HEAT EXCHANGER

Abstract

The heat exchanger includes: a plurality of fins disposed, spaced apart from each other; and a plurality of heat transfer tubes inserted in the plurality of fins. The plurality of heat transfer tubes have round profiles. The plurality of heat transfer tubes have outer circumferential surfaces in contact with the plurality of fins. The plurality of heat transfer tubes have outer diameters of 5.4 mm or less. The plurality of fins and the heat transfer tubes are disposed so that ratios of thicknesses of the plurality of fins to the outer diameters are 0.03 or greater.

Claims

1. A heat exchanger, comprising: a plurality of fins disposed, spaced apart from each other; and a plurality of heat transfer tubes inserted in the plurality of fins, wherein the plurality of heat transfer tubes have round profiles, the plurality of heat transfer tubes have outer circumferential surfaces in contact with the plurality of fins, the plurality of heat transfer tubes have outer diameters D.sub.0 of 5.4 mm or less, the plurality of fins and the heat transfer tubes are disposed so that ratios tf/D.sub.0 of thicknesses tf of the plurality of fins to the outer diameters D.sub.0 are 0.03 or greater, and the plurality of fins and the heat transfer tubes are disposed so that the outer diameters Do are greater than or equal to 5.2 mm and less than or equal to 5.4 mm, and the ratios tf/D.sub.0 are greater than or equal to 0.036 and less than or equal to 0.040.

2. (canceled)

3. A heat exchanger, comprising: a plurality of fins disposed, spaced apart from each other; and a plurality of heat transfer tubes inserted in the plurality of fins, wherein the plurality of heat transfer tubes have round profiles, the plurality of heat transfer tubes have outer circumferential surfaces in contact with the plurality of fins, the plurality of heat transfer tubes have outer diameters D.sub.0 of 5.4 mm or less, the plurality of fins and the heat transfer tubes are disposed so that ratios tf/D.sub.0 of thicknesses tf of the plurality of fins to the outer diameters D.sub.0 are 0.03 or greater, and the plurality of fins and the heat transfer tubes are disposed so that the outer diameters Do are greater than or equal to 3.6 mm and less than or equal to 3.8 mm and the ratios tf/D.sub.0 are greater than or equal to 0.038 and less than or equal to 0.058.

4. The heat exchanger according to claim 1, wherein the plurality of heat transfer tubes are each formed of a single material.

5. The heat exchanger according to claim 1, wherein materials comprising the plurality of fins include aluminum, and materials comprising the plurality of heat transfer tubes include copper.

6. The heat exchanger according to claim 3, wherein the plurality of heat transfer tubes are each formed of a single material.

7. The heat exchanger according to claim 3, wherein materials comprising the plurality of fins include aluminum, and materials comprising the plurality of heat transfer tubes include copper.

8. The heat exchanger according to claim 4, wherein materials comprising the plurality of fins include aluminum, and materials comprising the plurality of heat transfer tubes include copper.

9. The heat exchanger according to claim 6, wherein materials comprising the plurality of fins include aluminum, and materials comprising the plurality of heat transfer tubes include copper.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0009] FIG. 1 is a partial sectional view of a heat exchanger according to the present embodiment.

[0010] FIG. 2 is a partial sectional view for illustrating arrangement of multiple heat transfer tubes in the heat exchanger of FIG. 1.

[0011] FIG. 3 is a graph showing a ratio tf/D.sub.0 of a thickness tf of a fin to an outer diameter D.sub.0 of the heat transfer tube versus a ratio A.sub.0K/ΔP of heat exchange performance A.sub.0K to an extratube pressure loss ΔP.sub.0.

[0012] FIG. 4 is a graph showing the thickness tf of the fin where the heat transfer tube has the outer diameter D.sub.0 of 5.3 mm versus the ratio of the heat exchange performance A.sub.0K to the extratube pressure loss ΔP.sub.0.

[0013] FIG. 5 is a graph showing the thickness tf of the fin where the heat transfer tube has the outer diameter D.sub.0 of 5.3 mm versus the ratio of the heat exchange performance A.sub.0K to the extratube pressure loss ΔP.sub.0.

[0014] FIG. 6 is a graph showing an amount by which the heat transfer tube is expanded versus a contact heat transfer coefficient between the fin and the heat transfer tube.

[0015] FIG. 7 is a partial sectional view of a variation of the heat exchanger of FIG. 1.

[0016] FIG. 8 is a graph showing the outer diameter D.sub.0 of the heat transfer tube versus a ratio of a contact thermal resistance between the fin and the heat transfer tube to an entire thermal resistance of the heat exchanger.

DESCRIPTION OF EMBODIMENTS

[0017] Hereinafter, the present embodiment will be described, with reference to the accompanying drawings. Note that, for purpose of explanation, FIGS. 1 and 2 introduce a first direction X, a second direction Y, and a third direction Z, which intersect with each other.

[0018] As shown in FIGS. 1 and 2, a heat exchanger 10 according to the present embodiment is a plate-fin heat exchanger. The heat exchanger 10 includes multiple fins 20 and multiple heat transfer tubes 30.

[0019] The fins 20 are plate fins. The fins 20 each extend along the first direction X and the third direction Z. The fins 20 are disposed, spaced apart from each other in the second direction Y. The number of fins 20 may be two or more, for example, three or more fins 20. Three or more fins 20 are equidistantly disposed in the second direction Y, for example. Multiple fin collars are formed on each fin 20. Each fin collar is disposed so as to have one heat transfer tube 30 inserted therethrough. The center-to-center distance between adjacent two fins 20 in the second direction Y will be referred to as a fin pitch Fp.

[0020] As shown in FIG. 1, each fin 20 has a first surface 20A facing the first surface 20A of an adjacent fin 20 in the second direction Y, and a second surface 20B extending in a direction interesting with the first surface 20A and in contact with an outer circumferential surface of the heat transfer tube 30. The second surface 20B is an inner circumferential surface of the fin collar of each fin 20. The length of the second surface 20B in the second direction Y is equal to a thickness tf of the fin 20, for example.

[0021] The heat transfer tubes 30 are round tubes. Stated differently, each heat transfer tube 30 has a round profile. The heat transfer tube 30 is inserted into the fin 20. The outer circumferential surface of the heat transfer tube 30 is in contact with the fin 20. The heat transfer tubes 30 are expanded by a mechanical tube expansion system, for example. For example, one or more grooves are formed in the inner circumferential surface of each heat transfer tube 30. For example, the heat transfer tubes 30 are what is called grooved-heat transfer tubes. For example, the heat transfer tube 30 is not brazed to the fin 20. In this case, heat transfer tube 30 is formed of a single material, for example. Stated differently, the heat transfer tube 30 is not formed of a clad material. Note that the heat transfer tube 30 may be brazed to the fin 20. In this case, preferably, the heat transfer tube 30 is formed of a clad material. In the following, inside the heat transfer tube 30 will be referred to as intratube, and outside the heat transfer tube 30 will be referred to as extratube.

[0022] The heat transfer tubes 30 extend along the second direction Y. The number of heat transfer tubes 30 may be one or more, for example, four or more heat transfer tubes 30. The heat transfer tubes 30 are disposed spaced apart by a spacing Lp (see FIG. 2) from each other in the first direction X and spaced apart by a spacing D.sub.p (see FIG. 2) from each other in the third direction Z. Note that the arrangement of the heat transfer tubes 30 along the first direction X in which an air flows will be referred to as a column, and the arrangement of the heat transfer tubes 30 along the third direction Z will be referred to as an array. The number of columns of the heat transfer tubes 30 in the first direction X may be one or more, for example, three or more columns. The number of tiers of heat transfer tubes 30 in the third direction Z may be one or more, for example, three or more tiers. The distance between the central axes of adjacent two heat transfer tubes 30 in the first direction X will be referred to as a column pitch L.sub.P. The distance between the central axes of adjacent two heat transfer tubes 30 in the third direction Z will be referred to as a tier pitch D.sub.P.

[0023] Examples of the materials comprising the fins 20 include, but are not particularly limited to, copper (Cu) or aluminum (Al). Examples of the materials comprising the heat transfer tube 30, but are not particularly limited to, Cu or Al. For example, the material comprising each fin 20 includes Al, and the material comprising each heat transfer tube 30 includes Cu.

[0024] A flow passage R1 is formed between adjacent two fins 20 in the second direction Y. A first heat-transfer medium such as an air flows through the flow passage R1 in the first direction X. A flow passage R2 is formed inside each heat transfer tube 30. A second heat-transfer medium such as a refrigerant flows through the flow passage R2 in the second direction Y. The first heat-transfer medium exchanges heat with the second heat-transfer medium via the fin 20 and the heat transfer tube 30.

[0025] The fins 20 and the heat transfer tubes 30 are disposed so that a ratio tf/D.sub.0 of the thickness tf (unit: mm) of each fin 20 to the outer diameter D.sub.0 (unit: mm) of each heat transfer tube 30 is greater than or equal to 0.03. The outer diameters D.sub.0 of the heat transfer tubes 30 are for example, less than or equal to 5.4 mm. Stated from a different perspective, the fins 20 and the heat transfer tubes 30 are disposed so that a ratio A.sub.0K/ΔP of a heat exchanger performance AoK to an extratube pressure loss ΔP (unit: Pa) is not less than 100% when the outer diameters D.sub.0 of the fins 20 and the heat transfer tubes 30 are at least less than or equal to 5.4 mm.

[0026] The outer diameters D.sub.0 of the heat transfer tubes 30 are, for example, greater than or equal to 5.2 mm and less than or equal to 5.4 mm. In this case, preferably, the fins 20 and the heat transfer tubes 30 are disposed so that the ratio tf/D.sub.0 is greater than or equal to 0.03 and less than or equal to 0.034. Stated from a different perspective, the fins 20 and the heat transfer tubes 30 are disposed so that the ratio AoK/ΔP of the heat exchanger performance AoK to the extratube pressure loss ΔP is 102% or greater when the outer diameters D.sub.0 of the fins 20 and the heat transfer tubes 30 are greater than or equal to 5.2 mm and less than or equal to 5.4 mm.

[0027] The outer diameters D.sub.0 of the heat transfer tubes 30 are, for example, greater than or equal to 3.6 mm and less than or equal to 3.8 mm. In this case, preferably, the fins 20 and the heat transfer tubes 30 are disposed so that the ratio tf/D.sub.0 is greater than or equal to 0.034 and less than or equal to 0.058. Stated from a different perspective, the fins 20 and the heat transfer tubes 30 are disposed so that the ratio AoK/ΔP of the heat exchanger performance AoK to the extratube pressure loss ΔP is 102% or greater when the outer diameters D.sub.0 of the fins 20 and the heat transfer tubes 30 are greater than or equal to 3.6 mm and less than or equal to 3.8 mm. Methods of calculation of the heat exchanger performance A.sub.0K and the extratube pressure loss ΔP will be described below.

[0028] Note that, if the heat transfer tube 30 is expanded by the mechanical tube expansion system, the outer diameter D.sub.0 is the outer diameter of the expanded heat transfer tube 30. The tube expansion rate is, but not particularly limited to, greater than or equal to 5% and less than or equal to 8%, for example. The thickness tf and the outer diameter D.sub.0 are, while they can be measured by any method, measured by a vernier caliper, for example.

[0029] FIG. 3 is a graph showing the ratio tf/D.sub.0 versus the ratio AoK/ΔP, more specifically, a graph showing changes in ratio AoK/ΔP with varying fin thickness tf relative to the outer diameter D.sub.0 of the heat transfer tube. In FIG. 3, the ratio tf/D.sub.0 is indicated on the horizontal axis and the ratio AoK/ΔP is indicated on the vertical axis. As one example, FIG. 3 shows changes in ratio AoK/ΔP with varying thickness tf of the fin in contact with the heat transfer tube that has the outer diameter D.sub.0 of 3.7 mm, 5.3 mm, and 7.4 mm.

[0030] FIG. 4 is a graph showing changes in ratio AoK/ΔP with varying thickness tf of the fin in contact with the heat transfer tube that has the outer diameter D.sub.0 of 5.3 mm. FIG. 5 is a graph showing changes in ratio AoK/ΔP with varying thickness tf of the fin in contact with the heat transfer tube that has the outer diameter D.sub.0 of 3.7 mm. In FIGS. 4 and 5, the thickness tf is indicated on the horizontal axis, and the ratio AoK/ΔP is indicated on the vertical axis.

[0031] As shown in FIGS. 3, 4, and 5, the ratio AoK/ΔP is confirmed as being at its peak when the thickness tf is varied relative to a given outer diameter Do.

[0032] As shown in FIG. 3, when the outer diameter D.sub.0 is 5.3 mm, the ratio AoK/ΔP is 100% or greater if the ratio tf/D.sub.0 is greater than or equal to 0.021 and less than or equal to 0.04. The ratio AoK/ΔP is 102% or greater if the ratio tf/D.sub.0 is greater than or equal to 0.026 and less than or equal to 0.034. The ratio AoK/ΔP is 103% or greater if the ratio tf/D.sub.0 is 0.03.

[0033] As shown in FIG. 4, when the outer diameter DO is 5.3 mm, the ratio AoK/ΔP is 100% or greater if the thickness tf of the fin 20 is greater than or equal to 0.11 mm and less than or equal to 0.21 mm. The ratio AoK/ΔP is 102% or greater if the thickness tf is thicker than 0.12 mm and thinner than 0.20 mm. The ratio AoK/ΔP is 103% or greater if the thickness tf is greater than or equal to 0.15 mm and less than or equal to 0.17 mm. The ratio AoK/ΔP is at the maximum if the thickness tf is greater than or equal to 0.15 mm and less than or equal to 0.16 mm.

[0034] As shown in FIG. 3, when the outer diameter DO is 3.7 mm, the ratio A.sub.0K/ΔP is 100% or greater if the ratio tf/D.sub.0 is greater than or equal to 0.03. The ratio AoK/ΔP is 103% or greater if the ratio tf/D.sub.0 is greater than or equal to 0.034. The ratio AoK/ΔP is 108% or greater if the ratio tf/D.sub.0 is greater than or equal to 0.046 and less than or equal to 0.058.

[0035] As shown in FIG. 5, when the outer diameter D.sub.0 is 3.7 mm, the ratio AoK/ΔP is 100% or greater if the thickness tf of the fin 20 is greater than or equal to 0.11 mm and less than or equal to 0.21 mm. The ratio AoK/ΔP is 103% or greater if the thickness tf is greater than or equal to 0.12 mm. The ratio AoK/ΔP is 107% or greater if the thickness tf is greater than or equal to 0.15 mm. The ratio AoK/ΔP is 108% or greater if the thickness tf is greater than or equal to 0.17 mm and less than or equal to 0.21 mm. The ratio AoK/ΔP is at the maximum if the thickness tf is greater than or equal to 0.18 mm and less than or equal to 0.20 mm.

[0036] A method of calculation of the heat exchanger performance A.sub.0K and the pressure loss ΔP is as follows:

[0037] The heat exchanger performance AoK is defined by the following Equation (1), using an intratube heat transfer coefficient α.sub.i, a contact heat transfer coefficient α.sub.c, and an extratube heat transfer coefficient α.sub.a. Note that Equation (1) disregards the thermal resistance of the heat transfer tube in the direction of thickness thereof and the thermal resistance due to fouling in the tube because they are very small, as compared to the contact thermal resistance between the heat transfer tube and the refrigerant flowing therethrough, and the contact thermal resistance between the heat transfer tube and an air flowing outside the heat transfer tube. The heat transfer coefficients α.sub.i, α.sub.c, and α.sub.a are in unit of W/(m.sup.2.Math.K).

[00001]$\begin{array}{cc}\left[\mathrm{MATH}1\right]& \\ {AoK=(\frac{1}{A_{\mathrm{pi}}.Math.{\alpha }_i}+\frac{1}{A_{co}.Math.{\alpha }_c}+\frac{1}{\left(A_p+\eta .Math.A_F\right).Math.{\alpha }_a})}^{-1}& \left(1\right)\end{array}$

[0038] An intratube heat transfer area A.sub.pi, a contact area A.sub.co of the fin and the heat transfer tube, a surface area A.sub.P of the outer circumferential surface of the heat transfer tube, and a surface area A.sub.F of the fin in Equation (1) are set as specifications of the heat exchanger 10. These areas are in unit of m.sup.2.

[0039] The intratube heat transfer coefficient α.sub.i in Equation (1) is calculated using an equation by Koyama et. al, specifically, Equations (2) and (3):

[00002]$\begin{array}{cc}\left[\mathrm{MATH}2\right]& \\ {\alpha }_i=241.2.Math.X_{\mathrm{tt}}^{-0.2878}.Math.{\mathrm{Pr}}_l^{0.33}.Math.\frac{k_l}{d_i}& \left(2\right)\end{array}$$\begin{array}{cc}{X}_{\mathrm{tt}}={\left(\frac{1-x}{x}\right)}^{0.9}.Math.{\left(\frac{{\rho }_v}{{\rho }_l}\right)}^{0.5}.Math.{\left(\frac{{\mu }_l}{{\mu }_v}\right)}^{0.1}& \left(3\right)\end{array}$

[0040] A Prandtl number P.sub.rl of the refrigerant and a thermal conductivity k.sub.l (unit: W/m K) of the refrigerant in Equation (2), and a density ρ.sub.l (unit: g/m.sup.3) of a saturated liquid, a viscous modulus μ.sub.l of the saturated liquid, a density ρ.sub.v (unit: g/m.sup.3) of a saturated vapor, and a viscous modulus μ.sub.v of the saturated vapor in Equation (3) are physical property values of the refrigerant. An inner diameter d.sub.i (unit: m) of the heat transfer tube in Equation (3) is set as a specification of the heat exchanger 10. A vapor quality x in Equation (3) is a representative vapor quality, specifically, 0.5. In other words, the intratube heat transfer coefficient α.sub.i calculated using Equations (2) and (3) is an average heat transfer coefficient.

[0041] The contact heat transfer coefficient α.sub.c in Equation (1) is calculated using Equation (4):

[0042] [MATH 3]

α.sub.c=(2.Math.104d+2.5).Math.tf.Math.10.sup.7  (4)

[0043] where Δd (unit: m) is an amount of expansion of tube, that is, a difference between the outer diameter D.sub.0 of the expanded heat transfer tube 30, and the diameter of the fin collar before the heat transfer tube 30 is expanded.

[0044] The extratube heat transfer coefficient α.sub.a in Equation (1) is calculated using an equation by Seshita, specifically, Equation (5):

[00003]$\begin{array}{cc}\left[\mathrm{MATH}4\right]& \\ {\alpha }_{\alpha }=\mathrm{Nu}.Math.k_a/D_e& \left(5\right)\end{array}$$\begin{array}{cc}Re=\frac{\mathrm{Ve}.Math.\mathrm{De}}{v_a}& \left(6\right)\end{array}$$\begin{array}{cc}N{u}_{a\_\mathrm{lRe}}=C_o.Math.{\left(\frac{R{e}_{a\_\mathrm{lRe}}.Math.{\mathrm{Pr}}_a.Math.{\mathrm{De}}_c}{N_L.Math.L_p}\right)}^{C_1}& \left(7\right)\end{array}$$\begin{array}{cc}V{e}_{ac}=\frac{F_p.Math.D_p}{A_c}.Math.\left(\frac{{\rho }_{ai}}{{\rho }_a}\right).Math.{\mathrm{Ve}}_{af}& \left(8\right)\end{array}$$\begin{array}{cc}N{u}_{a\_\mathrm{hRe}}=C_o.Math.{\left(\frac{R{e}_{a\_h\mathrm{Re}}.Math.{\mathrm{Pr}}_a.Math.{\mathrm{De}}_{\min }}{N_L.Math.L_p}\right)}^{C_1}& \left(9\right)\end{array}$$\begin{array}{cc}V{e}_{a\max }=\frac{F_p.Math.D_p}{A_{\min }}.Math.\left(\frac{{\rho }_{ai}}{{\rho }_a}\right).Math.{\mathrm{Ve}}_{af}& \left(10\right)\end{array}$

[0045] An air Nusselt number Nu in Equation (5) is calculated using Equation (7), if a calculated value of an air Reynolds number Re defined by Equation (6) is less than a threshold (e.g., 400). An air Reynolds number Re.sub.a_IRc in Equation (7) is calculated by assigning an average velocity of fluid Ve.sub.ac at a free through-flow cross-sectional area A.sub.c calculated using Equation (8) to an average velocity of fluid Ve of Equation (6), and assigning a representative inter-fin length De.sub.c relative to a free-flow volume Vo to a representative inter-fin length De of Equation (6).

[0046] The air Nusselt number Nu of Equation (5) is calculated using Equation (9), if the calculated value of the air Reynolds number Re defined by Equation (6) is equal to or greater than or equal to the threshold (e.g., 400). The air Reynolds number Re.sub.a_hRe of Equation (9) is calculated by assigning an average velocity of fluid Ve.sub.max at a minimum flow passage cross-sectional area Amin calculated using Equation (10) to the average velocity of fluid Ve of Equation (6), and assigning a representative inter-fin length D.sub.min relative to a minimum free through-flow volume V.sub.min to the representative inter-fin length De of Equation (6).

[0047] The thermal conductivity k.sub.a (unit: W/(m k)) of air of Equation (5), the dynamic viscous modulus v.sub.a of air of Equation (6), and the Prandtl number Pra of air are physical property values of air that are determined depending on the temperature, pressure, etc. In Equations (8) and (10), ρ.sub.a and ρ.sub.ai are a density of air.

[0048] A number of columns N.sub.L of the heat transfer tube and the column pitch L.sub.P (unit: m) of the heat transfer tubes of Equations (7) and (9), and the fin pitch F.sub.p (unit: m) and the tier pitch D.sub.P (unit: m) of the heat transfer tubes of Equations (8) and (10) are set as specifications of the heat exchanger. In Equation (7), a constant C.sub.0 is 2.1, and a constant C.sub.1 is 0.38. In Equation (9), C.sub.0 is 0.12, and C.sub.1 is 0.64.

[0049] A fin efficiency η of Equation (1) is calculated using Equations (11) and (12) below. A thermal conductivity k.sub.f (unit: W/(m.Math.k)) of the fin, an equivalent diameter (unit: m) of the fin, and a diameter Dc (unit: m) of the fin collar before the heat transfer tube 30 is expanded in Equation (11) below are set as specifications of the heat exchanger.

[00004]$\begin{array}{cc}\left[\mathrm{MATH}5\right]& \\ \eta ={\left\{1+{\alpha }_a.Math.\frac{{\left(d_F-{\dot{d}}_c\right)}^2}{6.Math.k_F.Math.\mathrm{tf}}.Math.{\left(\frac{d_F}{{\dot{d}}_c}\right)}^{0.5}\right\}}^{-1}& \left(11\right)\end{array}$$\begin{array}{cc}{d}_F={\left\{\frac{4}{\pi }.Math.D_p.Math.L_p\right\}}^{0.5}& \left(12\right)\end{array}$

[0050] The extratube pressure loss ΔP is calculated using Equation (13) below, if the calculated value of the air Reynolds number Re defined by Equation (6) is less than the threshold (e.g., 400). The representative inter-fin length De.sub.c of Equation (13) is calculated relative to a free-flow volume V.sub.c. A flow-loss factor flRe of Equation (13) is calculated using Equation (14).

[0051] The extratube pressure loss ΔP is calculated using Equation (15) below, if a calculated value of the air Reynolds number Re defined by Equation (6) is greater than or equal to the threshold (e.g., 400). A representative inter-fin length De.sub.min of Equation (15) is calculated relative to a minimum flow passage cross-sectional area V.sub.min. The flow-loss factor flRe of Equation (15) is calculated using Equation (16).

[00005]$\begin{array}{cc}\left[\mathrm{MATH}6\right]& \\ \Delta P_{o\_\mathrm{lRe}}=f_{\mathrm{lRe}}.Math.\frac{2.Math.L_p.Math.N_L.Math.{\rho }_a.Math.{\mathrm{Ve}}_{ac}^2}{D{e}_c}& \left(13\right)\end{array}$$\begin{array}{cc}{f}_{lRe}=\left\{0.43+35.1{\left(\frac{R{e}_{a\_\mathrm{lRe}}.Math.{\mathrm{De}}_c}{L_p.Math.N_L}\right)}^{-1.07}\right\}.Math.\left(\frac{D{e}_c}{L_p.Math.N_L}\right)& \left(14\right)\end{array}$$\begin{array}{cc}\Delta P_{o\_\mathrm{hRe}}=f_{hRe}.Math.\frac{2.Math.L_p.Math.N_L.Math.{\rho }_a.Math.{\mathrm{Ve}}_{\mathrm{amax}}^2}{D{e}_{\min }}& \left(15\right)\end{array}$$\begin{array}{cc}{f}_{hRe}=0.026+27..Math.R{e}_{a\_\mathrm{hRe}}^{-1.27}& \left(16\right)\end{array}$

[0052] Next, operational advantages of the heat exchanger 10 according to the present embodiment are described.

[0053] As described above, the ratio of the contact thermal resistance to the entire thermal resistance of the heat exchanger is confirmed to increase with a reduction of the outer diameter of the heat transfer tube to less than 7 mm, as shown in FIG. 8. Therefore, deterioration of the heat exchange performance associated with an increase of the ratio above is a concern with a heat exchanger. In order to reduce the contact thermal resistance, it is contemplated to increase the contact heat transfer coefficient α.sub.c calculated from Equation (4). In view of Equation (4), at least one of an amount of tube expansion Δd and the fin thickness tf needs to be increased in order to increase the contact heat transfer coefficient α.sub.c.

[0054] FIG. 6 is a graph showing the amount of tube expansion Δd versus the contact heat transfer coefficient α.sub.c calculated from Equation (4), where the fin thickness tf is constant and the amount of tube expansion Δd is varied. Referring to Equation (4) and FIG. 6, the contact heat transfer coefficient α.sub.c is independent of the outer diameter Do of the heat transfer tube, and increases with an increase of the amount of tube expansion Δd. However, in practice, an increase of the amount of tube expansion increases the likelihood of causing failures in manufacturing such as breakage of fin collars. Thus, there is the upper limit for the amount of tube expansion.

[0055] In contrast, the contact heat transfer coefficient α.sub.c increases with an increase of the fin thickness tf. An increase of the wind speed of the air flowing between the fans, that is, increases of the average velocity of fluid Ve.sub.ac of Equation (8) and the average velocity of fluid Ve.sub.max of Equation (10) increase an extratube transfer coefficient α.sub.a, where the fin pitch F.sub.p is constant and the fin thickness tf is increased. However, an increase of the wind speed of the air flowing between the fins increases the average velocity of fluid Ve.sub.ac of Equation (13) and the average velocity of fluid Ve.sub.max of Equation (15), thereby increasing the extratube pressure loss ΔP.sub.0. Therefore, if a heat exchanger that has a high extratube pressure loss ΔP.sub.0 is mounted on a unit, the flow rate of the air flowing between the fans may be less than a sufficient rate expected, and the heat exchanger performance may therefore not be exercised as expected.

[0056] In contrast, in the heat exchanger 10, the outer diameters D.sub.0 of the heat transfer tubes 30 are less than or equal to 5.4 mm, and the fins 20 and the heat transfer tubes 30 are disposed so that the ratio tf/D.sub.0 of the thickness tf of each fin 20 to the outer diameter D.sub.0 of each heat transfer tube 30 is greater than or equal to 0.03. Therefore, the ratio AoK/ΔP of the heat exchanger 10 is 100% or greater, as shown in FIGS. 3 through 5. In other words, in the heat exchanger 10, the fins 20 and the heat transfer tubes 30 are disposed so that an increase of the extratube pressure loss ΔP.sub.0 can be inhibited, while increasing the heat exchanger performance A.sub.0K by increasing the contact heat transfer coefficient α.sub.c. As a result, the heat exchanger performance of the heat exchanger 10 is higher than the heat exchanger performance of the heat exchanger in which the fin thickness tf is simply increased in order to inhibit an increase of the contact thermal resistance.

[0057] While FIGS. 3 and 4 show only results of calculation when the outer diameter D.sub.0 is 5.3 mm, it should be noted that the inventors confirmed that the ratio AoK/ΔP is 100% or greater even when the outer diameter D.sub.0 is 5.4 mm, if the ratio tf/D.sub.0 is 0.03 or greater.

[0058] In one embodiment of the heat exchanger 10, multiple fins 20 and multiple heat transfer tubes 30 are disposed so that the outer diameter D.sub.0 is greater than or equal to 5.2 mm and less than or equal to 5.4 mm and the ratio tf/D.sub.0 is 0.034 or less.

[0059] In another one embodiment of the heat exchanger 10, multiple fins 20 and multiple heat transfer tubes 30 are disposed so that the outer diameter D.sub.0 is greater than or equal to 3.6 mm and less than or equal to 3.8 mm and the ratio tf/D.sub.0 is greater than or equal to 0.034 and less than or equal to 0.058.

[0060] In the heat exchanger 10, since the ratio tf/D.sub.0 is 0.03 or greater, the heat exchanger performance is enhanced even though the heat transfer tubes 30 are expanded by the mechanical tube expansion system. Therefore, there is no need for the heat exchanger 10 to have the heat transfer tubes 30 brazed to the fins 20 for the purpose of reducing the contact thermal resistance. When the heat transfer tubes 30 are not brazed to the fins 20, each heat transfer tube 30 may not be formed of a clad material. In other words, the heat transfer tube 30 may be formed of a single material. When the heat transfer tubes 30 are not brazed to the fins 20, the material cost of the heat transfer tubes 30 and the manufacturing cost associated with brazing can be reduced.

[0061] FIG. 7 is a cross-sectional view of a variation of the heat exchanger 10 of FIG. 1. As shown in FIG. 7, each fin 20 has the first surface 20A facing the first surface 20A of an adjacent fin 20 in the second direction Y, and the second surface 20B extending in a direction intersecting with the first surface 20A. A portion of the first surface 20A is an inner circumferential surface of the fin collar of each fin 20 and in contact with the outer circumferential surface of the heat transfer tube 30. The ratio AoK/ΔP in the heat exchanger 10 of FIG. 7 is 100% or greater if the ratio tf/D.sub.0 is greater than or equal to 0.03, as with the heat exchanger 10 of FIG. 1.

[0062] While the embodiment according to the present disclosure has been described as described above, the embodiment can be also modified in various ways. The scope of the present disclosure is not limited to the embodiment. The scope of the present disclosure is defined by the appended claims. All changes which come within the meaning and range of equivalency of the appended claims are to be embraced within their scope.

REFERENCE SIGNS LIST

[0063] 10 heat exchanger; 20 fin; 20A first surface; 20B second surface; and 30 heat transfer tube.