currently available enable the reliable production of finned tubes with fin densities of a maximum of 60 fins per inch. Furthermore, as the fin pitch decreases, the production speed of the tubes becomes slower and consequently the manufacturing cost becomes higher.

It is also known that performance-enhanced evaporation structures can be produced on the outside of the tube with the same fin density by introducing additional structural elements in the area of the groove base between the fins. Since the temperature of the rib is higher in the area of the bottom of the groove than in the area of the tip of the rib, structural elements for intensifying the formation of bubbles are particularly effective in this area. Examples of this are in EP 0 222 100 B1 ; U.S. 5,186,252 ; JP04039596A and U.S. 2007/0151715 A1 to find. What these inventions have in common is that the structural elements at the bottom of the groove do not have an undercut shape, which is why they do not sufficiently intensify the formation of bubbles. In EP 1 223 400 B1 and EP 2 101 136 B1 it is proposed to produce undercut secondary grooves at the bottom of the groove between the ribs, which extend continuously along the primary groove. The cross section of these secondary grooves can remain constant or be varied at regular intervals.

The pamphlets U.S. 2006/0213346 A1 and U.S. 2005/0145377 A1 disclose improved heat transfer surfaces that facilitate heat transfer from one side of the surface to the other. Described herein is another method of improving heat transfer surfaces by using a tool to cut the inner surface of a tube. The tool has at least one tip with a cutting edge and a lifting edge. Protrusions are formed by cutting the inner surface of a heat exchanger tube and raising the cut surface. Boiling surfaces produced in this way have a multiplicity of primary grooves, projections and secondary grooves, for example to form boiling cavities.

The invention is based on the object of specifying a performance-enhanced heat exchanger tube for evaporating liquids.

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

In both solutions according to the invention, the structured area can in principle also be formed on the outside of the tube.

The structures described can be used for both evaporator and condenser tubes. The structures are also suitable for single-phase fluid flows, such as water.

A cavity in adjacent projections is present when the respective shortest distance between adjacent projections decreases, starting from the tube wall to the point of the projections that is furthest away from the tube wall. In other words: the adjacent projections forming a cavity incline toward one another.

In other words, the cavity is formed with the opposing concave surfaces of adjacent projections. Thus, the surfaces of the adjacent projections forming a cavity extend over it in the manner of a vault. Protrusion height is conveniently defined as the dimension of a protrusion in the radial direction. The height of the projection is then, in the radial direction, the distance starting from the pipe wall to the point of the projection which is furthest away from the pipe wall.

The notch depth of the notches is the distance measured in the radial direction from the original rib tip to the deepest point of the notch. In other words: the notch depth is the difference between the original rib height and the remaining rib height at the deepest point of a notch.

The invention is based on the consideration that the cavities formed between the tube wall and the folded-over projections or between adjacent projections form the cavities according to the invention. To produce the cavities, the projections are cut and set up or folded over in such a way that they form such cavities. There are also possibilities, not according to the invention, in which the projections touch the tube wall or form cavities without direct contact. The production can take place directly via adapted cutting geometries or via a secondary forming process, whereby the secondary tool used can be smooth or have an additional structure.

In principle, the tubes can be arranged horizontally or vertically during evaporation, for example on the inside of the tube. There are also cases where the pipes are slightly inclined from the horizontal or the vertical. In refrigeration technology, evaporators with horizontal tubes are usually used. In contrast, vertical circulation evaporators are often used in chemical engineering to heat distillation columns. The evaporation of the substance takes place on the inside of vertical tubes.

In order to enable heat transport between the heat-emitting medium and the evaporating substance, the temperature of the heat-emitting medium must be higher than the saturation temperature of the substance. This temperature difference is called the driving temperature difference. The higher the driving temperature difference, the more heat can be transferred. On the other hand, the aim is usually to keep the driving temperature difference small, as this is advantageous for process efficiency.

The cavities according to the invention intensify the process of nucleate boiling in order to increase the heat transfer coefficient during evaporation. The formation of bubbles begins at nucleation sites. These nucleation sites are mostly small gas or vapor inclusions. When the growing bubble reaches a certain size, it detaches from the surface. If the nucleation site is flooded with liquid in the course of the bubble detachment, then the nucleation site is deactivated. The surface must therefore be designed as a cavity in such a way that when the bubble detaches, a small bubble remains, which then serves as the nucleus for a new cycle of bubble formation. This is achieved by arranging cavities on the surface in which a small bubble can remain after the bubble has detached.

According to the invention, the tips of at least two projections touch or cross one another along the course of the ribs. This is particularly advantageous in reversible operation during the phase change, since the projections for the liquefaction protrude far out of the condensate and form a kind of cavity for the evaporation.

In an embodiment not according to the invention, the tips of at least two projections can touch or cross one another across the primary groove. This is advantageous in reversible operation during the phase change, since the projections for the liquefaction in turn protrude far out of the condensate and form a kind of cavity for the evaporation.

In contrast, in a configuration not according to the invention it is also possible for the distance between the tip of the projection and the tube wall to be less than the remaining height of the ribs. This gives the projection a hook-like or loop-like shape directly above the pipe wall. Such rounded shapes are particularly advantageous for bubble nucleation in evaporation processes.

In another embodiment not according to the invention, at least one of the projections can be deformed in such a way that its tip touches the inside of the pipe. As a result, a bubble nucleus is formed by a hook-like or eyelet-like shape of the projection during the phase transition of a fluid heat transfer medium close to the tube wall. A particularly intensive heat exchange into the fluid takes place there via the tube wall.

In an advantageous embodiment of the invention, the indentations can be formed by cutting the inner ribs with a cutting depth transverse to the rib path to form rib layers and by raising the rib layers with a main orientation along the rib path between primary grooves.

The process-side structuring of the heat exchanger tube according to the invention can be produced using a tool which in the DE 603 17 506 T2 is already described. The disclosure of this reference DE 603 17 506 T2 is fully included in the available documents. As a result, the projection height and the distance can be made variable and individually adapted to the requirements, for example the viscosity of the liquid or the flow rate.

The tool used has a cutting edge for cutting through the fins on the inner surface of the tube to create layers of fins and a lifting edge for lifting the layers of fins to form the projections. In this way, the protrusions are formed without removing metal from the inner surface of the tube. The projections on the inner surface of the tube can be in the same or different processing than the

formation of the ribs are formed.

This allows the protrusion height and distance to be made variable and individually adapted to the requirements of the fluid in question, for example with regard to the viscosity of the liquid and the flow rate.

Advantageously, the projections can vary in projection height, shape and alignment. As a result, the individual projections can be specifically adapted to one another and varied in relation to one another, so that, particularly in the case of laminar flow, the different rib heights dip into the different boundary layers of the flow in order to dissipate the heat to the tube wall. The height of the protrusion and the distance can also be individually adapted to the requirements, for example the viscosity of the fluid or the flow rate.

In a preferred embodiment of the invention, a projection can have a pointed tip on the side facing away from the tube wall. This leads to optimized projection tip condensation in condenser tubes using two-phase fluids.

In a particularly preferred embodiment, a projection can have a curved tip on the side facing away from the pipe wall, the local radius of curvature of which is reduced as the distance from the pipe wall increases along the course of the projection. This has the advantage that, particularly in the event of condensation, the condensate that forms at the tip is transported more quickly to the base of the fins due to the convex curvature, thus optimizing the heat transfer during liquefaction. During the phase change, here in particular during the liquefaction, the main focus is on the liquefaction of the vapor and the discharge of the condensate away from the tip towards the bottom of the fins. A convexly curved projection forms an ideal basis for effective heat transfer. The base of the projection protrudes essentially radially from the tube wall. The same or similar structural elements can thus be equally suitable both for an evaporator tube and for a condenser tube.

Exemplary embodiments of the invention are explained in more detail with reference to the schematic drawings. The drawings 2, 3, 7 and 8, on the other hand, do not show embodiments according to the invention.

Fig. 1

schematisch eine Schrägansicht eines Rohrausschnitts des Wärmeübertragerrohrs mit einer erfindungsgemäßen Struktur auf der Rohrinnenseite;

Fig. 2

schematisch eine Schrägansicht eines Rohrausschnitts des Wärmeübertragerrohrs mit einer nicht erfindungsgemäßen Struktur;

Fig. 3

schematisch eine Schrägansicht eines Rohrausschnitts des Wärmeübertragerrohrs mit einer nicht erfindungsgemäßen Struktur auf der Rohrinnenseite;

Fig. 4

schematisch einen Rippenabschnitt mit unterschiedlicher Kerbtiefe;

Fig. 5

schematisch einen Rippenabschnitt mit zwei sich entlang dem Rippenverlauf sich gegenseitig berührenden Vorsprüngen;

Fig. 6

schematisch einen Rippenabschnitt mit zwei sich entlang dem Rippenverlauf sich gegenseitig überkreuzenden Vorsprüngen;

Fig. 7

schematisch einen Rippenabschnitt mit zwei sich über die Primärnut hinweg gegenseitig berührenden Vorsprüngen; und

Fig. 8

schematisch einen Rippenabschnitt mit zwei sich über die Primärnut hinweg gegenseitig überkreuzenden Vorsprüngen.

Show in it:

1

schematically an oblique view of a tube section of the heat exchanger tube with a structure according to the invention on the inside of the tube;

2

schematically an oblique view of a tube section of the heat exchanger tube with a structure not according to the invention;

3

schematically an oblique view of a tube section of the heat exchanger tube with a structure not according to the invention on the inside of the tube;

4

schematically a rib section with different notch depth;

figure 5

schematically a rib section with two mutually touching projections along the course of the ribs;

6

schematically a rib section with two mutually crossing projections along the course of the ribs;

7

schematically a rib section with two mutually touching projections across the primary groove; and

8

schematically shows a rib section with two mutually crossing projections across the primary groove.

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

1 shows schematically an oblique view of a pipe section of the Heat exchanger tube 1 with a structure according to the invention on the tube inside 22. The heat exchanger tube 1 has a tube wall 2, a tube outside 21 and a tube inside 22. On the tube inside 22, continuously running, helically circumferential ribs 3 are formed from the tube wall 2. The longitudinal axis A of the tube runs at a certain angle relative to the ribs 3 . Continuously extending primary grooves 4 are formed between each adjacent ribs 3 .

A plurality of projections 6 are deformed in pairs to one another to such an extent that cavities 10 are formed between adjacent projections 6 . In this case, the tips 61 of at least two projections 6 touch one another along the course of the ribs.

The projections 6 are formed by cutting the ribs 3 with a cutting depth across the rib path to form layers of ribs and raising the layers of ribs with a main orientation along the rib path between primary grooves 4 . The indentations 7 between the projections 6 can also be formed in a rib 3 with an alternating indentation depth.

2 1 schematically shows an oblique view of a tube section of the heat exchanger tube 1 with a structure not according to the invention. A plurality of projections 6 are deformed in pairs to one another to such an extent that cavities 10 are formed between adjacent projections 6 . In this case, the tips 61 of at least two projections 6 extend beyond the primary groove 4 and touch one another. However, the tips 61 of projections 6 deformed in pairs relative to one another can also still have a certain distance from one another. However, this is so small that effective cavities 10 are nevertheless formed.

The projections 6 are in turn made by cutting the ribs 3 with a cutting depth transverse to the rib course to form rib layers and by raising the rib layers with a main orientation along the Rib course between primary grooves 4 formed. The indentations 7 between the projections 6 can also be formed in a rib 3 with an alternating indentation depth.

3 1 schematically shows an oblique view of a tube section of the heat exchanger tube 1 with a further structure not according to the invention on the inside 22 of the tube.

In this case, the distance between the tips 61 of a projection and the tube wall is less than the residual rib height. As a result, a hook-like shape is created. However, a projection 6 can be deformed in such a way that its tip 61 touches the inside 22 of the tube. In this in figure 3 not shown case preferably creates a loop-like shape. The projections 6 are in turn by cutting the ribs 3 analogous to the Figures 1 and 2 educated.

4 1 schematically shows a rib section 31 with different notch depths t ₁ , t ₂ , t ₃ . Within the scope of the _invention , the terms _cutting depth and _notch depth represent the same terminology. Dashed is indicated in the 4 the original formed helical circumferential rib 3. From this the protrusions 6 are cut by cutting the rib 3 with a notching/cutting depth t ₁ , t ₂ , t ₃ transverse to the rib course to form rib layers and by raising the rib layers with a main orientation along the Shaped ribs. The different notch/cutting depths t ₁ , t ₂ , t ₃ are consequently dimensioned based on the notch depth of the original rib in the radial direction.

The projection height h is in 2 is plotted as the dimension of a protrusion in the radial direction. The projection height h is then in the radial direction Distance starting from the tube wall to the point of the projection that is furthest away from the tube wall.

The notch depth t ₁ , t ₂ , t ₃ is the distance measured in the radial direction, starting from the original rib tip to the deepest point of the notch. In other words:
The notch depth is the difference between the original rib height and the remaining rib height at the deepest point of a notch.

figure 5 FIG. 1 schematically shows a rib section 31 with two projections 6 touching one another along the course of the rib. FIG 6 shows schematically a rib portion 31 with two mutually crossing projections 6 along the course of the rib. Also 7 shows schematically a rib section 31, which is not according to the invention here, however, with two projections 6 touching one another across the primary groove. 8 shows schematically a further rib section 31 not according to the invention with two mutually crossing projections 6 beyond the primary groove.

At the in the Figures 5 to 8 The structural elements shown are advantageous, especially in reversible operation with two-phase fluids, that they form a type of cavity 10 for evaporation. The cavities 10 of this special type form the starting points for bubble nuclei of an evaporating fluid.

Reference List

1

heat exchanger tube

2

pipe wall

21

pipe exterior

22

pipe inside

3

rib

31

rib section

4

primary groove

6

head Start

61

Top

7

notches

10

cavity

A

longitudinal axis of the pipe

t1

first cutting depth

t2

second cutting depth

t3

third cutting depth

H

protrusion height

Claims (5)

1. Heat transfer pipe (1) having a longitudinal pipe axis (A), wherein

   - continually extending, axially parallel or helically circumferential ribs (3) are formed from the pipe wall (2) at the inner pipe side (22),

   - continuously extending primary grooves (4) are formed in each case between adjacent ribs (3),

   - the ribs (3) have at least one structured region at the inner pipe side (22),

   - the structured region has a plurality of projections (6) which protrude from the surface and which have a projection height (h),

   whereby the projections (6) are separated by notches (7), wherein a plurality of projections (6) are deformed with respect to each other in pairs to such an extent that cavities (10) are formed between adjacent projections, characterised in that the tips (61) of at least two projections (6) contact each other or intersect along the rib path.
2. Heat transfer pipe (1) according to claim 1, characterised in that the notches (7) are formed by cutting the inner ribs (3) with a cutting depth (t₁, t₂, t₃) transversely relative to the rib path in order to form rib layers and by raising the rib layers with a main orientation along the rib path between primary grooves (4).
3. Heat transfer pipe (1) according to claim 1 or claim 2, characterised in that the projections (6) vary relative to each other in terms of projection height (h), shape and orientation.
4. Heat transfer pipe (1) according to any one of claims 1 to 3, characterised in that a projection (6) has an acutely tapering tip (61) at the side facing away from the pipe wall (2) .
5. Heat transfer pipe (1) according to any one of claims 1 to 4, characterised in that a projection (6) at the side facing away from the pipe wall (2) has a curved tip (61) whose local radius of curvature is reduced with increasing spacing from the pipe wall (2) along the projection path.

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