LOW-PRESSURE REFRIGERANT CONTROL

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Application Serial No. 62/817,840, entitled“SYSTEMS AND METHODS FOR LOW-PRESSURE REFRIGERANT CONTROL,” filed March 13, 2019, which is herein incorporated by reference in its entirety for all purposes.

BACKGROUND

[0002] The present disclosure relates generally to heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems, and more particularly to systems and methods for low-pressure refrigerant control. Examples of HVAC&R systems may include water- cooled and/or air-cooled chillers, water-source/air-source reversing/non-reversing heat pumps, air conditioners, refrigerators, and so forth.

[0003] Vapor compression systems utilize a working fluid, typically referred to as a refrigerant, which changes phases between vapor, liquid, and combinations thereof in response to being subjected to different temperatures and pressures associated with operation of the vapor compression systems. Certain vapor compression systems direct the refrigerant between vessels disposed at various heights. However, to enable liquid refrigerant to flow through the certain vapor compression systems, liquid-carrying conduits may be restrictively designed to direct liquid refrigerant to flow in alignment with gravity and/or a pump may be used to direct the liquid refrigerant upward. Unfortunately, these certain vapor compression systems may require a large equipment cost, a large operating cost, and/or a large footprint within a facility to accommodate these liquid-refrigerant considerations. SUMMARY

[0004] In one embodiment of the present disclosure, a heating, ventilation, air conditioning, and refrigeration (HVAC&R) system includes a high-pressure vessel including a first shell and a fluid outlet extending from the first shell. The HVAC&R system also includes a low-pressure vessel including a second shell and a fluid inlet extending into the second shell. The low-pressure vessel is disposed above the high- pressure vessel. The HVAC&R system includes a conduit coupling the fluid outlet to the fluid inlet. The conduit is configured to direct a two-phase flow of a refrigerant upward within a vertical portion of the conduit.

[0005] In another embodiment of the present disclosure, a heating, ventilation, air conditioning, and refrigeration (HVAC&R) system includes a high-pressure refrigerant source configured to output a refrigerant. The HVAC&R system also includes a low- pressure refrigerant receiving system configured to receive the refrigerant. The low- pressure refrigerant receiving system is disposed above the high-pressure refrigerant source. The HVAC&R system also includes a conduit coupling the high-pressure refrigerant source to the low-pressure refrigerant receiving system. Additionally, the HVAC&R system includes a refrigerant control system configured to control direction of a two-phase flow of the refrigerant upward within a vertical portion of the conduit by vapor injection, vapor generation, valve control, or a combination thereof.

[0006] In a further embodiment of the present disclosure, heating, ventilation, air conditioning, and refrigeration (HVAC&R) system includes a high-pressure refrigerant source configured to provide a refrigerant. The high-pressure refrigerant source includes a horizontally-extending conduit or a high-pressure vessel. The HVAC&R system also includes a low-pressure refrigerant receiving system configured to receive the refrigerant. The low-pressure refrigerant receiving system is disposed above the high-pressure refrigerant source, and the low-pressure refrigerant receiving system includes a motor housing, a variable speed drive, a falling-film evaporator, a hybrid falling-film evaporator, a flash tank, or a separator. The HVAC&R system includes a conduit coupling the high-pressure refrigerant source to the low-pressure refrigerant receiving system. Additionally, the HVAC&R system includes a refrigerant control system configured to control direction of a two-phase flow of the refrigerant upward within a vertical portion of the conduit via vapor injection, vapor generation, valve control, or a combination thereof.

[0007] Other features and advantages of the present application will be apparent from the following, more detailed description of the embodiments, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the application.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 is a perspective view of an embodiment of a building that may utilize a heating, ventilation, air conditioning, and refrigeration (HVAC&R) system in a commercial setting, in accordance with the present techniques;

[0009] FIG. 2 is a perspective view of an embodiment of a vapor compression system, in accordance with the present techniques;

[0010] FIG. 3 is a schematic diagram of an embodiment of the vapor compression system, in accordance with the present techniques;

[0011] FIG. 4 is a schematic diagram of an embodiment of the vapor compression system, in accordance with the present techniques;

[0012] FIG. 5 is a schematic diagram of an embodiment of a refrigerant control system for the vapor compression system, in accordance with the present techniques;

[0013] FIG. 6 is a schematic diagram of an embodiment of the vapor compression system having the refrigerant control system for providing liquid level control and for injecting vapor into a conduit, in accordance with the present techniques; [0014] FIG. 7 is a schematic diagram of an embodiment of the vapor compression system having a flash tank and the refrigerant control system for liquid level control and for injecting vapor to the conduit, in accordance with the present techniques;

[0015] FIG. 8 is a flow chart of an embodiment of a method for operating the refrigerant control system, in accordance with the present techniques;

[0016] FIG. 9 is a schematic diagram of an embodiment of the vapor compression system having multiple vapor injection points into the conduit, in accordance with the present techniques;

[0017] FIG. 10 is a schematic diagram of an embodiment of the vapor compression system having a heat source for generating vapor within the conduit, in accordance with the present techniques;

[0018] FIG. 11 is a schematic diagram of an embodiment of the vapor compression system having multiple heat sources for generating vapor within the conduit, in accordance with the present techniques;

[0019] FIG. 12 is a schematic diagram of an embodiment of the vapor compression system having the refrigerant control system with multiple separators and a heat source to move refrigerant upward, in accordance with the present techniques; and

[0020] FIG. 13 is a schematic diagram of an embodiment of the vapor compression system having the refrigerant control system to move refrigerant upward at a low-head condition within a hermetic low-pressure embodiment of the vapor compression system, in accordance with the present techniques.

DETAILED DESCRIPTION

[0021] The present disclosure is directed to heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems and systems and methods for controlling refrigerant within the HVAC&R systems. In general, an HVAC&R system includes a closed refrigeration circuit, having both an evaporator configured to evaporate a refrigerant therein and a condenser configured to condense the refrigerant therein, to enable the HVAC&R system to condition an interior space. By employing a low-pressure refrigerant, such as R-1233zd(E), the HVAC&R system may employ a reduced amount of refrigerant and/or have an enhanced performance as compared to traditional HVAC&R systems. For example, employing low-pressure refrigerant may enable achievement of operation at lower-head conditions in the HVAC&R system that accordingly enables lower entering condenser water (or other working fluid) temperatures, without excessively or unsuitably low evaporator pressures. However, under certain conditions, a pressure difference alone between two points in the closed refrigeration circuit of the low-pressure refrigerant may not be sufficient to direct liquid flows of the low-pressure refrigerant upward through vertical conduits, such that packaging and control for the HVAC&R system may be more difficult or complicated. Indeed, when the low-pressure refrigerant is flowing upward through a vertical conduit, gravitational forces may cause a sizable hydrostatic pressure drop in the low-pressure refrigerant, thereby negatively affecting operation. To reduce the hydrostatic pressure drop, it is presently recognized that traditionally high-density, single-phase flows of liquid refrigerant may be altered to low-quality, two-phase flows having a substantially less average density, which reduces both the gravitational impact on momentum and the hydrostatic pressure drop. As used herein, quality or vapor quality refers to the mass fraction of a saturated mixture or fluid flow that is vapor, such that a saturated liquid has a quality of zero percent and a saturated vapor has a quality of one hundred percent.

[0022] Stated in other words, the present disclosure benefits operation by enabling an enthalpy change (e.g., isentropic specific enthalpy change) for a given pressure difference between points within the HVAC&R system to be greater with low-quality, two-phase flow than the enthalpy change would be with single-phase liquid. Indeed, because gravitational potential energy per unit mass is fixed for a given change in height, the greater enthalpy change enables two-phase flow to occur when liquid flow may otherwise be impractical or unachievable.

[0023] Accordingly, embodiments discussed herein may employ a refrigerant control system to reliably control expansion valves, drain valves, and/or other valves and control elements to improve an efficiency of an HVAC&R system and enable further reduction of a refrigerant charge (e.g., amount) of the HVAC&R system. For example, by reducing the refrigerant charge and controlling a two-phase flow of refrigerant between various components of the HVAC&R system, upward flow of the refrigerant without a pump may be achieved. For example, as discussed in more detail below, the refrigerant control system may use vapor injection, vapor generation, and/or valve control schemes to insert or allow small amounts of vapor into the upward flow of the refrigerant. Thus, the refrigerant control system may reduce a footprint and increase an operating efficiency of the HVAC&R system, as compared to systems without the disclosed refrigerant control system that may use more complicated and costly measures to direct refrigerant throughout the system. Additionally, the refrigerant control system may extend the range of possible operation of the HVAC&R system, which may be particularly beneficial for conditions in which there is a small difference (e.g., less than a threshold difference) between evaporation and condensation temperatures. These and other features will be described in detail below with reference to the drawings herein.

[0024] Turning now to the drawings, FIG. 1 is a perspective view of an embodiment of an environment for a heating, ventilation, air conditioning, and refrigeration (HVAC&R) system 10 in a building 12 for a typical commercial setting. The HVAC&R system 10 may include a vapor compression system 14 that supplies a chilled liquid, which may be used to cool the building 12. The HVAC&R system 10 may also include a boiler 16 to supply warm liquid to heat the building 12 and an air distribution system which circulates air through the building 12. The air distribution system can also include an air return duct 18, an air supply duct 20, and/or an air handler 22. In some embodiments, the air handler 22 may include a heat exchanger that is connected to the boiler 16 and the vapor compression system 14 by conduits 24. The heat exchanger in the air handler 22 may receive either heated liquid from the boiler 16 or chilled liquid from the vapor compression system 14, depending on the mode of operation of the HVAC&R system 10. The HVAC&R system 10 is shown with a separate air handler on each floor of building 12, but in other embodiments, the HVAC&R system 10 may include air handlers 22 and/or other components that may be shared between or among floors.

[0025] FIGS. 2 and 3 are embodiments of the vapor compression system 14 that can be used in the HVAC&R system 10. The vapor compression system 14 may circulate a refrigerant through a circuit starting with a compressor 32. The circuit may also include a condenser 34, an expansion valve(s) or device(s) 36, and a liquid chiller or an evaporator 38. The vapor compression system 14 may further include a control panel 40 that has an analog to digital (A/D) converter 42, a microprocessor 44, a non-volatile memory 46, and/or an interface board 48.

[0026] Some examples of fluids that may be used as refrigerants in the vapor compression system 14 are hydrofluorocarbon (HFC)-based refrigerants (e.g., R-410A, R-407C, R-134a), hydrochlorofluorocarbon refrigerants (e.g., R-123, R-22), hydrofluoroolefm (HFO) refrigerants (e.g., R-1336mzz(Z), R-1234yf, R-1234ze(E)), hydrochlorofluoroolefm (HCFO) refrigerants (e.g., R-1233zd(E), R-1224yd(Z)), chlorinated hydorocarbons (e.g., trans-1, 2-dichloroethylene, methylene chloride), methyl chloride azeotropic blends (e.g., R-514A, R-513A, R-515A),“natural” refrigerants (e.g., ammonia (NH3), R-717, carbon dioxide (C02), R-744, water vapor, R-718, hydrocarbon based refrigerants), or any other suitable refrigerant. In some embodiments, the vapor compression system 14 may be configured to efficiently utilize refrigerants having a normal boiling point of or above about 0 degrees Celsius or 32 degrees Fahrenheit at one atmosphere of pressure, also referred to as low-pressure refrigerants, versus a medium- pressure refrigerant, such as R-134a, or high pressure refrigerants such as R-410A. As used herein,“normal boiling point” may refer to a boiling point temperature measured at one atmosphere of pressure. [0027] In some embodiments, the vapor compression system 14 may use one or more of a variable speed drive (VSD) 52, a motor 50, the compressor 32, the condenser 34, the expansion valve or device 36, and/or the evaporator 38. The motor 50 may drive the compressor 32 and may be powered by a variable speed drive (VSD) 52. The VSD 52 receives alternating current (AC) power having a particular fixed line voltage and fixed line frequency from an AC power source, and provides power having a variable voltage and frequency to the motor 50. In other embodiments, the motor 50 may be powered directly from an AC or direct current (DC) power source. The motor 50 may include any type of electric motor that can be powered by a VSD or directly from an AC or DC power source, such as a switched reluctance motor, an induction motor, an electronically commutated permanent magnet motor, or another suitable motor.

[0028] The compressor 32 compresses a refrigerant vapor and delivers the vapor to the condenser 34 through a discharge passage. In some embodiments, the compressor 32 may be a centrifugal compressor. The refrigerant vapor delivered by the compressor 32 to the condenser 34 may transfer heat to a cooling fluid, such as water or air, in the condenser 34. The refrigerant vapor may condense to a refrigerant liquid in the condenser 34 as a result of thermal heat transfer with the cooling fluid. The liquid refrigerant from the condenser 34 may flow through the expansion device 36 to the evaporator 38. In the illustrated embodiment of FIG. 3, the condenser 34 is water cooled and includes a tube bundle 54 connected to a water cooling device like a cooling tower 56, which supplies the cooling fluid to the condenser 34.

[0029] The liquid refrigerant delivered to the evaporator 38 may absorb heat from another cooling fluid, which may or may not be the same cooling fluid used in the condenser 34. The liquid refrigerant in the evaporator 38 may undergo a phase change from the liquid refrigerant to a refrigerant vapor. As shown in the illustrated embodiment of FIG. 3, the evaporator 38 may include a tube bundle 58 having a supply line 60S and a return line 60R connected to a cooling load 62. The cooling fluid of the evaporator 38, such as water, ethylene glycol, calcium chloride brine, sodium chloride brine, or any other suitable fluid, enters the evaporator 38 via return line 60R and exits the evaporator 38 via supply line 60S. The evaporator 38 may reduce the temperature of the cooling fluid in the tube bundle 58 via thermal heat transfer with the refrigerant. The tube bundle 58 in the evaporator 38 can include a plurality of tubes and/or a plurality of tube bundles. In any case, the vapor refrigerant exits the evaporator 38 and returns to the compressor 32 by a suction line to complete the cycle.

[0030] FIG. 4 is a schematic diagram of the vapor compression system 14 with an intermediate circuit 64 incorporated between condenser 34 and the expansion device 36. The intermediate circuit 64 may have an inlet line 68 that is directly fluidly connected to the condenser 34. In other embodiments, the inlet line 68 may be indirectly fluidly coupled to the condenser 34. As shown in the illustrated embodiment of FIG. 4, the inlet line 68 includes a first expansion device 66 positioned upstream of an intermediate vessel 70. In some embodiments, the intermediate vessel 70 may be a flash tank or a flash intercooler. In other embodiments, the intermediate vessel 70 may be configured as a heat exchanger or a "surface economizer." In the illustrated embodiment of FIG. 4, the intermediate vessel 70 is used as a flash tank, and the first expansion device 66 is configured to lower the pressure of or expand the liquid refrigerant received from the condenser 34. During the expansion process, a portion of the liquid may vaporize, and thus, the intermediate vessel 70 may be used to separate the vapor from the liquid received from the first expansion device 66. Additionally, the intermediate vessel 70 may provide for further expansion of the liquid refrigerant because of a pressure drop experienced by the liquid refrigerant when entering the intermediate vessel 70, such as due to a rapid increase in volume experienced when entering the intermediate vessel 70. The vapor in the intermediate vessel 70 may be drawn by the compressor 32 through a suction line 74 of the compressor 32. In other embodiments, the vapor in the intermediate vessel may be drawn to an intermediate stage of the compressor 32 after the suction stage. The liquid that collects in the intermediate vessel 70 may be at a lower enthalpy than the liquid refrigerant exiting the condenser 34 because of the expansion in the expansion device 66 and/or the intermediate vessel 70. The liquid from intermediate vessel 70 may then flow in line 72 through a second expansion device 36 to the evaporator 38.

[0031] Moreover, in accordance with present techniques, a refrigerant control system may be incorporated in any of the HVAC&R systems illustrated in FIGS. 1-4. For example, the refrigerant control system may control refrigerant flowing within the HVAC&R system 10 of FIG. 1, the vapor compression system 14 of FIGS. 1-4, or any other suitable system employing refrigerant or fluid. As discussed herein, the refrigerant control system operates to maintain and/or direct a two-phase flow of refrigerant upward through conduits without a pump. Thus, the refrigerant control system may reduce a refrigerant charge, reduce a footprint or packaging size, reduce operating costs, and/or extend an operating map (e.g., performance map) of the HVAC&R system to improve operation of the HVAC&R system. The refrigerant control system will be described in greater detail below with reference to FIGS. 5-8 below.

[0032] FIG. 5 is a schematic diagram of an embodiment of a refrigerant control system 100 for the vapor compression system 14 of the HVAC&R system 10 discussed above. For example, the refrigerant control system 100 may operate to move a refrigerant 102, such as liquid refrigerant or liquid-rich, two-phase refrigerant, from a high-pressure vessel 110 of the vapor compression system 14 to a low-pressure vessel 112 of the vapor compression system 14. The high-pressure vessel 110 may generally have a pressure that is greater than a pressure of the low-pressure vessel 112 during operation of the vapor compression system 14. For example, in some embodiments, the high-pressure vessel 110 may be a condenser and the low-pressure vessel 112 may be a compressor motor housing, an evaporator, or a VSD component. However, it should be understood that any other suitable vessels having a suitable pressure difference therebetween, such as a pressure difference that is greater than zero or another predetermined threshold value, may be used by the techniques disclosed herein. Additionally, the refrigerant 102 may be any suitable low-pressure refrigerant having a normal boiling point of or above about 0 degrees Celsius at one atmosphere of pressure, such as R-123, R-1233zd(E), R1336mzz, R514A, R1233zd(Z), and so forth. While traditional low-pressure vapor compression systems may be arranged such that liquid refrigerant flows downward with gravity or is pumped upward via a pump or other device, the present refrigerant control system 100 obviates at least these restrictions of the traditional low-pressure vapor compression systems by facilitating upward flow of the refrigerant 102 as a two-phase flow from the high-pressure vessel 110 to the low-pressure vessel 112.

[0033] In particular, the high-pressure vessel 110 includes a high-pressure shell 114 and an outlet 116 (e.g., fluid outlet) extending from the high-pressure shell 114. A conduit 120 is coupled between the outlet 116 and an inlet 122 (e.g., fluid inlet), such as a nozzle, extending into a low-pressure shell 124 of the low-pressure vessel 112. Further, an expansion valve 126, such as the expansion device 36 discussed above, may be disposed along the conduit 120. As referenced herein, the high-pressure vessel 110 is installed “below” the low-pressure vessel 112, such that the high-pressure vessel 110 includes a high-pressure centerline 130 disposed a shell height difference 132 below a low-pressure centerline 134 of the low-pressure vessel 112. Moreover, as described herein, relative directional terms describing components of the vapor compression system 14 are intended to be interpreted relative to the direction of gravity and/or relative to other components of the vapor compression system 14 when the HVAC&R system is in an operating position or orientation. For example, such relative directional terms may include above, below, higher, lower, vertical or vertically-extending, horizontal or horizontally extending, and so forth. Thus, vertically-extending components are generally vertical with respect to gravity and/or their respective operating position and horizontally-extending components are generally horizontal with respect to gravity and/or their respective operating position.

[0034] Additionally, a liquid level 140 of the refrigerant 102 within the high-pressure vessel 110 is located a liquid height difference 142 below a top portion 144 of the inlet 122 within the low-pressure vessel 112. As such, based on the pressure within the high- pressure vessel 110, the refrigerant 102 may be directed through the outlet 116 of the high-pressure vessel 110, along the conduit 120, through the expansion valve 126, and into the inlet 122 of the low-pressure vessel 112. As discussed herein, the refrigerant control system 100 enables the refrigerant 102 to flow upward from the high-pressure vessel 110 to the low-pressure vessel 112 without assistance from auxiliary equipment, such as a pump.

[0035] For example, it is presently recognized that in the HVAC&R system 10 employing the low-pressure refrigerant 102, the pressure within the high-pressure vessel 110 alone may not be sufficient to motivate a subcooled liquid flow of the refrigerant 102 upward through the conduit 120 under certain conditions. As such, various considerations for the phase changes of the refrigerant 102 may be considered, such as when a subcooled liquid flow is directed upward, the refrigerant 102 may be unstable under certain conditions due to transitions between single-phase flow and two-phase flow, which may occur at lower values of subcooling. Then, under conditions in which flashing from single-phase flow to two-phase flow occurs, a mass flow rate from the high-pressure vessel 110 may be greatly reduced compared to a mass flow rate for single phase flow under most conditions, or such flow may not occur. Alternatively, to avoid or reduce concerns related to maintaining subcooled single-phase flow, a two-phase flow 150 of the refrigerant 102 may be directed through the inlet 122 of the low-pressure vessel 112. The inclusion of vapor within the refrigerant 102 reduces a density of the refrigerant 102 within a vertical portion 152 of the conduit 120, which allows upward flow to occur at low-head conditions where single-phase liquid may not reliably flow.

[0036] Accordingly, the refrigerant control system 100 operates to maintain or generate the two-phase flow 150 of the refrigerant 102 within the vertical portion 152 of the conduit 120. The refrigerant control system 100 may generate the two-phase flow of the refrigerant 102 by vapor injection, expansion valve control, or both. Additionally, the refrigerant control system 100 includes a controller 154 having a processor 156 and a memory 158 for executing instructions and receiving signals, as discussed herein. The controller 154 may be the control panel 40 discussed above, in certain embodiments. In order to generate and maintain a target mass flow rate of the refrigerant 102 under flashing conditions, valves, spray nozzles, piping, and any other components of the vapor compression system 14 may be sized, enlarged, and/or positioned for the two-phase flow condition. For example, by positioning the expansion valve 126 at a low height or a lowest point along the conduit 120 within the vapor compression system 14, the expansion valve 126 is more likely to receive a single-phase liquid flow or a lower- quality (liquid-rich) two-phase flow. As such, a size of the expansion valve 126 may be reduced as compared to an expansion valve that receives a flow having a higher-quality flow.

[0037] Accordingly, to provide vapor injection into the conduit 120, the refrigerant control system 100 includes a vapor injection line 160 or vapor injection conduit extending between the high-pressure shell 114 and the vertical portion 152 of the conduit 120. For example, the vapor injection line 160 may direct a vapor flow 162 of the refrigerant 102 from the high-pressure vessel 110 to join a liquid flow 164 of the refrigerant 102 flowing within the vertical portion 152 of the conduit 120. As referred to herein, it is to be understood that the vapor flow 162 may be either a vapor-rich (high- quality) two-phase flow or a vapor flow, and the liquid flow 164 may be a liquid-rich (low-quality) two-phase flow or a liquid flow. Thus, by mixing the two flows 162, 164 together, the two-phase flow 150 of the refrigerant having a low vapor quality is generated within the conduit 120 that has a lower density than the liquid flow 164. As such, the pressure of the high-pressure vessel 110 may move a greater quantity of the two-phase flow 150 upward through the conduit 120, as compared to the liquid flow 164. Additionally, in some embodiments, a vapor outlet 166 of the vapor injection line 160 may be oriented such that the vapor flow 162 out of the vapor injection line 160 is upward in a same direction as the liquid flow 164, which increases the benefit of the kinetic energy of the vapor flow 162 to the two-phase flow 150. For such embodiments, an eductor, injector, or the like, may be disposed at the vapor outlet 166. [0038] In some embodiments, the vapor injection line 160 may be actively or passively controlled. For passive control, the vapor injection line 160 may be designed with a fixed orifice or line size that regulates the vapor flow 162 entering the conduit 120. For active control that may enable improved performance over the fixed orifice, an optional vapor control valve 168 or vapor injection control valve communicatively coupled to the controller 154 may be disposed along the vapor injection line 160 to provide additional control options to the refrigerant control system 100, such as for increasing, decreasing, stopping, or permitting the vapor flow 162 to enter the conduit 120.

[0039] Vapor injection may find particular use in motor cooling applications in which the low-pressure vessel 112 is a compressor motor housing, VSD, or other component that is cooled by the refrigerant 102 or utilizes a liquid refrigerant or two-phase refrigerant for another purpose, such as bearing lubrication. For example, when the HVAC&R system 10 is at a maximum liquid supply condition, a pressure drop through the expansion valve 126 may be relatively low, which results in a relatively low vapor flow 162 through the vapor injection line 160. However, because the vapor injection line 160 is located above the liquid level 140 in the high-pressure vessel 110, which may be a condenser, at least a small amount of the vapor flow 162 may enter the conduit 120 even with the low- pressure drop through the expansion valve 126. Alternatively, at low liquid supply conditions, the expansion valve 126 may close, thus reducing the liquid flow 164 of the refrigerant 102 and allowing an increased flow rate for the vapor flow 162. The increased flow rate for the vapor flow 162 may increase a velocity of the refrigerant 102 entering the low-pressure vessel 112 and improve distribution of the refrigerant 102 therein. The details of control vapor injection depend on many factors, including the geometry of the lines, the relative height of the liquid level 140 in the high-pressure vessel 110 to the injection point within the low-pressure vessel 112, pressure losses through a subcooler and liquid line piping, etc. It is to be understood that testing and modeling may be required to establish optimum performance of the HVAC&R system 10. [0040] Various advantages are provided via employing this configuration for motor cooling a compressor or other device of the vapor compression system 14. For example, the refrigerant control system 100 improves stability of the vapor compression system 14 because there is a reduced or eliminated demand to transition between single-phase flow and two-phase flow. Additionally, vapor injection, as controlled by the refrigerant control system 100, may improve spray distribution from the inlet 122 or nozzle, which reduces or eliminates localized overcooling or other maldistribution of cooling. Moreover, the vapor injection enables the refrigerant 102 to flow upward in the conduit 120 flow for a very wide range of motor cooling requirements and head conditions. Operating modes of the HVAC&R system 10 with a small motor cooling requirement can approach 100% vapor cooling, which reduces or avoids potential sweating of a motor housing (e.g., the low-pressure vessel 112) because the vapor flow 162 enters the motor housing near the condensing temperature of the refrigerant 102. In addition to cosmetic issues associated with condensation of moisture, water condensation can pose reliability problems if electronics are located on the outside of the motor housing.

[0041] Additional considerations may be made in other embodiments in which the HVAC&R system 10 uses a flash tank economizer. For example, the available pressure difference between the condenser and evaporator is split between multiple expansion valves. Moreover, flash tank designs may include liquid entry at a top portion of the flash tank, which increases the effects of elevation differences between the various components. Further, packaging of the vapor compression system 14 may be improved, as the reduced demand for having liquid flows of the refrigerant 102 flow downward enables the packaging or footprint of the vapor compression system 14 to be reduced compared to that of conventional flash-tank configurations with less-flexible component locations.

[0042] In addition or as an alternative to injecting vapor into the conduit 120, the refrigerant control system 100 may supply the two-phase flow 150 through the expansion valve 126 to ensure that the two-phase flow 150 of the refrigerant 102 is present in the vertical portion 152 of the conduit 120 under various operating conditions. Indeed, in some embodiments, the vapor injection line 160 may be omitted and the two-phase flow 150 of the refrigerant 102 may be directed through the expansion valve 126. For the two- phase flow 150 to exit the high-pressure vessel 110, the refrigerant control system 100 may maintain the liquid level 140 of the refrigerant 102 within the high-pressure vessel 110 to be absent or below a lower level threshold, such that conventional sensor-based level control may not be practical. This condition may be referred to herein as“reverse operation.” Thus, the refrigerant control system 100 may control the liquid level 140 to be below the lower level threshold by determining a valve position for the expansion valve 126 that allows a small amount of vapor flow to escape with liquid leaving the high-pressure vessel 110. As discussed in more detail below, the valve position is determined based on an estimated mass flow rate through the vapor compression system 14, a pressure difference between the high-pressure vessel 110 and the low-pressure vessel 112, and/or known valve characteristics associated with the expansion valve 126. Indeed, embodiments of a method for adjusting an actual position of the expansion valve 126 to the target valve position is described with reference to FIG. 8 below.

[0043] Looking now to further examples of the techniques disclosed herein, FIG. 6 is an embodiment of a portion of the vapor compression system 14 of the HVAC&R system 10 having the refrigerant control system 100 for providing liquid level control and for injecting vapor to the conduit 120. In the present embodiment, the high-pressure vessel 110 discussed above is a condenser, such as the condenser 34 of FIG. 2, and the low- pressure vessel 112 discussed above is a housing 200 (e.g., the low-pressure shell 124) of a compressor, such as the compressor 32 of FIG 2. The housing 200 is preferably a housing for a hermetic motor that is part of the compressor 32. Indeed, as illustrated, the housing 200 of the compressor 32 is fluidly coupled at a top portion 202 of the conduit 120 to receive the refrigerant 102 for motor cooling. In some embodiments, the conduit 120 preferentially directs the refrigerant 102 to a top portion of the housing 200 to enable the refrigerant 102 to be sprayed over top of the motor and other compressor components within the housing 200 for cooling. The conduit 120 may further extend from the housing 200 of the compressor 32 into the evaporator 38 of FIG. 2. As such, a motor within the housing 200 may be cooled and/or lubricated by the refrigerant 102 directed therein. However, in some embodiments, the low-pressure vessel 112 may be any suitable component for receiving the refrigerant 102 or the refrigerant 102 may be directed directly to the evaporator 38 or the VSD. Moreover, a level sensor 210 is disposed within the condenser 34, the vapor injection line 160 and the vapor control valve 168 are disposed between the condenser 34 and the conduit 120, and a condenser drain valve 212, which serves as an expansion valve between the condenser and the flash tank, is disposed in a lower portion 214 of the conduit 120. Additionally, the controller 154 is communicatively coupled to the compressor 32, the level sensor 210, the vapor control valve 168, and the condenser drain valve 212. In some embodiments, the condenser 34 uses either a flash subcooler or uses no subcooler to reduce the refrigerant charge of the vapor compression system 14, reduce equipment costs, and reduce material costs. As shown, a condenser tube bundle 216 preferably extends to fill a bottom portion of the high-pressure shell 114 of the condenser 34. A suction line between the evaporator 38 and the compressor 32, a discharge line between the compressor 32 and the condenser 34, and a liquid line between the condenser 34 and the evaporator 38 are not shown in the present embodiment to enable the conduit 120 to be viewed more clearly.

[0044] In the present embodiment, the level sensor 210 monitors the liquid level 140 and transmits sensor signals to the controller 154. Then, the controller 154 controls the condenser drain valve 212 based on the liquid level 140 under most operating conditions. For example, in some embodiments, a target liquid level is a function of cooling load conditions of the vapor compression system 14. At high cooling load conditions (e.g., above an upper threshold cooling load), a void fraction or empty space fraction within the evaporator 38 is large, indicating that that a target liquid level in the condenser 34 is relatively high. At this condition, the refrigerant control system 100 may instruct the condenser drain valve 212 to modulate to maintain a target liquid level in the condenser, which results in partial coverage of the condenser tube bundle 216 and an amount of subcooling of the refrigerant 102 exiting the condenser 34. [0045] At an intermediate cooling load condition (e.g., between the upper threshold cooling load and a lower threshold cooling load), the void fraction within the evaporator 38 is lower and a greater mass of the refrigerant 102 is used for operating the evaporator 38. As such, the target liquid level in the condenser 34 is lower and subcooling of the refrigerant 102 is also reduced. At an even lower cooling load condition (e.g., below the lower threshold cooling load), the target liquid level in the condenser 34 may fall below a lower liquid level that enables reliable level control using the level sensor 210. At this condition, the refrigerant control system 100 may change to open-loop control, which allows the vapor flow 162 to leave the condenser 34 though the outlet 116 with the liquid flow 164, such that the two-phase flow 150 travels through the condenser drain valve 212 and up the vertical portion 152 of the conduit 120. To return to closed-loop control, the controller 154 may receive signals from the level sensor 210 indicating the liquid level 140 is above the lower liquid level, and then override the open-loop control.

[0046] Moreover, in some embodiments, the refrigerant control system 100 may control the vapor compression system 14 according to a related approach in which sensor feedback indicative of a liquid level within the evaporator 38 received from a level sensor disposed within the evaporator 38. The level sensor 210 may be set to monitor a lower or lowest liquid level within the evaporator 38 that can be reliably measured. When the refrigerant control system 100 determines that the liquid level within the evaporator 38 is below the lower liquid level, the control of the condenser drain valve 212 shifts to open- loop control until the liquid level within the evaporator 38 is above its setpoint value. In this manner, the refrigerant control system 100 blocks the condenser drain valve 212 from opening beyond a threshold value during conditions in which there is no longer a liquid level in the condenser 34.

[0047] To select a target refrigerant charge for the vapor compression system 14, a system designer may consider full-load and part-load performance of the vapor compression system 14, unit cost considerations, operation at low-head conditions, and other factors. For example, at full-load design conditions and a given evaporator level, increasing a charge of the refrigerant 102 in the vapor compression system 14 may increase the amount of subcooling for the refrigerant 102 leaving the condenser 34 and raise a pressure within the condenser 34, which may decrease an operating efficiency of the vapor compression system 14. Additionally, increasing the charge of the refrigerant 102 increases the cost of the vapor compression system 14, such that a target charge of the refrigerant 102 should consider this cost as well. Moreover, at part-load conditions of the vapor compression system 14, an operating benefit from subcooling is smaller and a greater charge of the refrigerant 102 may be used by the evaporator 38.

[0048] At low head conditions of the vapor compression system 14, sufficient storage capacity is present in the evaporator 38 to enable vapor to exit the condenser 34 without excessive liquid carryover from the evaporator 38. If high-load operation is required at low head, the vapor injection line 160 discussed above may be optionally included within the vapor compression system 14 of FIG. 6 to assure or improve movement of the two- phase flow 150 upward in the vertical portion 152 of the conduit 120, while a portion of the refrigerant 102 is stored in the condenser 34. As such, opening the vapor control valve 168 disposed along the vapor injection line 160 may enable operation where the entering condenser fluid temperature is lower than the leaving chilled fluid temperature, or inverted operation, because it is possible to starve the evaporator 38 while storing the refrigerant 102 in the condenser 34, while reducing or minimizing refrigerant pressure differences that may result in excessively or undesirably low suction pressures. The refrigerant control system 100 is therefore able to address considerations with directing or lifting the liquid flow 164 of the refrigerant 102 upward from the condenser 34 to the evaporator 38 for a wide range of operating conditions. Indeed, although the effect of liquid head is usually most important at low-head and low-load conditions, the refrigerant control system 100 addresses this concern by enabling a small amount of vapor to leave the condenser 34 to create low-quality, two-phase flow in the vertical portion 152 of the conduit 120. [0049] FIG. 7 is an embodiment of the vapor compression system 14 having a flash tank 250 and the refrigerant control system 100 for liquid level control and for injecting vapor to the conduit 120. The flash tank 250 is coupled along the conduit 120 between the condenser 34 and the evaporator 38. While the present embodiment includes the condenser 34 disposed above the flash tank 250, any other suitable configuration of the components within the HVAC&R system 10 may be controlled by the refrigerant control system 100. As previously described, the refrigerant control system 100 may control the condenser drain valve 212 via a combination of closed-loop control based on input from the level sensor 210 in the condenser 34 and open-loop control. Additionally, a flash tank drain valve 252 coupled along the conduit 120 is controlled based on open-loop control that enables a small amount of vapor to exit through a bottom portion 254 of the flash tank 250. The refrigerant control system 100 may receive sensor signals transmitted from a flash tank level switch 256 disposed within the flash tank 250 to determine whether a liquid level is present within the flash tank 250. Then, in response to determining that the liquid level is present, the refrigerant control system 100 may instruct the flash tank drain valve 252 to open. As shown, the controller 154 is communicatively coupled to the compressor 32, the vapor control valve 168, the level sensor 210, the condenser drain valve 212, the flash tank drain valve 252, and the flash tank level switch 256. As mentioned above, a suction line between the evaporator 38 and the compressor 32, a discharge line between the compressor 32 and the condenser 34, and a liquid line between the condenser 34 and the evaporator 38 are not shown in the present embodiment to enable the conduit 120 to be viewed more clearly. Additionally, the compressor 32 may include an open or air cooled motor or the motor may be cooled by suction gas, such that liquid flows of the refrigerant 102 may not be employed to cool the embodied compressor 32.

[0050] The present embodiment of the refrigerant control system 100 provides control over a wide range of operating conditions of the vapor compression system 14. As with the single-stage configuration discussed above, the condenser 34 has a liquid level at full load and subcooled liquid exits from the condenser 34 through the condenser drain valve 212. At part-load conditions, the refrigerant control system 100 may cause the refrigerant 102 to move from the condenser 34 to the evaporator 38. As such, the vapor compression system 14 shown in FIG. 7 is capable of directing the refrigerant 102 upward through the conduit 120, even at relatively low condenser water temperatures.

[0051] With the multi-stage system, various factors and options may be considered. For example, a reversed economizer flow may be directed from the compressor 32 to the flash tank 250, in some embodiments. Additionally, a control valve may be included along an economizer line to limit flow. An effect of liquid head on required pressure difference between the condenser 34 and the evaporator 38 may also be manipulated. Further, independent capacity control may be included for each of multiple compressor stages of the compressor 32. Additionally, vapor injection may also be included in the present embodiment, as represented by the vapor injection line 160.

[0052] FIG. 8 is a flow chart of an embodiment of a method 300 for operating the refrigerant control system 100 to control the two-phase flow 150 of the refrigerant 102 from the bottom of a flash tank or other suitable vessel. The method 300 of FIG. 8 is described with reference to the elements of FIGS. 1-7. It should be understood that the illustrated steps of the method 300 are illustrative of one example of operation of the refrigerant control system 100, as certain steps may be omitted, performed simultaneously, and/or performed in a different sequence from the sequence in FIG. 8. The method 300 may be performed by the controller 154 of the refrigerant control system 100 or by another suitable controller communicatively coupled to the refrigerant control system 100.

[0053] First, as indicated by block 302, the controller 154 sets a valve position for the expansion valve 126 to a default position. The default position may be a user-set, a technician-set, or a distributor- set value stored within the memory 158 of the controller 154. By instructing the expansion valve 126 to move to the default position after starting the method 300, the controller 154 enables the vapor compression system 14 to operate normally during the following determinations. [0054] As indicated by block 304, the controller 154 may next determine whether the compressor 32 is operating. Because the controller 154 is communicatively coupled to the compressor 32, the controller 154 may monitor signals sent by the controller 154, received by the compressor 32 or a motor of the compressor 32, or a combination thereof to determine whether the compressor 32 is operating. When operating, the compressor 32 moves the refrigerant 102 through the vapor compression system 14 to enable conditioning of the building 12. In response to determining that the compressor 32 is not operating, the controller 154 may return to set or maintain the valve position as the default position, as indicated by block 302.

[0055] In response to determining that the compressor 32 is operating, the controller 154 may determine whether a liquid level in the high-pressure vessel 110 is above a liquid level threshold, as indicated by block 306. In response to determining that the liquid level in the high-pressure vessel 110 is above the liquid level threshold, the controller 154 may instruct the expansion valve 126 to open, as indicated by block 308, and then return to determine whether compressor 32 is operating, as indicated by block 304.

[0056] In response to determining that the liquid level in the high-pressure vessel 110 is below the liquid level threshold, the controller 154 may determine an estimated mass flow rate within the vapor compression system 14, as indicated by block 310. As indicated by block 312, the controller 154 also determines a target valve position based on the estimated mass flow rate and an available pressure difference between the high- pressure vessel 110 and the low-pressure vessel 112. The calculated valve position may be determined by logic or algorithms that adjust the valve position based on high-level indications to better match actual chiller conditions.

[0057] For embodiments in which the HVAC&R system 10 includes a screw compressor and/or other suitable positive displacement devices, the estimated mass flow rate may be determined via equations based on a density of liquid, a pressure difference between the high-pressure vessel 110 and the low-pressure vessel 112, and the liquid height difference 142. Then, a target valve position for the expansion valve 126 may be selected via a look up table of the estimated mass flow rate versus valve position curve for the expansion valve 126. In some embodiments, instead of the look up table, calculation algorithms may be used to determine the target valve position for the expansion valve 126. In some embodiments, the refrigerant control system 100 may use a target pressure for the low- pressure vessel 112, or evaporator pressure, based on a leaving evaporator water temperature to block or prevent the expansion valve 126 from closing in response to starving the low-pressure vessel 112 or evaporator.

[0058] For embodiments in which the HVAC&R system 10 includes a centrifugal compressor, a different process may be used to calculate the estimated mass flow rate and therefore the target valve position. For example, in some embodiments, the controller 154 may receive various parameters, such as a compressor speed in units of inverter output frequency, suction and discharge pressures and temperatures, variable geometry diffuser (VGD) and/or vane position, and inverter output power. Then, the refrigerant control system 100 may use a curve fit equation that uses the various parameters combined with refrigerant properties and a geometry of the vapor compression system 14 to determine a dimensionless flow through the vapor compression system 14. The dimensionless flow may be used to calculate the estimated mass flow rate through the vapor compression system 14 based on an impeller tip diameter of a compressor of the vapor compression system 14, a density of a suction vapor into the compressor, and a speed of sound at a suction inlet of the compressor. Thus, the estimated mass flow rate may be used to determine the target valve position for the expansion valve 126.

[0059] Further, as indicated by block 314, the controller 154 instructs the expansion valve 126 to move to the target valve position, and returns to determine whether compressor 32 is operating, as indicated by block 304. The method 300 may be utilized to control valves for refrigerant leaving flash tanks or condensers without conventional subcoolers. Compared to conventional level control, the refrigerant control system 100 performing the method 300 may advantageously eliminate or reduce refrigerant charge associated with liquid reservoirs and provide for a controlled amount of vapor in the liquid line, which may further facilitate upward flow with low-pressure refrigerant.

[0060] Moreover, although discussed above with reference to vapor injection into the conduit 120 occurring through a single vapor injection line 160, it is to be understood that the vapor injection line 160 may alternatively branch into multiple injection points to provide a more uniform flow of the two-phase flow 150 of the refrigerant 102. For example, FIG. 9 is a schematic of an embodiment of the vapor compression system 14 having multiple vapor injection points 350 or vapor injection conduits to facilitate upward movement of the refrigerant 102 through the conduit 120. As illustrated, the conduit 120 extends between the compressor 32 and a liquid line 354 that is connected between the condenser 34 and the expansion valve 36. In other words, an inlet 360 of the conduit 120 may be connected downstream of the condenser 34 and upstream of the expansion valve 36. Additionally, an outlet 362 of the conduit 120 may be connected to the housing 200 of the compressor 32 or any suitable receiving system that uses the two- phase flow 150 of the refrigerant 102 for facilitating operation of the vapor compression system 14. The vapor injection points 350 or outlets may be disposed in parallel with one another to enable vapor to be injected at multiple positions, such as vertical positions, along the conduit 120. As such, the vapor injection points 350 adaptively enable a portion of a liquid flow of the refrigerant 102 from the condenser 34 to be moved upward into the compressor 32 for lubricating and/or cooling the motor of the compressor 32 or to the VSD for cooling of an electrical component. Another portion of the refrigerant 102 may continue from the condenser 34, through the expansion valve 36, through the evaporator 38, and into the compressor 32, as discussed above.

[0061] In the illustrated embodiment, the vapor injection points 350 are spaced vertically and are connected to the condenser shell, discharge line, or any other suitable source of higher-pressure vapor. Further, each vapor injection point 350 may include a corresponding valve 364 that may control flow of the higher-pressure vapor through the respective vapor injection point 350. In some embodiments, a manifold valve 366 may be included within along the vapor injection line 160 to control the vapor flow 162 of the refrigerant 102 to the vapor injection points 350. The manifold valve 366 may operate similar to the vapor control valve 168 discussed above with reference to FIG. 5.

[0062] At low-load conditions, a sufficient pressure difference between the condenser 34 and the liquid line 354 may not be present to provide flow to vapor injection points 350 having vertical positions located below the liquid level 140 of the refrigerant 102 in the condenser 34. In such conditions, the refrigerant control system 100 may inject the vapor flow 162 of the refrigerant 102 into the conduit 120 at vapor injection points 350 located above the liquid level 140. Once upward two-phase flow 150 of the refrigerant 102 is established via injection through upper vapor injection points 350 (e.g., vapor injection points 350 positioned vertically above the liquid level 140), it may be desirable to close the valves 364 for the upper vapor injection points 350 and open the valves 364 for the lower vapor injection points 350 (e.g., vapor injection points 350 positioned vertically below the liquid level 140).

[0063] Indeed, it may be desirable to inject vapor into the conduit 120 at multiple vapor injection points 350 simultaneously. For example, if the liquid flow 164 of the refrigerant 102 leaving the condenser 34 is subcooled, vapor injection at one vapor injection point 350 may result in vapor and liquid being present at non-equilibrium conditions, such that the vapor may eventually condense within the conduit 120 to create single-phase liquid flow. Thus, by injecting vapor further downstream relative to the path of the refrigerant 102 flowing upward through the conduit 120, the two-phase flow 150 of the refrigerant 102 may be reestablished or maintained to enable the refrigerant 102 to be directed upward, without assistance from auxiliary pumping equipment.

[0064] In some embodiments, self-regulation of the vapor flow 162 may be possible without all or a portion of the valves 364. For example, the vapor flow 162 to the vapor injection points 350 below the liquid level 140 within the condenser 34 may naturally fill with liquid refrigerant at some conditions. The liquid refrigerant may drain once the two- phase flow 150 of the refrigerant 102 is established in the vapor injection points 350 of conduit 120 above them, which then allows the vapor flow 162 through the vapor injection points 350 below the liquid level 140. Additionally, although described with reference to the multiple vapor injection points 350 branching from one vapor injection line 160, it is to be understood that each vapor injection point 350 may alternatively branch from a respective vapor injection line 160, may share a vapor injection line with only one other vapor injection point 350, and so forth.

[0065] Moreover, although discussed above with reference to injecting the vapor flow 162 of the refrigerant 102 into the conduit 120 and/or valve control that enable an existing vapor flow 162 to pass through expansion valves and into the conduit 120, it is to be understood that the present refrigerant control system 100 may also utilize any suitable vapor generation techniques to form the two-phase flow 150 of the refrigerant 102. That is, raising or pumping liquid flows of the refrigerant 102 in low-pressure refrigerant systems may present challenges, particularly as pressure differences between adjacent components of the vapor compression system 14 decreases. In the process of raising a subcooled liquid flow of the refrigerant 102 vertically in elevation, hydrostatic pressure loss can cause reduction in flow (or even complete loss of flow) when limited by driving pressure. By generating vapor within the liquid refrigerant, the hydrostatic loss may be reduced, thus enabling a greater liquid flow than in traditional systems without the present refrigerant control system 100. Indeed, in manner similar to that of air used in air lift pumps to motivate fluid upward, the present refrigeration control system 100 may introduce a vapor flow of refrigerant into a liquid flow to drive a two-phase flow of the refrigerant 102 upward within the conduit 120.

[0066] Additionally, in some embodiments, the present refrigeration control system 100 may use alternative methods to vapor injection into a vertical liquid line. The alternative methods discussed herein may reduce or eliminate complications with limited injection point pressure, which may be limited by vapor pressure and/or liquid pressure under some conditions. That is, in the absence of a vapor compression device to pressurize the vapor flow 162 of the refrigerant 102 discussed above, if the liquid pressure of a liquid flow of the refrigerant within a vertical conduit at a vapor injection point is higher than the vapor pressure of the vapor supplied to the injection point from the higher-pressure component, then the liquid pressure may be reduced to enable vapor to be drawn into the liquid flow 164. To further address these considerations, the refrigerant control system 100 of certain embodiments may include a vapor generation device in addition or alterative to the vapor injection and/or valve controls discussed above.

[0067] For example, FIG. 10 is a schematic diagram of an embodiment of the refrigerant control system 100 having a heat source 400, heater, or heating device disposed within a low point 402 or bottom portion of the conduit 120. That is, instead of injecting the vapor flow 162 into the vertically rising liquid flow 164, it is additionally recognized herein that supplying heat to the liquid flow 164 may generate vapor in-situ within the vertically rising refrigerant 102. To maximize or improve the available pumping effect, the liquid line may extend downward as low as possible within physical constrains of the vapor compression system 14, with the heat source 400 preferably located near a vertical portion of the conduit 120, directly downstream of the low point 402. The heat source 400 may include a heating element 410 or filament in thermal contact with the conduit 120 and a control module 412 communicatively coupled to the controller 154. However, in other embodiments, the heat source 400 may be any other suitably warm portion or heat source of the vapor compression system, such as a heat exchanger.

[0068] As shown, the heat source 400 may be included within the refrigerant control system 100 to apply heat to an outer surface 404 or wall of the conduit 120, within the flowing liquid within the conduit 120, or any other suitable location. The heat source 400 may then generate vapor bubbles within the refrigerant 102 to reduce or substantially reduce a density of the refrigerant 102 within the conduit 120 for the given liquid pressure. In some embodiments, the heat source 400 may also operate as a flow control device, similar to a valve, where, with addition of heat to the refrigerant 102, a flow rate of the resulting two-phase flow 150 of the refrigerant 102 may increase, peak, and then decline until choking or dropout within the refrigerant 102 occurs. [0069] As illustrated in FIG. 10, for further improved flow, the liquid flow 164 of the refrigerant 102 to be moved upward may be sourced from the high-pressure vessel 110 with a liquid pool of the refrigerant 102 at saturated conditions. Then, the liquid flow 164 may be brought to a lower elevation in the vapor compression system 14, thereby allowing the liquid pressure of the refrigerant 102 to rise due the momentum imparted by gravity (or due to the hydrostatic pressure gain) with a height difference 420 between the liquid level 140 within the high-pressure vessel 110 and an elevation 422 of the low point 402 of the conduit 120. At this low point 402, with high-pressure liquid (which is at a higher pressure than the source vessel), the heat source 400 may be positioned to generate bubbles and to allow the two-phase flow 150 of the refrigerant 102 to rise in elevation to another, lower pressure portion of the vapor compression system 14. Whether utilizing vapor injection or bubble generation, the present refrigerant control system 100 may efficiently and reliably introduce and/or form the vapor bubbles within the conduit 120. In certain conditions, if vertically raising or lifting a saturated liquid flow of the refrigerant 102, the refrigerant 102 may flash, in theory, if the refrigerant 102 is at thermodynamic equilibrium. However, under actual operating conditions, the refrigerant 102 may not flash. Thus, vapor generation assists the process of flashing and can be performed even when the liquid refrigerant 102 is subcooled.

[0070] Variants of the refrigerant control system 100 may include multiple heat sources 400 disposed vertically in series along the conduit 120, as illustrated in FIG. 11. As such, in the event of re-condensation of vapor bubbles within the two-phase flow 150 of the refrigerant 102, such as due to an originating liquid flow of the refrigerant 102 being sufficiently subcooled, the vapor portion of the two-phase flow 150 may be regenerated by a downstream heat source 400 relative to a flow direction of the two-phase flow 150 of the refrigerant 102 through the conduit 120.

[0071] Another variation of the refrigerant control system 100 is illustrated in FIG. 12, in which a multiple-stage pumping system 440 is included to further increase or improve an available pumping effect provided by the refrigerant control system 100. As shown, the conduit 120 or vertical riser directs the two-phase flow 150 of the refrigerant 102 into a separator 450 (e.g., first separator). The separator 450 may direct liquid flow 164 of the refrigerant 102 downward to heat source 400 in a low point 460 of another conduit 462, where a higher-pressure two-phase flow 470 is generated that moves to another separator 450 (e.g., second separator). A vapor flow 162 may be removed from each separator 450 though a vapor outlet line 472. In some embodiments, control valves 474 are disposed along the vapor outlet lines 472 and control the vapor flows 162 from the separators 450 based on actuation by the controller 154. While two separators 450 are shown, the concept may be extended to additional stages as well. As noted above, it is to be understood that the vapor flow 162 may be either a vapor-rich, two-phase flow or a vapor flow, and the liquid flow 164 may be a liquid-rich, two-phase flow or a liquid flow.

[0072] The refrigerant control system 100 may also use the separators 450 without a heat source 400 or vapor generator, where the liquid column extending from the low point 460 of the conduit 462 may supply high pressure liquid at some position below the liquid level within the separator 450 downstream of the conduit 462 relative to a flow direction of the refrigerant 102. In some embodiments, there may be a demand to employ vapor injection in combination with vapor generation or other control devices, such as valves. Additionally, it should be noted that once the liquid flow 164 or two-phase flow 150 of the refrigerant 102 is used in the system component to which the refrigerant 102 is sent, the refrigerant control system 100 may not necessarily return the refrigerant 102 to a lower pressure component. That is, because the refrigerant 102 may be supplied at higher pressure compared to other flows or pools of the refrigerant 102 within the vapor compression system 14 (due to momentum imparted to the refrigerant 102 by gravity) the refrigerant 102 may be directed to a high-pressure component of the vapor compression system 14, such as a motor housing operating at a higher-pressure, and then returned to another higher-pressure component, such as the condenser 34 from which the refrigerant 102 may have originated. [0073] Embodiments that include the heat sources 400 may include controls to prevent or reduce operation of the heat sources 400 that may result in excessively high temperatures. For example, the heat sources 400 may be thermostatically-controlled heating devices that operate in a limited or predetermined temperature range. Another, alternative control includes use of level sensors that detect the presence of adequate liquid for cooling a heat source 400 combined with control logic to prevent or reduce operation of the heat source 400 without adequate supply of liquid. Yet another alternative may include a simple high-temperature-limit switch or sensor that disables the heat source 400 if a temperature of the heat source 400 reaches a threshold temperature. Pressure relief may also be used in some embodiments to prevent or reduce development or increase of refrigerant pressure as a result of heater operation that is above a pressure threshold. Pressure relief may be achieved through piping or dedicated relief valves, depending on the location of shut-off valves within the vapor compression system 14.

[0074] FIG. 13 is a schematic of an embodiment of the refrigerant control system 100 configured to enable the two-phase flow 150 of the refrigerant 102 to move upward at a low-head condition within a hermetic low-pressure embodiment of the vapor compression system 14. As illustrated, a conduit 500 or liquid line connects a bottom portion 502 of the condenser 34 with a top portion 504 of the evaporator 38. Additionally, the expansion valve 36 may be disposed along the conduit 500 between the condenser 34 and the evaporator 38, as discussed above. The evaporator 38 may preferably be a falling-film or hybrid falling-film design evaporator, in some embodiments. The main refrigerant flow may also be sent to a higher flash tank or separator rather than a falling-film evaporator, in other embodiments. In some embodiments, a liquid flow of the refrigerant 102 may be drawn from a higher portion of the liquid line rather than from the high-pressure shell 114 of the condenser 34 or lower section of the conduit 500, which provides higher available pressure for cooling a hermetic motor. The present configuration may be especially suitable for use in low- pressure hermetic centrifugal chillers. [0075] During operation of the vapor compression system 14, a liquid or liquid-rich, two- phase flow of the refrigerant 102 may exit the condenser 34. If the condenser 34 includes a conventional subcooler, then subcooled liquid refrigerant 102 normally exits the condenser 34. If the condenser 34 has no subcooler or has a flash subcooler, then liquid refrigerant with little subcooling or liquid-rich, two-phase flow exits the condenser 34. As shown in FIG. 13, vapor injection through the vapor injection line 160 with or without the optional vapor control valve 168 and/or flashing of the liquid refrigerant results in a liquid-rich, two-phase flow 510 through a vertical section 512 of the conduit 500, which assists in carrying liquid flow of the refrigerant 102 upward. Alternatively, vapor may be supplied through the conduit 500 by lowering the liquid level 140 in the condenser 34 until the liquid reservoir in the condenser 34 is depleted and vapor exits with the outgoing liquid refrigerant. Details related to vapor injection or bypass of vapor to improve upward flow of liquid refrigerant are discussed above.

[0076] A top section 520 or horizontally-extending portion of the conduit 500 or liquid line directs the two-phase flow 510 horizontally and then downward toward the expansion valve 36 that feeds refrigerant 102 to the evaporator 38. The top section 520 also includes a liquid source line 524 or liquid source conduit that collects a working liquid flow 526 of the refrigerant for cooling, lubricating, or otherwise enhancing operation of a component of the vapor compression system 14. The liquid source line 524 draws the refrigerant liquid that naturally accumulates along an inner bottom surface 530 of the horizontal portion of the top section 520 of the conduit 500 by the action of buoyancy. The liquid source line 524 is sized for a low fluid velocity so that entrained vapor floats back upward to the conduit 500 and the remaining liquid flows downward to a heat source 400 located near a bottom portion 536 of the vapor compression system 14. An optional separator may be included at or an entrance 532 or connection point of the liquid source line 524 in some embodiments to further enhance separation of liquid refrigerant 102 from the vapor refrigerant 102, if desired. [0077] The heat source 400 at the bottom portion 536 of the liquid source line 524 may boil or vaporize a portion of the working liquid flow 526 of the refrigerant to create a liquid-rich, two-phase flow 150 upward through a vertical section 540 of the liquid source line 524, downstream of the heat source 400. A separator 542 disposed near the top of the vapor compression system 14 receives this two-phase flow 150. Liquid refrigerant 102 flows from a bottom 544 of the separator 542, through a liquid conduit 546 having a cooling control valve 550, and into a receiving system 548 or vessel of the vapor compression system 14. The receiving system 548 may be any suitable component or device that receives refrigerant 102 for any suitable operation. In the illustrated embodiment, the receiving system 548 preferably includes a motor of the compressor 32. The separator 542 is preferably located above the receiving system 548 to minimize flashing, in some embodiments. The cooling control valve 550 may be modulated by a controller (e.g., the controller 154 discussed above) to maintain an appropriate amount of refrigerant flow to the receiving system 548 for a set of appropriate operating limitations. The operating limitations may include situations where more or less than a target amount of the refrigerant liquid is supplied to the receiving system 548. In some embodiments, it may be desirable to supply the receiving system 548 with two-phase refrigerant at some specified vapor quality, rather than purely single-phase liquid. In such cases, any of the previous described methods of introducing vapor into a liquid may be utilized to provide the refrigerant 102 at a desired or target quality to the receiving system 548.

[0078] From a top portion 552 of the separator 542, a vapor conduit 554 having a vapor valve 556 or fixed orifice allows vapor or vapor-rich, two-phase flow 558 of the refrigerant 102 to leave the top of the separator 542. The separator 542 may be of a simple design, such as a section of larger diameter piping, and have a relatively small size compared to any other separators of the vapor compression system 14, because liquid carryover in the vapor conduit 554 may have little or no impact on performance. Instead, the refrigerant control system 100 may focus on reducing or minimizing vapor flow out of the liquid conduit 546 from the separator 542 to maximize available liquid flow to the receiving system 548. Superheated vapor or vapor-rich, two-phase flow may exit a bottom portion 560 of the system receiving the liquid, travel through a conduit 562, and drain or vent to the evaporator 38. Another option is to direct the vapor to another portion of the vapor compression system 14 that may use the vapor to assist with another objective before sending the vapor to the evaporator 38. Other options include returning the vapor to the liquid conduit 546, the liquid source line 524, the condenser 34, or other portion of the vapor compression system 14.

[0079] It is noted herein that the separator 542 and the associated vapor conduit 554 are optional. Furthermore, in certain embodiments, the separator 542 may not work on the principles of separating the liquid-phase from the vapor-phase, but may instead work partly or entirely on the basis of heat transfer to re-condense the vapor to liquid, such as via a heat exchanger or mixer. That is, liquid-rich, two-phase flow can go from the heat source 400, through the cooling control valve 550, and into the receiving system 548 without passing through a separator. In such embodiments, the cooling control valve 550, along with passages or spray nozzles through the vapor compression system 14 may be sized for the larger volume flow occurring due to the two-phase flow.

[0080] The above description of FIG. 13 assumes that two-phase flow of the refrigerant 102 occurs in the vertical section 512 of the conduit 500. Alternatively, if there is sufficient subcooling of the refrigerant 102 exiting the condenser 34 and sufficient pressure difference between the condenser 34 and the evaporator 38, then a liquid flow of the refrigerant 102 may naturally flow upward to the separator 542 at the top of the vapor compression system 14 without flashing. These conditions typically occur at or near a nominal design condition. Under such conditions, the heat source 400 may be deactivated or turned off. In addition, the vapor valve 556 from the separator 542 and vapor control valve 168 may be closed. Deactivating the heat source 400 and closing the vapor control valve 168 and the vapor valve 556 may improve system efficiency and capacity in some embodiments.

[0081] At lower head operation of the vapor compression system 14, the vapor control valve 168 may be set at or near a predetermined target or optimized position to minimize or reduce a pressure difference between the condenser 34 and the evaporator 38. In some embodiments, laboratory testing may be performed to determine the target position of the vapor control valve, which may be a function of vapor compression load or other parameters. The heat source 400 may be activated to generate the vapor in the two-phase flow 150 of the refrigerant 102 directed to the separator 542, and the vapor valve 556 may be opened to enable the vapor or vapor-rich, two-phase flow 558 to travel to the evaporator 38. Further, at lower head operation, the cooling control valve 550 may be opened to enable liquid or two-phase fluid to be directed to the receiving system 548.

[0082] In some embodiments, the heat source 400 may modulate the heat output to the refrigerant 102 to maintain control over the amount of liquid refrigerant provided to the receiving system 548 with the cooling control valve 550 fully open. Alternatively, improved control may be achieved by setting the heat output by the heat source 400 to a predetermined value that maximizes or increases the available refrigerant flow and allows the cooling control valve 550 to modulate appropriately.

[0083] This setup allows the pressure of the refrigerant 102 going into the receiving system 548 to be substantially higher than the pressure of the condenser 34. In a base- case design without a heat source 400 and with no flashing, the pressure at the receiving system 548 is reduced by an amount corresponding to a liquid head between the liquid level 140 in the condenser 34 and the inlet to the receiving system 548. In the system shown in FIG. 13, the liquid head between the vapor injection point and receiving system 548 inlet may be relatively small (e.g., lower than a threshold).

[0084] The present refrigeration control system 100 may advantageously draw liquid refrigerant from the bottom of the condenser 34 or the conduit 500 leaving the condenser 34. In this comparison, the advantage is a static liquid head, Ah, which corresponds to an elevation change 570 between the liquid surface in the condenser and the entrance 532 to the liquid source line 524 from the conduit 500. [0085] Additional features may be provided for further reliability and stability of the vapor compression system 14. For example, a filter or strainer may be positioned in the liquid source line 524 to prevent or reduce contaminants or particles from reaching the receiving system 548. Preferred locations for a filter may be directly upstream of the cooling control valve 550 or in the vertical section 540 of the liquid source line 524 between the heat source 400 and the liquid conduit 546. A differential pressure transducer or switch may also be included within the refrigerant control system 100 to sense a large pressure drop that indicates a clogged filter for possible notification messages or alerts to be transmitted.

[0086] Sensors may also be included in the refrigerant control system 100 to prevent or reduce overheating of the heat source 400. In some embodiments, the heat source 400 may be de-energized when the compressor 32 is not running. The heat source 400 may include a thermostat feature that may turn the heat source 400 off when a threshold or maximum temperature is reached. In addition to protecting the heat source 400, the maximum temperature is preferably at value that would prevent or reduce decomposition or other wear to the refrigerant 102. For R-1233zd(E), a value of approximately 250°F (121.1°C) to 300°F (148.9°C) may be used, in some embodiments.

[0087] A liquid reservoir of the refrigerant 102 may also be desirable in some embodiments to improve operation at transient conditions that may momentarily reduce flow of liquid from the conduit 500 into the liquid source line 524. The liquid reservoir may be preferably included as part of a separator at the entrance to the liquid source line 524 or in the separator 542. Advantages of the present embodiment for supplying liquid or two-phase flow may include a maximized or increased available pressure and flow without a pump, a simple design, a reduction in moving parts, reliable operation, and use of low-cost, readily available components.

[0088] Accordingly, the present techniques are directed to a refrigerant control system for enabling refrigerant to flow upstream as a two-phase flow. Because liquid static head is an important consideration for low-pressure refrigerants that are directed upward, certain traditional vapor compression systems may position components to avoid upward flow of the liquid refrigerant or may include pumps or pressure controls to provide force that moves the refrigerant liquid upward. In contrast, the present refrigerant control system may introduce or inject vapor refrigerant into upward-facing conduits or risers to improve operations that use upward movement of liquid refrigerant by reducing fluid density and increasing available momentum or energy for moving the liquid refrigerant upward. Moreover, the refrigerant control system may use open-loop control to enable vapor refrigerant to leave with liquid refrigerant from a bottom portion of a high-pressure vessel to move liquid refrigerant upward at low-head conditions. Additionally, the refrigerant control system may maintain operation with entering condenser fluid temperatures below the leaving chilled fluid temperatures combined with vapor injection for single-stage systems that use upward flow of the liquid refrigerant. A combination of optimized condenser level control for the condenser drain valve and open-loop control for the flash tank drain valve may be used for multi-stage systems. Accordingly, by using embodiments of the disclosed refrigerant control system, refrigerant can be directed upstream in various vapor compression systems as a two-phase flow, thus reducing packaging size and refrigerant charge for the various vapor compression systems.

[0089] While only certain features and embodiments of the present disclosure have been illustrated and described, many modifications and changes may occur to those skilled in the art, such as variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters including temperatures, pressures, and so forth, mounting arrangements, use of materials, orientations, and so forth without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the disclosure. Furthermore, in an effort to provide a concise description of the embodiments, all features of an actual implementation may not have been described, such as those unrelated to the presently contemplated best mode of carrying out the disclosure, or those unrelated to enabling the claimed features. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.