heat transfer tubes

Abstract

A heat transfer tube having a tube axis, a tube wall, a tube exterior, and a tube interior, wherein continuously extending, axially circular or helical circular internal ribs are formed from the tube wall on the tube interior, each internal rib having two rib side portions and a rib tip portion, and continuously extending grooves are formed in each case between adjacent internal ribs. The internal tube surface can be described by the following equation: Φ=e 2 / pd i During the ceremony, Φ is the dimensionless parameter severity factor, e is the height of the helical rib, p is the helical pitch, d i is the inner diameter of the pipe, N is the number of ribs counted on a cross section perpendicular to the tube axis. According to the present invention, the product ΦN 2 is greater than 16 and less than 70.

Description

[Technical Field]

[0001] The present invention relates to a heat transfer tube. [Background technology]

[0002] Heat transfer occurs in many branches of refrigeration and air conditioning technology, as well as in process and power engineering. These fields often use heat exchangers that use tubes for heat transfer. In many applications, a liquid or gaseous medium flows inside the tubes and is cooled or heated as a function of the direction of heat flow. Heat is distributed to or removed from a medium located outside the tubes.

[0003] To allow heat transfer between the heat distribution medium and the heat absorption medium, the temperature of the heat distribution medium must be higher than the temperature of the heat absorption medium. This temperature difference is called the driving temperature difference. The larger the driving temperature difference, the more heat can be transferred. On the other hand, it is often desirable to keep the driving temperature difference small, as this is beneficial in terms of process efficiency.

[0004] It is known that heat transfer can be enhanced by structuring the heat transfer surface. This allows for more heat to be transferred per unit area of ​​heat transfer surface than would be possible with a smooth surface. Furthermore, the driving temperature difference can be reduced, resulting in a more efficient process. In metallic heat transfer tubes, structuring the heat transfer surface is often achieved by forming ribs or similar elements from the material in the tube wall. These integrally formed ribs have a tight metallurgical bond with the tube wall and therefore allow optimal heat transfer.

[0005] A frequently used embodiment of a heat exchanger is a tube bundle heat exchanger. These devices often utilize structured tubes both inside and outside of the device. Structured heat transfer tubes for tube bundle heat exchangers typically have at least one structured region, smooth ends, and, optionally, a smooth middle section. The smooth ends or middle section define the boundaries of the structured region. To facilitate easy construction of the tubes into the tube bundle heat exchanger, the outer diameter of the structured region is preferably no larger than the outer diameter of the smooth ends and middle section.

[0006] Axially parallel or helical ribs are often used inside tubes to improve heat transfer characteristics. The formation of ribs increases the heat transfer area of ​​the tube's inner surface. Furthermore, in the case of helically arranged ribs, the turbulence of the medium flowing through the tube is increased, thus improving heat transfer. It is known that the heat transfer characteristics of axially parallel or helical ribs formed inside a tube can be improved by providing the internal ribs with V-shaped notches or grooves. Examples of this can be found in U.S. Pat. Nos. 5,623,499; 5,729,333; 5,729,349; and 5,829,503. The formation of V-shaped notches in the ribs results in a structure with staggered rib heights and lateral material protrusions on the rib sides. This structure further increases the turbulence of the medium flowing through the tube.

[0007] In particular, in refrigeration and air conditioning applications, the efficiency of refrigeration plant installations at partial loads is expected to become increasingly important. At partial loads, the throughflow rate of the heat transfer medium is often reduced, and therefore the velocity of the medium flowing through the tubes is significantly reduced. Since the main part of the heat transfer resistance is then transferred to the inside of the tubes, the currently known structures of the inside of the tubes need to be further improved. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] European Patent No. 1312885 [Patent Document 2] Chinese Patent No. 101556124 [Patent Document 3] Chinese Patent No. 101556125 [Patent Document 4] U.S. Patent No. 5,992,513 [Patent Document 5] U.S. Patent No. 6,018,963 [Patent Document 6] U.S. Patent No. 6,412,549 [Patent Document 7] European Patent No. 2339283 [Patent Document 8] U.S. Patent No. 5,697,430 Summary of the Invention [Problem to be solved by the invention]

[0009] The object of the present invention is to develop a heat transfer tube in terms of its heat transfer properties. [Means for solving the problem]

[0010] The present invention includes a heat transfer tube having a tube axis, a tube wall, a tube exterior, and a tube interior, wherein continuously extending, axially parallel or helical circular internal ribs are formed from the tube interior wall, each internal rib having two rib side portions and a rib tip portion, and in either case, continuously extending grooves are formed between adjacent internal ribs. The internal tube surface can be described by the following equation: Φ=e 2 / pd i During the ceremony, Φ is the dimensionless parameter severity-factor, e is the height of the helical rib, p is the helical pitch in the axial direction of the tube; d i is the inner diameter of the pipe, N is the number of ribs counted on a cross section perpendicular to the tube axis.

[0011] According to the present invention, the product ΦN 2is greater than 16 and less than 70.

[0012] The severity factor as a dimensionless parameter mainly refers to the enhancement factor on the tube-side heat transfer coefficient hi. Φ=Φhi=hi_ribbed surface / hi_exposed surface

[0013] Generally accepted and disclosed in the prior art is the relationship between the internal heat transfer coefficient, Ci, and a geometric parameter called the "severity factor," Φ, which is a dimensionless parameter that links the fin height, e, pitch, p, and inner diameter, di, via the equation shown above.

[0014] For example, as described in US Pat. No. 5,697,430, helical ribs on the interior surface of a tube have a predetermined rib height and pitch, and a predetermined pitch angle.

[0015] Pitch is defined as the distance between the fin tips of two adjacent fins measured axially.

[0016] In practice, in the air conditioning and process industries, highly reinforced surfaces have p / e ratios less than 5, because at p / e ratios greater than 5, the main flow of single-phase fluid reattaches to the wall at the tube side, increasing the boundary layer thickness and thereby negatively affecting heat transfer.

[0017] The focus of the study is on highly reinforced finned tubes with a given p / e of less than 5, with the fin spacing p being at most five times greater than the fin height e. It has been shown that under these conditions, no reattachment zones exist in the direction of flow along the tube wall. Essentially, only stable recirculation zones are located between the repeating ribs. In this case, very strong interactions occur between the vortex flow and the main flow above the lateral separation region, causing velocity gradients and turbulence to reach maximum values ​​radially along the tube. The main flow is then forced to "slide" past the ribs. A secondary flow develops between the ribs.

[0018] Regarding tubeside heat transfer enhancement, it is also very important to evaluate the heat transfer efficiency by combining the pressure drop due to the heat transfer improvement. The heat transfer efficiency can be estimated from empirical data where the efficiency index PEC is related to Φ and N as follows: PEC∝ΦN 2 (The mathematical symbol ∝ represents direct proportion.)

[0019] When p / e<5, the efficiency of the finned tube, PEC, is ΦN 2 It is roughly proportional to ΦN 2 The larger ΦN, the more efficient the optimized finned tube. 2 When ΦN is greater than 16, the PEC of the finned tube increases by at least 10% compared to the PEC of the prior art. 2 and why the range 16≦ΦN 2 This is a detailed explanation of whether it applies as ≦70.

[0020] In this application, we focus on highly surface-reinforced pipes (p / e<5). When p / e is reduced to a smaller range, the influence factor due to the rib shape cannot be ignored. The rib width then plays an important role. In practical applications, the rib shape is triangular or trapezoidal, including shapes with chamfered edges. Here, we introduce dimensions A and B measured at the height centerline of the rib height (e / 2 from the rib base platform) to illustrate the flow mechanism. In this way, B=pA In the formula, A: Rib width measured along the axial direction of the pipe at the centerline of the rib height B: Channel width between two adjacent ribs measured along the axial direction of the pipe at the centerline of the rib height is.

[0021] In tubeside heat transfer enhancement, it is also very important to evaluate the pressure drop tradeoff. One of the indicators is listed below.

[0022] Φf = f_ribbed surface / f_exposed surface From the available measurement data for the zone p / e<5, Φf is 2 On the other hand, Φf is proportional to d i 2 It is in an inverse relationship with in this case, Φf∝p 2 / d i 2 (p / e<5) get. From the geometry of the spiral rib surface, p tanθ=πd i / N is. In this way, Φf ∝ 1 / N 2 get. During the ceremony, θ: twist angle between the extending rib and the axial direction of the pipe, p...helical pitch, and d i ...inner diameter of the pipe, N: Number of ribs counted on a cross section perpendicular to the tube axis (X), ∝...the mathematical symbol for direct proportion is. In this way, the efficiency index PEC can be obtained as follows: PEC=Φ / Φf=ΦN 2

[0023] For highly reinforced pipes (p / e<5), the efficiency of the ribbed pipe is ΦN 2 It is roughly proportional to ΦN 2 The larger ΦN, the more efficient the optimized ribbed tube. 2 When is set to be greater than 16, the PEC of the ribbed tube increases by at least 10% compared to the prior art. The following is the expected heat transfer enhancement when 16≦ΦN 2 This is the reason why PEC is enhanced when ≦70.

[0024] a) Under the condition p / e<5, the adverse effects of the boundary layer development zone can be avoided. b) Φ is a severity factor directly related to the improved tube-side heat transfer coefficient. The actual heat transfer area with ribs is a monotonically increasing function of N (the number of circumferential ribs). Both parameters are directly proportional to hi (the tube-side heat transfer coefficient) when the area corresponding to hi is the nominal area (the area calculated assuming a smooth hole). c) It is a new discovery by the present inventors that for a fixed helix angle and a fixed rib height e, the tubeside pressure drop f decreases with increasing N when p / e<5. However, all prior art and literature studies have believed that the increasing relationship between the two parameters tends to be in the opposite direction. As N increases, p decreases, restricting the recirculation zone from a flat, elongated ellipse to a nearly round shape. The shear stress contact area between the main flow and the recirculating flow inside the adjacent rib also decreases. Thus, a smaller shear stress level exists between the main flow and the pipe wall, reducing the pressure drop on the pipe side. d)ΦN 2 When ΦN is greater than 70, the ratio p / e is usually less than 1.5, taking into account the width of the rib, and p / B is also less than 1, and the recirculation zone again narrows to form a circular shape. In this case, optimized PEC efficiency can be obtained. In addition, ΦN 2 As p / B increases further, it becomes smaller and smaller, forming two recirculation zones between the ribs: an upper zone and a lower zone that circulates in the opposite direction. The lower zone circulates at a lower velocity, resulting in reduced heat transfer. e) In addition, very small pitch values ​​increase the pipe rib weight, complicate tooling design, and increase ΦN 2 may pose a clogging hazard when is greater than 70. The following are the main advantages of the solution of the present invention:

[0025] ΦN 2 By setting PEC to be greater than 16 and less than 70, an efficiency PEC that is at least 10% greater than that of the prior art can be achieved.

[0026] Reducing the pressure drop penalty without sacrificing heat transfer performance is a major advance in the field of heat transfer enhancement.

[0027] The present invention provides a method for mounting tubes in a heat exchanger with enhanced heat transfer to reduce pressure drop and subsequent pumping power by a predetermined parameter ΦN. 2 The present invention provides a ribbed pipe having a ribbed design, which allows projects to utilize less pipe without increasing pressure drop, providing a cost reduction opportunity.

[0028] In a preferred embodiment of the present invention, the tangent circle having a radius R between adjacent rib side portions and the underline of the groove between adjacent ribs along the axial direction of the tube satisfies the relationship e / 4≦R≦e.

[0029] In other words, the reference plane on which the tangent circle touches the pipe surface constitutes the plane of the pipe cross section.

[0030] The radius of the tangent circle can be calculated from the normal direction of the rib using the following formula. R=(B / 2-e / 2×cot(β / 2))×tan(45+β / 4) During the ceremony, β: Incident angle of the rib in the plane of the pipe cross section [°] is.

[0031] Just as described in the previous paragraph, essentially only stable recirculation zones are located between the repeating ribs. In this case, very strong interactions occur between the vortex flow and the main flow above the lateral flow separation region, causing velocity gradients and turbulence to reach maximum values ​​in the radial direction of the pipe. The main flow is then forced to "slide" over the ribs. Secondary flows develop between the ribs. In this way, the radius of the tangent circle represents the scale of the secondary flows between adjacent ribs.

[0032] When R becomes smaller than e, there are no more reattachment sites between the ribs, and the recirculation zone is restricted to a round shape from a flattened elongated ellipse in the axial flow direction. And the shear stress contact area between the main flow and the recirculation flow inside the adjacent ribs is also reduced. In this way, there is a smaller shear stress between the main flow and the pipe wall, which reduces the pressure drop on the pipe side.

[0033] When R is further reduced below e / 4, there are two recirculation zones in the channel between the ribs, shaped as an upper zone and a lower zone. The lower zone circulates at a lower velocity, resulting in reduced heat transfer. Furthermore, the fluid stagnates in the channel, which is not good for heat transfer. In addition, a very small R can result in increased tube rib weight, more complex tooling design, and a higher risk of clogging.

[0034] In a further advantageous refinement of the invention, the product ΦN 2 can be greater than 19 and less than 55. 2 If ρ is greater than 19 and less than 55, only particularly stable recirculation zones are formed between adjacent ribs. Advantageously, the recirculation zones thereby assume a fairly rounded shape. The shear stress contact area between the main flow and the recirculation flow in adjacent fins is also optimized, further reducing the pressure drop on the tube side. The recirculation zones thus formed circulate at a faster rate, which leads to increased heat transfer.

[0035] In an advantageous manner, the distance B between two internal ribs can be between 0.0098 inches and 0.0236 inches. B is the channel width between two adjacent ribs, measured at the centerline of the rib height along the axial direction of the tube. Within the spacing defined for B, the primary flow forced to slide over the ribs and the secondary flow developing turbulence between the ribs reach a particularly stable and defined regime.

[0036] In preferred embodiments, the internal ribs can be shaped differently. Staggered surfaces on and along the ribs that increase surface area are beneficial in interfering with the recirculation zone and reducing tube-side pressure drop. Wavy rib tips and / or additional side structures are possible and help stabilize fluid flow.

[0037] In a preferred embodiment, an external structure can be formed on the outside of the tube. The integrated external ribs can advantageously extend around the outside of the tube parallel to the axial direction or in a manner that forms helical lines. In this case, a further aspect of the invention includes a method for producing a structured heat transfer tube, i.e., machined from the tube wall, with integrated external ribs that form helical lines and extend around the outside of the tube, by performing the following method steps: In a first forming zone, rib material is obtained by displacing material obtained from the tube wall by a first rolling step, and the resulting ribbed tube is rotated by a rolling force to extrude the ribs forward according to the ribs formed in the resulting helical lines, thereby forming the external ribs that extend in a manner that forms helical lines, the external ribs being formed with a rise height from an otherwise undeformed, smooth tube. In the first forming zone, the tube wall is supported by a rotatably mounted, contoured first rolling mandrel located within the tube, thereby building up the internal ribs. In another rolling step, external ribs are formed in another region spaced apart from the first forming region and have a further rise height, internal ribs are provided with secondary grooves, and the tube wall is also supported in the further forming region.

[0038] In the described approach, the external structure can be designed in the form of integral, spirally circular external fins.

[0039] In an advantageous manner, the ratio between the internal rib height and the external fin height can be between 0.62 and 0.80. The manufacturing method, with its dimensions obtained using different rolling tools, adds to the advantages of the invention already mentioned for the heat transfer tubes the further advantage that the internal and external structures of the ribbed tube can be set independently of each other. In this way, the internal and external structures can be optimally coordinated with each other for optimal heat transfer. Optimization can be achieved with a lower external rib height relative to the internal structure height.

[0040] In order to be able to compare previously known tubes with the improved tube of the present invention, Tables 1 and 2 are provided. These tables show essential tube parameters. The previously known prior art tubes are listed in Table 1. Table 2 contains details of the tubes studied and selected according to the present invention. The parameter ΦN of the prior art tube type 3 has the highest value of 13.021. 2 It can be seen from the table that the parameter ΦN is significantly lower than that of the tube according to the invention. 2 is found to be a criterion for optimizing the thermal properties of such a heat transfer tube.

[0041] [Table 1]

[0042] [Table 2] A…Rib width measured at the centerline [inches] B…Channel width between two adjacent ribs [inches] b...rib width along the axis [in] p...Axial pitch of ribs [inches] Φ…Severity factor (Φ=e 2 / pd i ) Fh: Outer fin height [inches] d i …inner diameter [inches] e...Rib height [inches] N: Number of rib starts counted on a cross section perpendicular to the tube axis θ...rib angle from axis [°] PEC: Performance Evaluation Index, efficiency of tube-side heat transfer enhancement at the expense of pressure drop

number

[0043] Exemplary embodiments of the invention will now be described in more detail with the aid of figures. [Brief explanation of the drawings]

[0044] [Figure 1] 1 shows a schematic representation of the flow pattern at the location of the internal rib. [Figure 2] 1 shows a schematic diagram of an internal rib with further geometric parameters of the internal structure. [Figure 3] A diagram of the efficiency factor of the heat transfer tube versus the parameter ΦN2=N2e2 / pdi is shown. DETAILED DESCRIPTION OF THE INVENTION

[0045] In all the figures, corresponding parts are given the same reference symbols.

[0046] FIG. 1 is a schematic diagram of the flow pattern around an internal rib 3 on the tube interior 22 of a heat transfer tube 1. Between the repeating ribs 3, an essentially stable recirculation zone RZ is located between two ribs 3. In this case, above the flow lateral separation region, a very strong interaction occurs between the vortices and the main flow MF, causing the velocity gradient and turbulence to reach a maximum value in the radial direction of the tube. The main flow MF is then forced to "slide" over the ribs 3.

[0047] FIG. 2 shows a schematic diagram of the internal rib 3, showing other geometric parameters of the internal structure. The tangent circle is tangent to the side surface 31 between the reference line of adjacent internal ribs 3 and grooves 33. This tangent circle has a radius R of at least one-quarter of the internal rib height e and an upper limit of the internal rib height e. In this region, as shown in FIG. 1, the fluid flow between two adjacent ribs 3 generates a stable recirculation zone RZ. The interaction between the vortex flow and the main flow results in a small pressure drop in the heat transfer tube 1, coupled with substantial heat transfer enhancement. Fins 3, each with a rib width A and a channel width B, extend axially in a periodic sequence within the heat transfer tube 1, following each other by a distance p.

[0048] Figure 3 shows the parameter N 2 Φ=N 2 e 2 / pd i 1 shows a diagram of the efficiency factor of the heat transfer tube as a function of

[0049] For improving tube-side heat transfer, it proves particularly important to evaluate the heat transfer efficiency by combining the pressure drop resulting from the heat transfer improvement.

[0050] For highly reinforced pipes (p / e<5), the efficiency of the ribbed pipe is ΦN 2 It is roughly proportional to ΦN 2 When ΦN is greater than 16, the efficiency factor PEC of the finned tube increases by at least 10% compared to the prior art. 2 ≦70.

[0051] ΦN greater than 70 2 At values ​​of , very small pitch values ​​can lead to undesirable tube weight increases, complex tooling designs, as well as the risk of clogging. [Explanation of symbols]

[0052] 1 Heat transfer tube 2 Pipe wall 21 Outside of the tube 22 Inside the pipe 3 Internal ribs 31 Rib side 32 Rib tip 33 Groove X tube axis A Rib width B: Distance between two internal ribs, channel width e Height of the internal spiral rib R Radius of the tangent circle MF Main flow RZ Recirculation Zone

Claims

1. A heat transfer tube (1) having a tube axis (X), a tube wall (2), a tube exterior (21), and a tube interior (22), wherein continuously extending, axially parallel or spirally circular internal ribs (3) are formed from the tube wall (2) at the tube interior (22), each internal rib (3) having two rib side portions (31) and a rib tip portion (32), and in each case, continuously extending grooves (33) are formed between adjacent internal ribs (3); The internal tube surface can be described by the following equation: Φ=e ２ / p / / ｉ During the ceremony, Φ is a dimensionless parameter, e is the height of the spiral rib; p is the helical pitch, d ｉ is the inner diameter of the tube, N is the number of the ribs counted on a cross section perpendicular to the tube axis (X). Product ΦN ２ is greater than 16 and less than 70 A heat transfer tube (1).

2. The tangent circle having a radius R between the adjacent rib side portions (31) and the line of the lowest point of the groove (33) between the adjacent ribs (3) along the pipe axis direction satisfies the relationship e / 4≦R≦e. A heat transfer tube (1) according to claim 1, characterized in that

3. The product ΦN ２ The heat transfer tube (1) according to claim 1 or 2, characterized in that is greater than 19 and less than 55.

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

5. The heat transfer tube (1) according to any one of claims 1 to 4, characterized in that the internal ribs (3) have different shapes.

6. A heat transfer tube (1) according to any one of claims 1 to 5, characterized in that the outside (22) of the tube is provided with an external structure.

7. 7. The heat transfer tube (1) according to claim 6, characterized in that the external structure is designed in the form of integral, spirally circular external fins.

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

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