Patent Application
20060218965
1. Field of the Invention
The present invention relates to refrigeration systems and, more specifically, to maintaining a relatively constant temperature of refrigerant passing through an evaporator, where the evaporator is exposed to a variable thermal load.
2. Description of the Related Art
In common refrigeration systems that operate at constant evaporating temperature under variable cooling load, the refrigerant is compressed in a variable speed compressor and then cooled in a condenser. After the refrigerant is cooled in the condenser, it is passed through an expansion device, or valve, to lower its pressure. The cooled, low-pressure refrigerant then enters an evaporator where the refrigerant absorbs thermal energy as its phase changes from a liquid to a vapor. Subsequently, the refrigerant in the evaporator is drawn into the compressor and re-cycled through the circuit.
Electronic components, such as microprocessors and laser diodes, perform better and more reliably when they are maintained at a constant, low temperature. Commonly, a refrigeration system is used to cool these electronic components by placing the evaporator near the components to absorb the heat that they produce. The heat produced by and emanating from these components may change over time depending on several factors. In order to maintain these components at a relatively constant temperature, the refrigeration system must be able to increase or decrease its cooling load in response to these changes.
To adjust the cooling load provided by the refrigeration circuit, the compressor may be cycled on and off which essentially starts and stops the working fluid from flowing through the circuit. However, cycling a compressor in this manner creates difficulties in the compressor lubrication system causing premature wear. Further, turning the refrigeration cycle on and off in this manner allows the temperature of the electronic components to fluctuate substantially. These substantial temperature swings may cause soldered connections to break or cause undesired condensation on the components.
Alternatively, variable speed compressors can be used to adjust the flow rate of the working fluid in the circuit to provide a variable, yet continuous, cooling load to the evaporator. However, variable speed compressors emit a variety of frequencies during operation which may cause nearby electronic components to malfunction. Further, variable speed compressors typically require additional electronics and hardware to convert AC power to DC power, thus increasing the cost of the refrigeration system.
What is needed is a refrigeration system which is an improvement over the foregoing.
The present invention provides a method and apparatus for maintaining a relatively constant temperature of a working fluid in a evaporator of a refrigeration system. In one form of the invention, the above can be accomplished by providing a constant volumetric displacement compressor and a heat exchanger for exchanging heat between the high pressure and low pressure portions of a refrigeration circuit to superheat, and hold substantially constant, the temperature of the refrigerant entering the compressor. In doing this, the pressure of the refrigerant in the low pressure portion of the circuit, including the evaporator, and the mass flow rate of the refrigerant remain substantially constant. As a result, the temperature of the saturated refrigerant in the evaporator remains substantially constant.
In this form of the invention, when the refrigerant in the evaporator is in a two-phase state, the pressure and temperature of the refrigerant in the evaporator uniquely correspond to one another, meaning, when the pressure is constant, so is the temperature regardless of the quality of the two-phase refrigerant. The quality of a refrigerant is the percentage of the refrigerant that is in a gaseous form. By holding the pressure relatively constant throughout the low-pressure side of the refrigeration circuit, the pressure and temperature of the refrigerant in the evaporator are held constant. The pressure is held constant in the low-pressure side of the circuit by using the aforementioned heat exchanger to control the properties of the refrigerant entering the compressor and the compressor which produces a constant mass flow rate for any given pressure of the low-pressure side refrigerant. In effect, the quality of the two-phase refrigerant in the evaporator will change as the cooling demand changes, however, as long as the refrigerant in the evaporator is in a two-phase state, the temperature of the two-phase refrigerant will remain constant.
In one form of the invention, the refrigeration system includes a compressor including an inlet and an outlet, a condenser including an inlet and an outlet, the condenser inlet in fluid communication with the compressor outlet, a sub-cooler, the sub-cooler having first and second fluid passages, the first passage having an inlet and an outlet, the second passage having an inlet and an outlet, the first passage inlet in fluid communication with the condenser outlet, the first passage and the second passage in a heat exchange relationship, an expansion device having an inlet and an outlet, the expansion device inlet in fluid communication with the sub-cooler first passage outlet; and an evaporator having an inlet and an outlet, the evaporator inlet in fluid communication with the expansion device outlet; the sub-cooler second passage inlet in fluid communication with the evaporator outlet, the second passage outlet in fluid communication with the compressor inlet, the temperature of the working fluid exiting the second passage outlet being substantially constant and substantially equal to the temperature of the working fluid entering the sub-cooler first passage inlet, where the mass flow rate of the working fluid is substantially constant and the pressure of the working fluid exiting the sub-cooler second passage outlet is substantially constant, whereby the pressure and temperature of the working fluid in the evaporator are substantially constant.
In an alternate form of the invention, the refrigeration circuit includes a constant volumetric displacement compressor for maintaining a substantially constant mass flow rate of a working fluid through the refrigeration circuit, an evaporator, and means for maintaining a substantially constant temperature of the working fluid in the evaporator.
In an alternate form of the invention, a method of operating a refrigeration cycle includes the steps of compressing a working fluid to a high-pressure working fluid with a compressor, cooling the high-pressure working fluid in a condenser, transferring the high-pressure working fluid from the condenser to an expansion device through a first passage in a heat exchanger, decompressing the high-pressure working fluid to low-pressure working fluid using the expansion device, heating the low-pressure working fluid in an evaporator, transferring the low-pressure working fluid from the evaporator to the compressor through a second passage in the heat exchanger while transferring heat between the high-pressure working fluid and the low-pressure working fluid in the heat exchanger; maintaining the temperature and mass flow rate of the low-pressure working fluid exiting the sub-cooler substantially constant, thereby maintaining the pressure and temperature of the low-pressure working fluid in the evaporator substantially constant.
In an alternate form of the invention, a method of operating a refrigeration cycle includes the steps of compressing a low-pressure working fluid to a high-pressure working fluid with a compressor, cooling the high-pressure working fluid in a condenser, decompressing the high-pressure working fluid to low-pressure working fluid using an expansion device, heating the low-pressure working fluid in an evaporator, placing the evaporator and the compressor in fluid communication, wherein the pressure of the low-pressure working fluid entering the compressor and the pressure of the low-pressure working fluid in the evaporator are proportionately related, maintaining the low-pressure working fluid entering into the compressor in a superheated thermodynamic state, maintaining the temperature, mass flow rate and pressure of the low-pressure working fluid entering the compressor substantially constant, maintaining the low-pressure working fluid in the evaporator in a two-phase thermodynamic state, and maintaining the pressure of the working fluid in the evaporator substantially constant, thereby maintaining the temperature of the working fluid in the evaporator substantially constant.
During the operation of the above refrigeration systems and circuits, the refrigerant may exit the evaporator in a superheated, or nearly superheated state. Accordingly, the low-pressure superheated refrigerant may not need to receive a significant amount of heat from the high-pressure refrigerant. Thus, a bypass device may be provided so that refrigerant, in some circumstances, may circumvent the sub-cooler or heat exchanger, or a portion thereof.
In one form of the invention, a heat exchanger includes a housing, including an inlet, an outlet, a first flow path in fluid communication with the inlet and the outlet, a second flow path in fluid communication with the inlet and the outlet, and porous media in fluid communication with the inlet, the porous media expandable when exposed to a working fluid, the working fluid substantially impeded from flowing through the first flow path when the media has expanded, whereby substantially all of the working fluid will flow through the second flow path to the outlet when the working fluid is substantially impeded from flowing through the first flow path.
In an alternative form of the invention, a valve includes a housing, including at least one inlet, at least one outlet, a primary flow path in fluid communication with the at least one inlet and the at least one outlet, a bypass flow path in fluid communication with the at least one inlet and the at least one outlet, and porous media, whereby liquid portions of a working fluid entering the housing through the at least one inlet is trapped by the porous media, the porous media expanded by the liquid portions, the primary flow path substantially obstructed by the porous media when the porous media expands, whereby the fluid will flow substantially through the bypass to the at least one outlet.
The above-mentioned and other features and objects of this invention will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplifications set out herein illustrate embodiments of the invention, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise form disclosed.
Included herein is a description of an exemplary refrigeration system in one form of the invention. Referring to
The refrigerant expelled from compressor 12 is communicated into condenser 14 through conduit 22. Conduit 22 may be a stainless steel or brass tube or any other conduit capable of withstanding elevated pressure and temperature. The compressed refrigerant enters condenser 14 from conduit 22 through inlet 15 and exits condenser 14 through outlet 17. Between inlet 15 and outlet 17, the refrigerant passes through a series of small tubes and conduits, or micro-channels, having fins or thin plates affixed thereto for dissipating thermal energy from the refrigerant contained within. As depicted in
Subsequently, the cooled, compressed refrigerant is communicated to expansion valve 16 through conduit 24. The refrigerant enters expansion valve 16 through inlet 23 and passes through an orifice into a larger chamber within expansion valve 16 allowing the refrigerant to expand and decompress. The cooled, low-pressure refrigerant exits expansion valve 16 through outlet 25 and is communicated to evaporator 18 through conduit 26. The refrigerant enters evaporator 18 from conduit 26 through inlet 27 and exits evaporator 18 through outlet 29. Similar to condenser 14, evaporator 18 may be a conventional heat exchanger where refrigerant passes between inlet 27 and outlet 29. However, unlike condenser 14 where the refrigerant is cooled, the refrigerant in evaporator 18 is heated. Evaporator 18 can be positioned near any heat emitting or conducting device, such as computer microchips or a circuit board, for example, so that the device may be cooled. Subsequently, the refrigerant exits evaporator 18 through outlet 29 and is communicated to compressor 12 through conduit 28, and the cycle described above is repeated. Although the above refrigeration process has been described by following a control mass through the refrigeration system, refrigerant is being cycled throughout the entire system as is well known in the art.
Also included in the refrigeration circuit is a third heat exchanger, sub-cooler 19. Sub-cooler 19 is a heat exchanger, or a series of heat exchangers, that exchanges thermal energy between the high pressure refrigerant that passes between condenser 14 and expansion valve 16 in conduit 24 and the low pressure refrigerant that passes between evaporator 18 and compressor 12 in conduit 28. Ultimately, sub-cooler 19 cools the high-pressure refrigerant before it passes to expansion device 16 and heats the low-pressure refrigerant before it enters compressor 12. In some embodiments, expansion device 16 is integral with sub-cooler 19. As will be discussed later, sub-cooler 19 is necessary to fix and control certain thermodynamic properties of the refrigeration cycle.
Sub-cooler 19 may be a tube-within-a-tube heat exchanger or any other heat exchanger. As illustrated in
The segment of line 100 to the left of point 102 defines the liquid saturation curve while the segment of line 100 to the right of point 102 defines the vapor saturation curve. Saturation curve 100 defines the boundary between the superheated, two-phase, and sub-cooled conditions of the refrigerant. Below liquid/vapor saturation curve 100 is a two-phase region where the refrigerant exists in a combined liquid and vapor, or two-phase, state, illustrated as region ST in
The operation of system 10 is represented in
Movement from point C to point D represents the cooling of the superheated refrigerant in condenser 14 at an essentially constant pressure. Point D represents the refrigerant at outlet 17 of condenser 14. The refrigerant at point D is in a two-phase state. The temperature of the refrigerant at point D is substantially equal to the temperature of the ambient air passing over condenser 14, which is represented by isotherm 106 in
Movement from point F to point G represents the drop in refrigerant pressure as it passes through expansion valve 16. The refrigerant at point G is in a substantially saturated liquid state and represents the refrigerant at expansion valve outlet 25. Movement from point G to point H represents the energy input converting the refrigerant from a liquid phase to a vapor phase in evaporator 18. The refrigerant at point H is in a two-phase state, however, the position of point H along isotherm 108 will depend on the amount of heat absorbed by the refrigerant while in evaporator 18. As illustrated in
The thermodynamic cycle illustrated in
In the forms of the invention discussed above, it is a goal of the invention to maintain the temperature of the refrigerant in the evaporator substantially constant regardless of the thermal energy absorbed by the refrigerant in the evaporator. To achieve this, the thermodynamic parameters of the refrigerant entering the compressor (point A) are held substantially constant, as discussed below.
In operation, the refrigerant passing through the evaporator may be a single-component refrigerant comprised of both gas and liquid, or in other words, the refrigerant will likely be in a two-phase state. As the single-component refrigerant passing through the evaporator is in a two-phase state, the pressure and temperature of the refrigerant will uniquely correspond to one another. More specifically, if the pressure of the two-phase refrigerant is held constant, its temperature will also be held constant. However, in some embodiments, a multi-component refrigerant may be used. A multi-component refrigerant is a mixture of at least two refrigerants commonly having different boiling points. As a result, the temperature of the mixture in the evaporator may drift although one of the refrigerants is in a two-phase state. This drift, also known as the temperature glide, is the difference between the temperature at which the mixture begins to evaporate (bubble-point temperature) and the temperature at which it has completely evaporated (dew-point temperature). This drift can be minimized by using refrigerants having close but different equal boiling points. These mixtures are called azeotropic refrigerants and may be used in some embodiments of the present invention.
As discussed above, by holding the pressure of the refrigerant in the evaporator at a constant level, the temperature of the refrigerant will also be held at a constant level. To hold the pressure of the refrigerant in the evaporator constant, the pressure of the refrigerant at the compressor inlet (point A) is maintained constant. These pressures are substantially linked together because the refrigerant entering the compressor and the refrigerant in the evaporator are in fluid communication through conduit 28. To hold the pressure of the refrigerant at point A constant, and to accommodate an economical compressor designed to compress only a gas, the refrigerant at point A is maintained in a superheated state. Unlike a refrigerant in a two-phase or saturated vapor state, the pressure and the temperature of a superheated refrigerant do not uniquely correspond. In a superheated state, a refrigerant has two degrees of freedom and thus two properties of the refrigerant needs to be held constant to hold constant the other properties of the refrigerant.
The Gibbs Phase Rule can be used to determine the degrees of freedom in a system and thereby indicate the number of parameters required to control the thermodynamic state of the fluid system and states:
p+f=c+2
wherein, p=the number of phases; f=number of degrees of freedom in the system, i.e., the number of independent parameters; and c=number of fluid components in the thermodynamic system. Thus, a single phase system, such as a superheated refrigerant, will have one more degree of freedom than a two-phase system, such as a saturated refrigerant. In these embodiments, two parameters, such as temperature, pressure, specific volume, mass flow rate, or density, are required to determine the other thermodynamic properties and physical parameters of a superheated refrigerant. Similarly, to hold the physical parameters of a superheated refrigerant constant, two thermodynamic parameters of the superheated refrigerant must be held constant.
Accordingly, to hold the pressure of the refrigerant constant at the compressor inlet (point A) in the present form, both the temperature and the mass flow rate of the refrigerant must be constant. To hold the temperature of the refrigerant at the compressor inlet constant (point A), sub-cooler 19 is used to assure that the temperature of the refrigerant exiting sub-cooler 19 through outlet 37 substantially equals the temperature of the refrigerant entering sub-cooler 19 through inlet 31. As discussed above, the temperature of the refrigerant entering inlet 31 (point D) substantially equals the temperature of the ambient air passing over condenser 14 in this form of the invention. Thus, the temperature of the refrigerant at point A substantially equals the temperature of the ambient air passing over the condenser, which itself is relatively constant. With one parameter fixed, for any given mass flow rate, i.e., the second parameter, there can be only one pressure of the refrigerant at point A. Thus, for any steady state operating condition, the refrigeration system will find an equilibrium with a substantially constant mass flow rate and compressor inlet refrigerant pressure when the compressor inlet refrigerant temperature is held constant. As a result, the pressure of the refrigerant in evaporator 18 is held constant, and accordingly, the temperature of the refrigerant in evaporator 18 is thereby held constant achieving the aim of the invention.
Sub-cooler 19 can also maintain the thermodynamic parameters of the refrigerant exiting through outlet 33 (point F) in a substantially sub-cooled, or saturated liquid, state. An advantage of maintaing the refrigerant at point F in a sub-cooled state is that a saturated liquid entering evaporator 18 at point G ensures the maximum possible cooling capacity for the refrigeration system.
Although the refrigeration process described above may not be the most efficient process, it is a process that can respond to a variable thermal load while maintaing a constant evaporating temperature with a low cost refrigeration system. In one application, it is important to hold the temperature of the refrigerant in the evaporator substantially constant to avoid undercooling computer microchips, which would allow the microchips to overheat, and/or overcooling the microchips, which would allow moisture in the ambient air to condense on them possibly causing a short circuit. System 10 can also be employed for other applications.
Other forms of the invention include using a variable capacity compressor in lieu of a constant capacity compressor. A variable capacity compressor can be operated at a constant operational speed while providing a range of output displacements. An axial piston pump in combination with an adjustable swash plate is a common variable capacity compressor. A variable capacity compressor provides the refrigeration system with the flexibility to accommodate a large range of cooling load demands in the evaporator without requiring changes in operational speed, and the accompanying changes in noise.
In alternative embodiments, refrigeration system 10 may include additional features or components such as a two stage compressor mechanism that employs an intercooler to cool the intermediate pressure refrigerant between the first and second compressor stages.
As discussed above, the state of the refrigerant exiting evaporator 18 under ordinary operating conditions may range between wet vapors and a superheated gas. When the refrigerant is substantially a gas, the refrigerant will not need to absorb a large quantity of heat while passing through sub-cooler 19. Accordingly, a form of the present invention includes a liquid-responsive device for shortening the path of the refrigerant through sub-cooler 19 to reduce the thermal energy transferred to the refrigerant when the refrigerant is mostly a gas. This device may include a sensor for sensing the quality of the fluid entering into sub-cooler 19 and may electronically switch the path of the refrigerant between a longer path and a shorter path with a solenoid or any other known switching device.
Alternatively, as illustrated in
Porous media 220, such as a solid having pores to trap a fluid, is contained within chamber 206 such that media 220 can expand to substantially fill the volume of chamber 206 when exposed to a liquid portion of the refrigerant. Thus, when refrigerant exiting evaporator 18 and entering sub-cooler 19′ is in a partially liquid state, the liquid portion of the refrigerant will be absorbed by porous media 220. As a result, porous media 220 will expand to substantially block the flow of the refrigerant through chamber 206 and a large portion of the refrigerant will flow through conduit 210. Conduit 210 comprises an extended path where the low-pressure refrigerant contained therein is exposed to the thermal energy of the high-pressure refrigerant passing through conduit 24 for a longer period of time than if the refrigerant had passed through shorter conduit 207. As a result of passing through conduit 210, in this form of the invention, the refrigerant will become superheated to the state represented by point A. Alternatively, when the refrigerant enters into sub-cooler 19′ in a mostly gaseous state, the porous media will not substantially expand and the refrigerant will be able to pass through chamber 206 and shorter conduit 207. In this condition, the refrigerant does not require as much thermal energy to achieve the state represented by point A and thus will require less exposure to the thermal energy provided by sub-cooler 19′.
While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.
