US20170299240A1

US20170299240A1 - Electronic expansion valve superheat recovery for a variable speed compressor system

Info

Publication number: US20170299240A1
Application number: US15/462,283
Authority: US
Inventors: Kevin Mercer
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2016-03-18
Filing date: 2017-03-17
Publication date: 2017-10-19

Abstract

A method of operating an electronic expansion valve of a heating, ventilation, air conditioning and refrigeration system includes detecting superheat of an evaporator of the heating, ventilation, air conditioning and refrigeration system and calculating a derivative of evaporator superheat with respect to time. The derivative of evaporator superheat with respect to time is compared to a selected derivative range, and the electronic expansion valve is closed at a rapid closure step increment higher than a normal closure step increment if the derivative is within the selected derivative range.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Application No. 62/310,304, filed Mar. 18, 2016, which is incorporated herein by reference in its entirety.

BACKGROUND

Embodiments relate generally to heating, ventilation, air conditioning and refrigeration (HVAC&R) systems, and more particularly to control of an electronic expansion valve for an HVAC&R system.
Typical HVAC&R operate on a vapor compression cycle and include a compressor, a condenser, an expansion valve and an evaporator through which a volume of refrigerant is circulated to produce a desired heating or cooling effect. To improve efficiency of such systems and to produce a more consistent and stable heating or cooling effect, some HVAC&R systems utilize variable speed compressors paired with an electronic expansion valve.
Compressors are lubricated by oil circulated through a compressor sump, with some such variable speed compressors being susceptible to high rates of oil discharge from the compressor sump and relatively low rates of oil return to the compressor sump under certain operating conditions.

SUMMARY

In one embodiment, a method of operating an electronic expansion valve of a heating, ventilation, air conditioning and refrigeration system includes detecting superheat of an evaporator of the heating, ventilation, air conditioning and refrigeration system and calculating a derivative of evaporator superheat with respect to time. The derivative of evaporator superheat with respect to time is compared to a selected derivative range, and the electronic expansion valve is closed at a rapid closure step increment higher than a normal closure step increment if the derivative is within the selected derivative range.
Additionally or alternatively, in this or other embodiments calculation of the derivative of evaporator sump superheat with respect to time is repeated at a selected time interval, comparison of the derivative of evaporator sump superheat with respect to time to the selected derivative range is repeated at the selected time interval, and the closure of the electronic expansion valve is maintained at the rapid closure step increment as long as the derivative is within the selected derivative range.
Additionally or alternatively, in this or other embodiments the selected time interval is in the range of about 2 seconds to 30 seconds.
Additionally or alternatively, in this or other embodiments the selected time interval is 5 seconds.
Additionally or alternatively, in this or other embodiments the electronic expansion valve is closed at the normal closure step increment slower than the rapid closure step increment if the derivative is outside of the selected derivative range.
Additionally or alternatively, in this or other embodiments the selected derivative range is between about 0.05 and 0.5 degrees Rankin/sec.
Additionally or alternatively, in this or other embodiments the rapid closure step increment is selected to minimize a time duration of low or constant evaporator sump superheat.
Additionally or alternatively, in this or other embodiments minimizing the time duration of low or constant evaporator sump superheat reduces oil depletion of a compressor sump of the heating, ventilation, air conditioning and refrigeration system.
In another embodiment, a heating, ventilation, air conditioning and refrigeration system includes a compressor, the compressor cooled via cooling fluid circulated therethrough from a compressor sump, a condenser, an expansion valve, an evaporator, a refrigerant pathway to fluidly connect the compressor, the condenser, the expansion valve and the evaporator, a volume of refrigerant circulating through the refrigerant pathway, and a controller operably connected to the evaporator and the expansion valve. The controller is configured to detect superheat of the evaporator, calculate a derivative of evaporator superheat with respect to time, compare the derivative of evaporator superheat with respect to time to a selected derivative range, and close the electronic expansion valve at a rapid closure step increment higher than a normal closure step increment if the derivative is within the selected derivative range.
Additionally or alternatively, in this or other embodiments the controller is configured to repeat calculation of the derivative of evaporator superheat with respect to time at a selected time interval, repeat comparison of the derivative of evaporator superheat with respect to time to the selected derivative range, and maintain the closure step increment of the electronic expansion valve at the rapid closure step increment as long as the derivative is within the selected derivative range.
Additionally or alternatively, in this or other embodiments the selected time interval is in the range of about 2 seconds to 30 seconds.
Additionally or alternatively, in this or other embodiments the selected time interval is 5 seconds.
Additionally or alternatively, in this or other embodiments the controller is configured to close the electronic expansion valve at the normal closure step increment slower than the rapid closure step increment if the derivative is outside of the selected derivative range.
Additionally or alternatively, in this or other embodiments the selected derivative range is between about 0.05 and 0.5 degrees Rankin/sec.
Additionally or alternatively, in this or other embodiments the rapid first rate is selected to minimize a time duration of low or constant compressor sump superheat.
Additionally or alternatively, in this or other embodiments minimizing the time duration of low or constant compressor sump superheat reduces oil depletion of the compressor sump.
Additionally or alternatively, in this or other embodiments the compressor is a variable speed compressor.
Additionally or alternatively, in this or other embodiments the expansion valve is an electronic expansion valve.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter is particularly pointed out and distinctly claimed at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic view of an embodiment of a heating, ventilation, air conditioning and refrigeration (HVAC&R) system;

FIG. 2 is a schematic view of an embodiment of a control method for an electronic expansion valve of the HVAC&R system; and

FIG. 3 is a sample plot of compressor sump superheat over time.

The detailed description explains embodiments, together with advantages and features, by way of example with reference to the drawings.

DETAILED DESCRIPTION

FIG. 1 depicts an embodiment of a heating, ventilation, air conditioning and refrigeration (HVAC&R) system 10. The HVAC&R system 10 is a vapor-compression system that includes a compressor 12 to compress a refrigerant from low to high pressure circulating through the HVAC&R system 10 in a refrigerant pathway 14. The HVAC&R system 10 further includes a condenser 16 an expansion valve 18 and an evaporator 20. The compressor 12 compresses a vapor refrigerant flow, and the refrigerant flow changes phase into liquid at a condenser 16 through thermal energy exchange with a condenser airflow 22, typically an ambient airflow flowed across the condenser 16 by a condenser fan 24. The condenser 16 is fluidly connected to the expansion valve 18. The expansion valve 18 is fluidly connected to the evaporator 20, where a compartment airflow 26 is cooled and the refrigerant flow is boiled through thermal energy exchange at the evaporator 20. In some embodiments, the compartment airflow 26 is urged across the evaporator 20 by one or more evaporator fans 28, and is urged to a compartment or space to be cooled (not shown). The vaporized refrigerant flow is then returned to a compressor inlet 30 of the compressor 12. The compressor 12 is cooled and the bearings lubricated by a flow of oil therethrough, circulated through the compressor 12 from a compressor sump 38.
In the embodiment of FIG. 1, the compressor 12 is a variable speed scroll or rotary, low or high-side compressor 12 and the expansion valve 18 is an electronic expansion valve (EXV) 18, with both components operably connected to a HVAC&R controller 32. The HVAC&R controller 32 utilizes sensor inputs and system feedback to direct a compressor 12 speed (possibly) and also to direct an open or closed position of the EXV 18. In particular, the EXV 18 is utilized to control a quantity of evaporator superheat at an evaporator exit 34. Thus the controller may utilize sensed temperature and/or pressure from one or more sensors 36 at the evaporator 20 to determine a current amount of evaporator superheat. The current amount of evaporator superheat is compared to a desired amount of evaporator superheat and a desired position of the EXV 18 is determined. The desired position is compared to a current position of the EXV 18, and a command to change the position of the EXV 18 from the current position to the desired position is initiated at the HVAC&R controller 32.
In some conditions, where high evaporator superheat is detected and communicated to the HVAC&R controller 32, the HVAC&R controller 32 commands opening of the EXV 18 to lower the evaporator superheat. In some conditions, where very low evaporator superheat is detected, or flooding of liquid refrigerant to the compressor 12 occurs, and is communicated to the HVAC&R controller 32, the HVAC&R controller 32 commands closure of the EXV 18 to raise the evaporator superheat to a selected set-point. Closure of the EXV 18 is done at a predetermined rate, and this closure of the EXV 18 is linked to oil depletion from the compressor sump 38, which negatively affects the lubrication of the bearings in compressor 12 and thus compressor 12 performance and service life. Further, oil depleted from the compressor sump 38 is often circulated through the refrigerant pathway 14 contaminating the refrigerant and may adversely affect HVAC&R system 10 performances. One cause of the oil depletion, but not all encompassing, from the compressor sump 38, is long durations of low compressor sump 38 superheat, in turn linked to slow closure of the EXV 18 under certain conditions.
To address the depletion of oil from the compressor sump 38, a schematic of an embodiment of an EXV 18 control methodology is illustrated in FIG. 2. Under this methodology, the HVAC&R controller 32 monitors evaporator superheat 34 over time at block 100. The HVAC&R controller 32 monitors a derivative of evaporator superheat 34 with respect to time (dSH/dt) at a selected time interval at block 102. In some embodiments, the time interval is between about 2 seconds and 30 seconds. In one embodiment, the time interval is 5 seconds. In block 104, dSH/dt is compared to a selected range. In some embodiments the selected range is 0.05 to 0.5 degrees Rankin/sec. If the derivative, dSH/dt is within the selected range, it is indicative of relatively low and possibly flooded compressor states based on evaporator superheat 34 reading. The EXV 18 has a normal open or close rate, or first closure step increment, that in some embodiments is 1 to 5 steps per time interval. In some embodiments, the time interval is between about 1 second and 15 seconds. To avoid dwell at the relatively low evaporator superheat 34, or flooded state, the HVAC&R controller 32 will command a rapid second closure step increment of the EXV 18 at block 106, faster than the first closure step increment. In some embodiments, this second closure step increment is between about 5 and 50 steps per time increment. In some embodiments, the second closure step increment is ten times the first closure step increment.
The calculation of dSH/dt at block 102 and comparison of dSH/dt to the selected range at block 104 is repeated at the selected time interval. If the value of dSH/dt falls outside of the selected range, at for example, greater than 0.5, dSH/dt is indicative of an inflection point, or an increase in compressor sump 38 superheat, and at block 108 the rate of closure of the EXV 18 is reduced to the normal first closure step increment, slower than the second closure step increment of the EXV 18.
A sample plot of compressor sump 38 superheat with respect to time is shown in FIG. 3. As shown, compressor sump 38 superheat is constant and low up to an inflection point 40. Up to this inflection point 40, the EXV 18 is commanded to close at the rapid first rate to minimize the time duration to reach the inflection point 40. Once the inflection point 40 is reached and compressor sump 38 superheat begins to increase, the EXV 18 is commanded to close at the slower or normal second rate to prevent overshoot of a desired EXV 18 position. This allows for accurate positioning of the EXV 18 while still minimizing the time duration of low compressor sump 38 superheat.
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

What is claimed is:

1. A method of operating an electronic expansion valve of a heating, ventilation, air conditioning and refrigeration system comprising:

detecting superheat of an evaporator of the heating, ventilation, air conditioning and refrigeration system;

calculating a derivative of evaporator superheat with respect to time;

comparing the derivative of evaporator superheat with respect to time to a selected derivative range; and

closing the electronic expansion valve at a rapid closure step increment higher than a normal closure step increment if the derivative is within the selected derivative range.

2. The method of claim 1, further comprising:

repeating calculation of the derivative of evaporator sump superheat with respect to time at a selected time interval;

repeating comparison of the derivative of evaporator sump superheat with respect to time to the selected derivative range; and

maintaining the closure of the electronic expansion valve at the rapid closure step increment as long as the derivative is within the selected derivative range.

3. The method of claim 2, wherein the selected time interval is in the range of about 2 seconds to 30 seconds.

4. The method of claim 3, wherein the selected time interval is 5 seconds.

5. The method of claim 1, further comprising closing the electronic expansion valve at the normal closure step increment slower than the rapid closure step increment if the derivative is outside of the selected derivative range.

6. The method of claim 1, wherein the selected derivative range is between about 0.05 and 0.5 degrees Rankin/sec.

7. The method of claim 1, wherein the rapid closure step increment is selected to minimize a time duration of low or constant evaporator sump superheat.

8. The method of claim 7, wherein minimizing the time duration of low or constant evaporator sump superheat reduces oil depletion of a compressor sump of the heating, ventilation, air conditioning and refrigeration system.

9. A heating, ventilation, air conditioning and refrigeration system, comprising:

a compressor, the compressor cooled via cooling fluid circulated therethrough from a compressor sump;

a condenser;

an expansion valve;

an evaporator;

a refrigerant pathway to fluidly connect the compressor, the condenser, the expansion valve and the evaporator, a volume of refrigerant circulating through the refrigerant pathway; and

a controller operably connected to the evaporator and the expansion valve configured to:

detect superheat of the evaporator;

calculate a derivative of evaporator superheat with respect to time;

compare the derivative of evaporator superheat with respect to time to a selected derivative range; and

close the electronic expansion valve at a rapid closure step increment higher than a normal closure step increment if the derivative is within the selected derivative range.

10. The heating, ventilation, air conditioning and refrigeration system of claim 9, wherein the controller is configured to:

Repeat calculation of the derivative of evaporator superheat with respect to time at a selected time interval;

repeat comparison of the derivative of evaporator superheat with respect to time to the selected derivative range; and

maintain the closure step increment of the electronic expansion valve at the rapid closure step increment as long as the derivative is within the selected derivative range.

11. The heating, ventilation, air conditioning and refrigeration system of claim 10, wherein the selected time interval is in the range of about 2 seconds to 30 seconds.

12. The heating, ventilation, air conditioning and refrigeration system of claim 11, wherein the selected time interval is 5 seconds.

13. The heating, ventilation, air conditioning and refrigeration system of claim 9, wherein the controller is configured to close the electronic expansion valve at the normal closure step increment slower than the rapid closure step increment if the derivative is outside of the selected derivative range.

14. The heating, ventilation, air conditioning and refrigeration system of claim 9, wherein the selected derivative range is between about 0.05 and 0.5 degrees Rankin/sec.

15. The heating, ventilation, air conditioning and refrigeration system of claim 9, wherein the rapid first rate is selected to minimize a time duration of low or constant compressor sump superheat.

16. The heating, ventilation, air conditioning and refrigeration system of claim 15, wherein minimizing the time duration of low or constant compressor sump superheat reduces oil depletion of the compressor sump.

17. The heating, ventilation, air conditioning and refrigeration system of claim 9, wherein the compressor is a variable speed compressor.

18. The heating, ventilation, air conditioning and refrigeration system of claim 9, wherein the expansion valve is an electronic expansion valve.