Abstract:
A method and apparatus for maintaining a relatively constant temperature of a working fluid in an evaporator of a refrigeration system 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.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     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.  
         [0003]     2. Description of the Related Art  
         [0004]     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.  
         [0005]     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.  
         [0006]     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.  
         [0007]     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.  
         [0008]     What is needed is a refrigeration system which is an improvement over the foregoing.  
       SUMMARY OF THE INVENTION  
       [0009]     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.  
         [0010]     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.  
         [0011]     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.  
         [0012]     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.  
         [0013]     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.  
         [0014]     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.  
         [0015]     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.  
         [0016]     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.  
         [0017]     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.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     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:  
         [0019]      FIG. 1  is a schematic view of a refrigeration system in accordance with an embodiment of the present invention;  
         [0020]      FIG. 2  is a sectional view through the sub-cooler of the refrigeration system of  FIG. 1 ;  
         [0021]      FIG. 3  is a schematic of the heat exchanger of the refrigeration system of  FIG. 1 ;  
         [0022]      FIG. 4  is a pressure-specific enthalpy diagram for a common refrigerant which illustrates the operation of the refrigeration system of  FIG. 1 ;  
         [0023]      FIG. 5  is a pressure-specific enthalpy diagram demonstrating a different mode of operation of the refrigeration system of  FIG. 1 ;  
         [0024]      FIG. 6  is a plan view of an alternative embodiment of the sub-cooler of the refrigeration system of  FIG. 1  in accordance with an embodiment of the present invention; and  
         [0025]      FIG. 7  is a detail view of a chamber containing porous media in the sub-cooler of  FIG. 6 . 
     
    
       [0026]     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.  
       DETAILED DESCRIPTION  
       [0027]     Included herein is a description of an exemplary refrigeration system in one form of the invention. Referring to  FIG. 1 , refrigeration system  10  includes, in serial order, constant volumetric displacement compressor  12 , a first heat exchanger, e.g., condenser  14 , an expansion device, e.g., expansion valve  16 , and a second heat exchanger, e.g., evaporator  18 , connected in series by fluid conduits. As is well known in the art, compressor  12  draws a refrigerant or working fluid, such as R-245fa, for example, through compressor inlet  11 , compresses the refrigerant, and expels the compressed refrigerant through compressor outlet  13 . R-245fa is a low density refrigerant that advantageously allows the refrigeration system to operate with a small pressure difference between the evaporator and the condenser. Compressor  12 , in this form of the invention, is a constant volumetric displacement compressor and may be any positive displacement compressor including a reciprocating piston, rotary, or scroll compressor.  
         [0028]     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  FIG. 3 , condenser  14  may be formed by a plurality of tubes  40  having radiating fins  42  mounted thereon as is well known in the art. The refrigerant within tubes  40  exchanges thermal energy with tubes  40  which, in turn, exchanges thermal energy with fins  42 . A second heat exchange medium, e.g., ambient air blown over fins  42  with an air blower, absorbs thermal energy from fins  42  to thereby cool the refrigerant within tube  40 . Alternatively, condenser  14  may be any type of heat exchanger including a shell-and-tube type heat exchanger where water or another refrigerant flows over the tube containing the system refrigerant.  
         [0029]     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.  
         [0030]     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.  
         [0031]     Sub-cooler  19  may be a tube-within-a-tube heat exchanger or any other heat exchanger. As illustrated in  FIG. 2 , a tube-within-a-tube heat exchanger may include small tube  34  passing through large tube  36 . High pressure refrigerant passes through small tube  34  between inlet  31  and outlet  33  while, simultaneously, low pressure refrigerant passes through large tube  36  between inlet  35  and outlet  37 . In this embodiment, heat is transferred from the high pressure refrigerant passing through tube  36  to the low pressure refrigerant passing through tube  34 . Ultimately, if tubes  34  and  36  were long enough, the temperature of the low pressure fluid exiting sub-cooler  19  through outlet  33  would substantially equal the temperature of the high pressure fluid entering sub-cooler  19  through inlet  31 . In most embodiments, the tubes are not long enough to equalize these temperatures, however, they will be substantially equalized to sufficiently effect the purposes of the invention as discussed further below.  
         [0032]      FIG. 4  illustrates the thermodynamic properties of a common refrigerant, the operation of system  10 , and the relationship between the pressure and specific enthalpy of the refrigerant in various thermodynamic states. In  FIG. 4 , the Y-axis represents the pressure of the refrigerant and the X-axis represents the specific enthalpy of the refrigerant. Line  100  represents the liquid/vapor saturation curve of the refrigerant. Point  102  is the critical point of the refrigerant and represents the point of maximum pressure on curve  100 . It is at thermodynamic state  102  when the refrigerant, at constant pressure, will instantaneously transition from liquid to gas without passing through a two-phase state. The isotherm passing through point  102 , represented by line  104 , has an inflection point only at point  102  where line  104  is horizontally tangent to curve  100  at point  102 .  
         [0033]     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  FIG. 4 . The states of the refrigerant represented to the right of saturation curve  100  are described as superheated states where the refrigerant is entirely in a gaseous form, illustrated as region SH in  FIG. 4 . The states of the refrigerant represented to the left of saturation curve  100  are described as sub-cooled states where the refrigerant is entirely in a liquid form, illustrated as region SCL in  FIG. 4 . The states of the refrigerant represented at a pressure higher that the pressure of point  102  are described as supercritical states where the refrigerant is entirely in a supercritical form, illustrated as region SC.  
         [0034]     The operation of system  10  is represented in  FIGS. 1 and 4  by cycle ABCDEFGH. Point A represents the condition of the refrigerant at outlet  37  of sub-cooler  19 . The refrigerant at point A is in a superheated state. As will be discussed in detail further below, it is a goal of this form of the invention to maintain point A in a substantially constant superheated state where the pressure and temperature of the refrigerant represented by point A is substantially constant during the operation of the refrigeration system. Movement from point A to point B in the refrigeration cycle represents the increase in temperature and energy that occurs when the refrigerant passes over the compressor housing before entering compressor inlet  11  to improve the efficiency of the refrigeration cycle. The refrigerant at point B is also in a superheated state. Movement from point B to point C represents the increase in pressure and temperature caused by the compression of the refrigerant in compressor  12 . If the compression of the refrigerant were to be adiabatic, meaning an ideal compression without losses, then the discharge state would be represented by point C′. The refrigerant at point C is also in a superheated state where point C represents the state of the refrigerant at condenser inlet  15 .  
         [0035]     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  FIG. 4 . The refrigerant at point D, in certain embodiments of the present invention, may be in a sub-cooled or superheated state depending on the design of condenser  14  and the amount of energy that can be dissipated. Movement from point D to point E, and from point E to point F, represents the continued cooling of the refrigerant as it passes through sections of sub-cooler  19 . In this embodiment, point E represents an intermediate step in the heat exchange process between two portions of sub-cooler  19 . Point E is illustrated as a point on the saturated liquid curve, however, the refrigerant at this state may also be a wet vapor or a sub-cooled liquid. Sub-cooler  19  may include one portion or as many portions that are necessary for any particular application. The refrigerant at point F may be in a sub-cooled state and represents the refrigerant at sub-cooler outlet  33 .  
         [0036]     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  FIG. 5  and discussed in further detail below, regardless of the position of point H, the refrigerant is heated from point H to point A in sub-cooler  19  to a superheated state. In a system used for cooling purposes, e.g., a refrigerated cabinet or air conditioning application, the length of the line GH represents the cooling capacity of the system and is coincident with isotherm  108 , the saturation temperature of the refrigerant in the evaporator.  
         [0037]     The thermodynamic cycle illustrated in  FIG. 5 , and represented by cycle ABCDEFGH′, reflects the operation of system  10  where the refrigerant in the evaporator absorbs more thermal energy than the refrigerant in the evaporator in cycle ABCDEFGH. As a result, the specific enthalpy of the refrigerant at point H′ is higher than the specific enthalpy at point H. In this embodiment, the refrigerant at point H′ is almost entirely a vapor and very little additional energy is required to achieve the superheated state represented by point A. As a result, the refrigerant passing from evaporator  18  to compressor  12  through sub-cooler  19  will absorb less energy in sub-cooler  19 . Regardless of the evaporator cooling load, the low-pressure vapor exits sub-cooler  19  at a substantially consistent temperature, the temperature of the ambient air passing over the condenser.  
         [0038]     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.  
         [0039]     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.  
         [0040]     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.  
         [0041]     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. 
 
         [0042]     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.  
         [0043]     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.  
         [0044]     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.  
         [0045]     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.  
         [0046]     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.  
         [0047]     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.  
         [0048]     Alternatively, as illustrated in  FIGS. 6 and 7 , sub-cooler  19 ′ includes housing  202 , a short refrigerant path, and a long refrigerant path for refrigerant to flow therethrough. The short path includes inlet  204 , chamber  206 , short conduit  207 , and outlet  208  where inlet  204  is in fluid communication with chamber  206  and chamber  206  is in fluid communication with outlet  208  through conduit  207 . The long path includes inlet  204 , a relatively long, serpentine-like conduit  210 , and outlet  208  where inlet  204  is in fluid communication with conduit  210  and conduit  210  is in fluid communication with outlet  208 . In this embodiment, a second fluid envelops conduits  207 ,  210  and  28  so that thermal energy may be conducted therebetween where conduits  207  and  210  are preferably in close proximity to conduit  28 . Alternatively, other heat exchangers may be used. In an alternative embodiment, similar to the heat exchanger illustrated in  FIG. 2  and described above, conduits  207  and  210  would pass through a larger tube containing the high pressure refrigerant.  
         [0049]     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 ′.  
         [0050]     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.