fig1 shows a refrigerating system having a set of basic components , which are : a compressor 1 with an economizing inlet 2 , a discharge line 3 , a condenser unit 4 , an evaporator 5 , and a suction line 6 . the condenser unit 4 has two condensation stages , an intermediate liquid outlet 7 associated with a first condensation stage and a liquid outlet 8 associated with a second condensation stage . the refrigerating system has two liquid lines : a main liquid line 9 and an economizing liquid line 10 outgoing from the condensation stage outlets 7 and 8 respectively . a liquid receiver 11 , a high - pressure side 12 a of an economizing heat exchanger 12 , a filter - drier 13 , a solenoid valve 14 , an expansion device 15 are installed on the liquid line 9 . if the expansion device 15 is a thermal expansion valve , then a sensing bulb 16 is installed at outlet from the evaporator 5 and a pressure equalization line is connected to the outlet from the evaporator 5 . if the expansion device 15 has an ability to stop liquid refrigerant in the main liquid line 9 during off - cycle or migration of refrigerant from the liquid line 9 to the suction line 6 is not an issue , then the solenoid valve 14 is not needed . a filter - drier 17 , a solenoid valve 18 , an expansion device 17 , a low - pressure side 12 b of the economizing heat exchanger 12 , and the economizing inlet 2 of the compressor 1 are installed on the economizing liquid line 10 . if the expansion device 19 is a thermal expansion valve , then a sensing bulb 20 is installed on the low - pressure side 12 b at outlet from the economizing heat exchanger 12 and a pressure equalization line is connected to outlet from the economizing heat exchanger 12 on the low - pressure side 12 b . if the expansion device 19 has an ability to stop liquid refrigerant in the liquid line 10 during off - cycle or migration of refrigerant from the liquid line 10 to the economizing inlet 2 is not an issue , then the solenoid valve 18 is not needed . it is important to underline that the refrigerating system with economizing cycle may be split in two major parts : a main refrigerant loop and an economizing refrigerant circuit . the main refrigerant loop includes the compressor 1 , the discharge line 3 , the condenser unit 4 , the intermediate liquid outlet 7 , the liquid line 9 and related components , the evaporator 5 , and the suction line 6 . the economizing refrigerant circuit includes the liquid line 10 outgoing from the liquid outlet 8 and components related to the liquid line 10 . the first condensation stage , is sized to provide liquid mass flow rate after the first condensation stage equal to required mass flow rate through the evaporator 5 . the second condensation stage is sized to provide liquid mass flow rate after the second condensation stage equal to mass flow rate through the economizing inlet 2 of the compressor 1 . the mass flow through the evaporator 5 is a derivative of the evaporator capacity . the mass flow through the economizing inlet 2 balances the economizing heat exchanger 12 to obtain the required sub - cooling of the liquid flow in the high - pressure side 12 a . liquid films in condensers cover up part of the internal surface of the refrigerant channels . also , the other side of liquid films contact vapor being condensed . liquid & amp ; vapor interface has saturated temperature and zero - sub - cooling . liquid & amp ; refrigerant channels interface is colder and has non - zero sub - cooling . the closer to the condenser exit the liquid film is , the larger the liquid fraction is condensed and the higher the sub - cooling degree is . therefore , the first condensation stage does not provide substantial sub - cooling . the second condensation stage may provide substantial sub - cooling . the staged condensation with removal of liquid refrigerant fraction or removal of a portion of this fraction between the stages reduces the amount of liquid refrigerant in the condenser unit . having less liquid in the condenser unit , the heat transfer area contacting the vapor being condensed is increased , overall heat transfer ratio is improved , temperature difference driving the condensation process is reduced , and the discharge pressure is reduced . as a result performance characteristics are enhanced if the heat transfer area parity is provided , or the cost is reduced if parity of the performance characteristics is provided . in fig1 the condenser unit has three two - stage condensation condenser coils 21 , 22 , and 23 . however , it is sufficient to have one coil with two condensation stages . such a coil is shown on fig2 . the coil has an inlet header 24 , an outlet header 25 , and a plurality of refrigerant channels 26 extending between the inlet and outlet headers 24 and 25 . the refrigerant channels 26 are sealed within the inlet and outlet headers 24 and 25 . the external surface of the channels is thermally exposed to a cooling fluid . the inlet header 24 has a vapor inlet 27 and an intermediate liquid outlet 28 associated with the main liquid line 9 . the outlet header 25 has a liquid outlet 29 . the inlet header 24 contains a phase separator 30 for splitting said inlet header into an upper chamber 31 and a lower chamber 32 . the upper chamber 31 is associated with the vapor inlet 24 and with the first condensation stage 33 . the lower chamber 32 is associated with the intermediate liquid outlet 28 and with the second condensation stage 34 . as refrigerant fills the upper chamber 32 , it contacts phase separator 30 , which selectively routes liquid - rich phase downwardly into the lower chamber 32 . the vapor - rich phase moves through refrigerant channels associated with the first condensation stage 33 to the outlet header 25 . each condensation stage may be circuited to have a number of passes . the coil in fig2 has one pass in each condensation stage . fig3 presents a coil having two passes 33 a and 33 b in a first condensation stage 33 and one pass in a second condensation stage 34 . an inlet header 24 has a phase separator 30 . the phase separator 30 splits the inlet header 24 into an upper chamber 31 associated with the vapor inlet 27 and a lower chamber 32 associated with an intermediate outlet 28 . an outlet header 25 has a phase separator 35 , which splits the outlet header into an upper chamber 36 and a lower chamber 37 . the upper chamber 36 is associated with the first condensation stage 33 . the lower chamber 37 is associated with the second condensation stage 34 and a liquid outlet 29 . it is possible to have a coil with multiple passes in each condensation stage . for example , fig4 represents two ( i . e . 33 a and 33 b ) passes in a first condensation stage 33 and three passes ( i . e . 34 a , 34 b , and 34 c ) in a second condensation stage 34 . phase separators 30 and 36 in an inlet header 24 and phase separators 35 and 37 in an outlet header 25 are employed . also , a collector 29 a is employed near a liquid outlet 29 . fig5 represents five ( i . e . 33 a , 33 b , 33 c , 33 d , and 33 e ) passes in a first condensation stage 33 and three passes ( i . e . 34 a , 34 b , and 34 c ) in a second condensation stage 34 . phase separators 30 , 36 , 38 , and 40 in an inlet header 24 and phase separators 35 , 37 , 39 , and 41 in an outlet header 25 are employed . also , a collector 29 a is employed near a liquid outlet 29 . in fig4 the intermediate liquid outlet 28 is located in the outlet header 25 and the liquid outlet 29 is located in the inlet header 24 , but in fig5 the intermediate liquid outlet 28 and the liquid outlet 29 are located in the outlet header 25 . also , there are possible constructions when the intermediate liquid outlet 28 is located in the inlet header 24 and the liquid outlet 29 is located in the outlet header 25 and constructions when the intermediate liquid outlet 28 and the liquid outlet 29 are located in the inlet header 24 . usually , the number of passes in the first condensation stage is larger than in the second condensation stage . however , in the current invention , the numbers of passes in each condensation stage and performance characteristics of the compressor 1 depend on each other . in the condenser coils shown in fig2 , fig3 , fig4 , and fig5 the refrigerant channels extending between the inlet header 24 and outlet header 25 , are oriented horizontally and the condensing refrigerant flow is routed from top to bottom . there is an option to use the condenser coils shown in fig2 , fig3 , fig4 , and fig5 in a reverse direction , wherein the vapor inlet is 29 instead of 27 , the vapor outlet is 27 instead of 29 and the intermediate liquid outlet 28 remains the same . in this case the condensing refrigerant flow is routed from bottom to top . configurations as mentioned in u . s . pat . no . 5 , 988 , 267 and in u . s . pat . no . 5 , 762 , 566 , are possible as well . fig6 demonstrates a two - stage condensation coil with condensing refrigerant flow routed from middle to top and from middle to bottom . the coil has an inlet header 24 , an outlet header 25 , and plurality of refrigerant channels 26 extending between the inlet and outlet headers 24 and 25 . the refrigerant channels 26 are sealed within the inlet and outlet headers 24 and 25 . the external surface of the channels is thermally exposed to a cooling fluid . the inlet header 24 has a vapor inlet 27 and an intermediate liquid outlet 28 associated with the main liquid line 9 . the outlet header 25 has a liquid outlet 29 . the inlet header 24 contains baffles 24 a and 24 b to route a pass 33 a into the outlet header 25 . the inlet header 24 has the following chambers : a chamber between the baffles 24 a and 24 b associated with the pass 33 a and the vapor inlet 27 ; a chamber between the baffle 24 a and top portion of the inlet header 24 associated with a pass 33 b ; a chamber between the baffle 24 b and bottom portion of the inlet header 24 associated with a pass 33 c , a pass 34 , and the intermediate liquid outlet 28 . the last two chambers of the inlet header 24 are in direct communication to allow liquid refrigerant to flow downwardly . the outlet header 25 has phase separator 35 and 36 and a baffle 25 a splitting the outlet header 25 into four chambers : a chamber between the phase separators 35 and 36 associated with the pass 33 a ; a chamber between the phase separator 35 and top portion of the outlet header 25 associated with the pass 33 b ; a chamber between the phase separators 36 associated with the pass 33 c ; a chamber between the baffle 25 a and bottom portion of the outlet header 25 associated with a pass 34 and the liquid outlet 29 . a first condensation stage includes the passes 33 a , 33 b , and 33 c . a second condensation stage contains the pass 34 . large chilling systems may have a number of whole condenser coils in a first condensation stage and another number of condenser coils in a second condensation stage . in fig7 the first condensation stage which includes coils 19 and 20 , is connected to a liquid line 9 through an intermediate liquid outlet 8 , and feeds a circuit with an evaporator 5 . the second condensation stage has a coil 21 , is connected to a liquid line 10 through a liquid outlet 7 , and feeds a circuit with an economizer inlet 2 of a compressor 1 . also , there is an option to have a combination of a number of whole coils and another number of staged coils in each condensation stage . in fig8 a first condensation stage includes a coil 19 and a portion 21 a of a coil 21 and is connected to a liquid line 9 through an intermediate liquid outlet 8 . a second condensation stage has a coil 20 and a portion 21 b of a coil 21 and is connected to a liquid line 10 through a liquid outlet 7 . the current invention may employ any other condenser coils and units as long as they have two - condensation stages , a vapor inlet , an intermediate liquid outlet , and a liquid outlet . it was mentioned that the first condensation stage is sized to provide liquid mass flow rate after the first condensation stage equal to the required mass flow rate through the evaporator 5 ; the second condensation stage is sized to provide liquid mass flow rate after the second condensation stage equal to the mass flow rate through the economizing inlet 2 of the compressor 1 . at some operation conditions the sized condensation stages may not provide the targeted equality of mass flow rates . a refrigerating system shown in fig9 has a compensation liquid line 10 a connecting the liquid line 9 outgoing from the intermediate liquid outlet 7 and the liquid line 10 outgoing from the liquid outlet 8 . one end of the compensation liquid line 10 a tees the liquid line 10 between the filter - drier 17 and the solenoid valve 18 . another end of the liquid lines tees the liquid line 9 between the filter - drier 13 and the solenoid valve 14 to avoid any refrigerant from flowing through the filtering and drying process twice . if inequality takes place at some operating conditions , the compensation liquid line 10 a allows redistribution of refrigerant flow between the liquid lines 9 and 10 to satisfy the requirements of the economizing inlet 2 and the requirements of the evaporator 5 . the compensation line 10 a may have a valve 10 b to disable and to enable mass exchange between the liquid lines 9 and 10 . a refrigerating system shown in fig1 employs a two - stage compressor . the two - stage compressor consists of a first compression stage 1 a , a second compression stage 1 b , and an economizer inlet 2 between these compression stages . the mass flow rate pumped by the first compression stage must satisfy the mass flow requirements for a circuit with an evaporator 5 . the mass flow rate pumped by a second compression stage must satisfy the mass flow requirements for a circuit with an economizer inlet 2 and for the circuit with the evaporator 5 . the system may employ a multi - stage compressor , and a number of the compression stages may serve as the first compression stage 1 a and the rest of the stages may serve as the second compression stage 1 b . the refrigerating system may have a liquid - to - suction heat exchanger 42 providing thermal contact between liquid refrigerant stream in a main liquid line 9 outgoing from an intermediate liquid outlet 7 of a first stage of a condenser unit 4 and a suction line 6 as shown on fig1 . the heat exchanger 42 provides additional sub - cooling of liquid refrigerant at the inlet to an expansion device 15 on account of superheating of vapor leaving the evaporator 5 . it is more efficient to use a liquid - to - suction heat exchanger 43 providing thermal contact between the liquid refrigerant in an economizing liquid line 10 outgoing from a liquid outlet 8 of a second stage of a condenser unit 4 and a suction line 6 as shown in fig1 . the heat exchanger 43 provides sub - cooling of liquid refrigerant at the inlet to an expansion device 19 on account of superheating of vapor leaving an evaporator 5 and has more room for the sub - cooling than the liquid - to - suction heat exchanger 42 in fig1 has . also , it is possible to employ both liquid - to - suction heat exchangers 42 and 43 as shown in fig1 . fig1 shows a refrigerating system accommodating a flash tank 44 . the flash tank 44 has two inlets 45 and 46 and two outlets 47 and 48 . an additional expansion device 49 is installed in a main liquid line 9 at the inlet 45 to the flash tank 44 . the additional expansion device 49 , the inlet 45 , the outlet 47 belong to the main liquid line 9 outgoing from an intermediate liquid outlet 7 . the inlet 46 and the outlet 48 belong to an economizing liquid line 10 outgoing from an intermediate liquid outlet 8 . the mass flow rate through the inlet 45 and the outlet 47 feeds a circuit with an evaporator 5 . the mass flow rate through the inlet 46 and the outlet 48 feeds an economizer inlet 2 . the flash tank 44 may have a float 50 , which indicates the level of liquid refrigerant in the flash tank 44 . based on a position of the float 50 a control device 51 reduces an opening of the additional expansion device 49 when the level of liquid refrigerant in the flash tank 44 is high and increases the opening of the additional expansion device 49 when the level of liquid refrigerant in the flash tank is low . refrigerating systems accommodating the flash tank 44 may employ the same options as refrigerating systems accommodating economizing heat exchangers : different two - stage condensation condensers as per fig2 , fig3 , fig4 , fig5 , fig6 , fig7 , and fig8 ; a compensation liquid line as per fig9 ; a multi - stage compressor as per fig1 ; and liquid - to - suction heat exchangers as per fig1 , fig1 , and fig1 . it is important to mention some features related to the use of liquid - to - suction heat exchangers 42 and 43 in refrigerating systems accommodating a flash tank 44 as shown in fig1 . the liquid - to - suction heat exchanger 42 provides thermal contact between liquid refrigerant stream in a main liquid line 9 outgoing from an intermediate liquid outlet 7 of a first condensation stage of a condenser unit 4 and a suction line 6 . the liquid - to - suction heat exchanger 42 is installed after a liquid outlet 47 of the flash tank 44 and prior to a filter - drier 17 . the liquid - to - suction heat exchanger 43 provides thermal contact between liquid refrigerant in an economizing liquid line 10 outgoing from an intermediate liquid outlet 8 of a second condensation stage of the condenser unit 4 and the suction line 6 . in accordance with the current invention the refrigerating system accommodating the flash tank 44 may use the liquid - to - suction heat exchanger 42 , the liquid - to - suction heat exchanger 43 , or both . however , it is important to use the liquid - to - suction heat exchangers 42 at least for the following reason . the flash tank 44 provides the same liquid temperature at the liquid outlet 47 as the economizing heat exchanger 12 in fig1 at the outlet from the high - pressure side 12 a ; however , the outgoing liquid does not have any sub - cooling . the absence of sub - cooling creates a potential risk for evaporating refrigerant in the filter - drier 17 and at the inlet to a main expansion valve 15 due to pressure drops in the liquid line outgoing from the liquid outlet 47 . the liquid - to - suction heat exchanger 42 eliminates this risk . while certain preferred embodiments of the present invention have been disclosed in detail , it is to be understood that various modifications in its structure may be adopted without departing from the spirit of the invention or the scope of the following claims .