CO2 refrigeration system

Abstract

A CO.sub.2 refrigeration system (1) includes: a compressor (3), a gas cooler (5), a temperature sensor (17) and an electronic control system (13), the electronic control system including a processor device (15) arranged to control operation of the compressor (3) according to input signals received from the temperature sensor (17), wherein the temperature sensor (17) is positioned to read an output temperature of the gas cooler. A method for controlling a compressor (3) in a CO.sub.2 refrigeration system (1) is also disclosed.

Claims

1. A CO.sub.2 refrigeration system, including: a compressor, a gas cooler, a temperature sensor, and an electronic control system, the electronic control system including a processor device arranged to control operation of the compressor according to input signals received from the temperature sensor, wherein: (a) the temperature sensor is positioned to read an output temperature of the gas cooler, (b) the processor device is arranged to compare the measured temperature of the gas cooler output with a lower threshold temperature value, and (c) in a condition where the processor device determines that the measured temperature is less than or equal to the lower threshold temperature value, the processor device is arranged to initiate an extended rest period for the compressor, and (d) the processor device is arranged to activate the compressor device when the extended rest period has ended.

2. The refrigeration system according to claim 1, wherein the temperature sensor is positioned to measure the temperature of the CO.sub.2 refrigerant directly as it exits the gas cooler.

3. The refrigeration system according to claim 1, wherein the temperature sensor is mounted on at least one of a gas cooler wall and an adjacent conduit.

4. The refrigeration system according to claim 1, including an upper threshold temperature value stored in memory in the processor device, wherein the processor device is arranged to compare the measured temperature of the gas cooler output with the upper threshold temperature value and wherein in a condition where the processor device determines that the measured temperature is greater than or equal to the upper threshold temperature value, the processor device is arranged to deactivate the compressor.

5. The refrigeration system according to claim 4, wherein the processor device is arranged to generate a gas cooler alarm signal each time the measured temperature is determined to be greater than or equal to the upper threshold temperature value and wherein the processor device is arranged to shut down the refrigeration system if the number of gas cooler alarms exceeds a predetermined value within a predetermined time period.

6. The refrigeration system according to claim 1, wherein the extended rest period is a fixed time period.

7. The refrigeration system according to claim 6, wherein the extended rest period lasts for at least 5 minutes.

8. The refrigeration system according to claim 6, wherein the extended rest period lasts for at least 8 minutes.

9. The refrigeration system according to claim 6, wherein the extended rest period lasts for at least 10 minutes.

10. The refrigeration system according to claim 1, the system further including a clock and the extended rest period is timed by the clock.

11. The refrigeration system according to claim 1, wherein the lower threshold temperature value is determined by subtracting an offset temperature value from the lower threshold temperature value.

12. The refrigeration system according to claim 11, wherein the offset temperature value is at least 5 C.

13. The refrigeration system according to claim 11, wherein the offset temperature value is at least 8 C.

14. The refrigeration system according to claim 11, wherein the offset temperature value is at least 10 C.

15. The refrigeration system according to claim 11, wherein the offset temperature value is at least 12 C.

16. The refrigeration system according to claim 1, wherein the temperature sensor is connected to an auxiliary input of the processor device.

17. The refrigeration system according to claim 1, the system further including a pressure sensitive device.

18. The refrigeration system according to claim 1, the system further including a rupturing device that is arranged to rupture when the operating pressure within the refrigeration system reaches a rupture pressure, wherein the processor device is arranged to control operation of the compressor to maintain the refrigeration system operating pressure at a value that is less than the rupture pressure.

19. The refrigeration system according to claim 1, wherein the processor device includes an interface that is arranged to enable the user to set at least one of the following parameters: the upper threshold temperature value, the lower threshold temperature value, and the length of the extended rest period for the compressor.

20. A method for controlling a compressor in a CO.sub.2 refrigeration system, said CO.sub.2 refrigeration system having a compressor, a gas cooler, a temperature sensor and an electronic control system including a processor device, wherein the method comprises: (a) measuring CO.sub.2 refrigerant temperature at the output of the gas cooler with the temperature sensor and using the processor device to control operation of the compressor according to input signals received by the processor device from the temperature sensor; (b) comparing the measured temperature of the gas cooler output with a lower threshold temperature value stored in memory in the processor device; (c) automatically initiating an extended rest period for the compressor when the processor device determines that the measured temperature is less than or equal to the lower threshold temperature value; and (d) activating the compressor when the extended rest period has ended.

21. The method according to claim 20, the method further including comparing the measured temperature of the gas cooler output with an upper threshold temperature value stored in a memory means and automatically deactivating the compressor when the processor device determines that the measured temperature is greater than or equal to the upper threshold temperature value.

22. The method according to claim 21, the method further including the processor device generating a gas cooler alarm signal each time the measured temperature is determined to be greater than or equal to the upper threshold temperature value.

23. The method according to claim 22, the method further including the processor device shutting down the refrigeration system if the number of gas cooler alarms exceeds a predetermined value within a predetermined time period.

24. The method according to claim 20, wherein the extended period is a fixed period of time.

25. The method according to claim 24, the wherein the extended rest period lasts for at least 3 minutes.

26. The method according to claim 20, the method further including calculating the lower threshold temperature value by subtracting an offset temperature value from the lower threshold temperature value.

27. The method according to claim 26, wherein the offset temperature value is at least 8 C.

28. The method according to claim 26, wherein the offset temperature value is at least 10 C.

29. The method according to claim 26, wherein the offset temperature value is at least 12 C.

30. The method according to claim 20, wherein the refrigeration system further includes a rupturing device that is arranged to rupture at when the operating pressure within the refrigeration system reaches a rupture pressure, wherein the processor device is arranged to control operation of the compressor to maintain the refrigeration system operating pressure at a value that is less than the rupture pressure.

31. The method according to claim 20, wherein the extended rest period lasts for at least 5 minutes.

32. The method according to claim 20, wherein the extended rest period lasts for at least 8 minutes.

33. The method according to claim 20, wherein the extended rest period lasts for at least 10 minutes.

Description

(1) An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

(2) FIG. 1 is an electrical circuit diagram for a prior art CO.sub.2 refrigeration system;

(3) FIG. 2 is a diagrammatic view of a CO.sub.2 refrigeration system in accordance with a first embodiment of the invention;

(4) FIG. 3 is a wiring diagram for the CO.sub.2 refrigeration system of FIG. 2;

(5) FIGS. 4 and 5 are graphs showing the relationship between pressure and the output temperature of a gas cooler, with varying amounts of gas cooler blockage;

(6) FIGS. 6 and 7 are graphs showing the relationship between pressure and the output temperature of a gas cooler, with varying ambient temperature;

(7) FIG. 8 is a flow diagram of a digital gas cooler alarm process for a programmable microcontroller, that is used to control operation of the first embodiment of the invention;

(8) FIG. 9 is a flow diagram of an analogue gas cooler alarm process for the programmable microcontroller, that is used to control operation of the first embodiment of the invention; and

(9) FIG. 10 is a flow diagram of a compressor reset time process.

(10) FIGS. 2 and 3 show a first embodiment of a CO.sub.2 refrigeration system 1 in accordance with the invention, in diagrammatic form. The refrigeration system 1 includes a compressor 3, gas cooler 5, heat exchanger 7, expansion valve 9 and evaporator 11, connected together in a refrigeration circuit, and a control system 13.

(11) The control system 13 includes a microcontroller 15 and a temperature sensor 17. The microcontroller 15 controls operation of the compressor 3, and optionally controls operation of at least one of the following components: an evaporator fan 19; a condenser fan 21, and system lights 23. Optionally, the microcontroller 15 may receive inputs from other parts of the refrigeration system such as a microRMD 25; an appliance sensor 27 such as a thermistor for measuring temperature in a refrigerator cooling compartment; and a door opening switch 29.

(12) The microcontroller 15 controls operation of the compressor according to inputs received from the appliance sensor 27, for example to maintain the cooling compartment within a desired temperature range.

(13) The temperature sensor 17 is electrically connected to an auxiliary input 33 of the microcontroller 15. The microcontroller 15 uses input signals received from the temperature sensor 17 to control operation of the compressor 3 to ensure that the refrigeration system operates within predetermined operating conditions, for example conditions that are considered to be safe for the application.

(14) The temperature sensor 17 is physically located such that it measures the temperature of the CO.sub.2 refrigerant T.sub.GC as it exits the gas cooler 5. The inventors have discovered that there is a relationship between the temperature of the CO.sub.2 refrigerant as it exits the gas cooler 5 and the pressure in the refrigeration system 1. This is illustrated in the graphs shown in FIGS. 4 to 7.

(15) FIG. 4 shows the relationship between the system discharge pressure (discharging from the compressor 3) and gas cooler output temperature for a Sanden Intercool gas cooler, at constant ambient temperature, as the percentage of blockage in the gas cooler increases. The refrigeration system used a 0.27 Kg charge of CO.sub.2. The inventors discovered that, as the gas cooler becomes increasingly blocked (thereby simulating a possible system failure), the temperature at the output of the gas cooler substantially tracks discharge pressure. That is, there is a substantially proportional relationship between the gas cooler output temperature and the refrigeration system pressure with increasing blockage of the gas cooler.

(16) FIG. 5 is a similar graph to FIG. 4, except that a Sanden Corporation gas cooler is used, together with a 0.28 Kg charge of CO.sub.2. The graph shows that the relationship holds true for different types of gas coolers.

(17) FIGS. 6 and 7 show that the temperature-pressure relationship holds for the Sanden Intercool and Sanden Corporation gas coolers, respectively, when the ambient temperature varies.

(18) The inventors have also found that the relationship between pressure and the output temperature of a gas cooler holds true, with varying ambient temperature, with a fixed amount of gas cooler blockage, for the Sanden Intercool and Sanden Corporation gas coolers.

(19) Thus the inventors have discovered that measuring the gas cooler output temperature T.sub.GC in the present invention can be used to indicate the pressure in the refrigeration system 1 in a reliable manner.

(20) The microcontroller 15 uses the signals received from the temperature sensor 17, which are indicative of the output temperature of the gas cooler T.sub.GC, to determine when to switch the compressor 3 on/off in order to maintain the pressure within the refrigeration system 1 within normal operating conditions, in a manner that prevents the compressor 3 from oscillating the refrigeration system 1. The microprocessor 15 is programmed with an upper temperature value T.sub.U and an offset temperature value X. A lower temperature value T.sub.L is determined by calculating T.sub.UX. Typically the value used for T.sub.U is in the range 40 C. to 60 C. Typically the value for X is in the range 3 C. to 30 C. For example, T.sub.U may be set at 50 C. and X may be set at 10 C. Of course it will be appreciated by skilled person that the values for the upper threshold temperature value T.sub.U and the offset temperature value X will depend on the specific application. An OEM manufacturer can determine the values according to its needs.

(21) The microprocessor 15 may be arranged such that at least one of T.sub.U and X is fixed (i.e. cannot be changed by the user after the microprocessor has been programmed). The microprocessor 15 may be arranged such that at least one of Tu and X is programmable by a user, for example via a user interface.

(22) The control logic for the microprocessor 15 is shown in the flow diagrams in FIGS. 8 to 10. As a safety check, the microprocessor 15 initially determines if it is receiving signals from the temperature sensor 17. If not, then the compressor 3 is shut down (see FIG. 9).

(23) When the temperature sensor 17 is operating correctly, the microprocessor 15 determines from the signals received from the temperature sensor 17 whether the output temperature of the gas cooler T.sub.GC is greater than or equal to the upper temperature value T.sub.U, by comparing T.sub.GC with the stored value for T.sub.U. When T.sub.GC is greater than or equal to T.sub.U the microprocessor 15 determines that the pressure within the refrigeration system 1 is at its maximum acceptable value and the microprocessor 15 cuts power to the compressor 3 by opening switch 1 (see FIG. 3), and signals a T.sub.U alarm (see FIG. 9). When the compressor 3 is switched off, the pressure within the refrigeration system 1, and hence the output temperature of the gas cooler T.sub.GC, begins to fall. Thus, there is a period during which the compressor 3 is switched off.

(24) When the microprocessor 15 determines from the signals received from the temperature sensor 17 that the output temperature of the gas cooler T.sub.GC has cooled by X C. to a temperature that is less than or equal to the lower temperature value T.sub.L, the microprocessor 15 resets the alarm and then initiates an extended rest time Y for the compressor 3 (see FIG. 8), for example monitored by reference to its internal clock, before switching the compressor 3 back on again. Thus, the microprocessor 15 is programmed to apply the extended rest time Y, in addition to the variable period of time that it takes T.sub.GC to cool by X C., in order to delay the operation of the compressor 3. The extended rest time Y is preferably fixed for the system. Typically Y is in the range 1 to 20 minutes although Y may be selected to suit the particular refrigeration system.

(25) The inventors have found that by delaying operation of the compressor 3 by the extended rest time Y, the system is prevented from oscillating since more time is provided to enable system pressures to equalise.

(26) If the number of gas cooler alarms exceeds a predetermined value within a predetermined time period, then the microprocessor 15 is programmed to shut down the refrigeration system 1 and to issue an error signal.

(27) Optionally, the refrigeration system 1 may include a pressure relief switch 31 located in the live input line 35 to the compressor 3 (see FIG. 3). Pressure relief switches are known in the art and any suitable conventional switch may be used.

(28) The pressure relief switch 31 may be connected to the microprocessor 15, for example an auxiliary input thereof, and the microprocessor 15 may be arranged to monitor the operational status of the pressure relief switch 31 and to control operation of the compressor 3 according to the signals received form the pressure relief switch.

(29) It will be apparent to the skilled person that modifications may be made to the above embodiment that falls within the scope of the invention. For example, the embodiment may include a bursting disc. In such an embodiment, the pressure in the refrigeration systems 1 may be controlled by the microprocessor 15 in order keep the pressure below the bursting disc rupture pressure, thereby preventing the bursting disc from rupturing and improving the safety of the systems.

(30) The refrigeration systems 1 may include high pressure pipe work, which is designed to withstand the highest pressure that can be generated by the system. This improves the safety of the systems.

(31) The microprocessor may be programmed such that, when the microprocessor is powered up, the compressor rest time must expire before allowing the compressor to restart. The microprocessor may be arranged to apply the extended compressor rest time rather than a standard rest time when the microprocessor is rebooted.

(32) The microprocessor may include a user interface to enable a user to: set parameterssuch as the maximum number of alarms, T.sub.U, T.sub.L, X, Y; cancel alarms; cancel error messages; and invert an input when in digital mode.

(33) The microprocessor may be arranged such that T.sub.U and T.sub.L are programmed, rather than specifying X and calculating T.sub.L on the basis of T.sub.UX. In this instance, T.sub.L is typically in the range 30 C. to 50 C.

(34) It is envisaged that the invention may be applicable to refrigeration systems that use a different refrigerant to CO.sub.2.