Cooling system with a plurality of subcoolers

Abstract

A cooling system, in particular for use on board an aircraft, includes a cooling circuit allowing circulation of a two-phase refrigerant therethrough, an evaporator disposed in the cooling circuit, and a condenser disposed in the cooling circuit. A plurality of subcoolers is arranged in series in the cooling circuit downstream of the condenser.

Claims

1. A cooling system, in particular for use on board an aircraft, the cooling system comprising: a cooling circuit allowing circulation of a two-phase refrigerant therethrough, an evaporator disposed in the cooling circuit, a condenser disposed in the cooling circuit, a plurality of subcoolers arranged in series in the cooling circuit downstream of the condenser, and a heat sink comprising a chiller having a primary circuit, wherein at least one of the plurality of subcoolers is associated with the condenser and with the heat sink, the heat sink being common to the at least one subcooler and the condenser, so as to form a condenser/subcooler/heat sink assembly unit to enable the assembly unit to be disconnected from the cooling system in a unitary fashion without opening the primary cooling circuit of the chiller.

2. The cooling system according to claim 1, wherein a plurality of condensers is arranged in parallel in the cooling circuit upstream of the plurality of subcoolers.

3. The cooling system according to claim 1, wherein an accumulator is disposed in the cooling circuit, in particular downstream of the condenser and upstream of the plurality of subcoolers.

4. The cooling system according to claim 1, further comprising a conveying device for discharging refrigerant from an accumulator and a storage container disposed in the cooling circuit downstream of the plurality of subcoolers and downstream of the conveying device, wherein the plurality of subcoolers include a refrigerant outlet and the storage container is connected to the refrigerant outlet via a first connecting line, and wherein a first valve is disposed in the first connecting line so as to control the supply of refrigerant from the refrigerant outlet of the subcoolers to the storage container.

5. The cooling system according to claim 4, wherein the plurality of subcoolers include a refrigerant inlet and the storage container is connected to the refrigerant inlet or the accumulator via a second connecting line, wherein a second valve is disposed in the second connecting line so as to control the supply of refrigerant from the storage container to the refrigerant inlet of the subcoolers or the accumulator.

6. The cooling system according to claim 5, further comprising at least one of a fill level detecting device which is adapted to detect a fill level of refrigerant in the accumulator, and a pressure detecting device which is adapted to detect a pressure of the refrigerant in the cooling circuit, and further comprising a valve control unit which is adapted to control at least one of the first and the second valve in dependence on signals provided to the valve control unit from at least one of the fill level detecting device and the pressure detecting device.

7. The cooling system according to claim 6, wherein the valve control unit is adapted to open the first valve disposed in the first connecting line if a signal provided to the valve control unit from the pressure detecting device indicates that a pressure difference between a pressure of the refrigerant in the cooling circuit upstream of the conveying device and a pressure of the refrigerant in the cooling circuit downstream of the conveying device exceeds a predetermined threshold value.

8. The cooling system according to claim 6, wherein the valve control unit is adapted to control the second valve disposed in the second connecting line so as to enable a flow of refrigerant from the storage container to the refrigerant inlet of the subcoolers or the accumulator if a signal provided to the valve control unit from the fill level detecting device indicates that the fill level of refrigerant in the accumulator is below a predetermined threshold value and a signal provided to the valve control unit from the pressure detecting device indicates that the pressure of the refrigerant in the cooling circuit is below a predetermined threshold value.

9. An aircraft comprising a cooling system according to claim 1.

10. A method of operating a cooling system, in particular for use on board an aircraft, the method comprising the steps: circulating a two-phase refrigerant through a cooling circuit, evaporating the refrigerant in an evaporator disposed in the cooling circuit, and condensing the refrigerant in a condenser disposed in the cooling circuit, wherein the refrigerant is subcooled in a plurality of subcoolers arranged in series in the cooling circuit downstream of the condenser, wherein at least one of the plurality of subcoolers is associated with the condenser and with a heat sink comprising a chiller having a primary circuit, the heat sink being common to the at least one subcooler and the condenser so as to form a condenser/subcooler/heat sink assembly unit to enable the assembly unit to be disconnected from the cooling system in a unitary fashion without opening the primary cooling circuit of the chiller.

11. The method according to claim 10, wherein the refrigerant is condensed in a plurality of condensers arranged in parallel in the cooling circuit upstream of the plurality of subcoolers.

12. The method according to claim 10, wherein the refrigerant is received in an accumulator disposed in the cooling circuit, in particular downstream of the condenser and upstream of the plurality of subcoolers.

13. The method according to claim 10, wherein the refrigerant is stored in a storage container disposed in the cooling circuit downstream of the plurality of subcoolers and in particular downstream of a conveying device for discharging refrigerant from the accumulator, wherein the storage container is connected to a refrigerant outlet of the plurality of subcoolers via a first connecting line, and further comprising controlling, by a first valve disposed in the first connecting line, the supply of refrigerant from the refrigerant outlet of the subcoolers to the storage container.

14. The method according to claim 13, further comprising controlling, by a second valve disposed in a second connecting line connecting the storage container to the refrigerant inlet of the subcoolers or the accumulator, the supply of refrigerant from the storage container to a refrigerant inlet of the subcoolers or the accumulator.

15. The method according to claim 14, further comprising at least one of detecting, by a fill level detector, a fill level of refrigerant in the accumulator, and detecting, by a pressure detector, a pressure of the refrigerant in the cooling circuit, and further comprising controlling, by a valve controller, at least one of the first valve and the second valve in dependence on signals provided to the valve controller from at least one of the fill level detector and the pressure detector.

16. The method according to claim 15, wherein the valve controller opens the first valve disposed in the first connecting line if a signal provided to the valve controller from the pressure detector indicates that a pressure difference between a pressure of the refrigerant in the cooling circuit upstream of the conveying device and a pressure of the refrigerant in the cooling circuit downstream of the conveying device exceeds a predetermined threshold value.

17. The method according to claim 15, wherein the valve controller controls the second valve disposed in the second connecting line so as to enable a flow of refrigerant from the storage container to the refrigerant inlet of the subcoolers or the accumulator if a signal provided to the valve controller from the fill level detector indicates that the fill level of refrigerant in the accumulator is below a predetermined threshold value and a signal provided to the valve controller from the pressure detector indicates that the pressure of the refrigerant in the cooling circuit is below a predetermined threshold value.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Preferred embodiments of the invention now are explained in more detail with reference to the enclosed schematic drawings wherein

(2) FIG. 1 shows a first embodiment of a cooling system suitable for operation with a two-phase refrigerant, and

(3) FIG. 2 shows a second embodiment of a cooling system suitable for operation with a two-phase refrigerant.

(4) 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 comprises a cooling circuit 12 allowing circulation of a two-phase refrigerant therethrough. The two-phase refrigerant circulating through the cooling circuit 12 may for example be CO.sub.2 or R134A. Two evaporators 14a, 14b are disposed in the cooling circuit 12. Each of the evaporators 14a, 14b comprises a refrigerant inlet and a refrigerant outlet. The refrigerant flowing through the cooling circuit 12 is supplied to the refrigerant inlets of the evaporators 14a, 14b in its liquid state of aggregation. Upon flowing through the evaporators 14a, 14b 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 evaporators 14a, 14b at their refrigerant outlets in its gaseous state of aggregation.

(5) The cooling system 10 usually is operated such that a dry evaporation of the refrigerant occurs in the evaporators 14a, 14b. This allows an operation of the cooling system 10 with a limited amount of refrigerant circulating in the cooling circuit 12. As a result, the static pressure of the refrigerant prevailing in the cooling circuit 12 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 14a, 14b, however, can only be ensured by an appropriate control of the amount of refrigerant supplied to the evaporators 14a, 14b in dependence on the operational state of the evaporators 14a, 14b, i.e. the cooling energy requirement of the cooling energy consumers coupled to the evaporators 14a, 14b.

(6) The supply of refrigerant to the evaporators 14a is controlled by respective valves 20a, 20b which are disposed in the cooling circuit 12 upstream of each of the evaporators 14a, 14b. The valves 20a, 20b may comprise a nozzle for spraying the refrigerant into the evaporators 14a, 14b and to distribute the refrigerant within the evaporators 14a, 14b. The spraying of the refrigerant into the evaporators 14a, 14b may be achieved, for example, by supplying refrigerant vapor from the evaporators 14a, 14b to the nozzles of the valves 20a, 20b and/or by evaporation of the refrigerant due to a pressure decrease of the refrigerant downstream of the valves 20a, 20b.

(7) To ensure occurrence of a dry evaporation in the evaporators 14a, 14b, a predetermined amount of refrigerant is supplied to the evaporators 14a, 14b by appropriately controlling the valves 20a, 20b. Then, a temperature TK1 of the refrigerant at the refrigerant inlets of the evaporators 14a, 14b and a temperature TA2 of the fluid to be cooled by the evaporators 14a, 14b, 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 14a, 14b or at the refrigerant outlets of the evaporators 14a, 14b is measured. If a temperature difference between the temperature TA2 of the fluid to be cooled by the evaporators 14a, 14b and the temperature TK1 of the refrigerant at the refrigerant inlets of the evaporators 14a, 14b exceeds a predetermined threshold value, for example 8K, and the pressure of the refrigerant in the evaporators 14a, 14b lies within a predetermined range, the refrigerant supplied to the evaporators 14a, 14b is thoroughly evaporated and possibly also super-heated by the evaporators 14a, 14b. Hence, the valves 20a, 20b again can be controlled so as to supply a further predetermined amount of refrigerant to the evaporators 14a, 14b.

(8) Further, the cooling system 10 comprises a first and a second condenser 22a, 22b which are arranged in parallel in the cooling circuit 12. Each condenser 22a, 22b has a refrigerant inlet and a refrigerant outlet. The refrigerant which is evaporated in the evaporators 14a, 14b, via a portion 12a of the cooling circuit 12 downstream of the evaporators 14a, 14b and upstream of the condensers 22a, 22b, is supplied to the refrigerant inlets of the condensers 22a, 22b in its gaseous state of aggregation. The supply of refrigerant from the evaporators 14a, 14b to the condensers 22a, 22b is controlled by means of a valve 28. The valve 28 is adapted to control the flow of refrigerant through the cooling circuit 12 such that a defined pressure gradient of the refrigerant in the portion 12a of the cooling circuit 12 between the refrigerant outlets of the evaporators 14a, 14b and the refrigerant inlets of the condensers 22a, 22b is adjusted. The pressure gradient of the refrigerant in the portion 12a of the cooling circuit 12 between the refrigerant outlets of the evaporators 14a, 14b and the refrigerant inlets of the condensers 22a, 22b induces a flow of the refrigerant from the evaporators 14a, 14b to the condensers 22a, 22b. By dosing the valve 28 the cooling circuit is separated into a high pressure portion and a low pressure portion.

(9) Each of the condensers 22a, 22b is thermally coupled to a heat sink 29a, 29b designed in the form of a chiller. The cooling energy provided by the heat sinks 29a, 29b in the condensers 22a, 22b is used to condense the refrigerant. Thus, the refrigerant exits the condensers 22a, 22b at respective refrigerant outlets in its liquid state of aggregation. Liquid refrigerant from the condensers 22a, 22b is supplied to an accumulator 30. Within the accumulator 30 the refrigerant is stored in the form of a boiling liquid.

(10) In the cooling circuit 12 the condensers 22a, 22b form a “low-temperature location” where the refrigerant, after being converted into its gaseous state of aggregation in the evaporators 14a, 14b, is converted back into its liquid state of aggregation. A particularly energy efficient operation of the cooling system 10 is possible, if the condensers 22a, 22b are installed at a location where heating of the condensers 22a, 22b by ambient heat is avoided as far as possible. When the cooling system 10 is employed on board an aircraft, the condensers 22a, 22b preferably are installed outside of the heated aircraft cabin behind the secondary aircraft structure, for example in the wing fairing, the belly failing or the tail cone. The same applies to the accumulator 30. Further, the condensers 22a, 22b and/or the accumulator 30 may be insulated to maintain the heat input from the ambient as low as possible.

(11) The accumulator 30 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 accumulator 30 is directed to a first subcooler 32a. The first subcooler 32a is associated with the first condenser 22a and the heat sink 29a so as to form a condenser/subcooler/heat sink assembly unit. Refrigerant exiting the first subcooler 32a is directed to a second subcooler 32b being arranged in series with the first subcooler 32a and being associated with the second condenser 22b and the heat sink 29b so as to form a condenser/subcooler/heat sink assembly unit. The subcoolers 32a, 32b 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 conveying device 34 for conveying refrigerant through the cooling circuit 12, which is embodied in the form of a pump, in its liquid state of aggregation. Thus, dry operation of the conveying device 34 and failure of the conveying device 34 can be prevented.

(12) The cooling system 10 further comprises a storage container 36 which is disposed in the cooling circuit 12a downstream of a refrigerant outlet of the subcoolers 32a, 32b and downstream of the conveying device 34. The supply of refrigerant exiting the subcoolers 32a, 32b to the storage container 36 is controlled by means of a valve 38 disposed in a first connecting line 40. A second connecting line 42 connects the storage container 36 to a refrigerant inlet of the subcoolers 32a, 32b. A valve 44 is adapted to connect the refrigerant inlet of the subcoolers 32a, 32b either to the accumulator 30 or to the storage container 36.

(13) A fill level detecting device 46 is adapted to detect a fill level of refrigerant in the accumulator 30. Further, the cooling system 10 comprises a pressure detecting device 48 which is adapted to detect a pressure of the refrigerant at different locations in the cooling circuit 12. The pressure detecting device may, for example, comprise a plurality of pressure sensors disposed at different locations in the cooling circuit 12. A valve control unit 50 serves to control the valves 38, 44 for regulating the flow of refrigerant to and from the storage container 36 in dependence on signals provided to the valve control unit 50 from the fill level detecting device 48 and the pressure detecting device 48.

(14) In particular, the valve control unit 50 opens the valve 38 disposed in the first connecting line 40 if a signal provided to the valve control unit 50 from the pressure detecting device 48 indicates that a pressure difference between a pressure of the refrigerant in the cooling circuit 12 upstream of the conveying device 34 and a pressure of the refrigerant in the cooling circuit 12 downstream of the conveying device 34 exceeds a predetermined threshold value of, for example, 6 bar. Further, the valve control unit 50 controls the valve 44 disposed in the second connecting line 42 so as to enable a flow of refrigerant from the storage container 36 to the refrigerant inlet of the subcoolers 32a, 32b if a signal provided to the valve control unit 50 from the fill level detecting device 46 indicates that the fill level of refrigerant in the accumulator 30 is below a predetermined threshold value and a signal provided to the valve control unit 50 from the pressure detecting device 48 indicates that the pressure of the refrigerant in the cooling circuit 12 is below a predetermined threshold value.

(15) The cooling system 10 according to FIG. 2 differs from the cooling system 10 of FIG. 1 in that the accumulator 30 is designed in the form of a spherical accumulator. In a spherical accumulator 30 the fill level detection is easier and more reliable than in an accumulator 30 having another shape, in particular if the accumulator 30 is installed on board an aircraft and hence during flight is positioned in different orientations. Further, the second connection line 42 is no longer connected to the refrigerant inlet of the subcoolers 32a, 32b, but to the accumulator 30. Hence, the valve 44, under the control of the valve control unit 50, enables or disables the supply of refrigerant from the storage container 36 to the accumulator 30 as described above. 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.

(16) Upon start-up of any one of the cooling systems 10 depicted in FIGS. 1 and 2 the heat sinks 29a, 29b are started. Further, the fill level of refrigerant in the accumulator 30 is checked. In case the fill level of refrigerant in the accumulator 30 exceeds a predetermined threshold value, refrigerant is directed from the accumulator 30 to the storage container 36 by appropriately controlling the valves 38, 44. Thereafter, refrigerant is condensed in the condensers 22a, 22b. The liquid refrigerant thus produced is conveyed to the storage container 36. Finally, the evaporators 14a, 14b are supplied with refrigerant.

(17) For controlling the supply of refrigerant to the evaporators 14a, 14b there are different options. As a first option, upon start-up of the cooling system 10, all evaporators 14a, 14b 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 14a, 14b upon start-up of the cooling system 100 such that at first only selected ones of the evaporators 14a, 14b are supplied with cooling energy until a predetermined target temperature of the selected evaporators 14a, 14b supplied with cooling energy is reached. Only then also the remaining evaporators 14a, 14b 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 14a, 14b are simultaneously supplied with cooling energy. Hence, heat sinks 29a, 29b 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 29a, 29b and the temperature of the cooling energy consumers.

(18) Finally, it is also conceivable to control the supply of cooling energy to the evaporators 14a, 14b upon start-up of the cooling system 10 such that at first all evaporators 14a, 14b are simultaneously supplied with cooling energy until a predetermined intermediate temperature of the evaporators 14a, 14b is reached. Immediately after start-up of the cooling system 10 the temperature difference between the operating temperature of heat sinks 29a, 29b 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 14a, 14b the operating temperature of the heat sinks 29a, 29b may be reduced and further cooling energy may be supplied only to selected ones of the evaporators 14a, 14b until a predetermined target temperature of the selected evaporators 14a, 14b supplied with cooling energy is reached. Finally, the remaining evaporators 14a, 14b may be supplied with cooling energy until a predetermined target temperature is reached also for these evaporators 14a, 14b. 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 29a, 29b and the temperature of the cooling energy consumers.

(19) Upon shut-down of the cooling system 10 the supply of refrigerant to the evaporators 14a, 14b is ceased, the fans of the evaporators 14a, 14b, however, are still operated. Further, the condensers maintain in operation at high load. Thereafter, valve 28 opens such that the pressure in the cooling circuit 12 in the region of the evaporators 14a, 14b is reduced to a predefined level. Finally, valve 28 closes so as to separate a low pressure portion of the cooling circuit 12 from a high pressure portion.

(20) In the embodiments of a cooling system 10 described above, the accumulator 30 and the storage container 36 fulfill the double function of storing liquid refrigerant exiting the condensers 22a, 22b and, in addition thereto, of reducing the system pressure in the cooling circuit 12. The pressure reducing effect of the accumulator 30 and the storage container 36 results from the additional volume the accumulator 30 and the storage container 36 add to the volume of the cooling circuit 12 and becomes more and more significant, as the volume of the accumulator 30 and the storage container 36 increases. The importance of the pressure reduction function of the accumulator 30 and the storage container 36 increases as the operating temperature of the cooling system 10 and hence the pressure in the cooling circuit 12 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.sub.2.

(21) Basically the cooling system 10 may comprise both, the accumulator 30 and the storage container 36 as described above, and both components may serve to store liquid refrigerant exiting the condensers 22a, 22b and to reduce the system pressure in the cooling circuit 12. It is, however, also conceivable to equip the cooling system 10 with only the acculumator 30 or only the storage container 36. The accumulator 30 or the storage container 36 which is provided in such a cooling system 10 then again fulfills the double function of storing liquid refrigerant exiting the condensers 22a, 22b and of reducing the system pressure in the cooling circuit 12. Finally, a configuration of the cooling system 10 is conceivable, wherein the accumulator 30 serves to collect and to store liquid refrigerant, whereas the storage container 36, due to its additional volume, serves to reduce the system pressure.

(22) In case the functions “storing liquid refrigerant” and “reducing system pressure” in the cooling system 10 are provided by two separate components, these components may be installed at different positions within the cooling circuit 12, 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 container 36 then preferably is installed in a high pressure portion of the cooling circuit 12 in order to reliably prevent the pressure in the high pressure portion of the cooling circuit 12 from exceeding a predetermined maximum value.

(23) Further, in case the storage container 36 merely serves to control the pressure in the cooling system 10, it is no longer necessary to provide for a direct fluid connection between the accumulator 30 and the storage container 36. Instead, the storage container 36 may be connected to the cooling circuit 12 via only a single line branching off from the cooling circuit 12, for example, upstream of one of the condensers 22a, 22b and downstream of the evaporators 14a, 14b. The line connecting the storage container 36 to the cooling circuit 12 preferably is connected to the storage container 36 at the geodetic lowest point of storage container 36. This configuration ensures that the storage container 36 is supplied only with gaseous refrigerant which is discharged from the cooling circuit 12 due to the pressure in the cooling circuit 12 exceeding a predetermined value. Of course, if desired, two storage containers 36 may be provided, in the cooling system 10, wherein a first storage container 36 may be connected to the cooling circuit 12 via a line branching off from the cooling circuit 12 upstream of the first condenser 22a and downstream of the evaporators 14a, 14b, and wherein a second storage container 36 may be connected to the cooling circuit 12 via a line branching off from the cooling circuit 12 upstream of the second condenser 22b and downstream of the evaporators 14a, 14b.