Heat exchanger tube

Abstract

Heat exchanger tube with a tube axis, a tube wall, a tube outside and a tube inside, and continuously running, axially parallel or helically circling inner ribs formed out of the tube wall on the tube insider. Each inner rib has two rib flanks and a rib tip, and a continuously extending groove formed between adjacent inner ribs. The internal tube surface can be described by the equation: Φ=e2 / pdi where: Φ is a dimensionless parameter, severity-factory; e is the height of the helical ribs; p is the helical pitch; and di is the tube inside diameter. N is the number of ribs counted in a cutting plane perpendicular to the tube axis. The product ΦN2 is greater than 16 and less than 70.

Description

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

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

[0003] To allow heat transport between the heat-dispensing medium and the heat-absorbing medium, the temperature of the heat-dispensing medium must be higher than the temperature of the heat-absorbing medium. This contrast in temperature is designated as the driving temperature difference. The higher the driving temperature difference is, the more heat can be transmitted. On the other hand, the aim is often to keep the driving temperature difference low since this has benefits for the efficiency of the process.

[0004] It is known that heat transmission can be intensified by means of the structuring of the heat transmission surface. What can be achieved thereby is that more heat can be transmitted per unit of heat transmission surface than in the case of a smooth surface. Furthermore, it is possible to reduce the driving temperature difference and consequently make the process more efficient. In metallic heat exchanger tubes, the structuring of the heat transmission surface is often obtained by the forming of ribs or similar elements out of the material in the tube wall. These integrally formed ribs have a firm metallic bond with the tube wall and can therefore transmit heat optimally.

[0005] An embodiment of heat exchangers which is often used is a tube bundle heat exchanger. These appliances frequently make use of tubes which are structured both on their inside and on their outside. Structured heat exchanger tubes for tube bundle heat exchangers usually possess at least one structured region and smooth end pieces and, if appropriate, smooth intermediate pieces. The smooth end pieces or intermediate pieces delimit the structured regions. So that the tube can easily be built into the tube bundle heat exchanger, the outside diameter of the structured regions should not be greater than the outside diameter of the smooth end pieces and intermediate pieces.

[0006] Axially parallel or helical ribs are often used on the inside of tubes to improve the heat transmission properties. The ribbing increases the heat transfer area of the inner surface of the tube. Furthermore, in the case of helically arranged ribs, the turbulence of the medium flowing in the tube is increased, and therefore heat transmission is improved. It is known that the heat transmission properties of axially parallel or helical ribbing on the tube inside can be improved by providing the inner ribs with notches or grooves. Examples of this are to be found in EP 1 312 885 B1, CN 101 556 124 A, CN 101 556 125 A, U.S. Pat. Nos. 5,992,513 A, 6,018,963 A, 6,412,549 B1, EP 2 339 283 B1 and U.S. Pat. No. 5,697,430 A. The notching of the ribs gives rise to a structure with an alternating rib height and with lateral material projections on the rib flank. This structure additionally increases the turbulence of the medium flowing in the tube.

[0007] Particularly in refrigerating and air conditioning technology applications, the efficiency of refrigerating plants in a part-load situation is assuming increasing importance. In a part-load situation, the through flow quantity of the heat transfer medium is often reduced, and therefore the velocity of the medium flowing in the tube decreases markedly. Since the main fraction of the heat transition resistance is then shifted onto the tube inside, it is necessary further to improve the now known structures on the tube inside.

[0008] The object on which the invention is based is to develop a heat exchanger tube in terms of the heat transmission properties.

[0009] The invention includes a heat exchanger tube with a tube axis, with a tube wall, with a tube outside and with a tube inside, continuously running, axially parallel or helically circling inner ribs being formed out of the tube wall on the tube inside, each inner rib having two rib flanks and a rib tip, a continuously extending groove being formed in each case between adjacent inner ribs. The internal tube surface can be described by the equation:Φ=e2 / pdiwhere:

[0011] Φ is a dimensionless parameter, severity-factor,

[0012] e is the height of the helical ribs,

[0013] p is the helical pitch in tube axial direction and

[0014] di is the tube inside diameter,whereby N is the number of ribs counted in a cutting plane perpendicular to the tube axis.

[0015] According to the invention for the product ΦN2 is greater than 16 and less than 70.

[0016] Severity factor as a dimensionless parameter mainly refers to enhancing factor in tube side heat transfer coefficient hiΦ=Φ⁢h⁢i=hi_ribbed⁢ surface / hi_bare⁢ surface

[0017] Generally acknowledged and disclosed in the prior art is the relationship between the internal heat transfer coefficient Ci and a geometric parameter, which is referred to as the “severity-factor”Φ. This parameter Φ is a dimensionless parameter which links the fin height e, the pitch p and the inner diameter di by the equation given above.

[0018] For example, as described in U.S. Pat. No. 5,697,430, the helical ribs on the inner surface of the tube have a predetermined rib height and pitch and a predetermined pitch angle.

[0019] The pitch is defined as the spacing between the fin tips of two adjacent fins, which spacing is measured in the axial direction.

[0020] In practice in air conditioning and process industry, all the highly enhanced surface has p / e ratio less than 5. Because when ratio p / e is greater than 5, the main flow of a single-phase fluid on the tube side reattaches to the wall, increasing the thickness of the boundary layer, which has an unfavorable effect on heat transfer.

[0021] The focus of the investigations is on highly enhanced finned tubes with a given p / e less than 5, in which the fin spacing p is at most five times as large as the fin height e as upper limit. Under this condition, it has been shown that there is no reattachment zone in the flow direction along the tube wall. Only an intrinsically stable recirculation zone is located between the repeating ribs. Here, a very strong interaction between vortex and main flow occurs on the upper side of a flow-side separation region, causing the velocity gradient and turbulent flow to reach a maximum value in the radial tube direction. The main flow is then forced to “slide” over the ribs. A second flow develops between the ribs.

[0022] For tube side heat transfer enhancement, it is also very important to evaluate the efficiency of heat transfer combining the pressure loss due to improved heat transfer. It can be deduced from the empirical data that the efficiency index PEC is related to Φ and N as follows:PEC∝Φ⁢N2(∝ Mathematical symbol representing positive proportional)

[0024] When p / e<5, the efficiency of finned tubes PEC is approximately in proportional to ΦN2. The larger ΦN2, the higher the efficiency of optimized finned tubes. When ΦN2 is greater than 16, PEC of the finned tube increases by at least 10% compared to that of prior art. Below are the detailed explanations on why the efficiency index as ΦN2 is chosen and on why the scope is applied as the 16≤ΦN2≤70.

[0025] In this application, the highly surface enhance tubes (p / e<5) would be the focus. When p / e is reduced in a smaller range, the influencing factor resulted from the rib shapes couldn't be ignored. The width of the ribs is now playing an important role. In practical applications, the rib shape is triangle or trapezoidal including the ones with chamfered edges. We introduce the dimension A and B measured at the center height line of rib height (e / 2 from the rib base plat form) to illustrate the flow mechanism here.

[0026] In this way,B=p-A

[0027] Where,

[0028] A . . . the rib width measured at center line of rib height along the tube axial direction,

[0029] B . . . channel width between two neighboring ribs measured at center line of rib height along the tube axial direction.

[0030] For the tube side enhanced heat transfer, it is very important to evaluate the pressure drop penalty as well. One of the indexes is listed below:Φ⁢f=f_ribbed⁢ surface / f_bare⁢ surface

[0031] From the measurement data available for zone p / e<5, Φf is in proportion to B2 value (the average channel spacing). Meanwhile, Φf is in revise ratio to di2

[0032] Then we have:Φ⁢f∝p2 / di2(p / e<5)

[0033] From the geometry of the helical rib surface,p.tan⁢θ=π⁢di / N

[0034] In this way, we have Φf ∝1 / N2

[0035] Where,

[0036] θ . . . helical angle between the ribs extending and the tube axial direction,

[0037] p . . . helical pitch, and

[0038] di . . . tube inside diameter,

[0039] N . . . number of ribs counted in a cutting plane perpendicular to the tube axis (X),

[0040] ∝ . . . mathematical symbol representing positive proportional.

[0041] In this way, we can have efficiency index PEC as follows:PEC=Φ / Φ⁢f=Φ⁢N2

[0042] For highly enhanced tubes (p / e<5), the efficiency of ribbed tubes is approximately in proportion to ΦN2. The larger ΦN2, the larger efficiency the optimized ribbed tubes will have. If ΦN2 is set to be greater than 16, PEC of the ribbed tube is at least increased 10% compared with prior art. Below are the reasons why PEC is enhanced when 16≤ΦN2≤70 from the enhanced heat transfer prospective:

[0043] a) Under the condition p / e<5 the negative influence of boundary layer development zone can be avoided.

[0044] b) Φ is a severity factor directly relates to an improved heat transfer coefficient in tube side. The actual heat transfer area with ribs is a monotone increasing function to N (rib number circumferentially). Both parameters are in positive ratio to hi (the tube side heat transfer coefficient) as corresponding area to hi is nominal area (area calculated assuming it is smooth bore).

[0045] c) It is a new finding from this invention that when p / e<5, tube side pressure drop f will decrease with increased N at fixed helix angle and fixed rib height e.

[0046] However, in all the prior art and literature studies, trend of growth relationship between the two parameters was believed to be in the opposite direction. When N is increased, p is decreased to restrict the recirculation zone from flat long ellipse to approximately round shape. And the shear stress contact area between the mainstream flow and the recirculation flow inside neighboring ribs is also reduced. In this way, there is less shear stress level between the main flow and the tube wall to reduce the pressured drop in tube side.

[0047] d) When ΦN2 is greater than 70, ratio of p / e is usually less than 1.5, considering width of ribs, p / B is even smaller than 1, the recirculation zone is narrowed again to form circular shape. We can have the optimized PEC efficiency here. Additionally, when ΦN2 is further increased, p / B is getting even smaller, two recirculation zone would be formed between the ribs which is the upper one and the bottom one which circulate in opposite directions. The bottom one is recirculating in less speed to result a decrease in heat transfer.

[0048] e) In addition, very small pitch value can result in increased tube rib weight, complexity in tooling design and fouling risk when ΦN2 is larger than 70.

[0049] Below are main advantages of the inventive solution:

[0050] By setting ΦN2 greater than 16 and less than 70, at least 10% more efficiency PEC can be achieved compared with prior art.

[0051] It is a big progress in enhanced heat transfer field to reduce the pressure drop penalty without sacrificing the heat transfer performance.

[0052] The invention has provided a ribbed tube with predetermined parameter ΦN2 to retube the heat exchangers with enhanced heat transfer and to reduced pressure drop and subsequent pumping power. And it has made it feasible to use less quantity of tubes without increasing the pressure drop in projects aiming at cost reduction opportunities.

[0053] In a preferred embodiment of the invention, a tangent circle between the neighboring rib flanks and the bottom line of the groove between adjacent ribs along tube axial direction with a radius R satisfies the relationship e / 4≤R≤e.

[0054] With other words, the reference plane in which the contact points of the tangent circle with the tube surface lies constitutes a plane of tube axial section.

[0055] The radius of tangent circle can be calculated in below formula from normal direction of ribs:R=(B / 2-e / 2⋆⁢C⁢O⁢T⁡(β / 2))⋆⁢T⁢A⁢N⁡(4⁢5+β / 4)Where,

[0057] β . . . include angle of rib in the tube axial section plane [degree]

[0058] Just as described in the previous paragraph, only an intrinsically stable recirculation zone is located between the repeating ribs. Here, a very strong interaction between vortex and main flow occurs on the upper side of a flow-side separation region, causing the velocity gradient and turbulent flow to reach a maximum value in the radial tube direction. The main flow is then forced to “slide” over the ribs. A second flow develops between the ribs. In this way, the radius of the tangent circle represents the scale of the second flow between neighboring ribs.

[0059] When R gets smaller than e, there is no longer a reattachment area between the ribs and the recirculation zone is restricted from a flat long ellipse to a round shape in the axial flow direction. And the shear stress contact area between the mainstream flow and the recirculation flow inside the neighboring ribs is also reduced. In this way, there is less shear stress level between the main flow and the tube wall to reduce the pressured drop in tube side.

[0060] When R is further reduced below to e / 4, there would be two recirculation zones in the channel between the ribs, shaped as upper zone and bottom zone. The bottom zone recirculates in less speed resulting a decrease in heat transfer. Moreover, the fluid stagnates in the channel, which is not good for heat transfer. In addition, a very small R-value can result in increased tube rib weight, complexity in tooling design and higher fouling risk.

[0061] In a further advantageous refinement of the invention, the product ΦN2 can be greater than 19 and less than 55. If ΦN2 assumes a value above 19 and below 55, only a particularly stable recirculation zone is formed between adjacent ribs. Advantageously, the recirculation zone thereby assumes a rather round shape. The shear stress contact area between the main flow and the recirculation flow in the adjacent fins is also optimized, further reducing the pressure drop on the tube side. The recirculation zone formed in this way circulates at a higher velocity, which leads to an increase in heat transfer.

[0062] In an advantageous manner the distance B between two inner ribs can be between 0.0236 inches and 0.0098 inches. B is the channel width between two neighboring ribs measured at center line of rib height along the tube axial direction. Within the defined interval for B, the main flow, which is forced to slide over the ribs, and the second flow, which develops turbulence between the ribs, reach a particularly stable and defined regime.

[0063] In a preferred embodiment the shapes of the inner ribs can be varying. A staggered surface on and along the ribs to increase surface area is beneficial to interfere with the recirculation zone and reduce the tube side pressure drop. Wavy rib tips and / or additional flank structure feasible and help stabilize the fluid flow. In a preferred embodiment an outer structure can be formed on the tube outside. Integral external ribs can advantageously run around the outside of the tube in an axially parallel or helical-line-shaped manner. For this case, a further aspect of the invention includes a method for producing a structured heat exchanger tube, with integral external ribs, i.e. machined from the tube wall, running around the outside of the tube in a helical-line-shaped, in which the following method steps are carried out. In a first forming region, external ribs running in a helical-line-shaped manner are formed on the outside of a smooth tube by the rib material being obtained by displacement of material from the tube wall by means of a first rolling step and the ribbed tube produced being caused to rotate by the rolling forces and being pushed forwards in accordance with the helical-line-shaped ribs produced, the external ribs being formed with a rising height from the otherwise undeformed smooth tube. In the first forming region, the tube wall is supported by a first roll mandrel which is situated in the tube, is mounted rotatably and is profiled, as a result of which the internal ribs are constructed. In further rolling steps, the external ribs are constructed in further regions spaced apart from the first forming region, with a further rising height, and the internal ribs are provided with secondary grooves, the tube wall being also supported in the further forming regions.

[0064] In the described manner the outer structure can be designed in the form of integral, spirally, circling outer fins.

[0065] In an advantageous manner the relation between inside rib height to outside fin height can be between 0.80 and 0.62. The advantages of the invention that have already been mentioned with regard to the heat exchanger tubes are added to by further advantages through the method of production by the dimensions, which are obtained with the different roll tools, of the internal and the external structure of the ribbed tube being able to be set independently of one another. Thus, for optimum passage of heat, the internal and the external structure can be optimally coordinated with each other. Optimization can be achieved with a lower rib height of the outer relative to the inner structural height.

[0066] To enable comparison of the improved tubes of the present invention, with previously known tubes, Tables 1 and 2 are provided. In these, the essential tube parameters are shown. The already known prior art tubes are listed in Table 1. Table 2 contain details of a selection of the tubes investigated in accordance with the invention. It can be seen from the tables that the parameter ΦN2 of the prior art tube type 3 with the highest value 13.021 is significantly lower than the values of the solution according to the invention. The parameter ΦN2 proves to be a benchmark for the optimization of the thermal properties of such heat exchanger tubes.TABLE 1(Prior Art):TypeTube 1Tube 2Tube 3Tube 4Tube 5Tube 6Tube 7Tube 8B0.0330.0330.0250.0490.0230.0320.0290.089Φ = e2 / pdi0.0080.0060.0090.0080.0080.0060.0070.002e / (p − b)0.6320.5230.7760.5680.8530.5730.7180.141e / p0.3090.3010.4100.2330.3300.2800.2960.086B / A1.7892.3872.5801.0771.0191.6821.3241.034p / e3.2383.3252.4404.2963.0283.5733.37311.67b / e2.0772.3431.7592.2281.5282.2411.9225.935B / (N*A)0.0530.0540.0680.0360.0270.0490.0350.103e / Fh0.7440.6670.6900.9170.5560.6740.3950.455N2Φ8.85612.5013.027.29211.317.27310.490.205PEC118%108%125%119%113%122%122%88%TABLE 2(Invention):TypeTube 9Tube 10Tube 11Tube 12Tube 13Tube 14B0.0190.0160.0190.0230.0270.025Φ = e2 / pdi0.0100.0110.0110.0090.0080.010e / (p − b)0.9361.1421.1310.7710.6710.849e / p0.4540.5160.4800.4120.3650.426B / A1.7801.6071.5412.0682.1531.963p / e2.2001.9392.0832.4292.7382.349b / e1.4091.1951.2631.6371.8701.556B / (N*A)0.0340.0320.0230.0430.0410.035e / Fh0.7290.7290.7920.7290.7500.667N2Φ26.86727.13947.78219.96722.20731.879PEC138%146%145%146%145%172%A . . . Rib width measured at center line [inches]B . . . Channel width between two neighboring ribs [inches]

[0069] b . . . Rib width along axis [inches]

[0070] p . . . Axial pitch of rib [inches]

[0071] Φ . . . Severity Factor (Φ=e2 / pdi)

[0072] Fh . . . Fin height outside [inches]

[0073] di . . . Inside diameter [inches]

[0074] e . . . Rib height [inches]

[0075] N . . . Number of rib starts counted in a cutting plane perpendicular to the tube axis

[0076] θ . . . Angle of rib from axis [degree]

[0077] PEC . . . Performance evaluation criteria, efficiency of tube side heat transfer enhancement versus pressure drop penalty factor.PEC=he⁢n⁢h⁢anced⁢ tube / hbare⁢ tubeΔ⁢P / Le⁢n⁢h⁢anced⁢ tube / Δ⁢P / Lbare⁢ tube)

[0078] Exemplary embodiments of the invention are explained in more detail by means of the diagrammatic drawings in which:

[0079] FIG. 1 shows a schematic illustration of the flow pattern in the area of the inner ribs,

[0080] FIG. 2 shows a schematic illustration of the inner ribs with further geometric parameters of the inner structure, and

[0081] FIG. 3 shows a diagram of the efficiency factor of the heat exchanger tubes vs. the parameter ΦN2=N2e2 / pdi.

[0082] Parts corresponding to one another are given the same reference symbols in all the figures.

[0083] FIG. 1 shows a schematic illustration of the flow pattern around the inner ribs 3 on the tube inside 22 of a heat exchanger tube 1. Between two ribs 3, an intrinsically stable recirculation zone RZ is located between the repeating ribs 3. Here, a very strong interaction between vortex and main flow MF occurs on the upper side of a flow-side separation region, causing the velocity gradient and turbulent flow to reach a maximum value in the radial tube direction. The main flow MF is then forced to “slide” over the ribs 3.

[0084] FIG. 2 shows a schematic illustration of the inner ribs 3 with further geometric parameters of the inner structure. A tangential circle is shown which adjoins the flanks 31 between adjacent inner ribs 3 and the base line of the groove 33. This tangential circle has a radius R which is at least a quarter of the inner rib height e and has at a maximum the inner rib height e as its upper limit. In this region, the fluid flow between two adjacent ribs 3 creates a stable recirculation zone RZ, as shown in FIG. 1. The interaction between vortex flow and main flow results in a small pressure drop in the heat exchanger tube 1 combined with a substantial increase in heat transfer. The fins 3 with a rib width A and a channel width B, which follow one another at a distance p, extend axially in periodic progression inside the heat exchanger tube 1.

[0085] FIG. 3 shows a diagram of the efficiency factor of the heat exchanger tubes as a function of the parameter N2Φ=N2e2 / pdi.

[0086] For the improvement of the tube-side heat transfer, it turns out to be particularly important to evaluate the efficiency of heat transfer by combining the pressure drop due to the improved heat transfer.

[0087] For highly enhanced tubes (p / e<5), the efficiency of ribbed tubes is approximately in proportion to ΦN2. When ΦN2 is greater than 16, the efficiency factor PEC of the finned tube increases by at least 10% compared to the prior art. The range of application according to the invention is given as 16≤ΦN2≤70.

[0088] For a value of ΦN2 greater than 70, a very small pitch value can lead to undesirable increased weight of the heat exchanger tube and complicated tool design, as well as a fouling risk.LIST OF DESIGNATIONS1 Heat exchanger tube

[0090] 2 Tube wall

[0091] 21 Tube outside

[0092] 22 Tube inside

[0093] 3 Inner ribs

[0094] 31 Rib flanks

[0095] 32 Rib tip

[0096] 33 Groove

[0097] X Tube axis

[0098] A Rib width

[0099] B Distance between two inner ribs, Channel width

[0100] e Height of the helical inner ribs

[0101] R Radius of tangent circle

[0102] MF Main flow

[0103] RZ Recirculation zone

Claims

1. A heat exchanger tube with a tube axis, with a tube wall, with a tube outside and with a tube inside, continuously running, axially parallel or helically circling inner ribs being formed out of the tube wall on the tube inside, each inner rib having two rib flanks and a rib tip, a continuously extending groove being formed in each case between adjacent inner ribs, whereby the internal tube surface can be described by the equation:Φ=e2 / pdiwhere:Φ is a dimensionless parameter,e is the height of the helical ribs,p is the helical pitch anddi is the tube inside diameter,whereby N is the number of ribs counted in a cutting plane perpendicular to the tube axis, wherein the product ΦN2 is greater than 16 and less than 70.

2. The heat exchanger tube according to claim 1, wherein a tangent circle between neighboring rib flanks and a bottom line of the groove between adjacent ribs along a tube axial direction with a radius R satisfies the relationship e / 4≤R≤e.

3. The heat exchanger tube according to claim 1, wherein the product Φ×N2 is greater than 19 and less than 55.

4. The heat exchanger tube according to claim 1, wherein a distance between two inner ribs is between 0.0236 inches and 0.0098 inches.

5. The heat exchanger tube according to claim 1, wherein the shapes of the inner ribs vary.

6. The heat exchanger tube according to claim 1, wherein an outer structure is formed on the tube outside.

7. The heat exchanger tube according to claim 6, wherein the outer structure is configured in the form of integral, spirally, circling outer fins.

8. The heat exchanger tube according to claim 7, wherein a relation between an inside rib height to an outside fin height is between 0.80 and 0.62.

Images