Patent Description:
In a refrigeration apparatus, a multistage compression mechanism using a plurality of compressors is recommended and used depending on working refrigerant. In the multistage compression mechanism using the plurality of compressors, it is important to control refrigerator oil in an appropriate amount in the plurality of compressors. In other words, the oil is to be controlled not to be extremely unevenly distributed in one compressor.

In Patent Literature <NUM> (<CIT>), a low-stage oil drain passage in a low-stage compressor and an oil return passage for returning oil discharged in a high-stage compressor to a suction pipe of the low-stage compressor are provided in order to keep an oil level of the low-stage and high-stage compressors constant.

Other examples of known multistage compression systems are disclosed in patent documents <CIT>, <CIT> and <CIT>. <CIT>, for example, discloses a multistage compression system using refrigerant and oil, the multistage compression system comprising: a low-stage compressor configured to compress the refrigerant; a high-stage compressor configured to further compress the refrigerant compressed by the low-stage compressor; refrigerant pipes configured to introduce the refrigerant compressed and discharged by the low-stage compressor into a suction part of the high-stage compressor;a pressure reducing element disposed between the refrigerant pipes; and an oil discharge pipe connecting the low-stage compressor, so that the oil discharge pipe discharges the oil.

In Patent Literature <NUM>, the low-stage oil drain passage is connected to a suction side of the high-stage compressor downstream of a high-stage accumulator. Further, an intercooler or a refrigerant merging point of an intermediate injection is not considered. However, if a pressure reducing element such as the intercooler or the refrigerant merging point of the intermediate injection is provided in a refrigerant pipe from a low-stage refrigerant discharge part to a high-stage refrigerant suction part, a pressure of the refrigerant pipe reduces. Thus, an amount of refrigerant and oil passing through the oil drain passage varies depending on a connection position of the oil drain passage, which greatly affects a refrigerant circuit. For example, if a large amount of refrigerant that bypasses an intercooler in a system using the intercooler, a refrigerant cooling amount may be insufficient.

Aim of the present invention is to provide a multistage compression system which improves the state of the art indicated above. This aim is achieved by the multistage compression system according to the corresponding appended claims.

A multistage compression system according to a first aspect uses refrigerant and oil. The multistage compression system comprises a low-stage compressor, a high-stage compressor, refrigerant pipes, a pressure reducing element, and an oil discharge pipe. The low-stage compressor compresses the refrigerant. The high-stage compressor further compresses the refrigerant compressed by the low-stage compressor. The refrigerant pipes introduces the refrigerant compressed and discharged by the low-stage compressor into a suction part of the high-stage compressor. The pressure reducing element is disposed between the refrigerant pipes. The oil discharge pipe discharges the oil in the low-stage compressor. The oil discharge pipe connects the low-stage compressor and a portion of the refrigerant pipes, which is an upstream side of the pressure reducing element.

In the multistage compression system according to the first aspect, the oil discharge pipe connects the low-stage compressor and a portion of the refrigerant pipes, which is an upstream side of the pressure reducing element. Thus, an amount of oil discharged from the oil discharge pipe is reduced, and an amount of oil in the low-stage compressor can be controlled appropriately.

A multistage compression system according to a second aspect is the system according to the first aspect, in which the low-stage compressor comprises a compression part, a motor, and a container. The compression part is a rotary type. The compression part has a compression chamber. The refrigerant is compressed in the compression chamber. The motor drives the compression part. The motor is disposed above the compression part. The container houses the compression part and the motor. The oil discharge pipe is connected to the container below the motor and above the compression chamber. When the low-stage compressor has two or more compression chambers having different heights, the compression chamber referred to here means a lowest compression chamber.

In the multistage compression system according to the second aspect, because the oil discharge pipe is connected to a position above the compression chamber of the container and below the motor, excess oil of the low-stage compressor can be discharged from the low-stage compressor without excess or deficiency.

A multistage compression system according to a third aspect is the system according to the first or second aspect, in which the pressure reducing element is an intercooler. The intercooler cools the refrigerant discharged by the low-stage compressor before the refrigerant is sucked into the high-stage compressor.

In the multistage compression system according to the third aspect, the oil discharge pipe is connected to the low-stage compressor and a portion of the refrigerant pipes, which is an upstream side of the intercooler. Thus, the amount of oil discharged from the oil discharge pipe is reduced, and the amount of oil in the low-stage compressor can be controlled appropriately.

A multistage compression system according to a fourth aspect is the system according to the third aspect, and further comprises a merging part of an intermediate injection passage. At the merging part of the intermediate injection passage, an intermediate-pressure refrigerant is injected into a portion of the refrigerant pipes. The merging part of the intermediate injection passage is connected to an upstream side of the intercooler. The oil discharge pipe is connected to a portion of the refrigerant pipes between the merging part and the intercooler.

In the multistage compression system according to the fourth aspect, because the oil discharge pipe is connected to a portion of the refrigerant pipes between the merging part and the intercooler, a pressure difference between the oil discharge pipe and a portion of the refrigerant pipes is appropriate, the amount of oil discharged by the oil discharge pipe can be controlled appropriately, and the amount of oil in the low-stage compressor can be controlled appropriately.

A multistage compression system according to a fifth aspect is the system according to the first or second aspect, and further comprises an intercooler. The intercooler is connected to a portion of the refrigerant pipes. The intercooler cools the refrigerant discharged by the low-stage compressor before the refrigerant is sucked into the high-stage compressor. The pressure reducing element is a downstream part of the intercooler.

In the multistage compression system according to the fifth aspect, the oil discharge pipe is connected to a portion of the intercooler. An oil discharge amount is appropriately controlled, and the amount of oil in the low-stage compressor can be controlled appropriately.

A multistage compression system according to a sixth aspect is the system according to the first or second aspect, in which the pressure reducing element is a merging part of an intermediate injection passage. The intermediate injection passage injects the intermediate-pressure refrigerant into the refrigerant pipe.

In the multistage compression system according to the sixth aspect, because the oil discharge pipe is connected to a portion of the refrigerant pipes, which is an upstream side of the merging part of the intermediate injection passage, a pressure reduction of the refrigerant pipes is small, the oil discharge amount from the oil discharge pipe is reduced, and the amount of oil in the low-stage compressor is controlled appropriately.

A multistage compression system according to a seventh aspect is the system according to the sixth aspect and further comprises an intercooler. The intercooler is disposed at an upstream side of the merging part of the intermediate injection passage. The intercooler cools the refrigerant discharged by the low-stage compressor before the refrigerant is sucked into the high-stage compressor. The oil discharge pipe is connected to a portion of the refrigerant pipes between the intercooler and the merging part.

In the multistage compression system of the seventh aspect, because the oil discharge pipe is connected between the intercooler and the merging part, the oil discharge amount can be appropriately controlled, and the amount of oil in the low-stage compressor can be appropriately controlled.

A multistage compression system according to an eighth aspect is the system according to any of the first to seventh aspects, in which the refrigerant is a refrigerant mainly including carbon dioxide, and the oil is an oil insoluble with carbon dioxide.

In the multistage compression system according to the eighth aspect, the refrigerant and the oil, which are insoluble with each other, are easily separated vertically in an oil reservoir of the low-stage compressor, and mainly the refrigerant is easily discharged from the oil discharge pipe.

<FIG> shows a refrigerant circuit configuration of a refrigeration apparatus <NUM> according to a first embodiment. The refrigeration apparatus <NUM> according to the present embodiment is an apparatus that performs a two-stage compression refrigeration cycle using carbon dioxide as refrigerant that operates in a supercritical region. The refrigeration apparatus <NUM> according to the present embodiment can be used for an air conditioner for heating and cooling, an air conditioner dedicated for cooling, a water cooler and heater, a refrigerator, a refrigeration storage apparatus, and the like.

The refrigeration apparatus <NUM> according to the present embodiment has a multistage compression system <NUM>, a four-way switching valve <NUM>, a heat source side heat exchanger <NUM>, a bridge circuit <NUM>, expansion mechanisms <NUM> and <NUM>, a use side heat exchanger <NUM>, and an economizer heat exchanger <NUM>.

The multistage compression system <NUM> compresses the refrigerant. Gas refrigerant is introduced into a first accumulator <NUM> at an inlet of a low-stage compressor <NUM> via the four-way switching valve <NUM> and a refrigerant pipe <NUM>. The refrigerant is compressed by the low-stage compressor <NUM> and a high-stage compressor <NUM>, and reaches the four-way switching valve <NUM> via a pipe <NUM>.

The four-way switching valve <NUM> switches directions in which the refrigerant from the multistage compression system <NUM> flows to the heat source side heat exchanger <NUM> or to the use side heat exchanger <NUM>. For example, when the refrigeration apparatus <NUM> is an air conditioner and is performing a cooling operation, the refrigerant flows from the four-way switching valve <NUM> to the heat source side heat exchanger <NUM> (condenser). The refrigerant flowing through the heat source side heat exchanger <NUM> (condenser) reaches a receiver <NUM> via a check valve 3a of the bridge circuit <NUM>, a pipe <NUM>, and a check valve 11e. The liquid refrigerant continues to flow from the receiver <NUM> through the pipe <NUM>, is decompressed by the expansion mechanism <NUM>, and flows to the use side heat exchanger <NUM> (evaporator) via a check valve 3c of the bridge circuit <NUM>. The refrigerant heated by the use side heat exchanger <NUM> (evaporator) passes through the four-way switching valve <NUM>, and is compressed again by the multistage compression system <NUM>. On the other hand, during a heating operation, the refrigerant flows from the four-way switching valve <NUM> to the use side heat exchanger <NUM> (condenser), a check valve 3b of the bridge circuit <NUM>, the pipe <NUM>, the receiver <NUM>, the expansion mechanism <NUM>, a check valve 3d of the bridge circuit <NUM>, the use side heat exchanger <NUM> (evaporator), and the four-way switching valve <NUM> in this order.

The economizer heat exchanger <NUM> is disposed between the receiver <NUM> and the expansion mechanism <NUM> in a middle of the refrigerant pipe <NUM>. At a branch 11a of the pipe <NUM>, a part of the refrigerant branches and is decompressed to an intermediate pressure at the expansion mechanism <NUM>. The intermediate-pressure refrigerant is heated by the high-pressure refrigerant flowing through the pipe <NUM> in the economizer heat exchanger <NUM> and injected into a merging part 15b of an intermediate pressure of the multistage compression system <NUM> via an intermediate injection pipe <NUM>. Further, a gas component of the refrigerant from the receiver <NUM> merges into the intermediate injection pipe <NUM> via the pipe <NUM>.

As shown in <FIG>, the multistage compression system <NUM> according to the present embodiment includes the first accumulator <NUM>, the low-stage compressor <NUM>, an intercooler <NUM>, a second accumulator <NUM>, the high-stage compressor <NUM>, an oil separator <NUM>, an oil cooler <NUM>, and a decompressor 31a.

In the present embodiment, the refrigerant compressed by the low-stage compressor <NUM> is further compressed by the high-stage compressor <NUM>. The compressors <NUM> and <NUM> are provided with the accumulator <NUM> and the accumulator <NUM>, respectively. The accumulators <NUM> and <NUM> play a role of storing the refrigerant before entering the compressor once and preventing the liquid refrigerant from being sucked into the compressor.

Next, a flow of the refrigerant and the oil in the multistage compression system <NUM> according to the present embodiment will be described with reference to <FIG>.

In the present embodiment, the low-pressure gas refrigerant heated by the evaporator (use side heat exchanger <NUM> or heat source side heat exchanger <NUM>) flows to the first accumulator <NUM> via the refrigerant pipe <NUM>. The gas refrigerant of the first accumulator <NUM> flows to the low-stage compressor <NUM> via a suction pipe <NUM>. The refrigerant compressed by the low-stage compressor <NUM> is discharged from a discharge pipe 15a, flows through intermediate pressure refrigerant pipes <NUM> to <NUM>, and reaches the second accumulator <NUM>.

The intercooler <NUM> is disposed between the intermediate pressure refrigerant pipes <NUM> and <NUM>. The intercooler <NUM> is a heat exchanger that cools the intermediate-pressure refrigerant with, for example, outdoor air. The intercooler <NUM> may be disposed adjacent to the heat source side heat exchanger <NUM> and exchange heat with air by a common fan. The intercooler <NUM> enhances efficiency of the refrigeration apparatus <NUM> by cooling the intermediate-pressure refrigerant.

Further, the intermediate-pressure refrigerant is injected into the merging part 15b of the intermediate pressure refrigerant pipe <NUM> from the intermediate injection pipe <NUM>. In the present embodiment, the merging part 15b of the intermediate injection pipe <NUM> with the pipe <NUM> is disposed downstream of the intercooler <NUM>. A temperature of the refrigerant injected by intermediate injection is lower than a temperature of the refrigerant flowing through the pipe <NUM>. Thus, the intermediate injection lowers the temperature of the refrigerant flowing through the pipe <NUM> and improves the efficiency of the refrigeration apparatus <NUM>.

The multistage compression system <NUM> according to the present embodiment further includes an oil discharge pipe <NUM> that discharges excess oil from the low-stage compressor <NUM>. The oil discharge pipe <NUM> connects the low-stage compressor <NUM> and the pipe <NUM> of an intermediate pressure. The oil discharge pipe <NUM> discharges not only the excess oil accumulated in an oil reservoir of the low-stage compressor but also excess refrigerant accumulated in the oil reservoir. A connection part of the oil discharge pipe <NUM> with the intermediate pressure refrigerant pipe <NUM> is a part upstream of the intercooler <NUM>.

The refrigerant sent to the second accumulator <NUM> by the pipe <NUM> is introduced into the high-stage compressor <NUM> from a suction pipe <NUM>. The refrigerant is compressed in the high-stage compressor <NUM> to a high pressure, and is discharged to a discharge pipe <NUM>.

The refrigerant discharged to the discharge pipe <NUM> flows to the oil separator <NUM>. The oil separator <NUM> separates the refrigerant from the oil. The separated oil is returned to the low-stage compressor <NUM> via an oil return pipe <NUM>.

The multistage compression system <NUM> according to the present embodiment further includes an oil discharge pipe <NUM> that discharges excess oil from the high-stage compressor. The oil discharge pipe <NUM> connects the high-stage compressor <NUM> and the discharge pipe <NUM> of the high-stage compressor <NUM>.

The decompressor 31a is disposed in a middle of the oil return pipe <NUM>. The decompressor 31a is for decompressing the high-pressure oil discharged from the oil separator <NUM>. Specifically, for example, a capillary tube is used for the decompressor 31a.

The oil cooler <NUM> is disposed in the middle of the oil return pipe <NUM>. The oil cooler <NUM> is a heat exchanger that cools the oil flowing through the oil return pipe <NUM>, for example, with the outdoor air. The oil cooler <NUM> is for cooling the high-temperature oil discharged from the oil separator <NUM>. The oil cooler <NUM> may be disposed, for example, near the heat source side heat exchanger <NUM> and may exchange heat with air by a common fan.

The oil (refrigerator oil) according to the present embodiment is not limited as long as the oil is refrigerator oil used as CO<NUM> refrigerant, but oil incompatible with the CO<NUM> refrigerant is particularly suitable. Examples of refrigerator oil include polyalkylene glycols (PAG) and polyolester (POE).

The refrigeration apparatus <NUM> according to the present embodiment performs two-stage compression with two compressors. Two or more stages of compression may be performed using three or more compressors. Further, three or more stages of compression may be performed.

In the present embodiment, the oil return pipe <NUM> returns the oil from the oil separator <NUM> to the low-stage compressor <NUM>. The oil return pipe <NUM> may directly return the oil discharged from the high-stage compressor <NUM> to the low-stage compressor <NUM>.

Both the low-stage compressor <NUM> and the high-stage compressor <NUM> according to the present embodiment are two-cylinder and oscillating rotary compressors. The compressors <NUM> and <NUM>, which have almost the same configuration, will be described in detail here using the low-stage compressor <NUM>.

<FIG> is a vertical sectional view of the low-stage compressor <NUM>, and <FIG> are horizontal sectional views taken along lines A-A to C-C in <FIG>, respectively. However, in the B-B sectional view in <FIG>, components of a motor <NUM> are not shown.

The low-stage compressor <NUM> has a container <NUM>, a compression part <NUM>, the motor <NUM>, a crankshaft <NUM>, and a terminal <NUM>.

The container <NUM> has a substantially cylindrical shape with an axis RA of the motor <NUM> as a center axis. The inside of the container is kept airtight, and an intermediate pressure is maintained in the low-stage compressor <NUM> and a high pressure is maintained in the high-stage compressor <NUM> during an operation. A lower part of the inside of the container <NUM> is the oil reservoir (not shown) for storing oil (lubricating oil).

The container <NUM> houses the motor <NUM>, the crankshaft <NUM>, and the compression part <NUM> inside. The terminal <NUM> is located above the container <NUM>. Further, the container <NUM> is connected to suction pipes 14a and 14b and the discharge pipe 15a of the refrigerant, the oil return pipe <NUM>, and the oil discharge pipe <NUM>.

The motor <NUM> is a brushless DC motor. The motor <NUM> generates power to rotate the crankshaft <NUM> around the axis RA. The motor <NUM> is disposed in a space inside the container <NUM>, below an upper space, and above the compression part <NUM>. The motor <NUM> has a stator <NUM> and a rotor <NUM>. The stator <NUM> is fixed to an inner wall of the container <NUM>. The rotor <NUM> rotates by magnetically interacting with the stator <NUM>.

The stator <NUM> has a stator core <NUM> and insulators <NUM>. The stator core <NUM> is made of steel. The insulator <NUM> is made of resin. The insulators <NUM> are disposed above and below the stator core <NUM>, and wires are wound around the insulators <NUM>.

The crankshaft <NUM> transmits power of the motor <NUM> to the compression part <NUM>. The crankshaft <NUM> has a main shaft <NUM>, a first eccentric part 62a, and a second eccentric part 62b.

The main shaft <NUM> is a part concentric with the axis RA. The main shaft <NUM> is fixed to the rotor <NUM>.

The first eccentric part 62a and the second eccentric part 62b are eccentric with respect to the axis RA. A shape of the first eccentric part 62a and a shape of the second eccentric part 62b are symmetrical with respect to the axis RA.

An oil tube <NUM> is provided at a lower end of the crankshaft <NUM>. The oil tube <NUM> pumps oil (lubricating oil) from the oil reservoir. The pumped lubricating oil rises in an oil passage inside the crankshaft <NUM> and is supplied to a sliding part of the compression part <NUM>.

The compression part <NUM> is a two-cylinder compression mechanism. The compression part <NUM> has a first cylinder <NUM>, a first piston <NUM>, a second cylinder <NUM>, a second piston <NUM>, a front head <NUM>, a middle plate <NUM>, a rear head <NUM>, and front mufflers 58a and 58b.

A first compression chamber <NUM> and a second compression chamber <NUM> are formed in the compression part <NUM>. The first and second compression chambers are spaces to which the refrigerant is supplied and compressed.

In the multistage compression system <NUM> according to the first embodiment, the compressors <NUM> and <NUM> are both two-cylinder compressors. Both or one of the compressors may be a one-cylinder compressor.

As shown in <FIG> or <FIG>, the first compression chamber <NUM> is a space surrounded by the first cylinder <NUM>, the first piston <NUM>, the front head <NUM>, and the middle plate <NUM>.

As shown in <FIG>, the first cylinder <NUM> is provided with a suction hole 14e, a discharge concave portion <NUM>, a bush housing hole 57a, and a blade moving hole 57b. The first cylinder <NUM> houses the main shaft <NUM> and the first eccentric part 62a of the crankshaft <NUM> and the first piston <NUM>. The suction hole 14e communicates the first compression chamber <NUM> with the inside of the suction pipe 14a. A pair of bushes 56c is housed in the bush housing hole 57a.

The first piston <NUM> has an annular part 56a and a blade 56b. The first eccentric part 62a of the crankshaft <NUM> is fitted into the annular part 56a. The blade 56b is sandwiched between the pair of bushes 56c. The first piston <NUM> divides the first compression chamber <NUM> into two. One of the divided chambers is a low pressure chamber 71a that communicates with the suction hole 14e. The other divided chamber is a high pressure chamber 71b that communicates with the discharge concave portion <NUM>. In <FIG>, the annular part 56a revolves clockwise, a volume of the high pressure chamber 71b becomes small, and the refrigerant in the high pressure chamber 71b is compressed. When the annular part 56a revolves, a tip of the blade 56b reciprocates between the blade moving hole 57b and the bush housing hole 57a.

As shown in <FIG>, the front head <NUM> is fixed to an inner side of the container <NUM> by an annular member 53a.

The front mufflers 58a and 58b are fixed to the front head <NUM>. The front mufflers reduce noise when the refrigerant is discharged.

The refrigerant compressed in the first compression chamber <NUM> is discharged to a first front muffler space 58e between the front muffler 58a and the front head <NUM> via the discharge concave portion <NUM>. After further moving to a second front muffler space 58f between the two front mufflers 58a and 58b, the refrigerant is blown out to a space below the motor <NUM> from discharge holes 58c and 58d (see <FIG>) provided in the front muffler 58b.

The refrigerant that has been compressed and blown out from the discharge holes 58c and 58d of the front muffler 58a moves to an upper space of the container <NUM> through a gap of the motor <NUM>, is blown out from the discharge pipe 15a, and proceeds to the high-stage compressor <NUM>.

The second compression chamber <NUM> is a space surrounded by the second cylinder <NUM>, the second piston <NUM>, the rear head <NUM>, and the middle plate <NUM>.

The flow of the refrigerant compressed in the second compression chamber <NUM>, which is almost similar to the flow of the refrigerant compressed in the first compression chamber <NUM>, will not be described in detail. However, the refrigerant compressed in the second compression chamber <NUM> is different in that the refrigerant is once sent to a rear muffler space 55a provided in the rear head <NUM>, and then further sent to the front muffler spaces 58e and 58f by the front mufflers 58a and 58b.

In the multistage compression system <NUM> according to the first embodiment, the rotary compression part of the compressor <NUM> has the first piston <NUM> in which the annular part 56a and the blade 56b are integrated. The rotary compression part may have a vane instead of a blade, and the vane and the piston may be separate bodies.

As shown in <FIG>, the oil return pipe <NUM> is connected to the container <NUM> such that an internal flow path communicates with the space above the compression part <NUM> below the motor <NUM>. The oil blown out of the oil return pipe <NUM> into the container <NUM> collides with the insulator <NUM> of the motor <NUM> and then falls on the front muffler 58b and the annular member 53a fixing the front head <NUM>, and further, merges into the oil reservoir at the lower part of the inside of the container <NUM>.

The oil return pipe <NUM> is preferably connected to a space above the second compression chamber <NUM>. If the oil return pipe <NUM> is connected to a space below the second compression chamber <NUM>, there is a high possibility that the connecting portion of the oil return pipe 31might be below an oil level of the oil reservoir, thereby causing foaming which is not preferable.

Further, the oil return pipe <NUM> may be connected to an upper part of the container <NUM>. For example, the oil return pipe <NUM> may be connected to a core cut part of the stator <NUM> of the motor <NUM>. However, the oil return pipe <NUM> is preferably connected to a lower part as close as possible to the oil reservoir, allowing the oil to be supplied to a sliding part (near the compression chambers <NUM> and <NUM>) more quickly.

An inner diameter of the oil return pipe <NUM> is, for example, <NUM> or more and <NUM> or less.

As shown in <FIG>, the oil discharge pipe <NUM> is connected to the container <NUM> such that the internal flow path communicates with the space above the compression part <NUM> below the motor <NUM>.

If the connection position of the oil discharge pipe <NUM> to the container <NUM> is below the compression chamber <NUM>, the oil may be lost excessively from the oil reservoir. If the connection position is above the motor <NUM>, a difference between the oil discharge pipe <NUM> and the discharge pipe 15a will be small, and separately providing the oil discharge pipe <NUM> will be meaningless.

Further, in the present embodiment, as shown in <FIG>, an attachment height position of the oil discharge pipe <NUM> with the container <NUM> is equivalent to an attachment height position of the oil return pipe <NUM> with the container <NUM>. This facilitates adjustment of the oil level of the oil reservoir.

Further, as shown in <FIG>, in a plain view, the attachment position of the oil discharge pipe <NUM> to the container <NUM> is a position opposite to the discharge holes 58c and 58d of the front muffler 58b with respect to the axis RA of the motor <NUM>. Here, the opposite position refers to a range of <NUM>° other than a total of <NUM>°, which is <NUM>° to left and right of the axis RA from the connection position of the oil discharge pipe <NUM>. Here, this means that half or more of an area of the discharge holes 58c and 58d is on the opposite side although a part of the discharge hole 58c is not in the opposite position in <FIG>.

In the present embodiment, the connection position of the oil discharge pipe <NUM> to the container <NUM> is separated from positions of the discharge holes 58c and 58d of the front muffler 58b. This can reduce the refrigerant discharged from the discharge holes 58c and 58d of the front muffler 58b to be discharged from the low-stage compressor <NUM> directly by the oil discharge pipe <NUM>.

An inner diameter of the oil discharge pipe <NUM> is equivalent to the inner diameter of the oil return pipe <NUM>. The oil discharge pipe <NUM> having a smaller inner diameter than the discharge pipe 15a is used. Specifically, the inner diameter of the oil discharge pipe <NUM> is, for example, <NUM> or more and <NUM> or less.

Further, as shown in <FIG>, in a planar positional relationship between the oil discharge pipe <NUM> and the oil return pipe <NUM>, the connection position of the oil discharge pipe <NUM> to the container <NUM> is separated from the connection position of the oil return pipe <NUM> to the container <NUM> by <NUM>° or more in a rotation direction of the motor <NUM> (a direction of an arrow in <FIG>). The connection position is preferably a position separated by <NUM>° or more. In the present embodiment, this angle is represented by θ. Theta is <NUM>° or more. Also, θ is to be <NUM>° or less.

In the present embodiment, the positions of the oil discharge pipe <NUM> and the oil return pipe <NUM> are sufficiently separated, and this reduces the oil introduced into the container <NUM> of the low-stage compressor <NUM> by the oil return pipe <NUM> to be discharged outside the container <NUM> directly by the oil discharge pipe <NUM>, thereby easily equalizing the oil in the low-stage compressor <NUM>.

In the multistage compression system <NUM> according to the first embodiment, the connection position of the oil return pipe <NUM> to the container <NUM> is as high as the connection position of the oil discharge pipe <NUM> to the container <NUM>. The connection position of the oil return pipe <NUM> to the container <NUM> may be higher than the connection position of the oil discharge pipe <NUM> to the container <NUM>.

In the multistage compression system <NUM> according to the present embodiment, the first accumulator <NUM> is disposed upstream of the low-stage compressor <NUM> and the second accumulator <NUM> is disposed upstream of the high-stage compressor <NUM>. The accumulators <NUM> and <NUM> once store the flowing refrigerant, prevent the liquid refrigerant from flowing to the compressor, and prevent liquid compression of the compressor. Configurations of the first accumulator <NUM> and the second accumulator <NUM> are almost the same, and thus the first accumulator <NUM> will be described with reference to <FIG>.

The low-pressure gas refrigerant heated by the evaporator flows through the refrigerant pipe <NUM> via the four-way switching valve <NUM> and is introduced into the accumulator <NUM>. The gas refrigerant is introduced into the first and second compression chambers <NUM> and <NUM> from the suction pipes 14a and 14b of the compressor <NUM>. The liquid refrigerant and the oil accumulate at a lower part inside the accumulator. Small holes 14c and 14d are formed in the suction pipes 14a and 14b at a lower part inside the accumulator. Diameters of the holes 14c and 14d are, for example, from <NUM> to <NUM>. The oil, together with the liquid refrigerant, merges with the gas refrigerant little by little through the holes 14c and 14d and is sent to the compression chamber.

(<NUM>-<NUM>)
The multistage compression system <NUM> according to the present embodiment is a system having the low-stage compressor <NUM>, the high-stage compressor <NUM>, the intermediate pressure refrigerant pipes <NUM> to <NUM> and <NUM>, a pressure reducing element, and the oil discharge pipe <NUM>. The intermediate pressure refrigerant pipes <NUM> to <NUM> and <NUM> introduce the refrigerant compressed and discharged by the low-stage compressor <NUM> into a suction part of the high-stage compressor <NUM>. The pressure reducing element is disposed in a middle of the refrigerant pipes <NUM> to <NUM>. The pressure reducing element reduces a pressure of the refrigerant flowing through the intermediate pressure refrigerant pipes. The oil discharge pipe <NUM> discharges the excess oil or liquid refrigerant from the low-stage compressor <NUM>. The oil discharge pipe <NUM> connects the low-stage compressor <NUM> and the intermediate pressure refrigerant pipe <NUM> upstream of the pressure reducing element.

In the present embodiment, the pressure reducing element is both or either of the intercooler <NUM> and/or the merging part 15b of an intermediate injection passage. The intercooler <NUM> lowers the temperature and pressure of the refrigerant itself. At the merging part 15b of the intermediate injection passage, the refrigerant having a relatively low temperature and low pressure and flowing through the intermediate injection pipe <NUM> merges into the refrigerant flowing through the intermediate pressure refrigerant pipe <NUM>, thereby decreasing the pressure of the refrigerant flowing through the intermediate pressure refrigerant pipe <NUM>.

In the multistage compression system <NUM> according to the present embodiment, the oil discharge pipe <NUM> is connected to a part of the intermediate pressure refrigerant pipe upstream of the pressure reducing element. Comparing the pressure of the refrigerant or oil in the intermediate pressure refrigerant pipe <NUM> and the oil discharge pipe <NUM>, there is a difference that relatively high-pressure refrigerant and oil compressed by the compression part <NUM> are discharged in the oil discharge pipe <NUM>, but the refrigerant discharged from the discharge pipe 15a after being slightly decompressed in the container <NUM> is in the intermediate pressure refrigerant pipe <NUM>. In other words, comparing the pressure of the part of the intermediate pressure refrigerant pipe <NUM> upstream of the pressure reducing element and the pressure of the oil discharge pipe <NUM>, the pressure of the oil discharge pipe <NUM> is slightly higher. Thus, the refrigerant and oil are discharged from the oil discharge pipe <NUM>.

However, a difference between the pressure of the part of the intermediate pressure refrigerant pipe <NUM> upstream of the pressure reducing element and the pressure of the oil discharge pipe <NUM> is small. Thus, the amount of the refrigerant and oil discharged from the oil discharge pipe <NUM> is not to be excessive and is suppressed. In particular, the amount of discharged refrigerant or oil is smaller than when the oil discharge pipe <NUM> is connected to the part of the intermediate pressure refrigerant pipes <NUM> and <NUM> downstream of the pressure reducing element. Thus, by connecting the oil discharge pipe <NUM> to the intermediate pressure refrigerant pipe <NUM> upstream of the pressure reducing element, the amount of oil in the low-stage compressor <NUM> can be appropriately controlled.

Further, when the pressure reducing element is the intercooler <NUM>, by connecting the oil discharge pipe <NUM> to upstream of the intercooler <NUM>, the intercooler <NUM> cools the refrigerant including the oil flowing in from the oil discharge pipe <NUM>. As a result, the temperature of the refrigerant flowing into the high-stage compressor <NUM> is lowered, which has an effect of protecting the high-stage compressor from overheating.

(<NUM>-<NUM>)
In the multistage compression system <NUM> according to the present embodiment, the oil discharge pipe <NUM> is connected to the container <NUM> above the compression chamber <NUM> and below the motor <NUM>. In the present embodiment, the low-stage compressor <NUM> is a two-cylinder compressor, and there are two compression chambers, the first compression chamber <NUM> and the second compression chamber <NUM>. In such a case, the term compression chamber refers to the second compression chamber <NUM>.

In the multistage compression system <NUM> according to the present embodiment, because the oil discharge pipe <NUM> is connected to a position above the compression chamber <NUM> of the container <NUM> and below the motor <NUM>, excess oil of the low-stage compressor <NUM> can be discharged from the low-stage compressor without excess or deficiency. Therefore, the amount of oil in the low-stage compressor can be controlled more quickly.

Further, in the multistage compression system <NUM> according to the present embodiment, as shown in <FIG>, an end of the discharge pipe 15a in the container <NUM> is disposed in a space above the motor <NUM> in the container <NUM>. As described above, the different arrangements of the discharge pipe 15a and the oil discharge pipe <NUM> form an internal pressure difference between the discharge pipe 15a and the oil discharge pipe <NUM>.

(<NUM>-<NUM>)
In the multistage compression system <NUM> according to the present embodiment, the refrigerant is a refrigerant mainly including carbon dioxide, and the oil is oil incompatible with carbon dioxide. Examples of oil incompatible with carbon dioxide are polyalkylene glycols (PAG) and polyolester (POE).

In such a mixed solution of incompatible oil and carbon dioxide refrigerant, when the refrigeration apparatus <NUM> is operated under normal temperature conditions (-<NUM> or higher), the oil is in a lower part and the refrigerant is in an upper part due to a specific gravity.

This makes it easy to collect the liquid refrigerant above in the oil reservoir in the low-stage compressor <NUM> and discharge the excess liquid refrigerant from the oil discharge pipe <NUM>.

(<NUM>-<NUM>)
The multistage compression system <NUM> according to the present embodiment further includes the oil return pipe <NUM>. The oil return pipe <NUM> returns the oil discharged from the high-stage compressor <NUM> to the low-stage compressor <NUM>.

The multistage compression system <NUM> according to the present embodiment has both the oil discharge pipe <NUM> and the oil return pipe <NUM>, and thus the amount of oil in the low-stage compressor <NUM> can be smoothly controlled.

In the multistage compression system <NUM> according to the first embodiment, the oil discharge pipe <NUM> is connected to upstream of the intercooler <NUM> on the intermediate pressure refrigerant pipe <NUM>. In Modification 1A, the oil discharge pipe <NUM> is connected between the intercooler <NUM> and the merging part 15b of the intermediate injection passage on the intermediate pressure refrigerant pipe <NUM>. At the merging part, a pressure difference between the oil discharge pipe <NUM> and the intermediate pressure refrigerant pipe is larger in Modification 1A than in the first embodiment. Therefore, the oil discharge amount is larger in Modification 1A than in the first embodiment. Consequently, the amount of oil in the low-stage compressor is controlled to be smaller in Modification 1A than in the first embodiment. The other configurations and characteristics are similar to those in the first embodiment.

In the multistage compression system <NUM> according to the first embodiment, the oil discharge pipe <NUM> is connected to upstream of the intercooler <NUM> on the intermediate pressure refrigerant pipe <NUM>. In Modification 1B, the oil discharge pipe <NUM> is connected to a middle of the intercooler <NUM>. At a connection part, a pressure difference between the oil discharge pipe <NUM> and the pipe in the middle of the intercooler <NUM> is larger than a pressure difference between the oil discharge pipe <NUM> and the pipe <NUM> upstream of the intercooler <NUM>. Therefore, the oil discharge amount is larger in Modification 1B than in the first embodiment. However, the oil discharge amount is smaller than in Modification 1A. Consequently, the amount of oil in the low-stage compressor is controlled to be smaller in Modification 1B than in the first embodiment. The other configurations and characteristics are similar to those in the first embodiment.

The multistage compression system <NUM> according to the first embodiment includes the intercooler <NUM> upstream of the intermediate pressure refrigerant pipe connected to the discharge pipe 15a of the low-stage compressor <NUM> and the merging part 15b of the intermediate injection passage downstream of the intermediate pressure refrigerant pipe. In the multistage compression system <NUM> of Modification 1C, only the intercooler <NUM> is provided in the intermediate pressure refrigerant pipe, and the merging part 15b of the intermediate injection passage is not provided. Modification 1C does not include the economizer heat exchanger <NUM>. The other configurations are similar to those in the first embodiment. The oil discharge pipe <NUM> is connected to upstream of the intercooler <NUM> on the intermediate pressure refrigerant pipe <NUM> as in the first embodiment.

Further, contrary to Modification 1C, the present disclosure is also effective when the multistage compression system <NUM> only includes the merging part 15b of the intermediate injection passage in the intermediate pressure refrigerant pipe and does not include the intercooler <NUM>.

In the multistage compression system <NUM> according to the first embodiment, the receiver <NUM> and the economizer heat exchanger <NUM> are disposed upstream of the intermediate injection pipe. In the multistage compression system <NUM> of Modification 1D, only the receiver <NUM> is provided upstream of the intermediate injection pipe <NUM>, and the economizer heat exchanger <NUM> is not provided. The other configurations are similar to those in the first embodiment.

The multistage compression system <NUM> of Modification 1D also has similar characteristics (<NUM>-<NUM>) to (<NUM>-<NUM>) to the multistage compression system <NUM> according to the first embodiment.

Further, contrary to Modification 1D, the present disclosure is also effective when the multistage compression system <NUM> only includes the economizer heat exchanger <NUM> upstream of the intermediate injection pipe <NUM> and does not include the receiver <NUM>.

The multistage compression system <NUM> according to the first embodiment includes the intercooler <NUM> upstream of the intermediate pressure refrigerant pipes <NUM> to <NUM> connected to the discharge pipe 15a of the low-stage compressor <NUM> and the merging part 15b of the intermediate injection passage downstream of the intermediate pressure refrigerant pipes <NUM> to <NUM>. As shown in <FIG>, the multistage compression system <NUM> of Modification 1E includes the merging part 15b of the intermediate injection passage upstream of the intermediate pressure refrigerant pipes <NUM> to <NUM> and the intercooler <NUM> downstream of the intermediate pressure refrigerant pipes <NUM> to <NUM>. The oil discharge pipe <NUM> is connected to upstream of the merging part 15b of the intermediate injection passage on the intermediate pressure refrigerant pipe <NUM>. The other configurations are the same as those in the first embodiment.

The multistage compression system <NUM> of Modification 1E also has similar characteristics (<NUM>-<NUM>) to (<NUM>-<NUM>) to the multistage compression system <NUM> according to the first embodiment.

Similarly to Modification 1E, as shown in <FIG>, the multistage compression system <NUM> of Modification 1F includes the merging part 15b of the intermediate injection passage upstream of the intermediate pressure refrigerant pipes <NUM> to <NUM> and the intercooler <NUM> downstream of the intermediate pressure refrigerant pipes <NUM> to <NUM>. In Modification 1E, the oil discharge pipe <NUM> is connected to upstream of the merging part 15b of the intermediate injection passage on the intermediate pressure refrigerant pipe <NUM>. In Modification 1F, the oil discharge pipe <NUM> is connected between the merging part 15b of the intermediate injection passage on the intermediate pressure refrigerant pipe <NUM> and the intercooler <NUM>. Other configurations are the same as those in Modification 1E.

At a connection part, a pressure difference between the oil discharge pipe <NUM> and the intermediate pressure refrigerant pipe <NUM> between the merging part 15b and the intercooler <NUM> is larger than a pressure difference between the oil discharge pipe <NUM> and the pipe <NUM> upstream of the merging part 15b. Therefore, the oil discharge amount is larger in Modification 1F than in Modification 1E. Consequently, the amount of oil in the low-stage compressor is controlled to be smaller in Modification 1F than in Modification 1E.

Similarly to Modification 1E, as shown in <FIG>, the multistage compression system <NUM> of Modification <NUM> includes the merging part 15b of the intermediate injection passage upstream of the intermediate pressure refrigerant pipes <NUM> to <NUM> and the intercooler <NUM> downstream of the intermediate pressure refrigerant pipes <NUM> to <NUM>. In Modification 1E, the oil discharge pipe <NUM> is connected to upstream of the merging part 15b of the intermediate injection passage on the intermediate pressure refrigerant pipe <NUM>. In Modification <NUM>, the oil discharge pipe <NUM> is connected to a middle of a refrigerant flow path of the intercooler <NUM>. Other configurations are the same as those in Modification 1E.

At a connection part, a pressure difference between the oil discharge pipe <NUM> and the middle of the refrigerant flow path of the intercooler <NUM> is larger than a pressure difference between the oil discharge pipe <NUM> and the pipe <NUM> upstream of the merging part 15b. Therefore, the oil discharge amount is larger in Modification <NUM> than in Modification 1E. Consequently, the amount of oil in the low-stage compressor is controlled to be smaller in Modification <NUM> than in Modification 1E.

The foregoing description concerns the embodiments of the present disclosure. It will be understood that numerous modifications and variations may be made without departing from the scope of the present invention which is defined in the appended claims.

Claim 1:
A multistage compression system (<NUM>) using refrigerant and oil, the multistage compression system comprising:
a low-stage compressor (<NUM>), configured to compress the refrigerant;
a high-stage compressor (<NUM>), configured to further compress the refrigerant compressed by the low-stage compressor;
refrigerant pipes (<NUM> to <NUM>, <NUM>) configured to introduce the refrigerant compressed and discharged by the low-stage compressor into a suction part of the high-stage compressor;
a pressure reducing element (<NUM>, 15b) disposed between the refrigerant pipes; and
an oil discharge pipe (<NUM>) connecting the low-stage compressor to a portion of the refrigerant pipes, said portion of the refrigerant pipes (<NUM>) providing an upstream side of the pressure reducing element (<NUM>), so that the oil discharge pipe (<NUM>) discharges oil from the low-stage compressor to said portion of the refrigerant pipes.