Pressure control for refrigerant system

A system includes a condenser and an evaporator. The condenser is configured to condense a working fluid, and the evaporator is configured to evaporate the working fluid. The system also includes piping that is configured to circulate the working fluid between the condenser and the evaporator. In addition, the system includes a low point configured to collect condensed working fluid. A controller is configured to selectively enable heating of the condensed working fluid collected within the low point based on a working fluid pressure of the low point.

BACKGROUND

The present disclosure relates generally to refrigerant systems, and more specifically, to systems and methods for pressure control within the refrigerant systems.

Refrigerants are used to transfer heat between fluids and may be employed in a variety of applications, such as heating, ventilating, air conditioning, and refrigeration (HVAC&R) systems, heat pumps, or power generation in Organic Rankine Cycles (ORC). The refrigerant is typically transported within a refrigerant piping system, which includes pipes, pipe fittings, valves, and the like. The refrigerant piping system transports the refrigerant between various vessels and equipment within the HVAC&R system, such as compressors, turbines, pumps, evaporators, condensers, and the like. It is now recognized that a leakage in the refrigerant piping system, the vessels, or the equipment may cause air to enter the HVAC&R system, thereby reducing the efficiency and operability of the HVAC&R system if such leakage occurs in a part of the refrigerant circuit at a pressure below atmospheric pressure. This leakage may occur in heat pumps or ORC systems, particularly when the system is not operating. In addition, moisture from the air may corrode the HVAC&R system, exacerbating the leakage.

DETAILED DESCRIPTION

The present disclosure is directed to systems and methods for pressure control of refrigerant systems. As used herein, the term “refrigerant system” includes any thermodynamic system that uses a working fluid (e.g., a refrigerant) to absorb and/or transfer energy. Accordingly, a refrigeration system may be an HVAC&R system, a heat pump system, an ORC system, or the like.

As noted earlier, leakages within the refrigerant piping system, the vessels, or the equipment of a refrigerant system may result in air ingress, particularly when refrigerant pressure is less than the ambient pressure. Air ingress reduces the efficiency and operability of the refrigerant system and may result in corrosion of the refrigerant piping system, the vessels, and/or the equipment. In addition, when air enters the refrigerant circuit, it may be desirable to purge the air out of the refrigerant system. Unfortunately, purging the air may induce undesired leakage of the refrigerant out of the refrigerant system.

It is now recognized that the refrigerant pressure may be controlled to reduce the possibility of air ingress into the refrigerant system. That is, the refrigerant pressure may be maintained above the ambient pressure, thereby reducing the driving force for air ingress into the refrigerant system. In particular, the refrigerant system includes one or more low points designed to collect liquid refrigerant. For example, gravity may pull the liquid refrigerant towards the one or more low points of the refrigerant system. A heating source may be employed to heat the liquid refrigerant collected within the one or more low points, thereby maintaining the refrigerant pressure above the ambient pressure and reducing the possibility of air ingress into the refrigerant system.

Turning now to the figures,FIG. 1illustrates an embodiment of an refrigerant system (e.g., a heat pump system10) with a pressure control system12configured to reduce the possibility of air ingress into the heat pump system10. The refrigerant system includes a compressor16(e.g., a screw compressor) and other equipment associated with operation of the compressor16, such as an oil separator18and/or a superheater60. It should be noted that the heat pump system10is given by way of example, and the present disclosure may be applied to a variety of refrigerant systems, such as organic Rankine cycle (ORC) systems, chillers, and the like. In addition, components of the heat pump system10may be implementation-specific. That is, the flow configurations and types of heat exchangers, the numbers and types of compressors, pumps, valves, and the like, may vary widely among embodiments.

The heat pump system10includes a refrigerant piping system14, which transports a working fluid (e.g., a refrigerant, such as R-245fa or R-236fa) between the various components of the heat pump system10. For example, the refrigerant enters the compressor16, which compresses and pressurizes the refrigerant. The pressurized refrigerant then flows to the oil separator18, which separates the refrigerant from lubrication oil of the compressor16. It should be noted that certain embodiments of the heat pump system10may not include the compressor16. For example, an organic Rankine cycle (ORC) system may employ a pump instead of the compressor16to pressurize and transport liquid refrigerant. In addition, certain embodiments may not employ the oil separator18. In other words, the refrigerant may flow from the outlet of the compressor16directly to a valve22or a condenser32instead of through the oil separator18.

As illustrated, the oil separator18is disposed at a low point20of the heat pump system10. That is, the oil separator18is disposed at a local minimum elevation between the compressor16and the valve22. Thus, liquid refrigerant may drain into the oil separator18by gravity flow, particularly when the heat pump system is not operating. As discussed earlier, it may be desirable to monitor and control the pressure of the refrigerant within the low point20to reduce the possibility of air ingress into the heat pump system10.

After the lubrication oil is separated out, the refrigerant flows through the valve22to the condenser32, where the refrigerant is condensed into a liquid phase. The condenser32is also disposed at a low point46of the heat pump system10between the valve22and a superheater60. A controller30may be employed to control the heating of liquid refrigerant that collects within the condenser32(i.e., low point46), when desired. As shown, the condenser32includes a bundle of tubes34, which are coupled to a coolant piping system36. The coolant piping system36transports a coolant (e.g., water) from a water source38to a water return40. For example, water from the water source38may flow through the tubes34, where the water absorbs heat from the refrigerant, thereby causing the refrigerant to condense into a liquid phase. Subsequently, the warmed water may flow to the water return40, where the warmed water is routed to downstream applications, such as cooling towers and the like.

The condensed refrigerant exits the condenser32and flows through the superheater60, a valve62, and a thermal expansion valve64, which is a metering device. The expansion valve64meters the flow of condensed refrigerant into an evaporator66, which evaporates the refrigerant into a vapor phase. However, certain embodiments may not include the thermal expansion valve64. It may be desirable for refrigerant to freely flow from the condenser32to the evaporator66. For example, ORC systems may include a turbine disposed between the evaporator66and the condenser32, without the thermal expansion valve64.

As shown, the evaporator66is also disposed at a low point68of the heat pump system10between the expansion valve64and the superheater60or shut-off valve78. As will be appreciated, during normal operation of the heat pump system10, the operating conditions of the evaporator66may maintain the refrigerant in a vapor phase. However, when the heat pump system10is not operational, the temperature of the refrigerant may gradually decrease, resulting in condensation of the refrigerant into a liquid phase. The liquid refrigerant may drain by gravity flow into the evaporator66and the low point68. Again, it may be desirable to monitor and control the pressure of the refrigerant within the low point68to reduce the possibility of air ingress into the heat pump system10, particularly when the heat pump system10is not operational (e.g., for a brief period due to a process upset or for a longer period during a shutdown).

As shown, the evaporator66includes a bundle of tubes70, which are coupled to an additional coolant piping system72. The coolant piping system72of the evaporator66is similar to the coolant piping system36of the condenser32. That is, the coolant piping system72transports a coolant (e.g., water) from a water source74through the tubes70, where the water expels heat to the refrigerant, thereby causing the refrigerant to evaporate. The cooled water then flows to a water return76, where the cooled water is routed to downstream applications, such as air conditioners and the like.

The vaporized refrigerant from the evaporator66flows into the superheater60, where the vaporized refrigerant is heated by the condensed refrigerant from the condenser32. The superheated refrigerant then flows through a suction valve78to the compressor16, where the heat pump cycle may essentially begin again. It should be noted that certain embodiments of the heat pump system10may not include the superheater60. That is, evaporated refrigerant may flow from an outlet of the evaporator66directly to the suction valve78or the compressor16rather than through the superheater60.

As illustrated, the valves22,62, and78may be used to divide the heat pump system10into three sections80,82, and84. Each of the sections80,82, and84is designed with at least one low point (e.g., low points20,46, and68) to collect liquid refrigerant by gravity flow. Although the oil separator18, the condenser32, and the evaporator66are illustrated as the respective low points20,46, and68, the heat pump system10may be designed with low points in other locations, such as the superheater60, the compressor16, or other designated liquid pockets within the heat pump system10. For example, the refrigerant piping system14may include a u-shaped pocket designed to collect liquid refrigerant by gravity flow. The controller30may be used to control the heating of the liquid refrigerant within the low points20,46, and68.

As illustrated, the controller30includes various components to implement the logic to heat the liquid refrigerant. In particular, the controller30includes one or more processors86and/or other data processing circuitry, such as memory88, to execute instructions to enable selective heating of the liquid refrigerant collected within the low points20,46, and68. These instructions may be encoded in software programs that may be executed by the one or more processors86. Further, the instructions may be stored in a tangible, non-transitory (i.e., not merely a signal), computer-readable medium, such as the memory88.

In certain embodiments, various operating parameters and thresholds may be encoded and stored within the memory88to be later accessed by the one or more processors88. For example, an ambient pressure sensor90may detect an ambient pressure around the heat pump system10. The processor86may calculate a threshold pressure based on the ambient pressure, and the threshold pressure may be stored within the memory88for later use in order to heat the low points20,46, and68, as will be discussed in greater detail below. The controller30may control the heating of the liquid refrigerant within each of the sections80,82, and84independently. It should be noted that certain embodiments may not include the ambient pressure sensor90. As will be appreciated, fluctuations of the atmospheric pressure are small compared to the pressure fluctuations within the refrigerant system. Accordingly, the atmospheric pressure may be assumed to be constant, thereby enabling the controller30to operate without the ambient pressure sensor90. However, in certain embodiments, such as heat pump systems at high elevations, the ambient pressure sensor90may be desirable, and the pressure threshold may be adjusted accordingly.

FIG. 2illustrates the section80of the heat pump system10between the valves78and22. The valves78and22may be closed to isolate the section80from the remainder of the heat pump system10. As explained earlier, liquid refrigerant may collect within the oil separator18(i.e. low point20) and become diluted in the oil, particularly when the heat pump system10is not operational. It may be desirable to heat the blend of oil and liquid refrigerant that collects within the oil separator18to reduce the possibility of air ingress into the section80. Accordingly, a heat source (e.g., an electrical heater or heating coil26) is coupled to the oil separator18. As shown, the heating coil26is submerged within a pool28of mixed oil and liquid refrigerant, and the heating coil26may supply heat directly to this mixture. In certain embodiments, additional or alternative heating sources may be used to heat the mixture. For example, heat tracing48(e.g., steam tracing or electrical tracing) that is externally coupled to the oil separator18may heat the oil separator18, thereby heating the liquid refrigerant within the oil separator18.

The controller30may selectively enable heating of the oil separator18using the heating coil26, the heat tracing48, or both, based on an operating condition (e.g., pressure) of the section80. As illustrated, a pressure sensor24is coupled to the oil separator18. The pressure sensor24detects a pressure within the oil separator18as an indication of the refrigerant pressure. In a presently contemplated embodiment, the controller30may compare the detected pressure from the pressure sensor24with a threshold pressure stored within the memory88to determine if heating the mixed oil and liquid refrigerant is desirable. For example, when the detected pressure is below the threshold pressure, the controller30may selectively enable the heating coil26, the heat tracing48, or both, to heat the mixture of refrigerant and oil to reduce the possibility or air ingress into the section80. In certain embodiments, the threshold pressure may be based at least in part on the ambient pressure, which may be assumed constant or may be detected by the ambient pressure sensor90. For example, the threshold pressure may be between approximately 100 to 300, 110 to 250, 150 to 200 percent of the ambient pressure, and all subranges therebetween.

The controller30may implement various logic to heat the mixture of liquid refrigerant and oil, in addition to the pressure-based control algorithm described above. For example, the controller30may selectively enable the heating coil26, the heat tracing48, or both, based on the temperature of the refrigerant, the amount of time the heat pump system10has been non-operational, or a combination thereof. The temperature-based and time-based control algorithms will be described in greater detail with respect toFIGS. 3 and 4.

FIGS. 3aand 3billustrate the section82between the valves22and62. The valves22and62may be closed to isolate the section82from the remainder of the heat pump system10. In certain embodiments, the valve62may occupy a lower elevation relative to the valve62inFIG. 3a(as illustrated inFIG. 3b). The elevation may affect the amount of accumulated refrigerant in the superheater60and the piping system when the valve62is closed. Additionally or alternatively, a bypass valve83may enable refrigerant to bypass the superheater60, which is also illustrated inFIG. 3b.

Again, because the condenser32is disposed at the low point46, liquid refrigerant may collect within the condenser32by gravity flow. However, in certain embodiments, the refrigerant from the superheater60may not drain to the condenser32due to a “neck” effect. For example, the superheater60may be a shell and tube heat exchanger with one or more baffles that may hold the condensed refrigerant when the heat pump system10is not operating. Additionally or alternatively, a static head of liquid (e.g., condensed refrigerant) may maintain a liquid level in the superheater60. Nevertheless, the condenser32generally contains a sufficient level of liquid to enable pressure control within the heat pump system10, as discussed below.

During normal operation of the heat pump system10, the pressure of the liquid refrigerant is generally sufficiently high (e.g., greater than the ambient pressure) to reduce the possibility of air ingress into the heat pump system10. However, when the heat pump system10is not operational, the temperature and pressure of the liquid refrigerant may gradually decrease, particularly in environments with low ambient temperatures. Accordingly, it may be desirable to heat the liquid refrigerant that collects within the condenser32to reduce the possibility of air ingress into the section82.

The heat may be provided from a variety of heat sources. For example, the heat tracing48that is externally coupled to the condenser32may provide the heat to the condenser32, thereby heating the liquid refrigerant within the condenser32. Additionally or alternatively, water from the coolant piping system36may heat the liquid refrigerant within the condenser32(i.e. low point46). For example, the water may flow from the water source38through the tubes34, releasing heat to the liquid refrigerant. In other words, the heat source may include a heat transfer fluid (e.g., water).

As illustrated, the coolant piping system36also includes control valves42and44, which are disposed along the water flow path between the water source38and the water return40. The control valves42and44may selectively enable or block the flow of water to the condenser32. For example, it may be desirable close the control valves42and44in order to perform maintenance on the tubes34of the condenser32. On the other hand, the controller30may open the control valves42and44to enable water to flow to the condenser32. In certain embodiments, the controller30may start up a pump92to increase the flow of water through the tubes of the condenser34, thereby increasing the rate at which the liquid refrigerant is heated.

In a presently contemplated embodiment, it may be desirable to increase the temperature of the water within the coolant piping system36, which enables faster heating of the liquid refrigerant within the low point46. To this end, the coolant piping system includes a pump50and a heat source (e.g., electrical heater52). The electrical heater52warms the water, and the pump50transports the water through the tubes34of the condenser32. In certain configurations, the controller30may close the control valves42and44, enabling the water to re-circulate through a continuous loop54between the electrical heater52and the tubes34. The continuous recirculation and heating of the water may increase the efficiency of the coolant piping loop36and reduce the water consumption of the heat pump system10.

A pressure sensor56is coupled to the condenser32, so that the controller30may implement the pressure-based control algorithm described previously. That is, the controller30may selectively enable the heat tracing48, the coolant piping system36, or both, to heat the refrigerant within the condenser based on the pressure detected by the pressure sensor56. As shown, the controller is communicatively coupled to the pressure sensor56, as well as the heat tracing48, the pump50, and the electrical heater52. It should be noted that in other embodiments, additional or alternative sources (e.g., heating coils) may be used.

The controller30may implement a time-based control algorithm in addition to the pressure-based control algorithm described above. For example, if the heat pump system10has been non-operational for a time period, the controller30may enable the water from the coolant piping system36to heat the liquid refrigerant within the condenser32. In particular, the controller30may open the control valves42and44and start up the pump92to enable water to flow through the tubes34of the condenser32. The water flow may increase the temperature and pressure of the liquid refrigerant. However, after a time delay, if the refrigerant pressure is still below the threshold pressure, the controller30may enable re-circulation of the water through the continuous loop54, as described above. That is, the controller30may close the control valves42and44and subsequently enable the electrical heater52and the pump50. The electrical heater52increases the temperature of the water, thereby increasing the rate at which the liquid refrigerant is heated.

In certain embodiments, the controller30may enable re-circulation of the water through the continuous loop54based on a temperature of the water (i.e., temperature-based control). As illustrated, a temperature sensor58is coupled to the coolant piping system36. The temperature sensor58detects a temperature of the water within the continuous loop54of the coolant piping system36. If the detected temperature is below a threshold temperature, it may be desirable to increase the water temperature to heat the liquid refrigerant more quickly. Thus, the controller30may enable re-circulation of the water through the continuous loop54when the detected temperature is below the threshold temperature. The threshold temperature may be based at least in part on a saturation temperature of the liquid refrigerant.

FIG. 4illustrates the section84between the valves62and78, as well as the coolant piping system72. The coolant piping system72of the evaporator66is similar to the coolant piping system36of the condenser32. That is, the coolant piping system72includes control valves94and96to selectively block or enable water flow through the tubes70of the evaporator66. In addition, the coolant piping system72includes a re-circulation loop98with a pump100and an electrical heater102. Further, the coolant piping system72includes a pump104, a pressure sensor106, and a temperature sensor108to implement the pressure-based, temperature-based, or time-based control algorithms, or any combination thereof, as described previously. It should be noted that the pressure thresholds, temperature thresholds, or other parameters of the control algorithms may vary between the coolant piping systems36and72. For example, the pressure threshold of the coolant piping system72may be higher than the pressure threshold of the coolant piping system36.

FIG. 5illustrates an embodiment of a method110to control the pressure within the low points20,46,68of the heat pump system10. The pressure sensors24,56, and106may detect (block112) a pressure of the respective low points20,46, and68. The controller30may determine (block114) if the detected pressure is less than a threshold pressure. In certain embodiments, the threshold pressure may be based on an assumed (e.g., constant) ambient pressure or an ambient pressure detected by the ambient pressure sensor90. In addition, the threshold pressure may be stored within the memory88of the controller30. When the detected pressure is less than the threshold pressure, the controller30may enable (block116) heating of the liquid refrigerant collected within the low points20,46,68using a heat source (e.g., water from coolant piping systems36and72, heat tracing48, heating coils26, or any combination thereof). After a time delay, the pressure sensors24,56, and106may re-detect (block118) the pressure of the respective low points. The controller30may then re-determine (block120) if the detected pressure is less than the threshold pressure. If the detected pressure is still less than the threshold pressure, the controller30may enable (block122) heating from an additional heat source (e.g., electrical heaters52and102). If the detected pressure is greater than or equal to the threshold pressure, the process may essentially begin again.