JPH0615951B2 - Heat transfer tube with internal groove - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heat transfer tube with an inner groove, which is used in an evaporator, a condenser and the like and has excellent heat transfer performance.

[Conventional technology]

BACKGROUND ART Conventionally, as a heat transfer tube having a high thermal conductivity and a small pressure loss during heat transfer involving evaporation and condensation, one having a structure in which a groove is formed on the inner surface of the tube is known.

As for the form of the groove to be formed, the sectional shape of the acute-angled groove 1 type shown in FIG. 1 (JP-A-52-38663) and the trapezoidal groove 2 type shown in FIG. 2 ( Tokkyo 1-1318
No. 95 gazette), and as a groove angle (Q) with respect to the pipe axis, a straight groove 3 having an angle of 0 ° is processed as shown in FIG. 3 and a spiral having a constant angle as shown in FIG. Some are processed into the groove 4.

The shape of these shaped grooves has advantages and disadvantages in each combination in relation to heat transfer efficiency. For example, considering a case where the refrigerant flow rate is constant, a trapezoidal shape is formed because the groove shape has a small accommodating capacity when the groove shape is small. In comparison, the liquid film at the groove bottom becomes thicker and the heat transfer resistance becomes larger. However, if it is formed as a spiral groove with an appropriate groove angle (Q), the refrigerant flows in a spiral shape along the groove, increasing the contact length between the refrigerant and the pipe, and as shown in FIG. It has been found that the thermal efficiency is improved.

[Problems to be Solved by the Invention] In order to reduce the weight and size of a heat transfer tube with an inner surface used in a heat exchanger of a recent air conditioner or the like, it is required to have a small diameter and a thin wall. However, the outer diameter of the inner surface grooved heat transfer tube was limited to about 9 mm due to restrictions of the tube manufacturing equipment.

On the other hand, when the outer diameter of the heat transfer tube with the inner groove is reduced (for example, the outer diameter of the heat transfer tube is 4 mm), the heat transfer characteristics tend to deteriorate when the groove angle is set, which has been well known in the past. It became impossible to apply the knowledge of heat transfer characteristics to the groove shape.

Therefore, an object of the present invention is to provide a heat transfer tube with an inner groove having an outer diameter of 10 mm or less, a trapezoidal groove, and excellent heat transfer characteristics.

[Means for Solving the Problems]

The heat transfer tubes used for heat exchangers of recent air conditioners are
Smaller diameters and thinner walls are required for lighter weight and smaller sizes, but heat transfer characteristics tend to deteriorate as the diameter decreases, and the well-known trapezoidal groove shape with good heat transfer characteristics is used. The inventors have studied to adopt it.

As a result, in the groove having a trapezoidal cross section, the liquid film becomes thinner than the acute-angled groove except for the corner portion of the groove bottom due to the action of the low surface tension of the refrigerant. However, according to the knowledge of the inventors, in the case where the groove angle (Q) is given in the case of the trapezoidal groove, the refrigerant is pressed against one corner of the groove bottom due to the swirling flow, and the liquid film at that portion is extremely thin. It was confirmed that the heat transfer efficiency was rather reduced due to the thickening. This trend is
In particular, it was found that it appears remarkably in the case of a heat transfer tube with a small tube diameter used at a refrigerant mass velocity of 400 kg / m ² s or less.

The present invention, based on the above findings, as a result of further multifaceted research, when forming a trapezoidal groove on the inner surface of the heat transfer tube used under the condition that the refrigerant mass velocity is 400 kg / m ² s or less, The development was carried out after confirming the groove angle and pipe diameter range that give the most effective heat transfer performance.

That is, the present invention provides a heat transfer tube with an internal groove that gives a high-performance heat transfer coefficient in the tube, and its structural feature is that the refrigerant mass velocity is 400 kg / m ² s (m ² : internal cross-sectional area of the pipe, s In the heat transfer tube that has been subjected to internal groove processing and is used for less than 10 seconds, the groove has a trapezoidal cross section and the groove angle (Q) with respect to the tube axis is 0.
~ 4 °, that is, a straight groove having an outer diameter of 2 to 7 mm.

A groove having a trapezoidal cross section in the present invention is formed in the same shape as in FIG. 2, but in this case, if a slight roundness (R: about 0.03 mm) is formed in the corner portion of the groove bottom, the average liquid film is thinned. Also, it effectively functions to reduce the shearing force of the refrigerant.

It is an important requirement of the present invention to set the groove angle (Q) with respect to the tube axis within the range of 0 to 4 °. If the groove is a spiral groove having an angle of more than 4 °, the heat transfer efficiency will be significantly reduced. The limit of 4 ° is due to the processing accuracy. The groove angle (Q) that gives the most effective heat transfer efficiency is 0 °, that is, when the groove is formed as a straight groove.

The heat transfer performance is also related to the outer diameter of the heat transfer tube, the pitch (P) of the grooves, and the number of threads, and good heat transfer characteristics can be exhibited in the range of the outer diameter of 2 to 7 mm. As for the groove pitch (P), if the tube diameter is 4.00 mm in outer diameter and 0.6 mm in wall thickness, for example, the evaporation heat transfer coefficient inside the tube increases in the range of 0.18 to 0.42 mm (25 to 60 rows). , The heat transfer coefficient of condensation in the pipe is also maintained at the upper level, but when it is less than 0.18 mm (the number of threads exceeds 60), these heat transfer coefficients tend to decrease. This is related to the balance between the wet edge length (heat transfer area) of the heat transfer tube and the total cross-sectional area of the groove (liquid trapping amount in the groove). Considering the condensation heat transfer coefficient, the wet edge length increases with the decrease of the groove pitch. However, since the average thickness of the groove in the cross section of the pipe is almost constant, the total cross-sectional area of the groove is reduced, and when the groove pitch becomes smaller and the number of threads exceeds a certain value, the drainage capacity of the condensate and the immersion of the groove This is considered to be based on the fact that it becomes noticeable and the performance deteriorates.

The heat transfer tube of the present invention can be formed by using a metal tube material having a high thermal conductivity such as copper, and inserting a grooved plug into the tube to perform drawing while reducing the diameter.

[Action]

According to the heat transfer tube of the present invention configured as described above, in addition to the trapezoidal cross section of the groove originally forming a thin liquid film, the groove angle (Q) with respect to the tube axis is set by the refrigerant at the groove bottom. Since it is set in the range of 0 to 4 °, which is not biased to the corners, these functions act synergistically and the refrigerant mass velocity 400
Under the conditions of less than kg / m ² s, the progress of evaporation and condensation is significantly accelerated. Therefore, smooth and efficient heat transfer is always performed.

〔Example〕

By inserting a grooved plug with a different number of threads into the inner surface of a copper pipe and drawing it while reducing the diameter, the pipe has an outer diameter (D ₀ ) of 4.00 mm and a maximum inner diameter (D ₁ ) of 3.4 mm. Pitch (P), number of threads
(J ₀ ), groove angle (Q), etc. were different.

Using each heat transfer tube in Table 1, an evaporation test and a condensation test were performed under the conditions shown in Table 2 using Freon gas (R22) as a refrigerant.

The relationship between the refrigerant mass velocity and the in-tube evaporation heat transfer coefficient of each heat transfer tube obtained in the test of Table 2 is shown in FIG. 5, and the relationship between the refrigerant mass speed and the in-tube condensation heat transfer coefficient is shown in FIG.

No. 1 and No. 4 both have a groove angle of 0 °, and the heat transfer characteristics are 300 kg for No. 1 as is clear from FIG.
/ m maximum at ² s are, No.4 maximum value is obtained at 240 kg / m ² s.

Further, the heat transfer coefficient tends to increase as the refrigerant mass velocity increases, but when the refrigerant mass velocity exceeds 400 kg / m ² s, the phenomena of reduced progress of evaporation and condensation occur, which is not preferable.

On the other hand, No. 5 of the comparative example has a groove angle of 8 °, and the heat transfer characteristics are on the upper right side with respect to the mass velocity of the refrigerant, but the invention is in the range of 200 to 300 kg / m ² s. The heat transfer rate was lower than the example.

Further, in No. 2 and No. 3, the groove angle was further set to 18 ° and 23 °, and the performance almost equal to that of No. 5 was obtained. No.6
Is a comparative example of a bare tube having no groove formed on the inner surface.

From the results shown in FIG. 5 and FIG. 6, the heat transfer coefficient of each of the examples of the present invention was relatively improved, and in particular, the straight groove having the groove angle (Q) of 0 ° was formed. It turns out that the effect is remarkable in 1 and 4.

At the same time, except that the refrigerant mass velocity was set to 250 kg / m ² s, the groove shape was trapezoidal, and the linear groove (groove angle 0 °) was used, the groove pitch (P) and the transmission were the same as when the above conditions were applied. The relationship of thermal performance is shown in FIG. From FIG. 7, groove pitch (P)
Was less than 0.18 mm (the number of rows exceeded 60), the heat transfer performance tended to decrease.

On the other hand, FIG. 8 shows the results of obtaining the heat transfer performance at a mass velocity of 250 kg / m ² s for a heat transfer tube having an outer diameter of 2 to 9.5 mm with a constant number of threads and different groove angles. The heat transfer tube has a groove pitch of 0.12 to 0.7 mm and a groove depth of 0.05 to 0.25 mm.

From FIG. 8, when the outer diameter exceeds 7 mm, the heat transfer coefficient of the heat transfer tube having the groove angle is high, but in the range of the outer diameter of 2 to 7 mm, the heat transfer coefficient of the heat transfer tube having the straight groove is high.

〔The invention's effect〕

As described above, if the heat transfer tube with the inner groove of the present invention is used,
Evaporation and condensation can be carried out very efficiently under the condition that the refrigerant mass velocity is 400 kg / m ² s or less. Therefore, it is suitable for evaporators and condensers that require high heat transfer performance.

[Brief description of drawings]

FIG. 1 is a partially enlarged cross-sectional view showing a groove having an acute-angled cross section, and FIG. 2 is a partially enlarged cross-sectional view showing a groove having a trapezoidal cross section. FIG. 3 is a partial plan sectional view showing a straight groove, and FIG. 4 is a partial plan sectional view showing a spiral groove. 5 is a diagram showing the relationship between the refrigerant mass velocity of each heat transfer tube and the in-tube evaporation heat transfer coefficient according to the embodiment, FIG. 6 is a diagram showing the relationship between the refrigerant mass speed and the in-tube condensation heat transfer coefficient, and FIG. FIG. 8 is a relationship diagram of groove pitch and heat transfer performance according to an example, FIG. 8 is a relationship diagram of pipe diameter and heat transfer coefficient, and FIG. 9 is a relationship diagram of groove angle and heat transfer efficiency. 1 ... Acute groove, 2 ... Trapezoidal groove 3 ... Straight groove, 4 ... Spiral groove P ... Groove pitch, Q ... Groove angle R ... Roundness of groove bottom corner

Claims (1)

[Claims] Claim: What is claimed is: 1. In a heat transfer tube having an inner surface grooved used at a refrigerant mass velocity of 200 to 300 kg / m ² s, the groove has a trapezoidal cross section, and the groove angle (Q) with respect to the tube axis is 0 to 0. A heat transfer tube with an inner groove, which is 4 ° (straight groove) and has an outer diameter of 2 to 7 mm.