Patent Description:
Conventionally, in a connection structure between a heat transfer tube and a pipe of a heat exchanger, there is a case where a method for inserting and brazing the pipe into the heat transfer tube is adopted. It has been conventionally required to ensure the strength of the coupling area between the heat transfer tube and the pipe, and the smaller the tube diameter of the heat transfer tube, the more important it is to ensure the strength of the coupling area.

For example, in Patent Literature <NUM> (<CIT>), by lengthening the expanded opening of a heat transfer tube, the area of overlap between the heat transfer tube and a pipe is lengthened to ensure the strength of the coupling area.

However, if the expanded opening of the heat transfer tube is lengthened, the brazed area is also lengthened, resulting in the need for more brazing filler metal and longer time required for brazing.

This pipe coupling structure, which is the coupling area between pipes, has the problem of ensuring the strength of the coupling area without lengthening the brazed area. Further pipe coupling structures are disclosed in the following prior art documents: <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>. In particular <CIT> discloses a pipe coupling structure with first and second refrigerant pipes both with a smaller diameter portion and a larger diameter portion, wherein the smaller diameter portions of both pipes face each other and the larger diameter portions of both pipes face each other and wherein said larger diameter portions are brazed.

Aim of the present invention is to provide a pipe coupling structure and a refrigeration cycle apparatus which improve the state of the art indicated above. This aim is achieved by the pipe coupling structure and the refrigeration cycle apparatus according to the corresponding appended claims.

A pipe coupling structure according to a first aspect includes: a first refrigerant pipe that has a first portion, a first expansion portion, a second portion, a second expansion portion, and a third portion arranged in order; and a second refrigerant pipe that is inserted into the first refrigerant pipe and has a fourth portion facing the second portion and a fifth portion facing the third portion. The second portion has an inner diameter larger than an inner diameter of the first portion, and the third portion has an inner diameter larger than the inner diameter of the second portion. The first expansion portion is an area of the first refrigerant pipe, an inner diameter of which gradually changes from the inner diameter of the first portion to the inner diameter of the second portion. The second expansion portion is an area of the first refrigerant pipe, an inner diameter of which gradually changes from the inner diameter of the second portion to the inner diameter of the third portion. The fifth portion has an outer diameter larger than an outer diameter of the fourth portion. The fifth portion and the third portion are brazed.

In the pipe coupling structure according to the first aspect, the second portion of the first refrigerant pipe and the fourth portion of the second refrigerant pipe face each other, thereby allowing an improvement in the strength of the coupling area. The fifth portion and the third portion are brazed together, and thus it is possible to prevent defects caused by an increase in the brazed area.

A pipe coupling structure according to a second aspect is the pipe coupling structure according to the first aspect, in which a first gap between the second portion and the fourth portion is larger than a second gap between the third portion and the fifth portion.

The pipe coupling structure according to the second aspect makes it easier to insert the fourth portion of the second refrigerant pipe into the second portion of the first refrigerant pipe, and to couple the pipes.

A pipe coupling structure according to a third aspect is the pipe coupling structure according to the first or second aspect, in which the first refrigerant pipe has a third expansion portion connected to the third portion and expanded in a tapered shape from the third portion.

A pipe coupling structure according to a fourth aspect is the pipe coupling structure according to any one of the first to third aspects, in which the first refrigerant pipe is a heat transfer tube of a heat exchanger, and the first portion and part of the second portion penetrate a heat transfer fin.

In the pipe coupling structure according to the fourth aspect, since the first portion and part of the second portion are supported by the heat transfer fin, the stress on the first portion of the first refrigerant pipe caused by the stress on the second refrigerant pipe can be reduced.

A pipe coupling structure according to a fifth aspect is the pipe coupling structure according to the fourth aspect, in which the second portion is in contact with a tube plate of the heat exchanger.

A pipe coupling structure according to a sixth aspect is the pipe coupling structure according to the fourth or fifth aspect, in which the other part of the second portion and the third portion are not in contact with the heat transfer fin.

The pipe coupling structure according to the sixth aspect can prevent heat from escaping to the heat transfer fin during the brazing of the third portion.

A pipe coupling structure according to a seventh aspect is the pipe coupling structure according to the fifth aspect, in which a distance along a tube axis direction from the third portion to the tube plate is greater than a length of the third portion along the tube axis direction.

The pipe coupling structure according to the seventh aspect can suppress the deterioration of the heat transfer fin due to the influence of brazing heat.

A pipe coupling structure according to an eighth aspect is the pipe coupling structure according to the fifth aspect, in which a distance along a tube axis direction from the third portion to the tube plate is greater than an outer diameter of the third portion.

The pipe coupling structure according to the eighth aspect can suppress the deterioration of the heat transfer fin due to the influence of brazing heat.

A pipe coupling structure according to a ninth aspect is the pipe coupling structure according to any one of the fourth to eighth aspects, in which the second refrigerant pipe is disposed at an outlet or inlet of the heat exchanger.

The pipe coupling structure according to the ninth aspect contributes to improvement of the durability of the heat exchanger by the second refrigerant pipe being placed at the outlet or inlet of the heat exchanger to which external stress is applied.

A pipe coupling structure according to a tenth aspect is the pipe coupling structure according to any one of the fourth to ninth aspects, in which the first refrigerant pipe has a thickness smaller than a thickness of the second refrigerant pipe.

The pipe coupling structure according to the tenth aspect contributes to improvement in performance of the heat exchanger by reducing the thickness of the heat transfer tube, and can compensate for the decrease in strength due to the reduced thickness of the heat transfer tube by means of the second refrigerant pipe.

A pipe coupling structure according to an eleventh aspect is the pipe coupling structure according to any one of the first to tenth aspects, in which a ratio of the inner diameter of the third portion to the inner diameter of the second portion is smaller than a ratio of the inner diameter of the second portion to the inner diameter of the first portion.

In the pipe coupling structure according to the eleventh aspect, the tube expansion rate from the second portion to the third portion is reduced to such an extent that allows positioning when inserting the second refrigerant pipe into the first refrigerant pipe, so that the first refrigerant pipe and the second refrigerant pipe are easy to manufacture.

A pipe coupling structure according to a twelfth aspect is the pipe coupling structure according to any one of the first to eleventh aspects, in which the inner diameter of the first portion is <NUM> or less.

A pipe coupling structure according to a thirteenth aspect is the pipe coupling structure according to any one of the first to twelfth aspects, in which the second refrigerant pipe has a fourth expansion portion between the fourth portion and the fifth portion. The fourth expansion portion is an area of the second refrigerant pipe, an outer diameter of which gradually changes from the outer diameter of the fourth portion to the outer diameter of the fifth portion. The fourth expansion portion is in contact with the second expansion portion.

In the pipe coupling structure according to the thirteenth aspect, positioning at the second expansion portion of the first refrigerant pipe enables accurate positioning.

A pipe coupling structure according to a fourteenth aspect is the pipe coupling structure according to any one of the first to thirteenth aspects, in which the second portion has a length longer than a length of the fourth portion.

In the pipe coupling structure according to the fourteenth aspect, when the second refrigerant pipe is inserted into the first refrigerant pipe, the second portion is longer than the fourth portion, so that it is possible to prevent a situation in which the leading end of the fourth portion abuts on the first refrigerant pipe and the second refrigerant pipe cannot be sufficiently inserted into the first refrigerant pipe.

A pipe coupling structure according to a fifteenth aspect is the pipe coupling structure according to any one of the first to fourteenth aspects, in which at least one of the first refrigerant pipe and the second refrigerant pipe is formed from aluminum or an aluminum alloy.

The pipe coupling structure according to the fifteenth aspect, can reinforce the strength of the soft aluminum or aluminum alloy with the second portion and the fourth portion that face each other and the third portion and the fifth portion that face each other, while using lightweight aluminum or aluminum alloy, and is less likely to break.

A refrigeration cycle apparatus according to a sixteenth aspect includes: a refrigerant circuit through which a refrigerant circulates; and the pipe coupling structure according to any one of the first to fifteenth aspects, provided in the refrigerant circuit.

<FIG> illustrates an air conditioner <NUM> as an example of a refrigeration cycle apparatus to which a pipe coupling structure is applied. In the present disclosure, the case where the refrigeration cycle apparatus is the air conditioner <NUM> will be described, but the refrigeration cycle apparatus is not limited to the air conditioner <NUM>. The refrigeration cycle apparatus is an apparatus that performs a refrigeration cycle. Examples of the refrigeration cycle apparatus include a refrigerator, a freezer, a water heater, a floor heating apparatus, and a heat pump apparatus.

The air conditioner <NUM> in <FIG> includes a compressor <NUM>, a four-way valve <NUM>, an outdoor heat exchanger <NUM>, an expansion valve <NUM>, an indoor heat exchanger <NUM>, an outdoor fan <NUM>, and an indoor fan <NUM>.

The compressor <NUM>, the four-way valve <NUM>, the outdoor heat exchanger <NUM>, the expansion valve <NUM>, and the indoor heat exchanger <NUM> are connected by a communicating tube <NUM> to form a refrigerant circuit <NUM>. In the refrigerant circuit <NUM>, a refrigerant circulates and a vapor compression refrigeration cycle is repeated. In other words, the refrigerant circulates in the refrigerant circuit <NUM> while alternating between decompression expansion and heat dissipation condensation. Examples of the refrigerant used in the refrigerant circuit <NUM> in which the vapor compression refrigeration cycle is performed include a hydrofluorocarbon (HFC)-based refrigerant, a hydrofluoroolefin (HFO), an unsaturated HFC-based refrigerant, and a natural refrigerant. Examples of the HFC-based refrigerant include R32, R410A, R407C, and R134a. Examples of the HFO include R1234ze and R1234yf. The natural refrigerant is, for example, R717.

The flow path of the refrigerant flowing in the refrigerant circuit <NUM> is switched by the four-way valve <NUM>. The air conditioner <NUM> can switch between cooling operation and heating operation by switching the flow path with the four-way valve <NUM>. During the cooling operation, the refrigerant flows through the compressor <NUM>, the four-way valve <NUM>, the outdoor heat exchanger <NUM>, the expansion valve <NUM>, the indoor heat exchanger <NUM>, the four-way valve <NUM>, and then the compressor <NUM> in this order. During the heating operation, the refrigerant flows through the compressor <NUM>, the four-way valve <NUM>, the indoor heat exchanger <NUM>, the expansion valve <NUM>, the outdoor heat exchanger <NUM>, the four-way valve <NUM>, and then the compressor <NUM> in this order.

The outdoor heat exchanger <NUM> and the indoor heat exchanger <NUM> are fin-and-tube heat exchangers. During the cooling operation, the outdoor heat exchanger <NUM> functions as a condenser, and the indoor heat exchanger <NUM> functions as an evaporator. During the heating operation, the outdoor heat exchanger <NUM> functions as an evaporator, and the indoor heat exchanger <NUM> functions as a condenser.

<FIG> schematically illustrates a configuration of the outdoor heat exchanger <NUM>. The outdoor heat exchanger <NUM> includes a first inlet/outlet <NUM>, a header collecting tube <NUM>, a plurality of connecting tubes <NUM>, a plurality of heat transfer tubes <NUM>, a U-tube <NUM>, a plurality of heat transfer fins <NUM>, a flow divider <NUM>, and a second inlet/outlet <NUM>.

In the outdoor heat exchanger <NUM> functioning as a condenser, the refrigerant discharged from the compressor <NUM> enters the outdoor heat exchanger <NUM> through the first inlet/outlet <NUM>. In the outdoor heat exchanger <NUM> functioning as an evaporator, the refrigerant subjected to heat exchange in the indoor heat exchanger <NUM> exits the outdoor heat exchanger <NUM> through the first inlet/outlet <NUM>. Regardless of whether the outdoor heat exchanger <NUM> functions as a condenser or an evaporator, the refrigerant passing through the first inlet/outlet <NUM> is mainly gasified refrigerant.

The header collecting tube <NUM> is connected to the first inlet/outlet <NUM> and the plurality of connecting tubes <NUM>. In the outdoor heat exchanger <NUM> functioning as a condenser, the refrigerant entering through the first inlet/outlet <NUM> is distributed by the header collecting tube <NUM> and flows into the plurality of connecting tubes <NUM>. In the outdoor heat exchanger <NUM> functioning as an evaporator, the refrigerant flowing through the plurality of connecting tubes <NUM> merges at the header collecting tube <NUM> and flows into the first inlet/outlet <NUM>. The connecting tubes <NUM> are connected to the heat transfer tubes <NUM>. The <NUM> heat transfer tubes <NUM> of the outdoor heat exchanger <NUM>, indicated by circles in <FIG>, are arranged in <NUM> columns and <NUM> rows. The number and array of the heat transfer tubes <NUM> of the outdoor heat exchanger <NUM> to which the technology according to the present disclosure is applied are not limited to the array illustrated in <FIG>. Note that the rows are aligned in the gravity direction D1, and the columns are aligned in the horizontal direction D2, as indicated by arrows in <FIG>.

<FIG> illustrates a circular cross section of the heat transfer tubes <NUM>. In other words, the heat transfer tubes <NUM> extend in a direction perpendicular to the paper surface of <FIG>. The U-tube <NUM> is connected to the end of the heat transfer tube <NUM> to which the connecting tube <NUM> is not connected. The U-tube <NUM> connects two heat transfer tubes <NUM>. The U-tube <NUM> allows the refrigerant flowing in one heat transfer tube <NUM> to turn around at the end of the outdoor heat exchanger <NUM> and flow into the other heat transfer tube <NUM>. In <FIG>, the U-tube <NUM> indicated by a solid line is disposed in the front, and the U-tube <NUM> indicated by a broken line is disposed in the back. All heat transfer tubes <NUM> penetrate vertically through the plurality of heat transfer fins <NUM>. The plurality of heat transfer fins <NUM> are arranged such that the main surfaces thereof are parallel to each other.

The plurality of heat transfer tubes <NUM> and the flow divider <NUM> are connected by the plurality of connecting tubes <NUM>. When the outdoor heat exchanger <NUM> in <FIG> functions as a condenser, the refrigerant flows into the flow divider <NUM> from six heat transfer tubes <NUM> through six connecting tubes <NUM>. The refrigerant that has flowed into the flow divider <NUM> from the six connecting tubes <NUM> and merged at the flow divider <NUM> flows out from one connecting tube <NUM>. In the outdoor heat exchanger <NUM> functioning as an evaporator, the refrigerant flowing into the flow divider <NUM> from one connecting tube <NUM> is distributed to the six connecting tubes <NUM> by the flow divider <NUM>. Between one connecting tube <NUM> of the flow divider <NUM> and the second inlet/outlet <NUM>, the plurality of heat transfer tubes <NUM>, the plurality of U-tubes <NUM>, and one connecting tube <NUM> are connected in series.

<FIG> schematically illustrates a configuration of the indoor heat exchanger <NUM>. The indoor heat exchanger <NUM> includes a connecting tube <NUM>, a plurality of heat transfer tubes <NUM>, a U-tube <NUM>, a plurality of heat transfer fins <NUM>, a plurality of connecting tubes <NUM>, a first flow divider <NUM>, a second flow divider <NUM>, and an inlet/outlet pipe <NUM>.

In the indoor heat exchanger <NUM> functioning as a condenser, the refrigerant discharged from the compressor <NUM> enters the heat transfer tubes <NUM> through the connecting tube <NUM>. In the indoor heat exchanger <NUM> functioning as an evaporator, the refrigerant subjected to heat exchange in the indoor heat exchanger <NUM> exits the indoor heat exchanger <NUM> through the connecting tube <NUM>. Regardless of whether the indoor heat exchanger <NUM> functions as a condenser or an evaporator, the refrigerant passing through the connecting tube <NUM> is mainly gasified refrigerant.

The connecting tube <NUM> is connected to the inlet/outlet (not illustrated) of the indoor heat exchanger <NUM> and the heat transfer tubes <NUM>. The <NUM> heat transfer tubes <NUM> of the indoor heat exchanger <NUM>, indicated by circles in <FIG>, are arranged in two columns. The number and array of the heat transfer tubes <NUM> of the indoor heat exchanger <NUM> to which the technology according to the present disclosure is applied are not limited to the array illustrated in <FIG>. Note that the indoor heat exchangers <NUM> are arranged in rows close to and far from a cross-flow fan <NUM> in <FIG>.

<FIG> illustrates a circular cross section of the heat transfer tubes <NUM>. In other words, the heat transfer tubes <NUM> extend in a direction perpendicular to the paper surface of <FIG>. The U-tube <NUM> is connected to the end of the heat transfer tube <NUM> to which the connecting tube <NUM> is not connected. The U-tube <NUM> connects two heat transfer tubes <NUM>. The U-tube <NUM> allows the refrigerant flowing in one heat transfer tube <NUM> to turn around at the end of the indoor heat exchanger <NUM> and flow into the other heat transfer tube <NUM>.

In <FIG>, the U-tube <NUM> indicated by a solid line is disposed in the front, and the U-tube <NUM> indicated by a broken line is disposed in the back. All heat transfer tubes <NUM> penetrate vertically through the plurality of heat transfer fins <NUM>. The plurality of heat transfer fins <NUM> are arranged such that the main surfaces thereof are parallel to each other.

One heat transfer tube <NUM> and the first flow divider <NUM> are connected by one connecting tube <NUM>. When the indoor heat exchanger <NUM> in <FIG> functions as a condenser, the refrigerant flows into the first flow divider <NUM> from one heat transfer tube <NUM> through one connecting tube <NUM>. The refrigerant flows into the first flow divider <NUM> from one connecting tube <NUM> and is distributed to two connecting tubes <NUM> by the first flow divider <NUM>. In the indoor heat exchanger <NUM> functioning as an evaporator, the refrigerant flowing into the first flow divider <NUM> from two connecting tubes <NUM> merges at the first flow divider <NUM> and flows out from one connecting tube <NUM> into the heat transfer tube <NUM>.

Between the two connecting tubes <NUM> connected to the first flow divider <NUM> and the two connecting tubes <NUM> connected to the second flow divider <NUM>, two flow paths are formed by the plurality of heat transfer tubes <NUM> and the plurality of U-tubes <NUM> connected in series.

Two heat transfer tubes <NUM> and the second flow divider <NUM> are connected by the two connecting tubes <NUM>. When the indoor heat exchanger <NUM> illustrated in <FIG> functions as an evaporator, the refrigerant flowing into the second flow divider <NUM> from one inlet/outlet pipe <NUM> is distributed to the two connecting tubes <NUM> by the second flow divider <NUM>, and flows out from the two connecting tubes <NUM> to the two heat transfer tubes <NUM>. When the indoor heat exchanger <NUM> functions as a condenser, the refrigerant flows into the second flow divider <NUM> from the two heat transfer tubes <NUM> through the two connecting tubes <NUM>. The refrigerant flowing into the second flow divider <NUM> from the two connecting tubes <NUM> merges at the first flow divider <NUM> and flows out from one inlet/outlet pipe <NUM>.

<FIG> illustrates a cross section of a pipe coupling structure <NUM>. The pipe coupling structure <NUM> is the structure of the area where a heat transfer tube <NUM> and a connecting tube <NUM> are coupled together. The heat transfer tube <NUM> is a first refrigerant pipe having a first portion <NUM>, a first expansion portion <NUM>, a second portion <NUM>, a second expansion portion <NUM>, and a third portion <NUM> arranged in order. The heat transfer tube <NUM> is expanded in the first stage from the first portion <NUM> through the first expansion portion <NUM> to the second portion <NUM>. Furthermore, the heat transfer tube <NUM> is expanded in the second stage from the second portion <NUM> through the second expansion portion <NUM> to the third portion <NUM>. This two-stage expansion of the heat transfer tube <NUM> is performed, for example, by flaring. The connecting tube <NUM> is inserted into the heat transfer tube <NUM> serving as the first refrigerant pipe. The connecting tube <NUM> is a second refrigerant pipe having a fourth portion <NUM> that faces the second portion <NUM> of the heat transfer tube <NUM> and a fifth portion <NUM> that faces the third portion <NUM> of the heat transfer tube <NUM>.

An inner diameter ID2 of the second portion <NUM> of the heat transfer tube <NUM> is larger than an inner diameter ID1 of the first portion <NUM>. In addition, an inner diameter ID3 of the third portion <NUM> of the heat transfer tube <NUM> is larger than the inner diameter ID2 of the second portion <NUM>. An outer diameter OD2 of the second portion <NUM> is larger than an outer diameter OD1 of the first portion <NUM>. In addition, an outer diameter OD3 of the third portion <NUM> is larger than the outer diameter OD2 of the second portion <NUM>. Furthermore, the thickness of the heat transfer tube <NUM> is the same in the first portion <NUM>, the second portion <NUM>, and the third portion <NUM>. The first expansion portion <NUM> of the heat transfer tube <NUM> is the area of the heat transfer tube <NUM>, the inner diameter of which gradually changes from the inner diameter ID1 of the first portion <NUM> to the inner diameter ID2 of the second portion <NUM>. The second expansion portion <NUM> of the heat transfer tube <NUM> is the area of the heat transfer tube <NUM>, the inner diameter of which gradually changes from the inner diameter ID2 of the second portion <NUM> to the inner diameter ID3 of the third portion <NUM>.

An outer diameter OD5 of the fifth portion <NUM> of the connecting tube <NUM> is larger than an outer diameter OD4 of the fourth portion <NUM>. In addition, an inner diameter ID5 of the fifth portion <NUM> of the connecting tube <NUM> is larger than an inner diameter ID4 of the fourth portion <NUM>. Furthermore, the thickness of the connecting tube <NUM> is the same in the fourth portion <NUM> and the fifth portion <NUM>. The fourth portion <NUM> of the connecting tube <NUM> is formed, for example, by narrowing the outer diameter OD5 of the fifth portion <NUM> to the outer diameter OD4. For example, the fourth portion <NUM> can be narrowed by swaging the fourth portion <NUM>.

The heat transfer tube <NUM> is formed from, for example, aluminum, an aluminum alloy, or copper. The heat transfer tube <NUM> formed from aluminum or aluminum alloy is preferable in that the weight of the outdoor heat exchanger <NUM> or the indoor heat exchanger <NUM> can be reduced. In addition, the connecting tube <NUM> is formed from, for example, aluminum, an aluminum alloy, or copper. The connecting tube <NUM> formed from aluminum or aluminum alloy is preferable in that the weight of the outdoor heat exchanger <NUM> or the indoor heat exchanger <NUM> can be reduced. The thickness of the heat transfer tube <NUM> (first refrigerant pipe) is thinner than the thickness of the connecting tube <NUM> (second refrigerant pipe). This thin-walled heat transfer tube <NUM> is reinforced by the overlap with the connecting tube <NUM>.

In the pipe coupling structure <NUM>, the fifth portion <NUM> of the connecting tube <NUM> and the third portion <NUM> of the heat transfer tube <NUM> are brazed together. The total length of the second portion <NUM>, the second expansion portion <NUM>, and the third portion <NUM> is, for example, within the range of <NUM> to <NUM>. The length of the third portion <NUM> is, for example, within the range of <NUM> to <NUM>. The end of the fourth portion <NUM> is preferably inserted to the vicinity of the boundary between the first expansion portion <NUM> and the second portion <NUM>. The length of the second portion <NUM> of the heat transfer tube <NUM> is longer than the length of the fourth portion <NUM> of the connecting tube <NUM>.

A first gap In1 between the second portion <NUM> of the heat transfer tube <NUM> and the fourth portion <NUM> of the connecting tube <NUM> is larger than a second gap In2 between the third portion <NUM> of the heat transfer tube <NUM> and the fifth portion <NUM> of the connecting tube <NUM>. By making the first gap In1 larger than the second gap In2, it is easier to insert the connecting tube <NUM> into the heat transfer tube <NUM>.

The heat transfer tube <NUM> has a third expansion portion <NUM> connected to the third portion <NUM> and expanded in a tapered shape from the third portion <NUM>. Since the third expansion portion <NUM> is expanded in the tapered shape, the gap between the third expansion portion <NUM> and the fifth portion <NUM> of the connecting tube <NUM> is even wider than the second gap In2. A brazing filler metal <NUM> is poured through the gap between the expanded third expansion portion <NUM> and the fifth portion <NUM>. The tapered expansion of the third expansion portion <NUM> facilitates brazing.

As illustrated in <FIG>, the entire first portion <NUM> of the heat transfer tube <NUM> penetrates the heat transfer fins <NUM>. In addition, part 102a of the second portion <NUM> of the heat transfer tube <NUM> penetrates the heat transfer fins <NUM>. The part 102a of the second portion <NUM> is the area of the second portion <NUM> which is closer to the first expansion portion <NUM> than the second expansion portion <NUM>. The other part 102b of the second portion <NUM> is the area of the second portion <NUM> other than the part 102a and the tube plate <NUM>. The other part 102b of the second portion <NUM> is the area of the second portion <NUM> closer to the second expansion portion <NUM>. The other part 102b of the second portion <NUM> is not in contact with the heat transfer fins <NUM>. In addition, the third portion <NUM> of the heat transfer tube <NUM> is also not in contact with the heat transfer fins <NUM>. As illustrated in <FIG>, the second portion <NUM> of the heat transfer tube <NUM> is in contact with the tube plate <NUM>. More specifically, the second portion <NUM> is in contact with the tube plate <NUM> between the part 102a and the other part 102b of the second portion <NUM>.

A distance di1 along the tube axis direction D3 from the third portion <NUM> to the tube plate <NUM> is greater than a length L1 of the third portion <NUM> along the tube axis direction D3. In addition, the distance di1 along the tube axis direction D3 from the third portion <NUM> to the tube plate <NUM> is greater than the outer diameter OD3 of the third portion <NUM>.

The ratio of the inner diameter ID3 of the third portion <NUM> to the inner diameter ID2 of the second portion <NUM> is smaller than the ratio of the inner diameter ID2 of the second portion <NUM> to the inner diameter ID1 of the first portion <NUM>. In other words, the relationship of (ID3/ID2) < (ID2/ID1) holds. It is preferable that <NUM> ≥ ID3/ID2 ≥ <NUM>. In addition, the ratio of the outer diameter OD3 of the third portion <NUM> to the outer diameter OD2 of the second portion <NUM> is smaller than the ratio of the outer diameter OD2 of the second portion <NUM> to the outer diameter OD1 of the first portion <NUM>. In other words, the relationship of (OD3/OD2) < (OD2/OD1) holds. The inner diameter ID1 of the first portion <NUM> is <NUM> or less.

The connecting tube <NUM> has a fourth expansion portion <NUM> between the fourth portion <NUM> and the fifth portion <NUM>. The fourth expansion portion <NUM> is the area of the connecting tube <NUM>, the outer diameter of which gradually changes from the outer diameter OD4 of the fourth portion <NUM> to the outer diameter OD5 of the fifth portion <NUM>. The fourth expansion portion <NUM> is in contact with the second expansion portion <NUM>.

The contact between the fourth expansion portion <NUM> and the second expansion portion <NUM> enables positioning of the heat transfer tube <NUM> and the connecting tube <NUM> in the tube axis direction D3. In addition, the contact between the fourth expansion portion <NUM> and the second expansion portion <NUM> prevents the brazing filler metal <NUM> from entering the first gap In1 between the second portion <NUM> of the heat transfer tube <NUM> and the fourth portion <NUM> of the connecting tube <NUM>. Therefore, the inner surface of the second portion <NUM> and the outer surface of the fourth portion <NUM> are not brazed.

(<NUM>-<NUM>)
In the pipe coupling structure <NUM>, the second portion <NUM> of the heat transfer tube <NUM> serving as a first refrigerant pipe and the fourth portion <NUM> of the connecting tube <NUM> serving as a second refrigerant pipe face each other. In this manner, the second portion <NUM> and the fourth portion <NUM> face each other, thereby allowing an improvement in the strength of the coupling area. The third portion <NUM> of the heat transfer tube <NUM> and the fifth portion <NUM> of the connecting tube <NUM> are brazed together, but the second portion <NUM> of the heat transfer tube <NUM> and the fourth portion <NUM> of the connecting tube <NUM> are not brazed. Thus, the pipe coupling structure <NUM> can prevent defects caused by an increase in the brazed area.

(<NUM>-<NUM>)
In the pipe coupling structure <NUM>, the first gap In1 between the second portion <NUM> of the heat transfer tube <NUM> and the fourth portion <NUM> of the connecting tube <NUM> is larger than the second gap In2 between the third portion <NUM> of the heat transfer tube <NUM> and the fifth portion <NUM> of the connecting tube <NUM>. This structure makes it easier to insert the fourth portion <NUM> of the connecting tube <NUM> into the second portion <NUM> of the heat transfer tube <NUM>, and to couple the pipes.

(<NUM>-<NUM>)
The third expansion portion <NUM> of the pipe coupling structure <NUM> is expanded in a tapered shape from the third portion <NUM>. The third expansion portion <NUM>, which is expanded in the tapered shape in this manner, facilitates the insertion of the connecting tube <NUM> into the heat transfer tube <NUM>. In addition, the brazing filler metal <NUM> can be easily poured through the third expansion portion <NUM> expanded in the tapered shape.

(<NUM>-<NUM>)
In the pipe coupling structure <NUM>, the first refrigerant pipe is the heat transfer tube <NUM> of the outdoor heat exchanger <NUM> or the indoor heat exchanger <NUM>. The first portion <NUM> and the part 102a of the second portion <NUM> of the heat transfer tube <NUM> penetrate the heat transfer fins <NUM>. With this structure, the first portion <NUM> and the part 102a of the second portion <NUM> are supported by the heat transfer fins <NUM>. As a result, the stress on the first portion <NUM> of the heat transfer tube <NUM> caused by the stress on the connecting tube <NUM> serving as the second refrigerant pipe can be reduced. The reduced stress on the first portion <NUM> makes it less likely to occur the damage in the pipe coupling structure <NUM>.

(<NUM>-<NUM>)
In the pipe coupling structure <NUM>, the second portion <NUM> is in contact with the tube plate <NUM> of the outdoor heat exchanger <NUM> or the indoor heat exchanger <NUM>. The second portion <NUM> is a non-brazed area, and even if the tube plate <NUM> is in contact with the second portion <NUM>, defects such as difficulty in brazing due to escape of heat during brazing to the tube plate <NUM> are less likely to occur. Even if heat is transferred from the first portion <NUM> to the second portion <NUM> by brazing in the first portion <NUM>, heat escapes to the tube plate <NUM>, thereby allowing a reduction in the risk of deterioration of the heat transfer fins <NUM> due to exposure to the heat of brazing.

(<NUM>-<NUM>)
In the pipe coupling structure <NUM>, the other part 102b of the second portion <NUM> and the third portion <NUM> are not in contact with the heat transfer fins <NUM>. Since the pipe coupling structure <NUM> has this structure, it is possible to prevent heat from escaping to the heat transfer fins <NUM> during the brazing of the third portion <NUM>. As a result, it becomes easier to perform good brazing between the third portion <NUM> of the heat transfer tube <NUM> and the fifth portion <NUM> of the connecting tube <NUM>.

(<NUM>-<NUM>)
In the pipe coupling structure <NUM>, the distance di1 along the tube axis direction D3 from the third portion <NUM> to the tube plate <NUM> is greater than the length L1 of the third portion <NUM> along the tube axis direction D3. Since the distance di1 from the third portion <NUM> to the tube plate <NUM> makes it difficult for the brazing heat to be transferred to the heat transfer fins <NUM>, it is possible to suppress the deterioration of the heat transfer fins <NUM> due to the influence of the brazing heat.

(<NUM>-<NUM>)
In the pipe coupling structure <NUM>, the distance di1 along the tube axis direction D3 from the third portion <NUM> to the tube plate <NUM> is greater than the outer diameter OD3 of the third portion <NUM>. Since the distance di1 from the third portion <NUM> to the tube plate <NUM> makes it difficult for the brazing heat to be transferred to the heat transfer fins <NUM>, it is possible to suppress the deterioration of the heat transfer fins <NUM> due to the influence of the brazing heat.

(<NUM>-<NUM>)
External stress may be applied to the connecting tube <NUM> connected to the second inlet/outlet <NUM> of the outdoor heat exchanger <NUM> in <FIG> and the connecting tube <NUM> connected to the inlet/outlet (not illustrated) of the indoor heat exchanger <NUM> in <FIG>. However, such connecting tube <NUM> connected to the outlet or inlet is reinforced by the pipe coupling structure <NUM>, which contributes to improvement of the durability of the outdoor heat exchanger <NUM> or the indoor heat exchanger <NUM>.

(<NUM>-<NUM>)
The thickness of the heat transfer tube <NUM> serving as the first refrigerant pipe is thinner than the thickness of the connecting tube <NUM> serving as the second refrigerant pipe. By reducing the thickness of the heat transfer tube <NUM>, the heat of the refrigerant is more easily transferred to the heat transfer fins <NUM>, and the performance of the outdoor heat exchanger <NUM> and the indoor heat exchanger <NUM> can be improved. Meanwhile, the decrease in the strength of the heat transfer tube <NUM> having a thinner wall thickness can be compensated for by overlapping with the connecting tube <NUM> having a thicker wall thickness.

(<NUM>-<NUM>)
In the pipe coupling structure <NUM>, the ratio of the inner diameter ID3 of the third portion <NUM> to the inner diameter ID2 of the second portion <NUM> is smaller than the ratio of the inner diameter ID2 of the second portion <NUM> to the inner diameter ID1 of the first portion <NUM>. As described above, the tube expansion rate (ID3/ID2) from the second portion <NUM> to the third portion <NUM> is reduced to such an extent that allows positioning when inserting the connecting tube <NUM> into the heat transfer tube <NUM>, so that the heat transfer tube <NUM> and the connecting tube <NUM> are easy to manufacture.

(<NUM>-<NUM>)
In the pipe coupling structure <NUM>, the inner diameter ID1 of the first portion <NUM> is <NUM> or less. Even if the thin heat transfer tube <NUM> of <NUM> or less is used, the pipe coupling structure <NUM> requires only a small amount of drawing, and can suppress an increase in pressure loss caused by the flow of the refrigerant and the generation of refrigerant noise.

(<NUM>-<NUM>)
In the pipe coupling structure <NUM>, the fourth expansion portion <NUM> of the connecting tube <NUM> serving as the second refrigerant pipe is in contact with the second expansion portion <NUM> of the heat transfer tube <NUM> serving as the first refrigerant pipe. Positioning by the contact between the fourth expansion portion <NUM> and the second expansion portion <NUM> enables accurate positioning.

(<NUM>-<NUM>)
The length of the second portion <NUM> of the heat transfer tube <NUM> is longer than the length of the fourth portion <NUM> of the connecting tube <NUM>. With this structure, when inserting the connecting tube <NUM> into the heat transfer tube <NUM>, the fourth expansion portion <NUM> of the connecting tube <NUM> abuts on the second expansion portion <NUM> of the heat transfer tube <NUM> before the leading end of the fourth portion <NUM> abuts on the first expansion portion <NUM> of the heat transfer tube <NUM>. As described above, since the second portion <NUM> is longer than the fourth portion <NUM>, it is possible to prevent a situation in which the connecting tube <NUM> cannot be sufficiently inserted into the heat transfer tube <NUM>.

(<NUM>-<NUM>)
If the heat transfer tube <NUM> is formed from aluminum or an aluminum alloy, the strength of the soft aluminum or aluminum alloy can be reinforced by the second portion <NUM> and the fourth portion <NUM> that face each other and the third portion <NUM> and the fifth portion <NUM> that face each other, while using lightweight aluminum or aluminum alloy, thereby making it less likely to cause damage. Similarly, if the connecting tube <NUM> is formed from aluminum or an aluminum alloy, the strength of the soft aluminum or aluminum alloy can be reinforced by the second portion <NUM> and the fourth portion <NUM> that face each other and the third portion <NUM> and the fifth portion <NUM> that face each other, while using lightweight aluminum or aluminum alloy, thereby making it less likely to cause damage. In particular, if the heat transfer tube <NUM> and the connecting tube <NUM> are formed from the same aluminum or the same aluminum alloy, the electrolytic corrosion that occurs between dissimilar metals can be prevented.

Although the embodiment of the present disclosure have been described above, it will be understood that various changes in form and details can be made without departing from the scope of the present disclosure described in the claims.

Claim 1:
A pipe coupling structure (<NUM>) comprising:
a first refrigerant pipe (<NUM>) that has a first portion (<NUM>), a first expansion portion (<NUM>), a second portion (<NUM>), a second expansion portion (<NUM>), and a third portion (<NUM>) arranged in order; and
a second refrigerant pipe (<NUM>) that is inserted into the first refrigerant pipe and has a fourth portion (<NUM>) facing the second portion and a fifth portion (<NUM>) facing the third portion, wherein
the second portion has an inner diameter (ID2) larger than an inner diameter (ID1) of the first portion, and the third portion has an inner diameter (ID3) larger than the inner diameter of the second portion,
the first expansion portion is an area of the first refrigerant pipe, an inner diameter of which gradually changes from the inner diameter of the first portion to the inner diameter of the second portion,
the second expansion portion is an area of the first refrigerant pipe, an inner diameter of which gradually changes from the inner diameter of the second portion to the inner diameter of the third portion,
the fifth portion has an outer diameter (OD5) larger than an outer diameter (OD4) of the fourth portion, and
the fifth portion and the third portion are brazed.