Abstract:
Apparatus for compressing gaseous refrigerant for use in a refrigeration circuit of a liquefaction plant comprises a refrigeration circuit ( 1 ) and two compressors ( 10, 20 ) that are functionally connected to the refrigeration circuit. One of the compressors is provided with a double suction configuration, and the outlets and the inlets of the first and second compressors are connected at least partly in a mutually parallel flow configuration such that the refrigerant flow that leaves the refrigeration circuit from a plurality of outlets thereof is distributed between the two compressors before joining at the inlet of the refrigeration circuit.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a refrigeration compression system for use in a refrigeration circuit of a liquefaction plant, and in particular to a refrigeration compression system that can be constructed as a compact unit for a given production rate. 
       BACKGROUND ART 
       [0002]    Natural gas is known to be environmentally favorable as compared with other fossil fuels and nuclear fuels, but is required to be processed in an appropriate manner before it can be delivered to the final users. The cost for such processing is highly important for the natural gas to be competitive with other forms of fuels. Most significantly, natural gas is most often required to be liquefied for the convenience of storage and delivery to the final users, and the cost of liquefaction accounts for a large part of the cost of natural gas. In large liquefaction plants, liquefaction of natural gas is usually accomplished by compressing the natural gas by using a series of compressors which are powered by gas turbine drivers, and removing heat from the compressed natural gas by using heat exchangers. 
         [0003]    In general, the cost of liquefaction can be reduced by using a refrigerant system with a higher refrigeration duty. The C3 MR (propane pre-cooled mixed refrigerant process) and the optimized cascade process are the two processes that are currently most widely used. These processes use C3 refrigerant for the pre-cooling of the process flow, and it is the pre-cooling process that imposes the most critical design issue in increasing the production rate of the system. 
         [0004]    The production rate of the pre-cooling process can be increased by using larger compressors that can accommodate larger inlet volumes, but there is a limit to the size of the compressors for the given driver speed. A simple method to increase the compressor inlet volume is to increase the impeller diameter. However, the required yield strength of the impeller tip would increase so sharply as the impeller diameter is increased that the difficulty and cost of manufacture become unacceptable once a certain limit is reached. Also, as the inlet flow Mach number is increased, the operating range of the compressor is narrowed, and the efficiency of the compressor starts dropping sharply. The problem is particularly acute because the sonic velocity in LP C3 refrigerant is very low, typically approximately 230 m/sec. As a result, the impeller size at the LP stage would be limited below 1,320 mm when the rotor speed is selected at the typically speed of 3,600 rpm. 
         [0005]    Also, the last stage of the compressor must treat a high volume flow particularly when a side-stream compressor machine is used as is often the case in large refrigeration plants. This will lead to a drop in aerodynamic performance because of the need to select a large flow coefficient impeller in order to handle the large flow rate with a limited size of the impeller. 
         [0006]    The flow coefficient is one of the dimensionless quantities used by the manufacturers to show the impeller performance, and is expressed by the formula given below. 
         [0000]        F=Q   0 /{(pi/4) D   2   U 2} 
         [0007]    where F: flow coefficient 
         [0008]    Q: suction volume flow (m 3 /sec) 
         [0009]    D: impeller diameter (m) 
         [0010]    U2: impeller tip speed (m/s) 
         [0011]    The proven range of the flow coefficient is below 0.155. 
         [0012]    A liquefaction plant typically employs a number of identical compressor trains. Therefore, designing the compressor trains in an optimum fashion is highly important in increasing the efficiency of the liquefaction plant. 
         [0013]    The production rate of a C3 can be increased by using two compressors of a limited size in separate casings in a 50%-50% parallel scheme. However, this scheme requires a large amount of piping in an exactly symmetric 3D configuration, and the necessary material and labor cost prevents it from becoming a practical solution. 
         [0014]    It was proposed in U.S. Pat. No. 6,637,238 and U.S. Pat. No. 6,962,060 to use two compressors with the aim of minimizing the mass flow rate at the higher pressure ends. The outlets of the two compressors are commonly connected, and the main and side inlets of the two compressors are connected to various outlets of the refrigeration circuit. However, the previous proposals are not entirely satisfactory in limiting the flow velocity at the lower pressure inlet of the corresponding compressor. 
       SUMMARY OF THE INVENTION 
       [0015]    In view of such problems of the prior art, a primary object of the present invention is to provide apparatus for compressing gaseous refrigerant for use in a refrigeration circuit of a liquefaction plant which is highly compact and can still maximize the product output rate. 
         [0016]    A second object of the present invention is to provide apparatus for compressing gaseous refrigerant which can maximize the product output rate by using readily available, relatively inexpensive compressors. 
         [0017]    A third object of the present invention is to provide apparatus for compressing gaseous refrigerant which can maximize the product output rate without suffering from the problem of reduced efficiency. 
         [0018]    According to the present invention, such objects can be at least partly accomplished by providing apparatus for compressing gaseous refrigerant for use in a refrigeration circuit of a liquefaction plant, the apparatus comprising: a refrigeration circuit including an inlet for refrigerant at a refrigeration pressure, a low pressure outlet for gaseous refrigerant at a low pressure, a high pressure outlet for gaseous refrigerant at a high pressure and at least one intermediate pressure outlet for gaseous refrigerant at an intermediate pressure; a first compressor received in a first casing and provided with a double suction configuration including at least two main inlets and one outlet; a second compressor received in a second casing separate from the first casing and having an at least one inlet and an outlet, the outlet of the second compressor being connected to the inlet of the refrigeration circuit; and a common power source including an output shaft for driving the first and second compressors; wherein the first and second compressors are provided with inlets and outlets that are functionally connected to the inlet and the outlets of the refrigeration circuit, and the outlets and the inlets of the first and second compressors are connected at least partly in a mutually parallel flow configuration. 
         [0019]    The use of a double suction compressor for the first compressor allows the flow rate at the main inlets of the first compressor to be maximized without excessively increasing the flow velocity at the inlets. Typically, the two main inlets of the first compressor are commonly connected in a symmetric configuration. The outlets and the inlets of the first and second compressors being connected at least partly in a mutually parallel flow configuration means that the refrigerant flow that leaves the refrigeration circuit from a plurality of outlets thereof is distributed between the two compressors before joining at the inlet of the refrigeration circuit. 
         [0020]    If the two main inlets of the first compressor are connected to the low pressure outlet of the refrigeration circuit, the flow rate of the low pressure gaseous refrigerant to be maximized without excessively increasing the flow velocity of the low pressure gaseous refrigerant. If desired, the two main inlets of the first compressor may be connected to the intermediate pressure outlet of the refrigeration circuit. 
         [0021]    According to a certain aspect of the present invention, the first compressor is further provided with a pair of side inlets in a symmetric configuration that are connected to the intermediate pressure outlet of the refrigeration circuit. 
         [0022]    The second compressor may be provided with either a straight suction configuration or a double suction configuration. If the second compressor is provided with a straight suction configuration, the outlet of the second compressor may be connected to the inlet of the refrigeration circuit via an economizer so that an economical operation of the refrigeration compression system can be achieved even when the available power of the compressor driver is not sufficient. The two main inlets of the first compressor may also be connected to different outlets of the refrigeration circuit. In such a case, the first compressor having a double suction configuration may be provided with either a symmetric or asymmetric configuration. 
         [0023]    The refrigeration circuit typically comprises two intermediate pressure outlets for gaseous refrigerant at intermediate pressures, but may also be provided with one, three or more intermediate pressure outlets. 
         [0024]    If the outlet of the first compressor as well as the outlet of the second compressor is connected to the inlet of the refrigeration circuit, the mass flow rate of the highest pressure gaseous refrigerant can be divided between the two compressors, and the mass flow rate at the outlet of each compressor can be avoided from become excessive. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]    Now the present invention is described in the following with reference to the appended drawings, in which: 
           [0026]      FIG. 1 a    is a schematic diagram of a first embodiment of the apparatus for compressing gaseous refrigerant according to the present invention; 
           [0027]      FIG. 1 b    is a view similar to  FIG. 1 a    showing a modification of the first embodiment; 
           [0028]      FIG. 2 a    is a view similar to  FIG. 1 a    showing a second embodiment of the present invention; 
           [0029]      FIG. 2 b    is a view similar to  FIG. 2 a    showing a modification of the second embodiment; and 
           [0030]      FIGS. 3 to 20  are views similar to  FIG. 1 a    showing other embodiments of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]      FIG. 1 a    shows a first embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. This embodiment, as well as the other embodiments discussed in the following, is particularly suited for use in the C3MR process. For the details of the C3 MR (propane pre-cooled mixed refrigerant process), reference may be made to U.S. Pat. No. 5,832,745. However, the present invention may also be used in other applications where refrigerant is cooled by using a refrigeration circuit having a plurality of outlets and a compressor system including a plurality of stages corresponding to the outlets of the refrigeration circuit. 
         [0032]    This apparatus comprises a propane (C3) refrigeration circuit  1  which includes an inlet  2  and four outlets  3 ,  4 ,  5  and  6  for refrigerant at different temperatures and pressures. In the illustrated embodiment, the four outlets consist of a low pressure (LP) outlet  3  for refrigerant at −40 deg. C and 112.5 kPa, a medium pressure (MP) outlet  4  for refrigerant at −21 deg. C and 234.5 kPa, a high pressure (HP) outlet  5  for refrigerant at −6.6 deg. C and 384.8 kPa, and a high high pressure (HHP) outlet  6  for refrigerant at 16.3 deg. C and 757.7 kPa. 
         [0033]    This apparatus comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  is received in a single casing receiving two sets of impellers in a symmetric arrangement such that a pair of main inlets  12  and  13  are defined on either axial end of the casing, and an outlet  11  is defined in an axially middle part of the casing. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0034]    The second compressor  20  is also received in a single casing, and includes a plurality of impeller disks arranged in series along the axial length thereof and typically supported by a common shaft. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a first inlet  22  at the other axial end thereof, and three more additional four inlets  23 ,  24  and  25  in axially intermediate positions thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . It is also possible to use other drive sources such as an electric motor or electric motors, instead of the gas turbine driver. 
         [0035]    The LP outlet of the refrigeration circuit  1  is connected to the two main inlets  12  and  13  of the first compressor  10 , and the outlet  11  of the first compressor  10  is connected to the high high high (HHHP) inlet  25  of the second compressor  20 . The MP outlet  5  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 , and the HP outlet  6  and the HHP outlet  7  of the refrigeration circuit  1  are connected to the HP and HHP inlets  23  and  24  of the second compressor  20 , respectively. The outlet  21  of the second compressor  20  is connected to the inlet  2  of the refrigeration circuit  1 . 
         [0036]    Alternatively, the HHHP inlet  25  may be omitted, and the outlet  11  of the first compressor  10  may be directly connected to the inlet  2  of the refrigeration circuit  1  as indicated by the dotted line in  FIG. 1 a   . It should be noted that connecting the outlet of the first compressor  10  to an HHHP inlet of the second compressor  20 , instead of the inlet  2  of the refrigeration circuit  1  is an option also in other embodiments, wherever applicable, which will be described hereinafter. 
         [0037]      FIG. 1 b    shows a modification of the first embodiment which is similar to the first embodiment except for the provision of an economizer circuit in the outlet circuit that can be used for reducing the flow rate of the refrigerant and hence the power consumption of the gas turbine driver  30 . 
         [0038]    The outlet  21  of the second compressor  20  is connected to an inlet of a desuperheater  41 , instead of being connected directly to the inlet  2  of the refrigeration circuit  1 . The outlet of the desuperheater  41  is connected to an economizer  44  via a condenser  42 , an accumulator  43  and an adjustment valve  45 , in that order. The economizer  44  is also connected to a high high high (HHHP) inlet  25  of the second compressor  20  (or the outlet  11  of the first compressor  10 ), and to the inlet  2  of the refrigeration circuit  1 . 
         [0039]    In this embodiment, by suitably adjusting the adjustment valve  45 , the refrigerant flow can be adjusted depending on the demand for the refrigerant. This economizer circuit can also be optionally included in any of the following embodiments which will be described hereinafter, where the second compressor  20  is provided with a straight suction configuration. 
         [0040]      FIG. 2 a    shows a second embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the second embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing, a pair of side inlets  14  and  15  and an outlet  11  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0041]    The second compressor  20  includes a plurality of impeller disks arranged in series along the axial length thereof. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a main inlet  22  at the other axial end thereof and a single side inlet  23  in an axially intermediate position thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0042]    The LP outlet  6  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the two main inlets  12  and  13  of the first compressor  10 . The HP outlet  4  of the refrigeration circuit  1  is connected to the two side inlets  14  and  15  of the first compressor  10 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the side inlet  23  of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0043]      FIG. 2 b    shows a modification of the second embodiment which is similar to the second embodiment except for the provision of an economizer circuit in the outlet circuit that can be used for reducing the flow rate of the refrigerant and hence the power consumption of the gas turbine driver  30 . 
         [0044]    The outlet  21  of the second compressor  20  is connected to an inlet of a desuperheater  41 , instead of being connected directly to the inlet  2  of the refrigeration circuit  1 . The outlet of the desuperheater  41  is connected to an economizer  44  via a condenser  42 , an accumulator  43  and an adjustment valve  45 , in that order. The economizer  44  is also connected to a high high high (HHHP) inlet  25  of the second compressor  20  and to the inlet  2  of the refrigeration circuit  1 . The outlet  11  of the first compressor  10  is connected to the high high high (HHHP) inlet  25  of the second compressor  20 . 
         [0045]    In this embodiment, by suitably adjusting the adjustment valve  45 , the refrigerant flow can be adjusted depending on the demand for the refrigerant. This economizer circuit can also be optionally included in any of the following embodiments which will be described hereinafter, where the second compressor  20  is provided with a straight suction configuration. 
         [0046]      FIG. 3  shows a third embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. In  FIG. 3 , the parts corresponding to those of the previous embodiment without necessarily repeating the description of such parts. 
         [0047]    The apparatus of the third embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing, a pair of side inlets  14  and  15  and an outlet  11  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0048]    The second compressor  20  includes a plurality of impeller disks arranged in series along the axial length thereof and typically supported by a common shaft. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a main inlet  22  at the other axial end thereof, and a single side inlet  23  in an axially intermediate position thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0049]    The LP outlet  6  of the refrigeration circuit  1  is connected to the two main inlets  12  and  13  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the two side inlets  14  and  15  of the first compressor  10 . The HP outlet  4  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the side inlet  23  of the second compressor  20 . The outlet  21  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0050]      FIG. 4  shows a fourth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. In  FIG. 4  and other drawings showing different embodiments of the present invention which are to be described hereinafter, the parts corresponding to those of the preceding embodiments are denoted with like numerals without necessarily repeating the description of such parts. 
         [0051]    The apparatus of the fourth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  also having a double suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing, a pair of side inlets  14  and  15  and an outlet  11  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0052]    The second compressor  20  also includes a pair of main inlets  22  and  23  defined on either axial end of the casing, a pair of side inlets  24  and  25  and an outlet  21  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0053]    The LP outlet  6  of the refrigeration circuit  1  is connected to the two main inlets  12  and  13  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the two side inlets  14  and  15  of the first compressor  10 . The HP outlet  4  of the refrigeration circuit  1  is connected to the main inlets  22  and  23  of the second compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the side inlets  24  and  25  of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0054]      FIG. 5  shows a fifth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the fifth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  also having a double suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing, a pair of side inlets  14  and  15  and an outlet  11  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0055]    The second compressor  20  also includes a pair of main inlets  22  and  23  defined on either axial end of the casing, a pair of side inlets  24  and  25  and an outlet  21  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0056]    The LP outlet  6  of the refrigeration circuit  1  is connected to the two main inlets  12  and  13  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the main inlets  22  and  23  of the second compressor  20 . The HP outlet  4  of the refrigeration circuit  1  is connected to the side inlets  24  and  25  of the second compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the two side inlets  14  and  15  of the first compressor  10 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0057]      FIG. 6  shows a sixth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the sixth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  also having a double suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing, a pair of side inlets  14  and  15  and an outlet  11  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0058]    The second compressor  20  also includes a pair of main inlets  22  and  23  defined on either axial end of the casing, a pair of side inlets  24  and  25  and an outlet  21  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0059]    The LP outlet  6  of the refrigeration circuit  1  is connected to the two main inlets  12  and  13  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the main inlets  22  and  23  of the second compressor  20 . The HP outlet  4  of the refrigeration circuit  1  is connected to the two side inlets  14  and  15  of the first compressor  10 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the side inlets  24  and  25  of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0060]      FIG. 7  shows a seventh embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the seventh embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing, a pair of side inlets  14  and  15 , and an outlet  11  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0061]    The second compressor  20  includes a plurality of impeller disks arranged in series along the axial length thereof. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a main inlet  22  at the other axial end thereof and a single side inlet  23  in an axially intermediate position thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0062]    The LP outlet  6  of the refrigeration circuit  1  is connected to the two main inlets  12  and  13  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 , and the HP outlet  4  of the refrigeration circuit  1  is connected to the two side inlets  14  and  15  of the first compressor  10 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the side inlet  23  of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0063]      FIG. 8  shows an eighth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the eighth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing, a pair of side inlets  14  and  15  and an outlet  11  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0064]    The second compressor  20  includes a plurality of impeller disks arranged in series along the axial length thereof. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a main inlet  22  at the other axial end thereof and a single side inlet  23  in an axially intermediate position thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0065]    The LP outlet  6  of the refrigeration circuit  1  is connected to the two main inlets  12  and  13  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 . The HP outlet  4  of the refrigeration circuit  1  is connected to the side inlet  23  of the second compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the two side inlets  14  and  15  of the first compressor  10 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0066]      FIG. 9  shows a ninth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the ninth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0067]    The second compressor  20  includes a plurality of impeller disks arranged in series along the axial length thereof. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a main inlet  22  at the other axial end thereof, and a pair of side inlets  23  and  24  in axially intermediate positions thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0068]    The LP outlet  6  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the two main inlets  12  and  13  of the first compressor  10 . The HP outlet  4  of the refrigeration circuit  1  is connected to the one of the side inlets  23  (lower pressure side) of the second compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the other side inlet  24  (higher pressure side) of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0069]      FIG. 10  shows a tenth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the tenth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing, preferably all in a symmetric arrangement. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0070]    The second compressor  20  includes a plurality of impeller disks arranged in series along the axial length thereof. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a main inlet  22  at the other axial end thereof and a pair of side inlets  23  and  24  in axially intermediate positions thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0071]    The LP outlet  6  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 , and the MP outlet  5  of the refrigeration circuit  1  is connected to one of the side inlets  23  (lower pressure side) of the second compressor  20 . The HP outlet  4  of the refrigeration circuit  1  is connected to the two main inlets  12  and  13  of the first compressor  10 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the other side inlet  24  (higher pressure side) of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0072]      FIG. 11  shows an eleventh embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the eleventh embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0073]    The second compressor  20  includes a plurality of impeller disks arranged in series along the axial length thereof. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a main inlet  22  at the other axial end thereof and a single side inlet  23  in an axially intermediate position thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0074]    The LP outlet  6  of the refrigeration circuit  1  is connected to one of the main inlets  12  of the first compressor  10 , and the MP outlet  4  of the refrigeration circuit  1  is connected to the other main inlet  13  of the first compressor  10 . The HP outlet  4  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the side inlet  23  of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0075]      FIG. 12  shows a twelfth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the twelfth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  also having a double suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0076]    The second compressor  20  also includes a pair of main inlets  22  and  23  defined on either axial end of the casing, a pair of side inlets  24  and  25  and an outlet  21  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the second compressor  20  are preferably symmetric to each other. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0077]    The LP outlet  6  of the refrigeration circuit  1  is connected to one of the two main inlets  12  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the other main inlet  13  of the first compressor. The HP outlet  4  of the refrigeration circuit  1  is connected to the main inlets  22  and  23  of the second compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the two side inlets  24  and  25  of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0078]      FIG. 13  shows a thirteenth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the thirteenth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  also having a double suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0079]    The second compressor  20  also includes a pair of main inlets  22  and  23  defined on either axial end of the casing and an outlet  21  defined in an axially middle part of the casing. In this case also, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0080]    The LP outlet  6  of the refrigeration circuit  1  is connected to one of the two main inlets  12  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the other main inlet  12  of the first compressor  10 . The HP outlet  4  of the refrigeration circuit  1  is connected to one of the two main inlets  22  of the first compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the other main inlet  23  of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0081]      FIG. 14  shows a fourteenth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the fourteenth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0082]    The second compressor  20  includes a plurality of impeller disks arranged in series along the axial length thereof. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a main inlet  22  at the other axial end thereof and a single side inlet  23  in an axially intermediate position thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0083]    The LP outlet  6  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 , and the MP outlet  4  of the refrigeration circuit  1  is connected to one of the main inlets  12  of the first compressor  10 . The HP outlet  4  of the refrigeration circuit  1  is connected to the other main inlet  13  of the first compressor  10 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the side inlet  23  of the second compressor  20 . The outlet  11  of the first compressor and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0084]      FIG. 15  shows a fifteenth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the fifteenth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  also having a double suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0085]    The second compressor  20  also includes a pair of main inlets  22  and  23  defined on either axial end of the casing, a pair of side inlets  24  and  25 , and an outlet  21  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the second compressor  20  are preferably symmetric to each other. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0086]    The LP outlet  6  of the refrigeration circuit  1  is connected to the two main inlets  22  and  23  of the second compressor  20 , and the MP outlet  5  of the refrigeration circuit  1  is connected to one of the main inlet  12  of the first compressor. The HP outlet  4  of the refrigeration circuit  1  is connected to the other main inlet  13  of the first compressor  10 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the two side inlets  24  and  25  of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0087]      FIG. 16  shows a sixteenth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the sixteenth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0088]    The second compressor  20  includes a plurality of impeller disks arranged in series along the axial length thereof. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a main inlet  22  at the other axial end thereof and a single side inlet  23  in an axially intermediate position thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0089]    The LP outlet  6  of the refrigeration circuit  1  is connected to one of the main inlets  12  of the first compressor  10 , and the MP outlet  4  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 . The HP outlet  4  of the refrigeration circuit  1  is connected to the side inlet  23  of the second compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the other main inlet  13  of the first compressor  10 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0090]      FIG. 17  shows a seventeenth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the seventeenth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  also having a double suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0091]    The second compressor  20  also includes a pair of main inlets  22  and  23  defined on either axial end of the casing, a pair of side inlets  24  and  25  and an outlet  21  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the second compressor  20  are preferably symmetric to each other. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0092]    The LP outlet  6  of the refrigeration circuit  1  is connected to one of the two main inlets  12  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to the main inlets  22  and  23  of the second compressor  20 . The HP outlet  4  of the refrigeration circuit  1  is connected to the two side inlets  24  and  25  of the second compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the other main inlet  13  of the first compressor  10 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0093]      FIG. 18  shows an eighteenth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the eighteenth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  also having a double suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0094]    The second compressor  20  also includes a pair of main inlets  22  and  23  defined on either axial end of the casing and an outlet  21  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0095]    The LP outlet  6  of the refrigeration circuit  1  is connected to one of the two main inlets  12  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to one of the two main inlets  22  of the second compressor  20 . The HP outlet  4  of the refrigeration circuit  1  is connected to the other main inlet  23  of the second compressor  20 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the other main inlet  12  of the first compressor  10 . The outlet  11  of the first compressor  10  is connected to the inlet  2  of the refrigeration circuit  1 . Likewise, the outlet  21  of the second compressor  120  is connected to the inlet  2  of the refrigeration circuit  1 . 
         [0096]      FIG. 19  shows a nineteenth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the nineteenth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  having a straight suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0097]    The second compressor  20  includes a plurality of impeller disks arranged in series along the axial length thereof. The casing of the second compressor  20  defines an outlet  21  at an axial end thereof, a main inlet  22  at the other axial end thereof and a single side inlet  23  in an axially intermediate position thereof. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0098]    The LP outlet  6  of the refrigeration circuit  1  is connected to one of the main inlets  12  of the first compressor  10 , and the MP outlet  4  of the refrigeration circuit  1  is connected to the main inlet  22  of the second compressor  20 . The HP outlet  4  of the refrigeration circuit  1  is connected to the other main inlet  13  of the first compressor  10 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the side inlet  23  of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0099]      FIG. 20  shows a twentieth embodiment of the apparatus for compressing gaseous refrigerant according to the present invention. The apparatus of the twentieth embodiment comprises a first compressor  10  having a double suction configuration and a second compressor  20  also having a double suction configuration. The first compressor  10  includes a pair of main inlets  12  and  13  defined on either axial end of the casing and an outlet  11  defined in an axially middle part of the casing. In this case, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. 
         [0100]    The second compressor  20  also includes a pair of main inlets  22  and  23  defined on either axial end of the casing and an outlet  21  defined in an axially middle part of the casing. In this case also, the two parts of the dual suction configuration of the first compressor  10  may be either symmetric or asymmetric depending on different design considerations. Each set of impellers may include any number of impeller disks which are typically supported by a common shaft. The two compressors  10  and  20  are driven by an output shaft  31  of a common gas turbine driver  30 . 
         [0101]    The LP outlet  6  of the refrigeration circuit  1  is connected to one of the two main inlets  12  of the first compressor  10 , and the MP outlet  5  of the refrigeration circuit  1  is connected to one of the two main inlets  22  of the second compressor  20 . The HP outlet  4  of the refrigeration circuit  1  is connected to the other main inlet  12  of the first compressor  10 , and the HHP outlet  3  of the refrigeration circuit  1  is connected to the other main inlet  23  of the second compressor  20 . The outlet  11  of the first compressor  10  and the outlet  21  of the second compressor  20  are both connected to the inlet  2  of the refrigeration circuit  1 . 
         [0102]    The present invention can be implemented in a number of different ways as discussed above. In the foregoing embodiments, the refrigeration circuit  1  included four outlets LP, MP, HP and HHP which are denoted with numerals  6 ,  5 ,  4 ,  3  and  2 , respectively. Using notations A, D and S to denote “asymmetric double suction configuration”, “symmetric double suction configuration” and “straight suction configuration” in combination with the associated outlets  6 ,  5 ,  4 ,  3  and  2  of the refrigeration circuit  1 , each of the illustrated embodiments can be designated by the following notations. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Embodiment 
                 Drawing 
                 Notation 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 FIGS. 1a and 1b 
                 D6-S543 
               
               
                 2 
                 FIGS. 2a and 2b 
                 D54-S63 
               
               
                 3 
                 FIG. 3 
                 D65-S43 
               
               
                 4 
                 FIG. 4 
                 D65-D43 
               
               
                 5 
                 FIG. 5 
                 D63-D54 
               
               
                 6 
                 FIG. 6 
                 D64-D53 
               
               
                 7 
                 FIG. 7 
                 D64-S53 
               
               
                 8 
                 FIG. 8 
                 D63-S54 
               
               
                 9 
                 FIG. 9 
                 D5-S643 
               
               
                 10 
                 FIG. 10 
                 D4-S653 
               
               
                 11 
                 FIG. 11 
                 A65-S43 
               
               
                 12 
                 FIG. 12 
                 A65-D43 
               
               
                 13 
                 FIG. 13 
                 A65-A43 
               
               
                 14 
                 FIG. 14 
                 A54-S63 
               
               
                 15 
                 FIG. 15 
                 A54-D63 
               
               
                 16 
                 FIG. 16 
                 A63-S54 
               
               
                 17 
                 FIG. 17 
                 A63-D54 
               
               
                 18 
                 FIG. 18 
                 A63-A54 
               
               
                 19 
                 FIG. 19 
                 A64-S53 
               
               
                 20 
                 FIG. 20 
                 A64-A53 
               
               
                   
               
             
          
         
       
     
         [0103]    These notations are helpful in sorting out different combinations of the compressors and the connections between the compressors and the refrigeration circuit. Also, by using these notations, it is also possible to consider different other combinations of the compressors and the connections between the compressors and the refrigeration circuit. Such other combinations which are not covered by the foregoing embodiments are also part of the present invention. 
         [0104]    Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. The contents of the prior art references mentioned in this application are incorporated in this application by reference.