JPH04260792A - Small-diameter heat transfer tube - Google Patents

Abstract

PURPOSE:To enhance heat transfer performance in a tube, to prevent collapse of a groove due to an extension of the tube when fins are mounted on an outside of the tube, and to limit a decrease in the heat transfer performance to a minimum limit by specifying-the shape of the groove of a small-diameter heat transfer tube for a heat exchanger. CONSTITUTION:In a small-diameter heat transfer tube 1 having 3-6mm of an outer diameter, a groove 3 spiral or continuous in an axial direction is formed on an inner surface of the tube, the depth H of the groove 3 is formed 0.15 < H < 0.25m, the width W1 of the bottom is formed 0.10<=W1<=0.20mm, and a ratio t/D of the thickness (t) of the bottom to the outer diameter D of the tube is formed 0.025<=t/D<=0.075.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvements in small-diameter heat exchanger tubes used in heat exchangers for refrigerators, cavity machines, and the like.

0002

[Prior Art and its Problems] In recent years, there has been a strong demand for energy saving and space saving for heat pump air conditioners, and it has become important to make the heat exchanger, which is the main part of the air conditioner, more efficient and more compact. Heat pump air conditioners use aluminum fins with louvers, etc. cut out on the surface for heat exchange with the air on the outside of the tube.A tube expansion plug is passed inside the heat transfer tube, the tube is expanded, and the aluminum fin is brought into close contact with the aluminum fin. Cross-fin type heat exchangers are mainly used, which are installed vertically in the fins and flow a refrigerant such as Freon into the tubes. In the past, smooth tubes were used as heat transfer tubes. However, recently, an internally grooved tube in which a large number of fine spiral grooves are formed on the inner surface of the tube has been developed, and this has improved the heat transfer performance within the tube. Currently, due to improvements in heat exchangers, the main
．． 53 mm and 7.00 mm internally grooved tubes are used. Recently, there has been a demand for more compact heat exchangers, and in response, the development of compact heat exchangers that effectively use thinner heat exchanger tubes with an outer diameter of 4 mm, etc., is progressing. A small-diameter heat exchanger tube was announced in 1982-98200. However, simply using small-diameter heat exchanger tubes only increases the pressure loss within the tubes and does not improve the efficiency of the heat exchanger. In order to use small diameter tubes more effectively,
In addition to optimizing the groove shape of small-diameter pipes, it is necessary to develop an effective groove shape with consideration to workability. Another problem is the tube expansion problem when assembling the heat exchanger. That is, if the wall thickness is the same, the smaller the tube diameter, the more the ridges on the inner surface of the tube will be crushed, and the groove will be deformed. Typically,
It is known that the groove depth greatly affects the heat transfer performance within the tube, and in order to improve the efficiency of the heat exchanger, it is necessary to minimize the deterioration in performance due to groove deformation.

0003

[Problems to be Solved by the Invention] In view of this, as a result of various studies, the present invention has dramatically superior heat transfer performance, and the present invention is capable of reducing heat transfer performance by collapsing the grooves during pipe expansion for assembling a heat exchanger. We have developed a small-diameter internally grooved tube that can minimize the

0004

[Means for Solving the Problems] The present invention has an outer diameter of 3 to 6 m.
A small-diameter heat exchanger tube with a groove depth of 0.15<H<0.25mm and a groove bottom width of 0.10≦W1≦0.20mm, spirally formed on the inner surface of the tube or continuous in the axial direction of the tube. This is a small-diameter heat exchanger tube characterized in that the ratio of the bottom wall thickness to the outside diameter of the tube is 0.025≦t/D≦0.075.

[0005]

[Function] According to the small-diameter heat transfer tube of the present invention, the heat transfer performance inside the tube can be dramatically improved, and even when the tube is expanded and brought into close contact with the outer fin, the performance deterioration due to the collapse of the groove can be kept to a minimum. Can be done. This makes it possible to manufacture a compact heat exchanger that is significantly more compact and efficient than conventional heat exchangers. In the heat exchanger tube of the present invention, the outer diameter D is set to 3 to 6 mm because if it is less than 3 mm, it will be difficult to form the prescribed grooves, and if it exceeds 6 mm, it will not contribute to making the heat exchanger more compact. This is because it almost disappears. Also, the groove depth H is 0.15<H<0.25
The reason why the groove bottom width W1 is set to 0.10 to 0.20 mm is to achieve the same processability and cost as conventional internally grooved tubes, and to optimize the heat transfer performance. In addition, the bottom wall thickness t for the tube outer diameter D is 0.025≦t/D≦0.
．． The reason for setting the diameter to 075 is to minimize performance deterioration due to groove collapse. Note that the groove apex angle α is 20°<
It is preferable that α<50°.

[0006]

[Embodiment] An embodiment of the present invention will be described below. Various small-diameter heat exchanger tubes made of phosphorus-deoxidized copper and having an outer diameter of 4 mm as shown in FIG. 1 were manufactured by rolling using a grooved plug. Table 1 shows the specifications of typical heat exchanger tube shapes.

[0007]

[Table 1]

These small-diameter heat exchanger tubes were assembled into a double-tube heat exchanger, Freon R-22 was flowed inside the heat exchanger tubes, water to be cooled was flowed outside the tubes, and evaporation and evaporation inside the tubes were conducted under the measurement conditions shown in Tables 2 and 3. The condensing heat transfer coefficient and its pressure drop were measured.

[0009]

[Table 2]

0010

[Table 3]

FIGS. 2 and 3 show the pressure loss during evaporation and condensation versus the refrigerant mass flow rate in the tube. As can be seen from FIG. 3, the pressure loss during condensation is approximately 1.8 times greater than that of a smooth tube due to the effect on the grooves. However, there is almost no difference in groove shape such as groove depth. Furthermore, as shown in FIG. 2, during evaporation, there was also little difference due to the groove shape, which was about 1.4 times that of the smooth tube. 4 and 5 show the heat transfer coefficients in the tube during evaporation and condensation with respect to the groove bottom width W1. The refrigerant mass flow rate at this time is 400 kg/m2S. In FIG. 4, it can be seen that when the groove depth is increased, the optimum value exists around H=0.1 to 0.15 mm. This is because when the number of grooves is increased for a constant groove depth, the inner peripheral length of the heat transfer tube increases, and the heat transfer performance improves accordingly. However, if the number of grooves is increased too much, the groove bottom width will be extremely reduced, making it difficult to form a liquid film within the grooves, and the grooves will constantly be filled with liquid, resulting in a decrease in performance. That is, the optimum value of the inner peripheral length of the heat transfer tube and the amount of liquid film in the groove exists in the vicinity of 0.1 to 0.15 mm. Next, FIG. 6 shows the best performance for each groove depth obtained in FIGS. 4 and 5. According to this, condensation performance increases almost in proportion to groove depth, whereas evaporation performance tends to increase rapidly at groove depth H=0.15 mm or more. Furthermore, considering that the pressure loss during condensation is 1.8 times that of a smooth tube, it is desired that the heat transfer performance is at least twice that of a smooth tube. Considering this, it is possible to form the groove depth so that H>0.15mm.
is necessary. In addition, the groove depth is set to H>0.15 mm,
When considering performance improvement, the groove bottom width is set to 0.
By forming 10≦W1≦0.20mm, performance nearly twice that of a smooth tube can be obtained during condensation, and H
It can be seen that a dramatic improvement in performance can be expected compared to ≦0.15.

Next, in order to examine the effect of pipe expansion on a small diameter pipe, the bottom wall thickness was measured using the same method as described above for a shape with an outer diameter of 4 mm, 36 grooves, a groove depth of 0.22 mm, and a groove bottom width of 0.15 mm. Various types of small diameter tubes were manufactured. after that,
After each sample was annealed, a tube expansion plug having an outer diameter 0.6 mm larger than the minimum inner diameter of the internally grooved tube was inserted into the groove in the axial direction to expand the tube. Figure 7 shows the collapse of the groove at this time by Δ
h (difference in groove depth before and after expansion) and bottom wall thickness t relative to outer diameter
/D relationship was shown. As expected, groove collapse increases with increasing bottom wall thickness. Also, t/D≦0.025
However, the bottom wall thickness was too thin, and the pipe broke during groove processing. Next, the sample after tube expansion was subjected to heat transfer measurement during evaporation using the method described above. The results are shown in Figure 8, where the groove collapse Δ
It is shown as performance against t. The figure also shows the highest performance value with the same groove depth as the groove depth after pipe expansion obtained from Figures 4 and 5. According to this, when Δt<0.04, the performance after pipe expansion shows a decrease in performance due to the decrease in groove depth, but when Δt>0.04, the decrease in groove depth causes the performance to deteriorate at the peak. The shape becomes a greatly collapsed trapezoid shape that does not retain its original shape, and the performance decreases more than the decrease in groove depth, resulting in a significant decrease in performance compared to the optimum shape with the same groove depth. Taking these things into consideration, we can see from Fig. 7 that the groove collapse Δ
When t=0.04, t/D≒0.075, so the ratio of bottom wall thickness to tube outer diameter, t/D, is 0.025≦t/D≦0.
．． It can be seen that it is desirable to set the value to 075.

[0013]

[Effects of the Invention] As described above, according to the small diameter heat exchanger tube of the present invention,
The heat transfer performance within the tube can be dramatically improved, and even when the tube is expanded and brought into close contact with the outer fins, the deterioration in performance due to groove collapse can be minimized. This makes it possible to manufacture a compact heat exchanger that is significantly more compact and efficient than conventional heat exchangers.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a main part of a small-diameter heat exchanger tube according to an embodiment of the present invention.

FIG. 2 is a diagram showing pressure loss during evaporation.

FIG. 3 is a diagram showing pressure loss during condensation.

FIG. 4 is a diagram showing groove bottom width and evaporative heat transfer coefficient.

FIG. 5 is a diagram showing groove bottom width and condensation heat transfer coefficient.

FIG. 6 is a diagram showing groove depth and heat transfer coefficient.

FIG. 7 is a diagram showing the relationship between the ratio of the bottom wall thickness and the tube outer diameter and the collapse of the groove during tube expansion.

FIG. 8 is a diagram showing a decrease in heat transfer performance after pipe expansion.

[Explanation of symbols]

1. Small diameter heat exchanger tube 2 Yamabe 3 Groove

Claims (1)

[Claims] 1. A small diameter heat exchanger tube with an outer diameter of 3 to 6 mm, the inner surface of the tube having a groove depth of 0.0 mm that is spiral or continuous in the axial direction of the tube.
15<H<0.25mm, groove bottom width 0.10≦W1≦0
．． A groove of 20 mm is formed, and the ratio of the bottom wall thickness to the outside diameter of the tube is 0.0.
A small diameter heat exchanger tube characterized in that 25≦t/D≦0.075.