Heat exchanger tube

Abstract

Heat exchanger tube having a tube axis, a tube wall, a tube outer side and a tube inner side, from the tube wall on the tube inner side forming continuously extending, axially parallel or helically encircling inner ribs, each inner rib having two rib flanks and a rib tip, in each case forming a continuously extending groove between adjacent inner ribs. Thereby, the inner tube surface can be described by the following equation: [phi] = e2 / pdi, where: [phi] is a dimensionless parameter, severity factor, e is the height of the helical ribs, p is the helical pitch, and di is the tube inner diameter, whereby N is the number of ribs counted in a cutting plane perpendicular to the tube axis. According to the invention, the product [phi] N2 is greater than 16 and less than 70.

Description

[0001] The present invention relates to a heat exchanger tube.

[0002] Heat transfer occurs in many sectors of refrigeration and air-conditioning technology as well as in process and power engineering. Heat exchangers with tubes are often used in these fields for heat transfer. In many applications, in this case, a liquid or gaseous medium flows inside the tube and is cooled or heated depending on the direction of the heat flow. Heat is distributed to or extracted from the medium located outside the tube.

[0003] In order to permit heat transfer between the heat-distributing medium and the heat-absorbing medium, the temperature of the heat-distributing medium must be higher than the temperature of the heat-absorbing medium. This temperature difference is designated as the driving temperature difference. The higher the driving temperature difference, the more heat can be transferred. On the other hand, the aim is often to keep the driving temperature difference low, since this is beneficial for the efficiency of the process.

[0004] It is known that heat transfer can be enhanced by structuring the heat transfer surface. It is thereby possible to achieve that, compared to the case of a smooth surface, more heat can be transferred per unit of heat transfer surface. In addition, the driving temperature difference can be reduced, making the process more efficient. In metal heat exchanger tubes, structuring of the heat transfer surface is often obtained by forming ribs or similar elements from the material in the tube wall. These integrally formed ribs have a firm metallic bond with the tube wall and can therefore transfer heat optimally.

[0005] An often used embodiment of heat exchangers is the tube bundle heat exchanger. These devices often utilize tubes that are structured on both their inside and outside. Structured heat exchanger tubes for tube bundle heat exchangers usually have at least one structured region and smooth end parts and, if appropriate, smooth intermediate parts. The smooth end parts or intermediate parts delimit the structured region. So that the tube can be easily incorporated into a tube bundle heat exchanger, the outer diameter of the structured region should not be greater than the outer diameters of the smooth end parts and intermediate parts.

[0006] Axially parallel or helical ribs are often used on the inner side of a tube to improve the heat transfer characteristics. The rib structure increases the heat transfer area of the inner surface of the tube. In addition, in the case of helically arranged ribs, the turbulence of the medium flowing in the tube is increased, and thus the heat transfer is improved. It is known that by providing internal ribs with grooves or indentations, the heat transfer characteristics of axially parallel or helical rib structures on the inner side of the tube can be improved. Embodiments of this situation can be found in EP 1312 885B1, CN 101 556 124A, CN 101556 125A, US 5 992 513 A, US 6 018 963A, US 6 412 549 B1, EP 2 339 283 B1 and US 5697 430 A. The grooving of the ribs results in a structure that has alternating rib heights and lateral material protrusions on the rib flanks. This structure additionally increases the turbulence of the medium flowing in the tube.

[0007] Particularly in refrigeration and air-conditioning technology applications, it is assumed that the efficiency of refrigeration equipment in part-load situations is becoming increasingly important. In part-load situations, the through-flow of the heat transfer medium often decreases, and thus the velocity of the medium flowing in the tube decreases significantly. Since the main part of the heat transfer resistance is then transferred to the inner side of the tube, it is necessary to further improve the currently known structures on the inner side of the tube.

[0008] The object on which the present invention is based is to develop a heat exchanger tube with respect to heat transfer characteristics.

[0009] The present invention includes a heat exchanger tube having a tube axis, a tube wall, an outer tube side and an inner tube side, from which continuously extending, axially parallel or helically surrounding internal ribs are formed on the inner side of the tube, each internal rib having two rib flanks and a rib tip, and in each case a continuously extending groove is formed between adjacent internal ribs. The inner tube surface can be described by the following equation:

[0010] Φ = e 2 / pd i

[0011] where:

[0012] Φ is a dimensionless parameter, the severity factor,

[0013] e is the height of the helical rib,

[0014] p is the helix pitch in the tube axial direction, and

[0015] d i is the inner diameter of the tube,

[0016] whereby N is the number of ribs counted in a cutting plane perpendicular to the tube axis.

[0017] According to the present invention, for the product ΦN 2 is greater than 16 and less than 70.

[0018] The severity factor, as a dimensionless parameter, mainly refers to the enhancement factor in the tube-side heat transfer coefficient hi

[0019] Φ = Φhi = hi_finned surface / hi_naked surface

[0020] In the prior art, the relationship between the internal heat transfer coefficient Ci and geometric parameters is generally recognized and disclosed, and this relationship is called the "severity factor" Φ. This parameter Φ is a dimensionless parameter, which relates the fin height e, pitch p, and inner diameter d i by the equation given above.

[0021] For example, as described in U.S. Patent No. 5697430, the spiral ribs on the inner surface of the tube have a predetermined rib height and pitch and a predetermined pitch angle.

[0022] The pitch is defined as the distance between the fin tips of two adjacent fins, and this distance is measured in the axial direction.

[0023] In fact, in the air-conditioning and processing industries, all highly enhanced surfaces have a p / e ratio less than 5. Because when the ratio p / e is greater than 5, the main flow of the single-phase fluid on the tube side reattaches to the wall, increasing the thickness of the boundary layer, which has an adverse effect on heat transfer.

[0024] The focus of the research is on highly enhanced finned tubes with a given p / e less than 5, where the fin pitch p is at most five times the upper limit of the fin height e. Under such conditions, it has been shown that there is no reattachment region along the flow direction of the tube wall. Only inherently stable recirculation zones are located between the repeating ribs. Here, a very strong interaction between the eddy current and the main flow is generated on the upper side of the flow-side separation region, such that the velocity gradient and turbulence reach a maximum in the radial tube direction. Then, the main flow is forced to "slide" on the ribs. A secondary flow develops between the ribs.

[0025] For enhancing tube-side heat transfer, it is also very important to evaluate the heat transfer efficiency in combination with the pressure loss due to improved heat transfer. From empirical data, it can be deduced that the efficiency index PEC is related to Φ and N as follows:

[0026] PEC ∝ ΦN 2

[0027] (∝ represents the mathematical symbol of direct proportion)

[0028] When p / e < 5, the efficiency of the finned tube PEC is approximately proportional to ΦN2 Proportional. ΦN 2 The larger ΦN is, the higher the efficiency of the optimized finned tube. When ΦN 2 is greater than 16, the PEC of the finned tube increases by at least 10% compared to the PEC of the prior art. The following is an explanation of why the efficiency index ΦN is selected 2 and why the application range is 16 ≤ ΦN 2 ≤ 70.

[0029] In this application, highly surface-enhanced tubes (p / e < 5) will be the focus. When p / e decreases within a small range, the influencing factors caused by the rib shape cannot be ignored. The width of the rib now plays an important role. In practical applications, the rib shape is triangular or trapezoidal, including triangles or trapezoids with chamfered edges. We introduce the dimensions A and B measured at the center height line of the rib height (e / 2 from the rib base platform) to illustrate the flow mechanism here.

[0030] In this way,

[0031] B = p - A

[0032] where,

[0033] A... the rib width measured at the center line of the rib height along the tube axial direction,

[0034] B... the channel width between two adjacent ribs measured at the center line of the rib height along the tube axial direction,

[0035] For tube-side enhanced heat transfer, it is also very important to evaluate the pressure drop penalty. One of the indicators is listed below:

[0036] Φf = f_finned surface / f_naked surface

[0037] From the measurement data available for the region p / e < 5, Φf is proportional to the B 2 value (average channel spacing). At the same time, Φf is inversely proportional to d i 2 inversely proportional.

[0038] Then, we have:

[0039] Φf ∝ p 2 / d i 2 (p / e < 5)

[0040] According to the geometry of the helical rib surface,

[0041] p.tanθ = πd i / N

[0042] In this way, we have Φf ∝ 1 / N2

[0043] wherein,

[0044] θ... the helix angle between the extending ribs and the tube axial direction,

[0045] p... the helix pitch, and

[0046] d i ... the inner diameter of the tube,

[0047] N... the number of ribs counted in a cutting plane perpendicular to the tube axis (X),

[0048] ∝... the mathematical symbol representing direct proportionality.

[0049] In this way, we can obtain the following efficiency index PEC:

[0050] PEC = Φ / Φf = ΦN 2

[0051] For highly enhanced tubes (p / e < 5), the efficiency of the finned tube is approximately proportional to ΦN 2 . The larger ΦN 2 , the greater the efficiency of the optimized finned tube will be. If ΦN 2 is set to be greater than 16, the PEC of the finned tube will increase by at least 10% compared with the prior art. The following is the reason for enhancing PEC expected from enhanced heat transfer when 16 ≤ ΦN 2 ≤ 70:

[0052] a) Under the condition of p / e < 5, the adverse effects of the boundary layer development region can be avoided.

[0053] b) Φ is a severity factor directly related to the improvement of the heat transfer coefficient on the tube side. The actual heat transfer area with ribs is a monotonically increasing function of N (the number of circumferential ribs). The two parameters are in direct proportion to hi (the heat transfer coefficient on the tube side) because the corresponding area of hi is the nominal area (calculated assuming it is a smooth hole).

[0054] c) The new discovery of the present invention is that when p / e < 5, at a fixed helix angle and a fixed rib height e, the tube side pressure drop f will decrease with the increase of N.

[0055] However, in all prior art and literature studies, the trend of the growth relationship between the two parameters is considered to be in the opposite direction. When N increases, p decreases in order to limit the recirculation region from a flat long ellipse to an approximately circular shape. Also, the shear stress contact area between the mainstream flow and the recirculation flow inside the adjacent ribs decreases. In this way, there is a lower shear stress level between the mainstream and the tube wall to reduce the pressure drop on the tube side.

[0056] d) When ΦN 2 is greater than 70, the p / e ratio is usually less than 1.5. Considering the width of the rib, p / B is even less than 1, and the recirculation area narrows again to form a circular shape. Here, we can have optimized PEC efficiency. In addition, when ΦN 2 is further increased, p / B becomes smaller, and two recirculation areas will be formed between the ribs, which are the upper recirculation area and the bottom recirculation area that circulate in opposite directions. The bottom recirculation area has a lower recirculation speed, resulting in reduced heat transfer.

[0057] e) In addition, when ΦN 2 is greater than 70, a very small pitch value can lead to increased tube rib weight, complexity of tool design, and fouling risk.

[0058] The following are the main advantages of the solution of the present invention:

[0059] By setting ΦN 2 to be greater than 16 and less than 70, a PEC with at least 10% higher efficiency can be achieved compared with the prior art.

[0060] Reducing the pressure drop penalty without sacrificing heat transfer performance is a great progress in the field of enhanced heat transfer.

[0061] The present invention has provided a finned tube with a predetermined parameter ΦN 2 for tube replacement of a heat exchanger, having enhanced heat transfer, reduced pressure drop, and subsequent pumping power. And in a project aiming at opportunities to reduce costs, a smaller number of tubes can be used without increasing the pressure drop.

[0062] In a preferred embodiment of the present invention, there is a tangent circle with a radius R between the adjacent rib flanks and the bottom line of the groove between the adjacent ribs along the tube axial direction, satisfying the relationship e / 4 ≤ R ≤ e.

[0063] In other words, the reference plane where the contact point of the tangent circle and the tube surface is located constitutes the plane of the tube axial section.

[0064] The radius of the tangent circle can be calculated from the normal direction of the rib by the following formula:

[0065] R = (B / 2 - e / 2 * COT(β / 2)) * TAN(45 + β / 4)

[0066] Where,

[0067] β... the angle of the rib included in the plane of the tube axial section [degrees]

[0068] As described in the previous paragraphs, only inherently stable recirculation regions are located between the repeating ribs. Here, a very strong interaction between the eddy current and the mainstream is generated on the upper side of the flow-side separation region, such that the velocity gradient and turbulence reach a maximum in the radial tube direction. Then, the mainstream is forced to "slide" over the ribs. A secondary flow develops between the ribs. In this way, the radius of the tangent circle represents the scale of the secondary flow between adjacent ribs.

[0069] When R becomes smaller than e, there is no longer a reattachment region between the rib and the recirculation region, and this reattachment region is restricted from a flat long ellipse to a circular shape in the axial flow direction. Also, the shear stress contact area between the mainstream flow and the recirculation flow within adjacent ribs is reduced. In this way, there is a lower shear stress level between the mainstream and the tube wall to reduce the pressure drop in the tube side.

[0070] When R is further reduced to below e / 4, there will be two recirculation regions in the channel between the ribs, shaped as an upper region and a bottom region. The bottom region recirculates at a lower speed, resulting in reduced heat transfer. In addition, the fluid stagnates in the channel, which is not conducive to heat transfer. Furthermore, a very small R value can lead to increased tube rib weight, complexity of tool design, and a higher risk of fouling.

[0071] In another advantageous improvement of the present invention, the product ΦN 2 can be greater than 19 and less than 55. If ΦN 2 takes a value higher than 19 and lower than 55, only particularly stable recirculation regions are formed between adjacent ribs. Advantageously, the recirculation regions thus exhibit a rather circular shape. The shear stress contact area between the mainstream and the recirculation flow in adjacent fins is also optimized, further reducing the pressure drop on the tube side. The recirculation regions formed in this way circulate at a higher speed, which results in an increase in heat transfer.

[0072] Advantageously, the distance B between the two inner ribs can be between 0.0236 inches and 0.0098 inches. B is the channel width between two adjacent ribs measured at the centerline of the rib height in the tube axial direction. Within the defined range of B, the mainstream forced to slide over the ribs and the secondary flow developing turbulence between the ribs reach a particularly stable and defined state.

[0073] In a preferred embodiment, the shape of the internal ribs can vary. Interleaved surfaces on and along the ribs to increase the surface area are beneficial for disturbing the recirculation regions and reducing the tube side pressure drop. Wavy rib tips and / or additional flank structures are feasible and contribute to stabilizing the fluid flow.

[0074] In a preferred embodiment, an external structure can be formed on the outer side of the tube. The integral external ribs can advantageously extend around the outer side of the tube in an axially parallel or helical manner. For this case, another aspect of the present invention includes a method for producing a structured heat exchanger tube having integral external ribs, i.e., external ribs machined from the tube wall and extending helically around the outer side of the tube, wherein the following method steps are performed. In a first forming region, by a first rolling step, rib material obtained by displacing material from the tube wall is formed into external ribs extending in a helical manner on the outer side of the smooth tube, and the resulting ribbed tube is rotated by the rolling pressure and is pushed forward according to the resulting helical ribs, and the external rib material is formed to have a raised height from the otherwise undeformed smooth tube. In the first forming region, the tube wall is supported by a first roller mandrel located in the tube, the first roller mandrel being rotatably mounted and shaped, thereby constructing internal ribs. In a further rolling step, the external ribs are constructed in another region spaced apart from the first forming region, having a further raised height, and the internal ribs are provided with second grooves, and the tube wall is also supported in this other forming region.

[0075] In this way, the external structure can be designed in the form of integral, helically surrounding external fins.

[0076] Advantageously, the ratio between the inner rib height and the outer fin height can be between 0.80 and 0.62. In terms of the advantages of the present invention regarding the heat exchanger tube already mentioned above, further advantages are added by the production method, namely, the dimensions of the internal and external structures of the ribbed tube that can be set independently of each other and obtained with different rolling tools. Thus, for the best passage of heat, the internal and external structures can be optimally coordinated with each other. With a lower rib height on the outside relative to the internal structure height, optimization can be achieved.

[0077] To be able to compare the improved tubes of the present invention with previously known tubes, Table 1 and Table 2 are provided. In these tables, the basic tube parameters are shown. The known prior art tubes are listed in Table 1. Table 2 contains details of the selection of tubes studied according to the present invention. It can be seen from the tables that the parameter ΦN of the prior art tube type 3 with the highest value of 13.021 2 is significantly lower than the value of the solution according to the present invention. The parameter ΦN 2 has proven to be a benchmark for optimizing the thermal characteristics of such heat exchanger tubes.

[0078] Table 1 (Prior Art):

[0079] Type Pipe 1 Pipe 2 Pipe 3 Pipe 4 Pipe 5 Pipe 6 Pipe 7 Pipe 8 B 0,033 0,033 0,025 0,049 0,023 0,032 0,029 0,089 <![CDATA[Φ = e 2 / pdi]]> 0,008 0,006 0,009 0,008 0,008 0,006 0,007 0,002 e / (p - b) 0,632 0,523 0,776 0,568 0,853 0,573 0,718 0,141 e / p 0,309 0,301 0,410 0,233 0,330 0,280 0,296 0,086 B / A 1,789 2,387 2,580 1,077 1,019 1,682 1,324 1,034 p / e 3,238 3,325 2,440 4,296 3,028 3,573 3,373 11,67 b / e 2,077 2,343 1,759 2,228 1,528 2,241 1,922 5,935 B / (N * A) 0,053 0,054 0,068 0,036 0,027 0,049 0,035 0,103 e / Fh 0,744 0,667 0,690 0,917 0,556 0,674 0,395 0,455 <![CDATA[N 2 Phi]]> 8,856 12,50 13,02 7,292 11,31 7,273 10,49 0,205 PEC 118％ 108％ 125％ 119％ 113％ 122％ 122％ 88％

[0080] Table 2 (Present Invention):

[0081]

[0082]

[0083] A … Rib width measured at the centerline [inches]

[0084] B … Channel width between two adjacent ribs [inches]

[0085] b … Rib width along the axis [inches]

[0086] p … Axial pitch of the ribs [inches]

[0087] Φ … Severity factor (Φ = e 2 / pd i )

[0088] Fh … Finned height on the outside [inches]

[0089] d i … Inside diameter [inches]

[0090] e … Rib height [inches]

[0091] N … Number of ribs counted starting in a cutting plane perpendicular to the tube axis

[0092] θ … Angle of the ribs with the axis [degrees]

[0093] PEC... Performance evaluation criterion, efficiency of enhanced heat transfer on the tube side versus pressure drop penalty factor.

[0094]

[0095] The exemplary embodiments of the present invention are explained in more detail with the aid of the schematic drawings, in which:

[0096] Figure 1 A schematic diagram of the flow pattern in the region of the internal ribs is shown,

[0097] Figure 2 A schematic diagram of the internal ribs with other geometric parameters of the internal structure is shown, and

[0098] Figure 3 A diagram showing the efficiency factor of the heat exchange tube versus the parameter ΦN 2 = N 2 e 2 / pd i .

[0099] In all the drawings, parts corresponding to each other are given the same reference numerals.

[0100] Figure 1Shows a schematic diagram of the flow pattern on the inner tube side 22 around the heat exchanger tube 1 of the internal ribs 3. Between the two ribs 3, an inherently stable recirculation region RZ is located between the repeating ribs 3. Here, a very strong interaction between the eddy current and the mainstream MF is generated on the upper side of the flow-side separation region, resulting in the maximum values of the velocity gradient and turbulence in the radial tube direction. Then, the mainstream MF is forced to "slide" on the ribs 3.

[0101] Figure 2 Shows a schematic diagram of the internal ribs 3 with other geometric parameters of the internal structure. A tangent circle is shown, which is adjacent to the baseline of the flank 31 and the groove 33 between the adjacent internal ribs 3. The tangent circle has a radius R, which is at least one-fourth of the internal rib height e and has a maximum upper limit of the internal rib height e. In this region, the fluid flow between two adjacent ribs 3 generates a stable recirculation region RZ, as Figure 1 shown. The interaction between the eddy current and the mainstream results in a small pressure drop in the heat exchanger tube 1, combined with a significant increase in heat transfer. The fins 3 with the rib width A and the channel width B follow each other at a distance p and axially extend in a periodically progressive manner on the inner side of the heat exchange tube 1.

[0102] Figure 3 Shows the efficiency factor of the heat exchange tube as a function of the parameter N 2 Φ = N 2 e 2 / pd i graph.

[0103] To improve the heat transfer on the tube side, it is particularly important to prove that the efficiency of heat transfer is evaluated by combining the pressure drop due to the improved heat transfer.

[0104] For highly enhanced tubes (p / e < 5), the efficiency of the finned tube is approximately proportional to ΦN 2 When ΦN 2 is greater than 16, the efficiency factor PEC of the finned tube increases by at least 10% compared with the prior art. The application range according to the present invention is given as 16 ≤ ΦN 2 ≤ 70.

[0105] For ΦN 2 values greater than 70, very small pitch values can lead to an undesired increase in the weight of the heat exchanger tube and a complex tool design, as well as a risk of fouling.

[0106] List of names

[0107] 1 Heat exchanger tube

[0108] 2 Tube wall

[0109] 21 Outer tube side

[0110] Inner side of the 22 tubes

[0111] 3 Internal ribs

[0112] 31 Rib flank

[0113] 32 Rib tip

[0114] 33 Groove

[0115] X Tube axis

[0116] A Rib width

[0117] B Distance between two internal ribs, channel width

[0118] e Height of the internal ribs of the helix

[0119] R Radius of the tangent circle

[0120] MF Main flow

[0121] RZ Recirculation zone

Claims

1. A heat exchanger tube (1) having a tube axis (X), a tube wall (2), an outer tube side (21) and an inner tube side (22), with continuously extending, axially parallel or helically wound internal ribs (3) formed from the tube wall (2) on the inner tube side (22), each internal rib (3) having two rib flanks (31) and a rib tip (32), and in each case a continuously extending groove (33) being formed between adjacent internal ribs (3), whereby the internal tube surface can be described by the following equation: Φ = e 2 / pd i where: Φ is a dimensionless parameter, e is the height of the helical rib, p is the helix pitch, and d i is the inner diameter of the tube, where N is the number of ribs counted in a cutting plane perpendicular to the tube axis, Characterized in that the product ΦN 2 is greater than 16 and less than 70.

2. The heat exchanger tube (1) according to claim 1, characterized in that the tangent circle between adjacent rib flanks (31) and the bottom line of the groove (33) between adjacent ribs (3) along the tube axial direction has a radius R, satisfying the relation e / 4 ≤ R ≤ e.

3. The heat exchanger tube (1) according to claim 1 or 2, characterized in that Product Φ × N 2 Greater than 19 and less than 55.

4. The heat exchanger tube (1) according to any one of claims 1 to 3, characterized in that the distance (B) between two internal ribs (3) is between 0.0236 inches and 0.0098 inches.

5. The heat exchanger tube (1) according to any one of claims 1 to 4, characterized in that the shape of the internal ribs (3) is variable.

6. The heat exchanger tube (1) according to any one of claims 1 to 5, characterized in that an external structure is formed on the outer tube side (22).

7. The heat exchanger tube (1) according to claim 6, characterized in that the external structure is designed in the form of an integral, helically wound external fin.

8. The heat exchange tube (1) according to claim 7, characterized in that the relationship ratio between the internal rib height and the outer fin height is between 0.80 and 0.62.

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