US20100011788A1

US20100011788A1 - Off-season start-ups to improve reliability of refrigerant system

Info

Publication number: US20100011788A1
Application number: US12/440,786
Authority: US
Inventors: Alexander Lifson; Michael F. Taras
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2006-09-12
Filing date: 2006-09-12
Publication date: 2010-01-21
Also published as: CN101512254A; WO2008033123A1; HK1135167A1; CN101512254B

Abstract

A vapor compression system includes a timer and a controller for periodically starting up the system during the off-season periods (primarily non-cooling months) in which the system would normally be shut down. This provides periodic lubrication to the compressor components and prevents severe flooded starts due to excessive accumulation of refrigerant in the compressor (compressor oil sump in particular) and other system components. Provision is also made to sequentially turn on and off system components such as the compressor, the evaporator fan and the condenser fan to enhance the system operation. The timing sequences are provided for the time intervals between the startups, as well as the off-season operation cycle times.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to vapor compression systems and, more particularly, to a method and apparatus for preventing severe flooded starts caused by the system operational seasonality.
A typical air conditioning system includes, in serial flow communication, a compressor, a condenser, an expansion device and an evaporator. The compressor compresses refrigerant and passes this high pressure refrigerant vapor to the condenser where it is desuperheated, condensed and typically subcooled, as a result of heat transfer interaction with a secondary fluid such as air or water. The liquid refrigerant then flows to the expansion device where it is expanded to a lower pressure and temperature forming a two-phase (liquid and vapor) refrigerant mixture at the expansion device exit, while a portion of the refrigerant is flashed to vapor. This vapor and liquid refrigerant mixture then flows to the evaporator, where heat is absorbed by the refrigerant while cooling another secondary fluid that is typically delivered to the space to be conditioned, with the resultant evaporated and typically superheated refrigerant vapor passing to the compressor to complete the cycle.
In seasonal climate zones, or in the geographic regions that have cooling and heating seasons typically requiring both cooling and heating components of HVAC (air conditioning, heating and ventilation) equipment, the air conditioning cooling systems are mostly used during the hot and/or humid summer months, but they are typically shut down during the winter seasons (and potentially during fall and summer months as well) for the prolonged time intervals measured in months. Since refrigerant tends to migrate to the coldest spot within the system, and since the compressor is normally located in the outdoor section of the unit, during the winter months, the liquid refrigerant accumulates in the outdoor components, including the condenser and compressor. Therefore over the prolonged periods when the unit has been shut down, the liquid refrigerant will fill the volumes of the outdoor system components. In particular, the lubricating oil in the compressor sump is diluted and mixed with this liquid refrigerant such that its lubrication characteristics are diminished. If the system is then started up in the summer or spring with the compressor sump and other compressor elements being flooded with the refrigerant, the liquid refrigerant will slug through the compressor and can result in compressor damage. Furthermore, after prolonged periods of time without the startups or even intermittent operation, all the residual lubricating oil which normally is collected on the compressor contact services during operation (i.e. bearings, scroll elements in the case of the scroll compressors, piston rings in the case of the reciprocal compressors, rotors in the case of the screw compressors, etc.) is no longer present and has been washed off by the refrigerant or completely drained from the surfaces. Such conditions will exacerbate the problem of potential compressor damage or its performance degradation.
As known in the art, the attempts have been made to isolate the condenser from the compressor by means of flow control devices such as solenoid or check valves to reduce the severity of the flooded starts. As also known, these flow control devices leak over time and there is still a significant volume of refrigerant trapped between the compressor and evaporator. Compressor crankcase heaters have also been used in the prior art to heat the compressor oil sump before the startup and evaporate at least some liquid refrigerant that has been accumulated in the sump since the last shutdown. Although this technique is helpful, it doesn't affect other components within the vapor compression system that are filled with the liquid refrigerant. Also crankcase heaters present their own reliability problems, and may fail after several years of operation. Crankcase heaters also add extra cost and reduce overall system efficiency. Therefore, it is desired to provide a reliable and inexpensive method to reduce or eliminate severe flooded starts caused by seasonal operational pattern of the air conditioning equipment.

SUMMARY OF THE INVENTION

Briefly, in accordance with one aspect of the invention, a timer is installed in communication with or integrated into the system control so as to periodically start the system during the off-season months to prevent the excessive migration of refrigerant to the compressor sump and thus eliminate severe flooded startups.
In according with another aspect of the invention, the environmental conditions are recorded to determine if sufficient amount of liquid refrigerant could accumulate within the compressor volume, and the compressor sump in particular, to execute an off-season startup procedure. Such environmental conditions may include (but are not limited to) the ambient and indoor temperatures.
In accordance with yet another aspect of the invention, vapor compression system components would be started in a predetermined sequence that is defined by environmental and operational parameters. For instance, at certain conditions, the compressor, the evaporator fan and the condenser fan may start simultaneously. On the other hand, typically when the ambient temperature falls below a certain threshold, the compressor and the evaporator fan would start first, followed by the delayed start of the condenser fan or some of the condenser fans. Such operational and environmental parameters may include, for instance, discharge pressure, suction pressure, ambient temperature and indoor temperature.
In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary vapor compression system with the present invention incorporated therein.

FIG. 2 is a flow chart showing the method in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A basic vapor compression system 10 normally includes a compressor 11, a condenser 12, an expansion device 13 and an evaporator 14 interconnected in serial refrigerant flow communication.
A refrigerant vapor from the evaporator 14 is delivered to the compressor 11 where it is compressed, and the compressed vapor then flows to the condenser 12 where it is desuperheated, condensed and typically subcooled by a secondary fluid such as ambient air. Then the liquid refrigerant passes to the expansion device 13 where it is expanded to a lower pressure and temperature to form a two-phase (liquid and vapor) mixture with a portion of the refrigerant being flashed to a vapor. A vapor and liquid refrigerant mixture than passes to the evaporator 14 where it is evaporated and typically superheated by another secondary fluid such as air to be delivered to a conditioned space, while cooling this secondary fluid. The refrigerant vapor then passes to the compressor 11 to complete the cycle. It should be noted that the basic air conditioning system 10 of FIG. 1 is exemplary and could include a number of different options and enhancement features. All these various system configurations are within the scope of the invention. Also, as known in the art, if a vapor compression system employs the refrigerant with a relatively low critical point such as CO₂, the condenser 12 becomes a single-phase gas cooler in the transcritical (rather than conventional subcritical) refrigerant cycle. These systems as well could equally benefit from the invention.
A condenser fan 16 circulates ambient air over the condenser 12 to provide heat transfer interaction with the refrigerant flowing within its passages (heat is transferred from the refrigerant to air), and an evaporator fan 17 circulates air, to be cooled and delivered to a conditioned space, over the evaporator 14 to provide heat transfer interaction with the evaporating refrigerant and to cool the air. The air conditioning system 10 shown in the FIG. 1 embodiment is a so-called air-to-air system, where one stream of air is cooled and delivered to a conditioned space while another stream of air (typically ambient air) is heated by the refrigerant. As known in the art, there are vapor compression systems, where water or glycol solutions are used as secondary fluids instead of air. In these systems, each of the fans 16 and 17 is replaced by a liquid pump to circulate these secondary fluids. These systems are also within the scope of the invention and can equally benefit from it.
As discussed hereinabove, the refrigerant in the system tends to migrate toward the coolest component (or components) within the vapor compression system. During so-called off-season or winter months, the liquid refrigerant will accumulate in the outdoor section of the unit, and typically within the condenser 12 and compressor 11. Therefore over the prolonged periods of time when the unit has been shut down, the liquid refrigerant fills the volumes of the outdoor system components. In particular, the lubricating oil in the compressor sump is diluted and mixed with this liquid refrigerant such that its lubrication characteristics are diminished. When the system is then started up in the summer or spring with the compressor sump and other compressor elements being severely flooded with the refrigerant, the liquid refrigerant will slug through the compressor and can result in compressor damage. Furthermore, after prolonged periods of time without startups or even intermittent operation, all the residual lubricating oil which normally is collected on the compressor contact services during operation (i.e. bearings, scroll elements in the case of the scroll compressors, piston rings in the case of the reciprocal compressors, rotors in the case of the screw compressors, etc.) is no longer present and has been washed off by the refrigerant or completely drained from the surfaces. Such conditions will exacerbate the problem of potential compressor damage or its performance degradation.
In order to address the above-described problem, a timer 18 has been added to a control 19. In operation, the timer 18 is started when the system is shut down. After a predetermined sufficiently long period of time during an off-season, the system is turned on and allowed to operate for a second predetermined relatively short period of time. This timed operation allows the compressor components to be lubricated as well as it allows refrigerant to circulate, redistribute and at least partially evaporate any liquid refrigerant accumulated within the compressor sump and other components throughout the system. After the expiration of the second predetermined period of time, the system is shut down and the timer 18 is reset for the first predetermined period of time after which the off-season startup cycle is repeated.
If the vapor compression system 10 is equipped with the sensors to sense environmental conditions and to communicate the sensed values to the system control 19, the off-season startup procedure can be improved. The sensed environmental conditions are monitored and recorded to determine if sufficient amount of liquid refrigerant could accumulate within the system components such as the compressor 11 (and the compressor sump in particular), condenser 12 and evaporator 14 to execute the off-season startup procedure. If it is determined that sufficient amount of liquid refrigerant could have been accumulated, the off-season startup procedure is executed. Otherwise, the timer is reset once again, with the time interval potentially adapted to the sensed environmental conditions (colder temperature would suggest shorter time intervals between the startups). Such environmental conditions may include (but are not limited to) the ambient temperature T_AMBsensed by a temperature sensor 21 and the indoor temperature T_INDOORsensed by a temperature sensor 22. The temperature sensors can be, for instance, of a thermistor or thermocouple type.
Further, instead of starting the compressor 11 and the fans 16 and 17 simultaneously, these components of the vapor compression system 10 could be started by the control 19 in a predetermined sequence defined by the environmental and operational parameters. Such operational and environmental parameters may include, for instance, the discharge pressure P_Dmeasured by a sensor 23, the suction pressure P_Smeasured by a sensor 24, the ambient temperature T_AMBmeasured by the sensor 21, the indoor temperature T_INDOORmeasured by the sensor 22 or a combination of thereof. The compressor heater 20, typically inserted into the oil sump or wrapped around the compressor shell at the oil sump location, could be switched on first, usually for a time period of a few hours, to boil off at least some of the refrigerant accumulated in the oil sump of the compressor 11 prior to the compressor startup. Then, for instance, if the ambient temperature falls below a predetermined threshold, the compressor 11 and the evaporator fan 17 are started first, while the discharge pressure P_Dis monitored by the sensor 23 and communicated to the control 19. In case the discharge pressure P_Dexceeds the upper limit, the condenser fan (or some of the condenser fans) 16 are turned on to move air over the condenser 12 and thus reduce this pressure. Further, if the discharge pressure P_Dmeasured by the sensor 23 and monitored by the control 19 falls below the lower limit, the condenser fan (or some of the condenser fans) 16 are turned off to maintain the discharge pressure P_Dbetween the upper and the lower limits, as desired. As known to a person ordinary skilled in the art, the time intervals between the oil sump heater 20 and compressor 11 startup and the compressor 11 and condenser fan 16 startup depends on a particular system configuration and refrigerant charge amount.
The time intervals between subsequent startups can be adjusted based on various factors such as vapor compression system configuration and schematic, amount of refrigerant charge, ambient temperature, ambient temperature swings, etc. Typically, the time between the subsequent startups (i.e. the first predetermined time as described above) will be in the range of three days to four weeks.
Similarly, the time of operation (i.e. the second predetermined period of time) can be adjusted based on the various factors as set forth above. Typically the running time of operation would be in the range of 2-15 minutes.
In addition to the factors discussed hereinabove, the selection of the appropriate time for the off-season unit startups can also take into consideration the occupancy schedule, such as to avoid periods of time when the building is occupied and execute the off-season startup procedure during the nighttime, on the weekends or holidays to cause minimum disruption and occupant discomfort. Referring now to FIG. 2, the methodology and the control logic of the present invention is shown in the flowchart format. In a block 21, the control determines whether the vapor compression system is sophisticated enough to have the provisions for sensing environmental and operating conditions indicative of a need of the off-season startup and to communicate these conditions to the system control to initiate the off-season startup procedure in order to avoid the severe flooded conditions and associated problems as discussed hereinabove. That is, if the system, for instance, does not include various sensors mentioned above to sense the temperatures and/or pressures at various locations associated with the vapor compression system that are indicative of the problem, then it is presumed that the need exists to proceed with the present off-season startup method and the control steps to a block 24. If the system is of a type that does include the various sensors that will be indicative of a need to take an action, then the method proceeds to a block 22 wherein such operational parameters are sensed. Such environmental conditions may include (but are not limited to) the ambient temperature T_AMBand the indoor temperature T_INDOOR.
Based on the results of the sensed parameters, the control determines in a block 23 whether the off-season startup is required. For instance, if the ambient temperature T_AMBfalls below 40° F. then the determination could be made that the off-season startup is required. If the determination is made that the off-season startup is not required, then the control steps to a block 28 to reset the timer, after which the process is repeated. It should be pointed out that the reset timer interval could be potentially adapted based on sensed environmental conditions (colder temperature would suggest shorter time intervals between the startups).
If the control determines in the block 23 that the off-season startup is required, the compressor 11 and the evaporator fan 17 are started first. After it is determined in a block 26 that the discharge pressure has exceeded the upper threshold, then the operational sequence for the condenser fan (or fans) 16 is started to keep the discharge pressure P_Dbetween the upper and lower limits.
After a second predetermined period of time, the system is shut down and the timer is reset (possibly to a new value based on the environmental conditions, as discussed above) at a block 28. After the first predetermined period of time has expired, the process is repeated.

Claims

1. A method of operating a vapor compression system during an off-season period during which the system is normally shut down, comprising the steps of:

providing a timer for timing the time interval during which the system has been shut down;

starting the compressor if the time interval exceeds a first predetermined threshold;

shutting off the compressor after the time interval exceeds a second predetermined threshold.

2. A method as set forth in claim 1 and including the additional steps of starting and shutting off an evaporator fan.

3. A method as set forth in claim 2 wherein said evaporator fan is turned on and off at substantially the same times as the compressor is turned on and off.

4. A method as set forth in claim 1 and including the additional steps of turning on and off at least one condenser fan.

5. A method as set forth in claim 4 wherein said at least one condenser fan is turned on after the compressor is started.

6. A method as set forth in claim 4 wherein said condenser fan is turned off before the compressor is turned off.

7. A method as set forth in claim 1 wherein the additional step is provided of sensing at least one parameter to determine whether the compressor startup is required.

8. A method as set forth in claim 7 wherein said at least one parameter is selected from the group consisting of: ambient temperature, indoor temperature, suction pressure, discharge pressure, saturation suction temperature, saturation discharge temperature or a combination of thereof.

9. A method as set forth in claim 1 wherein the timer setting is determined by at least one parameter.

10. A method as set forth in claim 9 wherein said at least one parameter is selected from the group consisting of: ambient temperature and indoor temperature.

11. A method as set forth in claim 1 wherein the first predetermined threshold is between 2 days and 4 weeks.

12. A method as set forth in claim 1 wherein the second predetermined threshold is between 2 and 15 minutes.

13. A vapor compression system, comprising:

a compressor for compressing refrigerant;

a condenser for rejecting heat of compressed refrigerant;

an expansion device for expanding the compressed refrigerant to a lower pressure and temperature;

an evaporator for receiving refrigerant from said expansion device and then transferring heat from air to be cooled to the refrigerant;

a timer for determining a period of time during which the vapor compression system has been shut down; and

a controller for starting the compressor after the system has been shut down for a predetermined period of time.

14. A vapor compression system as set forth in claim 13 wherein an evaporator fan is operatively associated with said evaporator and said controller also turns on and off said evaporator fan.

15. A vapor compression system as set forth in claim 14 wherein said controller turns on and off said evaporator fan at substantially the same times as it turns on and off the compressor.

16. A vapor compression system as set forth in claim 13 wherein a condenser fan operatively associated with said condenser and said controller also turns on and off said condenser fan.

17. A vapor compression system as set forth in claim 16 wherein said controller turns on said condenser fan after it has turned on said compressor.

18. A vapor compression system as set forth in claim 16 wherein said controller turns off said condenser fan before it turns off said compressor.

19. A vapor compression system as set forth in claim 13 and including at least one sensor for sensing at least one parameter to determine whether the compressor startup is required.

20. A vapor compression system as set forth in claim 19 wherein at least one parameter is selected from the group consisting of: ambient temperature, indoor temperature, suction pressure, discharge pressure, saturation suction temperature, saturation discharge temperature or a combination of thereof.

21. A vapor compression system as set forth in claim 13 wherein the timer setting is determined by at least one parameter.

22. A vapor compression system as set forth in claim 21 wherein at least one parameter is selected from the group consisting of: ambient temperature and indoor temperature.

23. A vapor compression system as set forth in claim 13 wherein the period of time during which the vapor compression system has been shut down is between 2 days and 4 weeks.

24. A vapor compression system as set forth in claim 13 wherein the vapor compression system operation between consecutive shutdowns is between 2 and 15 minutes.