Maintaining superheat conditions in a compressor

An illustrative example refrigerant system includes a compressor configured to pressurize a refrigerant fluid. The compressor includes a sump portion. A heater is situated to heat at least the sump portion. A controller is configured to selectively operate the heater to apply heat to at least the sump portion while the compressor is off to establish and maintain a superheat condition in the compressor.

BACKGROUND

Air conditioning and refrigeration systems are well known. A typical refrigerant circuit includes a compressor, a condenser, an expansion valve and an evaporator. While such circuits have proven useful and reliable, there are certain conditions that may occur that can adversely affect the system.

For example, under some conditions, such as when the system is idle or shut down, liquid refrigerant tends to migrate to the coldest parts of the system. The compressor is often the coldest component because it is typically within the outdoor equipment. If liquid refrigerant is left in the compressor it is possible for the liquid refrigerant to mix with oil in the compressor. One problem associated with such a mixture is that it may develop into a foam when the compressor begins to operate, and oil may be introduced into other portions of the circuit, depleting the oil in the compressor and increasing the risk of damage or premature wear of compression elements. Another problem that may arise is that the refrigerant may dilute the lubricating capacity of the oil, which is needed for proper compressor operation over time.

SUMMARY

An illustrative example embodiment of a refrigerant system includes a compressor configured to pressurize a refrigerant fluid. The compressor includes a sump portion. A heater is situated to heat at least the sump portion. A controller is configured to selectively operate the heater to apply heat to at least the sump portion while the compressor is off to maintain a superheat condition in the compressor.

In an embodiment having one or more features of the system of the previous paragraph, the controller is configured to determine whether the superheat condition exists in the compressor based on a temperature and a pressure associated with the compressor.

In an embodiment having one or more features of the system of any of the previous paragraphs, the compressor includes a shell and the pressure is inside the shell.

In an embodiment having one or more features of the system of any of the previous paragraphs, the temperature is at least one of inside or on the shell.

In an embodiment having one or more features of the system of any of the previous paragraphs, the controller is configured to determine a minimum temperature to maintain the superheat condition based on the pressure.

In an embodiment having one or more features of the system of any of the previous paragraphs, the controller is configured to determine at least one of the temperature and the pressure based on a temperature or pressure of another component of the refrigerant system in fluid communication with the compressor.

In an embodiment having one or more features of the system of any of the previous paragraphs, the controller is configured to operate the heater to apply a first amount of heat when a current temperature of the compressor is below a minimum temperature needed for the superheat condition, the controller is configured to operate the heater to apply a second amount of heat when the superheat condition exists, and the first amount of heat is greater than the second amount of heat.

An illustrative example method of controlling a temperature of a compressor of a refrigerant system includes operating a heater for heating at least a sump portion of the compressor while the compressor is off to maintain a superheat condition in the compressor.

An embodiment having one or more features of the method of the previous paragraph includes determining whether the superheat condition exists in the compressor based on a temperature and a pressure associated with the compressor.

In an embodiment having one or more features of the method of any of the previous paragraphs, the compressor includes a shell and the pressure is inside the shell.

In an embodiment having one or more features of the method of any of the previous paragraphs, the temperature is at least one of inside or on the shell.

An embodiment having one or more features of the method of any of the previous paragraphs includes determining a minimum temperature to maintain the superheat condition based on the pressure.

An embodiment having one or more features of the method of any of the previous paragraphs includes determining at least one of the temperature and the pressure based on a temperature or pressure of another component of the refrigerant system in fluid communication with the compressor

An embodiment having one or more features of the method of any of the previous paragraphs includes operating the heater to apply a first amount of heat when a current temperature of the compressor is below a minimum temperature needed for the superheat condition and operating the heater to apply a second amount of heat when the superheat condition exists. The first amount of heat is greater than the second amount of heat.

An illustrative example refrigerant system controller includes a processor and memory including instructions that are executable by the processor to operate a heater for heating at least a sump portion of a compressor while the compressor is off to maintain a superheat condition in the compressor.

In an embodiment having one or more features of the controller of the previous paragraph, the instructions include instructions that are executable by the processor to determine whether the superheat condition exists in the compressor based on a temperature and a pressure associated with the compressor.

In an embodiment having one or more features of the controller of any of the previous paragraphs, the instructions include instructions that are executable by the processor to determine a minimum temperature to maintain the superheat condition based on the pressure.

In an embodiment having one or more features of the controller of any of the previous paragraphs, the instructions include instructions that are executable by the processor to determine at least one of the temperature and the pressure based on a temperature or pressure of another component of the refrigerant system in fluid communication with the compressor.

In an embodiment having one or more features of the controller of any of the previous paragraphs, the compressor includes a shell, the pressure is inside the shell, and the temperature is at least one of inside or on the shell.

In an embodiment having one or more features of the controller of any of the previous paragraphs, the instructions include instructions that are executable by the processor to operate the heater to apply a first amount of heat when a current temperature of the compressor is below a minimum temperature needed for the superheat condition, and operate the heater to apply a second amount of heat when the superheat condition exists. The first amount of heat is greater than the second amount of heat.

The various features and advantages of at least one disclosed example embodiment will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

DETAILED DESCRIPTION

FIG.1schematically illustrates a system20that includes a refrigerant circuit capable of operating as a heat pump or providing air conditioning or refrigeration, for example. The refrigerant circuit includes a first heat exchanger22, a compressor24, a second heat exchanger26and an expansion valve28that operate in a known manner. In some implementations, the first heat exchanger22is configured to be situated within a temperature conditioned space, such as a building or a residence, and the second heat exchanger26is configured to be situated outside the space. The direction of refrigerant fluid flow through the circuit will be consistent with the intended operation as a heat pump or air conditioner.

A controller30, which includes a processor or another computing device and memory, is configured to control operation of the compressor. In some situations, the compressor24remains idle or inoperative. Under certain circumstances, such as when cooling is needed, the controller30turns on the compressor24and causes it to operate such that the compressor24pressurizes refrigerant fluid within the circuit in a known manner.

A heater32is associated with the compressor24. In the illustrated example system, the compressor24includes a sump portion34and a shell36. The heater32is situated to heat at least the sump portion34of the compressor24. The controller30is configured to selectively operate the heater32. While the compressor24is off, the controller30causes the heater32to operate to maintain a superheat condition in the compressor24.

FIG.2is a flowchart diagram40that summarizes an example control strategy. At42, the compressor24turns off, which may be based on a command from the controller30.

The controller30determines a temperature and a pressure associated with the compressor24and, at44, determines if the temperature and pressure correspond to a superheat condition in the compressor24. Although not illustrated, known temperature and pressure sensors may be included in various locations within the system20to provide such information to the controller30. In the illustrated example embodiment, the controller30determines a pressure within the shell36of the compressor24and a temperature on or in the shell36. In some embodiments, the controller30determines a pressure near the compressor24and a corresponding temperature.

The controller30uses the temperature and pressure information to determine whether a superheat condition exists in the compressor24. A superheat condition is that which includes a temperature and pressure that is above the saturation point of the refrigerant. The superheat condition ensures that any refrigerant in the compressor24is in a vapor state and no liquid refrigerant is allowed to accumulate in the compressor24. There are known pressure and temperature relationships that correspond to superheat conditions and the controller30uses at least one such relationship to determine whether the determined temperature satisfies a minimum temperature requirement to maintain superheat conditions given the determined pressure.

At46, the controller30causes the heater32to operate to apply a first amount of heat when the temperature and pressure do not correspond to a superheat condition. The first amount of heat is intended to raise the temperature of at least the sump portion34of the compressor24to establish superheat conditions in the compressor24. The first amount of heat may be sufficient, for example, to vaporize any liquid refrigerant in the compressor24.

The controller30continues to monitor the pressure and temperature at44until a superheat condition exists in the compressor24. When that condition exists, the controller30operates the heater at48to apply a second, lower amount of heat to maintain the superheat condition in the compressor24.

In the illustrated example embodiment, the controller30continues the operation of the heater32as long as the compressor is off. The controller30in some embodiments dynamically adjusts the heat supplied by the heater32to maintain the superheat condition in the compressor24while using as little energy as possible.

One aspect of the illustrated example embodiment is that it minimizes or eliminates the possibility of liquid refrigerant collecting in the compressor24while the compressor is off. Maintaining a superheat condition in the compressor24also minimizes or eliminates the possibility of refrigerant condensation as the compressor24starts up at the beginning of a subsequent operating cycle. Keeping liquid refrigerant out of the compressor24enhances system efficiency and extends the useful life of the compressor components and the oil used to lubricate those components. The example embodiment is also more energy efficient than systems that apply heat for other reasons or based on other conditions because only as much heat as is needed to maintain a superheat condition in the compressor24will be applied.