JP5495601B2 - Heat transfer tube with inner groove for evaporator - Google Patents

Description

  TECHNICAL FIELD The present invention relates to an internally grooved tube for an evaporator of a fin-and-tube heat exchanger, and more particularly to an internally grooved evaporator for an evaporator using a carbon dioxide refrigerant containing 0.1 to 10% of a polyalkylene glycol oil as a refrigerating machine oil. It relates to heat transfer tubes.

  Conventionally, chlorofluorocarbon refrigerants have been used in heat exchangers installed in air conditioners, car air conditioners, refrigerators, refrigerators, water heaters and vending machines. Emissions are regulated as substances that affect liquefaction, and natural refrigerants with low toxicity and flammability, are safe, inexpensive, and have a low environmental impact are drawing attention.

  Carbon dioxide, a kind of such natural refrigerant, has high liquid constant pressure specific heat and liquid heat transfer coefficient that greatly affect the thermal characteristics, and has a high heat exchange efficiency, and therefore has better heat transfer performance than CFC refrigerant. . 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, heat transfer is greater than when chlorofluorocarbon refrigerants are used. Performance is improved. Furthermore, since carbon dioxide has a lower liquid viscosity and density than chlorofluorocarbon refrigerants, if carbon dioxide is used as a refrigerant in a heat exchanger, pressure loss can be kept small even if the flow rate of the refrigerant is increased. Yes, high thermal conductivity can be obtained. Furthermore, since the carbon dioxide has a large vapor density and latent heat and the refrigeration effect per unit excluded volume is larger than that of the chlorofluorocarbon refrigerant, the heat exchanger using carbon dioxide as a refrigerant is advantageous for downsizing.

  On the other hand, carbon dioxide has a low critical temperature, and on the high-pressure side of an air conditioner using carbon dioxide as a refrigerant, condensation accompanying heat dissipation is not performed, and the dryness at the entrance of the evaporator is increased, so in a simple cycle of cooling and heating There is a problem that the theoretical performance is low. For this reason, when using carbon dioxide as a refrigerant, high heat transfer performance is required for the heat transfer tube through which the refrigerant passes.

Therefore, in a heat exchanger using a CO ₂ refrigerant, an internally grooved tube in which a groove is formed on the inner surface of a heat transfer tube made of copper or copper alloy is used, and contact between the heat transfer tube and the refrigerant is made by this inner surface groove. The area is increased and the heat transfer performance is improved.

  For example, Patent Document 1 discloses the relationship between the tube outer diameter and the bottom wall thickness of the internally grooved heat transfer tube, the relationship between the groove depth and the groove sectional area per unit groove, and the relationship between the number of grooves and the maximum tube inner diameter. By optimizing the above, an internally grooved heat transfer tube for a cross fin tube type heat exchanger is disclosed in which the heat transfer coefficient in the tube is improved while maintaining sufficient pressure resistance.

  Here, the refrigeration oil is used for the purpose of lubrication, wear reduction, and internal sealing of the compressor in the refrigeration air conditioner. Refrigerating machine oil is required to have a high fluidity in a low-temperature environment because it is difficult to generate precipitates, the refrigerant dissolves well. That is, the deterioration of the oil, the formation of precipitates in a low temperature environment, or the solidification of the refrigerating machine oil in a low temperature environment with a small amount of solvent dissolution should not occur. For example, polyalkylene glycol oils are excellent in compressibility, thermal conductivity, and fluidity, and do not react with iron or copper to form precipitates, so that they are suitably used as refrigerating machine oils.

In the heat exchanger using carbon dioxide containing the refrigerating machine oil as the refrigerant, the CO ₂ refrigerant is changed from a liquid to a gas in the evaporator, but at the inlet of the evaporator, the liquid CO ₂ and the liquid of the lubricant (refrigerating machine oil) are Two layers are separated and supplied without mixing. In the heat transfer tube described in Patent Document 1, depending on the combination of the tube outer diameter and the bottom wall thickness, the two-layer separated refrigerating machine oil may adhere to the tube surface and inhibit CO ₂ liquid evaporation.

  Patent Document 2 discloses a carbon dioxide refrigerant containing refrigerating machine oil by optimizing the groove depth and the lead angle formed by the tube shaft and the inner surface groove in the inner surface grooved heat transfer tube having an outer diameter of 3 to 7 mm. Even in the case of using an evaporator, an internally grooved heat transfer tube for an evaporator has been proposed which does not increase pressure loss and has excellent evaporation heat transfer performance.

  However, when the small-diameter evaporator inner grooved tube disclosed in Patent Document 2 is used, fins are formed between the grooves by forming the grooves in the tube, and the cross-sectional area of the flow path of the tube is reduced. The flow rate increases and the pressure loss increases. Also, in the internally grooved tube disclosed in Patent Document 2, the refrigeration oil contained in the carbon dioxide refrigerant stays and the inner surface of the tube is easily covered with the refrigeration oil film. As a result, the thermal resistance of the tube wall is reduced. Increases and lowers the evaporation performance.

  In the patent document 3, the inventors of the present application minimize the formation of a refrigerator oil film on the inner surface of a small-diameter inner grooved tube, and prevent the heat transfer performance from being deteriorated by the oil film. A grooved tube was proposed. These internally grooved pipes proposed in Patent Documents 2 and 3 are advantageous for downsizing because the pipe outer diameter is small.

  Fin-and-tube heat exchangers used in air conditioners, heat exchangers, etc. are made of aluminum or stainless steel plates (referred to as heat exchanger fins) arranged in parallel at an appropriate length. A common through hole is provided in the heat exchanger fins, and after passing through the heat transfer tubes, a spreading tool (hereinafter referred to as a burette) is inserted into the heat transfer tube, and the inner surface of the tube is mechanically expanded by the burette. The heat transfer tube and the heat exchanger fin are brought into close contact with each other. By providing heat exchanger fins outside the heat transfer tubes, the contact area between the heat transfer tubes and the fluid outside the tubes is increased, and between the fluid that flows through the heat transfer tubes and the fluid that flows through the heat exchanger fins side. However, heat transfer performance is further improved by using the above-mentioned internally grooved tube for this heat transfer tube.

  When an internally grooved tube is used in a fin-and-tube heat exchanger manufactured by such a manufacturing method, if the fin formed on the tube inner surface does not have sufficient strength, the fin is expanded during mechanical tube expansion by a burette. Crushing and fin collapse occur. In particular, when a heat transfer tube with an increased bottom wall thickness is mechanically expanded, fin collapse and fin collapse are likely to occur, and heat transfer performance deteriorates when a fin-and-tube heat exchanger is assembled. There is.

  On the other hand, commercial air conditioners, refrigerators, etc. have a large heat capacity, and it is necessary to pass a large amount of refrigerant through the tubes used for these heat exchangers. However, the inner grooved tubes disclosed in Patent Documents 2 and 3 described above have a small outer diameter of 7 mm or less. When a large amount of refrigerant is allowed to flow inside these small-diameter inner grooved tubes, the flow velocity of the refrigerant flowing on the inner surface of the tube increases, and fins formed on the inner surface of the tube act as resistance to increase pressure loss. However, there is a problem that the evaporation performance deteriorates.

  Further, when the inner grooved tube is incorporated in a device having a large heat capacity, it is necessary to set the ratio of the surface area in the tube to the surface area outside the tube to a certain extent in order to ensure a certain performance. When the outer diameter of the inner grooved tube is increased, the bottom wall thickness must be increased. As a result, there is a problem that workability at the time of rolling the fin on the inner surface of the pipe is deteriorated, and workability at the time of performing secondary processing such as hairpin bending on the inner grooved pipe.

  That is, the inner surface grooved tube used in a device having a large heat capacity has a large amount of refrigerant flowing through the tube, and if the tube has a small diameter, the evaporation performance decreases due to an increase in pressure loss. When the outer diameter of the pipe is increased in order to ensure the evaporation performance, fin collapse and fin collapse occur during mechanical pipe expansion due to an increase in the bottom wall thickness, and further, the secondary workability of the pipe decreases. The above-mentioned Patent Documents 1 to 3 are not intended to solve the problems inherent in the pipe having a large diameter.

JP 2006-162100 A JP 2006-64311 A JP 2006-20166 A

In the Japanese Patent Application No. 2008-072266, in order to prevent fin collapse and fin collapse that occur in the machine expansion process during heat exchanger manufacture, the inventors of the present application are not able to retain the refrigerating machine oil and have sufficient CO ₂ evaporation. We proposed an internally grooved heat transfer tube that is less susceptible to fin crushing and fin collapse while maintaining heat transfer performance.

  However, similarly to Patent Documents 2 and 3, the invention of this prior application also relates to a small-diameter pipe, and has a large diameter that increases pressure loss and decreases evaporation performance when used in a commercial air conditioner having a large refrigerant flow rate. It does not solve the problems inherent to tubes.

The present invention has been made in view of such problems, and an object thereof is to provide an internally grooved heat transfer tube with good workability while maintaining sufficient CO ₂ evaporation heat transfer performance even when the diameter is increased. To do.

An inner grooved heat transfer tube for an evaporator according to the present invention is an inner grooved heat transfer tube for an evaporator of a fin-and-tube heat exchanger that uses carbon dioxide as a refrigerant, and extends in a direction parallel to or inclined with respect to the tube axis. A plurality of grooves are formed, and fins are formed between these grooves. The outer diameter D of the tube is 7.94 to 13 mm , the height H of the fin is 0.1 to 0.4 mm, The lead angle β is 0 to 35 °, the ratio H / D of the fin height H to the tube outer diameter D is 0.01 to 0.05, the number N of the grooves is 40 to 100, and the slopes of the grooves are The apex angle θ is 10 to 40 °, the tensile strength of the pipe material is σB [N / mm ² ], the root radius of the fin is R [mm], and the bottom wall thickness that is the wall thickness at the bottom of the groove of the pipe is When t [mm], parameter α = (H / D) obtained by multiplying the above H / D by the bottom wall thickness t t = 0.022 to 0.04, the ratio of the fin height H to the fin root radius R multiplied by the bottom wall thickness t. γ = (H / R) × t is 1.93 to 4.0, the bottom The wall thickness t is t ≧ 25.5 × D / [2 × (0.8 × σB + 25.5)].

According to the inner surface grooved heat transfer tube for an evaporator of the present invention, by optimizing the tube outer diameter D, fin height H, lead angle β, number of grooves N, and peak angle θ, the inner surface having an outer diameter exceeding 7 mm. Even in a grooved tube, sufficient CO ₂ evaporation heat transfer performance can be obtained. Moreover, the inner surface grooved heat transfer tube for an evaporator of the present invention defines these optimum values by associating the fin height H, the tube outer diameter D, the fin root radius R, and the bottom wall thickness t with each other. The workability is good not only when machining into an internally grooved tube but also during secondary machining. And by defining the R value of the fin base, the fin crushing and the fin collapse that occur when the heat transfer tube is mechanically expanded are prevented. Even when incorporated in an exchanger, excellent heat transfer performance can be obtained.

(A) is a longitudinal cross-sectional view including the tube axis | shaft in the inner surface grooved heat exchanger tube which concerns on embodiment of this invention, (b) is sectional drawing which shows a part of inner surface grooved heat exchanger tube in a tube axis orthogonal cross section. It is a figure which shows the R part of a fin base. 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 shown in FIG.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings. Fig.1 (a) is a longitudinal cross-sectional view including the tube axis in the internally grooved heat transfer tube according to the embodiment of the present invention, and Fig.1 (b) shows a part of the internally grooved heat transfer tube in the tube axis orthogonal cross section. A sectional view and FIG. 2 are partially enlarged sectional views of FIG.

  In the internally grooved heat transfer tube 21, a plurality of grooves 22 are formed in a spiral shape on the tube inner surface, and a fin 23 extending in a spiral shape is formed as a protrusion between the spiral grooves 22. The formation of the grooves 22 and the fins 23 increases the contact area between the inner surface of the heat transfer tube and the refrigerant, thereby improving the heat transfer performance.

  Next, the shape of the internally grooved heat transfer tube of the present invention will be described. As shown in FIG. 1A, the lead angle of the groove 22 with respect to the tube axis direction is β, and the outer diameter of the tube 21 is D as shown in FIG. The height is H, the bottom wall thickness is t, the apex angle of the fin 23 is θ, and the root radius of the fin 23 is R as shown in FIG. At this time, in the present invention, the outer diameter D of the tube 21 is more than 7 mm and not more than 13 mm, the height H of the fin 23 is 0.1 to 0.4 mm, and the lead angle β that the groove 22 has with respect to the tube axis. Is 0 to 35 °. Further, the ratio H / D between the fin height H and the tube outer diameter D is 0.01 to 0.05, the number N of grooves 22 formed in the tube is 40 to 100, and the inclined surfaces of the grooves 22 form. The apex angle θ is 10 to 40 °. Further, the parameter α = (H / D) × t obtained by multiplying the H / D by the bottom wall thickness t is 0.04 or less, and the ratio of the fin height H and the fin root radius R is set to the bottom wall thickness t. The multiplied parameter γ = (H / R) × t is 0.5 to 4.0.

Furthermore, assuming that the tensile strength of the pipe material is σB [N / mm ² ], the bottom wall thickness t satisfies t ≧ 25.5 × D / [2 × (0.8 × σB + 25.5)]. Here, when phosphorous-deoxidized copper is used as the pipe material, the tensile strength σB of the pipe material is 240.5 [N / mm ² ] based on the refrigeration security rule related illustration standard.

  Hereinafter, the reason for the above numerical limitation will be described.

"Pipe outer diameter D: Over 7mm and 13mm or less"
By making the heat transfer tube an internally grooved tube, the heat transfer area in the tube is increased, and furthermore, the CO ₂ refrigerant is agitated by the fins formed in the tube, so the heat transfer performance is improved. As will be described later, the inner surface grooved tube is formed by rolling the outer surface of the tube at the position of the grooved plug that is rotatably connected to the reduced diameter plug in the tube. It is manufactured by a rolling process in which the groove shape is transferred to the inner surface of the pipe by pressing with a roll. By subjecting the inner surface grooved tube to which the groove shape has been transferred to a diameter reduction process, an inner surface grooved tube having a predetermined diameter is manufactured. Since the inner grooved tube has a higher heat transfer performance as the cross-sectional area of the tube wall portion in the tube cross section is smaller, a smaller tube outer diameter is advantageous in terms of heat transfer performance. However, the present invention is directed to an internally grooved tube for an evaporator used in a heat exchanger having a relatively large capacity such as a commercial air conditioner. Therefore, it is assumed that the inner diameter of the inner grooved tube exceeds 7 mm. When the working pressure of the refrigerant flowing through the inner grooved pipe is constant, the thickness of the heat transfer pipe must be increased in proportion to the outer diameter so that the heat transfer pipe does not break down due to the working pressure of the refrigerant. . If the outer diameter of the tube exceeds 13 mm, it is disadvantageous for downsizing, and it is necessary to ensure a large thickness in order to ensure the pressure resistance of the tube, so the mass of the evaporator incorporating the inner grooved tube increases, It is disadvantageous for weight reduction. Further, when the thickness of the pipe increases, the tool load increases during the grooving to the inner surface of the pipe and the diameter reduction processing of the pipe, and the productivity of the inner grooved pipe becomes worse. Therefore, the tube outer diameter D is more than 7 mm and not more than 13 mm.

“Fin height H: 0.1 to 0.4 mm”
If fin height is made high, the contact area of a heat exchanger tube and a refrigerant will increase, and heat transfer performance will improve. However, if the fin height exceeds 0.4 mm, the fin becomes a resistance that hinders the flow of the refrigerant in the pipe and increases the pressure loss. As a result, the evaporation heat transfer performance decreases. On the other hand, when the fin height is less than 0.1 mm, the contact area between the heat transfer tube and the refrigerant increases, but the effect of the fins stirring the refrigerant is reduced, and the heat transfer coefficient is not improved. Accordingly, the fin height H is 0.1 to 0.4 mm. Furthermore, if importance is placed on the reduction of pressure loss, the fin height H is preferably 0.30 mm or less.

“Lead angle β: 0 to 35 °”
The evaporative heat transfer performance of the internally grooved tube is greatly influenced by the lead angle formed by the groove formed on the tube inner surface with respect to the tube axis, and the fin of the heat transfer tube has a lead angle β with respect to the tube axis. Since a velocity component perpendicular to the fins of the refrigerant flowing in the pipe is generated, the heat transfer area is increased and the heat transfer performance is improved. However, if the lead angle exceeds 35 °, the fin itself acts as a resistance that hinders the flow of the refrigerant in the pipe, and the pressure loss increases more than the improvement of the evaporation heat transfer performance. Therefore, the lead angle β is 0 to 35 °.

“Ratio H / D of fin height H to outer diameter D: 0.01 to 0.05”
When the ratio of the fin height H to the tube outer diameter D is less than 0.01, the heat transfer coefficient is not improved, and only the evaporation heat transfer performance equivalent to that of the smooth tube can be obtained. On the other hand, if the ratio of the fin height H to the pipe outer diameter D exceeds 0.05, the heat transfer area is increased and the heat transfer performance is improved, but the ratio of the fin height to the pipe outer diameter is increased. Fin collapse and fin collapse during tube expansion are likely to occur. Therefore, the ratio H / D between the fin height H and the outer diameter D is 0.01 to 0.05.

“Number of grooves on the inner surface of tube: 40 to 100”
As the number of grooves present on the inner surface of the tube increases, the inner surface area of the inner surface of the tube increases and the amount of heat transfer increases. However, if the number of grooves exceeds 100, the fin becomes a resistance that obstructs the flow of the refrigerant in the pipe, and the increase in the pressure loss becomes more remarkable than the improvement in the heat transfer coefficient due to the increase in the heat transfer area. descend. On the other hand, if the number of grooves is less than 40, an increase in the heat transfer area does not lead to an improvement in the heat transfer coefficient, and only an evaporation heat transfer performance equivalent to that of the smooth tube can be obtained. Accordingly, the number N of grooves on the inner surface of the tube is 40 to 100.

“Fine vertical angle θ: 10 to 40 °”
The smaller the top angle of the fin, the wider the groove bottom width, the more refrigerant can be held in the groove, and the more the heat transfer area, the higher the heat transfer rate, and the fins orthogonal to the refrigerant flow in the pipe. Evaporative heat transfer performance is improved because the cross-sectional area is reduced and the pressure loss is reduced. However, if the peak angle is less than 10 °, the fin width becomes smaller. Therefore, when the machine is expanded to bring the heat exchanger fin and the inner grooved tube into close contact, fin collapse and fin collapse occur, reducing the heat transfer area. As a result, the heat transfer performance is reduced. On the other hand, if the peak angle exceeds 40 °, the groove bottom width decreases, so the amount of refrigerant flowing between adjacent grooves decreases, and the evaporation heat transfer performance decreases. Furthermore, since the cross-sectional area of the fins orthogonal to the refrigerant flow in the pipe increases and the pressure loss increases, the evaporation heat transfer performance decreases. Accordingly, the apex angle θ of the fin is 10 to 40 °.

“Parameter α = (H / D) × t: 0.04 or less” obtained by multiplying ratio H / D of fin height H and outer diameter D by bottom wall thickness t
In the mechanical pipe expansion process performed when the inner grooved pipe and the heat exchanger fin are brought into close contact with each other, the force required for the pipe expansion increases as the bottom wall thickness t increases, and the higher the fin height H, the more the load is applied to the fin. Since the applied force increases, fin collapse and fin collapse tend to occur. In the present invention, in order to define an appropriate bottom thickness and fin height, a parameter α obtained by multiplying the ratio H / D between the fin height and the outer diameter by the bottom thickness t is used. That is, the greater the bottom wall thickness, the greater the force required for pipe expansion, so the fin height must be reduced. The higher the fin height, the greater the force applied to the fin base, so the outer diameter of the pipe is increased. There is a need to. If α exceeds 0.04, the fins are collapsed and collapsed during mechanical pipe expansion, and the evaporation heat transfer performance is drastically reduced. Accordingly, the parameter α = (H / D) × t obtained by multiplying the ratio H / D between the fin height H and the outer diameter D by the bottom wall thickness t is 0.04 or less.

“Parameter γ = (H / R) × t: 0.5 to 4.0 obtained by multiplying ratio H / R of fin height H and fin root radius R by bottom wall thickness t”
In the mechanical pipe expansion process performed when the inner grooved pipe and the heat exchanger fin are brought into close contact with each other, the force required for the pipe expansion increases as the bottom wall thickness t increases, and the higher the fin height H, the more the load is applied to the fin. Since the applied force increases, fin collapse and fin collapse tend to occur. Therefore, the resistance against the collapse of the fin and the collapse of the fin is enhanced by connecting the fin slope and the groove bottom with a smooth arc. In the present invention, in order to define an appropriate R value, a parameter γ = (H / R) × t obtained by multiplying the ratio H / R between the fin height H and the fin root radius R by the bottom wall thickness t is used. I will do it. That is, the fin root radius R is proportional to the bottom wall thickness t and the fin height H because the fin root radius R needs to be increased as the force required for machine pipe expansion increases or the fin height increases. It is defined as a value. When γ is less than 0.5, the fin base radius R is larger than the bottom wall thickness and fin height, so that the inner surface area of the tube is reduced and the amount of heat transfer is reduced. On the other hand, if γ is larger than 4.0, the fin base radius R becomes smaller than the bottom wall thickness and the fin height, the fin crushing and the fin collapse occur, and the evaporation heat transfer performance decreases rapidly. Therefore, the parameter γ = (H / R) × t obtained by multiplying the ratio H / R between the fin height H and the fin base radius R by the bottom wall thickness t is 0.5 to 4.0.

  Next, the manufacturing method of this pipe | tube is demonstrated. The manufacturing apparatus includes a reduced diameter portion, a rolled portion, and a diameter adjusting portion. An element pipe made of copper or a copper alloy is first inserted into a reduced diameter portion, and is reduced in diameter by a reduced diameter die whose inner diameter is tapered and a reduced diameter plug inserted into the pipe. The diameter-reduced element pipe enters the rolling section, and is subjected to diameter reduction and groove processing by a rolling ball or a rolling roll that rotates planetarily while being in rolling contact with the outer surface of the pipe at the position of the grooved plug disposed on the inner surface of the pipe. The At this time, the grooved plug is rotatably connected to the reduced-diameter plug and is held in that position in a floating state against the pulling force of the raw tube, and the outer surface of the tube is pressed by a rolling ball or a rolling roll. As a result, the tube wall on the inner surface of the raw tube enters the spiral groove formed on the outer surface of the grooved plug, and a groove is formed on the inner surface of the tube. Then, the grooved tube is reduced in diameter by the diameter adjusting portion, and becomes an internally grooved tube having a predetermined outer diameter. By appropriately combining the outer diameter, the wall thickness, the diameter reduction ratio in the reduced diameter section, the diameter reduction ratio in the rolled section, and the diameter reduction ratio in the diameter adjusting section, the inner surface of the predetermined outer diameter and wall thickness. Grooved tubes can be manufactured.

The effects of the embodiments of the present invention will be described below in comparison with comparative examples. Tables 1 to 3 below show the shape conditions of Examples and Comparative Examples. For the production of the internally grooved pipes of Examples and Comparative Examples, the phosphorus deoxidized copper having a tensile strength σB of 240.5 [N / mm ² ] or 267.0 [N / mm ² ] (Example 8). By using a copper alloy and changing the diameter reduction ratio in the reduced diameter part, the type of grooved plug to be inserted into the tube during the rolling process, and the diameter reduction ratio in the shaping part, various examples and comparative examples An internally grooved tube was obtained. Moreover, after rolling the groove on the inner surface of the tube, the formability of the groove was investigated. Tables 1 to 3 show the case where the shape of the groove (fin) formed in the tube was good, and the case where it was not good as x.

The performance ratio was evaluated based on the heat transfer coefficient when compared with a smooth tube having the same diameter and bottom wall thickness. The heat transfer coefficient measurement was performed using the apparatus shown in FIG. FIG. 4 shows the configuration of the evaporator. As shown in FIG. 3, the expansion to 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, a CO ₂ refrigerant An expansion valve 7 for lowering the temperature and an evaporator 1 as a test unit are provided. An oil separator 3a that separates refrigeration oil in the refrigerant is provided at the outlet of the compressor 2, and an accumulator that eliminates pulsation of the refrigerant at the inlet of the compressor 2 and the outlet of the gas cooler 4, respectively. 5a and 5b are provided.

In this embodiment, polyalkylene glycol-based oil is used as the compressor oil for the compressor, and the oil separator 3b provided at the outlet of the evaporator 1 is used to separate the refrigerator oil in the CO ₂ refrigerant. Is cooled by the oil cooler 10 and then added to the CO ₂ refrigerant again via the oil pump 11 and the flow meter 12, so that the refrigerator oil content in the portion immediately before the evaporator 1 is 0.1 mass%. Adjusted. 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. In this example, the preferred range of the refrigeration oil content in the CO ₂ refrigerant is 0.2% by mass or less.

Further, as shown in FIG. 4, three double pipes in which the internal grooved pipes of the example or the comparative example are arranged in the evaporator 1 are connected in series, and the internal grooved pipes of the examples or comparative examples are connected. CO ₂ was allowed to flow inside, and cooling water was allowed to flow between these heat transfer tubes and outer tubes. At that time, the flow direction of the CO ₂ refrigerant was made opposite to the flow direction of the cooling water, and the exchange heat quantity was measured by measuring the temperatures on the inlet side and the outlet side of the cooling water. Using this exchange heat quantity, the heat transfer coefficient of the tube was measured.

  The performance ratio is the heat transfer coefficient of the internally grooved tube / heat transfer coefficient of the smooth tube.

  Pipe expandability is evaluated by mechanically expanding the inner grooved tube of the example or comparative example and the aluminum heat exchanger fins, and investigating the inner surface shape of the tube after the tube expansion, and whether the fin is crushed or collapsed Made by confirming.

  As shown in Tables 1 to 3, since Examples 1 to 8 have a tube outer diameter D that satisfies the scope of the present invention, they are less smooth than Comparative Examples 1 and 2 that do not satisfy the scope of the present invention. The performance ratio of the pipe is high. In Comparative Example 8, the tube outer diameter D exceeded the range of the present invention, and the groove formability and tube expandability were lowered. Moreover, since the fin height H satisfies the range of the present invention in Examples 1 to 8, the fin height H is superior in performance to the comparative example 14 in which the fin height H is lower than the range of the present invention. However, the groove formability is excellent as compared with Comparative Example 10 exceeding the range of the present invention. In Comparative Example 17, the lead angle β exceeded the range of the present invention, and the groove formability and tube expandability deteriorated. In Examples 1 to 8, since the number N of grooves satisfies the range of the present invention, the performance is higher than Comparative Examples 3 and 4 that do not satisfy the range of the present invention. In Comparative Example 9, the peak angle θ exceeded the range of the present invention, and the performance deteriorated. On the other hand, in Comparative Example 6, the summit angle θ was below the range of the present invention, and when forming a groove in the pipe, it was poorly formed, and the evaluation of the tube expandability and the performance ratio could not be performed.

  In Examples 1 to 8, the ratio H / D between the fin height H and the outer diameter D satisfies the range of the present invention, and the performance is higher than that of Comparative Example 15 that falls below the range of the present invention. The tube expandability is superior to Comparative Example 11 exceeding the range. In Comparative Example 12, the ratio H / D between the fin height H and the outer diameter D satisfies the range of the present invention, but the parameter α obtained by multiplying H / D by the bottom wall thickness t satisfies the range of the present invention. The groove formability and tube expandability deteriorated. In Examples 1 to 8, since the parameter γ obtained by multiplying the ratio of the fin height H and the fin base radius R by the bottom wall thickness t satisfies the range of the present invention, it is compared with Comparative Example 16 which is less than the range of the present invention. And the performance is high. On the other hand, in Comparative Examples 7 and 13, since the parameter γ obtained by multiplying the ratio of the fin height H and the fin base radius R by the bottom wall thickness t exceeds the range of the present invention, the groove formability and the tube expandability are deteriorated. Among these, about the comparative example 7, it became a molding defect when shape | molding a groove | channel in a pipe | tube, and evaluation of pipe expandability and evaluation of performance ratio were not able to be implemented. In Comparative Examples 5 and 8, when the fin height H exceeded the range of the present invention, other parameters increased, and the groove formability and tube expandability deteriorated.

  DESCRIPTION OF SYMBOLS 1; Evaporator, 2; Compressor, 3a, 3b; Oil separator, 4; Gas cooler, 5a, 5b; Accumulator, 6, 12; Flow meter, 7: Expansion valve, 8: Preheater, 9; DESCRIPTION OF SYMBOLS 10; Oil cooler, 11; Oil pump, 13; Heat source, 14; Sampling port, 15a-15c; Double pipe, 16a-16f; Heat source water mixing chamber, 17a-17d; Differential pressure converter, 18a-18e Refrigerant mixing chamber, 19a to 19e; thermocouple, 20a to 20e; pressure converter, 21: heat transfer tube with inner groove, 22: groove, 23: fin, β: lead angle, D: tube inner diameter, H: fin height T, bottom thickness, θ: peak angle, R: fin root radius

Claims (1)

In a heat transfer tube with an inner surface groove for an evaporator of a fin-and-tube heat exchanger using carbon dioxide as a refrigerant, a plurality of grooves extending in a direction parallel to or inclined with respect to the tube axis are formed on the inner surface of the tube, and between these grooves Fins are formed, the tube outer diameter D is 7.94 to 13 mm , the fin height H is 0.1 to 0.4 mm, the fin lead angle β is 0 to 35 °, and the fin height. The ratio H / D of H to the outer diameter D of the pipe is 0.01 to 0.05, the number N of the grooves is 40 to 100, and the apex angle θ formed by the inclined surfaces of the grooves is 10 to 40 °. When the tensile strength of the material is σB [N / mm ² ], the base radius of the fin is R [mm], and the bottom wall thickness that is the wall thickness at the groove bottom of the tube is t [mm], the above H / D Parameter α = (H / D) × t multiplied by bottom wall thickness t is 0.022 to 0.04, fin height Parameter γ = (H / R) × t obtained by multiplying the ratio of H to fin root radius R by bottom wall thickness t is 1.93 to 4.0, and bottom wall thickness t is t ≧ 25.5 × D / [ 2 × (0.8 × σB + 25.5)], an internally grooved heat transfer tube for an evaporator.

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