Abstract:
A system and method of loading and unloading a compressor in a cooling system. The method includes detecting a temperature, determining a compressor should be turned on/off to supply/stop supplying cooling based on the temperature, turning the compressor on/off, and opening/closing a plurality of valves when the compressor is turned on/off.

Description:
BACKGROUND 
       [0001]    The invention relates to cycling of compressors, specifically rapid cycling of scroll compressors. 
         [0002]    Compressors are integral parts of cooling systems (e.g., air conditioners, refrigerators, etc.). Compressors compress refrigerant which later expands and draws heat out of the environment. The amount the refrigerant is compressed is directly related to the amount of heat the evaporating refrigerant can remove from the environment. The compressors are turned on or off (loaded/unloaded) to control the pressure of the refrigerant and the cooling capacity of the system. The turning on and off of a compressor causes wear and tear on the compressor that can lead to higher maintenance costs and reduce the life of the compressor. The wear and tear is increased when the compressor is cycled on and off too rapidly. Thus, compressors are controlled to have minimum-cycle-times (e.g., a minimum of three minutes on and a minimum of three minutes off) to reduce the wear and tear on the compressor. These-cycle-times reduce the ability to tightly control the cooling effects of the system (e.g., resulting in excessively wide temperature swings), and reduce the efficiency of the system (e.g. resulting in increased energy usage). 
       SUMMARY 
       [0003]    In one embodiment, the invention provides a method of loading and unloading a compressor in a cooling system. The method includes detecting a temperature, determining a compressor should be turned on to supply cooling based on the temperature, determining a point in time when the impact of turning on a motor of the compressor is minimized using point-on-wave analysis, and turning on the compressor at about the determined point in time. 
         [0004]    In another embodiment the invention provides a method of loading and unloading a compressor in a cooling system. The method includes detecting a temperature, determining a compressor should be turned on to supply cooling based on the temperature, turning on the compressor, and opening a plurality of valves when the compressor is turned on. 
         [0005]    In another embodiment the invention provides a cooling system. The cooling system includes a compressor; a temperature sensor, a compressor intake valve, a compressor output valve, and a controller. The temperature sensor is configured to provide an indication of a temperature. The compressor intake valve is coupled to an input of the compressor. The compressor output valve is coupled to an output of the compressor. The controller is coupled to the compressor, the temperature sensor, the compressor intake valve, and the compressor output valve. The controller is also configured to receive the indication of the temperature from the temperature sensor, determine that the compressor should be turned off to stop providing cooling based on the indication of temperature received from the temperature sensor, turn off the compressor, close the compressor intake valve, and close the compressor output valve, wherein closing the compressor intake valve and the compressor output valve maintains a pressure of refrigerant across the compressor while the compressor is off. 
         [0006]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a block diagram of a single compressor cooling system. 
           [0008]      FIG. 2  is a block diagram of a multiple compressor cooling system. 
           [0009]      FIG. 3A  is a graph showing the operation of a prior-art cooling system. 
           [0010]      FIG. 3B  is a graph showing the operation of a cooling system employing the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0012]    The examples described below show various cooling systems. However, the invention has application in other constructions such as heat pumps as well. 
         [0013]      FIG. 1  is a block diagram of a cooling system  100  (e.g., a residential air-conditioner). The system  100  includes a compressor  105 , a condenser  110 , a controller  115 , an expansion valve  120 , an evaporator  125 , a temperature sensor  130 , a first valve  135  (a compressor intake valve), a second valve  140  (a compressor output valve), and a third valve  145  (an evaporator valve). 
         [0014]    The controller  115  receives an indication of a temperature from the temperature sensor  130 . Depending on the system, the temperature can be an air temperature (e.g., a direct cooling system) or a temperature of a coolant (e.g., chiller water or a refrigerant). 
         [0015]    The controller  115  determines if cooling is needed, turning on the compressor  105  when cooling is needed, and turning off the compressor  105  when cooling is not needed. In some embodiments, the controller  115  anticipates the need for cooling, turning the compressor  105  on prior to the temperature reaching a turn-on set-point, and turning off the compressor  105  prior to reaching a turn-off set-point. In some constructions, the controller uses a proportional-integral-derivative (PID) control scheme to operate the compressor  105 . U.S. Pat. No. 5,415,346, filed Jan. 28, 1994, and entitled “Apparatus and Method for Reducing Overshoot in Response to the Setpoint Change of an Air Conditioning System,” the entire content of which is hereby incorporated by reference, describes such a method of controlling the operation of an air conditioning system. In some embodiments, as described below, the controller  115  also controls the compressor  105  using a scheme designed to reduce wear and tear on the compressor  105 . 
         [0016]    When the controller  115  turns the compressor  105  on, the compressor  105  compresses a refrigerant in the cooling system  100  to provide cooling capacity for the system  100 . The refrigerant flows through piping to the condenser  110  which condenses the refrigerant into a liquid. The refrigerant continues on to the expansion valve  120 . The expansion value  120  causes the refrigerant to expand and transform into a gas. This process occurs as the refrigerant passes through the evaporator  125 . As this happens, the refrigerant, in the evaporator  125 , removes heat from the air surrounding the evaporator  125 , resulting in the air (or water) being cooled. The refrigerant then continues on back to the compressor  105 . 
         [0017]    In addition to turning the compressor  105  on and off, the controller  115  also opens (when turning on the compressor  105 ) and closes (when turning off the compressor  105 ) the first, second, and third valves  135 ,  140 , and  145 . As the pressure of the refrigerant varies significantly throughout the cooling system  100 , closing the valves  135 ,  140 , and  145  traps the pressure of the refrigerant in zones or sections of the system  100 . This enables the refrigerant exiting the compressor  105  to achieve its full pressure nearly immediately upon the compressor  105  being turned on, improving the performance of the system  100 . Other schemes are contemplated as well, including sequencing of the opening and closing of the valves  135 ,  140 , and  145 , and timing the opening and closing of the valves  135 ,  140 , and  145  such that they open or close before or after the compressor  105  is turned on/off. 
         [0018]    In some constructions, the temperature sensor  130  is a thermostat. The thermostat  130  provides a signal to the controller  115  (e.g., a motor controller) indicating whether the controller  115  should turn on the compressor  105  or turn off the compressor  105  based on a temperature set-point, and a dead-band. The thermostat  130  may or may not have intelligence enabling the thermostat  130  to anticipate the thermal inertia of the area to be cooled. 
         [0019]      FIG. 2  is a block diagram of an exemplary large-scale cooling system  200  (e.g., for cooling a commercial building, for cooling a plurality of refrigerated display cases, etc.). The cooling system  200  includes at least one compressor  205 , a condenser  210 , a receiver  215  (optional), a controller  220 , a suction header  230 , a plurality of expansion valves  235 , a plurality of evaporators  240 , a plurality of intake valves  245 , and a plurality of output valves  250 . In some constructions, where all of the compressors  205  are operated in unison (i.e., all the compressors  205  are turned on and turned off at the same time), a single intake valve  245  is used prior to the suction header  230  and/or a single output valve  250  is used after common piping for the compressors  205 . In addition, the system  200  includes an evaporator valve  255  between the receiver  215  and the expansion valves  235 . In some constructions, multiple evaporator valves  255  are used, e.g., an evaporator valve  255  positioned before each expansion valve  235 . 
         [0020]    One or more temperature sensors may be used to detect the temperature of an area or a coolant cooled by the evaporators  240 . The controller  220  receives an indication of the temperature from the sensor, and controls the compressors  205  based on the temperature as described above with respect to cooling system  100 . 
         [0021]    The compressor  205  compresses a refrigerant in the cooling system  200  to provide cooling capacity for the system. In a cooling system  200  with more than one compressor  205 , the compressors  205  can turn on and off at the same or different times to meet the demand required by the system. In some constructions, all of the compressors  205  are of one or more fixed capacities, and the controller  220  stages or loads the compressors  205  into the system as necessary, for example as described in U.S. Pat. No. 5,123,256, filed May 7, 1991, and entitled “Method of Compressor Staging for a Multi-Compressor Refrigeration System,” the entire content of which is hereby incorporated by reference. When a compressor  205  is turned off, the intake valve  245  and the output valve  250  associated with the compressor  205  are closed, maintaining high-side and low-side pressures within the evaporator  110  and condenser  125 . When the compressor  205  is turned on, the intake valve  245  and the output valve  250  associated with the compressor  205  are opened, and the compressor  205  gets to operating pressure nearly immediately. If all of the compressors  205  in the system  200  are turned off, the evaporator valve  255  is also closed. 
         [0022]    In some embodiments, the controller  220  controls the compressors  105 / 205  using a scheme designed to reduce wear and tear on the compressors  205 . U.S. Pat. No. 7,812,563, the entire content of which is hereby incorporated by reference, discloses a technology referred to a point-on-wave (POW) switching. POW switching determines when to power (i.e., switch on) a winding (i.e., a phase) of a motor based on the relationship between the wave of the phase of AC power to be supplied with the wave(s) of the phase(s) of AC power presently supplied to the other winding(s) of the motor. The invention monitors each phase of AC voltage supplied to the windings of the motor(s) of the compressor(s) through precision DC contactors (although AC contactors could be used instead), switching a contactor and powering a phase only when the relationship between the phases will result in the least amount of stress on the compressor motor. 
         [0023]    The use of POW switching, and the maintaining of pressure zones using valves, enables the invention to reduce or eliminate cycling delays for compressors of cooling systems, increasing efficiency and comfort. Some prior art cooling systems have used multiple smaller compressors to improve the performance of the cooling system (e.g., to narrow the temperature control range). The invention enables the use of a single larger compressor while achieving the same or better levels of performance and efficiency, than achieved using multiple smaller compressors. 
         [0024]      FIG. 3A  shows a graph of temperature TEMP versus a set point, and a related on/off indication  400  of a compressor of a prior art system. When the temperature TEMP is above the set point, the controller turns the compressor on if a minimum off-cycle-time has been met. Conversely, when the temperature TEMP is below the set point, the controller turns the compressor off if a minimum on-cycle-time has been met. In the graph shown in  FIG. 3A , the temperature is changing faster than the-cycle-times. Thus, there is a delay of ΔD 1  between when the temperature TEMP rises above the set point, and when the controller turns the compressor on. A similar time delay ΔD 2  occurs when the temperature TEMP drops below the deadband before the controller turns the compressor off. 
         [0025]    These delays, caused by the cycle times, result in the compressor running longer (ΔD 2 ) than necessary, wasting energy. In addition, the delays (ΔD 1  and ΔD 2 ) cause the temperature range (ΔT 1 ) to be greater than necessary, potentially causing discomfort to occupants of the area cooled by the cooling system. 
         [0026]    As shown in  FIG. 3B , by being able to turn compressors on and off at any time (i.e., without cycle delays), and maintaining pressure in the compressors when they are off, the delays (ΔD 1  and ΔD 2 ) are eliminated. In fact, the compressors can be turned on and off in anticipation of the temperature being above/below the set-point/deadband (ΔD 3  and ΔD 4 ). This reduces energy usage, and results in a much narrower temperature range (ΔT 2  versus ΔT 1 ) increasing the comfort of occupants. 
         [0027]    Various features and advantages of the invention are set forth in the following claims.