Patent Publication Number: US-2018031291-A1

Title: Vfd control for multiple circuit refrigeration system

Description:
FIELD OF THE INVENTION 
     The subject matter disclosed herein relates to refrigerant systems and, more specifically, to control of compressors for refrigerant systems associated with transport containers having multiple compartments. 
     BACKGROUND 
     Variable speed control of a compressor in a refrigeration or air conditioning application is often accomplished using a variable speed drive. This allows the removal of all unloading hardware from the compressor system. In typical applications involving more than one compressor, such as multiple circuit chillers, multiplexed compressor chillers, refrigeration, compressor racks, a variable speed control could be used with each compressor to selectively unload compressors as necessary based on system demand. Variable speed drives are expensive and therefore, multiple compressor systems requiring multiple variable speed drives also become extensively more expensive. In addition, the use of multiple speed drives adds to the complexity and space requirements of the air conditioning or refrigeration system. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect, a refrigeration system is provided. The refrigeration system includes a first refrigerant circuit having a first compressor, and a second refrigerant circuit having a second compressor, the second refrigerant circuit separate from the first refrigeration circuit. A variable frequency drive (VFD) is in signal communication with the first compressor and the second compressor and is configured to enable a variable speed operation of the first and second compressors. A controller is in signal communication with the VFD. The controller is programmed to selectively switch the first and second compressors between a variable speed operation using the VFD and a fixed speed operation such that in a first mode the first compressor is operated in the fixed speed operation and the second compressor is operated in the variable speed operation, and in a second mode the second compressor is operated in the fixed speed operation and the first compressor is operated in the variable speed operation. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include: wherein the refrigeration system only comprises a single VFD; wherein the first compressor and the second compressor are the only compressors in signal communication with the single VFD; a transport container comprising a first cargo compartment and a second cargo compartment, wherein the first refrigerant circuit is thermally coupled to the first cargo compartment and the second refrigerant circuit is thermally coupled to the second cargo compartment; wherein the first cargo compartment is a frozen cargo compartment and the second cargo compartment is a perishable cargo compartment; wherein the controller is programmed to switch the first and second compressors between the variable speed operation and the fixed speed operation when a predetermined condition occurs; wherein the predetermined condition is when a temperature in at least one of the first and second cargo compartments reaches a predetermined setpoint temperature; wherein the predetermined condition is when an ambient temperature reaches a predetermined temperature; and/or a first sensor disposed in the first cargo compartment and a second sensor disposed in the second cargo compartment, the first and second sensors configured to send respective signals to the controller indicative of at least one condition in the respective first and second cargo compartments, wherein the controller is programmed to switch the first and second compressors between the variable speed operation and the fixed speed operation based on the signals. 
     In another aspect, a method is provided of controlling a temperature in a first compartment and a second compartment with a refrigeration system comprising a first refrigerant circuit having a first compressor, a second refrigerant circuit having a second compressor, a variable frequency drive (VFD) in signal communication with the first compressor and the second compressor, and a controller in signal communication with the VFD. The method includes operating, with the controller and the VFD, the refrigeration system in a first mode such that the first compressor is operated at a variable speed and the second compressor is operated at a fixed speed, monitoring the first and second compartments for the occurrence of a predetermined condition, and switching, with the controller and the VFD, upon the occurrence of the predetermined condition, the refrigeration system to operate in a second mode such that the second compressor is operated at a variable speed and the first compressor is operated at a fixed speed. 
     In addition to one or more of the features described above, or as an alternative, further embodiments may include: operating only one of the first and second compressors at the variable speed during each of the first and second modes; switching between the first and second modes with only a single VFD; wherein the step of monitoring the first and second compartments comprises monitoring a first sensor and a second sensor respectively disposed in the first compartment and the second compartment for the occurrence of a predetermined condition; wherein the predetermined condition is when a monitored temperature in at least one of the first and second cargo compartments reaches a predetermined setpoint temperature; and/or wherein the predetermined condition is when an ambient temperature reaches a predetermined temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic illustration of an exemplary refrigeration system; and 
         FIG. 2  is a schematic illustration of an exemplary variable frequency drive that may be used with the refrigeration system shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a simplified schematic diagram of an exemplary container refrigeration system  10  operatively associated with a transport container  12  having multiple temperature controlled compartments or zones defined therein. For example, as illustrated, transport container  12  includes a first compartment  14  and a second compartment  16 . However, transport container  12  may have more than two compartments. 
     Refrigeration system  10  is configured to provide a cooling capacity to temperature controlled compartments  14 ,  16 , which may store perishable commodities and/or frozen or chilled products. Transport container  12  may include, for example, truck trailers for road transport, railroad cars, and shipping container bodies for land and sea service. Refrigeration system  10  may be secured to a portion of transport container  12  such as a wall thereof. 
     Refrigeration system  10  generally includes a first refrigerant circuit  20 , a second refrigerant circuit  30 , a variable frequency drive (VFD)  40 , and a controller  50 . First refrigerant circuit  20  generally includes a compressor  22 , a condenser  24 , an expansion device  26 , and an evaporator  28 . Second refrigerant circuit  30  is fluidly separate from circuit  20  and generally includes a compressor  32 , a condenser  34 , an expansion device  36 , and an evaporator  38 . First refrigerant circuit  20  is operably associated with first compartment  14  and is configured to provide cooling demand thereto. Similarly, second refrigerant circuit  30  is operably associated with second compartment  16  and is configured to provide cooling demand thereto. 
     Operation of first and second refrigerant circuits  20 ,  30  is similar. For example, operation of circuit  20  may begin at compressor  22  where suction gas (refrigerant) is compressed to a higher temperature and pressure. The refrigerant gas is supplied to condenser  24  where it is cooled and condensed by a heat transfer fluid such as air flowing across condenser  24 . By removing latent heat, the gas condenses to a high pressure, high temperature liquid and flows to expansion device  26  (e.g., an expansion valve). 
     The liquid refrigerant passing through expansion device  26  is expanded or reduced in pressure and may be flashed or vaporized into a gas. The expanded refrigerant is subsequently supplied to evaporator  28  where it is further vaporized against a heat transfer fluid such as return air. The transport container air to be cooled is passed across tubes/fins of evaporator  28 , and heat is removed from the air as it is absorbed by the refrigerant in evaporator  28 . This causes the temperature of the low pressure refrigerant vapor to increase before it is returned to compressor  22  to complete the refrigeration cycle. 
     Refrigeration system  10  may be charged with a relatively low critical temperature refrigerant such as carbon dioxide (R744). Because carbon dioxide has a low critical temperature, most refrigeration systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical pressure regime. In systems operating in a transcritical cycle, condensers  24 ,  34  function as gas coolers rather than as condensers and operate at a refrigerant temperature and pressure in excess of the refrigerant&#39;s critical point, while evaporators  28 ,  38  operate at a refrigerant temperature and pressure in the subcritical range. 
     VFD  40  selectively controls the capacity of compressors  22 ,  32  by varying the speed at which the compression devices operate by varying the frequency of AC electric current supplied to a drive motor of the compressor (not shown). VFD  40  receives electric current at line frequency, selectively alters that frequency, and outputs the DC current to the drive motor at the selected frequency. In the transport refrigeration application described herein, the power source comprises an electric current generator  42  driven by an internal combustion engine  44 , such as the engine of a truck or trailer tractor associated with transport container  12  or a separate internal combustion engine dedicated to driving electric current generator  42 . 
     Controller  50  is in signal communication with VFD  40 , one or more sensors  52  operably associated with first compartment  14 , and one or more sensors  54  operably associated with second compartment  16 . As used herein, the term controller refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     In some embodiments, sensors  52 ,  54  are temperature sensors configured to respectively sense an internal ambient temperature of cargo compartments  14 ,  16 . As such, controller  50  may monitor the temperatures within compartments  14 ,  16  and adjust operation of refrigeration system  10  to meet a desired cooling demand within compartments  14 ,  16 , as described herein in more detail. Alternatively sensors  52 ,  54  may be refrigerant temperature sensors thermally coupled to, for example, evaporator  38 . 
     Controller  50  is programmed to control VFD  40  to selectively switch compressors  22 ,  32  between a variable speed operation and a fixed speed operation such that only one compressor  22 ,  32  is operated at a variable speed while the other compressor  22 ,  32  is operated at a fixed speed. Controller  50  determines which compressor requires the most cooling capacity at any given time (e.g., based on signals from sensors  52 ,  54 ), and subsequently enables VFD  40  to switch variable speed operation between compressors  22 ,  32 . Accordingly, controller  50  is programmed to selectively switch refrigeration system  10  between a first mode where compressor  22  is operated at variable speed and compressor  32  and any remaining compressors (not shown) are operated at fixed speed, and a second mode where compressor  32  is operated at variable speed and compressor  22  and any remaining compressors (not shown) are operated at fixed speed. 
     Controller  50  may be programmed to switch the operation of compressors  22 ,  32  between the fixed and variable speed operations when various predetermined conditions occur. For example, first compartment  14  may be a deep frozen compartment and second compartment  16  may be a perishable products compartment that has a higher setpoint temperature than frozen compartment  14 . Controller  50  may be programmed to operate in the first mode with compressor  22  in a variable speed operation and compressor  32  in a fixed speed operation. Controller  50  may subsequently automatically switch the compressor operation into the second mode when sensor  52  and/or  54  senses a predetermined condition (e.g., deep frozen first compartment  14  achieves a temperature set point). In some embodiments, refrigeration system  10  may only include two compressors  22 ,  32  and a single VFD  40 . 
     Controller  50  may be programmed to switch the operation of compressors  22 ,  32  between the fixed and variable speed operations when other predetermined conditions occur. For example, predetermined conditions may include a predetermined system pressure, a predetermined system temperature, a predetermined temperature in each of compartments  14 ,  16 , a predetermined ambient temperature, a predetermined user setpoint (time, temperature, and/or pressure), a box temperature differential (e.g., difference between the compartment temperature and the ambient, or difference between supply air and return air), based on refrigeration system properties (e.g., temperature, pressure), when within a predetermined percentage of a user setpoint, when one compartment has a greater pull-down than another, compressor protection (low mass flow, high suction temperature, suction liquid slugging), and/or noise reduction. 
       FIG. 2  illustrates one embodiment of VFD  40 . In the exemplary embodiment, VFD  40  generally includes a housing  60 , a VFD control board  62 , a first relay  64 , a second relay  66 , a third relay  68 , a fourth relay  70 , a first set of output terminals  72 , and a second set of output terminals  74 . Relays  64 ,  66 ,  68 , and  70  are selectively activated by controller  50  to selectively operate compressor  22 ,  32  in one of the variable speed operation and the fixed speed operation. As illustrated, compressor  22  is operated in the variable speed operation and compressor  32  is operated in the fixed speed operation. In this configuration, relays  64 ,  66  are not energized, and VFD control board  62  provides a current to output terminals  72  via relays  64 ,  66 . VFD control board  62  determines compressor speed based on controller demand. 
     A compressor contactor  76  is utilized to aid a start/stop control mode in order to shut down compressor  22  when the box setpoint has been met. Start/stop mode is a selectable configuration that will shut down compressor  22  when the box setpoint has been met, facilitating saving on load draw from the generator  42  and fuel savings from the engine  44 . Compressor contactor  76  may be external or internal to VFD housing  60 , or could be controlled via VFD control board  62 . 
     Relays  68 ,  70  are energized and a power source  78  provides power to compressor  32  via fourth relay  70  and output terminals  74 . Power source  78  may be an AC current power electrical source such as generator  42 , or an outside power source such as house power or electrical source during ferry transport. Compressor terminals  4 - 6  of compressor  32  are shorted at third relay  68 , which causes compressor  32  to run at the fixed speed. A compressor contactor  80  is utilized to aid a start/stop control mode in order to shut down compressor  32  when the box set point has been met. 
     Described herein are systems and methods for controlling the cooling capacity of multiple compartments of a multi-temperature transport container. The systems include a single VFD selectively switching two or more compressors between a variable speed operation and a fixed speed operation. The compressor associated with a compartment requiring the most cooling capacity is operated in the variable speed operation while the remaining compressor(s) are operated at the fixed speed operation. Accordingly, only a single VFD is used rather than using a dedicated VFD for each compressor in the system. As such, the systems described herein reduce total product cost, weight, and fuel consumption while requiring less space and providing capacity requirements needed for each compartment&#39;s temperature control. 
     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.