Patent Publication Number: US-2013233003-A1

Title: Reliable cooling system for operation with a two-phase refrigerant

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is entitled to and claims the benefit of European Patent Application No. 12 001 230.7 and U.S. Provisional Application No. 61/602,624, both filed Feb. 24, 2012, the disclosures of which, including the specification, drawings and abstract, are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The invention relates to a cooling system for operation with a two-phase refrigerant which is in particular suitable for use on board an aircraft. Further, the invention relates to a method of operating a cooling system of this kind. 
     BACKGROUND 
     Cooling systems for operation with a two-phase refrigerant are known from DE 10 2006 005 035 B3, WO 2007/088012 A1, DE 10 2009 011 797 A1 and US 2010/0251737 A1 and may be used for example to cool food that is stored on board a passenger aircraft and intended to be supplied to the passengers. Typically, the food provided for supplying to the passengers is kept in mobile transport containers. These transport containers are filled and precooled outside the aircraft and after loading into the aircraft are deposited at appropriate locations in the aircraft passenger cabin, for example in the galleys. In order to guarantee that the food remains fresh up to being issued to the passengers, in the region of the transport container locations cooling stations are provided, which are supplied with cooling energy from a central refrigerating device and release this cooling energy to the transport containers, in which the food is stored. 
     In the cooling systems known from DE 10 2006 005 035 B3, WO 2007/088012 A1, DE 10 2009 011 797 A1 and US 2010/0251737 A1 the phase transitions of the refrigerant flowing through the circuit that occur during operation of the system allow the latent heat consumption that then occurs to be utilized for cooling purposes. The refrigerant mass flow needed to provide a desired cooling capacity is therefore markedly lower than for example in a liquid cooling system, in which a one-phase liquid refrigerant is used. Consequently, the cooling systems described in DE 10 2006 005 035 B3, WO 2007/088012 A1, DE 10 2009 011 797 A1 and US may have lower tubing cross sections than a liquid cooling system with a comparable cooling capacity and hence have the advantages of a lower installation volume and a lower weight. What is more, the reduction of the refrigerant mass flow makes it possible to reduce the conveying capacity needed to convey the refrigerant through the cooling circuit of the cooling system. This leads to an increased efficiency of the system because less energy is needed to operate a corresponding conveying device, such as for example a pump, and moreover less additional heat generated by the conveying device during operation of the conveying device has to be removed from the cooling system. 
     SUMMARY 
     The invention is directed to the object to provide a cooling system for operation with a two-phase refrigerant which has a high operational reliability and hence is in particular suitable for use on board an aircraft. Further, the invention is directed to the object to provide a method of operating a cooling system of this kind. 
     These objects are achieved by a cooling system having features of attached claims and a method of operating a cooling system having features of attached claims. 
     A cooling system, which is in particular suitable for use on board an aircraft for cooling heat generating components or food comprises a first cooling circuit allowing circulation of a two-phase refrigerant therethrough. The two-phase refrigerant circulating in the first cooling circuit is a refrigerant, which upon releasing cooling energy to a cooling energy consumer is converted from the liquid to the gaseous state of aggregation and is then converted back to the liquid state of aggregation. The two-phase refrigerant may for example be CO 2  or R134A (CH 2 F—CF 3 ). Electric or electronic systems, such as avionic systems or fuel cell systems usually have to be cooled at a higher temperature level than food. For cooling these systems, for example Galden® can be used as a two-phase refrigerant. The evaporating temperature of Galden® at a pressure of 1 bar is approximately 60° C. 
     A first evaporator of the cooling system, which forms an interface between the first cooling circuit and a first cooling energy consumer, is disposed in the first cooling circuit. The first evaporator may, for example, comprise a heat exchanger which provides for a thermal coupling of the refrigerant flowing through the first cooling circuit and a fluid to be cooled, such as for example air to be supplied to mobile transport containers for cooling food stored in the mobile transport containers or any heat generating component on board the aircraft. The two-phase refrigerant is supplied to the first evaporator in its liquid state of aggregation. Upon releasing its cooling energy to the first cooling energy consumer, the refrigerant is evaporated and thus exits the first evaporator in its gaseous state of aggregation. 
     The cooling system further comprises a first condenser disposed in the first cooling circuit. The refrigerant which is evaporated in the first evaporator, via a portion of the first cooling circuit downstream of the first evaporator and upstream of the first condenser, is supplied to the first condenser in its gaseous state of aggregation. In the first condenser, the refrigerant is condensed and hence exits the first condenser in its liquid state of aggregation. A first heat sink is adapted to provide cooling energy to the first condenser. The first heat sink may be a chiller or any other suitable heat sink. For example, in a cooling system employing Galden® as the two-phase refrigerant flowing through the first cooling circuit the first condenser may be operated without a chiller. The first heat sink then may, for example, be formed as a fin cooler or outer skin heat exchanger which is cooled by ambient air. 
     The cooling system further comprises a second cooling circuit allowing circulation of a two-phase refrigerant therethrough, a second evaporator disposed in the second cooling circuit, a second condenser disposed in the second cooling circuit and a second heat sink adapted to provide cooling energy to the second condenser. The components of the cooling system which are associated with the second cooling circuit may be designed as described above for the respective components of the cooling system which are associated with the first cooling circuit. 
     A cooling energy transfer arrangement of the cooling system is adapted to transfer cooling energy provided by the first heat sink and/or the first condenser to the second cooling circuit or to transfer cooling energy provided by the second heat sink and/or the second condenser to the first cooling circuit. Preferably, the cooling energy transfer arrangement is adapted to couple the first heat sink and/or the first condenser to the second cooling circuit or to couple the second heat sink and/or the second condenser to the first cooling circuit in dependence on the operating state of the cooling system. In particular, the cooling energy transfer arrangement preferably is adapted to transfer at least a part of the cooling energy provided by the first heat sink and/or the first condenser to the second cooling circuit, if the amount cooling energy provided by the second heat sink and/or the second condenser is not sufficient to ensure proper operation of the second evaporator. By contrast, in operational states of the cooling system wherein the amount cooling energy provided by the first heat sink and/or the first condenser is not sufficient to ensure proper operation of the first evaporator the cooling energy transfer arrangement preferably is adapted to transfer at least a part of the cooling energy provided by the second heat sink and/or the second condenser to the second cooling circuit. The cooling energy transfer arrangement thus allows to at least partially compensate for a capacity overload, malfunctioning or failure of one or more of the cooling energy generating components in one of the two cooling circuits. The cooling system thus is distinguished by a high operational reliability rendering the cooling system suitable for use on board an aircraft. 
     In a preferred embodiment of the cooling system a third condenser is disposed in the first cooling circuit. The third condenser may be arranged in the first cooling circuit in parallel to the first condenser such that the refrigerant flowing through the first cooling circuit may be supplied in parallel to the first and the third condenser. Preferably, the first and the third condenser are controllable and operable is independently. Further, the cooling system may be equipped with a third heat sink which is adapted to provide cooling energy to the third condenser. The redundant design of a first cooling circuit comprising two condensers and two heat sinks ensures that the first evaporator can be supplied with a sufficient amount of cooling energy even in case of malfunctioning or failure of one or more of the cooling energy generating components in the first cooling circuit. Further, in case of failure of the second condenser and/or the second heat sink the second evaporator may be provided with cooling energy by the first heat sink, the first condenser, the third heat sink and/or the third condenser, as required. As a result, the operational reliability of the overall system may be enhanced. 
     The cooling system further may comprise one or more subcooler(s) which serve(s) to subcool refrigerant exiting the condenser(s). Preferably each of the condensers of the cooling system is coupled to a subcooler which provides for an appropriate subcooling of the refrigerant exiting its associated condenser. The condensers and their associated subcoolers may be designed in the form of condenser/subcooler assembly units. Subcooling the refrigerant exiting the condensers ensures that the refrigerant is supplied to a conveying device discharging refrigerant from the condensers in its liquid state of aggregation and sufficiently subcooled such that cavitation in the conveying device due to an unintended evaporation of the refrigerant within the conveying device is prevented. As a result, excess wear of the conveying device due to cavitation can be avoided. 
     Further, the cooling system may comprise a first accumulator which is disposed in the first cooling circuit and with serves to store refrigerant condensed in the first condenser prior to being supplied to the subcooler associated with the first condenser. The first accumulator may also be used to store refrigerant condensed in the third condenser prior to being supplied to the subcooler associated with the third condenser. If desired, it is, however, also possible to connect the third condenser to a separate accumulator for receiving refrigerant condensed in the third condenser. Similarly, in the second cooling circuit a second accumulator may be provided for receiving refrigerant condensed in the second condenser prior to being supplied to the subcooler associated with the second condenser. 
     The cooling energy transfer arrangement may comprise a heat exchanger which is adapted to be connected to the first cooling circuit such that refrigerant exiting the first condenser is thermally coupled to the refrigerant flowing through the second cooling circuit. Further, the heat exchanger may be adapted to be connected to the second cooling circuit such that refrigerant exiting the second condenser is thermally coupled to the refrigerant flowing through the first cooling circuit. Preferably, the heat exchanger is adapted to thermally couple the first and the second cooling circuit while maintaining a hermetic separation of the first and the second cooling circuit. This may be achieved by providing the heat exchanger with separate and hermatically sealed flow path fields for the refrigerant flowing through the first cooling circuit and the refrigerant flowing through the second cooling circuit. A hermetic separation of the first and the second cooling circuit enhances the operational reliability of the cooling system, since a leakage in one the two cooling circuits does not affect the operability of the other cooling circuit. Further, if desired, different refrigerants may be employed in the first and the second cooling circuit. 
     The heat exchanger may comprise a condenser wherein refrigerant flowing through the first cooling circuit may be condensed by means of cooling energy transfer from the refrigerant flowing through the second cooling circuit or wherein refrigerant flowing through the second cooling circuit may be condensed by means of cooling energy transfer from the refrigerant flowing through the first cooling circuit. Further, the heat exchanger may comprise a subcooler which serves to subcool refrigerant exiting the condenser of the heat exchanger. Subcooling the refrigerant exiting the heat exchanger ensures that the refrigerant is supplied to a conveying device discharging refrigerant from the heat exchanger in its liquid state of aggregation and sufficiently subcooled such that cavitation in the conveying device due to an unintended evaporation of the refrigerant within the conveying device is prevented. As a result, excess wear of the conveying device due to cavitation can be avoided. 
     The heat exchanger may be adapted to be connected to the first cooling circuit such that refrigerant flowing through the first cooling circuit is directed in series first through the condenser and thereafter through the subcooler of the heat exchanger. Further, the coupling between the heat exchanger and the first cooling circuit may be designed such that refrigerant exiting the heat exchanger is recirculated to the first condenser. If a third condenser is provided in the first cooling circuit, the heat exchanger may be adapted to be connected to the first cooling circuit such that the refrigerant exiting the heat exchanger is recirculated in parallel to the first and the third condenser. Further, the heat exchanger may be adapted to be connected to the second cooling circuit such that refrigerant flowing through the second cooling circuit first is directed through the condenser of the heat exchanger and thereafter is discharged to the second accumulator which also serves to store refrigerant condensed in the second condenser. The coupling between the heat exchanger and the first cooling circuit further may be designed such that refrigerant from the second accumulator may be directed in series first through the subcooler associated with the second condenser and thereafter through the subcooler of the heat exchanger. After exiting the subcooler of the heat exchanger the refrigerant flowing through the second cooling circuit may be directed to the second evaporator. 
     The cooling energy transfer arrangement further may comprise a valve which in its closed state is adapted to disconnect the heat exchanger from the first cooling circuit and which in its open state is adapted to connect the heat exchanger to the first cooling circuit. A further valve may be provided which in its closed state is adapted to disconnect the heat exchanger from the second cooling circuit and which in its open state is adapted to connect the heat exchanger to the second cooling circuit. A thermal coupling between the first and the second cooling circuit thus, by means of the valve(s), may be enabled or disabled, as desired. 
     Alternatively or additionally to a heat exchanger the cooling energy transfer arrangement of the cooling system may comprises a tubing system which is adapted to connect the first evaporator to the second condenser. This allows the first evaporator to be supplied with liquid refrigerant condensed in the second condenser. Further, the tubing system may be adapted to connect the second evaporator to the first condenser so as to allow the second evaporator to be supplied with liquid refrigerant condensed in the first condenser. Preferably, the tubing system is adapted to maintain a hermetic separation of the first and the second cooling circuit. If a third condenser and a third heat sink is provided in the first cooling circuit, the tubing system of the cooling energy transfer arrangement preferably also is adapted to connect the second evaporator to the third condenser while maintaining a hermetic separation of the first and the second cooling circuit. 
     Further, the cooling system may comprise a third accumulator which is disposed in the second cooling circuit and which is adapted to store refrigerant condensed in the first condenser prior to being supplied to the subcooler associated with the first condenser. A fourth accumulator may be disposed in the first cooling circuit and be adapted to receive refrigerant condensed in the second condenser. The third accumulator may also be used to store refrigerant condensed in the third condenser prior to being supplied to the subcooler associated with the third condenser. If desired, it is, however, also possible provide the cooling system with a fifth accumulator disposed in the second cooling circuit and being adapted to receive refrigerant condensed in the third condenser and/or a sixth accumulator disposed in the first cooling circuit and being adapted to receive refrigerant condensed in the third condenser. 
     The cooling system may comprise a first conveying device for conveying refrigerant through the first cooling circuit. The first conveying device may be connected to the first and the fourth accumulator. If also a sixth accumulator is disposed in the first cooling circuit, the first conveying device preferably also is connected to the sixth accumulator. This configuration of the cooling system allows the use of a single conveying device for conveying the refrigerant through the first cooling circuit. Alternatively or additionally thereto, the cooling system may comprise a second conveying device for conveying refrigerant through the second cooling circuit. The second conveying device may be connected to the second and the third accumulator. If also a fifth accumulator is disposed in the second cooling circuit, the second conveying device preferably also is connected to the fifth accumulator. This configuration of the cooling system allows the use of a single conveying device for conveying the refrigerant through the second cooling circuit. The cooling system, however, also may comprise a plurality of conveying devices. For example, a selected number of accumulators or each of the accumulators provided in the cooling system may be connected to a conveying device for discharging refrigerant from the associated accumulator. 
     The cooling system further may comprise a first interruption device adapted to interrupt a connection between a conveying device, in particular the first conveying device and the first accumulator, if a refrigerant level in the first accumulator falls below a predetermined threshold value, a second interruption device adapted to interrupt a connection between a conveying device, in particular the second conveying device and the second accumulator, if a refrigerant level in the second accumulator falls below a predetermined threshold value, a third interruption device adapted to interrupt a connection between a conveying device, in particular the second conveying device and the third accumulator, if a refrigerant level in the third accumulator falls below a predetermined threshold value, a fourth interruption device adapted to interrupt a connection between a conveying device, in particular the first conveying device and the fourth accumulator, if a refrigerant level in the fourth accumulator falls below a predetermined threshold value, a fifth interruption device adapted to interrupt a connection between a conveying device, in particular the second conveying device and the fifth accumulator, if a refrigerant level in the fifth accumulator falls below a predetermined threshold value, and/or a sixth interruption device adapted to interrupt a connection between a conveying device, in particular the first conveying device and the sixth accumulator, if a refrigerant level in the sixth accumulator falls below a predetermined threshold value. 
     The interruption devices ensure that the conveying devices do not convey gaseous refrigerant from accumulators which do not contain a sufficient amount of liquid refrigerant. Thereby, excessive wear of the conveying devices due to cavitation can be prevented. The interruption devices may comprise respective controllable valves which may be disposed in the first and/or the second cooling circuit(s) and which may be controlled in dependence of the refrigerant levels in the accumulators measured, for example, by appropriate fill level sensors. Alternatively or additionally thereto, the interruption devices may comprise float valves which are disposed in the accumulators and with close a respective refrigerant outlet of the accumulators, as soon as a refrigerant level in the accumulators falls below a predetermined threshold value. 
     In a further embodiment of the cooling system the cooling energy transfer arrangement may comprise a tubing and valve system which is adapted to establish a fluid connection between the first and the second cooling circuit. The cooling system then can be of a particularly low-volume and light-weight design while still being operable with the desired redundance and hence reliability. 
     The cooling system then preferably further comprises a first detection device adapted to detect the amount of refrigerant circulating in the first cooling circuit, and/or a second detection device adapted to detect the amount of refrigerant circulating in the second cooling circuit. A control unit may be adapted to control the tubing and valve system of the cooling energy transfer arrangement in dependence on signals provided to the control unit by the first and/or the second detection device. In particular, the control unit may be adapted to control the tubing and valve system of the cooling energy transfer arrangement such that a fluid connection between the first and the second cooling circuit is only established, if the signals provided to the control unit by the first and/or the second detection device indicate that an amount of refrigerant circulating in the first cooling circuit and/or an amount of refrigerant circulating in the second cooling circuit exceed(s) a predetermined threshold value. This ensures that a fluid connection between the first and the second cooling circuit is not established in case of a leakage in one of the cooling circuits. 
     Further, the control unit may adapted to control the tubing and valve system of the cooling energy transfer arrangement such that a fluid connection between the first condenser and the first evaporator and/or a fluid connection between the third condenser and the first evaporator is/are interrupted, in case a fluid connection between the first and the second cooling circuit is established in the event of failure of the cooling energy supply to the first evaporator via the first cooling circuit. Alternatively or additionally thereto, the control unit may adapted to control the tubing and valve system of the cooling energy transfer arrangement such that a fluid connection between the second condenser and the second evaporator is interrupted, in case a fluid connection between the first and the second cooling circuit is established in the event of failure of the cooling energy supply to the second evaporator via the second cooling circuit. 
     Moreover, the control unit may be adapted to interrupt the operation of a first conveying device for conveying refrigerant through the first cooling circuit, in case a fluid connection between the first and the second cooling circuit is established in the event of failure of the cooling energy supply to the first evaporator via the first cooling circuit, and/or to interrupt the operation of a second conveying device for conveying refrigerant through the second cooling circuit, in case a fluid connection between the first and the second cooling circuit is established in the event of failure of the cooling energy supply to the second evaporator via the second cooling circuit. 
     In a method of operating a cooling system, in particular for use on board an aircraft, a two-phase refrigerant is circulated through a first cooling circuit. The refrigerant circulating through the first cooling circuit is evaporated in a first evaporator disposed in the first cooling circuit. During normal operation of the cooling system the refrigerant circulating through the first cooling circuit is condensed in a first condenser disposed in the first cooling circuit. During normal operation of the cooling system cooling energy is providing to the first condenser from a first heat sink. Further, a two-phase refrigerant is circulated through a second cooling circuit. The refrigerant circulating through the second cooling circuit is evaporated in a second evaporator disposed in the second cooling circuit. During normal operation of the cooling system the refrigerant circulating through the second cooling circuit is condensed in a second condenser disposed in the second cooling circuit. During normal operation of the cooling system cooling energy is providing to the second condenser from a second heat sink. In the event of failure of the cooling energy supply to the second evaporator via the second cooling circuit cooling energy provided by the first heat sink and/or the first condenser is transferred to the second cooling circuit. Alternatively or additionally thereto, in the event of failure of the cooling energy supply to the first evaporator via the first cooling circuit cooling energy provided by the second heat sink and/or the second condenser is transferred to the first cooling circuit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Preferred embodiments of the invention now are explained in more detail with reference to the enclosed schematic drawings wherein 
         FIG. 1  shows a first embodiment of a cooling system suitable for operation with a two-phase refrigerant, 
         FIG. 2  shows a second embodiment of a cooling system suitable for operation with a two-phase refrigerant, 
         FIG. 3  shows a third embodiment of a cooling system suitable for operation with a two-phase refrigerant, 
         FIG. 4  shows an accumulator arrangement which may be employed in the cooling system of  FIG. 3 , and 
         FIG. 5  shows a fourth embodiment of a cooling system suitable for operation with a two-phase refrigerant. 
     
    
    
       FIG. 1  depicts a cooling system  10  which on board an aircraft, for example, may be employed to cool food provided for supplying to the passengers. The cooling system  10  of  FIG. 1  comprises a first cooling circuit  12   a  allowing circulation of a two-phase refrigerant therethrough. The two-phase refrigerant circulating through the first cooling circuit  12   a  may for example be CO 2  or R134A. Two first evaporators  14   a  are disposed in the first cooling circuit  12   a . Each of the first evaporators  14   a  comprises a refrigerant inlet and a refrigerant outlet. The refrigerant flowing through the first cooling circuit  12   a  is supplied to the refrigerant inlets of the first evaporators  14   a  in its liquid state of aggregation. Upon flowing through the first evaporators  14   a  the refrigerant releases its cooling energy to a cooling energy consumer which in the embodiment of a cooling system  10  depicted in  FIG. 1  is formed by the food to be cooled. Upon releasing its cooling energy, the refrigerant is evaporated and hence exits the first evaporators  14   a  at their refrigerant outlets in its gaseous state of aggregation. 
     The cooling system  10  of further comprises a second cooling circuit  12   b  allowing circulation of a two-phase refrigerant therethrough. The two-phase refrigerant circulating through the second cooling circuit  12   a  may also for example be CO 2  or R134A. Two second evaporators  14   b  are disposed in the second cooling circuit  12   b . Each of the second evaporators  14   b  comprises a refrigerant inlet and a refrigerant outlet. The refrigerant flowing through the second cooling circuit  12   b  is supplied to the refrigerant inlets of the second evaporators  14   b  in its liquid state of aggregation. Upon flowing through the second evaporators  14   b  the refrigerant releases its cooling energy to a cooling energy consumer which in the embodiment of a cooling system  10  depicted in  FIG. 1  is formed by the food to be cooled. Upon releasing its cooling energy, the refrigerant is evaporated and hence exits the second evaporators  14   b  at their refrigerant outlets in its gaseous state of aggregation. 
     The cooling system  10  usually is operated such that a dry evaporation of the refrigerant occurs in the evaporators  14   a ,  14   b . This allows an operation of the cooling system  10  with a limited amount of refrigerant circulating in the cooling circuits  12   a ,  12   b . As a result, the static pressure of the refrigerant prevailing in the cooling circuit  12   a ,  12   b  in the non-operating state of the cooling system  10  is low, even at high ambient temperatures. Further, negative effects of a leakage in the cooling system  10  are limited. Occurrence of a dry evaporation in the evaporators  14   a ,  14   b , however, can only be ensured by an appropriate control of the amount of refrigerant supplied to the evaporators  14   a ,  14   b  in dependence on the operational state of the evaporators  14   a ,  14   b , i.e. the cooling energy requirement of the cooling energy consumers coupled to the evaporators  14   a ,  14   b.    
     The supply of refrigerant to the first evaporators  14   a  is controlled by respective valves  20   a , which are disposed in the first cooling circuit  12   a  upstream of each of the first evaporators  14   a . Similarly, the supply of refrigerant to the second evaporators  14   b  is controlled by respective valves  20   b , which are disposed in the second cooling circuit  12   b  upstream of each of the second evaporators  14   b . The valves  20   a ,  20   b  may comprise a nozzle for spraying the refrigerant into the evaporators  14   a ,  14   b  and to distribute the refrigerant within the evaporators  14   a ,  14   b . The spraying of the refrigerant into the evaporators  14   a ,  14   b  may be achieved, for example, by supplying refrigerant vapor from the evaporators  14   a ,  14   b  to the nozzles of the valves  20   a ,  20   b  and/or by evaporation of the refrigerant due to a pressure decrease of the refrigerant downstream of the valves  20   a ,  20   b.    
     To ensure occurrence of a dry evaporation in the evaporators  14   a ,  14   b , a predetermined amount of refrigerant is supplied to the evaporators  14   a ,  14   b  by appropriately controlling the valves  20   a ,  20   b . Then, a temperature TK 1  of the refrigerant at the refrigerant inlets of the evaporators  14   a ,  14   b  and a temperature TA 2  of the fluid to be cooled by the evaporators  14   a ,  14   b , for example air supplied to the cooling energy consumers, is measured, preferably while a fan conveying the fluid to be cooled to the cooling energy consumers is running. Further, the pressure of the refrigerant in the evaporators  14   a ,  14   b  or at the refrigerant outlets of the evaporators  14   a ,  14   b  is measured. If a temperature difference between the temperature TA 2  of the fluid to be cooled by the evaporators  14   a ,  14   b  and the temperature TK 1  of the refrigerant at the refrigerant inlets of the evaporators  14   a ,  14   b  exceeds a predetermined threshold value, for example 8K, and the pressure of the refrigerant in the evaporators  14   a ,  14   b  lies within a predetermined range, the refrigerant supplied to the evaporators  14   a ,  14   b  is thoroughly evaporated and possibly also super-heated by the evaporators  14   a ,  14   b . Hence, the valves  20   a ,  20   b  again can be controlled so as to supply a further predetermined amount of refrigerant to the evaporators  14   a ,  14   b.    
     Further, the cooling system  10  comprises a first condenser  22   a  which is disposed in the first cooling circuit  12   a . A second condenser  22   b  is disposed in the second cooling circuit  12   b . Finally, a third condenser  22   c  is disposed in the first cooling circuit  12   a  in addition to the first condenser  22   a . Each condenser  22   a ,  22   b ,  22   c  has a refrigerant inlet and a refrigerant outlet. The refrigerant which is evaporated in the first evaporators  14   a , via a portion of the first cooling circuit  12   a  downstream of the first evaporators  14   a  and upstream of the condensers  22   a ,  22   c , is supplied to the refrigerant inlets of the condensers  22   a ,  22   c  in its gaseous state of aggregation. The supply of refrigerant from the first evaporators  14   a  to the condensers  22   a ,  22   c  is controlled by means of a valve  28   a . The valve  28   a  is adapted to control the flow of refrigerant through the first cooling circuit  12   a  such that a defined pressure gradient of the refrigerant in the portion of the first cooling circuit  12   a  between the refrigerant outlets of the first evaporators  14   a  and the refrigerant inlets of the condensers  22   a ,  22   c  is adjusted. The pressure gradient of the refrigerant in the portion of the first cooling circuit  12   a  between the refrigerant outlets of the first evaporators  14   a  and the refrigerant inlets of the condensers  22   a ,  22   c  induces a flow of the refrigerant from the first evaporators  14   a  to the condensers  22   a ,  22   c.    
     The refrigerant which is evaporated in the second evaporators  14   b , via a portion of the second cooling circuit  12   b  downstream of the second evaporators  14   b  and upstream of the second condenser  22   b , is supplied to the refrigerant inlet of the second condenser  22   b  in its gaseous state of aggregation. The supply of refrigerant from the second evaporators  14   b  to the second condenser  22   b  is controlled by means of a valve  28   b . The valve  28   b  is adapted to control the flow of refrigerant through the second cooling circuit  12   b  such that a defined pressure gradient of the refrigerant in the portion of the second cooling circuit  12   b  between the refrigerant outlets of the second evaporators  14   b  and the refrigerant inlet of the second condenser  22   b  is adjusted. The pressure gradient of the refrigerant in the portion of the second cooling circuit  12   b  between the refrigerant outlets of the second evaporators  14   b  and the refrigerant inlet of the second condenser  22   b  induces a flow of the refrigerant from the second evaporators  14   b  to the second condenser  22   b.    
     Each of the condensers  22   a ,  22   b ,  22   c  is thermally coupled to a heat sink  29   a ,  29   b ,  29   c  designed in the form of a chiller. The cooling energy provided by the heat sinks  29   a ,  29   b ,  29   c  in the condensers  22   a ,  22   b ,  22   c  is used to condense the refrigerant. Thus, the refrigerant exits the condensers  22   a ,  22   b ,  22   c  at respective refrigerant outlets in its liquid state of aggregation. Liquid refrigerant from the first and the third condenser  22   a ,  22   c  is supplied to a first accumulator  30   a . Liquid refrigerant from the second condenser  22   b  is supplied to a second accumulator  30   b . Within the accumulators  30   a ,  30   b  the refrigerant is stored in the form of a boiling liquid. 
     In the cooling circuits  12   a ,  12   b  the condensers  22   a ,  22   b ,  22   c  form a “low-temperature location” where the refrigerant, after being converted into its gaseous state of aggregation in the evaporators  14   a ,  14   b , is converted back into its liquid state of aggregation. A particularly energy efficient operation of the cooling system  10  is possible, if the condensers  22   a ,  22   b ,  22   c  are installed at a location where heating of the condensers  22   a ,  22   b ,  22   c  by ambient heat is avoided as far as possible. When the cooling system  10  is employed on board an aircraft, the condensers  22   a ,  22   b ,  22   c  preferably are installed outside of the heated aircraft cabin behind the secondary aircraft structure, for example in the wing fairing, the belly fairing or the tail cone. The same applies to the accumulators  30   a ,  30   b . Further, the condensers  22   a ,  22   b ,  22   c  and/or the accumulators  30   a ,  30   b  may be insulated to maintain the heat input from the ambient as low as possible. 
     The first accumulator  30   a  may, for example, be an accumulator as it is described in the non-published German patent application DE 10 2011 014 943. Liquid refrigerant from a sump of the first accumulator  30   a  is directed to a first subcooler  32   a . The first subcooler  32   a  is associated with the first condenser  22   a . The second accumulator  30   b  may, for example, also be an accumulator as it is described in the non-published German patent application DE 10 2011 014 943. Liquid refrigerant from a sump of the second accumulator  30   b  is directed to a second subcooler  32   b . The second subcooler  32   b  is associated with the second condenser  22   b . Refrigerant exiting the first subcooler  32   a  is directed to a third subcooler  32   c  associated with the third condenser  22   c . The subcoolers  32   a ,  32   b ,  32   c  serve to subcool the liquid refrigerant and to thus prevent an undesired evaporation of the refrigerant. This ensures that the refrigerant is supplied to a first conveying device  34   a  for conveying refrigerant through the first cooling circuit  12   a , which is embodied in the form of a pump, and to a second conveying device  34   b  for conveying refrigerant through the second cooling circuit  12   b , which also is embodied in the form of a pump, in its liquid state of aggregation. Thus, dry operation of the conveying devices  34   a ,  34   b  and failure of the conveying devices  34   a ,  34   b  can be prevented. 
     The cooling system  10  further comprises a first storage container  36   a  which is disposed in the first cooling circuit  12   a  downstream of the first conveying device  34   a , wherein the supply of refrigerant to the first storage container  36   a  is controlled by means of a valve  40   a . A second storage container  36   b  is disposed in the second cooling circuit  12   b  downstream of the second conveying device  34   b , wherein the supply of refrigerant to the second storage container  36   b  is controlled by means of a valve  40   b . The storage containers  36   a ,  36   b  serve as backup reservoir for operational situations of the cooling system  10 , wherein the volume of the first and the second accumulator  30   a ,  30   b , respectively, is not sufficient so as to receive the entire amount of liquid refrigerant provided by the condensers  22   a ,  22   b ,  22   c . Valves  38   a ,  38   b  serve to control the supply of refrigerant from the storage containers  36   a ,  36   b  to the first and the second accumulator  30   a ,  30   b , respectively. 
     Finally, the cooling system  10  comprises a cooling energy transfer arrangement  42  which is adapted to transfer cooling energy between the first and the second cooling circuit  12   a ,  12   b . In the cooling system  10  of  FIG. 1  the cooling energy transfer arrangement  42  comprises a heat exchanger  44  including a condenser  46  and a subcooler  48 . The heat exchanger  44  is permanently coupled to the second cooling circuit  12   b  such that refrigerant flowing through the second cooling circuit  12   b , after exiting the second condenser  22   b , first is directed through the condenser  46  of the heat exchanger  44  and thereafter is discharged to the second accumulator  30   b  which also serves to store refrigerant condensed in the second condenser  22   b . The refrigerant from the second accumulator  30   b  is directed in series first through the second subcooler  32   b  associated with the second condenser  22   b  and thereafter through the subcooler  48  of the heat exchanger  44 . After exiting the subcooler  48  of the heat exchanger  44  the refrigerant flowing through the second cooling  12   b , by means of the second conveying device  34   b , is conveyed to the second evaporators  14   b.    
     Further, the cooling energy transfer arrangement  42  comprises a valve  50  which in its closed state is adapted to disconnect the heat exchanger  44  from the first cooling circuit  12   a  and which in its open state is adapted to connect the cooling heat exchanger  44  to the first cooling circuit  12   a . When the valve  50  opens the connection between the heat exchanger  44  from the first cooling circuit  12   a , refrigerant flowing through the first cooling circuit  12   a  downstream of the first conveying device  34   a  is directed in series first through the condenser  46  and thereafter through the subcooler  48  of the heat exchanger  44 . Further, the coupling between the heat exchanger  44  and the first cooling circuit  12   a  is designed such that refrigerant exiting the heat exchanger  44  is recirculated in parallel to the first and the third condenser  22   a ,  22   c.    
     The heat exchanger  44  is adapted to thermally couple the first and the second cooling circuit  12   a ,  12   b  while maintaining a hermetic separation of the first and the second cooling circuit  12   a ,  12   b . This is achieved by providing the heat exchanger  44  with separate and hermatically sealed flow path fields for the refrigerant flowing through the first cooling circuit  12   a  and the refrigerant flowing through the second cooling circuit  12   b.    
     The valve  50 , by means of a control device not shown in  FIG. 1 , is controlled in dependence on the operating state of the cooling system  10 . In particular, the valve  50  is controlled so as to connect the cooling heat exchanger  44  to the first cooling circuit  12   a , if the amount cooling energy provided by the second heat sink  29   b  and/or the second condenser  22   b  is not sufficient to ensure proper operation of the second evaporators  14   b . Within the heat exchanger  44  then at least a part of the cooling energy provided by the first heat sink  29   a , the first condenser  22   a , the third heat sink  29   c  and/or the third condenser  22   c  is transferred to the second cooling circuit  12   b . Similarly, the valve  50  is controlled so as to connect the cooling heat exchanger  44  to the first cooling circuit  12  also in operational states of the cooling system  10  wherein the amount cooling energy provided by the first heat sink  29   a , the first condenser  22   a , the third heat sink  29   c  and/or the third condenser  22   c  is not sufficient to ensure proper operation of the first evaporators  14 . Within the heat exchanger  44  then at least a part of the cooling energy provided by the second heat sink  29   b  and/or the second condenser  22   b  is transferred to the first cooling circuit  12   a.    
     The cooling system  10  according to  FIG. 2  differs from the cooling system  10  of  FIG. 1  in that the cooling energy transfer arrangement  42  comprises a tubing system  52  which is adapted to connect the first evaporators  14   a  to the second condenser  22   b . This allows the first evaporators  14   b  to be supplied with liquid refrigerant condensed in the second condenser  22   b . Further, the tubing system  52  is adapted to connect the second evaporators  14   b  (not shown in  FIG. 2 ) to the first condenser  22   a  so as to allow the second evaporators  14   b  to be supplied with liquid refrigerant condensed in the first condenser. Finally, the tubing system  52  of the cooling energy transfer arrangement  42  also is adapted to connect the second evaporators  14   b  to the third condenser  22   c . The tubing system  52  is adapted to maintain a hermetic separation of the first and the second cooling circuit  12   a ,  12   b.    
     Further, the cooling system  10  of  FIG. 2  comprises a third accumulator  30   c  which is disposed in the second cooling circuit  12   b  and which is adapted to store refrigerant condensed in the first condenser  22   a  prior to being supplied to the subcooler  32   a  associated with the first condenser  22   a . A fourth accumulator  30   d  is disposed in the first cooling circuit  12   a  and is adapted to receive refrigerant condensed in the second condenser  22   b . The cooling system  10  further is provided with a fifth accumulator  30   e  disposed in the second cooling circuit  12   b  and being adapted to receive refrigerant condensed in the third condenser  22   c  and a sixth accumulator  30   f  disposed in the first cooling circuit  12   a  and being adapted to receive refrigerant condensed in the third condenser  22   c . The first conveying device  34   a  for conveying refrigerant through the first cooling circuit  12   a  is connected to the first, the fourth accumulator and the sixth accumulator  30   a ,  30   d ,  30   f . The second conveying device  34   b  for conveying refrigerant through the second cooling circuit  12   b  is connected to the second, the third accumulator and the fifth accumulator  30   b ,  30   c ,  30   e.    
     The cooling system  10  of  FIG. 2  further comprises a first interruption device  54   a  adapted to interrupt a connection between the first conveying device  34   a  and the first accumulator  30   a , if a refrigerant level in the first accumulator  34   a  falls below a predetermined threshold value, a second interruption device  54   a  adapted to interrupt a connection between the second conveying device  34   b  and the second accumulator  30   b , if a refrigerant level in the second accumulator  30   b  falls below a predetermined threshold value, a third interruption device  54   c  adapted to interrupt a connection between the second conveying device  34   b  and the third accumulator  30   c , if a refrigerant level in the third accumulator  30   c  falls below a predetermined threshold value, a fourth interruption device  54   d  adapted to interrupt a connection between the first conveying device  34   a  and the fourth accumulator  30   d , if a refrigerant level in the fourth accumulator  30   d  falls below a predetermined threshold value, a fifth interruption device  54   e  adapted to interrupt a connection between the second conveying device  34   b  and the fifth accumulator  30   e , if a refrigerant level in the fifth accumulator  30   e  falls below a predetermined threshold value, and a sixth interruption device  54   f  adapted to interrupt a connection between the first conveying device  34   a  and the sixth accumulator  30   f , if a refrigerant level in the sixth accumulator falls below a predetermined threshold value. 
     The interruption devices  54   a - 54   f  ensure that the conveying devices  34   a ,  34   b  do not convey gaseous refrigerant from accumulators  30   a - 30   f  which do not contain a sufficient amount of liquid refrigerant. Each of the interruption devices  54   a - 54   f  comprises a float valve which is disposed in one of the accumulators  30   a - 30   f  and with closes a respective refrigerant outlet of the accumulator  30   a - 30   f , as soon as a refrigerant level in the accumulator  30   a - 30   f  falls below a predetermined threshold value. Otherwise the structure and the function of the cooling system  10  according to  FIG. 2  correspond to the structure and the function of the cooling system  10  of  FIG. 1 . 
     The cooling system  10  according to  FIG. 3  differs from the cooling system  10  of  FIG. 2  in that the first accumulator  30   a  is used to store refrigerant condensed in the first and the third condenser  22   a ,  22   c . Similarly, the third accumulator  30   c  is also used to store refrigerant condensed in the first and third condenser  22   a ,  22   c . Hence, the fifth and the sixth accumulator  30   e ,  30   f  present in the cooling system  10  of  FIG. 2  can be dispensed with. Further, in the cooling system according to  FIG. 3  the first conveying device  34   a  only serves to discharge refrigerant from the first accumulator  30   a  while the second conveying device  34   b  only serves to discharge refrigerant from the second accumulator  30   b . A third conveying device  34   c  is provided to discharge refrigerant from the third accumulator  30   c  and a fourth conveying device  34   d  is provided to discharge refrigerant from the fourth accumulator  30   d . Otherwise the structure and the function of the cooling system  10  according to  FIG. 3  correspond to the structure and the function of the cooling system  10  of  FIG. 2 . 
     An accumulator arrangement depicted in  FIG. 4  can be employed in the cooling system  10  of  FIG. 3 . The accumulator arrangement of  FIG. 4 , however, also is suitable for use in any one of cooling system  10  of  FIG. 1 ,  2  or  5 . In the accumulator arrangement a first accumulator  30   a  is connected to two redundant first conveying devices  34   a . Suction lines  56  of the conveying devices  34   a  which connect the refrigerant inlets of the conveying devices  34   a  to the accumulator  30   a  are disposed at least partially within the accumulator  30   a , ensuring that refrigerant is supplied to the conveying devices  34   a  from the accumulator in its liquid state of aggregation. Excess wear of the conveying devices  34   a  due to cavitation thus can be prevented. 
     In the cooling system according to  FIG. 5  the cooling energy transfer arrangement  42  comprises a tubing and valve system  58  which is adapted to establish a fluid connection between the first and the second cooling circuit  12   a ,  12   b . The tubing and valve system  58  comprises connecting lines  60 ,  62 ,  64  connecting the first and the second cooling circuit  12   a ,  12   b  as well as respective valves  66 ,  68 ,  70  for controlling the flow refrigerant between the first and the second cooling circuit  12   a ,  12   b  through the connecting lines  60 ,  62 ,  64 . 
     Further, the cooling system  10  comprises a first detection device  72  adapted to detect the amount of refrigerant circulating in the first cooling circuit  12   a , and a second detection device  74  adapted to detect the amount of refrigerant circulating in the second cooling circuit  12   b . A control unit  76  is adapted to control the tubing and valve system  58  in dependence on signals provided to the control unit  76  by the first and the second detection device  72 ,  74 . In particular, the control unit  76  is adapted to control the tubing and valve system  58  such that a fluid connection between the first and the second cooling circuit  12   a ,  12   b  is only established, if the signals provided to the control unit  76  by the first and/or the second detection device  72 ,  74  indicate that an amount of refrigerant circulating in the first cooling circuit  12   a  and an amount of refrigerant circulating in the second cooling circuit  12   b  exceed a predetermined threshold value. This ensures that a fluid connection between the first and the second cooling circuit  12   a ,  12   b  is not established in case of a leakage in one of the cooling circuits  12   a ,  12   b.    
     Further, the control unit  76  is adapted to control the tubing and valve system  58  such that a fluid connection between the first condenser  22   a  and the first evaporators  14   a  and a fluid connection between the third condenser  22   c  and the first evaporators  14   a  are interrupted, in case a fluid connection between the first and the second cooling circuit  12   a ,  12   b  is established in the event of failure of the cooling energy supply to the first evaporators  14   a  via the first cooling circuit  12   a . Additionally thereto, the control unit  76  is adapted to control the tubing and valve system  58  such that a fluid connection between the second condenser  22   b  and the second evaporators  14   b  is interrupted, in case a fluid connection between the first and the second cooling circuit  12   a ,  12   b  is established in the event of failure of the cooling energy supply to the second evaporators  14   b  via the second cooling circuit  12   b.    
     Moreover, the control unit  76  is adapted to interrupt the operation of the first conveying device  34   a  for conveying refrigerant through the first cooling circuit  12   a , in case a fluid connection between the first and the second cooling circuit  12   a ,  14  is established in the event of failure of the cooling energy supply to the first evaporators  14   a  via the first cooling circuit  12   a , and to interrupt the operation of the second conveying device  34   b  for conveying refrigerant through the second cooling circuit  12   b , in case a fluid connection between the first and the second cooling circuit  12   a ,  12   b  is established in the event of failure of the cooling energy supply to the second evaporators  14   b  via the second cooling circuit  12   b . Otherwise the structure and the function of the cooling system  10  according to  FIG. 5  correspond to the structure and the function of the cooling systems  10  of  FIGS. 1 to 3 . 
     For controlling the start-up of any one of the cooling systems  10  depicted in  FIGS. 1 to 3  and  5  there are different options. As a first option, upon start-up of the cooling system  10 , all evaporators  14   a ,  14   b  are simultaneously supplied with cooling energy. Typically the cooling system  10  will be designed for this start-up mode of operation. It is, however, also conceivable to control the supply of cooling energy to the evaporators  14   a ,  14   b  upon start-up of the cooling system  100  such that at first only selected ones of the evaporators  14   a ,  14   b  are supplied with cooling energy until a predetermined target temperature of the selected evaporators  14   a ,  14   b  supplied with cooling energy is reached. Only then also the remaining evaporators  14   a ,  14   b  may be supplied with cooling energy. In this start-up mode of operation the amount of heat to be discharged by means of the cooling system  10  is smaller than in a mode of operation wherein all evaporators  14   a ,  14   b  are simultaneously supplied with cooling energy. Hence, heat sinks  29   a ,  29   b ,  29   c  designed in the form of chillers can be operated at lower temperatures allowing heat to be discharged from the cooling energy consumers rather quickly due to the large temperature difference between the operating temperature of the heat sinks  29   a ,  29   b ,  29   c  and the temperature of the cooling energy consumers. 
     Finally, it is also conceivable to control the supply of cooling energy to the evaporators  14   a ,  14   b  upon start-up of the cooling system  10  such that at first all evaporators  14   a ,  14   b  are simultaneously supplied with cooling energy until a predetermined intermediate temperature of the evaporators  14   a ,  14   b  is reached. Immediately after start-up of the cooling system  10  the temperature difference between the operating temperature of heat sinks  29   a ,  29   b ,  29   c  designed in the form of chillers and the temperature of the cooling energy consumers still is high allowing a quick removal of heat from the cooling energy consumers. After reaching the predetermined intermediate temperature of the evaporators  14   a ,  14   b  the operating temperature of the heat sinks  29   a ,  29   b ,  29   c  may be reduced and further cooling energy may be supplied only to selected ones of the evaporators  14   a ,  14   b  until a predetermined target temperature of the selected evaporators  14   a ,  14   b  supplied with cooling energy is reached. Finally, the remaining evaporators  14   a ,  14   b  may be supplied with cooling energy until a predetermined target temperature is reached also for these evaporators  14   a ,  14   b . Again a quick removal of heat from the cooling energy consumers may be achieved due to the large temperature difference between the operating temperature of the heat sinks  29   a ,  29   b ,  29   c  and the temperature of the cooling energy consumers. 
     In the embodiments of a cooling system  10  described above, the accumulators  30   a ,  30   b  and the storage containers  36   a    36   b  fulfill the double function of storing liquid refrigerant exiting the condensers  22   a ,  22   b ,  22   c ,  46  and, in addition thereto, of reducing the system pressure in the cooling circuits  12   a ,  12   b . The pressure reducing effect of the accumulators  30   a ,  30   b  and the storage containers  36   a ,  36   b  results from the additional volume the accumulators  30   a ,  30   b  and the storage containers  36   a ,  36   b  add to the volume of the cooling circuits  12   a ,  12   b  and becomes more and more significant, as the volume of the accumulators  30   a ,  30   b  and the storage containers  36   a ,  36   b  increases. The importance of the pressure reduction function of the accumulators  30   a ,  30   b  and the storage containers  36   a ,  36   b  increases as the operating temperature of the cooling system  10  and hence the pressure in the cooling circuits  12   a ,  12   b  increases and is of particular relevance if the cooling system  10  is operated with a refrigerant causing a high system pressure such as, for example, CO 2 . 
     Basically the cooling system  10  may comprise both, the accumulators  30   a ,  30   b  and the storage containers  36   a , 36   b  as described above, and both components may serve to store liquid refrigerant exiting the condensers  22   a ,  22   b ,  22   c ,  46  and to reduce the system pressure in the cooling circuits  12   a ,  12   b . It is, however, also conceivable to equip the cooling system  10  with only the acculumators  30   a ,  30   b  or only the storage containers  36   a ,  36   b . The accumulators  30   a ,  30   b  or the storage containers  36   a ,  36   b  which are provided in such a cooling system  10  then again fulfill the double function of storing liquid refrigerant exiting the condensers  22   a ,  22   b ,  22   c ,  46  and of reducing the system pressure in the cooling circuits  12   a ,  12   b . Finally, a configuration of the cooling system  10  is conceivable, wherein the accumulators  30   a ,  30   b  serve to collect and to store liquid refrigerant, whereas the storage containers  36   a ,  36   b , due to their additional volume, serve to reduce the system pressure. 
     In case the functions “storing liquid refrigerant” and “reducing system pressure” in the cooling system  10  are provided by separate components, these components may be installed at different positions within the cooling circuits  12   a ,  12   b , allowing to more efficiently use the available installation space and to limit the size of the individual components of the cooling system  10 . However, the pressure reducing storage containers  36   a ,  36   b  then preferably are installed in a high pressure portion of the cooling circuits  12   a ,  12   b  in order to reliably prevent the pressure in the high pressure portion of the cooling circuits  12   a ,  12   b  from exceeding a predetermined maximum value. 
     Further, in case the storage containers  36   a ,  36   b  merely serve to control the pressure in the cooling system  10 , it is no longer necessary to provide for a direct fluid connection between the accumulators  30   a ,  30   b  and the storage containers  36   a ,  36   b . Instead, the storage containers  36   a ,  36   b  may be connected to the cooling circuits  12   a ,  12   b  via only a single line branching off from the cooling circuits  12   a ,  12   b , for example, upstream of one of the condensers  22   a ,  22   b ,  22   c ,  46  and downstream of the evaporators  14   a ,  14   b . The line connecting the storage containers  36   a ,  36   b  to the cooling circuits  12   a ,  12   b  preferably is connected to the storage containers  36   a ,  36   b  at the geodetic lowest point of storage containers  36   a ,  36   b . This configuration ensures that the storage containers  36   a ,  36   b  are supplied only with gaseous refrigerant which is discharged from the cooling circuits  12   a ,  12   b  due to the pressure in the cooling circuits  12   a ,  12   b  exceeding a predetermined value. Of course, if desired, only one storage container may be provided, in the cooling system  10 .