EP1156294B1 - Tube for evaporative heat exchanger with pores having different size - Google Patents

Description

The invention relates to a metallic heat exchange tube, in particular for 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 as well as in process and power engineering. In the art are often tube bundle heat exchangers used in which liquids of pure substances or mixtures Evaporate on the outside of the pipe and on the inside of the pipe a brine or cool water. Such apparatuses are referred to as flooded evaporators.

By intensifying the heat transfer on the outside of the pipe and inside the pipe The size of the evaporator can be greatly reduced. Take this the manufacturing cost of such apparatus. In addition, the necessary decreases Filling amount of refrigerant used in the chlorine-free today used predominantly Safety refrigerants a not inconsiderable cost share of the total investment costs. For toxic or flammable refrigerants can be further reduced by a reduction in the amount of potential hazards decrease. The today usual high performance pipes are about a factor of three more efficient than smooth tubes of the same diameter.

The present invention relates to structured pipes in which the Heat transfer coefficient is intensified on the outside of the pipe. Because of this the majority of the heat transfer resistance often on the inside The heat transfer coefficient must be on the inside in the Usually also be intensified. Heat exchanger tubes for tube bundle heat exchanger usually have at least one structured area as well smooth end pieces and possibly smooth intermediate pieces. The smooth end or Spacers limit the structured areas. So that the tube easily may be installed in the tube bundle heat exchanger, the outer may Diameter of the structured areas should not be greater than the outer diameter the smooth end and intermediate pieces.

To increase the heat transfer during evaporation, the process of Bubble boiling intensified. It is known that the formation of bubbles at germinal sites starts. 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 releases itself from the surface. If in the course of bladder detachment, the germinal site by inflowing Liquid is flooded, u.U. the gas or steam inclusion by Liquid displaced. In this case, the germinal site is inactivated. This is possible avoid by a suitable design of the germinal sites. For this it is necessary that the opening of the germinal site is smaller than the underlying cavity, such as e.g. at undercut structures.

It is state of the art to have such structures based on integrally rolled To produce finned tubes. Under integrally rolled finned tubes are finned Tubes understood in which the ribs of the wall material of a smooth tube were formed. Here, various methods are known with which the be sealed between adjacent ribs channels, that connections between the channel and the environment in the form of pores or slits stay. Through this, the transport of liquid and steam can take place. In particular, such substantially closed channels are bent over or folding the rib (US 3,696,861, US 5,554,548), by columns and Pinching the rib (DE 2,758,526, US 4,577,381), and by notching and upsetting rib (US 4,660,630, EP 0,713,072, US 4,216,826).

The well-known property rights are aimed at the most uniform, defined Channel and pore size to produce. U.S. Patent 5,054,548 each according to the medium to be evaporated (high-pressure or low-pressure refrigerant) given different sized optimal pore sizes. This consideration sets that the pore system is best built from equally sized pores.

In JP-OS 63-172.892 a method is described, with the self-contained, large and small cavities are generated. This happens through Widening of the rolled rib channels at regular intervals. The single ones Cavities are connected by different sized pores with the outside space; however, large and small cavities are separated. The goal of the JP-OS 63-172.892 is the creation of a structure that, at different Heizflächenbelastungen, expressed by wall overheating, work consistently should. The large cavities and pores should be at high wall overheating the Heat transfer ensure the separated therefrom small cavities and Pores, however, with small wall overheating. This approach sets Also, again, assume that each one pore size for a given Operating state (heating surface load, saturation conditions, evaporating Fabric) is optimal. The widening of the ribs is by a toothed Achieved slice that is thicker than the channel width between the ribs. hereby The ribs are pressed apart at the expansion point on both sides. As a result, the two adjacent channels will be at this point closed, resulting in individual separate cavities. At The expansion point creates a relatively large opening.

In JP-OS 54-16,766 a heat exchanger surface with large and suggested small pore openings, wherein the pores are arranged so that all the big pores on one side of the tube and all the small pores on the tube located on the other side of the pipe. Such a tube is for horizontal Installation provided in a tube bundle apparatus. The installation must be so Make sure the big pores are up and the little pores are down are directed. The liquid is then sucked through the small pores, the Steam expelled up through the large pores. Such an installation in However, a predetermined orientation is in a mass production of Heat exchangers not feasible, since the pipes usually by a Rolling be connected to the apparatus and in this Einwalzvorgang the tube is rotated about its axis by an uncontrollable angle. It should also be noted that in this pipe concept from fluidic Reasons the channels must have a very large volume. This conditionally disadvantageous high pipe weights and a large layer thickness of the outer structure. The latter leads to a small inner tube cross-section and thus to undesirably high pressure drop of the pipe side flowing medium.

No. 5,597,039 (or US Pat. No. 5,896,660) describes an evaporator tube with bent-over tubes Rib tips, with the rib tips before bending over with notches be provided. Here, each adjacent notches of a rib have different Shape and / or size, so that a system of different Pore openings formed. The decisive factor is that immediate adjacent openings are different sizes. Depending on the operating condition, expressed by the Heizflächenbelastung, which is for the operating condition be activated most favorable type of pores. The many different pores serve to provide the tube with good vaporization properties over a wide range of operating conditions to rent. However, the non-active pores do not carry to the evaporation process. On the other hand, they reduce the density of the active ones Bubble nuclei and can thus the heat transfer properties of the Pipe even deteriorate.

The invention is based on the object, a heat exchanger tube of said Type with improved heat transfer properties Evaporation of substances on the tube outside to produce. The heat transfer properties should be on the properties of the substance to be vaporized and be adaptable to the operating condition.

The object is achieved by the characterizing features of Claim 1 solved.

The size of a single pore can be precisely defined and measured. Due to the manufacturing process and due to tolerances in material and Tools have two arbitrarily chosen pores practically never the same shape and shape Size. The pore size is subject to statistical fluctuations. It is because of that useful to divide the pores according to their size in size classes, where the pores with a finite distribution range group around frequency maxima. Different sized pores according to the invention should then be present if, in the frequency diagram according to FIG. 5, the abscissa values are adjacent Maxima of the frequency distribution by at least 50% of the smallest pore class different abscissa value.

For metrological determination of pore size and frequency distribution, for example, a suitable image processing system consisting of optical image capture unit and digital evaluation unit is used. The surface of the tube is captured photographically and the image is sorted in grayscale. By a suitable choice of a gray level threshold, the image of the tube surface is divided into pore areas and areas of metallic surface. The pore areas are then geometrically measured and evaluated digitally. FIG. 5 shows the frequency distribution of the pore size determined by means of such a system on a tube pattern according to the invention (compare the numerical example discussed later). The pore size is characterized by the area of the pore opening, measured in μm ² . One recognizes two maxima in the histogram. The class of small pores is grouped around the maximum at a pore area A _k , the class of large pores is grouped around the maximum at a pore area A _g . The values A _g and A _k can thus be interpreted in each case as the average pore size of the two pore classes. The frequency ratio N _k / N _g (number N _{k of} the small pores to the number N _{g of} the large pores) is designated by m.

According to the invention, the channels located between the ribs pass through Material of the upper rib areas substantially closed, the so resulting cavities are connected by pores with the surrounding space. These pores are designed to be typically in two size categories can be divided. After a regular, repetitive scheme follow along the channels to each a certain number of small pores one or several large pores. Through this structure, a directed flow in created the channels. Fluid gets through the small pores with support drawn in by the capillary pressure and wets the channel walls, creating thin films be generated. The steam accumulates in the center of the canal and escapes in the places with the lowest capillary pressure.

These are the large pores arranged at certain intervals. Size ratio A _g / A _k and frequency distribution m of the pores are chosen so that the steam can escape without too much liquid penetrates into the channels and flooding them, whereby the thermally efficient very thin film evaporation would come to a standstill. On the other hand, the vapor pores must be large enough that the vapor is not dammed back in the pores.

Preferred embodiments of the invention are subject matter of claims 2 to 4. In particular. Claim 4 is directed to the ratio F _k / F _g .

For the ratio of the total opening area F _{k of} all small pores to the total opening area F _{g of} all large pores, the following applies:

The ratio of the total opening area must be matched to the properties of the medium used at the operating point. In particular, when designing the pore geometry, it should be borne in mind that this ratio should be proportional to the square root of the density ratio of vapor ρ _D and liquid ρ _FI : F k F G ~ ρ D ρ Fl

Thus, the pore structure of the properties of the material used and the Operating state, in particular the pressure, adaptable.

The invention further provides a process for the preparation of the invention Heat exchange tube.

Starting from the method according to US-PS 5,896,660 according to the preamble of Claim 5, the method according to the invention is characterized in that that the notch by large and small arranged on the circumference of the notch disc Teeth is effected; the notched rib tips are due to radial pressure up compressed to the level of notch.

An apparatus for carrying out the method according to the preamble of claim 6 is characterized in that the notched disc has small and large teeth on the circumference in a regular arrangement, each followed by a large number of small teeth or a large tooth and the teeth Ratio of the number of small teeth to the number of large teeth m = 12: 1 to 1: 5; on the notch disc follows a compression roller.
(The ratio is naturally identical to m = N _k / N _g .)

Claims 7 to 12 relate to preferred embodiments of the device.

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

Figur 1

schematisch die Oberfläche eines erfindungsgemäßen Wärmeaustauschrohres mit zwei Porenklassen,

Figur 2

eine Vorrichtung zur Herstellung des Wärmeaustauschrohres,

Figur 3

den Teilausschnitt einer Kerbscheibe,

Figur 4

schematisch die gerichtete Dampfströmung entlang eines Rippenkanals,

Figur 5

beispielhaft die Häufigkeitsverteilung von großen und kleinen Poren,

Figur 6

den Wärmeübergangskoeffizienten auf der Rohraußenseite als Funktion der Heizflächenbelastung für drei unterschiedlich gestaltete Porensysteme.

It shows:

FIG. 1

2 shows schematically the surface of a heat exchange tube according to the invention with two pore classes,

FIG. 2

a device for producing the heat exchange tube,

FIG. 3

the partial section of a notched disc,

FIG. 4

schematically the directed vapor flow along a rib channel,

FIG. 5

for example, the frequency distribution of large and small pores,

FIG. 6

the heat transfer coefficient on the pipe outside as a function of the Heizflächenbelastung for three differently designed pore systems.

The integrally rolled finned tube 1 according to FIGS. 1 and 2 has helical circumferential ribs 2 on the tube outside, between which a groove 3 is formed. Material of the rib tips 2 'is displaced such that the rib gaps are closed to form channels 4 except for large pores 5 (area A _g ) and small pores 6 (area A _k ). The channels 4 run around with a substantially uniform cross-section.

The production of the finned tube 1 according to the invention is carried out by a rolling process (see U.S. Patent Nos. 1,865,575 / 3,327,512) by means of that shown in FIG Contraption.

It is used a device consisting of n = 3 tool holders 7, in the in each case a rolling tool 8 and at least one downstream notching disc 9 and a compression roller 10 are integrated (in Figure 2, only one tool holder 7th shown. But it can, for example, four or more tool holder. 7 be used). The tool holder 7 are each offset by α = 360 ° / n on Circumference of the finned tube arranged. The tool holder 7 are radially deliverable.

They are in turn arranged in a stationary (not shown) rolling head (According to another variant, the tube is only rotating with rolling head axially advanced).

The running in the direction of arrow in the device smooth tube 1 'is characterized by the on Circumferentially arranged, driven rolling tools 8 set in rotation, wherein the axes of the rolling tools 8 extend obliquely to the tube axis. The rolling tools 8 exist in a conventional manner of several side by side arranged rolling disks 11 whose diameter increases in the direction of the arrow. The Centrally arranged rolling tools 8 form the helical circumferential ribs 2 from the pipe wall of the smooth tube 1 ', wherein the smooth tube 1 'here by a rolling dome 12 is supported.

The rib tips 2 'are notched by means of the notching disc 9, which according to FIG 3 regularly and circumferentially distributed large and small teeth 13 and 14 respectively having. In the notching disk 9 shown in Figure 3 follows in each case three similar small teeth 14 a big tooth 13.

Finally, the upsetting of the notched rib tips by the compression roller takes place 10, whereby two pore classes, namely the large pores 5 and the small Pores 6, arise. The large pores 5 are thereby formed at the points on where the big teeth 13 of the notching disc 9 have left their mark.

In addition, in FIG. 3, the width b at the tip of the small teeth 14 is the width B registered at the top of the large teeth 13 and the flank angle β indicated.

Bring the tube outer surface with a liquid to be evaporated in contact (See Fig. 4), then by the inventive design of the channels 4 and of the pores 5, 6 reaches that of the channel walls 15 of a liquid film 16 be wetted.

The phase change from liquid to vapor then does not happen Bubble evaporation, but by thin-film evaporation at the channel walls 15. In this case, the pore system has two different tasks to fulfill. The liquid must first be in the area below the outer tube surface Channels 4 are transported. After evaporation, the resulting Steam 17 can escape to the outside.

So that the evaporation process can be maintained, the equal amounts of liquid and steam 17 in opposite directions Directions are transported through the pores 5,6. Otherwise, the Channels 4 are either flooded with liquid or they dry out. In both cases the evaporation process is greatly impaired or comes in the channels. 4 to a halt.

In order to transport the generated steam 17 from the channels 4, must in the channels 4 a higher pressure prevail than in the outer space. This overpressure is determined by the vapor pressure curve of the medium to be evaporated by the Overtemperature of the pipe wall set.

Usually liquids are used, which wet the pipe material well. Such a liquid can penetrate into the channels 4 due to the capillary effect through the pores 5, 6 in the outer tube surface against an overpressure. In each pore 5, 6 a liquid meniscus is formed, on the curved surface of which, due to the surface tension, a discontinuity of the pressure arises. This pressure difference is called capillary pressure p _k and is determined for spherically curved liquid surfaces by the following relationship: P k = 2 · σ r

In this equation, σ is the surface tension and r is the radius of curvature of the meniscus surface. The radius of curvature r depends on the contact angle Θ and the pore shape. For pores 5, 6 of circular cross section and pore radius R _{p, the} following applies: P k = 2 · σ · cos R p

For pores 5, 6 with non-circular cross section, similar relationships can be derived. It can be seen that the largest capillary pressure at the pores 6 with can set the smallest radius. The liquid penetrates through the small one Pore 6 in the channel 4, forms a thin film 16 on the channel walls 15 and evaporates under heat. The steam 17 escapes through the larger ones Pores 5, since the capillary pressure is lower at these. So it forms one of the small pores 6 toward the large pores 5 directed flow. This is shown schematically in Figure 4.

So that sufficient liquid can be transported into the channels 4, must have a sufficient amount of smallest possible pores 6 available be put. At the same time, the large pores 5 must be dimensioned that the steam 17 can escape sufficiently quickly and the channels 4 not dry out. The size and frequency of the steam pores 5 in relation to the smaller liquid spores 6 are therefore extremely critical sizes.

It may prove convenient to add more than two size classes of pores use. The fluid always penetrates through the pores of the smallest Class in the channel while the steam escapes through the larger ones.

Numerical example:

The influence of the design of the pore system on the performance of the Tube 1, expressed by the heat transfer coefficient on the tube outside depending on the Heizflächenbelastung, is based on three different designed pore systems are shown.

The helical circumferential channels 4 have a pitch of 0.5 mm and a height of 0.75 mm total. The outer diameter of the tube 1 is about 19 mm.

The geometric data of the notched discs 9 used are summarized in Table 1; FIG. 3 shows a schematic illustration of such a notch disk 9. The larger the width B at the tip of the large teeth 13, the larger the pore area of the large pores 5. No. Notch division t Flank angle β Width b Width B Frequency ratio m 1 0.50 mm 25 ° 0.20 - - 2 0.50 mm 25 ° 0.20 0.40 8: 1 3 0.50 mm 25 ° 0.20 0.60 8: 1

The effect on the external heat transfer coefficient as a function of Heating surface load is exemplary for the refrigerant R-22 at 14.4 ° C saturation temperature shown in FIG.

Compared to a notched disc 9 with constant tooth width (see no. 1), i.e. Pores of the same size, you get an improvement in notched disc No. 2 the heat transfer by about 30%.

FIG. 5 shows the frequency distribution of the pore size determined on the tube pattern according to the invention. The class of small pores 6 is grouped around the maximum at a pore area of about A _k = 30000 μm ² , the class of large pores 5 is grouped around the maximum at a pore area of about A _g = 75000 μm ² .

If you continue to enlarge the pores of the steam 5, as in the case of notched disc No. 3, you get compared to the uniform pores, a 25 to 45% reduced heat transfer coefficient on the outside of the pipe. In this case, the steam pores 5 too large, the channels 4 are flooded with liquid and the thin film evaporation collapses.

It turns out that the dimensions of the pores 5, 6 as well as the frequency of larger steam pores 5 decisive influence on the function and thus the Having efficiency of structure.

The present observations show that less the channel size, but rather, the pore size for the function and thus the heat transfer prevail are. Due to the lack of widening of the channels (see JP-OS 63-172,892, local figure 5 and 7) are each adjacent channels not in a negative way affected.

U.S. Patent 4,729,155 relates to adjacent channels passing through smaller transverse openings are interconnected. The present invention however, refers to closed channels, where a directed flow present as described above. Cross connections between the channels lead to a disorder of directional flow and are therefore not for this concept useful.

Claims (12)

1. Metal heat exchange tube, in particular for evaporating fluids comprising pure substances or mixtures on the outer side of the tube, having integral ribs (2) which extend in an annular or helical manner on the outer side of the tube and which are shaped to form substantially closed channels (4),
   the channels (4) extending with a substantially identical cross-section and being open outwardly in alternation by means of pores (5, 6) having at least two different sizes, characterised by the following features:

   a) the reciprocal relationship of the mean pore surface-area A_k (6) of the smallest pore class relative to the mean pore surface-area A_g (5) of the next-largest pore class is: Ag/Ak = from 1.5 to 4;

   b) the frequency relationship m = number N_k of pores (6) of the smallest pore class relative to the number N_g of pores (5) of the next-largest pore class is: m = Nk Ng = from 12:1 to 1:5.
2. Heat exchange tube according to claim 1, characterised in that Ag/Ak = from 2 to 3 and m = Nk Ng = from 9:1 to 1:3.
3. Heat exchange tube according to claim 1 or 2, characterised in that it has two pore classes.
4. Heat exchange tube according to claim 3, characterised in that
   the relationship of the entire opening surface-area F_k of all the small pores (6) relative to the entire opening surface-area F_g of all the large pores (5) is co-ordinated with the properties of the medium used by means of: Fk Fg ∼ ρD ρFl ; with

   

   and ρ_D = density of the vapour and ρ_Fl = density of the fluid.
5. Method for producing a heat exchange tube having integral ribs (2) which extend in a helical manner on the outer side according to any one or more of claims 1 to 4, wherein the following method steps are carried out:

   a) helically extending ribs (2) are rolled on the outer surface of a smooth tube (1') by the rib material being obtained by material being displaced outwards from the tube wall by means of a rolling operation and the resultant ribbed tube (1) being caused to rotate by the rolling forces and/or being advanced in accordance with the resultant ribs (2), the ribs (2) being formed from the otherwise unformed smooth tube (1') with increasing height,

   b) the smooth tube (1') is supported by a roll mandrel (12) located therein,

   c) after the ribs (2) are formed, the rib tips (2') are notched by a notched disc (9),
      characterised by:

   c') the notching is brought about by large and small teeth (13, 14) which are arranged on the periphery of the notched disc (9),

   d) the notched rib tips (2') are swaged by radial pressure as far as the level of the notching.
6. Device for carrying out the method according to claim 5 in order to produce a heat exchange tube according to any one or more of claims 1 to 4, having the following features:

   a) at least two radially adjustable tool holders (7) which are arranged in a stationary rolling head in a mutually staggered manner are provided at the periphery of the ribbed tube (1),

   b) the tool holders (7) each have a driven rolling tool (8) which comprises a plurality of rolling discs (11) and which has an axis which is inclined relative to the tube axis,

   c) the rolling discs (11) having an increasing diameter,

   d) a notched disc (9) is connected downstream of the rolling tool (8) in at least one tool holder (7),
   characterised by the following features:

   d') the notched disc (9) has, in a regular arrangement on the periphery, large and small teeth (13, 14), a large tooth (13) or a plurality of large teeth (13) following a given number of small teeth (14) and the relationship of the number of small teeth (14) relative to the number of large teeth (13) being m = from 12:1 to 1:5, the relationship A_g/A_k being fixed between 1.5 and 4 by the teeth (13, 14),

   e) a swaging roller (10) follows the notched disc (9).
7. Device according to claim 6, characterised in that the relationship m = from 9:1 to 1:3.
8. Device according to claim 6 or 7, characterised in that the notched disc (9) has from 8 to 25 teeth (13, 14) per cm of periphery.
9. Device according to any one or more of claims 6 to 8, characterised in that,
   when the teeth (13, 14) are of trapezoidal form, the relationship of the width B of the tip of a large tooth (13) to the width b of the tip of a small tooth (14) B/b = from 1.2 to 4.
10. Device according to claim 9, characterised in that the relationship B/b = from 1.5 to 3.
11. Device according to any one or more of claims 6 to 10, characterised in that
    the notched disc (9) is indented in a straight manner.
12. Device according to any one or more of claims 6 to 10, characterised in that the notched disc (9) is indented in an inclined manner.

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