RU2450880C1 - Method of fabricating heat exchange bimetallic ribbed tube - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to metal forming and may be used in production of bimetallic ribbed tubes for gas-to-fluid heat exchangers for whatever industry. Helical crosswise ribs are extruded in cold state from thick-wall external tube fitted with minor clearance on bearing tube. Prior to cold rolling, helical bulged skew ledged are made on internal bearing tube outer surface to form V-shape herringbone relief.

EFFECT: recesses between V-shape relief are filled by external tube metal to increase contact surface area and reduce thermal contact resistance, hence, to increase heat transfer factor.

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Description

The invention relates to methods for processing metals by pressure and can be used in the production of heat-exchange bimetallic ribbed pipe.

Bimetal finned tubes (BRT) with spiral transverse ribs have been used for many years in steam boilers [1], in evaporators and condensers [2] of vapor compression refrigeration units and heat pumps, in air heaters and air heaters [3] of large air conditioning systems, in nuclear reactor heat exchangers [4] with gas coolant.

However, the most famous and widely used were the BRT with spiral rolling aluminum ribs. Such pipes consist of tube bundles of heat-exchange sections of air-cooling apparatuses [5] of the country's fuel and energy complex, air-cooled oil-coolers of the electric power industry, general-purpose air heaters, intermediate and end gas compressor refrigerators, condensing heat exchangers for deep cooling of natural gas combustion products in industrial heating boilers, diesel charge air coolers [6]. It is difficult to find industries and national economies that do not use heat-exchanging BRT with rolling aluminum fins. The number of pipes produced to date is measured in millions of meters.

Structurally, the BRT [5, 6] consists of a metal inner supporting pipe and a ribbed sheath mechanically tightly connected to it with spiral transverse ribs, made mainly of highly heat-conducting ductile metal, most often aluminum and its alloys. The advantage of BRT is that the ribs are made of metal with high thermal conductivity. While the carrier pipe is made of cheaper, mechanically strong and corrosion-resistant metal (carbon, stainless steel, cupronickel, etc.). Mechanical loads during operation of the BRT are absorbed by the carrier tube, and the ribbed shell practically does not experience mechanical stresses, and the mechanical characteristics of its material are not determining. Outside, the ribbed shell flows around with a transverse stream of air (gas) of atmospheric pressure or close to it. Due to the imperfection of the mechanical contact of the inner surface of the ribbed shell and the outer surface of the supporting pipe (the presence of micro- and macro-roughness, waviness of the mating surfaces, oxide film, etc.) during the transfer of heat flux Q, W, a temperature drop Δt _k , K appears in the contact zone The perfection of the mechanical connection is estimated by the value of thermal contact resistance (TCS) R _k , m ² · K / W, calculated by the formula [5]

R _k = Δt _k / q _k

where q _k = Q / F _k is the heat flux density in the contact zone of the BRT, W / m ² ;

F _to - the nominal area of the contacting surfaces, m ² .

When metallurgical connection of the contacting surfaces of the TCS is absent.

The presence of TCS is the main disadvantage of BRT in comparison with a monometallic ribbed pipe [7]. In case of unreliable contact with the surface of the supporting pipe, the finning efficiency is significantly reduced due to a decrease in the BRT heat transfer coefficient. The value of TCS can be very significant in comparison with other thermal resistance of the BRT and act as a “blocking” thermal resistance when transferring heat flow, and minimize the heat transfer intensification by fins.

When designing heat exchangers, the choice of the type of pipe finning depends on the manufacturability, operational reliability and assimilation of production by industry. Pipes with spiral transverse (screw) finning made by cold rolling with three rolls [8, 9] satisfy the above requirements.

The method described in [8, 9] for cross-helical rolling of BRT by cold extrusion of spiral ribs from the wall of the outer pipe has a significant drawback - it is not possible to provide clearance-free contact between the outer surface of the inner supporting pipe and the inner surface of the outer pipe during rolling. The contact of the surfaces has a discrete character, and during heat transfer there is a significant TCS due to the presence of air with an inherent low thermal conductivity in the gap. The heat transfer coefficient of the pipe is reduced. To transfer the heat flux Q = idem to the heat exchanger, it is necessary to increase the heat exchange surface area with all the ensuing negative consequences.

A known method [10] for the production of BRT, according to which cold rolling is carried out of a pipe billet consisting of an internal carrier and an external pipe, with three rolls recruited from disks of variable profile. The following disadvantages are characteristic of this method. It is not possible to achieve a gapless contact between the outer and inner pipes; the average gap thickness can reach 0.1 ÷ 0.2 mm. At an economically feasible speed of rolling the pipe 4 ÷ 5 m / min or more under the ribs on the inner surface of the ribbed shell there is a metal tightening with the formation of annular air cavities. At such rolling speeds, along with plastic extrusion (outflow) of the metal into the formed ribs, an axial movement of the outer pipe occurs. The magnitude of the axial displacement of the outer pipe (hood) is 12–16% of the length of the carrier pipe and depends on the geometric parameters of the spiral rib. Axial movement increases the metal consumption of the original outer pipe, since in this case it is necessary to assign a larger wall thickness to this pipe so that its material consumption is sufficient to produce the ribs of the required parameters.

There is a known method [5] of cold rolling of a tube stock by three rolls for the production of BRT, in which the negative phenomenon in the form of a metal tensile is eliminated. For this, the BRT rolling technology provides for additional metal compression between the ribs with special crimping disks mounted on the same axis as the main profile-forming disks. Crimp disks carry out additional compression of the ribbed shell and ribs in the final stage of deformation after the final formation of the profile and size of the ribs of the main disks. The diameter of the crimping discs is 0.15 ÷ 0.5 mm larger than the diameter of the last main disc. As a result, the air annular cavities are eliminated and the thickness of the air gap decreases, and the contact pressure of the mating surfaces increases. Our studies established a decrease in TCS by 1.7 ÷ 2 times and, as a result, an increase in the BRT heat transfer coefficient by 7 ÷ 11% for the finning coefficient φ = 15.2.

The disadvantages of the method are the need to stop the line for rolling the BRT to replace the crimp discs due to their rapid wear, reduced labor productivity, increased cost of manufacturing the BRT. Also not eliminated axial exhaust of the outer pipe.

A known method [11] for the production of BRT, in which the rolling of a tube stock consisting of an internal supporting and an outer tube is carried out by three rolls drawn from variable disks forming gauges. To reduce the axial displacement of the outer pipe and increase the thermal efficiency of the BRT before rolling, a spiral spiral groove 1/3 ÷ 1/5 of the thickness of the wall of the carrier pipe and a step of 2/3 ÷ 3/2 of the finning step are applied to the outer surface of the inner supporting pipe. The groove has a rectangular cross section. For pipes ABO, the main consumer of BRT, the depth of the groove is 0.3 ÷ 0.5 mm. Thermal and metallographic studies of the BRT made by this method, performed by us [5], revealed that the metal does not fill the groove of the outer pipe at the corners at its base with a depth greater than 0.5 mm, air cavities remain, which reduces the potential of this method for intensifying heat transfer. Rolling such grooves requires very great effort on the rolls, the groove rolling speed is significantly inferior to the rolling speed of the tube stock, because of this it is not possible to create a single in-line production of BRT. The axial exhaust of the outer pipe has actually decreased, but not completely eliminated, since the direction of the helical groove coincides with the direction of cross-helical rolling of the BRT. This method [11] is the closest in technical essence and the achieved result, and we adopted for the prototype.

The objective of the invention is the intensification of heat transfer in a bimetallic ribbed pipe (BRT) with spiral transverse ribs, reducing the axial displacement of the outer pipe in the pipe billet, reducing the BRT metal consumption, expanding the temperature range of its applicability, increasing the operational reliability of BRT.

The objective is achieved in that the proposed method for the production of a heat-exchange bimetallic ribbed pipe, including assembling a pipe billet consisting of an internal supporting and an external pipe, and rolling it to form spiral transverse ribs by extruding the metal of the outer pipe with three rolls drawn from variable-profile disks, differs in that before assembling the tubular billet, continuous oblique protrusions are applied along the entire length of the inner surface of the inner supporting pipe to form a V-shaped chevron relief permanent type, wherein when forming predetermined edges geometrical parameters rolls create such a pressure that the depressions between the projections V-shaped relief completely filled metal outer pipe.

The principal distinguishing feature of the invention from the prototype is the application of continuous rows of oblique (angular) protrusions, which form a V-shaped chevron-shaped relief [12] with an angle β at the top of the relief formed by converging oblique protrusions of a separate chevron, before rolling on the outer surface of the carrier pipe. Structurally, the V-shaped relief represents the so-called ultimate roughness in the form of alternating corrugations of a protrusion-depression (groove) on the surface of the carrier pipe, and they are applied mechanically, for example, by rolling in a rotating carrier pipe in cylindrical rollers. On the surface of the rollers made in one way or another V-shaped relief of the required geometric parameters.

Own research by the authors found that regardless of the wall thickness of the supporting pipe, the pitch of the raised protrusions is set equal to s _p = 0.6 ÷ 2 mm, their height h _p = 0.15 ÷ 4 mm. It is thermotechnically feasible to develop the area of mechanical contact of the contacting surfaces by increasing the depth of the grooves while decreasing their number around the circumference of the supporting pipe. The thickness at the base of the protrusions is chosen approximately equal to the height, but it is not of fundamental importance for the implementation of the process and the intensification of heat transfer. With this geometry of the V-shaped relief, guaranteed filling of the outer pipe of the hollows (grooves) with metal, and the speed of their application is not less than the rolling speed of the tube billet. The angle β is assigned in the range of 30 ÷ 120 °. Width b of the left or right track of the V-shaped chevron, i.e. the width of the base of the oblique protrusion, it is advisable to take equal to the step s of the spiral ribs (b = s).

Applying a V-shaped relief allows you to increase the surface area of the contacting surfaces in 1.5 ÷ 2.6 times, while the temperature drop Δt _k at the BRT contact is reduced and the value of the TCS of the pipe is reduced with other constant values of the individual thermal resistances. As a result, heat transfer is intensified, the growth of heat transfer coefficient reaches 10 ÷ 14%. The intensification of heat transfer by improving the mechanical contact in the BRT is an energy-efficient technological process, since it does not require additional power costs for moving the gas coolant along the ribbed side.

When rolling a tube stock in rolls in the case of a V-shaped relief of oblique (angular) protrusions on the left and right tracks, its resulting axial forces of approximately the same magnitude have the opposite direction, which means that the outer pipe does not undergo a significant resulting axial force. Therefore, the axial movement (exhaust) of the outer pipe in the workpiece noticeably becomes smaller and does not exceed 3 ÷ 5%. A dominant radial outflow of the metal of the wall of the outer pipe into the formed ribs is observed, which allows one to reduce the initial wall thickness of the outer pipe, and hence the metal consumption of the pipe with spiral ribs.

The increased surface mating surface in the contact zone determines a large amount of adhesion force between them and thereby ensures the peelability (separability) of these surfaces relative to one another when a cooled medium is supplied inside the BRT with a temperature of 310 ÷ 320 ° C.

The increased mechanical adhesion of the ribbed shell to the carrier pipe eliminates the negative influence of multiple alternating heat-cooling cycles that occur during operation of the heat exchanger, which means that the operational reliability of the heat exchanger from such BRTs increases.

Our experiments indicate an increased contact pressure of the BRT compared with the BRT made according to the prototype. This prevents the separation of metal under the rib from the carrier pipe, the emergence of air voids, which further increases their energy efficiency and contributes to the growth of productivity of lines for the production of BRT.

Thus, in the inventive method for the production of heat-exchange BRT, the formed tasks are completely solved.

Figure 1 shows the inner supporting tube 1 of length l and outer diameter d, as well as V-shaped relief projections 2; figure 2 is the same, a cross section along aa of figure 1; figure 3 - node I in figure 2, in which d ₁ - the inner diameter of the carrier pipe; h _p , s _p - respectively, the height (depth) and the step of the raised protrusions; figure 4 shows a pipe billet with a length of 1 _{n of the} outer pipe 3, its outer diameter d _n and the wall thickness δ _n ; figure 5 is the same, a cross section along BB of figure 4; Fig.6 shows options for the profile (cross section) of the relief projections; Fig.7 shows a cross section of the BRT with spiral transverse ribs 4 of the outer diameter D, height h and diameter at the base d ₀ = D-2h; on Fig shows the appearance of the BRT with the step of the ribs s and its average thickness Δ. The length of 1 _{n of the} outer pipe 3 is chosen to be less by the value of its axial drawing in comparison with the length l of the inner supporting pipe 1, since 1 _n <l. The inner diameter of the outer pipe equal to (d _n -2δ _n ) is always somewhat larger (approximately 1.4 ÷ 1.6 mm) of the outer diameter d of the inner supporting pipe.

The method is implemented as follows.

On the outer surface of the inner supporting pipe 1, continuous oblique (angular) protrusions of the V-shaped relief 2 of chevron type are applied along its entire length. Then, the outer pipe 3 is freely put on with a small gap on the inner support pipe 1 and the formed tube stock is sent to three rolls, and the bimetallic workpiece is cold rolled to form spiral ribs 4 of predetermined geometric parameters by extruding the metal of the outer tube by rollers and completely filling the hollows with metal ( grooves) between the protrusions of the V-shaped relief. The required pressure on these operations is created by rolls drawn from variable disks [5]. The rolls are located around a circle with an angular distance between them of 120 °, and their axes are inclined to the axis of rolling of the tube stock at an angle of 0 ... 6 °, called the feed angle. The free gap between the pipes in the tube stock is selected during the rolling process.

The proposed method creates a tight mechanical contact of the inner supporting pipe and the ribbed shell with an increased contact area, which is accompanied by a significant reduction in the TCS, intensifies the heat transfer of the pipe and, as a result, reduces the heat transfer surface area, dimensions and mass of the heat exchanger when transmitting the same heat flux Q = idem in comparison with the prototype. The metal filling of the outer tube of the cavities between the protrusions of the V-shaped relief eliminates the occurrence of axial force during rolling acting on the outer pipe, the axial movement of the pipe is minimized, and the transverse roll disks experience lower stresses, which increases the reliability of the rolling tool. The radial outflow of metal into the extruded ribs is also intensified, the temperature range of applicability of such BRTs is expanded to a temperature of 320 ° C along the cooled medium inside the pipe without reducing the operational reliability of the heat exchanger, the material consumption is reduced due to the use of a smaller thickness δ _n of the outer pipe wall to produce the ribs of the same geometric parameters .

Example.

The initial tube billet is an external aluminum pipe with a diameter of d _n = 37 mm, a wall thickness of δ _n = 5.3 mm, a length of 1 _n = 3.85 m, and an internal supporting pipe with a diameter of d = 25 mm with V-shaped on its outer surface chevron-type reliefs with a depth (height) of the protrusion h _p = 0.35 mm, a pitch of s _p = 1.8 mm, an angle β = 60 °, a track width of the relief (base of the oblique protrusion) b = 2.5 mm. The inner diameter of the carrier pipe d ₁ = 20 mm, its length l = 4 m.

When rolling the initial bimetallic tube billet with three rolls drawn from transverse disks of variable profile, a heat-exchange bimetallic finned tube with spiral transverse ribs with an outer diameter of the rib D = 57 mm, a diameter at the base of the ribs d _o = 26.4 mm, height, pitch and average rib thickness, respectively, h = 15.2 mm, s = 2.5 mm, Δ = 0.55 mm. The coefficient of finning of the pipe is φ = 20.4, and its length is 1 = 4 m. The heat-exchanging BRT with this finning geometry is the main type in the heat-exchanging sections of ABO, the largest consumer of bimetal pipes.

The method can be used by chemical and power engineering plants for the manufacture of bimetallic ribbed tubes with spiral ribs used in the air-fuel complex of the country's fuel and energy complex, in air and gas coolers of turbogenerators, in oil coolers of electric transformers, air heaters, in nuclear power heat exchangers.

Information sources

1. Levchenko G.I., Liseikin I.D., Kopeliovich A.M. et al. Finned heating surfaces of steam boilers. - M .: Energoatomizdat, 1986. - P.24-35.

2. Danilova G.N., Bogdanov S.N., Ivanov O.P. and others. Heat exchangers of refrigeration units. - L .: Mechanical engineering, 1986. - S.120-124, 180-184.

3. Fraas A., Otsisik M. Calculation and design of heat exchangers. - M .: Atomizdat, 1971. - S.13-14, 29-33.

4. Andreev P.A., Gremilov D.I., Fedorovich E.D. Heat exchangers of nuclear power plants. - L .: Shipbuilding, 1969. - P. 41-43, 94-99.

5. Sleepless A. N., Dreitser G. A., Kuntysh V. B. et al. Fundamentals of calculation and design of air-cooled heat exchangers: Reference book / Ed. ed. V. B. Kuntysh and A. N. Bessonogo. - St. Petersburg: Nedra, 1996. - S.28-89, 89-101, 254-272.

6. Bazhan P.I., Kanevets G.E., Seliverstov V.M. Handbook of heat exchangers. - M .: Mechanical Engineering, 1989. - P.62-66.

7. A.S. USSR 1838746, IPC F28F 1 / 26.1993.

8. Barbarich M.V., Kirpichnikov V.P. New methods of transverse and transverse helical rolling of metals. - M.: VINTI AN USSR, 1957, - 80 p.

9. Vasilchikov M.V., Volkov M.M. Cross-helical rolling of products with a helical surface. - M.: Mechanical Engineering, 1968, - 140 p.

10. A.S. USSR 56645, IPC B21C 37/20, 1978.

11. A.S. USSR 1016003, IPC B21H 3/12, B21C 37/26, 1983 (prototype).

12. Polytechnical dictionary. - M .: Soviet Encyclopedia, 1977. - P.563.

Claims (1)

A method of manufacturing a heat-exchanging bimetallic ribbed pipe, including assembling a tube stock consisting of an internal carrier and an outer tube, and rolling it to form spiral transverse ribs by extruding the outer tube metal by three rolls drawn from variable disks, characterized in that before assembling the tube stock continuous oblique protrusions are made along the outer surface of the inner supporting pipe along its entire length with the formation of a V-shaped relief of the chevron type, moreover, when forming eber predetermined geometric parameters rolls pressurized at which the troughs between the projections V-shaped relief completely filled metal outer pipe.

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