Refrigeration system

Abstract

A refrigeration system includes a main refrigeration circuit for holding refrigerant fluid, the main refrigeration circuit including: a compression device 12, a heat rejecting heat exchanger 14, an expansion device 18 and a heat absorbing heat exchanger 16. In addition, the refrigeration system includes a buffer tank 20 attached to the main refrigeration circuit, with valves 22, 24 for controlling flow of refrigerant fluid between the main refrigeration circuit and the buffer tank 20. The refrigeration system is arranged such that the valves 22, 24 are controlled to transfer refrigerant fluid between the main refrigeration circuit and the buffer tank 20 based on a measure of sub-cooling in the main refrigeration circuit.

Claims

1. A refrigeration system comprising: a main refrigeration circuit for holding refrigerant fluid, the main refrigeration circuit including: a compression device, a heat rejecting heat exchanger, an expansion device and a heat absorbing heat exchanger; a sub-cooling sensor for obtaining the measure of sub-cooling; wherein the refrigeration system includes a buffer tank attached to the main refrigeration circuit, with valves for controlling flow of refrigerant fluid between the main refrigeration circuit and the buffer tank; a controller configured to control the valves; and wherein the refrigeration system is arranged such that the valves are controlled by the controller to transfer refrigerant fluid between the main refrigeration circuit and the buffer tank based on a measure of sub-cooling in the main refrigeration circuit; wherein the valves comprise a first valve for controlling flow to or from a higher pressure point on the refrigerant circuit, and a second valve for controlling flow to or from a lower pressure point on the refrigerant circuit; wherein the second valve is a valve with a controllable degree of opening and the controller is configured to vary the degree of opening whilst the refrigerant fluid is being transferred from the buffer tank to the main refrigeration circuit; wherein the controller is configured to control the degree of opening of the second valve is used in order to control the amount of liquid refrigerant fluid at an inlet of the compression device such as to avoid any liquid refrigerant that may damage the compression device.

2. A refrigeration system as claimed in claim 1, wherein the buffer tank is connected to the main refrigeration circuit in parallel with the expansion device, with fluid connections to a higher pressure point prior to expansion, and a lower pressure point after expansion.

3. A refrigeration system as claimed in claim 1, wherein the sub-cooling sensor is located on the main refrigeration circuit after the heat rejecting heat exchanger and before the expansion device.

4. A refrigeration system as claimed in claim 1, comprising a controller for controlling the valves, wherein the controller is configured such that when the measure of sub-cooling indicates there is excessive sub-cooling then refrigerant fluid is directed into the buffer tank from the main refrigeration circuit, and when there is insufficient sub-cooling then refrigerant fluid is emptied from the buffer tank into the main refrigeration circuit to thereby refill the main refrigeration circuit.

5. A refrigeration system as claimed in claim 1, wherein the refrigeration system is arranged such that when it is desired to decrease charge levels in the main refrigeration circuit then the first valve is opened to fill the buffer tank from the higher pressure point, and when it is desired to increase charge levels in the main refrigeration circuit then the second valve is opened to empty the buffer tank to the lower pressure point.

6. A refrigeration system as claimed in claim 1, wherein the expansion device is arranged to provide a controllable degree of expansion, and the refrigeration system is arranged to control the degree of opening of the expansion valve in reaction to changes in the behaviour of the refrigerant circuit.

7. A method for operating a refrigeration system as claimed in claim 1, the method comprising controlling the valves to transfer refrigerant fluid between the main refrigeration circuit and the buffer tank based on a measure of sub-cooling in the main refrigeration circuit.

8. A method as claimed in claim 7, wherein controlling of the valves is done to vary the refrigerant charge level of the main refrigeration circuit in order to allow for one or more of: increased efficiency during changes in operating conditions, an enlarged operating envelope of the refrigerant system, and/or adjustments as refrigerant charge is depleted over time.

9. A method as claimed in claim 7, including controlling the valves for directing refrigerant fluid into the buffer tank from the main refrigeration circuit when there is excessive sub-cooling, and for directing refrigerant fluid from the buffer tank into the main refrigeration circuit when there is insufficient sub-cooling.

10. A method as claimed in claim 7, including: when there is over-sub-cooling, opening a first valve with a second valve being closed; when there is insufficient sub-cooling, opening the second valve with the first valve being closed; and when it is required to keep the refrigerant charge level of the main circuit unchanged, keeping both the first valve and second valve closed.

11. A method as claimed in claim 7, including varying the degree of opening of the respective valve whilst the refrigerant fluid is being transferred from the buffer tank to the main refrigeration circuit in order to control the amount of liquid refrigerant fluid at an inlet of the compression device.

12. A refrigeration system as claimed in claim 1, wherein the first valve is connected to the main refrigeration circuit between the heat rejecting heat exchanger and the expansion device and the second valve is connected to the main refrigeration circuit between the expansion device and the heat absorbing heat exchanger.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Certain preferred embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which:

(2) FIG. 1 is a schematic diagram for a refrigeration system using a main refrigeration circuit and a buffer tank connected to the main refrigeration circuit; and

(3) FIG. 2 shows example plots of efficiency against charge level for various operating conditions.

DETAILED DESCRIPTION

(4) As seen in FIG. 1, a refrigeration system includes a compression device 12, a heat rejecting heat exchanger 14, an expansion device 18 and a heat absorbing heat exchanger 16 that together form a main refrigeration circuit. The main refrigeration circuit contains a refrigerant fluid and circulation of the refrigerant fluid via the compression device 12 enables the refrigeration system to utilise a refrigeration cycle (or heat pump cycle) to satisfy a cooling (or heating) load. In this example the compression device 12 is a compressor 12 for compression of gaseous refrigerant fluid, the heat rejecting heat exchanger 14 is a condenser for at least partially condensing the refrigerant fluid, the expansion device 18 is an expansion valve for expanding the refrigerant fluid, and the heat absorbing heat exchanger 16 is an evaporator for at least partially evaporating the refrigerant fluid. The refrigeration system may advantageously be arranged so that the fluid is fully condensed at the condenser 14, and fully evaporator at the evaporator 16. In many cases it is beneficial to avoid the presence of liquid at the inlet to the compressor 12.

(5) The example refrigeration system further includes a buffer tank 20 attached to the main refrigerant circuit in parallel with the expansion device 18. The buffer tank 20 provides a receiver/reservoir for the refrigerant fluid and is connected to the main refrigeration circuit via a first valve 22 for controlling flow to or from a higher pressure point on the refrigerant circuit and a second valve 24 for controlling flow to or from a lower pressure point on the refrigerant circuit. As shown in FIG. 1, in this example the buffer tank 20 is connected to the main refrigeration circuit in parallel with the expansion device 18, with fluid connections via the first valve 22 to a higher pressure point prior to expansion, and via the second valve 24 to a lower pressure point after expansion.

(6) The valves 22, 24 are controlled by a controller 26 in order to control the flow of refrigerant fluid between the main refrigerant circuit and the buffer tank 20. The controller 26 can also be used for control of other elements of the refrigeration system, such as the compressor 12. The valves 20, 22 are controlled to transfer refrigerant fluid between the main refrigerant circuit and the buffer tank 20 based on a measure of sub-cooling obtained from a sub-cooling sensor 28 in the main refrigerant circuit. In this example the sub-cooling sensor 28 is placed on a refrigerant fluid pathway between the heat rejecting heat exchanger (condenser) 14 and the expansion device (expansion valve) 18.

(7) When a refrigerant pressure in the buffer tank 20 is lower or higher than a refrigerant pressure in the main refrigeration circuit then opening the respective valve 22, 24 will allow for transfer of refrigerant fluid to or from the buffer tank 20. For example, when it is desired to decrease charge levels in the main refrigeration circuit then the first valve 22 may be opened to fill the buffer tank 20 from the higher pressure point, and when it is desired to increase charge levels in the main refrigeration circuit then the second valve 24 may be opened to empty the buffer tank 20 to the lower pressure point. The first valve 22 is a solenoid valve 22, with an open state and a closed state. The second valve 24 is a valve 24 with a controllable degree of opening, such as a PMV 24, in order to allow the controller 26 to vary the degree of opening whilst the refrigerant fluid is being transferred from the buffer tank 20 to the main refrigeration circuit. The controller 26 can control the degree of opening of the second valve 24 in order to control the amount of liquid refrigerant fluid at an inlet of the compressor 12, such as to avoid any liquid refrigerant that may damage the compressor 12.

(8) The refrigerant charge level is varied using the valves 22, 24 in order to control the sub-cooling within the main refrigerant circuit, as assessed through a sub-cooling value obtained via the sub-cooling sensor 28. In case of over-sub-cooling, the controller opens the first valve 22 to reduce the refrigerant charge. Some refrigerant will be stored into the receiver/buffer tank 20. In case of a low sub-cooling value, the second valve 24 is opened so that the main refrigerant circuit will be refilled from the buffer tank 20. As noted above, the degree of opening of the second valve 24 can be controlled, such as via a PMV 24, in order to avoid liquid at the compressor suction inlet. The thresholds for opening and closing the first and second valves 22, 24 are based on a sub-cooling curve optimisation that allows for varying refrigerant charge depending on the condition and unit load. This gives various advantages as discussed above, including increased efficiency and the potential for an enlarged operating envelope. It also increases the available refrigerant charge and allows for corrections to maintain effective operation as the refrigeration charge levels decrease over time, i.e. during use of the refrigerant system. A further advantage of this is a reduced maintenance burden.

(9) It has been realised that a better control of optimal performance can be achieved by allowing for a varying refrigerant charge level controlled depending on the sub-cooling within the main refrigeration circuit. As is known, an optimal sub-cooling value will exist for a given refrigeration circuit, dependent on various factors including the specification of the compression device and the superheat value selected for the expansion device 18 (typically a fixed value). Operating at the optimal sub-cooling value will result in the maximum efficiency for the refrigeration circuit. It is possible to assess the refrigerant charge level required to achieve this optimum. FIG. 2 shows sample curves of efficiency against refrigerant charge level for three different operating conditions, such as operating conditions relating to differing internal or external temperature.

(10) In a typical refrigeration system, absent the buffer tank 20 proposed herein, the refrigerant charge level is optimised for one specific condition, such as by identifying a peak on the relevant curve of FIG. 2. However, this generally involves a compromise in which the charge level is set based on an estimate of the most likely condition, and/or by assessing a central condition within a range of operating conditions. In real world operation, the refrigeration system will operate at many different conditions, with different external temperatures and different loads during operation. For all those conditions that vary from the assumed operating condition, the performance of the refrigeration system is not optimised in relation to the refrigerant charge level. As a consequence, prior systems always involve a compromise, for example by deciding on a refrigerant charge of 6 kg. This would be ideal for Condition 3, but the refrigeration system would lose 10% efficiency for condition 1 and 5% for condition 2 versus the most optimum value for those conditions. With the use of a buffer tank 20 as discussed above, the controller 26 can adjust the refrigerant charge via use of the valves 22, 24 in order to fit the refrigerant charge level to the optimum values for different operating conditions. The controller 26 may use any suitable control system to vary the refrigerant charge level with the target of keeping the sub-cooling value at the sub-cooling sensor at the optimal level, which will hence then adjust the refrigerant charge levels to track the peak efficiency values shown an FIG. 2 as the operating conditions vary. As discussed above, the controller 26 may allow for some hysteresis with respect to threshold values using during control of the valves 22, 24. Other control systems may also be present.

(11) The refrigeration system may include other elements not shown in FIG. 1, such as an economiser line or other more complex additions to the refrigeration circuit, such as in order adapt a refrigeration cycle for particular requirements. The refrigeration system may operate for satisfying a cooling load, or it may be used as a heat pump to provide heating.