Abstract:
An air conditioning system includes a first circulation module and a second circulation module. Two circulation modules are joined by a heat exchanger. The first circulation is a modular refrigeration system includes a compressor, expansion device, and heat exchangers. The second circulation module includes a main liquid refrigerant tank, a number of distributed liquid refrigerant tanks, liquid pumps and a plurality of indoor units which includes a heat exchange device and a vapor propelling device. The heat exchange device is connected to the main liquid tank. The vapor propelling device propels the working fluid in a saturated vapor state to the first heat exchanger, thus forming a working fluid loop. It can be switched between the heating and cooling modes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No(s). 098108335 filed in Taiwan, R.O.C. on Mar. 13, 2009, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to an air conditioning system, and more particularly to an air conditioning system capable of adjusting temperature through a secondary circulation. 
     2. Related Art 
     Variable Refrigerant Volume (VRV) also known as Variable refrigerant flow (VRF) consists of a number of air handling units connected to a modular external condensing unit, and allows refrigerant flow to be varied using either an inverter controlled variable speed compressor, or multiple compressors of varying capacity in response to changes in the cooling or heating requirements within the air conditioned space. A sophisticated control system enables switching between the heating and cooling modes. This type of system requires no internal plant room space and offers great flexibility through the many types of air handling units available. Applications vary from office, retail, hotel, luxury apartments, industrial, new and retrofitted buildings. 
     However, as the outdoor unit and the indoor units of the conventional VRV air conditioning system belong to the same circulation, i.e., the outdoor unit and the indoor units are all located in the same refrigerant circulation loop, the conventional VRV air conditioning system has the following problems in operation. 
     Generally, in order to make the compressor operate normally, in the prior art, a lubricant is used to lubricate the compressor. As the lubricant applied to the compressor usually exists in an oil tank of the compressor or in the refrigerant circulation system in a liquid state, and the refrigerant returning to the compressor after passing through the evaporator is usually in a vapor state. When the compressor is in operation, a part of the lubricant is usually discharged from the compressor with the refrigerant. As the outdoor unit and the indoor units of the conventional VRV air conditioning system all belong to the same circulation, the lubricant carried out of the compressor flows in the circulation refrigeration system with the refrigerant. 
     To solve the above problem, in the prior art, a device is added to the refrigeration system for retaining lubricant (for example, a high-efficiency oil separator is mounted at the outlet of the refrigerant on the compressor, so as to intercept the lubricant and prevent the lubricant from being carried out of the compressor with the vaporous refrigerant), or a more complicated control method is employed (for example, the running speed of the compressor is increased under a low load and at a specific time to accelerate the flow of the refrigerant, thus providing adequate vapor velocity to assure oil return), such that the lubricant flows back to the compressor. However, such a design makes the system more complicated. 
     The aforementioned refrigerating circulation is generally used in a large building, and thus the compressor is placed above the evaporator by 30 meters or even higher. When the height difference between the evaporator and the compressor is too big or the compressor is unloaded due to lower cooling demands from the air conditioned object, the vaporous refrigerant may not carry the lubricant any longer due to insufficient velocity of vapor flowing through the suction line, so that the lubricant is accumulated within the refrigerating tubes and the pipes and progressive loss of oil from the compressor. Insufficient oil is left to properly lubricate and cool the compressor, thereby causing the compressor to fail. As a result, a vertical restraint of the piping design in the prior art appears. 
     Similarly, when the horizontal length of the pipe is too long, the vaporous refrigerant may not carry the lubricant any longer due to insufficient velocity of vapor flowing through the suction line, so that the lubricant is easily accumulated within the refrigerating tubes and pipes. As a result, a horizontal restraint of the piping design in the prior art appears. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an air conditioning system capable of adjusting a temperature through a secondary circulation, so as to solve the problem in the prior art that the lubricant is carried out of the compressor and retained in the big refrigeration system and thus cannot return to the compressor. 
     An air conditioning system comprising a first circulation module and a second circulation module is provided. A first working fluid and a second working fluid are respectively circulated in the first circulation module and the second circulation module. The first circulation module comprises a compressor, a first heat exchanger, an expansion device, and a second heat exchanger. The compressor is used for compressing the first working fluid from a low-pressure vapor state to a high-pressure vapor state. The first heat exchanger is connected to the compressor. The expansion device is connected to the first heat exchanger. The second heat exchanger is connected between the expansion device and the compressor. The second circulation module comprises multiple third heat exchangers. Each of the third heat exchangers comprises a heat exchange device and a vapor propelling device. The heat exchange device has a first end and a second end. The vapor propelling device is communicated between the first end and the third heat exchanger, and the second end is connected to the third heat exchanger, to form a working fluid loop between the fourth heat exchanger and the third heat exchangers. The vapor propelling device propels the second working fluid in a saturated vapor state to flow between the heat exchange device and the second heat exchanger, wherein in the second heat exchanger, heat exchange is performed between the second working fluid and the first working fluid. 
     According to a preferred embodiment of the present invention, the air conditioning system further comprises a main liquid storage tank connected between the third heat exchanger and the second end. Preferably, the main liquid storage tank is placed higher than the fourth heat exchangers. 
     According to a preferred embodiment of the present invention, in addition to the main liquid storage tank, the air conditioning system further comprises a pump and a control device. The pump is placed lower than the second end of the heat exchange devices, and is communicated with the main liquid storage tank. The control device is communicated between the main liquid storage tank, the second end, and the pump, and has a first status and a second status. In the first status, the control device guides the second working fluid into the second end; in the second status, the control device guides the second working fluid into the pump. Preferably, the control device is a valve placed at a height between the second end and the pump. 
     According to a preferred embodiment of the present invention, in addition to the pump and the control device, the air conditioning system further comprises a fourth heat exchanger and a control device module. The fourth heat exchanger is located in the main liquid storage tank. The control device module is communicated between the expansion device, the second heat exchanger, and the fourth heat exchanger, and has a first status and a second status. In the first status, the control device module guides the second working fluid into the second heat exchanger; in the second status, the control device module guides the second working fluid into the fourth heat exchanger. Preferably, the control device module comprises a first valve and a second valve. The first valve is located in a first flow path extending from the expansion device to the main liquid storage tank through the second heat exchanger. The second valve is located in another flow path extending from the expansion device to the main liquid storage tank without passing through the second heat exchanger. 
     According to a preferred embodiment of the present invention, in addition to the pump and the control device, the air conditioning system further comprises a liquid-vapor separation tank. An upper side of the liquid-vapor separation tank is communicated with the vapor propelling device and the main liquid storage tank, and a lower side of the liquid-vapor separation tank is communicated with the control device. Preferably, a secondary liquid storage tank is communicated between the valve and the pump. Preferably, the air conditioning system further comprises a liquid level sensor located in the liquid-vapor separation tank, for measuring a liquid level of the second working fluid in the liquid-vapor separation tank. 
     According to a preferred embodiment of the present invention, in addition to the pump and the control device, each of the fourth heat exchangers further comprises a valve communicated between the main liquid storage tank and the heat exchange device. 
     According to a preferred embodiment of the present invention, the vapor propelling device is a fan or a blower. 
     According to a preferred embodiment of the present invention, the fourth heat exchangers are located below the first circulation module. 
     According to a preferred embodiment of the present invention, the air conditioning system further comprises a pump located between the main liquid storage tank and the fourth heat exchangers. 
     According to a preferred embodiment of the present invention, the second circulation module of the air conditioning system does not comprise a compressor. 
     In view of the above, as the present invention adopts a first circulation module and a second circulation module independent from each other, and the second circulation module does not have a compressor for compressing the working fluid from a low-pressure vapor state to a high-pressure vapor state, the problem that the lubricant is retained in the circulation refrigeration system may not occur in the second circulation module. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given herein below for illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic view of an air conditioning system according to an embodiment of the present invention; 
         FIG. 2  is a schematic enlarged view of a fourth heat exchanger in  FIG. 1 ; 
         FIG. 3  is a schematic view of the air conditioning system in  FIG. 1  in a cooling mode; 
         FIG. 4  is a schematic view of the air conditioning system in  FIG. 1  in a heating mode; 
         FIG. 5  is a schematic view of the air conditioning system in  FIG. 1  in a pre-cooling mode; 
         FIG. 6  is a schematic view of the air conditioning system in  FIG. 1  in a part load mode; and 
         FIG. 7  is a schematic view of an air conditioning system according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic view of an air conditioning system according to an embodiment of the present invention.  FIG. 2  is a schematic enlarged view of a third heat exchanger  320  in  FIG. 1 . Referring to  FIGS. 1 and 2  together, the air conditioning system  100  comprises a first circulation module  200  and a second circulation module. The first circulation module  200  comprises a compressor  210 , a first heat exchanger  220 , an expansion device  230 , and a second heat exchanger  240 . The first heat exchanger  220  is connected to the compressor  210 . The expansion device  230  is connected to the first heat exchanger  220 . The second heat exchanger  240  is connected between the expansion device  230  and the compressor  210 . A first working fluid R 1  (not shown) is circulated between the compressor  210 , the first heat exchanger  220 , the expansion device  230 , and the second heat exchanger  240 . The first working fluid R 1  is R-134a, R-12, R-22, or other types of refrigerants. The number of the first circulation module  200  is not limited herein. In this embodiment, as shown in  FIG. 1 , the air conditioning system  100  may comprise multiple first circulation modules  200  connected in parallel to enhance the cooling capability of the air conditioning system  100 . 
     The second circulation module comprises multiple third heat exchangers  320 . Each of the third heat exchangers  320  is located below the first circulation module  200 , and comprises a heat exchange device  322  and a vapor propelling device  324 . The heat exchange device  322  has a first end  322   a  and a second end  322   b . The vapor propelling device  324  located at an outlet of the first end  322   a  is communicated between the first end  322   a  and the second heat exchanger  240 , and the second heat exchanger  240  is connected to the second end  322   b , so as to form a loop of a second working fluid R 2  between each of the third heat exchangers  320  and the second heat exchanger  240 . The second working fluid R 2  is circulated between the second heat exchanger  240 , the heat exchange device  322 , and the vapor propelling device  324  through the loop. The second working fluid R 2  is R-134a, R-12, R-22, or other types of refrigerants. 
     Based on the above structure, the air conditioning system  100  of this embodiment may perform refrigerating air conditioning on a space, so as to reduce the temperature of this space, i.e., the air conditioning system  100  is in a cooling mode.  FIG. 3  is a schematic view of the air conditioning system  100  in a cooling mode. In this embodiment, the adjustment of the temperature of a space inside a building B is taken as an example for the convenience of illustration. In this embodiment, the first circulation module  200  and the second heat exchanger  240  are located on the roof of the building B, and the third heat exchangers  320  is located inside the building B. When the air conditioning system  100  is in the cooling mode, the first heat exchanger  220  functions as a condenser, and the second heat exchanger  240  functions as an evaporator. 
     Based on the above structure of the first circulation module  200 , the first working fluid R 1  in a low-pressure vapor state is compressed by the compressor  210  into a high-pressure vapor state. Then, the first working fluid R 1  in the high-pressure vapor stat enters the first heat exchanger  220  and dissipates heat to the external environment, and is thus changed into a high-pressure liquid state. In this embodiment, the heat generated by the first working fluid R 1  in the high-pressure liquid state is dissipated by a fan  250  to the ambient environment. Afterward, the first working fluid R 1  in the high-pressure liquid state enters the expansion device  230  and is expanded into a saturated low-pressure state. The first working fluid R 1  in the low-pressure liquid state enters the second heat exchanger  240  after flowing through the expansion device  230 , so as to receive the heat of the second working fluid R 2  (which will be described later) to become the first working fluid R 1  in a low-pressure vapor state, which then returns to the compressor  210  to complete a circulation of the first working fluid R 1 . 
     It should be noted that, compared with the prior art, the compressor  210 , the first heat exchanger  220 , the expansion device  230 , and the second heat exchanger  240  in the first circulation module  200  are all substantially located at the same height, such that the compressor  210  can provide sufficient kinetic energy for the first working fluid R 1 , so as to bring the lubricant carried out of the compressor  210  by the first working fluid R 1  back to the compressor  210 . 
     In this embodiment, the second heat exchanger  240  functions as a condenser for the second circulation module, and the third heat exchangers  320  function as an evaporator. Based on such configuration, the second working fluid R 2  in a saturated vapor state performs heat exchange with the first working fluid R 1  in the second heat exchanger  240  so as to transfer heat to the first working fluid R 1 , and is thus changed into a saturated liquid state. Afterward, as the second heat exchanger  240  is placed higher than the third heat exchangers  320 , the second working fluid R 2  in a saturated liquid state enters the heat exchange devices  322  of the third heat exchangers  320  under the effect of gravity, and absorbs the heat in the space of the building B to be changed into the saturated vapor state again. The second working fluid R 2  in the saturated vapor state is then propelled by the vapor propelling device  324  back to the second heat exchanger  240 , thus completing a circulation of the second working fluid R 2 . 
     It should be noted that, in the second circulation module, the vapor propelling device  324  is adapted to propel the second working fluid R 2  in the saturated vapor state from the third heat exchangers  320  to the second heat exchanger  240 , so the vapor propelling device  324  is a fan or a blower. Moreover, not provided with a compressor, the second circulation module does not have the problem in the prior art that the lubricant is carried in the second working fluid R 2 . 
     Next, referring to  FIGS. 1 ,  2 , and  3  together, in order to make the second working fluid R 2  flow more smoothly, in this embodiment, the air conditioning system  100  further comprises a main liquid storage tank  330 . The main liquid storage tank  330  is communicated between the second heat exchanger  240  and the second end  322   b  of each of the heat exchange devices  322 , and an opening  332  of the main liquid storage tank  330  is communicated with the second end  322   b . Therefore, in this embodiment, a part of the second working fluid R 2  is accommodated in the main liquid storage tank  330 . Preferably, the main liquid storage tank  330  is placed higher than the third heat exchangers  320 , so that the second working fluid R 2  in the main liquid storage tank  330  is distributed to each of the third heat exchangers  320  under the effect of gravity. 
     In addition, referring to  FIGS. 1 and 2  again, in this embodiment, the air conditioning system  100  further comprises a pump  340  and a control device  350 . The pump  340  is placed lower than the second end  322   b  of the heat exchange devices  322 , and is communicated with the main liquid storage tank  330  via the opening  334  of the main liquid storage tank  330 . The control device  350  is communicated between the main liquid storage tank  330 , the second end  322   b , and the pump  340 . In this embodiment, the control device  350  is a valve placed at a height between the second end  322   b  and the pump  340 . The control device  350  has a first status (closed status) and a second status (open status). In the first status, the control device  350  guides the second working fluid R 2  into the second end  322   b ; in the second status, the control device  350  guides the second working fluid into the pump  340 . Based on the above design, the air conditioning system  100  may not only operate in a cooling mode, but also in a heating mode to raise the temperature in the building B. 
     Referring to  FIGS. 1 ,  2 , and  4  together,  FIG. 4  is a schematic view of the air conditioning system  100  in a heating mode. When the air conditioning system  100  is in the heating mode, the first heat exchanger  220  functions as an evaporator, and the second heat exchanger  240  functions as a condenser. That is, the first working fluid R 1  in a low-pressure vapor state is compressed by the compressor  210  into a high-pressure vapor state. Then, the first working fluid R 1  in the high-pressure vapor state enters the second heat exchanger  240  and dissipates heat to the external environment, and is thus changed into a high-pressure liquid state. Afterward, the first working fluid R 1  in the high-pressure liquid state enters the expansion device  230  and is expanded into a saturated low-pressure state. The first working fluid R 1  in the high-pressure liquid state enters the first heat exchanger  220  after flowing through the expansion device  230 , so as to receive the heat of the second working fluid R 2  (which will be described later) to become the first working fluid R 1  in a low-pressure vapor state, which then returns to the compressor  210  to complete a circulation of the first working fluid R 1 . 
     Moreover, when the air conditioning system  100  is in the heating mode, the control device  350  is in the second status, and the flow path between the opening  332  of the main liquid storage tank  330  and end portions of the third heat exchangers  320  is closed by a valve  326 . The second heat exchanger  240  functions as an evaporator, and the third heat exchangers  320  function as a condenser. In particular, the second working fluid R 2  in a saturated liquid state performs heat exchange with the first working fluid R 1  in the second heat exchanger  240 , and absorbs the heat of the first working fluid R 1  to be changed into a saturated vapor state. Afterward, the second working fluid R 2  in the saturated vapor state is propelled by the vapor propelling device  324  to enter the heat exchange devices  322  of the third heat exchangers  320  via the first end  322   a , and dissipates heat into the space of the building B to be changed into the saturated liquid state again. Finally, the second working fluid R 2  in the saturated liquid state is drawn by the pump  340  back to the second heat exchanger  240 , thus completing a circulation of the second working fluid R 2 . 
     In addition, referring to  FIGS. 1 and 2  again, in an embodiment of the invention, the air conditioning system  100  further comprises a fourth heat exchanger  360  and a control device module  370 . The fourth heat exchanger  360  is located in the main liquid storage tank  330 . The control device module  370  is communicated between the expansion device  230 , the second heat exchanger  240 , and the fourth heat exchanger  360 . Specifically, in this embodiment, the control device module  370  comprises a valve  371  and a valve  372 . The valve  371  is located in a flow path extending from the expansion device  230  to the main liquid storage tank  330  through the second heat exchanger  240 . The valve  372  is located in another flow path extending from the expansion device  230  to the main liquid storage tank  330  without passing through the second heat exchanger  240 . 
     The control device module  370  has a first status and a second status. When the control device module  370  is in the first status, the valve  371  is open and the valve  372  is closed, so the control device module  370  guides the second working fluid R 2  into the second heat exchanger  240 . When the control device module  370  is in the second status, the valve  371  is closed and the valve  372  is open, so the control device module  370  guides the first working fluid R 1  into the fourth heat exchanger  360 . 
     Referring to  FIGS. 1 ,  2 , and  5  together,  FIG. 5  is a schematic view of the air conditioning system  100  in a pre-cooling mode. Based on the above design, the air conditioning system  100  may not only operate in a cooling mode or a heating mode, but also in a pre-cooling mode before cooling. When the air conditioning system  100  operates in the pre-cooling mode, the fourth heat exchanger  360  functions as an evaporator, the first heat exchanger  220  functions as a condenser, the control device module  370  is in the second status, and the control device  350  is also in the second status. Thereby, the first circulation module  200  can reduce the temperature of the second working fluid R 2  in the main liquid storage tank  330  by using the fourth heat exchanger  360 . Further, when the fourth heat exchanger  360  reduces the temperature of the second working fluid R 2  in the main liquid storage tank  330 , as the control device module  350  is in the second status, the second working fluid R 2  flowing from the opening  332  of the main liquid storage tank  330  is directly drawn back to the main liquid storage tank  330  by the pump  340 , so as to complete a circulation of the second working fluid. It should be noted that, in this circulation, the second working fluid R 2  does not flow into the heat exchange devices  322  through the second end  322   b  of the heat exchange devices  322 . In the pre-cooling mode, as the second working fluid R 2  does not enter the heat exchange devices  322 , most of the second working fluid R 2  in the second circulation module is drawn by the pump  340  into the main liquid storage tank  330 , such that the air conditioning system  100  can reduce the temperature of most of the second working fluid R 2  to a preset value within a short period of time. After the second working fluid R 2  reaches the preset temperature, the control device module  370  and the control device  350  are both switched from the second status to the first status, such that the air conditioning system  100  is switched from the pre-cooling mode to the cooling mode to reduce the temperature inside the building B. 
     Further, in order to make the second working fluid R 2  flow more smoothly, in an embodiment of the invention, the air conditioning system  100  further comprise a secondary liquid storage tank  390 , which is communicated between the control device  350  and the pump  340 , and is adapted to store a part of the second working fluid R 2 . 
     Moreover, referring to  FIGS. 1 and 2  again, in the air conditioning system  100 , in an embodiment of the invention, each of the third heat exchangers  320  further comprise a valve  326 , such that the air conditioning system  100  can operate in a part load mode. The valves  326  are located between the main liquid storage tank  330  and the second end  322   b  of the heat exchange devices  322 . Specifically, in the air conditioning system  100 , a main working fluid conduit  380  and multiple secondary working fluid conduit  382   a  are disposed between the main liquid storage tank  330  and the third heat exchangers  320 . One end of the main working fluid conduit  380  is communicated with the main liquid storage tank  330 . One end of each of the secondary working fluid conduit  382  is communicated with the main working fluid conduit  380 , and the other end is respectively communicated with the vapor propelling device  324  of each of the third heat exchangers  320 . The valves  326  are located in the secondary working fluid conduit  382 . When the air conditioning system  100  is in the part load mode, a part of the third heat exchangers  320  are in operation, while the others are shut down. 
     Referring to  FIGS. 1 ,  2 , and  6  together,  FIG. 6  is a schematic view of the air conditioning system  100  in a part load mode. In the part load mode, the air conditioning system  100  operates in a way similar to the cooling mode. However, different from the cooling mode, when the air conditioning system  100  is in the part load mode, the path for communicating the main liquid storage tank  330  with the third heat exchangers  320  in an OFF-status is cut off, i.e., the valves  326  of the third heat exchangers  320  in the OFF-status are closed, such that the second working fluid R 2  is unable to enter the third heat exchangers  320  in the OFF-status (as shown by the two third heat exchangers  320  circled by a dashed line in  FIG. 6 ) from the main liquid storage tank  330 . Moreover, the path for communicating the main liquid storage tank  330  with the third heat exchangers  320  in an ON-status (as shown by the plurality of third heat exchangers  320  not circled by any dashed line in  FIG. 6 ) is open, i.e., the valves  326  of the third heat exchangers  320  in the ON-status are open, such that the second working fluid R 2  can enter the third heat exchangers  320  in the ON-status from the main liquid storage tank  330 . In addition, the second working fluid R 2  remaining in the third heat exchangers  320  in the OFF-status is drawn by the pump  340  into the main liquid storage tank  330 . 
     Based on the above structure, in this embodiment, the third heat exchangers  320  at a specific position are turned on or off, or the number of the third heat exchangers  320  in the ON-status are adjusted according to air conditioning requirements, such that the air conditioning system  100  achieves a high utilization efficiency of the second working fluid R 2  with a small amount of the second working fluid R 2 . 
     Moreover, in an embodiment of the invention, each of the third heat exchangers  320  further comprises a liquid-vapor separation tank  328 . An upper side of the liquid-vapor separation tank  328  is communicated with the vapor propelling device  324  and the main liquid storage tank  330 , and a lower side of the liquid-vapor separation tank  328  is communicated with the control device  350 . Through the design of the liquid-vapor separation tanks  328 , a part of the second working fluid R 2  is accommodated in the liquid-vapor separation tanks  328 , and the air conditioning system  100  detects the liquid level of the second working fluid R 2  in the liquid-vapor separation tanks  328 , for example, through a liquid level sensor (not shown). The air conditioning system  100  in the cooling mode determines the degree of opening the valves  326  by measuring the liquid level of the second working fluid R 2  in the liquid-vapor separation tanks  328  of the third heat exchangers  320 , so as to adjust the amount of the second working fluid R 2  flowing into the third heat exchangers  320 . 
     In the above embodiment, it is not limited that the first circulation module  200  must be placed higher than the fourth heat exchangers.  FIG. 7  is a schematic view of an air conditioning system according to another embodiment of the present invention, in which like reference numerals represent the same members in  FIG. 1 . Referring to  FIG. 7 , the air conditioning system  100 ′ of this embodiment mainly differs from the air conditioning system  100  in  FIG. 1  in that, the first circulation module  200  is not placed higher than the third heat exchangers  320 . In order to make the second working fluid R 2  in the main liquid storage tank  330  uniformly distributed to each of the third heat exchangers  320 , the air conditioning system  100 ′ further comprises a pump  395 . The pump  395  is disposed between the main liquid storage tank  330  and the third heat exchangers  320 , so that when the air conditioning system  100 ′ is in the cooling, pre-cooling, or part load mode, the second working fluid R 2  is uniformly distributed by the pump  395  to each of the third heat exchangers  320 . 
     In view of the above, as the present invention uses the first circulation module and the second circulation module independent from each other, and the second circulation module does not have a compressor for compressing the working fluid from the liquid state into the vapor state, the problem that the lubricant is retained in the circulation conduit may not occur to the second circulation module. Therefore, compared with the prior art, in the present invention, the design of the second circulation conduit is not limited by the vertical height or horizontal length. 
     In addition, as the fourth heat exchangers of the present invention are located below the first circulation module, the liquid second working fluid may be uniformly distributed into each of the fourth heat exchangers under the effect of gravity. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.