JP2006064211A - Heat transfer pipe for evaporator and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat transfer pipe for an evaporator and its manufacturing method, superior in evaporating performance even when carbon dioxide is used as a refrigerant. <P>SOLUTION: A pipe material is manufactured by hot extruding a billet composed of copper or copper alloy. The pipe material is rolled by placing a mandrel in the pipe and allowing a roll to be rotated and brought into contact with an outer face of the pipe. Then, a floating plug is placed inside of the pipe, and the pipe material after rolling process is radially reduced by using radial reduction dies. Then drawing process is performed two or more times without using the floating plug, here, a sectional reduction rate of the pipe material in the first drawing process is 5-30% to achieve average surface roughness in the circumferential direction of a pipe inner surface of 0.2-20 μm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a heat exchanger tube for an evaporator of a heat exchanger that uses carbon dioxide as a refrigerant and a method for producing the same, and in particular, a heat exchanger tube for an evaporator that uses carbon dioxide having a refrigeration oil content of 0.5% by mass or less. It relates to the manufacturing method.

Conventionally, chlorofluorocarbon solvents have been used for heat exchangers installed in air conditioners, car air conditioners, refrigerators, refrigerators, water heaters, vending machines, etc. Recently, natural refrigerants such as carbon dioxide, which are safe, inexpensive, non-toxic and flammable, and have little addition to the environment, are attracting attention. Table 1 below shows physical property values of the fluorocarbon refrigerants (R22, R134a, R410A) and carbon dioxide (CO ₂ ). In addition, each physical property value shown in the following Table 1 is a value when the saturation temperature is 0 ° C.

  As shown in Table 1 above, carbon dioxide has a high liquid constant pressure specific heat and liquid thermal conductivity that greatly affect the thermal characteristics, and has better heat transfer performance than CFC-based refrigerants (R22, R134a, R410A). Also, since carbon dioxide has a low surface tension, bubbles are more likely to be generated than chlorofluorocarbon refrigerants, and nucleate boiling is promoted. Therefore, when carbon dioxide is used as the refrigerant, it is transmitted compared to the case where chlorofluorocarbon refrigerant is used. Thermal performance is improved. Furthermore, since carbon dioxide has a smaller liquid viscosity and density than a chlorofluorocarbon refrigerant, the pressure loss is small. Furthermore, carbon dioxide has a feature that the vapor density and latent heat are large, and the refrigeration effect per unit excluded volume is larger than that of the fluorocarbon refrigerant.

  On the other hand, carbon dioxide has a problem of low theoretical performance in a simple cycle of cooling and heating. For this reason, when carbon dioxide is used as the refrigerant, high heat transfer performance is required for the heat transfer pipe through which the refrigerant flows. Conventionally, heat transfer pipes for evaporators of heat exchangers using carbon dioxide as the refrigerant are Since a smooth tube having a low transmission rate was used, the evaporator had to be enlarged in order to obtain a large evaporation performance (see, for example, paragraphs 0003 and 0004 of Patent Document 1). As a method of improving the evaporation performance of such a heat transfer tube for an evaporator, there is a method of forming a groove on the inner surface to expand the heat transfer area (see, for example, Patent Documents 2 and 3). In the evaporator, the heat transfer rate of the heat transfer tube is increased by using a heat transfer tube provided with a plurality of protrusions extending in the longitudinal direction of the tube on the inner wall of the tube, thereby reducing the size of the evaporator.

JP 2003-343492 A Japanese Patent No. 2524983 JP-A-4-18231

  However, the conventional techniques described above have the following problems. That is, when carbon dioxide is used as the refrigerant, nucleate boiling greatly affects the evaporation performance of the evaporator of the heat exchanger. Therefore, as in Patent Documents 1 to 3, a groove is formed on the inner surface of the heat transfer tube. Even if the thermal area is enlarged, there is a problem that a sufficient effect cannot be obtained. Moreover, since the pressure in a heat exchanger tube will become high when carbon dioxide is used for a refrigerant | coolant, it is necessary to make the thickness of a heat exchanger tube thicker than the case where a fluorocarbon refrigerant is used. For this reason, when forming a groove | channel in the inner surface of a heat exchanger tube, there also exists a problem that the groove depth is restrict | limited.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an evaporator heat transfer tube having excellent evaporation performance even when carbon dioxide is used as a refrigerant, and a method for manufacturing the same.

  The heat transfer tube for an evaporator according to the first invention of the present application is a heat transfer tube for an evaporator of a heat exchanger that uses carbon dioxide having a refrigerating machine oil content of 0.5% by mass or less as a refrigerant, and the inner circumferential direction of the tube The average surface roughness at is from 0.2 to 20 μm.

In the present invention, since the average surface roughness in the tube circumferential direction of the inner surface is 0.2 to 20 μm, nucleate boiling is promoted by the cavity formed on the inner surface of the tube, and even when CO ₂ refrigerant is used, Excellent heat transfer performance can be obtained.

  The average surface roughness is preferably 0.4 to 15 μm. Thereby, heat-transfer performance can be improved more. Further, the evaporator heat transfer tube can be formed of, for example, copper or a copper alloy.

  The method for manufacturing a heat exchanger tube for an evaporator according to the second invention of the present application is a method for manufacturing a heat exchanger tube for an evaporator of a heat exchanger that uses carbon dioxide having a refrigerating machine oil content of 0.5 mass% or less as a refrigerant. , A step of reducing the diameter of a raw tube made of copper or a copper alloy using a floating plug and a diameter reducing die, and a step of emptying the diameter-reduced element pipe twice or more without using a floating plug The cross-sectional reduction rate of the raw tube in one empty drawing process is 5 to 30%.

  In the present invention, since the drawing process with a cross-sectional reduction rate of 5 to 30% is performed twice or more using only a reduced diameter die, the average surface roughness in the pipe circumferential direction of the inner surface is 0.2 to 20 μm. Thus, a heat transfer tube with excellent heat transfer performance is obtained.

According to the present invention, by roughening the average surface roughness in the circumferential direction of the pipe of the pipe surface than the conventional smooth tube, because it optimizes the state of the cavity formed in the tube surface, CO ₂ is a refrigerant Nucleate boiling is promoted, and excellent heat transfer performance is obtained.

Hereinafter, an evaporator heat transfer tube according to an embodiment of the present invention will be specifically described with reference to the accompanying drawings. The heat transfer tube of the present embodiment is a smooth tube made of a metal material such as copper or copper alloy, and has an average surface roughness Ra in the tube circumferential direction of the inner surface of 0.2 to 20 μm. Then, the heat transfer tube is incorporated carbon dioxide content of the refrigerating machine oil is not more than 0.5 mass% (CO ₂₎ to the evaporator of the heat exchanger to be used as a refrigerant. The outer diameter of the heat transfer tube of this embodiment is, for example, 4 to 7 mm, the thickness is, for example, 0.3 to 1.0 mm, and fins or protrusions are formed on the outer surface, for example.

  Hereinafter, the reason for the numerical limitation in the heat transfer tube for an evaporator of the present embodiment will be described.

Average surface roughness Ra in the pipe circumferential direction of the inner surface: 0.2 to 20 μm
When the refrigerant circulating in the heat transfer tube boils, bubbles are generated from fine depressions (hereinafter referred to as cavities) on the inner surface of the tube. Yes. When carbon dioxide is used as a refrigerant, such nucleate boiling greatly affects the evaporation characteristics of the evaporator heat transfer tube. Therefore, the evaporation performance of the heat transfer tube can be improved by facilitating nucleate boiling.

  However, if the average surface roughness Ra of the inner surface of the heat transfer tube is less than 0.2 μm, the wettability of the refrigerant on the tube surface is reduced and nucleate boiling is not promoted. On the other hand, when the average surface roughness (Ra) of the inner surface of the tube exceeds 20 μm, the opening of the cavity widens, that is, the cavity has an open structure, and bubbles are hardly generated. Therefore, in the heat transfer tube of this embodiment, the average surface roughness Ra in the tube circumferential direction of the inner surface is set to 0.2 to 20 μm. The average surface roughness Ra in the tube circumferential direction of the inner surface is more preferably 0.4 to 15 μm. Thereby, the nucleate boiling of the refrigerant | coolant which distribute | circulates the inside of a pipe | tube is further accelerated | stimulated, and the evaporation characteristic of a heat exchanger tube is improved significantly.

Refrigerating machine oil content in the CO ₂ refrigerant: 0.5% by mass or less Usually, the refrigerating machine oil such as polyalkylene glycol oil is contained in the CO ₂ refrigerant circulating in the heat transfer tube. This refrigerating machine oil is a lubricant for compressors, improves the lubricity of moving parts such as bearings and cylinders, and cools moving parts by absorbing heat generated by friction, making the compressor better It is intended to operate. However, if the refrigeration oil content in the CO ₂ refrigerant exceeds 0.5 mass%, the heat transfer performance is improved even if the average surface roughness (Ra) in the tube circumferential direction of the inner surface of the heat transfer tube is 0.2 to 20 μm. Decrease significantly. Therefore, the refrigerating machine oil content in the CO ₂ refrigerant to be circulated in the heat transfer tube of the present embodiment is 0.5% by mass or less.

  Next, the manufacturing method of the heat exchanger tube for evaporators of this embodiment is demonstrated. First, a billet made of copper or a copper alloy and having a diameter of, for example, 300 mm is hot-extruded to produce an element tube having an outer diameter of 100 mm and a wall thickness of 10 mm, for example. Next, a mandrel is placed in the raw tube obtained by extrusion, and a roll is rolled on the outer surface of the tube, for example, rolled so that the outer diameter of the tube is 40 mm and the wall thickness is 2 to 4 mm. . Subsequently, the drawn pipe is subjected to a drawing process. Specifically, in a state where the floating plug is arranged inside the tube, the tube is pulled out from a die hole having a diameter smaller than the outer diameter of the tube, for example, the outer diameter of the tube is set to 15 mm and the wall thickness is set to 2 mm. Then, without using a floating plug, a drawing process (hereinafter referred to as empty drawing) is performed using only a die, for example, to obtain a smooth tube having an outer diameter of 7 mm and a wall thickness of 1 mm.

  In the method of manufacturing an evaporator heat transfer tube of the present embodiment, the cross-section reduction rate Re (= the cross-sectional area before emptying / the cross-sectional area after emptying) in one empty drawing is set to 5 to 30%. The drawing process is performed at least twice. Thereby, since the surface roughness of the inner surface of the tube becomes rough, the average surface roughness in the tube circumferential direction can be set to 0.2 to 20 μm. However, if the cross-section reduction rate Re in a single blanking process is less than 5%, the productivity is reduced and the floating plug is not used. To do. On the other hand, when the cross-section reduction rate Re exceeds 30%, the diameter is rapidly reduced, and therefore, depending on the ratio between the outer diameter and the wall thickness and the tempering (mechanical properties), there is a case where turning-up occurs. Note that “turn-up” refers to a state in which a part of the tube is depressed and the portions on both sides of the depressed portion are turned toward each other.

  In the method for manufacturing an evaporator heat transfer tube according to the present embodiment, the inner surface of the pipe is roughened by performing the emptying process with a cross-section reduction rate Re of 5 to 30% twice or more, so that the pipe circumferential direction The average surface roughness can be 0.2 to 20 μm. As a result, the performance of the heat transfer tube can be improved as compared with the conventional smooth tube.

Next, the operation of the evaporator heat transfer tube of the present embodiment configured as described above will be described. The heat exchanger tube for an evaporator according to the present embodiment is incorporated in an evaporator of a heat exchanger such as an air conditioner, a refrigerator, a water heater, a refrigerator, and a vending machine. And inside the heat transfer tube, a CO ₂ refrigerant having a refrigeration oil content of 0.5% by mass or less, for example, at a pressure of 3 to 5 MPa in the case of a water heater and an air conditioner, It distribute | circulates at the pressure of 3 Mpa or less. At this time, CO ₂ boils and vaporizes in a heat transfer tube incorporated in the evaporator. As a result, heat is removed from the heat transfer tube, and the air around the heat transfer tube is further cooled.

Evaporator heat exchanger tube of the present embodiment, since the average surface roughness Ra in the circumferential direction of the pipe of the inner surface to 0.2 to 20 [mu] m, CO ₂ refrigerant amount of the refrigerating machine oil is not more than 0.5 mass% Even when incorporated in an evaporator of a heat exchanger that uses nuclei, nucleate boiling is promoted by the cavities formed on the inner surface of the tube, so that excellent heat transfer performance can be obtained.

  Hereinafter, the effect of the Example of this invention is demonstrated compared with the comparative example which remove | deviates from the scope of the present invention. First, as a heat transfer tube for an evaporator according to Example 1 of the present invention, a copper tube specified in JIS H3300 C1220 is rolled with a mandrel and a roll, then drawn with a floating plug and a die, and then is emptied after drawing the plug. Drawing was performed twice to produce a smooth tube having an outer diameter of 6.00 mm and a wall thickness of 0.60 mm. Table 2 shows the blanking conditions at that time.

  In addition, as a heat transfer tube for an evaporator according to Comparative Example 1 of the present invention, a copper tube defined in JIS H3300 C1220 is rolled with a mandrel and a roll, and then drawn with a floating plug and a die so that the outer diameter is 6 A smooth tube having a thickness of 0.000 mm and a wall thickness of 0.60 mm was produced.

  Next, SEM (Scanning Electron Microscope) observation of the inner surface of the heat transfer tube of Example 1 and Comparative Example 1 was performed. 1A is an SEM photograph (magnification: 250 times) showing the inner surface of the heat transfer tube of Example 1, and FIG. 1B is an SEM photograph (magnification: 250 times) showing the inner surface of the heat transfer tube of Comparative Example 1. ). As shown in FIGS. 1 (a) and 1 (b), the heat transfer tube of Example 1 that was subjected to empty drawing twice after plug drawing was finished by plug drawing without empty drawing. The surface was rougher than the heat transfer tube of Comparative Example 1. Therefore, when the average surface roughness Ra in the tube circumferential direction of the inner surface of these heat transfer tubes was measured, the heat transfer tube of Example 1 was 0.5 μm, and the heat transfer tube of Comparative Example 1 was 0.08 μm. FIG. 2 is a diagram showing the measurement results of the surface roughness in the tube circumferential direction of the inner surface of the heat transfer tube of Example 1.

Next, the heat transfer coefficient and pressure loss of the heat transfer tubes of Example 1 and Comparative Example 1 were measured, and the heat transfer performance was evaluated. FIG. 3 is a diagram showing a configuration of an apparatus used for measurement of heat transfer coefficient and pressure loss, and FIG. 4 is a diagram showing a configuration of the evaporator. As shown in FIG. 3, the measuring apparatus used in this embodiment includes a compressor 2 to a high temperature by compressing a CO ₂ refrigerant, the gas cooler 4 is a condenser, expands the CO ₂ refrigerant An expansion valve 7 for lowering the temperature and an evaporator 1 as a test part are provided. A preheater 8 and a superheater 9 for heating the CO ₂ refrigerant are respectively provided at the entrance and exit of the evaporator 1. An oil separator 3a that separates refrigeration oil in the refrigerant is provided at the outlet of the compressor 2, and further, pulsation of the refrigerant is eliminated at the inlet of the compressor 2 and the outlet of the gas cooler 4, respectively. Accumulator 5a and 5b are provided.

In this example, polyalkylene glycol oil was used as the refrigerating machine oil for the compressor 2. Then, the refrigeration oil in the CO ₂ refrigerant is separated by the oil separator 3 b provided at the outlet of the evaporator 1, and the separated refrigeration oil is cooled by the oil cooler 10, and then passes through the oil pump 11 and the flow meter 12. Then, by adding again to the CO ₂ refrigerant, the refrigerant oil content in the refrigerant immediately before the evaporator 1 was adjusted to 0.1 mass%. Incidentally, the refrigerating machine oil content of CO ₂ in the refrigerant, the CO ₂ refrigerant is collected in the sampling port 14 immediately before the preheater 8, was determined by measuring its mass by a precision analytical balance. Further, while supplying heat source water to the evaporator 1, cooling water was supplied to the gas cooler 4 and the oil cooler 10. Further, a direct current was supplied to the preheater 8 and the superheater 9.

  In this embodiment, a micro motion type mass flow meter 6 with an accuracy of ± 0.4% is provided between the gas cooler 4 and the expansion valve 7, and the flow rate of the refrigerant is measured by the flow meter 6. . Further, between the evaporator 1 and the superheater 9, between the oil separator 3b and the compressor 2, between the compressor 2 and the gas cooler 4, between the gas cooler 4 and the accumulator 5b, and the flow meter 6 and expansion. Refrigerant mixing chambers 18a to 18e are provided between the valves 7 respectively. The temperature and pressure of the refrigerant are respectively determined by the 0.5 mm chromel-alumel-coated thermocouples 19a to 19e and the pressure transducers 20a to 20e having an accuracy of 0.02 MPa provided in the refrigerant mixing chambers 18a to 18e. It was measured. At that time, the thermocouples 19a to 19e were calibrated in advance so that the error was within ± 0.05K.

Furthermore, as shown in FIG. 4, the evaporator 1 has three double tubes in which the heat transfer tubes of Example 1 or Comparative Example 1 are arranged inside an outer tube having a diameter of 18 mm and an inner diameter of 12 mm. 15a to 15c are connected in series. These double tubes 15a to 15c have a length of 0.688 m and an effective heat transfer length of 0.5 m. Then, the inside of the heat transfer tube of Example 1 or Comparative Example 1 was circulated CO ₂ refrigerant, and between these heat exchanger tubes and the outer pipe is circulated cooling water. At that time, the flow direction of the CO ₂ refrigerant and the flow direction of the cooling water were made opposite to each other.

  The evaporator 1 is provided with a heat source 13, and the flow rate of the cooling water was measured with a gear type flow meter with an accuracy of ± 0.5% provided in the heat source 13. Further, heat source water mixing chambers 16a to 16f are provided at both ends of the double pipes 15a to 15c, respectively, and the temperature of the cooling water is 2 outside diameters installed in the heat source water mixing chambers 16a to 16f. Measured with a resistance thermometer of 0 mm. At that time, each resistance thermometer was calibrated so that the error was within ± 0.02K. Furthermore, the pressure difference between the double tubes 15a to 15c was measured by differential pressure transducers 17a to 17d with an accuracy of ± 0.25%. Furthermore, the temperature of the outer wall of each heat transfer tube was measured by placing copper-constantan thermocouples formed of a copper wire and a constantan wire having an outer diameter of 0.1 mm on the top, bottom, left and right of the outer surface of the heat transfer tube. At that time, these thermocouples were calibrated so that the error was within ± 0.05K.

The temperature T _wi of the inner wall of each heat transfer tube was determined from the following formula 1.

In the above formula 1, T _wo is the temperature of the outer wall of the heat transfer tube, λ _w is the heat conductivity of the heat transfer tube, d _wi and d _wo are the inner diameter and outer diameter of the heat transfer tube, respectively, and Q is 2 The thermal conductivity of the heavy pipes 15a to 15c, and Δz is the effective heat transfer length of the double pipes 15a to 15c.

  Further, the flow rate of the refrigerant was changed between 10 and 40 kg / hour, and the pressure at the inlet of the test part was adjusted to be 3 to 5 MPa. Furthermore, the total heat balance, that is, the ratio of the increase in the amount of heat due to the cooling water and the loss of the amount of heat due to the refrigerant was less than 5%. Furthermore, the thermodynamic properties and transport performance of carbon dioxide are reported in REFPROP Ver. 7.0 (M. O. McLinden, 2 others, 2002, Peskin AP, NIST thermodynamic properties of reactants and refrigerant mixtures database (REFPROP), Ver. 7.0) was used for calculation.

  Further, the local heat transfer coefficient α of the heat transfer tube was obtained from the following formula 2.

Here, _{T b} in the above equation 2 is the temperature of the CO ₂ refrigerant during normal pressure in double tube 15a to 15c. Further, q is a heat flux in the double tubes 15a to 15c, and is obtained by the following mathematical formula 3.

  Note that the uncertainty of the local heat transfer coefficient α obtained by Equation 2 is less than 10%, and the uncertainty of the heat flux q obtained by Equation 3 is less than 6%.

FIG. 5 is a graph showing the performance of the heat transfer tubes of the examples and comparative examples, with the horizontal axis representing the dryness and the vertical axis representing the heat transfer coefficient. FIG. 6 is a graph showing the performance of the heat transfer tubes of the example and the comparative example, with the horizontal axis representing the dryness and the vertical axis representing the pressure loss. Note that values shown in FIGS. 5 and 6, 4 MPa refrigerant inlet pressure _{P in,} is a value when the refrigerant flow rate Gr and 362 kg / ^{m 2} sec, the dryness is the gas phase mass flow rate to the total mass flow rate Is the ratio. As shown in FIGS. 5 and 6, the heat transfer tube of Example 1 had the same pressure loss as the heat transfer tube of Comparative Example 1, and the heat transfer coefficient was higher than that of the heat transfer tube of Comparative Example 1.

Next, using the heat transfer tube of Example 1, the refrigerating machine oil content in the CO ₂ refrigerant was changed, and the heat transfer performance was evaluated by the method described above. FIG. 7 is a graph showing the relationship between the refrigerating machine oil content and the heat transfer performance, with the horizontal axis representing the amount of refrigerating machine oil in the CO ₂ refrigerant and the vertical axis representing the performance reduction rate. As shown in FIG. 7, the heat transfer tube of Example 1 was able to suppress a decrease in heat transfer performance when the refrigeration oil content in the CO ₂ refrigerant was 0.5 mass%.

  Next, a plurality of heat transfer tubes having different average surface roughness Ra on the inner surface of the tube were produced, and the heat transfer performance was evaluated by the method described above. FIG. 8 is a graph showing the relationship between the average surface roughness Ra of the pipe inner surface and the heat transfer performance, with the horizontal axis representing the average surface roughness Ra in the pipe circumferential direction of the heat transfer pipe inner surface and the vertical axis representing the heat transfer coefficient. FIG. As shown in FIG. 8, excellent heat transfer performance was obtained with a heat transfer tube having a surface roughness Ra of 0.2 to 20 μm in the tube circumferential direction on the inner surface of the tube.

(A) is the SEM photograph (magnification: 250 times) which shows the inner surface of the heat exchanger tube of Example 1, (b) is the SEM photograph (magnification: 250 times) which shows the inner surface of the heat exchanger tube of the comparative example 1. It is a figure which shows the surface roughness in the pipe peripheral direction of the inner surface of the heat exchanger tube of Example 1. FIG. It is a figure which shows the structure of the apparatus used for the measurement of a heat transfer rate and a pressure loss. It is a figure which shows the structure of the evaporator of the measuring apparatus shown in FIG. It is a graph which shows the performance of the heat exchanger tube of an Example and a comparative example, taking a dryness on a horizontal axis and taking a heat transfer coefficient on a vertical axis | shaft. It is a graph which shows the performance of the heat exchanger tube of an Example and a comparative example, taking a dryness on a horizontal axis and taking pressure loss on a vertical axis | shaft. The horizontal axis represents the refrigerating machine oil of CO ₂ refrigerant, and the vertical axis represents the performance degradation rate is a graph showing the relationship between the refrigerating machine oil content and the heat transfer performance. It is a graph which shows the relationship between the average surface roughness of a pipe inner surface, and heat transfer performance, taking the average surface roughness in the pipe peripheral direction of a heat transfer pipe inner surface on a horizontal axis, and taking a heat transfer coefficient on a vertical axis | shaft.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1; Evaporator 2; Compressor 3a, 3b; Oil separator 4; Gas cooler 5a, 5b; Accumulator 6, 12; Flowmeter 7; Expansion valve 8; Preheater 9; Superheater 10; Oil cooler 11; Pump 13; Heat source 14; Sampling ports 15a to 15c; Double pipes 16a to 16f; Heat source water mixing chamber 17a to 17d; Differential pressure converter 18a to 18f; Refrigerant mixing chamber 19a to 19f; Thermocouple 20a to 20f; vessel

Claims (4)

A heat exchanger tube for an evaporator of a heat exchanger that uses carbon dioxide having a refrigerating machine oil content of 0.5% by mass or less as a refrigerant, and the average surface roughness in the tube circumferential direction of the inner surface is 0.2 to 20 μm A heat transfer tube for an evaporator characterized by being. The heat transfer tube for an evaporator according to claim 1, wherein the average surface roughness is 0.4 to 15 µm. The heat transfer tube for an evaporator according to claim 1 or 2, wherein the heat transfer tube is made of copper or a copper alloy. A method of manufacturing a heat exchanger tube for an evaporator of a heat exchanger using carbon dioxide having a refrigerating machine oil content of 0.5% by mass or less as a refrigerant, wherein a raw tube made of copper or a copper alloy has a floating plug and a reduced diameter A process for reducing the diameter using a die, and a process for emptying the reduced diameter pipe at least twice without using a floating plug. A method of manufacturing a heat transfer tube for an evaporator, wherein the cross-sectional reduction rate of the tube is 5 to 30%.

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