Method for sensing the liquid level in a compressor

• US 20080250798A1
• Filed: 04/11/2007
• Published: 10/16/2008
• Est. Priority Date: 04/11/2007
• Status: Active Grant

• Current Assignee: Trane International Inc. (Trane Technologies plc)
• Original Assignee: American Standard Companies Incorporated (Ideal Standard International Topco SCA)
• Inventors: Okoren, Ronald W., Brown, Jerry E., Klingemann, Steven K., Nelson, Charles E., VanderZee, Joel C., DeGroot, Jeffrey J.
• Granted Patent: US 7,874,724 B2
• Time in Patent Office: Days
• US Class Current: 62/129
• CPC Class Codes: F04B 39/02 Lubrication of machines or engines in general F01M
• CPC Class Codes: F04B 39/0207 with lubrication control systems
• CPC Class Codes: G01F 23/247 for discrete levels
• CPC Class Codes: G01K 1/16 Special arrangements for conducting heat from the object to the sensitive element
• CPC Class Codes: G01K 3/10 in respect of time, e.g. reacting only to a quick change of temperature

Abstract

Two vertically offset thermistors for sensing a fluid such as oil and refrigerant in a compressor shell are monitored by a method that takes into account rapidly changing conditions within the shell. The system can determine the fluid'"'"'s sump temperature, high/low liquid levels, and can determine whether the thermistors are sensing the fluid as a liquid, gas, or a mixture of the two, such as a foam or mist of liquid and gas. For greater accuracy, thermistor readings can be dithered and filtered to provide temperature or voltage values having more significant digits than the readings originally processed through a limited-bit A/D converter. For faster response, limited microprocessor time is conserved by sampling thermistor readings at strategic periods that enable the microprocessor to identify certain conditions and temperatures via simple delta-temperature ratios and undemanding equations rather than resorting to exponential functions or lookup tables to determine time constants.

Claims

1. A method for sensing a fluid in a compressor, wherein the fluid can exist in a plurality of states including at least one of a liquid state, a gaseous state, and a gas/liquid mixture, the method comprising:

positioning a thermistor in heat transfer relationship with the fluid;

electrically heating the thermistor to a heated temperature that is higher than a fluid temperature of the fluid;

after electrically heating the thermistor, allowing the thermistor to cool toward the fluid temperature, wherein the thermistor cools at a cooling rate; and

determining the fluid temperature based on the cooling rate.

2. The method of claim 1, wherein the fluid temperature is determined prior to the thermistor actually reaching the fluid temperature.

3. The method of claim 1, further comprising determining in which state the fluid exists based on the cooling rate.

4. The method of claim 3 including the further step of establishing a delta temperature ratio as a function of the cooling rate.

5. The method of claim 4 wherein a delta temperature ratio below about 1.0 indicates a liquid state, a delta temperature ratio above about 1.2 indicates a gaseous state, and a delta temperature ratio between about 1.0 and 1.2 indicates a foamy or misty state.

6. The method of claim 1, further comprising cyclically electrically heating and thermally cooling the thermistor so that the thermistor experiences a plurality of temperatures during a plurality of heating and cooling cycles.

7. The method of claim 6, further comprising determining in which state the fluid exists based on the plurality of temperatures during the plurality of heating and cooling cycles.

8. The method of claim 7 including the further step of establishing a delta temperature ratio as a function of the cooling rate.

9. The method of claim 5 wherein a delta temperature ratio below about 1.0 indicates a liquid state, a delta temperature ratio above about 1.2 indicates a gaseous state, and a delta temperature ratio between about 1.0 and 1.2 indicates a foamy or misty state.

10. The method of claim 8, further comprising applying a reduced voltage to the thermistor, thereby allowing the thermistor to cool toward the fluid temperature.

11. The method of claim 10, wherein the step of determining the fluid temperature is accomplished independent of any calculated time constant, exponential function, or exponential look up table.

12. The method of claim 11, wherein the thermistor is a first thermistor and further comprising shielding the first thermistor within a tubular sheath disposed within a hermetically sealed shell of the compressor.

13. The method of claim 1, further comprising applying a reduced voltage to the thermistor, thereby allowing the thermistor to cool toward the fluid temperature.

14. The method of claim 1, wherein the step of determining the fluid temperature is accomplished independent of any calculated time constant, exponential function, or exponential look up table.

15. The method of claim 1, wherein the thermistor is a first thermistor cools at a first cooling rate and further comprising shielding the first thermistor within a tubular sheath disposed within a hermetically sealed shell of the compressor.

16. The method of claim 15 including:

providing a second shielded thermistor within the tubular sheath at an elevation or location different from the first thermistor;

positioning the second thermistor in heat transfer relationship with the fluid;

electrically heating the second thermistor to a heated temperature that is higher than a fluid temperature of the fluid;

after electrically heating the second thermistor, allowing the second thermistor to cool toward the fluid temperature, wherein the second thermistor cools at a second cooling rate; and

determining the fluid temperature based on the second cooling rate.

17. A method for sensing a fluid in a compressor, wherein the fluid can exist in a plurality of states including at least one of a liquid state, a gaseous state, and a gas/liquid mixture, the method comprising:

cyclically electrically heating and thermally cooling a thermistor that is in heat transfer relationship with the fluid so that the thermistor experiences a plurality of temperatures during a plurality of heating and cooling cycles; and

based on the plurality of temperatures during the plurality of heating and cooling cycles, determining in which state the fluid exists.

18. The method of claim 17, further comprising:

providing a signal that varies with a thermistor temperature of the thermistor; and

within a single heating and cooling cycle, sampling the signal at a first point in time, a second point in time, and a third point in time, wherein a first period extending from the first point in time to the second point in time is shorter than a second period extending from the second point in time to the third point in time, and the step of determining in which state the fluid exists is further based on the signal at the first point in time, the second point in time, and the third point in time.

19. The method of claim 18, wherein the step of determining in which state the fluid exists is accomplished independent of any calculated time constant, exponential function, or exponential lookup table.

20. The method of claim 18, wherein the second period is substantially twice as long as the first period.

21. The method of claim 18, further comprising:

electrically heating the thermistor to a heated temperature that is higher than a fluid temperature of the fluid; and

after electrically heating the thermistor, reducing a voltage applied to the thermistor, thereby allowing the thermistor to cool toward the fluid temperature, wherein the thermistor cools at a cooling rate.

22. The method of claim 21 including the further step of establishing a delta temperature ratio as a function of a cooling rate.

23. The method of claim 22 wherein a delta temperature ratio below about 1.0 indicates a liquid state, a delta temperature ratio above about 1.2 indicates a gaseous state, and a delta temperature ratio between about 1.0 and 1.2 indicates a foamy or misty state.

24. The method of claim 21, further comprising determining the fluid temperature based on the cooling rate.

25. The method of claim 24, wherein the thermistor is a first thermistor cooling at a first cooling rate and further comprising shielding the first thermistor within a tubular sheath disposed within a hermetically sealed shell of the compressor.

26. The method of claim 25 including:

providing a second shielded thermistor within the tubular sheath at an elevation or location different from the first thermistor;

positioning the second thermistor in heat transfer relationship with the fluid;

electrically heating the second thermistor to a heated temperature that is higher than a fluid temperature of the fluid;

after electrically heating the second thermistor, allowing the second thermistor to cool toward the fluid temperature, wherein the second thermistor cools at a second cooling rate; and

determining the fluid temperature based on the second cooling rate.

27. A method for sensing a fluid in a compressor, wherein the fluid can exist in a plurality of states including at least one of a liquid state, a gaseous state, and a gas/liquid mixture, the method comprising:

positioning a thermistor in heat transfer relationship with the fluid, wherein the thermistor has a thermistor temperature that can vary;

providing a feedback signal that varies with the thermistor temperature;

adding a ripple signal to the feedback signal to create a dithered input;

calculating a dither-based temperature value based on the dithered input; and

based on the dither-based temperature value, determining at least one of the following;

a) a state of the fluid, and b) a temperature of the fluid.

28. The method of claim 27, further comprising determining the state of the fluid as being at least one of the liquid state, the gaseous state and the gas/liquid mixture.

29. The method of claim 28 including the further step of establishing a delta temperature ratio as a function of a cooling rate of the thermistor.

30. The method of claim 29 wherein a delta temperature ratio below about 1.0 indicates a liquid state, a delta temperature ratio above about 1.2 indicates a gaseous state, and a delta temperature ratio between about 1.0 and 1.2 indicates a foamy or misty state.

31. The method of claim 29, further comprising cyclically electrically heating and thermally cooling the thermistor so that the thermistor experiences a plurality of thermistor temperatures during a plurality of heating and cooling cycles.

32. The method of claim 27, further comprising:

repeatedly sampling the feedback signal;

calculating a plurality of dither-based temperature values;

calculating a delta-temperature ratio that factors in the plurality of dither-based temperature values; and

determining in which state the fluid exists based at least partially on the delta-temperature ratio.

33. The method of claim 32, wherein the thermistor is a first thermistor cooling at a first rate and further comprising shielding the first thermistor within a tubular sheath disposed within a hermetically sealed shell of the compressor.

34. The method of claim 33 including;

providing a second shielded thermistor within the tubular sheath at an elevation or location different from the first thermistor;

positioning the second thermistor in heat transfer relationship with the fluid;

electrically heating the second thermistor to a heated temperature that is higher than a fluid temperature of the fluid;

after electrically heating the second thermistor, allowing the second thermistor to cool toward the fluid temperature, wherein the second thermistor cools at a second cooling rate; and

determining the fluid temperature based on the second cooling rate.