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 
       [0001]    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. 
         [0002]    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. 
         [0003]    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. 
         [0004]    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&#39;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 
       [0005]    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. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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 
         [0009]      FIG. 1  is an exemplary vapor compression system with the present invention incorporated therein. 
           [0010]      FIG. 2  is a flow chart showing the method in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0011]    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. 
         [0012]    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 CO 2 , 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. 
         [0013]    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. 
         [0014]    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. 
         [0015]    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. 
         [0016]    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 AMB  sensed by a temperature sensor  21  and the indoor temperature T INDOOR  sensed by a temperature sensor  22 . The temperature sensors can be, for instance, of a thermistor or thermocouple type. 
         [0017]    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 D  measured by a sensor  23 , the suction pressure P S  measured by a sensor  24 , the ambient temperature T AMB  measured by the sensor  21 , the indoor temperature T INDOOR  measured 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 D  is monitored by the sensor  23  and communicated to the control  19 . In case the discharge pressure P D  exceeds 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 D  measured 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 D  between 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. 
         [0018]    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. 
         [0019]    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. 
         [0020]    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 AMB  and the indoor temperature T INDOOR . 
         [0021]    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 AMB  falls 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). 
         [0022]    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 D  between the upper and lower limits. 
         [0023]    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.