Abstract:
The invention relates to refrigerating systems, primarily, to refrigerating systems employing compressors with economizing inlets and multi-pass condensers. In accordance with the invention a refrigerating system with economizing cycle employs a compressor unit with an economizer inlet and a condenser unit having a first condensation stage, a second condensation stage, and means to remove liquid refrigerant portion between the condensation stages. An intermediate liquid outlet from the first condensation stage feeds a circuit with the evaporator and a liquid outlet from the second condensation stage feeds a circuit with the economizer inlet. The invention provides a high efficiency refrigerating system incorporating of advantages of cost-effectiveness provision of liquid sub-cooling or/and liquid temperature inherent for refrigerating systems with economizing cycle and cost-effectiveness advantages of two-stage condensation condensers.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 60/649,423, filed Feb. 2, 2005, and entitled REFRIGERATING SYSTEM WITH ECONOMIZING CYCLE, which application is incorporated herein by this reference. 
     
    
     TECHNICAL FIELD  
       [0002]     The invention relates generally to refrigerating systems and, more particularly, to refrigerating systems employing compressors with economizing inlets and multi-pass condensers.  
       BACKGROUND OF THE INVENTION  
       [0003]     Liquid refrigerant condensed inside refrigerant channels occupies an insignificant part of the entire internal condenser volume, but it sticks to the condenser walls and covers up significant part of its heat transfer area. As a result, vapor refrigerant, which occupies a significant part of the entire internal volume, does not contact the condenser walls and overall heat transfer ratio is substantially reduced.  
         [0004]     A number of patents (U.S. Pat. No. 5,988,267 and U.S. Pat. No. 5,762,566) addressed this by splitting condensers in a number of passes and removing of a condensed portion from refrigerant stream after each pass. This reduces mass flow rate in each following pass, increases the heat transfer area interfacing with the condensing vapor, improves overall heat transfer ratio, reduces temperature difference required for the condenser duty, and reduces discharge pressure. As a result, performance characteristics are enhanced if heat transfer area parity is provided or the cost is reduced if parity of the performance characteristics is provided.  
         [0005]     Such condensers may provide substantial sub-cooling in the last condensation pass only. When liquid refrigerant streams outgoing from all condenser passes are mixed, liquid sub-cooling of the entire refrigerant stream is reduced. If the liquid line is too long and/or pressure drop in the liquid line is substantially high, then at certain operating conditions there is potential risk of evaporation of liquid refrigerant at the expansion device inlet. Evaporation of liquid refrigerant at the expansion device inlet results in unstable operation of the entire refrigerating system and degradation of performance characteristics.  
         [0006]     In U.S. Pat. No. 5,752,566 a condenser has a plurality of headers having baffles and/or phase separators positioned therein. The refrigerant strikes a sidewall of one of the headers and respective phases are separated by gravity. Additionally, phase separators may be used to selectively route the vapor and liquid phases to specific locations in the condenser. This patent implies that refrigerant after the condenser is directed to a liquid refrigerant receiver and then to a sub-cooling section. The sub-cooling section handles the entire refrigerant mass flow rate and carries thermal load associated with the entire refrigerant mass flow rate. Since the temperature difference driving the heat transfer process is significantly lowered, the sub-cooling section may be classified as an inefficient heat exchanging device in comparison with the condensation section.  
         [0007]     U.S. Pat. No. 6,385,981 B1 relates to refrigerating systems accommodating the economizing cycle. The economizing cycle utilizes an economizing heat exchanger providing thermal contact between liquid refrigerant in the liquid line and evaporating refrigerant at a pressure lower than the discharge pressure and higher than the suction pressure. Such a heat exchanger has substantially high overall heat transfer ratio between liquid and evaporating refrigerant streams and, therefore, provides very efficient sub-cooling duty. This significantly reduces initial cost of means providing the adequate sub-cooling. However, refrigerating systems utilizing the economizing cycle require increased refrigerant mass flow rate through the condenser and, therefore, demand higher condenser capacities and sizes. Additionally, they elevate potential risk of evaporation of liquid refrigerant at the expansion device inlet.  
         [0008]     U.S. Pat. No. 5,692,389 relates to refrigerating systems accommodating the economizing cycle with a flash tank. The flash tank has vapor and liquid outlets. The liquid outlet feeds a circuit with an evaporator. The vapor outlet feeds a circuit with the economizer inlet at a pressure lower than the discharge pressure and higher than the suction pressure. The flash tank provides liquid refrigerant at the liquid outlet at the same temperature as an economizing heat exchanger does in the above-mentioned example. The refrigerating systems utilizing an economizing cycle with a flash tank require increased refrigerant mass flow rate through the condenser, demand higher refrigerant mass flow rate through the condenser, higher condenser capacities and sizes than refrigerating systems utilizing an economizing cycle with an economizing heat exchanger.  
       DISCLOSURE OF THE INVENTION  
       [0009]     The purpose of the invention is to incorporate advantages of cost-effectiveness provision of liquid sub-cooling or/and liquid temperature by refrigerating systems with economizing cycle and cost-effectiveness advantages of multi-pass condensers. This allows creating a high efficiency refrigerating system.  
         [0010]     In accordance with the invention a refrigerating system with economizing cycle comprises a main refrigerant loop and an economizing refrigerant circuit. The main refrigerant loop consists of an evaporator, a suction line, a compressor unit with an economizer inlet, a condenser unit, a main liquid line, and the economizing refrigerant circuit. The liquid line includes a liquid receiver, an economizing heat exchanger, and a main expansion device. The economizing refrigerant circuit consists of an economizing expansion device and the economizing heat exchanger. The economizing heat exchanger has a high-pressure side and a low-pressure side. The high-pressure side is associated with the main refrigerant loop and the low-pressure side is associated with the economizing refrigerant circuit. The condenser unit comprises a vapor inlet, an intermediate liquid outlet, and a liquid outlet. A first condensation stage is associated with part of the refrigerant channels and with the intermediate liquid outlet. A second condensation stage is associated with other part of the refrigerant channels and with the liquid outlet. The main liquid line carries liquid refrigerant outgoing from the first condensation stage and feeds a circuit with the evaporator. The economizing liquid line carries liquid refrigerant outgoing from the second condensation stage and feeds a circuit with the economizer inlet. 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. 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 of the compressor.  
         [0011]     Another aspect of the current invention is a refrigerating system with economizing cycle and with a flash tank. The flash tank comprises an inlet and an outlet associated with the main liquid line, and an inlet and an outlet associated with the economizing refrigerant circuit.  
         [0012]     The liquid line comprises an additional expansion device, the flash tank, and a main expansion device. The economizing refrigerant circuit includes an economizing expansion device, and the flash tank.  
         [0013]     The flash tank may have a float indicating level of liquid refrigerant in the flash tank. Based on a position of the float a controller reduces an opening of the additional expansion device when level of liquid refrigerant in the flash tank is high and increases the opening of the additional expansion device when level of liquid refrigerant in the flash tank is low.  
         [0014]     There are different options associated with the above-mentioned major aspects of the inventions.  
         [0015]     In accordance with the invention, both aspects may employ a one-stage compressor or a multi-stage compressor with the economizing inlet.  
         [0016]     One liquid-to-suction heat exchanger provides thermal contact between liquid refrigerant stream in the main liquid line and superheated refrigerant stream leaving the evaporator. Another liquid-to-suction heat exchanger provides thermal contact between liquid refrigerant stream in the economizing refrigerant circuit and superheated refrigerant stream leaving the evaporator. There is an option to have either liquid-to-suction heat exchanger or both of them. If both liquid-to-suction heat exchangers are applied, the second liquid-to-suction heat exchanger provides thermal contact between liquid refrigerant stream in the economizing refrigerant circuit and superheated refrigerant stream leaving the first liquid-to-suction heat exchanger.  
         [0017]     An expansion valve with a sensing bulb located at outlet from the evaporator is used as the main expansion device. An expansion valve with a sensing bulb located at outlet from the low-pressure side of the economizing heat exchanger is used as the economizing expansion device.  
         [0018]     A main solenoid valve is installed on the main liquid line. An economizing solenoid valve is installed on the economizing liquid line. Use of the both solenoid valves is an option as well.  
         [0019]     A main filter-drier is installed on the main liquid line and an economizing filter-drier is installed on the economizing liquid line.  
         [0020]     The condenser unit has a two-stage condensation coil with a vapor inlet, an inlet header, an outlet header, plurality of refrigerant channels extended between the inlet and outlet headers and sealed inside the inlet and outlet headers, an intermediate liquid outlet, a liquid outlet, and means to route refrigerant flow from the vapor inlet to the intermediate liquid and liquid outlets. A first condensation stage associated with one part of the refrigerant channels and with the intermediate liquid outlet. A second condensation stage is associated with other part of the refrigerant channels and with the liquid outlet. Also, the coil has means to remove a condensed liquid portion after the first condensation stage. The means to route refrigerant flow from the vapor inlet to the intermediate liquid and liquid outlets are baffles, phase separators, and a collector inside the inlet and outlet headers. The means to remove condensed liquid portion after the first condensation stage are baffles, phase separators, and a collector inside the inlet and outlet headers.  
         [0021]     When plurality of coils is applied the vapor inlets of each coil are connected to the vapor inlet of the condenser unit, intermediate liquid outlets of each coil are connected to the intermediate liquid outlet of the condenser unit, and liquid outlets of each coil are connected to the liquid outlet of the condenser unit.  
         [0022]     The portion of refrigerant channels related to the first condensation stage and the portion of refrigerant channels related to the second condensation stage are usually oriented horizontally and condensing refrigerant flow is routed from top to bottom, from bottom to top, or a portion of condensing refrigerant flow is routed downwards and another portion is routed upwards.  
         [0023]     The portion of refrigerant channels related to the first condensation stage and the portion of refrigerant channels related to the second condensation stage are oriented vertically. In this case the inlet header is located at the top and the outlet header is located at the bottom or the inlet header is located at the bottom and the outlet header is located at the top.  
         [0024]     In some applications at least one whole coil in the first condensation stage and at least one whole coil in the second condensation stage are applied. Also, it is possible to have in each condensation stage a combination of at least one whole coil and a portion of refrigerant channels associated with at least one two-stage condensation coil. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  is a schematic illustration of a refrigerating system with an economizing cycle utilizing an economizing heat exchanger and two-stage condensation condenser coils in accordance with one aspect of the invention;  
         [0026]      FIG. 2  is a schematic illustration of a two-stage condensation coil with one pass in each condensation stage;  
         [0027]      FIG. 3  is a schematic illustration of a two-stage condensation coil with two passes in the first condensation stage and one pass in the second condensation stage;  
         [0028]      FIG. 4  is a schematic illustration of a two-stage condensation coil with two passes in the first condensation stage and three passes in the second condensation stage;  
         [0029]      FIG. 5  is a schematic illustration of a two-stage condensation coil with five passes in the first condensation stage and four passes in the second;  
         [0030]      FIG. 6  is a schematic illustration of a two-stage condensation coil with condensing refrigerant flow routed from middle to top and from middle to bottom;  
         [0031]      FIG. 7  is a schematic illustration of a refrigerating system with economizing cycle utilizing an economizing heat exchanger and whole condenser coils;  
         [0032]      FIG. 8  is a schematic illustration of a refrigerating system with economizing cycle utilizing an economizing heat exchanger and a combination of whole condenser coils and two-stage condensation condenser coils;  
         [0033]      FIG. 9  is a schematic illustration of a refrigerating system with economizing cycle utilizing an economizing heat exchanger, two-stage condensation condenser coils, and a compensation liquid line;  
         [0034]      FIG. 10  is a schematic illustration of a refrigerating system with economizing cycle utilizing a two-stage compressor, an economizing heat exchanger and two-stage condensation condenser coils;  
         [0035]      FIG. 11  is a schematic illustration of a refrigerating system with economizing cycle utilizing an economizing heat exchanger, two-stage condensation condenser coils and a liquid-to-suction heat exchanger;  
         [0036]      FIG. 12  is a schematic illustration of a refrigerating system with economizing cycle utilizing an economizing heat exchanger, two-stage condensation condenser coils and another liquid-to-suction heat exchanger;  
         [0037]      FIG. 13  is a schematic illustration of a refrigerating system with economizing cycle utilizing an economizing heat exchanger, two-stage condensation condenser coils and two liquid-to-suction heat exchangers;  
         [0038]      FIG. 14  is a schematic illustration of a refrigerating system with economizing cycle utilizing a flash tank and two-stage condensation condenser coils;  
         [0039]      FIG. 15  is a schematic illustration of a refrigerating system with economizing cycle utilizing a flash tank, two-stage condensation condenser coils, and two liquid-to-suction heat exchangers. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0040]      FIG. 1  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 .  
         [0041]     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.  
         [0042]     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.  
         [0043]     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.  
         [0044]     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 .  
         [0045]     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.    
         [0046]     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.  
         [0047]     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.  
         [0048]     In  FIG. 1  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  FIG. 2 . 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 .  
         [0049]     Each condensation stage may be circuited to have a number of passes. The coil in  FIG. 2  has one pass in each condensation stage.  FIG. 3  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 .  
         [0050]     It is possible to have a coil with multiple passes in each condensation stage. For example,  FIG. 4  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 .  
         [0051]      FIG. 5  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 .  
         [0052]     In  FIG. 4  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  FIG. 5  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 .  
         [0053]     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.  
         [0054]     In the condenser coils shown in  FIG. 2 ,  FIG. 3 ,  FIG. 4 , and  FIG. 5  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  FIG. 2 ,  FIG. 3 ,  FIG. 4 , and  FIG. 5  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.  
         [0055]     Configurations as mentioned in U.S. Pat. No. 5,988,267 and in U.S. Pat. No. 5,762,566, are possible as well.  FIG. 6  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 .  
         [0056]     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  FIG. 7  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 .  
         [0057]     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  FIG. 8   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 .  
         [0058]     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.  
         [0059]     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  FIG. 9  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 .  
         [0060]     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 .  
         [0061]     A refrigerating system shown in  FIG. 10  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 .  
         [0062]     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.    
         [0063]     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  FIG. 11 . 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 .  
         [0064]     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  FIG. 12 . 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  FIG. 11  has.  
         [0065]     Also, it is possible to employ both liquid-to-suction heat exchangers  42  and  43  as shown in  FIG. 13 .  
         [0066]      FIG. 14  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 .  
         [0067]     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.  
         [0068]     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  FIG. 2 ,  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6 ,  FIG. 7 , and  FIG. 8 ; a compensation liquid line as per  FIG. 9 ; a multi-stage compressor as per  FIG. 10 ; and liquid-to-suction heat exchangers as per  FIG. 11 ,  FIG. 12 , and  FIG. 13 .  
         [0069]     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  FIG. 14 .  
         [0070]     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 .  
         [0071]     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 .  
         [0072]     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.  
         [0073]     The flash tank  44  provides the same liquid temperature at the liquid outlet  47  as the economizing heat exchanger  12  in  FIG. 1  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.  
         [0074]     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.