EP1156294A2 - Tube for evaporative heat exchanger with pores having different size - Google Patents

Abstract

The heat exchanger pipe (1) has integral annular or helical ribs (2) on the outer side of the pipe and deformed to form closed channels (4). The channels have continuous cross-section and are opened by outer pores (5,6) in at least two different sizes. The reciprocal ratio of the average sizes of the smallest sized pores (6) to the average sizes of the next largest sized pores (5) is 1.5 - 4. The frequency ratio of the number of smallest sized pores to the number of the next largest sized pores is between 12:1 and 1:5. An Independent claim is included for a method to manufacture the heat exchanger pipe.

Description

The invention relates to a metallic heat exchange tube, in particular for Evaporation of liquids from pure substances or mixtures on the outside of the pipe, 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 often used in technology used in which liquids of pure substances or mixtures Evaporate on the outside of the pipe and a brine on the inside of the pipe or cool water. Such devices are referred to as flooded evaporators.

By intensifying the heat transfer on the outside and inside of the pipe the size of the evaporators can be greatly reduced. Take this through the manufacturing costs of such apparatus. In addition, the necessary one drops The amount of refrigerant used in the chlorine-free products that are mainly used today Safety refrigerants have a not negligible share of the costs can account for total investment costs. With toxic or flammable refrigerants the hazard potential can also be reduced by reducing the filling quantity belittle. Today's high-performance pipes are around a factor of three more powerful than smooth pipes of the same diameter.

The present invention relates to structured tubes in which the Heat transfer coefficient is intensified on the outside of the pipe. Because of this the main part of the thermal resistance is often on the inside is shifted, the heat transfer coefficient on the inside in the Usually also be intensified. Heat exchanger tubes for shell and tube heat exchangers usually have at least one structured area as well smooth end pieces and possibly smooth intermediate pieces. The smooth end or Intermediate pieces limit the structured areas. So that the pipe easily can be installed in the shell and tube heat exchanger, the outer The diameter of the structured areas must not be larger than the outer diameter of 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 germ sites begins. These germ sites are mostly small gas or vapor inclusions. Such germ sites can already be created by roughening the surface. When the growing bladder has reached a certain size, it comes off the surface. If in the course of the blister detachment the germ site through inflowing If liquid is flooded, the gas or steam inclusion through Displaced liquid. In this case the germ site is inactivated. This can be done avoid by a suitable design of the germ sites. For this it is necessary that the opening of the germ site is smaller than the cavity below, such as e.g. with undercut structures.

It is prior art to make such structures on the basis of integrally rolled To manufacture finned tubes. Fins are finned under integrally rolled finned tubes Pipes understood in which the fins are made of the wall material of a smooth pipe were formed. Various methods are known with which the channels located between adjacent ribs are closed in such a way that connections between channel and environment in the form of pores or slits stay. These can be used to transport liquid and steam. In particular, such essentially closed channels are made by bending or folding the rib (US 3,696,861, US 5,054,548), by splitting and Compression of the rib (DE 2,758,526, US 4,577,381), and by notching and compression the rib (US 4,660,630, EP 0.713.072, US 4,216,826).

The known property rights aim to ensure that the definition is as uniform as possible Generate channel and pore size. In US-PS 5,054,548 each depending on the medium to be evaporated (high-pressure or low-pressure refrigerant) different sized optimal pore sizes specified. This consideration sets assumes that the pore system is best built up from pores of the same size.

JP-OS 63-172.892 describes a method with which separate, large and small cavities are generated. This happens through Widen the rolled rib channels at regular intervals. The single ones Cavities are connected to the outside by pores of different sizes; however, large and small cavities are separated. The goal of the JP-OS 63-172.892 is the creation of a structure with different heating surface loads, expressed by wall overheating, work consistently should. The large cavities and pores are intended to prevent the wall from overheating Ensure heat transfer, the small cavities and Pores, however, when the wall overheats. This approach sets again again ahead that a pore size for a given Operating condition (heating surface load, saturation conditions, evaporating Fabric) is optimal. The widening of the rib channels is through a toothed Disc reached, which is thicker than the channel width between the ribs. Hereby the ribs at the point of expansion are pushed far apart on both sides. As a result, the two adjacent channels are at this point closed, which creates separate cavities. On the expansion site has a relatively large opening.

In JP-OS 54-16.766 a heat exchanger surface with large and proposed small pore openings, the pores being arranged so that all the large pores on one side of the tube and all the small pores on the the other side of the pipe. Such a tube is for the horizontal Installation in a tube bundle provided. However, the installation must be so that the large pores face up and the small pores face down are directed. The liquid is then sucked in through the small pores Steam expelled up through the large pores. Such an installation in However, a predetermined orientation is in the case of large series production of Heat exchangers not feasible, since the pipes are usually through a Rolling process are connected to the apparatus and this rolling process the tube rotates around its axis by an uncontrollable angle. It should also be borne in mind that this pipe concept is based on fluid dynamics Reasons the channels must have a very large volume. This causes disadvantageously 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 an undesirably high pressure drop in the medium flowing on the pipe side.

US 5,597,039 (or US Pat. No. 5,896,660) describes an evaporator tube with bent Rib tips, taking the rib tips with notches before bending be provided. Here, adjacent notches of a rib have different ones Shape and / or size, so that a system of different Pore openings arise. It is considered decisive that immediate neighboring openings are of different sizes. Depending on the operating state, expressed by the heating surface load, that for the operating state cheapest pore type must be activated. The many different pores serve it also gives the tube good evaporation properties over a wide range of operating conditions to rent. However, the inactive pores do not wear to the evaporation process. However, they reduce the density of active ones Bladder germ sites and can thus the heat transfer properties of the Pipe even deteriorate.

The invention has for its object a heat exchanger tube of the above Kind with improved properties regarding heat transfer Evaporation of substances on the outside of the pipe. The heat transfer properties are aimed at the properties of the substance to be evaporated and be adaptable to the operating state.

The object is achieved by the characterizing features of the 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 Tool, two arbitrarily selected pores practically never have the same shape and Size. The pore size is subject to statistical fluctuations. It is therefore makes sense to divide the pores into size classes according to their size, whereby the pores are grouped around frequency maxima with a finite distribution width. Different sized pores in the sense of the invention should then exist if in the frequency diagram according to FIG. 5 the abscissa values are more adjacent Maxima of the frequency distribution by at least 50% of the smallest pore class distinguish the corresponding abscissa value.

A suitable image processing system, consisting of an optical image acquisition unit and a digital evaluation unit, is used, for example, to determine the pore size and frequency distribution by measurement. The pipe surface is captured photographically and the image sorted in grayscale. By appropriately choosing a gray value threshold, the image of the pipe surface is broken down into pore areas and areas of metallic surface. The pore areas are then geometrically measured and digitally evaluated. FIG. 5 shows the frequency distribution of the pore size determined using a system of this type on a tube pattern according to the invention (cf. the numerical example discussed later). The pore size is characterized by the area of the pore opening, measured in µm ² . Two maxima can be seen 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 as the average pore size of the two pore classes. The frequency ratio N _k / N _g (number N _k of small pores to number N _g of large pores) is denoted by m.

According to the invention, the channels located between the ribs are through Material of the upper rib areas essentially closed, the so emerging voids are connected to the surrounding space by pores. These pores are designed to be of typically two size classes can be classified. According to a regular, repeating scheme follow a certain number of small pores along the channels or several large pores. This structure creates a directional flow in the channels. Liquid is supported through the small pores of capillary pressure is drawn in and wets the channel walls, creating thin films be generated. The steam accumulates in the center of the channel and escapes at the points with the lowest capillary pressure.

These are the large pores arranged at certain intervals. The size ratio A _g / A _k and the frequency distribution m of the pores are selected so that the steam can escape without too much liquid penetrating into the channels and flooding them, which would bring the thermally very effective thin-layer evaporation to a standstill. On the other hand, the vapor pores must be chosen large enough that the vapor is not backed up in the pores.

Preferred embodiments of the invention are the subject 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 ratio of the total opening areas must be matched to the properties of the medium used at the operating point. When designing the pore geometry, particular attention must be paid to the fact 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

So the pore structure is related to the properties of the substance used as well as the Operating state, in particular the pressure, adjustable.

The invention further relates to a method for producing the invention Heat exchange tube.

Based on the method according to US Pat. No. 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 notched disk Teeth is effected; the notched rib tips are up to by radial pressure compressed to the level of the notch.

A device for performing the method according to the preamble of claim 6 is characterized in that the notched disk has small and large teeth on the circumference in a regular arrangement, each being followed by a large tooth or a plurality of large teeth after a certain number of small teeth, and wherein Ratio of the number of small teeth to the number of large teeth m = 12: 1 to 1: 5; a compression roller follows the notched disk.
(The ratio is naturally identical to m = N _k / N _g .)

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

The invention is explained in more detail using the following exemplary 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:

Figure 1

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

Figure 2

a device for producing the heat exchange tube,

Figure 3

the partial section of a notched disk,

Figure 4

schematically the directional steam flow along a rib channel,

Figure 5

exemplary the frequency distribution of large and small pores,

Figure 6

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

The integrally rolled finned tube 1 according to FIGS. 1 and 2 has, on the outside of the tube, ribs 2 running helically, between which a groove 3 is formed. Material of the rib tips 2 'is shifted in such a way that the rib interspaces are closed with the formation of 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 finned tube 1 according to the invention is produced by a rolling process (see US Pat. Nos. 1,865,575 / 3,327,512) by means of those shown in FIG Contraption.

A device is used which consists of n = 3 tool holders 7 into which in each case one rolling tool 8 and at least one subsequent notched disk 9 and an upsetting roller 10 are integrated (in FIG. 2 there is only one tool holder 7 shown. However, four or more tool holders 7 can be used, for example be used). The tool holder 7 are each offset by α = 360 ° / n on The circumference of the finned tube is arranged. The tool holder 7 are radially adjustable.

For their part, they are arranged in a stationary roller head (not shown) (According to another variant, the tube is only with the rolling head rotating axially advanced).

The smooth tube 1 'entering the device in the direction of the arrow is replaced by the Circumferentially arranged, driven rolling tools 8 set in rotation, wherein the axes of the rolling tools 8 run obliquely to the tube axis. The rolling tools 8 consist of several side by side in a manner known per se arranged roller disks 11, the diameter of which increases in the direction of the arrow. The Centrally arranged rolling tools 8 form the helical shape circumferential ribs 2 from the tube wall of the smooth tube 1 ', the smooth tube 1 'is supported here by a roll dome 12.

The rib tips 2 'are notched by means of the notched disk 9, which according to FIG 3, large and small teeth 13 and 14 distributed regularly around the circumference having. In the notched disk 9 shown in FIG. 3, three identical ones follow each small teeth 14 a large tooth 13.

Finally, the notched rib tips are compressed by the compression roller 10, whereby two pore classes, namely the large pores 5 and the small ones Pores 6 arise. The large pores 5 are formed at the points which the large teeth 13 of the notched disk 9 have left their imprint.

In FIG. 3, the width b at the tip of the small teeth 14 is also the width B entered at the tip of the large teeth 13 and the flank angle β indicated.

If the outer surface of the pipe is brought into contact with a liquid to be evaporated (see FIG. 4), then the channels 4 and of the pores 5, 6 achieves that the channel walls 15 are covered by a liquid film 16 be wetted.

The phase change from liquid to vapor does not then occur Bubble evaporation, but by thin-film evaporation on the channel walls 15. In this case, the pore system has two different tasks. The liquid must first be placed in the pipe surface under the outer surface Channels 4 are transported into it. After evaporation, the resultant Steam 17 can escape to the outside.

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

In order to be able to transport the generated steam 17 from the channels 4, in the channels 4 have a higher pressure than in the exterior. This overpressure is corresponding to the vapor pressure curve of the medium to be evaporated by the Overtemperature of the pipe wall set.

Liquids that wet the pipe material well are usually used. Due to the capillary effect, such a liquid can penetrate into the channels 4 through the pores 5, 6 in the outer tube surface against an overpressure. A liquid meniscus forms in each pore 5, 6, on the curved surface of which a discontinuity in pressure arises due to the surface tension. 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 means the surface tension and r the radius of curvature of the meniscus surface. The radius of curvature r depends on the contact angle Θ and the pore shape. The following applies to pores 5, 6 with a circular cross section and pore radius R _p : p k = 2 · σ · cosΘ R p

Similar relationships can be found for pores 5, 6 with a non-circular cross section deduce. It can be seen that the greatest capillary pressure at the pores 6 can set the smallest radius. The liquid penetrates through the little ones Pores 6 into the channel 4, forms a thin film 16 on the channel walls 15 and evaporates with the addition of heat. The steam 17 escapes through the larger ones Pores 5, since the capillary pressure is lower at these. So one of forms the small pores 6 to the large pores 5 directed flow. This is shown schematically in FIG.

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

It may prove useful to have more than two size classes of pores use. The liquid 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 pipe 1, expressed by the heat transfer coefficient on the outside of the pipe as a function of the heating surface load, is to be demonstrated using three differently designed pore systems.

The screw-like circumferential channels 4 have a pitch of 0.5 mm and a total height of 0.75 mm. The outer diameter of the tube 1 is approx. 19 mm.

The geometric data of the notched disks 9 used are summarized in Table 1; FIG. 3 shows a schematic illustration of such a notched 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. Score 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 3rd 0.50 mm 25 ° 0.20 0.60 8: 1

The effect on the external heat transfer coefficient depending on the The heating surface load is exemplary for the refrigerant R-22 at a saturation temperature of 14.4 ° C shown in Figure 6.

In comparison to a notched disk 9 with a constant tooth width (see No. 1), i.e. Pores of the same size, an improvement is obtained with notched disk No. 2 heat transfer by approx. 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 approximately A _k = 30,000 μm ² , the class of large pores 5 is grouped around the maximum at a pore surface of approximately A _g = 75000 μm ² .

If the steam pores 5 are enlarged further, as in the case of notched disk No. 3, the result is compared to the uniform pores a heat transfer coefficient reduced by 25 to 45% on the outside of the pipe. In this case, the vapor pores 5 too large, the channels 4 are flooded with liquid and the thin film evaporation collapses.

It can be seen that the dimensions of the pores 5, 6 and the frequency of larger steam pores 5 have a decisive influence on the function and thus the Have structure performance.

The present observations show that the channel size is less rather, the pore size is decisive for the function and thus the heat transfer are. Due to the lack of widening of the channels (see JP-OS 63-172.892, Figures 5 and 7 there) are not adjacent channels in a negative manner influenced.

The US-PS 4,729,155 relates to adjacent channels through smaller transverse openings are interconnected. The present invention however, refers to closed channels in which a directional flow is present as described above. Cross connections between the channels lead to a disturbance of the directional flow and are therefore not for this concept useful.

Claims (12)

Metallic heat exchange tube, in particular for the evaporation of liquids made of pure substances or mixtures on the outside of the tube, with integral ribs (2) encircling the outside or around the tube, which are deformed to form essentially closed channels (4),
the channels (4) having a substantially uniform cross-section and being alternately open to the outside through pores (5, 6) with at least two different sizes,
characterized by the following features: a) the reciprocal ratio of the average size A _{k of} the pores (6) of the smallest pore class to the average size A _{g of} the pores (5) of the next larger pore class is: A G / A k = 1.5 to 4; b) the frequency ratio m = number N _k of pores (6) of the smallest pore class to number N _g of pores (5) of the next larger pore class is: m = N k N G = 12: 1 to 1: 5. Heat exchange tube according to claim 1, characterized in that A _g / A _k = 2 to 3 and m = N k N G = 9: 1 to 1: 3. Heat exchange tube according to claim 1 or 2, characterized in that it has two classes of pores. Heat exchange tube according to claim 3, characterized in that the ratio of the total opening area F _{k of} all small pores (6) to the total opening area F _{g of} all large pores (5) is matched to the properties of the medium used by: F k F G ∼ ρ D ρ Fl ; With

and ρ _D = density of the vapor and ρ _FI = density of the liquid. Method for producing a heat exchange tube with integral ribs (2) encircling the outside on the outside according to one or more of claims 1 to 4, in which the following method steps are carried out: a) Helical ribs (2) are rolled out on the outer surface of a smooth tube (1 '), in that the rib material is obtained by displacing material from the tube wall to the outside by means of a rolling process and the resulting rib tube (1) is rotated by the rolling forces is displaced and / or advanced according to the resulting ribs (2), the ribs (2) being formed from the otherwise undeformed 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) have been shaped out, the rib tips (2 ') are notched by a notched disk (9),
characterized by : c ') the notching is effected by large and small teeth (13, 14) arranged on the circumference of the notched disk (9), d) the notched rib tips (2 ') are compressed by radial pressure to the level of the notch. Device for carrying out the method according to claim 5 with the following features: a) on the circumference of the finned tube (1) there are at least two mutually offset and radially adjustable tool holders (7) arranged in a stationary roller head, b) the tool holders (7) each have a driven rolling tool (8) consisting of several rolling disks (11) with an axis lying obliquely to the pipe axis, c) the rolling disks (11) having an increasing diameter, d) in at least one tool holder (7), a notched disk (9) is connected downstream of the rolling tool (8),
characterized by the following features: d ') the notched disk (9) has large and small teeth (13, 14) on the circumference in a regular arrangement, each being followed by a large tooth (13) or a plurality of large teeth (13) after a certain number of small teeth (14) and the ratio of the number of small teeth (14) to the number of large teeth (13) is m = 12: 1 to 1: 5, e) a compression roller (10) follows the notched disk (9). Device according to claim 6, characterized in that the ratio is m = 9: 1 to 1: 3. Device according to claim 6 or 7, characterized in that the notched disk (9) has 8 to 25 teeth (13, 14) per cm circumference. Device according to one or more of claims 6 to 8, characterized in that, in the case of trapezoidal teeth (13, 14), the ratio 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 = 1.2 to 4. Device according to claim 9, characterized in that the ratio B / b = 1.5 to 3. Device according to one or more of claims 6 to 10, characterized in that the notched disk (9) is straight toothed. Device according to one or more of claims 6 to 10, characterized in that the notched disk (9) is helically toothed.

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