EP2863151B1

EP2863151B1 - Two-stage compression cycle

Info

Publication number: EP2863151B1
Application number: EP14188877.6A
Authority: EP
Inventors: Yoshiaki Miyamoto; Yoshiyuki Kimata; Youhei Hotta; Hajime Sato
Original assignee: Mitsubishi Heavy Industries Thermal Systems Ltd
Current assignee: Mitsubishi Heavy Industries Thermal Systems Ltd
Priority date: 2013-10-18
Filing date: 2014-10-14
Publication date: 2019-10-02
Anticipated expiration: 2034-10-14
Also published as: JP6301101B2; EP2863151A2; EP2863151A3; JP2015078804A

Description

{Technical Field}

The present invention relates to a two-stage compression system in which a lower-stage compressor and a higher-stage compressor are disposed in series.

{Background Art}

In a two-stage compression system in which two compressors are connected in series, and a higher-stage compressor takes in refrigerant gas compressed by a lower-stage compressor, thus performing two-stage compression, in order to prevent oil from accumulating in one of the compressors, oil separators are individually provided on the discharge sides of the respective compressors and oil separated at the oil separators is returned to the respective compressors, or an oil-equalizing pipe is provided between the individual compressors.
On the other hand, Patent Literature 1 discloses a system in which a single oil separator is provided in an discharge pipe of a higher-stage compressor, oil-returning circuits are provided so as to individually connect the oil separator to the lower-stage compressor and the higher-stage compressor, and thus, oil is returned to the individual compressors without being affected by a pressure difference, which eliminates an imbalance in oil levels in the individual compressors, and in which a second oil-returning circuit is also provided between the higher-stage compressor and the lower-stage compressor, thus making the imbalance in oil levels between the lower-stage compressor and the higher-stage compressor smaller.

{Citation List}

{Patent Literature}

{PTL 1} Japanese Unexamined Patent Application, Publication No. Hei 7-260263
US 2012/023978 discloses a refrigerator according to the preamble of claim 1.

{Summary of Invention}

{Technical Problem}

As described above, with a system in which oil separators are individually provided on the discharge sides of the individual compressors or a system in which an oil-equalizing pipe is provided between the individual compressors, a plurality of oil separators are necessary, and the configuration thereof and oil equalizing control become complicated, thus giving rise to problems such as an increase in cost or the like. On the other hand, with the system described in Patent Literature 1, it suffices to install just one oil separator, and the configuration thereof can be simplified; however, subtle adjustment of the return-oil volume in the individual oil-returning circuits is essential in order to eliminate the imbalance in the oil levels in the individual compressors, and there is a problem in that, in order to make the imbalance smaller, it is necessary to provide a second oil-returning circuit between the two compressors, or the like.
The present invention has been conceived in light of the above-described circumstances, and an object thereof is to provide a two-stage compression system that has no need for oil equalization between a lower-stage compressor and a higher-stage compressor, making it possible to simplify the configuration, and that makes it possible to reliably ensure an appropriate oil level in both the lower-stage compressor and the higher-stage compressor.

{Solution to Problem}

In order to solve the above-described problems, a two-stage compression system of the present invention provides the following solutions.
Specifically, a two-stage compression system according to an aspect of the present invention is a two-stage compression system as defined by claim 1, in which a lower-stage compressor and a higher-stage compressor are disposed in series, and an oil separator is provided on an discharge side of the higher-stage compressor, the two-stage compression system including a first oil-returning circuit and a second oil-returning circuit that are independent of each other and that are provided so as to be connected between the oil separator and the lower-stage compressor and the higher-stage compressor, respectively, the relationship V1 > V2 being satisfied, wherein V1 is the volume of oil returned to the lower-stage compressor from the first oil-returning circuit, and V2 is the volume of oil returned to the higher-stage compressor from the second oil-returning circuit is V2.
With the above-described aspect, in the two-stage compression system in which the lower-stage compressor and the higher-stage compressor are disposed in series, the oil separator is provided on the discharge side of the higher-stage compressor; the first oil-returning circuit and the second oil-returning circuit, which are independent of each other, are provided so as to be connected between the oil separator and the lower-stage compressor and the higher-stage compressor; because V1 > V2 is satisfied, assuming that the volume of oil returned to the lower-stage compressor from the first oil-returning circuit is V1, and the volume of oil returned to the higher-stage compressor from the second oil-returning circuit is V2, by returning the oil separated at the oil separator provided on the discharge side of the higher-stage compressor to the lower-stage compressor via the first oil-returning circuit and also to the higher-stage compressor via the second oil-returning circuit, it is possible to ensure a certain oil level in the lower-stage compressor and the higher-stage compressor, respectively; and, additionally, by making the volume V1 of the oil returned to the lower-stage compressor greater than the volume V2 of the oil returned to the higher-stage compressor (i.e., V1 > V2), it is possible to increase the amount of oil that circulates to the oil separator from the lower-stage compressor via the higher-stage compressor, and, by doing so, it is possible to more easily ensure an appropriate oil level in the higher-stage compressor. Among the lower-stage compressor, the higher-stage compressor, and the oil separator, because "pressure difference ΔP1 = HP (high pressure) - LP (low pressure)" is always greater than "pressure difference ΔP2 = HP (high pressure) - MP (medium pressure)", it is possible to more easily adjust the return-oil volumes between the first oil-returning circuit and the second oil-returning circuit, for example, as in a case in which the return-oil volumes can be always maintained so that "V1 > V2" is satisfied, even if the same kind of tubes are used as capillary tubes for adjusting the return-oil volumes or the like. Therefore, no oil equalization is necessary between the lower-stage compressor and the higher-stage compressor, it is possible to simplify the configuration of the oil-returning circuits, and it is also possible to reliably ensure necessary oil levels in both the lower-stage compressor and the higher-stage compressor.
In the above-described two-stage compression system, the first oil-returning circuit is provided with a first capillary tube for adjusting the return-oil volume V1, the second oil-returning circuit is provided with a second capillary tube for adjusting the return-oil volume V2, and V1 > V2 is achieved by setting a flow reduction level of the first capillary tube ≤ a flow reduction level of the second capillary tube.
In the above-described two-stage compression system, a switching means may be provided in an discharge pipe of the lower-stage compressor, and, by using the switching means, it may be allowed to switch between a single-stage compression operation, in which the lower-stage compressor is independently operated, and a two-stage compression operation by guiding refrigerant gas compressed by the lower-stage compressor to the oil separator by bypassing the higher-stage compressor through an discharge bypass circuit.
With such a configuration, because the switching means is provided in the discharge pipe of the lower-stage compressor, and, because, by using this switching means, it is possible to switch between the single-stage compression operation, in which the lower-stage compressor is independently operated, and the two-stage compression operation by guiding the refrigerant gas compressed by the lower-stage compressor to the oil separator by bypassing the higher-stage compressor through the discharge bypass circuit, the operation of the two-stage compression system can be switched to a single-stage compression system, in which the lower-stage compressor is independently operated, by switching the switching means provided in the discharge pipe of the lower-stage compressor, thereby guiding the refrigerant gas compressed by the lower-stage compressor to the oil separator by bypassing the higher-stage compressor through the discharge bypass circuit. Therefore, in the case in which the high-pressure-difference operation, in which the two-stage compression system is used, is not necessary due to the operating conditions, and more efficient operation is possible by using the single-stage compression operation, it is possible to switch the operation to the single-stage compression operation, in which the lower-stage compressor is used, in a simple manner in accordance with the load.
In the above-described two-stage compression system, a switching means may be provided in an discharge pipe of the lower-stage compressor, and, by using the switching means, it may be allowed to switch between a single-stage compression operation, in which the higher-stage compressor is independently operated, and a two-stage compression operation by guiding low-pressure refrigerant gas to the higher-stage compressor from an intake side of the lower-stage compressor by bypassing the lower-stage compressor through an intake bypass circuit.
With the above-described aspect, because the switching means is provided in the discharge pipe of the lower-stage compressor, and, because, by using this switching means, it is possible to switch between the single-stage compression operation, in which the higher-stage compressor is independently operated, and the two-stage compression operation by guiding the low-pressure refrigerant gas to the higher-stage compressor from the intake side of the lower-stage compressor by bypassing the lower-stage compressor through the intake bypass circuit, the operation of the two-stage compression system can be switched to a single-stage compression system, in which the higher-stage compressor is independently operated, by switching the switching means provided in the discharge pipe of the lower-stage compressor, thereby guiding the low-pressure refrigerant gas to the higher-stage compressor from the intake side of the lower-stage compressor by bypassing the lower-stage compressor through the intake bypass circuit, and compressing the low-pressure refrigerant gas. Therefore, in the case in which the high-pressure-difference operation, in which the two-stage compression system is used, is not necessary due to the operating conditions, and more efficient operation is possible by using the single-stage compression operation, it is possible to switch the operation to the single-stage compression operation, in which the higher-stage compressor is used, in a simple manner in accordance with the load.
With any one of the two-stage compression systems described above, a first solenoid valve and a second solenoid valve may be provided in the first oil-returning circuit and the second oil-returning circuit, respectively, and the oil-returning circuit leading to a stopped compressor may be closed during the independent operation of the lower-stage compressor or the higher-stage compressor.
Because the first solenoid valve and the second solenoid valve are provided in the first oil-returning circuit and the second oil-returning circuit, respectively, and because the oil-returning circuit leading to the stopped compressor can be closed during the independent operation of the lower-stage compressor or the higher-stage compressor, by closing the solenoid valve provided in the oil-returning circuit leading to the stopped compressor during the independent operation of the lower-stage compressor or the higher-stage compressor, it is possible to prevent oil accumulation in the stopped compressor. Therefore, it is possible to reliably ensure an appropriate oil level in the operating compressor even when the lower-stage compressor or the higher-stage compressor is independently operated.
With any one of the two-stage compression systems described above, an intercooler may be provided in the discharge pipe of the lower-stage compressor that guides the refrigerant gas compressed by the lower-stage compressor to the higher-stage compressor.
Because the intercooler is provided in the discharge pipe of the lower-stage compressor that guides the refrigerant gas compressed by the lower-stage compressor to the higher-stage compressor, when performing two-stage compression, the refrigerant gas compressed by the lower-stage compressor can be taken into the higher-stage compressor after the refrigerant gas is cooled by the intercooler. Therefore, the compression efficiency at the higher-stage compressor can be increased and the COP (Coefficient Of Performance) of the two-stage compression system can be enhanced.
With any one of the two-stage compression systems described above, the intercooler is provided so as to allow heat exchange with an evaporator.
Because the intercooler is provided so as to allow heat exchange with the evaporator, the refrigerant gas discharged from the lower-stage compressor can be cooled by the intercooler to a necessary and sufficient level by using the heat of evaporation of the evaporator as a heat source. Therefore, the compression efficiency at the higher-stage compressor can be increased by reliably cooling the refrigerant gas discharged from the lower-stage compressor.

{Advantageous Effects of Invention}

With the present invention, by returning oil separated by the oil separator provided on the discharge side of the higher-stage compressor to the lower-stage compressor via the first oil-returning circuit and also to the higher-stage compressor via the second oil-returning circuit, it is possible to ensure a certain oil level in the lower-stage compressor and the higher-stage compressor, respectively; and, additionally, by making the volume V1 of the oil returned to the lower-stage compressor greater than the volume V2 of the oil returned to the higher-stage compressor (i.e., V1 > V2), it is possible to increase the amount of oil that circulates to the oil separator from the lower-stage compressor via the higher-stage compressor, and, by doing so, it is possible to more easily ensure an appropriate oil level in the higher-stage compressor. Among the lower-stage compressor, the higher-stage compressor, and the oil separator, because "pressure difference ΔP1 = HP (high pressure) - LP (low pressure)" is always greater than "pressure difference ΔP2 = HP (high pressure) - MP (medium pressure)", it is possible to more easily adjust the return-oil volumes between the first oil-returning circuit and the second oil-returning circuit, for example, as in a case in which the return-oil volumes can be always maintained so that "V1 > V2" is satisfied, even if the same kind of capillary tubes are used to adjust the return-oil volumes or the like, and therefore, no oil equalization is necessary between the lower-stage compressor and the higher-stage compressor, it is possible to simplify the configuration of the oil-returning circuits, and it is also possible to reliably ensure necessary oil levels in both the lower-stage compressor and the higher-stage compressor.

{Brief Description of Drawings}

{Fig. 1} Fig. 1 is a diagram showing the configuration of a two-stage compression system according to a first embodiment of the present invention.
{Fig. 2} Fig. 2 is a diagram showing the configuration of a two-stage compression system according to a second embodiment of the present invention.
{Fig. 3} Fig. 3 is a diagram showing the configuration of a two-stage compression system according to a third embodiment of the present invention.
{Fig. 4} Fig. 4 is a P-h diagram for the two-stage compression system shown in Fig. 3.

{Description of Embodiments}

Embodiments according to the present invention will be described below with reference to the drawings.

{First Embodiment}

A first embodiment of the present invention will be described below by using Fig. 1.
Fig. 1 is a diagram showing the configuration of portions in the vicinity of compressors of a two-stage compression system according to this embodiment.
A two-stage compression system 1 according to this embodiment is formed of a refrigeration system employing CO2 refrigerant, in which a lower-stage compressor 2 and a higher-stage compressor 3 are connected in series. The lower-stage compressor 2 and the higher-stage compressor 3 may be compressors employing any configuration or system having an oil sump in an airtight housing thereof.
The lower-stage compressor 2 is connected to an intake pipe 4B for taking in low-pressure gas refrigerant via an accumulator 5 provided in an exit refrigerant pipe 4A of an evaporator (not shown). This lower-stage compressor 2 is connected to the higher-stage compressor 3 via an discharge pipe 4C, and the higher-stage compressor 3 takes in the medium-pressure refrigerant gas compressed by the lower-stage compressor 2, thus performing two-stage compression. The high-pressure refrigerant gas compressed by the higher-stage compressor 3 is guided to an oil separator 7 via a higher-stage discharge pipe 4D and a check valve 6, and is guided to a condenser (not shown) via a high-pressure gas pipe 4E after oil in the refrigerant gas is separated.
A switching means 8 formed of a three-way valve or the like is provided in the discharge pipe 4C of the lower-stage compressor 2, and, by switching this switching means 8, it is possible to switch to a high-pressure-difference operation, in which the two-stage compression system is used, by guiding the refrigerant gas compressed by the lower-stage compressor 2 directly to the higher-stage compressor 3, or to single-stage compression, in which only the lower-stage compressor 2 is used, by guiding the refrigerant gas compressed by the lower-stage compressor 2 to the oil separator 7 via an discharge bypass circuit 9 and a check valve 10 by bypassing the higher-stage compressor 3.
The oil separator 7 is provided on the downstream side of a point at which the higher-stage discharge pipe 4D from the higher-stage compressor 3 and the discharge bypass circuit 9 join, and has functions for separating oil contained in the refrigerant gas discharged from the higher-stage compressor 3 or the lower-stage compressor 2 and for returning this oil to the lower-stage compressor 2 and the higher-stage compressor 3. As for this oil separator 7, it is possible to employ a known oil separator 7, such as a centrifugal separation system or the like.
An independent first oil-returning circuit 11 and second oil-returning circuit 12 for retuning oil to the lower-stage compressor 2 and the higher-stage compressor 3 are connected to the bottom portion of the oil separator 7. The first oil-returning circuit 11 and the second oil-returning circuit 12 may be connected to the oil sumps inside the lower-stage compressor 2 and the higher-stage compressor 3, or they may be connected to intake pipes of the lower-stage compressor 2 and the higher-stage compressor 3, that is, the intake pipe 4B, which serves as the intake pipe of the lower-stage compressor 2, and the discharge pipe 4C from the lower-stage compressor 2, which serves as the intake pipe of the higher-stage compressor 3.
The first and second oil-returning circuits 11 and 12 are provided with a first solenoid valve 13 and a second solenoid valve 14 that are opened and closed in accordance with whether or not the lower-stage compressor 2 and the higher-stage compressor 3 are in operation or stopped and a flow-volume adjusting first capillary tube 15 and second capillary tube 16 for adjusting return-oil volumes. The return-oil volumes of the first and second oil-returning circuits 11 and 12 are set so that "V1 > V2" is satisfied, assuming that the volume of oil returned to the lower-stage compressor 2 from the first oil-returning circuit 11 is V1, and the volume of oil returned to the higher-stage compressor 3 from the second oil-returning circuit 12 is V2.
In order to set the return-oil volumes of the first and second oil-returning circuits 11 and 12 so that "V1 > V2" is satisfied, among the lower-stage compressor 2, the higher-stage compressor 3, and the oil separator 7, because "pressure difference ΔP1 = HP (high pressure) - LP (low pressure)" is always greater than "pressure difference ΔP2 = HP (high pressure)- MP (medium pressure)", it is possible to always achieve "V1 > V2" by setting flow reduction levels of the first and second capillary tubes 15 and 16 so that "the flow reduction level of the first capillary tube 15 ≤ the flow reduction level of the second capillary tube 16", as in a case in which return-oil volumes can always be maintained so that "V1 > V2" is satisfied, even if the same kind of tubes are used as the first and second capillary tubes 15 and 16 for adjusting the return-oil volumes.
With the configuration described above, this embodiment affords the following operational advantages.
When performing a two-stage compression operation by using the above-described two-stage compression system 1, it suffices to switch the switching means (three-way valve) 8 in the discharge pipe 4C so as to open toward the higher-stage compressor 3, and, by doing so, the low-pressure gas refrigerant taken into the lower-stage compressor 2 via the accumulator 5 is compressed to medium pressure by the lower-stage compressor 2, is discharged into the discharge pipe 4C, and is guided to the higher-stage compressor 3 via the switching means (three-way valve) 8. This medium-pressure refrigerant gas is subjected to the second compression at the higher-stage compressor 3, is discharged into the higher-stage discharge pipe 4D as high-pressure gas, and is subsequently fed to the oil separator 7 via the check valve 6.
After the oil contained in the gas is centrifugally separated in the oil separator 7, the high-pressure refrigerant gas fed to the oil separator 7 is guided to the condenser (not shown) via the high-pressure gas pipe 4E, thus circulating in the refrigeration system. On the other hand, the oil separated from the refrigerant gas at the oil separator 7 is returned to the lower-stage compressor 2 and the higher-stage compressor 3 via the first oil-returning circuit 11 and the second oil-returning circuit 12 because the first solenoid valve 13 and the second solenoid valve 14 are both open.
The volumes V1 and V2 of the oil that are returned to the lower-stage compressor 2 and the higher-stage compressor 3 via the first oil-returning circuit 11 and the second oil-returning circuit 12 are always "V1 > V2" because flow reduction levels are set so that "the flow reduction level of the first capillary tube 15 ≤ the flow reduction level of the second capillary tube 16" by using the same kind of capillary tubes as the flow-volume adjusting first capillary tube 15 and second capillary tube 16.
Specifically, among the lower-stage compressor 2, the higher-stage compressor 3, and the oil separator 7, because "pressure difference ΔP1 = HP (high pressure) - LP (low pressure)" is always greater than "pressure difference ΔP2 = HP (high pressure) - MP (medium pressure)", by setting the flow reduction levels of the flow-volume adjusting first capillary tube 15 and second capillary tube 16 as described above, the volumes of oil returned from the first oil-returning circuit 11 and the second oil-returning circuit 12 to the lower-stage compressor 2 and the higher-stage compressor 3 can always be maintained so that "V1 > V2" is satisfied.
Therefore, with this embodiment, by returning the oil separated at the oil separator 7 provided on the discharge side of the higher-stage compressor 3 to the lower-stage compressor 2 via the first oil-returning circuit 11 and also to the higher-stage compressor 3 via the second oil-returning circuit 12, it is possible to ensure a certain oil level in the lower-stage compressor 2 and the higher-stage compressor 3, respectively, and, additionally, by making the volume V1 of the oil returned to the lower-stage compressor 2 greater than the volume V2 of the oil returned to the higher-stage compressor 3 (i.e., V1 > V2), it is possible to increase the amount of oil that is discharged toward the higher-stage compressor 3 from the lower-stage compressor 2.
Because of this, while it is possible to more easily ensure an appropriate oil level in the higher-stage compressor 3, it is also possible to more easily adjust the return-oil volumes between the first oil-returning circuit 11 and the second oil-returning circuit 12. Accordingly, no oil equalization is necessary between the lower-stage compressor 2 and the higher-stage compressor 3, it is possible to simplify the configuration of the oil-returning circuits 11 and 12, and it is also possible to reliably ensure necessary oil levels in both the lower-stage compressor 2 and the higher-stage compressor 3.
In this embodiment, because the switching means (three-way valve) 8 is provided in the discharge pipe 4C of the lower-stage compressor 2 of the two-stage compression system 1, and because, by using this switching means 8, it is possible to switch between the single-stage compression operation, in which the lower-stage compressor 2 is independently operated, and the two-stage compression operation by guiding the refrigerant gas compressed by the lower-stage compressor 2 to the oil separator 7 by bypassing the higher-stage compressor 3 through the discharge bypass circuit 9, the operation of the two-stage compression system 1 can be switched to the single-stage compression system, in which the lower-stage compressor 2 is independently operated, by switching the switching means 8 provided in the discharge pipe 4C of the lower-stage compressor 2, thereby guiding the refrigerant gas compressed by the lower-stage compressor 2 to the oil separator 7 by bypassing the higher-stage compressor 3 through the discharge bypass circuit 9.
Therefore, in the case in which the high-pressure-difference operation, in which the two-stage compression system 1 is used, is not necessary due to the operating conditions, and more efficient operation is possible by using the single-stage compression operation, it is possible to switch the operation to the single-stage compression operation, in which the lower-stage compressor 2 is used, in a simple manner in accordance with the load. For example, in the case in which this two-stage compression system 1 is applied to a heat pump for supplying hot water, the operation by using the two-stage compression cycle is performed during a boiling operation, and the operation is subsequently switched to the single-stage compression cycle during temperature-retaining operation after water boiling is completed. Alternatively, in the case in which the two-stage compression system 1 is applied to a heat-pump air conditioner or the like, the operation using the two-stage compression cycle is performed during winter when the high-pressure-difference operation is required due to low outdoor temperatures or the like, and the operation is switched to the single-stage compression cycle during the intermediate seasons (spring and fall).
With this embodiment, because the first solenoid valve 13 and the second solenoid valve 14 are provided in the first oil-returning circuit 11 and the second oil-returning circuit 12, respectively, and because the oil-returning circuit 12 leading to the stopped compressor (higher-stage compressor 3) can be closed during the independent operation of the lower-stage compressor 2, by closing the solenoid valve 14 provided in the oil-returning circuit 12 leading to the higher-stage compressor 3 during the independent operation of the lower-stage compressor 2, it is possible to prevent oil accumulation in the higher-stage compressor 3. Accordingly, it is possible to reliably ensure an appropriate oil level in the operating compressor even when the lower-stage compressor 2 is independently operated.

{Second Embodiment}

Next, a second embodiment of the present invention will be described by using Fig. 2.
This embodiment differs from the first embodiment described above in that it is possible to independently operate the higher-stage compressor 3 during the single-stage compression operation. Because other points are the same as those of the first embodiment, descriptions thereof will be omitted.
With the configuration in this embodiment, in order to switch to the independent operation of the higher-stage compressor 3 when operating the two-stage compression system 1 in the single-stage compression operation, instead of the discharge bypass circuit 9 in the first embodiment, an intake bypass circuit 17 branched off from the intake pipe 4B is connected to a switching means (three-way valve) 8A provided in the discharge pipe 4C of the lower-stage compressor 2, as shown in Fig. 2, and, during the independent operation of the higher-stage compressor 3, the low-pressure refrigerant gas from the intake pipe 4B can directly be taken into the higher-stage compressor 3 by bypassing the lower-stage compressor 2 through the intake bypass circuit 17.
As described above, in the two-stage compression system 1 in which the lower-stage compressor 2 and the higher-stage compressor 3 are connected in series, by employing the configuration in which the switching means (three-way valve) 8A is provided in the discharge pipe 4C of the lower-stage compressor 2, and in which, by using this switching means 8A, the low-pressure refrigerant gas can directly be taken into the higher-stage compressor 3 from the intake pipe 4B of the lower-stage compressor 2 by bypassing the lower-stage compressor 2 through the intake bypass circuit 17, the operation of the two-stage compression system 1 can be switched to a single-stage compression system in which the higher-stage compressor 3 is independently operated.
Therefore, in the case in which the high-pressure-difference operation, in which the two-stage compression system 1 is used, is not necessary due to the operating conditions, and more efficient operation is possible by using the single-stage compression operation, it is possible to switch the operation to the single-stage compression operation, in which the higher-stage compressor 3 is used, in a simple manner in accordance with the load.
Because the first solenoid valve 13 and the second solenoid valve 14 are provided in the first oil-returning circuit 11 and the second oil-returning circuit 12, and because the oil-returning circuit 11 leading to the stopped compressor (lower-stage compressor 2) can be closed during the independent operation of the higher-stage compressor 3, by closing the solenoid valve 13 provided in the oil-returning circuit 11 leading to the lower-stage compressor 2 during the independent operation of the higher-stage compressor 3, it is possible to prevent oil accumulation in the lower-stage compressor 2. Accordingly, it is possible to reliably ensure an appropriate oil level in the operating compressor even when the higher-stage compressor 3 is independently operated.

{Third Embodiment}

Next, a third embodiment of the present invention will be described by using Figs. 3 and 4.
This embodiment differs from the first embodiment described above in that, during the two-stage compression cycle, medium-pressure refrigerant gas compressed by the lower-stage compressor 2 is taken into the higher-stage compressor 3 after the refrigerant gas is cooled by an intercooler 18. Because other points are the same as those of the first embodiment, descriptions thereof will be omitted.
As shown in Fig. 3, with the configuration of this embodiment, the medium-pressure refrigerant gas that is compressed by the lower-stage compressor 2 and is guided to the higher-stage compressor 3 via the switching means (three-way valve) 8 and the discharge pipe 4C is taken into the higher-stage compressor 3 via the intercooler 18 that is provided so as to allow heat exchange with an evaporator 19.
As described above, by employing the configuration in which the intercooler 18 is provided in the discharge pipe 4C of the lower-stage compressor 2, which guides the refrigerant gas compressed by the lower-stage compressor 2 to the higher-stage compressor 3, when subjecting the low-pressure refrigerant gas to two-stage compression, as shown in Fig. 4, the medium-pressure refrigerant gas compressed by the lower-stage compressor 2 can be taken into the higher-stage compressor 3 after the refrigerant gas is cooled by the intercooler 18. Therefore, the compression efficiency at the higher-stage compressor 3 can be increased and the COP (Coefficient Of Performance) of the two-stage compression system 1 can be enhanced.
In particular, because the intercooler 18 is provided so as to allow heat exchange with the evaporator 19, the refrigerant gas discharged from the lower-stage compressor 2 can be cooled by the intercooler 18 to a necessary and sufficient level by using the heat of evaporation of the evaporator 19 as a heat source, and, by doing so, the compression efficiency at the higher-stage compressor 3 can be increased by reliably cooling the refrigerant gas discharged from the lower-stage compressor 2.
The present invention is not limited to the invention according to the above-described embodiments, and appropriate modifications are possible within a range that does not depart from the scope thereof. For example, in the above-described embodiments, it is possible to operate the lower-stage compressor 2 in the single-stage compression operation by means of the system configuration shown in Fig. 1, and it is possible to operate the higher-stage compressor 3 in the single-stage compression operation by means of the system configuration shown in Fig. 2; naturally, however, it is possible to selectively allow one of the lower-stage compressor 2 and the higher-stage compressor 3 to be operated in the single-stage compression operation by using a single system configuration.
Although examples in which a three-way valve is used as the switching means of switching between the two-stage compression cycle and the single-stage compression cycle have been described, the switching means 8 is not limited to a three-way valve, and a switching means that uses two solenoid valves in combination or the like can be employed as an alternative.
The intercooler 18 is not limited to the one that performs heat exchange with the evaporator 19, and the intercooler 18 may be configured so as to perform heat exchange with the low-pressure refrigerant pipe 4A or the like.

{Reference Signs List}

1: two-stage compression system
2: lower-stage compressor
3: higher-stage compressor
4B: intake pipe
4C: discharge pipe
4D: higher-stage discharge pipe
7: oil separator
8, 8A: switching means (three-way valve)
9: discharge bypass circuit
11: first oil-returning circuit
12: second oil-returning circuit
13: first solenoid valve
14: second solenoid valve
15: first capillary tube
16: second capillary tube
17: intake bypass circuit
18: intercooler
19: evaporator

Claims

A two-stage compression system (1) in which a lower-stage compressor (2) and a higher-stage compressor (3) are disposed in series, and an oil separator (7) is provided on an discharge side of the higher-stage compressor (3), the two-stage compression system comprising:
a first oil-returning circuit (11) and a second oil-returning circuit (12) that are independent of each other and that are provided so as to be connected between the oil separator (7) and the lower-stage compressor (2) and the higher-stage compressor (3), respectively, wherein

the first oil-returning circuit (11) is provided with a first capillary tube (15) for adjusting a return-oil volume V1, V1 being the volume of oil returned to the lower-stage compressor (2) from the first oil-returning circuit (11)

the second oil-returning circuit (12) is provided with a second capillary tube (16) for adjusting a return-oil volume V2, V2 being the volume of oil returned to the higher-stage compressor (3) from the second oil-returning circuit (12), characterised in that

the relationship V1 > V2 is satisfied by setting a flow reduction level of the first capillary tube (15) ≤ a flow reduction level of the second capillary tube (16).
A two-stage compression system according to Claim 1, wherein a switching means (8) is provided in a discharge pipe of the lower-stage compressor (2), the switching means (8) is configured to switch between a single-stage compression operation, in which the lower-stage compressor (2) is independently operated, and a two-stage compression operation by guiding refrigerant gas compressed by the lower-stage compressor (2) to the oil separator (7) by bypassing the higher-stage compressor (3) through a discharge bypass circuit (9).
A two-stage compression system according to Claim 1, wherein a switching means (8A) is provided in a discharge pipe of the lower-stage compressor (2), the switching means (8A) is configured to switch between a single-stage compression operation, in which the higher-stage compressor (3) is independently operated, and a two-stage compression operation by guiding low-pressure refrigerant gas to the higher-stage compressor (3) from an intake side of the lower-stage compressor (2) by bypassing the lower-stage compressor (2) through an intake bypass circuit (17).
A two-stage compression system (1) according to Claim 2 or 3, wherein a first solenoid valve (13) and a second solenoid valve (14) are provided in the first oil-returning circuit (11) and the second oil-returning circuit (12), respectively, and the oil-returning circuit (11, 12) leading to a stopped compressor (2, 3) is closed during the independent operation of the lower-stage compressor or the higher-stage compressor.
A two-stage compression system (1) according to any one of Claims 1 to 4, wherein an intercooler (18) is provided in the discharge pipe of the lower-stage compressor (2) that guides the refrigerant gas compressed by the lower-stage compressor (2) to the higher-stage compressor (3).
A two-stage compression system (1) according to Claim 5, wherein the intercooler (18) is provided so as to allow heat exchange with an evaporator (19).