Heat transfer tube, heat exchanger, and method for manufacturing heat transfer tube

1. A heat transfer tube made of aluminum, comprising:

a streak-shaped Zn diffusion layer which is spirally formed on an outer peripheral surface of the tube along a longitudinal direction,

wherein an outer diameter of the heat transfer tube is from 4 mm to 15 mm,

wherein a bottom wall thickness of the heat transfer tube is from 0.2 mm to 0.8 mm,

wherein a plurality of fins which are spirally formed along the longitudinal direction are provided on an inner peripheral surface of the heat transfer tube, and

satisfying the following expression:

tan

⁢

⁢

θ

⁢

⁢

2

=

(

α

+

2

⁢

π

⁢

⁢

β

)

⁢

tan

⁢

⁢

θ

⁢

⁢

1

α

,

wherein α indicates an inner peripheral length,

β indicates the bottom wall thickness,

θ1 indicates a lead angle of a fin, and

θ2 indicates a lead angle of the streak-shaped Zn diffusion layer.

2. The heat transfer tube according to claim 1 ,

wherein the streak-shaped Zn diffusion layer is provided in a region of 50% or more of the outer peripheral surface.

3. The heat transfer tube according to claim 1 ,

wherein an average Zn concentration of the outer peripheral surface is from 3 mass % to 12 mass %,

wherein the average Zn concentration of the outer peripheral surface is obtained by:

cutting the heat transfer tube to obtain a suitable length in the longitudinal direction, and opening and deploying the heat transfer tube from a cut surface, and crushing horizontally to obtain a plate-like sample,

placing the plate-like sample so that a cross section perpendicular to the longitudinal direction becomes a measurement surface, filling with a resin around the plate-like sample, polishing, and buffing,

dividing the measurement surface into 72 equally spaced intervals, and performing a line analysis on a part of the plate-like sample from a surface layer on the outer peripheral side of each heat transfer tube to the inner peripheral side, and measuring on Al strength and the Zn concentration in 70 points at 5 μm pitch by Electron Probe Micro Analyzer; and

calculating an average value of Zn concentration of 72 points as the average Zn concentration.

4. The heat transfer tube according to claim 1 ,

wherein an average diffusion depth of 0.3% Zn concentration is from 80 μm to 285 μm.

5. The heat transfer tube according to claim 1 ,

wherein the lead angle of the streak-shaped Zn diffusion layer is 8° or more.

6. The heat transfer tube according to claim 1 ,

wherein the heat transfer tube is inserted into insertion holes of a plurality of heat sinks which are arranged to be parallel to each other at predetermined intervals, is expanded in a diameter, and thereby is connected to the heat sinks.

7. A heat exchanger comprising:

the heat transfer tube according to claim 1 ; and

a heat sink which is connected to the heat transfer tube.

8. A method for manufacturing the heat transfer tube of claim 1 , the method comprising:

performing Zn thermal spraying on an outer periphery of an aluminum raw tube in a linear streak shape along a longitudinal direction, wherein the aluminum raw tube has a plurality of fins linearly extending along the longitudinal direction on an inner peripheral surface of the aluminum raw tube;

performing a heat treatment on the aluminum raw tube to diffuse Zn into the aluminum raw tube and forming a Zn diffusion layer;

twisting the aluminum raw tube to form the plurality of fins and the Zn diffusion layer in a spiral shape along the longitudinal direction; and

performing the heat treatment on the aluminum raw tube.

9. A method for manufacturing a heat transfer tube, the method comprising:

performing Zn thermal spraying on an outer periphery of an aluminum raw tube in a linear streak shape along a length direction, wherein the aluminum raw tube has a plurality of fins linearly extending along a length direction on an inner peripheral surface of the heat transfer tube;

twisting the aluminum raw tube to form the fins and a Zn sprayed layer in a spiral shape along the length direction; and

performing a heat treatment on the aluminum raw tube to diffuse Zn into the aluminum raw tube, form a Zn diffusion layer, and form an O- materialized aluminum raw tube.

10. The method according to claim 8 ,

wherein the twisting comprises,

using a first drawing die having a first direction as a drawing direction, a second drawing die having a second direction opposite to the first direction as a drawing direction, and a revolution flyer which reverses a pipeline of a tube material between the first drawing die and the second drawing die from the first direction to the second direction and rotates around at least one selected from the group consisting of the first drawing die and the second drawing die,

causing the aluminum raw tube having a plurality of linear grooves formed on an inner surface along the longitudinal direction to pass through the first drawing die, winding the aluminum raw tube around the revolution flyer, and revolving the aluminum raw tube to reduce a diameter of the aluminum raw tube and twist the aluminum raw tube so as to form an intermediate twisted tube, and

causing the intermediate twisted tube rotating together with the revolution flyer to pass through the second drawing die to reduce a diameter of the intermediate twisted tube and twist the intermediate twisted tube.

11. The method according to claim 9 ,

wherein the twisting comprises,

using a first drawing die having a first direction as a drawing direction, a second drawing die having a second direction opposite to the first direction as a drawing direction, and a revolution flyer which reverses a pipeline of a tube material between the first drawing die and the second drawing die from the first direction to the second direction and rotates around any one of the first drawing die and the second drawing die,

causing the aluminum raw tube having a plurality of linear grooves formed on an inner surface along the length direction to pass through the first drawing die, winding the aluminum raw tube around the revolution flyer, and revolving the aluminum raw tube to reduce a diameter of the aluminum raw tube and twist the aluminum raw tube so as to form an intermediate twisted tube, and

causing the intermediate twisted tube rotating together with the revolution flyer to pass through the second drawing die to reduce a diameter of the intermediate twisted tube and twist the intermediate twisted tube.