Abstract:
A system comprising a compressor coupled to a first coil through a first valve and a second coil through a second valve, wherein the first coil and the second coil are coupled to a third coil. The system further comprises a fan operable to blow ambient air across the first coil, a first expansion valve coupled to and positioned between the first coil and the third coil and a second expansion valve coupled to and positioned between the second coil and the third coil. The system comprises a controller operable to monitor a pressure of the refrigerant, operate the first expansion valve to reduce refrigerant flow into the first, and operate the second expansion valve to reduce refrigerant flow through the second coil.

Description:
RELATED APPLICATION 
       [0001]    This application claims benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 62/089,817, titled “Variable Refrigerant Flow System Operation in Low Ambient Conditions,” filed Dec. 9, 2014. This application is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This application is directed, in general, to HVAC (heating, ventilating, and air conditioning) systems, and more specifically to variable refrigerant flow system operation in low ambient conditions. 
       BACKGROUND 
       [0003]    HVAC systems often need to be able to operate in varying environmental conditions. Present HVAC systems operate ineffectively or not at all when an establishment has a cooling demand in conditions where the ambient environmental temperature is also relatively cool. Thus, methods and systems are needed for HVAC systems cool effectively in low ambient temperature conditions. 
       SUMMARY OF THE DISCLOSURE 
       [0004]    A system comprising a compressor coupled to a first coil through a first valve and a second coil through a second valve, wherein the first coil and the second coil are further coupled to a third coil, the compressor being operable to compress refrigerant and pump the refrigerant out of a first compressor opening into the first and second coils and receive the refrigerant through a second compressor opening after the refrigerant has passed through the third coil is disclosed. The system further comprises a fan operable to blow ambient air across the first coil, a first expansion valve coupled to and positioned between the first coil and the third coil and a second expansion valve coupled to and positioned between the second coil and the third coil. Additionally, the system comprises a controller operable to trigger a low ambient temperature mode to monitor a pressure of the refrigerant, in response to determining that the refrigerant pressure is below a threshold pressure, operate the first expansion valve to reduce refrigerant flow into the first coil and increase refrigerant flow through the second coil and into the third coil, and in response to determining that the refrigerant pressure is above a maximum threshold pressure, operate the second expansion valve to reduce refrigerant flow through the second coil and increase refrigerant flow through the first coil and into the third coil. 
         [0005]    The present embodiment presents several technical advantages. First, the present embodiment discloses an HVAC system that is operable to effectively cool an environment even when the ambient temperature is low. Second, the HVAC system of the present embodiment is able to function effectively in both low ambient temperatures and regular temperatures. Third, the HVAC system of the present embodiment can be regulated with an intelligent controller which may be adjusted for different temperature settings. 
         [0006]    Certain embodiments of the present disclosure may include some, all, or none of these advantages. One or more other technical advantages may be readily apparent to those skilled in the art from the figures, descriptions, and claims included herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying drawings, in which: 
           [0008]      FIG. 1  is a block diagram of an outdoor section of a VRF system; 
           [0009]      FIG. 2  is a flow chart of a method of control of the VRF system in cooling operation during low, or extra low, ambient conditions; and 
           [0010]      FIG. 3  is a refrigerant flow diagram during operation of VRF system in cooling operation during low, or extra low, ambient conditions. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Cooling operation of HVAC systems can be problematic when the outdoor ambient air temperature is low. Operation during low ambient condition causes the refrigerant pressure throughout the HVAC system to drop, potentially freezing the evaporator coil and leading to unsafe operating conditions for the HVAC system. 
         [0012]    Variable Refrigerant Flow (VRF) systems are a type of HVAC system consisting of multiple indoor units and one, or more, outdoor units. VRF systems may be configured for heat pump operation, capable of providing either heating or cooling supply air to a conditioned space through use of a reversing valve, which may change the direction of refrigerant flow through the system components. 
         [0013]    Each indoor unit of a VRF system comprises an indoor coil and is configured to condition supply air for delivery to a specific zone of conditioned space within a building. Refrigerant may evaporate as it passes through the indoor coil and absorbs heat from air blown across the indoor coil. Each indoor unit of a VRF system may be paired with an outdoor unit assembly consisting of one or more outdoor units, forming a refrigerant flow circuit. Each outdoor unit may comprise one or more separate outdoor coils. Refrigerant may condense as it passes through the outdoor coils and releases heat to air blown across the outdoor coils. Each of the one or more indoor units of a VRF system may provide conditioned air to a specific, and separate, zone within a building. Each indoor unit may operate independently of the other indoor units, such that some, none, or all of the indoor units may be in operation simultaneously. 
         [0014]    The ability to safely accommodate cooling operation in low, or even extra low, ambient outdoor temperatures is an especially desirable feature of VRF systems, particularly VRF systems having several separate indoor units. In a building cooled by a VRF system having several indoor units, for example, a single cooling zone may place a cooling demand on the VRF system while the other zones place no cooling demand on the system. The single zone placing cooling demand on the VRF system may, perhaps, be a server room. In such a setting, the server room may place a cooling demand on the VRF system regardless of the outdoor ambient air temperature. 
         [0015]    Present HVAC systems do not accommodate HVAC system cooling operation in extra low ambient outdoor temperatures. The present embodiment addresses this limitation of present HVAC systems without requiring the addition of components to the HVAC system specifically for use only during low ambient operation, such as bypass piping and metering devices, adding to the cost of the HVAC system. The present VRF system accommodates cooling demand at even extra low ambient outdoor air temperatures with little, or no, additional components required to specifically accommodate cooling operation in extra low ambient outdoor temperatures. 
         [0016]    Referring to  FIG. 1 , a block diagram of the outdoor section components and piping arrangement of a VRF system  1000  according to an embodiment of the present invention is shown. The VRF system  1000  may be a three pipe VRF system configured for heat pump operation and comprising a single outdoor unit, having two outdoor coils, coupled with one or more indoor units (not shown). The VRF system  1000  may include a compressor assembly  100 , two valves  200 A and  200 B, two outdoor coils  300 A and  300 B, two fan assemblies  400 A and  400 B, two metering device  500 A and  500 B, a reversing valve  600 , a controller  700 , and an indoor coil  800 . 
         [0017]    The embodiment shown in  FIG. 1  corresponds to simplified system components and piping for a single refrigerant flow circuit. In other embodiments, the apparatus and method described herein may be utilized in multi-stage VRF systems have multiple refrigerant flow circuits. 
         [0018]    In alternative embodiments, VRF system  1000  may include additional, fewer, or different components than those shown in  FIG. 1 . For example, in an alternative embodiment, VRF system  1000  may be provided with more than one compressor  100 , with more than two valves  200 , more than two outdoor coils  300 , more than two fan assemblies  400 , with more than two metering devices  500 , with more than one reversing valve  600 , and/or with more than one indoor coil  800  and the like. The VRF system  1000  may, in alternative embodiments, be provided with additional components and associated piping, such as one or more oil separators, one or more crankcase heaters, one or more check valves, one or more refrigerant accumulators, one or more pressure and/or temperature sensors, and the like. 
         [0019]    Further, VRF system  1000  components may be located in different sections of the VRF  1000  system than shown. For example, some, none, or all of the system components such as the compressor  100 , the valves  200 , the metering devices  500 , the reversing valve  600 , and the controller  700  may be located elsewhere in the VRF system  1000 , such as in an indoor section, for example, and not within the outdoor section. 
         [0020]    As shown in  FIG. 1 , the VRF system  1000  may include a compressor assembly  100  for pumping refrigerant from the low pressure to the high pressure sides of a VRF system  1000 . The compressor assembly  100  may be configured to pump refrigerant through the VRF system  1000  at a variable flow rate, configured to match VRF system  1000  demand. The compressor assembly  100  may operatively connect to, and receive power and control signals from, the system controller  700 . 
         [0021]    The compressor assembly  100  may comprise a compressor  102  operatively coupled to a variable speed drive  104  for varying the speed of the compressor  102 . The compressor  102  may be of any type, such as a scroll compressor, a reciprocating compressor, or the like. The compressor  102  may include refrigerant temperature and pressure sensors, which may be internal or external to the compressor  102 , for sensing one or more operating parameters of the compressor  102 , such as refrigerant pressure and/or temperature at the suction and/or discharge of the compressor  102 . The sensed operating parameters may be communicated to the controller  700  via wired or wireless communication means. 
         [0022]    The variable speed drive  104  may adjust the speed of the compressor  102 , varying the flow rate of refrigerant through the compressor  102 . The variable speed drive  104  may adjust the compressor  102  speed through any suitable method, such as through frequency modulation of an incoming power signal, voltage modulation of an incoming power signal, or other suitable methods. In an alternative embodiment than that shown in  FIG. 1 , the variable speed drive  104  may be an internal component of the compressor  102  or, alternatively, incorporated within the system controller  700 . 
         [0023]    The VRF system  1000  may include valves  200 A and  200 B for routing refrigerant flow received from compressor assembly  100  through the VRF system  1000 . As shown in  FIG. 1 , the valves  200 A and  200 B may each be four-way valves configured to route refrigerant flow through the valves  200  along one of two paths, as desired. The valves  200 A and  200 B may be four way valves of any other suitable type of valve. The valves  200 A and  200 B may be operatively connected to the system controller  700  for receiving control signals setting the position of valves  200 A and  200 B. 
         [0024]    As shown in  FIG. 1 , the valve  200 A may be paired with the outdoor coil  300 A while the valve  200 B may be paired with the outdoor coil  300 B. This configuration may allow for refrigerant flow to be directed from the discharge of the compressor assembly  100  to either, or both, of the outdoor coils  300 A and  300 B, depending on the heating or cooling demand to which the VRF system  1000  is operating in response to as well as in response to the ambient outdoor air temperature. 
         [0025]    During cooling operation in low, or extra low, ambient air temperatures, the valves  200 A and  200 B may both be configured to allow refrigerant flow from the discharge of the compressor assembly  100  to both of the outdoor coils  300 A and  300 B. Allowing refrigerant to flow to both outdoor coils  300 A and  300 B may provide a “bypass” for a portion of the refrigerant flow during cooling operation in low, or extra low, ambient outdoor air temperatures, as described further below. Those skilled in the art will appreciate that in an alternative embodiment, the four-way valves  200 A and  200 B may be replaced with a series of shutoff valves, check valves, or the like, and configured to permit refrigerant flow along a desired path in a manner consistent with the methods of the VRF system operation described herein. 
         [0026]    Returning to  FIG. 1 , the VRF system  1000  may include outdoor coils  300 A and  300 B and indoor coil  800 . Outdoor coils  300 A and  300 B and indoor coil  800  may allow for heat transfer between VRF system  1000  refrigerant by passing outdoor air over outdoor coils  300 A and B and indoor air over indoor coil  800 . In an embodiment, the outdoor coils  300 A and  300 B may be identical to one another and to indoor coil  800 . Alternatively, in an embodiment, one or more outdoor coils  300  and indoor coil  800  may vary in size, shape, piping configuration, and/or heat transfer capacity from each other. 
         [0027]    The outdoor coils  300 A and  300 B and indoor coil  800  may be implemented with one or more sensor devices for sensing operational conditions of the VRF system  1000 , such as refrigerant temperature and pressure, ambient outdoor air temperature, refrigerant flow rate, and the like. These operational conditions may be communicated to the system controller  700  through a wired, or wireless, connection for use in control of the VRF system  1000  components. 
         [0028]    The VRF system  1000  may include fans  400 A and  400 B. The fans  400 A and  400 B may induce airflow across the outdoor coils  300 A and  300 B. The fans  400 A and  400 B may include a plurality of blades that may be rotated about a hub in response to a control signal input to a motor. The fans  400 A and  400 B may be configured to operate at different speeds and in one of two directions, as desired, to push air across, or draw air through, the outdoor coils  300 A and  300 B. In some embodiments, one or more indoor fans may also induce airflow across indoor coil  800 . 
         [0029]    As shown in  FIG. 1 , the fan  400 A may be paired with the outdoor coil  300 A while the fan  400 B may be paired with the outdoor coil  300 B. In alternative embodiments, more or fewer fans  400  may be provided. For example, in an embodiment, a single fan  400  may be provided for inducing airflow across all of the outdoor coils  300 . In an alternative embodiment, each outdoor coil  300  may be paired with multiple fans  400 . In such an embodiment, the fans  400  may be controlled by the system controller  700  independently, or in concert. Further, the fans  400 A and  400 B may be configured to operate independently of one another, such that one or more fans  400  may be energized and operated at a desired speed while one or more other fans  400  are de-energized and not rotating. 
         [0030]    The fans  400 A and  400 B may be operably connected to, and may receive control and power signals from, the system controller  700  via a wired or wireless connection. The fans  400 A and  400 B may be configured for variable speed operation in response to heating and cooling demand on the VRF system  1000  and in response to ambient outdoor air temperatures. 
         [0031]    The electrical input to the fans  400 A and  400 B may be a direct current (DC) input or an alternating current (AC) input. The control signal may be a pulse-width modulated (PWM) signal in which the relative width of pulses determines the level of power applied to the fans  400 A and  400 B. The revolutions per minute (RPM) of the fans  400 A and  400 B may have a direct relationship to the width of PWM pulses. Alternatively, the control signal may be the power applied to the fans  400 A and  400 B which may be switched on and off, with the controller  700  setting the amplitude of the power signal to control the speed of the fans  400 A and  400 B. Alternatively, the speed of the fans  400 A and  400 B may be controlled using any suitable methods of fan speed control. 
         [0032]    The fans  400 A and/or  400 B may be operated, in an embodiment, at higher speed to induce more airflow over the outdoor coils  300 A and/or  300 B, increasing the rate of heat transfer between the VRF system  1000  refrigerant and the outdoor air and reducing the refrigerant head pressure. Operation of the fans  400  at higher speeds may accommodate higher heating or cooling demand on the VRF system  1000  and/or may be in response to sufficiently high ambient outdoor air temperatures, allowing for greater heat transfer at the outdoor coil, or coils, while still maintaining the refrigerant head pressure within a safe range for VRF system  1000  operation. 
         [0033]    Conversely, in an embodiment, the fans  400 A and/or  400 B may be operated at lower speeds, or turned off, to reduce the airflow over the outdoor coils  300 A and/or  300 B, reducing the rate of heat transfer between the VRF system  1000  refrigerant and the outdoor air, causing an increase in refrigerant head pressure. Operation of one or more of the fans  400  at lower speeds, or turning one or more of the fans  400  off, may accommodate low heating or cooling demand on the VRF system and/or may be in response to cooling operation at low, or extra low, ambient outdoor air temperatures. 
         [0034]    During cooling operation at low, or extra low, ambient outdoor air temperatures one or more fans  400  may be turned off, or reduced to a low, or lowest, speed setting, decreasing the heat transfer rate between the VRF system  1000  refrigerant and the ambient outdoor air and reducing the amount of refrigerant head pressure loss as the refrigerant passes through the outdoor coil, or coils,  300 A and  300 B. According to the embodiment shown in  FIG. 1 , for example, during cooling operation in low, or extra low, ambient outdoor air temperatures, the fan, or fans,  400 A may be configured to rotate at the lowest speed setting while the fan, or fans,  400 B may be de-energized. 
         [0035]    According to this configuration, the outdoor coil  300 A can be described as the “active coil,” in which heat transfer between the refrigerant of the VRF system  1000  and the ambient outdoor air is induced through operation of the fan, or fans,  400 A. The outdoor coil  300 B can be described as the “inactive coil,” in which little to no heat transfer between the VRF system  1000  refrigerant and the ambient outdoor air is induced since the fan, or fans,  400 B are not energized. Operation of the fans  400  in this manner may allow for continued VRF system cooling operation in low, or extra low, ambient outdoor air temperatures while still maintaining refrigerant head pressures within a safe range for VRF system  1000  component operation since the “inactive coil” functions as a hot gas bypass for the portion of the refrigerant passing through it. 
         [0036]    As shown in  FIG. 1 , the VRF system  1000  may include two metering devices  500 A and  500 B for controlling the rate of refrigerant flow between VRF system  1000  components and causing a pressure drop of the refrigerant fluid while the VRF system  1000  is operating in heating mode, as part of the vapor compression cycle. In cooling mode, the metering devices  500 A and  500 B may be, typically, in the fully open position. Either or both of the metering devices  500 A and  500 B may be expansion valves. These expansions valves may be of any suitable type including electronic expansion valves (EXV). The expansion valves may any valves that regulates the flow of the refrigerant fluid inside VRF system  1000 . 
         [0037]    In an embodiment, the metering devices  500 A and  500 B may both be EXVs which may each be operatively connected to, and receive control signals from, the system controller  700  by a wired or wireless connection. The system controller  700  may control each metering device  500 A and  500 B, adjusting the size of the opening through the metering devices  500 A and  500 B that the VRF system  1000  refrigerant may flow. The desired setting of each EXV may be determined by the controller  700  in response to received data from temperature and pressure sensors within the VRF system  1000  and system components, sensing ambient outdoor air temperature, refrigerant temperature, refrigerant pressure, and the like. EXV control during operation at low, or extra low, ambient outdoor air temperatures may be provided in accordance with any suitable methods of EXV control. 
         [0038]    System controller  700  may have an interface  702 , processor  704 , and memory  706  for performing the functions of system controller  700 . The system controller  700  memory may store VRF system  1000  characteristics such as a maximum pressure level, a threshold pressure level, and a threshold temperature value for triggering a low ambient temperature mode in memory  706 . Memory  706  may include any one or a combination of volatile or non-volatile local or remote devices suitable for storing information. For example, memory  706  may include RAM, ROM, flash memory, magnetic storage devices, optical storage devices, network storage devices, cloud storage devices, solid state devices, or any other suitable information storage device or a combination of these devices. Memory  706  may store, either permanently or temporarily, data, operational software, other information for system controller  700 . Memory  706  may store information in one or more databases, file systems, tree structures, relational databases, any other suitable storage system, or any combination thereof. Furthermore, different information stored in memory  706  may use any of these storage systems. The information stored in memory  706  may be encrypted or unencrypted, compressed or uncompressed, and static or editable. Memory  706  may store information in one or more caches. 
         [0039]    Interface  702  may receive and transmit signals and inputs from and to users, remote sensors, or any other component of VRF system  1000 . Interface  702  may also communicate with processor  704  and memory  706 . Interface  702  may be any port or connection, real or virtual, including any suitable hardware and/or software, including protocol conversion and data processing capabilities, to communicate through a LAN, WAN, or other communication system that allows system controller  700  to exchange information with any user or component of VRF system  1000 . For example, interface  702  may be operable to receive temperature information or pressure information from remote temperature and pressure sensors. A temperature sensor may be any thermometer or other temperature sensing device. The temperature sensor may be alcohol based, mercury based or based on any other suitable material. 
         [0040]    Processor  704  may be any electronic circuitry, including, but not limited to microprocessors, application specific integrated circuits (ASIC), application specific instruction set processor (ASIP), and/or state machines, that communicatively couples interface  702  and memory  706  and controls the operation of system controller  700 . In some embodiments, processor  704  may be single core or multi-core having a single chip containing two or more processing devices. Processor  704  may be 8-bit, 16-bit, 32-bit, 64-bit or of any other suitable architecture. Processor  704  may comprise an arithmetic logic unit (ALU) for performing arithmetic and logic operations, processor registers that supply operands to the ALU and store the results of ALU operations, and a control unit that fetches instructions from memory and executes them by directing the coordinated operations of the ALU, registers and other components. Processor  704  may include other hardware and software that operates to control and process information. Processor  704  may execute computer-executable program instructions stored in system controller  700  memory. Processor  704  may not be limited to a single processing device and may encompass multiple processing devices. 
         [0041]    During cooling operation during low, or extra low, ambient outdoor air temperatures, the metering devices  500 A and  500 B may be commanded to a desired setting by the system controller  700 . The desired settings may be those corresponding to a rate of refrigerant flow passing through each of the “active” and “inactive” outdoor coils. As shown in  FIG. 1 , for example, the size of the opening through the metering device  500 A may be increased, or decreased, to permit more, or less, of the VRF system  1000  refrigerant from the outdoor coil  300 A, which may be configured to be an “active coil,” to flow to the indoor sections of the VRF system  1000 . Similarly, the size of the opening through the metering device  500 B may be increased, or decreased, to permit more, or less, of the VRF system  1000  refrigerant from the outdoor coil  300 B, which may be configured to be an “inactive coil,” to flow to the indoor sections of the VRF system  1000 . 
         [0042]    In this manner, the controller  700  may adjust the mix of refrigerant flow passing from the outdoor coils  300 A and  300 B to the indoor sections of the VRF system  1000 , controlling the amount of refrigerant “bypassing” the “active coil” to influence the overall head pressure of the mixed refrigerant routed to the indoor units of the VRF system  1000 . According to the example described above, and in reference to  FIG. 1 , adjusting the settings of the metering devices  500 A and  500 B to permit more refrigerant flow through the “active coil”  300 A may cause a reduction in refrigerant head pressure for the mixed refrigerant routed to the indoor units of the VRF system  1000 . Conversely, adjusting the settings of the metering devices  500 A and  500 B to permit more refrigerant flow through the “inactive coil”  300 B may cause an increase in refrigerant head pressure for the mixed refrigerant routed to the indoor units of the VRF system  1000 . 
         [0043]    The VRF system  1000  may include a reversing valve  600  for setting the direction of flow of refrigerant in the VRF system in one of two directions, as desired, and in accordance with any suitable methods of heat pump operation. Although the VRF system  1000  shown is configured for heat pump operation, the present disclosure may be implemented in a VRF system not comprising a reversing valve  600  and configured to accommodate refrigerant flow in only one direction. 
         [0044]    The VRF system  1000  may be provided with a system controller  700  for controlling operation of VRF system  1000  components, including the compressor assembly  100  components, the valves  200 A and  200 B, the fans  400 A and  400 B, the metering devices  500 A and  500 B, and the reversing valve  600 , as well as other components comprising the VRF system  1000  not shown in  FIG. 1 . The controller  700  may be connected to the VRF system  1000  components via wired or wireless connections. The controller  700  may be implemented with hardware, software, or firmware defining methods of VRF system  1000  control operation. Further, the controller  700  may be implemented with logic for VRF system  1000  control during cooling operation in low, or extra low, ambient outdoor air temperatures in accordance with the method shown in  FIG. 2 . 
         [0045]    Turning now to  FIG. 2 , the controller  700  may control the VRF system  1000  components according to the flowchart shown in  FIG. 2  during cooling operation in low, or extra low, ambient outdoor air temperatures. At step  201 , the VRF system  1000  may enter low ambient cooling mode in response to input to the controller  700  from one or more system sensors sensing temperature, pressure, VRF system demand mode, and the like in accordance with control logic defining the VRF system  1000  operation that may be stored within the controller  700  memory. The VRF system  1000  may be configured to enter low ambient mode at times when the VRF system  1000  is operating in normal cooling mode, with the fans  400  set to their lowest speed settings, and upon the controller  700  sensing that the refrigerant head pressure in the VRF system  1000  is too low for safe operation in normal cooling mode. 
         [0046]    At step  201 , the controller  700  may configure the valves  200 A and  200 B to route refrigerant flow from the compressor assembly  100  discharge to the outdoor coils  300 A and  300 B. The controller  700  may set the metering devices  500 A and  500 B to the fully open settings, allowing maximum refrigerant flow through each leg of the VRF system  1000  piping. 
         [0047]    At step  202 , the controller  700  may configure the outdoor coil  300 B to be an “inactive coil” by de-energizing the fans  400 B so that no ambient outdoor air flow is induced over the outdoor coil  300 B. At step  203 , the controller may monitor VRF system  1000  refrigerant pressure throughout the VRF system  1000  using system sensors sensing refrigerant pressures and temperatures according to any suitable methods. The controller  700  may compare the sensed refrigerant pressure to a range of threshold values of refrigerant pressure defining a safe range of refrigerant pressures for the VRF system  1000  to continue cooling operation. 
         [0048]    If the controller  700  determines that the refrigerant head pressure of the VRF system  1000  is too low for safe cooling operation, the controller  700  may generate a control signal adjusting the setting of the metering device  500 A at step  204 . The controller  700  may close the “active” EXV by a predetermined number of steps, routing less of refrigerant flow passing through the “active coil,” to the indoor units of the VRF system  1000 . Choking the refrigerant flow from the “active coil” in this manner may increase the refrigerant pressure of the mixture of refrigerant flows from the outdoor coils that is routed to the indoor units of the VRF system  1000  as the ratio of condensed refrigerant to “bypassed” refrigerant is adjusted to increase the relative amount of “bypassing” refrigerant. 
         [0049]    Alternatively, at step  204 , the controller  700  may determine that the head pressure of the VRF system  1000  is lower than desired. The controller  700  may adjust the setting of the “inactive” EXV, opening it to allow more refrigerant routed through the “inactive coil” to pass through to the indoor units of the VRF system  1000 , causing an increase in system head pressure. This alternative control option may only be available to the controller  700  in instances where the operating conditions are fluctuating, such that the controller  700  may have closed the “inactive” EXV in response to sensed conditions at some earlier point in VRF system  1000  operation. 
         [0050]    The controller  700  may detect the setting of the metering device  500 A at step  205 . If the “active” EXV has been closed in response to the controller detecting too low refrigerant head pressure to the point where the “active” EXV is fully closed, the controller  700  may cease cooling operation of the system  700  at step  206 , by de-energizing the compressor assembly  100  and fans  400 A, to prevent damage to system components that may be caused by operation at refrigerant pressures outside of a defined safe range. Alternatively, the controller  700  may respond by altering the setting of the “inactive” EXV, partially closing it to choke the refrigerant flow through the “inactive” EXV to increase the head pressure. Alternatively, the controller  700  may continue cooling operation with the “active” EXV closed for a period of time while monitoring the refrigerant pressure. If the “active” EXV is not in the fully closed setting, the controller  700  may continue to monitor refrigerant head pressure at step  203 . 
         [0051]    If, at step  203 , the controller  700  detects that the refrigerant head pressure is too high, the controller  700  may adjust the metering device  500 B to reduce flow through the “inactive coil,” the outdoor coil  300 B. The controller  700  may close the “inactive” EXV by a predetermined number of steps, routing less of refrigerant flow passing through the “inactive coil,” to the indoor units of the VRF system  1000 . Choking the refrigerant flow from the “inactive coil” in this manner may decrease the refrigerant pressure of the mixture of refrigerant flows from the respective outdoor coils that is routed to the indoor units of the VRF system  1000  by manipulating the ratio of “condensed” refrigerant and “bypassed” refrigerant to reduce the amount of “bypassing” refrigerant. 
         [0052]    Alternatively, or additionally, at step  207  the controller  700  may increase the speed of the fans  400 A, inducing more ambient air flow over the “active coil,” the outdoor coil  300 A, and causing a reduction in the refrigerant head pressure for the portion of the refrigerant routed through the “active coil,” the outdoor coil  300 A. 
         [0053]    In yet another alternative, at step  207  the controller  700  may adjust the setting of the “active” EXV to allow more refrigerant routed through the “active coil,” the outdoor coil  300 A to pass through to the indoor units of the VRF system  1000 . This alternative control option may only be available to the controller  700  in instances where the operating conditions are fluctuating, such that the controller  700  may have closed the “active” EXV in response to sensed conditions at some earlier point in VRF system  1000  operation. 
         [0054]    The controller  700  may detect the setting of the metering device  500 B at step  208 . If the “inactive” EXV has been closed in response to the controller  700  detecting too high refrigerant head pressure to the point where the “inactive” EXV is fully closed, the controller  700  may cease cooling operation of the system  700  in low ambient mode, and commence operation in normal cooling mode at step  209 . 
         [0055]    Turning now to  FIG. 3 , the refrigerant flow routing through the VRF system  1000  during low ambient operation, as described by the method  2000  of  FIG. 2 , is shown. As shown in  FIG. 3 , the VRF system  1000  refrigerant may be routed along the path shown in solid lines, and in the directions indicated by arrows. Refrigerant may be configured to flow from the compressor assembly  100 , through both valves  200 A and  200 B to the outdoor coils  300 A and  300 B. The metering devices  500 A and  500 B may be adjusted by the controller  700  to control the flow of refrigerant from each outdoor coil,  300 A and  300 B, respectively, permitted to pass to the indoor units of the VRF system  1000 . Controlling the respective rates of refrigerant flow in this manner may allow the controller  700  to adjust the refrigerant mixture ratio to manipulate the refrigerant head pressure in the VRF system  1000 , maintaining the refrigerant head pressure within a safe range for VRF system  1000  operation when operating in cooling mode in low, or extra low, ambient outdoor air temperatures. 
         [0056]    Modifications, additions, or omissions may be made to the systems, apparatuses, and processes described herein without departing from the scope of the disclosure. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. The methods may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. Additionally, operations of the systems and apparatuses may be performed using any suitable logic. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
         [0057]    Although several embodiments have been illustrated and described in detail, it will be recognized that substitutions and alterations are possible without departing from the spirit and scope of the present disclosure, as defined by the appended claims. To aid the Patent Office, and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants do not intend any of the appended claims to invoke 35 U.S.C. §112(f) as it exists on the date of filing hereof unless the words “means for” or “step for” are explicitly used in the particular claim.