JPS5924311B2 - Heat transfer tube with multiple internal ridges - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a metal tube material for the purpose of heat transfer, and in particular to a metal tube material whose inner surface has been given a special shape in order to improve its properties.

U.S. Patent No. 3.217.799, U.S. Patent No. 3.463.9
No. 97, No. 3.481.394, No. 3.559.
No. 4.37 and 1967 U.S. Patent Application No. 674.61
No. 1, 197] No. 224.095 of the same year, and 1972
No. 232.571 of the same year, the degree of thermal transition is significantly enhanced by imparting a special shape to the inner and/or outer surface of the tube compared to a flat tube. can do.

Regarding the external surface, externally finned tubes significantly increase the external surface and the fins can give rise to advantageous properties of significantly increasing the external film coefficient with respect to heat transfer. There is technology.

In other words, further logical improvements are required for the heat transfer device consisting of the following: by deforming the inner surface of the tube having fins on the outside.

One such attempt was made in addition to the above-mentioned US patents, as well as US Patent Nos. 2181927, 2220726, and 24
No. 32308, No. 2913009, No. 30884
As shown in No. 94 and No. 3,612,175, a spiral or annular ridge is provided on the inner surface in order to increase the vortex flow of the fluid.

In order to compare the tube-side heat transfer properties of different tubes with different internal geometries, a 5iedev-
It is sufficient to use Tate's formula.

hi d i/ni=Ci (diG/μ)”8(Cpμ
/K)3(p,/vw)”4・−・(Formula 1) hi=internal thermal transition coefficient, Btu/hr−sqft−
oF.

di - inner diameter of tube (ft).

K - Thermal conductivity of the fluid in the pipe at the total fluid temperature, (B
tu/hr-sqft-oF)/ftsCi=
In-tube heat transfer constant (dimensionless), G = mass velocity (1
b/hr −5qft), Cp-specific heat (Btu
/ 1 b oF), μ - viscosity of the fluid in the pipe at the average overall fluid temperature (1b/ f t-hr ), μ W - the viscosity of the fluid in the pipe at the average wall temperature, the above equation uses the correction coefficient Ci. If possible, it can be applied to single-phase fluids flowing dropwise in flat or internally ridged tubes.

The dimensionless inner heat transfer constant "CI" for a particular tube consisting of [Industrial Eng.
ineeringChemistry Process
Design & Development J 1st
0, No. 1 (1971), J.G.

In a paper entitled "Condensation of Vapor in Vertically Aligned Horizontal Wave-Shaded and Smooth Tubes" by Reiser et al.
It can be determined experimentally by a modified Wilson curve drawing method.

Although it is desirable to design the tube so that Ci is maximized, there are many cases where it would be better to have a force where Ci is a low but constant value.

This last condition is mainly found in cases where the permissible pressure drop is severely limited.

If the designer is constrained in selecting internal feature elements by limitations in metal processing capabilities or by the need for material conservation, the designer may choose to maximize Ci, rather than ultimately maximizing Ci. , it is important to keep it within the current constraints.

That is, it is highly desirable to be able to predict thermal transition properties as a function of geometry number.

It is therefore an object of the present invention to provide a metallic heat transfer tube having an internal morphology with improved heat transfer properties.

Accordingly, a metal tube with an improved rate of heat transfer to or from a fluid flowing therein provided by the present invention is characterized in that the metal tube has a constant pitch length and lead angle and multiple starting points. having a plurality of integral internal helical ridges extending radially inwardly from the inner wall of the metal tube, said plurality of internal helical ridges (measured from the perpendicular toward the tube axis); a) a lead angle of less than 60° and a pitch distance that is greater than the pitch distance of at least one external fin, provided that the lead angle of each of said at least one external fin and said plurality of internal ridges is , the length and/or direction of the inner wall of the tube is different from each other, and the inner wall of the tube is located between the flat inner wall portion of the tube between the two adjacent peaks in the vertical cross-sectional shape. together with said ridges defining an intermediate flat between adjacent inner ridges, i.e. a portion of the actual inner wall of the tube, having a cross-sectional profile including a pair of lateral boundaries connecting the apices of the ridges; The lateral boundary portion is formed of a recessed portion and a protruding portion, and these portions are formed such that:
They are characterized in that they are interconnected at curvature turning points radially outward from the tip and at a distance from the tip less than or equal to the height of the ridge.

That is, the above aspects are achieved by the metal heat transfer tube according to the invention, in which a spiral ridge having a plurality of starting points is integrally formed on the inside of its cylindrical shape.

The effect of the ridges is to disturb the fluid flowing within the tube so that the fluid cannot create a boundary layer along the tube wall that would prevent heat transfer between the fluid and the tube wall.

Although the prior art has also suggested significant geometrical considerations that affect the thermal transfer characteristics, the response of the thermal transfer coefficient within the tube to changes in geometry can be predicted in a manner that It has not been possible to relate it to geometric properties.

No. 3,217,799 cited the ratio of the axial spacing dimension between adjacent ridges to the height dimension of the ridges as an important variable.

Although this method is also an important consideration, it is not clear enough to limit the most advantageous tube design to the extent that the internal heat transfer characteristics can be predicted or made extremely thick.

Next, 1978 Patent Application No. 20720 (Reference Document A)
In , a geometrical variable called the strict coefficient and C
The relationship between i has been announced.

This coefficient φ belongs to dimensionless and is expressed by the following equation: height e), pitch (p), and inner diameter (d
i) Consisting of:

φ−e2/pti (Equation 2) In reference A above, for a tube with a single starting intratubular ridge, there is a maximum possible Ci, and over the range of values of φ consisting of Rather, it is derived that the above maximum value occurs at a specific value of φ.

For single thread internal ridge tube, φ20.365X10-
Since the thickest values of Ci have been found to occur when As such, it can be formed.

For tubes with a single helical ridge inside, Ci vs. φ
The above relationship is also of interest in the design of tubes with internal multiple starting ridges.

That is, the latter tube has a higher thermal transfer coefficient for the given conditions, both in terms of severity and in terms of pressure drop, than in a tube with a single helical internal ridge. It turns out that it is possible to design.

Therefore, the present invention also includes the strictness coefficient φ as a basic element.

This fact suggests that for tubes with multiple starting internal ridges,
In improving the heat transfer characteristics within the tube, the above technology is advanced by clarifying the role of the ridge and the dimensions of the ridge and the tube.

In particular, the present invention is directed to the connecting portion between the intermediate flat portion between the inner raised portions and the convex curve and the concave curve between the tip of the raised portion and the intermediate flat portion in the longitudinal cross-sectional shape. It pertains to an internal canal wall formed to define.

Further, as disclosed in the aforementioned U.S. Pat. No. 3,481,394,
In some tube embodiments, the tube includes a single internal rib or ridge and a plurality of external fins.

The pitch of the single internal rib in the above US patent is, of course,
The ribs are more dog-like than the pitch of the multiple external fins,
Since it is formed according to the valley of the groove that defines two adjacent external fins, it has coexistence with the fins.

The helical ridge progression is more canine than the fin progression due to the reduced diameter of the outer root of the tube adjacent to the internal ridge.

The result is that conventional tubes are less rigid (more susceptible to vibrations) than the tube material of the present invention.

The improved tube according to the present invention also improves the heat transfer and pressure drop characteristics of the tube without the size, shape, number of starting points, and advance angle of the internal ridges being fixed relative to the external fins. The range of design choices is also expanded.

Currently used tubing has a uniform wall thickness at the bottom of the fin, except for the thicker areas at the ridges, but at least one method (U.S. Pat. No. 35,594)
No. 37), U.S. Patent No. 3481
It follows that the tube according to No. 394 may have a reduced wall thickness near the internal ridges.

That is, for a given strength, the strength of the tube of the present invention requires less material.

After designing and testing a large number of tubes with different multi-starting ridge cross-sectional shapes and different dimensional features, a mathematical model, i.e. It is now possible to create an equation.

Conversely, if a particular value of the constant Ci is desired, it is possible to predict the variables of the tube, such as the base width of the ridge, that will give the desired constant.

It has been found that within the scope of the above equation, as the height of the ridge increases, and as the width of the ridge decreases, the thermal transfer properties are enhanced.

However, there are many factors that affect the size of the ridge.

For example, the processing characteristics of the metal may limit the extent to which the metal substance of the tube can be moved radially inward, thus limiting the maximum height of the ridge.

If narrow ridges are desired, problems may arise in fabricating the appropriate metalworking tools to form the ridges.

In external forces, if the width of the ridge is narrower, it is easier to achieve a deeper depth than if the width is narrower, and it is also more resistant to wear when in contact with corrosive fluids. Become more of a dog.

However, these advantages can only be obtained by oversizing the tubing or by losing some external heat transfer surface.

The above equation created to predict the coefficient Ci can be written as follows.

0.0264+(22,1)(φ)(1-b/p) for C
(e/y)a (Formula 3) where, φ = Stringency factor (Formula 2), b - Basal width of the ridge (measured in the axial direction), C - On the adjacent ridge in the axial direction pitch measured between two corresponding points; e - height of the ridge; y - height of the cap of the ridge measured radially from the apex of the ridge to the point of curvature at its cross-sectional boundary;

The above equations can be applied to internal helical ridged tubes having an intermediate flat or cylindrical inner wall between adjacent internal ridges.

The limitations on the application of the above equation to make a good tube are as follows.

That is, b/p should be between 0.10 and 0.20.
φ should be less than 0.25 x 10''-2, and e/y should be between 1.50 and 5.00.

Next, the functions and effects of the present invention will be explained in more detail with reference to the drawings showing embodiments of the present invention.

FIG. 1 represents an axial cross-section of a tube, indicated generally at 10, made in accordance with the present invention.

Tube 10 includes a plurality of external fins 12,14 and a plurality of multi-start internal ridges 16,18.

The external fins 12, 14 and the internal raised portions 16°18 are:
Preferably, it can be integrally and simultaneously formed from the wall portion 20 of the tube on a grooved straight shaft (not shown).

The inner wall 22 of the tube is adapted to have a cylindrical cross-section, ie to form an intermediate flat, except where it is interrupted by inner ridges 16,18.

The width of the raised part is "b", the pitch of the raised part is "...", and the helical angle is "θ".

This θ is the angle measured from a plane perpendicular to the axis.

The specific dimensions of a practical tube with a cross-section as shown in FIG.
33" (8,46mm), di=0.820" (20,83mm), φ20.11
6X10", b=0.064" (1,62mm), y=o, o, os9" (0,226mm), b/p
=0.2.

e/y=2.00, Ci (expected value)=0.05
2°Ci (actual value) = 0.052. θ=39°.

Fin starting point-3, ridge starting point-6, Material - Copper.

FIG. 2 is a characteristic diagram showing the relationship between the internal heat transfer coefficient and the strictness coefficient for a number of tubes.

The lower curve 26 represents the characteristics for a single helical corrugated tube with a curved inner wall cross-section, as published in Reference Example A, supra.

The upper curve 28 represents the characteristics for a multi-helical, internally ridged tube with a flat inner wall cross section in accordance with the invention.

Both curves show that when φ=O, i.e. for a flat tube, the value Ci
They intersect at 20.0264.

Both curves 26 and 28 generally represent the heat transfer properties of both tubes and their respective improvements when compared to a flat tube.

The relationship between the degree of heat transfer and the pressure drop (in Fig. 12,
It is expressed using Ci and a friction coefficient f.

The pressure drop is proportional to the coefficient of friction when comparing tubes of a given diameter at the same Reynolds number.

According to FIG. 12, the degree of improvement in Ci at a given pressure drop for the tube of the invention (curve 29) compared to the tube according to the prior art (curve 30) is clear. .

The prior art tube represented by curve 30 is of the type published in Reference A above, which corresponds to a single-start, internally raised tube with a curved internal wall cross-section.

FIG. 3 shows a diagram for an internally raised tube with a large number of multiple starting points, having an intermediate flat internal wall profile and varying ridged morphology, countering the thermal transfer coefficient predicted by Equation 3 above. This is a graph showing the inner thermal transition coefficient Ci as a curve, which was created experimentally.

This graph shows a fairly good relationship between the predicted and experimental values, as evidenced by the proximity of the points representing the various test results on the graph to the 45° line 32 representing perfect agreement. It shows that there is a match.

As seen from equations 2 and 3, the inner ridge 1
The height, width, and shape of tubes 6 and 18 individually play an important role in determining the heat transfer and pressure drop characteristics of a particular tube design.

Figures 4 to 7 show the common ridge width b (Cos θ) and the common ridge height e, i.e., the innermost tip, when viewed from a direction perpendicular to the ridge passage. and the intermediate plateau.

In these examples, when =bCosθ, 4
The cross-sectional shapes of two different ridges are shown.

A pair of lateral force boundaries of each of the above-mentioned ridge cross-sections, that is, a surface portion connecting the apex and root of the ridge are determined by the recessed curve 36 and the protrusion curve 38.

The concave curve and the protruding curve merge at the curvature exchange point, and the fourth
The curvature transformation point is shown as 40 in the figure.

The ridge 44, which has a cross-sectional shape defined by the combination of the two curves 36, 38, has a width portion 46 with a height ryJ equal to the radial distance between the apex of the ridge and the point of curvature transformation 40. It is growing.

The base portion 50 of the raised portion 44 has a width b (Cos θ) and a height e−y).

The cross-sectional shapes of the various ridges shown in FIGS. 4 to 1 are as follows:
The re/yJ values of the four drawings are 1.50 and 2.0, respectively.
0, 3.00 and 4.00.
They are different in that their dimensions are different.

In the cross-sectional shape of the ridge in FIG. 8, the top of the ridge is shown as a flat portion 48, and the shape is similar to that in FIG. 5, except that the cap and base are made wider by the dimension fc (cos θ). are the same.

Since FIG. 8 is a cross-sectional view of the protrusion in one axial plane, the arc in FIG. 5 is an elliptical arc stretched along fc (Cos θ) by a factor of 1/Cos θ. It has become.

Within the range of tubes for which the values of stringency factor and ridge pitch are given, the base of the ridge section in FIG. Although the coefficient of movement is lower, it is advantageous in terms of manufacturing.

It is easier to prepare a core with a wide groove than to have a core with a narrow groove, and it is easier to form the core to form a wide ridge than a narrow ridge. , the force of moving the metal of the tube is easier.

When corrosive fluids flow through the tube, the force of increasing the width of the ridges increases wear life.

It is extremely difficult to polish the grooves in the core that produce the curved cross-sectional shapes shown in FIGS. 4-8.

Therefore, for example, the curved cross-sectional shapes 36 and 38 in FIG.
Dotted line 36'.

It has been found that even when approximated by straight lines such as 36", 38', and 38", sufficiently satisfactory results can be obtained.

In this case, on the line connecting lines 36', 36", β・8', and 38", point 40 is the same as in the case of curve 36.38.
can be seen to be a curved transition point in a continuous straight line.

The advantage of applying a linear approximation to curves is that if the extremely thin grinding wheel blades used to form the grooves have a straight cross-section, this will be easier to fabricate than if they were curved. It is also easy to maintain.

Figure 9 shows the relationship between the strictness coefficient φ and the heat transfer constant Ci listed above when Equation 3 is drawn with e/y as a variable and the value of b/p as 0.15. It represents.

This graph shows that the value of CI increases as the value of e/y is increased from 1.5 to 5.0, given the stringency factor φ.

Lines 51, 52, 53 and 55 indicate the value of e/y, respectively.
．． 5, 2, 3, and 5 are shown.

Using a single graph, such as the graph shown in Figure 9, one can obtain the degree of transition of a given heat for a given stringency and a given value of b/p. The form of the ridge used in the process can be easily determined.

For example, according to FIG. 9, for a strictness factor of 0.15X10-2, using the ridge shape with e/y of 30 as shown in FIG. 7, the value Ci =0.067fj will be obtained.

The shape of the protuberance in Figure 4, where the cap is wider and easier to manufacture and has an e/y value of 1.5, is 0.15XIQ-2.
will yield a value of Ci =0.059 for the stringency coefficient of .

FIG. 10 is a graph similar to FIG. 9, with the only difference from FIG. 3 being that various values of b/p are plotted as opposed to Ci.

This graph, drawn with a constant stringency factor of φ = 0, I y
It is shown that the value of Ci decreases as the value of b/p increases with respect to the value of .

That is, this graph shows that by reducing the width of the ridges relative to the pitch p, the heat transfer efficiency is improved.

FIG. 11 shows the 9
FIG. 10 is a graph similar to FIG.

0. This curve, drawn for a constant value of the strictness factor φ, I X I F2, given the value of b/p,
It is shown that as e/y increases, the value of the thermal transition coefficient Ci increases.

Both lines 71 and 72 have b/p values of o, i, and 0.2, respectively.

As can be seen from Equation 3 and the graphs of FIGS. 9, 10 and 11, the ridges are superior in effectiveness to conventional tubes and can be made to have a specific value of the thermal transfer coefficient Ci. It is possible to design external fins and multiple internal ridged tubes with flat intermediate walls in between.

For example, if we have a tube with an internal diameter of 0.8" (21,6 mm) and assume that the value of ICi is equal to 0.056, representing the special heat transfer requirements, then the width of the ridge is This can be determined by the following steps:

a) Based on known metal processing limitations, assume the maximum height of the ridge that can be fabricated, e20.0175".

b) Assume that the minimum pitch p that can be created in the form of the six starting point ridges is 0.3" (7,62 mm).

Once the diameter, height of the ridge, number of starting points and pitch are known,
The helical angle of the ridge is almost fixed.

C) Calculate the value of φ from Equation 2.

φ=e''/pdi=o, 128X10-2d) Select a ridge shape that is easy to mold and wear resistant, such as the e/y2 shape shown in FIG.

e) Solve Equation 3.

Ci =0.0264+22.1(φK l b/p
)(e/yl)30.05620.0264(22,1
)(0,00128)(1-b/p)(2g1, b/p=0.833 b/p=0.167.

Since p20.3, b=0.167 (0,3) 20,050" (1,27mm) Next, we will compare the effect of using the heat transfer tube of the present invention compared to the conventional equipment. Comparison is made with that of a metal tube member with inner and outer ribs manufactured by the manufacturing method disclosed in Japanese Patent Publication No. 1756/1983.

The specifications of both tubes are as follows, but in the tube of the above-mentioned publication, what corresponds to the "intermediate flat part" on the inside of the tube of the present invention is not substantially formed, and the inner raised part is curved. I can't see the conversion point.

The advance angle is 42.95° at the base of the raised part, 44.60° at the tip, and 2.4° at the outer part of the fin.
4 degrees, and 2.94 degrees at the base, the fin pitch was 0.0503 inches, and the number of fins was 2.

As a result of tests conducted on pressure drop and heat transfer efficiency, according to the present invention, the pressure drop for the same amount of water flow under the same conditions is 50% lower than that of Japanese Patent Publication No. 1756/1983. The heat transfer efficiency was also found to increase by 16-24%.

[Brief explanation of the drawing]

FIG. 1 is a partial longitudinal section of a tube made according to the invention; FIG. 2 is a function of the severity variable φ for an internal multiple helical ridge tube with an internal flat intermediate wall profile consisting of: Figure 3 shows the experimental versus expected values for the internal heat transfer coefficient Ci for several multi-helical internal ridge tubes with internal intermediate flat wall cross-sections. Graphs 4 to 7 depicting the relationship between the number FIG. 8 is a view similar to FIG. 5, except that the ridge is widened and the axial plane is the cross-section, and FIGS. Figure 11 shows the equation Ci −=0.264−4−(22,■)(φ)(1−
b/p) (e/p) Graph showing the effect of varying the strict dust φ, the ridge dimensions eeh/p and e/y on the inner thermal transfer coefficient 01 , FIG. 12 is a graph comparing the thermal transfer coefficient versus pressure drop characteristics for two different types of internal helical ridged tubes. In the figure, 10: tube according to the invention, 12, 14: external fins, 16, 18: internal ridges, 20: wall portion of the tube,
22: Inner wall of tube.

Claims (1)

[Scope of Claims] 1. A metal tube having an improved heat transfer rate from and to a source of fluid flowing therein, wherein the metal tube has a constant pitch length, a certain lead angle, and a plurality of starting points. a plurality of internal helical integral ridges extending radially inwardly from an inner wall of the metal tube having a constant advance angle of not more than 60° (measured from the plane) and a pitch distance that is greater than the pitch distance of at least one external fin, provided that said at least one external fin and said plurality of internal ridges , the leading angles of the tubes are different in thickness and/or direction, and the inner wall of the tube is intermediate between adjacent inner ridges in the vertical cross-section. said raised portion is formed to define a flat portion, said raised portion having a raised cross-sectional shape having a pair of lateral force boundaries connecting said intermediate flat portion and a tip of said raised portion; ,
the lateral force boundary comprises a recess and a protrusion, and at a curved transformation point where the recess and the protrusion are at a distance from the apex radially outwardly from the apex less than or equal to the height of the protuberance;
Metal tubes with improved heat transfer rate, characterized in that they are interconnected.