Pressure controlled switching valve for refrigeration system
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to a commonly assigned co-pending application Ser. No. 07/612,290, filed on Nov. 9, 1990, titled Refrigeration System and Refrigerant Flow Control Apparatus Therefor. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to refrigeration systems, and more particularly relates to refrigeration systems with multiple evaporators having pressure controlled autonomous switching valves for conveying refrigerant from the multiple evaporators to a compressor unit of such refrigeration systems. 
     BACKGROUND OF THE INVENTION 
     In a typical refrigeration system, refrigerant circulates continuously through a closed circuit. The term &#34;circuit&#34;, as used herein, refers to a physical apparatus whereas the term &#34;cycle&#34; as used herein refers to operation of a circuit, e.g., refrigerant cycles in a refrigeration circuit. The term &#34;refrigerant&#34;, as used herein, refers to refrigerant in liquid, vapor and/or gas form. Components of the closed circuit cause the refrigerant to undergo temperature/pressure changes. The temperature/pressure changes of the refrigerant result in energy transfer. Typical components of a refrigeration system include, for example, compressors, condensers, evaporators, control valves, and connecting piping. Details with regard to some known refrigeration systems are set forth in Baumeister et al., Standard Handbook for Mechanical Engineers, McGraw Hill Book Company, Eighth Edition, 1979, beginning at page 19-6. 
     Energy efficiency is one of the important factors in the assessment of refrigeration systems. Particularly, an ideal refrigeration system operates at an ideal refrigeration effect. However in practice, an actual refrigeration system operates at less than the ideal refrigeration effect. 
     Increased energy efficiency is typically achieved by utilizing more expensive and more efficient refrigeration system components, adding extra insulation adjacent to the area to be refrigerated, or by other costly additions. Increasing the energy efficiency of a refrigeration system therefore usually results in an increase in the cost of the system. It is therefore, desirable to increase the efficiency of a refrigeration system and minimize any increase as a result thereof in the cost of the system. 
     In some apparatus utilizing refrigeration systems, more than one area needs to be refrigerated, and at least one area requires more refrigeration than another area. A typical household refrigerator, which includes a freezer compartment and a fresh food compartment, is one example of such an apparatus. The freezer compartment is preferably maintained between -10° Fahrenheit (F.) and +15° F., and the fresh food compartment is preferably maintained between +33° F. and +47° F. 
     To meet these temperature requirements, a typical refrigeration system includes a compressor coupled to an evaporator disposed within the household refrigerator. The terms &#34;coupled&#34; and &#34;connected&#34; are used herein interchangeably. When two components are coupled or connected, this means that the components are linked, directly or indirectly in some manner in refrigerant flow relationship, even though another component or components may be positioned between the coupled or connected components. For example, even though other components such as a pressure sensor or an expander are connected or coupled in the link between the compressor and evaporator, the compressor and evaporator are still coupled or connected. 
     Referring again to the refrigeration system for a typical household refrigerator, the evaporator is maintained at about -10° F. (an actual range of about -30° F. to 0° F. is typically used) and air is blown across the coils of the evaporator. The flow of the evaporator-cooled air is controlled, for example, by barriers. A first portion of the evaporator-cooled air is directed to the freezer compartment and a second portion of the evaporator-cooled air is directed to the fresh food compartment. To cool a fresh food compartment, rather than utilizing evaporator-cooled air from an evaporator operating at about -10° F., it is possible to utilize an evaporator operating at, for example, about +25° F. (or a range of about +15° F. to +32° F.). A typical refrigeration system utilized in household refrigerators, therefore, produces its refrigeration effect by operating an evaporator at a temperature which is appropriate for the freezer compartment but lower than it needs to be for the fresh food compartment. 
     It is well-known that the energy required to maintain an evaporator at about -10° F. is greater than the energy required to maintain an evaporator at about +25° F. in a refrigerator. A typical household refrigerator therefore uses more energy to cool the fresh food compartment than is necessary, operating at reduced energy efficiency. 
     The above referenced household refrigerator example is provided for illustrative purposes only. Many apparatus other than household refrigerators utilize refrigeration systems which include an evaporator operating at a temperature below a temperature at which the evaporator actually needs to operate. 
     Refrigeration systems which operate at reduced energy consumption are described in commonly assigned U.S. Pat. Nos. 4,910,972 and 4,918,942. The patented systems utilize at least two evaporators and a plurality of compressors or a compressor having a plurality of stages. For example, in a dual, i.e., two, evaporator circuit for household refrigerators, a first evaporator operates at +25° F. and a second evaporator operates at -10° F. Air cooled by the first evaporator is utilized for the fresh food compartment and air cooled by the second evaporator is utilized for the freezer compartment. Utilizing the dual evaporator refrigeration system in a household refrigerator results in increased energy efficiency. Energy is conserved by operating the first evaporator at the temperature (e.g., +25° F.) required for the fresh food compartment rather than operating an evaporator for the fresh food compartment at -10° F. Other features of the patented systems also facilitate increased energy efficiencies. 
     To drive the plurality of evaporators in the refrigeration systems described in U.S. Pat. Nos. 4,910,972 and 4,918,942, and as mentioned above, a plurality of compressors or a compressor including a plurality of stages are utilized. Utilizing a plurality of compressors or utilizing a compressor having a plurality of stages results in increasing the cost of the refrigeration system over the cost, at least initially, of refrigeration systems utilizing one evaporator and one single stage compressor. It is therefore desirable to provide improved energy efficiency achieved by using a plurality of evaporators and to minimize, if not eliminate, the increase in cost associated with a plurality of compressors or a compressor having a plurality of stages. 
     STATEMENT OF THE INVENTION 
     The present invention is directed to a refrigerant flow switching device for alternately conveying refrigerant from either high pressure or low pressure evaporator means to compressor means of a refrigeration system, the device comprising, a first flow controller positioned in a refrigerant flow relationship between the high pressure evaporator means and the compressor means, and comprising expandable enclosure means responsive to pressure from the high pressure evaporator means for compelling the expandable enclosure means to move from a first position to a second position against a force provided by a first biased means, activating means for preventing flow of refrigerant from the high pressure evaporator means to the compressor means when the expandable enclosure means is at the first position and for allowing flow of refrigerant from the high pressure evaporator means to the compressor means when the expandable enclosure means is at the second position, and a second flow controller positioned in a refrigerant flow relationship between the low pressure evaporator means and the compressor means for allowing flow of refrigerant from the low pressure evaporator means to the compressor means only when the first flow controller prevents flow of refrigerant from the high pressure evaporator means to the compressor means. 
     The present invention is also directed to a refrigerator, comprising, compressor means, condenser means connected to receive refrigerant discharged from the compressor means, a fresh food compartment, first evaporator means for refrigerating the fresh food compartment and connected to receive at least part of the refrigerant discharged from the condenser means, a freezer compartment, second evaporator means for refrigerating the freezer compartment and connected to receive at least part of the refrigerant discharged from the condenser means; and the aforedescribed refrigerant flow switching device for alternately conveying refrigerant from either the high pressure or the low pressure evaporator means to the compressor means. 
     The present invention provides increased energy efficiency by utilizing a plurality of evaporators which operate at desired, respective, refrigeration temperatures. Further, by utilizing, in one embodiment, a single-stage compressor rather than a plurality of compressors or a compressor having a plurality of stages, increased costs associated with improved energy efficiency are minimized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a refrigerant flow switching device and a compressor unit. 
     FIG. 2A illustrates a refrigeration system utilizing the refrigerant flow switching device of the preferred embodiment. 
     FIG. 2B shows, in more detail, the refrigerant flow switching device included in the refrigeration system of FIG. 2A at a first position (STATE 1). 
     FIG. 2C shows, in more detail, the refrigerant flow switching device included in the refrigeration system of FIG. 2A at a second position (STATE 2). 
     FIG. 2D is a partial 3-dimensional view of the refrigerant flow switching device of the preferred embodiment. 
     FIG. 2E is a partial 3-dimensional view of the gate member used in the refrigerant flow switching device of FIG. 2A. 
     FIG. 3 is a block diagram illustration of a household refrigerator incorporating a refrigeration system having a fresh food evaporator and a freezer evaporator. 
     FIG. 4 is a block diagram illustration of a refrigeration system with multiple evaporators, incorporating the refrigerant flow switching device of the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention, as described herein, is believed to have its greatest utility in refrigeration systems and particularly in household refrigerator freezers. The present invention, however, has utility in other refrigeration applications such as control of multiple air conditioner units. The term refrigeration systems, as used herein, therefore not only refers to refrigerator/freezers but also to many other types of refrigeration applications. 
     Referring now more particularly to the drawings, FIG. 1 shows a block diagram 100 illustrating a refrigerant flow switching device or devices 102 and a compressor unit 104 in accordance with the present invention. A plurality of inputs INPUT 1-INPUT N are shown as being supplied to switching devices 102. The inputs to switching devices 102 are typically refrigerants. Refrigerant conduits, for example, are coupled to or formed integral with switching devices 102 for supplying input refrigerant. More details with regard to alternate embodiments for refrigerant flow switching devices 102 are provided hereinafter, particularly with reference to FIGS. 2B-2E, 3 and 4. 
     The output from switching devices 102 is supplied as input to compressor unit 104. Compressor unit 104 comprises means for compressing refrigerant, such as a single-stage compressor, a compressor having a plurality of stages, or a plurality of compressors, which provides, as output, compressed refrigerant. Embodiments of the present invention wherein a single stage compressor is utilized, are believed to have greatest utility. 
     FIG. 2A illustrates a refrigeration system 200 in accordance with the preferred form of the present invention. Refrigeration system 200 includes a compressor unit 202 coupled to a condenser 204. A capillary tube 206 is coupled to the outlet of condenser 204, and a first evaporator 208, also known as a high pressure evaporator, is coupled to the outlet of capillary tube 206. The outlet of first evaporator 208, also known as a high pressure evaporator, is coupled to the inlet of a phase separator 210, which includes a screen 212 disposed adjacent to the inlet of phase separator 210, a gas- or vapor-containing portion 214 and a liquid-containing portion 216. Although sometimes referred to herein as vapor-containing portion 214 or simply as vapor portion 214, it should be understood that this portion of phase separator 210 may have gas and/or vapor disposed therein. Vapor portion 214 is coupled to supply a high pressure refrigerant, as a first input, to a refrigerant flow switching device 218. Particularly, the intake of conduit 220 is so positioned in vapor portion 214 that liquid refrigerant passing through vapor portion 214 to liquid-containing portion 216 does not enter said intake. The outlet of liquid-containing portion 216 is coupled to an expansion device 222, such as an expansion valve or a capillary tube. The expansion device 222 is sometimes referred to herein as a throttle. A second evaporator 224, also known as a low pressure evaporator, is coupled to the outlet of expansion device 222, and the outlet of second evaporator 224 is coupled to provide a low pressure refrigerant, as a second input, to refrigerant flow switching device 218. 
     A thermostat 227, which is preferably user adjustable, receives current flow from an external power source designated by the legend &#34;POWER IN&#34; and it is connected to compressor unit 202. When cooling is required, thermostat 227 provides an output signal which activates compressor unit 202. In a household refrigerator, for example, thermostat 227 is preferably disposed in the freezer compartment. 
     Capillary tube 206 is shown in thermal contact with conduit 220 which connects phase separator vapor portion 214 with refrigerant flow switching device 218. Capillary tube 206 is also in thermal contact with a conduit 230 which couples second evaporator 224 to refrigerant flow switching device 218. Thermal contact is achieved, for example, by soldering the exterior of capillary tube 206 and a portion of the exterior of conduits 220 and 230, together side-by-side. Capillary tube 206, in FIG. 2A, is shown as being wrapped around conduits 220 and 230 in a schematic representation of a heat transfer relationship. The heat transfer occurs in a counterflow arrangement, i.e., the refrigerant flowing in capillary tube 206 proceeds in a direction opposite to the flow of refrigerant in conduits 220 and 230. As is well known in the art, using a counterflow heat exchange arrangement, rather than a heat exchange arrangement wherein the flows proceed in a same direction, increases the heat exchange efficiency. 
     In operation, and by way of example, first evaporator 208 contains refrigerant at a temperature of approximately +25° F. The second evaporator 224 contains refrigerant at a temperature of approximately -10° F. Expansion device 222 is adjusted to provide barely superheated vapor flow at the outlet of second evaporator 224. A capillary tube (not shown) having an appropriate bore size and length or an expansion valve can be used as expansion device 222. 
     Switching device 218 controls the flow of refrigerant passing through respective evaporators 208 and 224 to compressor unit 202. When refrigeration is called for, thermostat 227 activates compressor unit 202. Vapor from second evaporator 224 enters compressor unit 202 through refrigeration flow switching device 218, when switching device 218 is configured to allow conduits 230 and 232 to be in flow communication. Alternatively, vapor from phase separator 210 enters compressor unit 202 through refrigeration flow switching device 218 when switching device 218 is configured to allow conduits 220 and 232 to be in flow communication. For ease of reference, when switching device 218 is configured to provide flow communication between conduits 230 and 232, or similarly disposed conduits, this condition is hereinafter referred to as STATE 1. When switching device 218 is configured to provide flow communication between conduits 220 and 232, or similarly disposed conduits, this condition is hereinafter referred to as STATE 2. 
     In the exemplified operation, and using refrigerant R-12 (dichlorodifluoromethane), refrigerant at about 20 pounds per square inch absolute (psia) is disposed in conduit 230 and refrigerant at about 40 psia is disposed in conduit 220. The inlet pressure to compressor unit 202 is about 20 psia when switching device 218 is in STATE 1 and about 40 psia when switching device 218 is in STATE 2. 
     At the time of transition from STATE 1 to STATE 2, flow communication between conduit 230 and conduit 232 is switched &#34;off&#34;, to discontinue flow of refrigerant from second evaporator 224 and communication between conduit 220 and conduit 232 is switched &#34;on&#34; to allow refrigerant to flow from first evaporator 208. At the time of transition from STATE 2 to STATE 1, as the flow communication between conduit 220 and conduit 232 is switched off, liquid refrigerant from phase separator 210 begins flowing through second evaporator 224 but some refrigerant continues to flow through first evaporator 208, albeit at a slower rate. 
     More particularly, when thermostate 227 activates compressor unit 202, such as when the temperature of the freezer compartment rises above some predetermined set temperature, high pressure gas at high temperature discharged from the compressor unit 202, is condensed in condenser 204. Capillary tube 206 is preferably sized to obtain some subcooling of the liquid exiting condenser 204. Subcooling is defined as cooling of a given fluid below its saturation temperature. By subcooling a fluid below its saturation temperature, more BTUs (British Thermal Unit) can be removed by the refrigeration system. Capillary tube 206 is generally a fixed length, small bore tube. Due to the tube diameter of capillary tube 206, a high pressure drop occurs across the capillary tube length thus reducing the pressure of refrigerant to its saturation pressure. Some of the refrigerant evaporates in the capillary tube 206 and at least some of the refrigerant evaporates in first evaporator 208 and changes to a vapor. Capillary tube 206 meters the flow of refrigerant and maintains a pressure difference between condenser 204 and first evaporator 208. 
     The direct contact between the outside of capillary tube 206 into which the warm condensed liquid from condenser 204 enters and the outside of conduit 220 from phase separator 210 causes cooler conduit 220 to warm up and capillary tube 206 to cool down. Without the heating provided by capillary tube 206, the temperatures for conduits 220 and 230 in STATE 1 and STATE 2, respectively, in the preferred embodiment are about -10° F. and +25° F., respectively. Additionally, without the heating provided by capillary tube 206, moisture from air at room temperature will condense on conduits 220 and 230. Such condensed moisture tends to drip and create a flooding problem. Conduit heating by means of capillary tube 206 warms conduits 220 and 230 sufficiently to avoid condensation and it also cools the refrigerant in capillary tube 206 flowing to first evaporator 208. Even though the warming of refrigerant in conduits 220 and 230 adversely affects the system efficiency, the beneficial effect provided by the cooling of refrigerant in capillary tube 206, far outweighs such a loss of system efficiency. 
     The expansion of the liquid refrigerant in first evaporator 208 causes part of liquid refrigerant to evaporate. Refrigerant in liquid and vapor phases exiting from first evaporator 208 then enters phase separator 210. Liquid refrigerant accumulates in liquid-containing portion 216 and vapor accumulates in vapor portion 214 of phase separator 210. Conduit 220 supplies vapor from vapor portion 214 to switching device 218. Vapor from phase separator 210 is at generally at about +25° F. 
     When thermostat 227 activates compressor unit 202, and when switching device 218 is in STATE 1, liquid from liquid-containing portion 216 of phase separator 210 evaporates as it flows through throttle 222 into second evaporator 224. Thus, the temperature and pressure of refrigerant entering second evaporator 224 from throttle 222 significantly drop and any remaining liquid refrigerant evaporates in second evaporator 224, and further cools second evaporator 224 to about -10° F. As previously stated, refrigerant flows, albeit at a slow rate, through first evaporator 208 when switching device 218 is in STATE 1. A sufficient refrigerant charge is typically supplied to system 200 to maintain liquid refrigerant phase separator 210 at a desired level. 
     The pressure at the input of compressor unit 202 when switching device 218 is in STATE 1, is determined by the pressure at which refrigerant exists in a two-phase equilibrium at -10° F. The pressure at compressor unit 202 when switching device 218 is in STATE 2 is determined by the saturation pressure of refrigerant at +25° F. 
     The temperature of condenser 204 has to be greater than ambient temperature for condenser 204 to function as a condenser. The refrigerant within condenser 204, for example, may be at +105° F. The pressure of refrigerant in condenser 206, of course, depends upon the refrigerant selected. 
     Compressor unit 202 is any type of compressor or mechanism which provides a compressed refrigerant output. For example, compressor unit 202 is a single stage compressor, a plurality of compressors, a compressor having a plurality of stages, or any combination of compressors. Compressor unit 202 is, for example, a rotary or reciprocating type compressor. A compressor with a small volume inlet chamber is preferred since gases at two different pressures are alternately being compressed. For example, a rotary compressor with an inlet chamber volume of one cubic inch that gets compressed to 0.28 cubic inches per compressor revolution, is satisfactory. If a compressor with a large inlet chamber is used, there is a substantial delay between the time when the high pressure refrigerant stops flowing to the compressor and the time when the inlet compressor pressure is reduced sufficiently to start compressing the lower pressure refrigerant. Using a large inlet chamber also reduces the system efficiency. 
     FIGS. 2B, 2C and 2D and 2E illustrate, in more detail, a preferred embodiment of refrigerant flow switching device 218. Particularly, device 218 is shown as being integrally formed with conduits 220, 230 and 232. However, device 218 may be provided with inlet conduits and an outlet conduit which are coupled to conduits 220, 230 and 232, respectively by joining methods, such as welding, soldering, or mechanical coupling. 
     First flow controller 226 is shown as being disposed, at least partially, within conduit 220. In FIG. 2B, first flow controller 226 is shown as being closed, so that refrigerant cannot flow from conduit 220 to conduit 232, i.e., STATE 1. In FIG. 2C, first flow controller 226 is shown as being open so that refrigerant can flow from conduit 220 to conduit 232, i.e. STATE 2. First flow controller 226 comprises a chamber 231 to which conduit 220 and 232 are attached. 
     First controller 226 further comprises activating means 233 having expandable enclosure means 229, preferably a bellows 252 positioned within chamber 231. Bellows 252 are sealably attached to receive at least part of refrigerant from conduit 220 via a passage way 239. To one skilled in the art, it will be apparent to use some other expandable enclosure means, such as a flexible membrane sealably attached to the bottom of chamber 231. 
     Activating means 233 prevent flow of refrigerant from high pressure evaporator means, such as first evaporator 208 shown in FIG. 2A to compressor means, such as compressor unit 202 shown in FIG. 2A when bellows 252 is at a first position shown in FIG. 2B (STATE 1), and allow flow of refrigerant from first evaporator 208, shown in FIG. 2A to compressor unit 202 when bellows 252 is at a second position shown in FIG. 2C (STATE 2). 
     As shown in FIG. 2B, when first flow controller 226 is in STATE 1, bellows 252 is compressed by the force provided by a first biased means, such as a compression spring 266 placed within a centering well 272, whose constant downward force may be adjusted by a pressure regulator knob 268 threaded into chamber 231 and connected to a pressure plate 270 placed over compression spring 266. Compression spring 266 also prevents back flow of refrigerant from compressor unit 202 to first evaporator 208 when compressor unit 202 is not running. A chamber opening 274 in chamber 231 communicates with either air at atmospheric pressure or preferably the low pressure refrigerant from conduit 230 to allow air or the low pressure refrigerant to &#34;breathe&#34; in and out of chamber 231 as bellows 252 reciprocates between the first and the second position. To one skilled in the art, it would be apparent to replace compression spring 266 with a pressure regulating fluid introduced into chamber 231 chamber opening 274. The fluid pressure may then be adjusted by increasing or decreasing the fluid volume within chamber 231. 
     Activating means 233 further comprise a gate member 234, which preferably comprises a pair of substantially planar parallel blades 240 A and B, each having an orifice 236 A and B therein, respectively. FIGS. 2D and 2E show the details of gate member 234. Orifice 236 A and B on each blade 240 A and B, respectively, are preferably aligned for providing unrestricted passage to flow of refrigerant. Gate member 234 is positioned between two substantially parallel faces 238 A and B located in conduits 232 and 220, respectively. Each face 238 A and B is provided with a portal 237 A and B, respectively. Portals 237 A and B are preferably substantially similar in size and shape to orifices 236 A and B such that when gate member 234 is in an open position orifices 236 A and B on blades 240 A and B align with portals 237 A and B on faces 238 A and B, thereby allowing flow of refrigerant and when gate member 234 is in a closed position, orifices 236 A and B do not align with portals 237 A and B, thereby preventing flow of refrigerant. 
     In order to prevent leakage of refrigerant, each blade 240 A and B is preferably held against each face 238 A and B, by the force provided by a third biased means, such as a leaf spring 242. FIG. 2E further shows details of blade 240. 
     Activating means 233 further comprise a reciprocating member 254 whose first end 256 is preferably connected to gate member 234 by means of an axle 258. A second end 260 of reciprocating member 254 is moveably engaged to the first prongs of a plurality of second biased means, such as &#34;U&#34; shaped toggle springs 262. Second end 260 is preferably disc shaped with slots in which the first prongs of &#34;U&#34; shaped toggle springs 262 are hooked. The second prongs of &#34;U&#34; shaped toggle springs 262 are moveably engaged to anchoring means 264 affixed to bellows 252. Anchoring means 264 is preferably cylindrical in shape and preferably has a lip on which slots are provided for hooking in the second prongs of preferably two &#34;U&#34; shaped toggle springs 262 in a linear relationship. Once &#34;U&#34; springs 262 are hooked into position, they are under stress that provides an outward force along the first and second prongs. 
     Activating means 233 are provided with means for limiting motion of reciprocating member 254, such as a motion limiting bracket 255 and a motion limiting plate 257. Motion limiting bracket 255 and motion limiting plate 257 provide stops which correspond to the open and closed position of gate member 234, respectively. 
     Rather than being constructed as shown in FIGS. 2B-C, it is contemplated that chamber 231 of first flow controller 226 may be constructed, for example, from a single block of material, such as polymer or steel. Many other techniques, such as plastic molding, could be also utilized to make chamber 231. 
     The location and type of flow controller used as first flow controller 226, of course, may differ from the location and type shown in FIGS. 2B-C. For example, first flow controller 226 may be located anywhere along the length of conduit 220. However, to minimize any delay between switching from one refrigerant flow to another, it is desirable to locate flow controller 226 as close as possible to conduit 232, as shown in FIGS. 2B and 2C, in order to minimize the volume between flow controller 226 and compressor unit 202. 
     Device 218 includes a second flow controller 228, shown as a check valve, disposed in the conduit 230. FIG. 2B shows check valve 228 as being in an open position, i.e., refrigerant can flow between conduit 230 and conduit 232. Particularly, check valve 228 may include a ball 244 and a ball seat 246 having an opening 248. A cage 250 prevents ball 244 from escaping when the pressure in conduit 230 is greater than the pressure in conduit 232. When ball 244 is forced into the seat 246 from the pressure of refrigerant in conduit 232, check valve 228 is closed and refrigerant cannot flow between conduit 230 and conduit 232. The location and type of flow controller for second flow controller 228, of course, may differ from the location and type shown in FIG. 2B. For example, second flow controller 228 may be located anywhere along the length of conduit 220. However, for minimizing any delay between switching from one refrigerant flow to another, as shown in FIG. 2B, it is desirable to locate flow controller 228 as close as possible to conduit 232, in order to minimize the volume between flow controller 228 and compressor unit 202. 
     In operation, and by way of example, conduit 230 has refrigerant at low pressure, e.g., about 20 psia, flowing therethrough and conduit 220 has refrigerant at a higher pressure, e.g., about 40 psia, flowing therethrough. The conduit 232 side of face 238 A is at pressure P1 where pressure P1 is equal to the pressure of refrigerant disposed in conduit 232. Pressure P1 is sometimes referred to herein as the compressor unit inlet pressure. Pressure P1 would alternate, in this example, from about 40 psia to 20 psia, depending upon which flow controller is open. 
     The conduit 220 side of face 238 B is at pressure P2, where pressure P2 is equal to the pressure of the high pressure refrigerant supplied by conduit 220. Pressure P2 in this example is about 40 psia or above. When first flow controller 226 is closed (STATE 1), pressure P1 will stabilize at 20 psia, since second flow controller 228 is in open position and compression unit 202 receives flow of refrigerant from second evaporator 224 (low pressure evaporator). At the same time, pressure P2 at 40 psia building up inside bellows 252, starts to push bellows 252 from the first position, as shown in FIG. 2B, to the second position as shown in FIG. 2C. Bellows 252 pushes against the constant force of compression spring 266 as it from the first position to the second position. The selection of particular springs, bellows, and first controller chamber size of first flow controller 226 is matched to the desired operating characteristics. 
     In the present examples, the initial conditions (STATE 1) as shown in FIG. 2B are as follows: second flow controller 228 is open; bellows 252 is in compressed state due to the force exerted by compression spring 266; orifices 236 A and B on blades 240 A and B, respectively are not lined up with portals 237 A and B on faces 238 A and B, respectively, i.e. first flow controller 226 is closed and bellows 252 is starting to expand against the constant force exerted by compression spring 266. 
     As bellows 252 expands, anchoring means 264 affixed to bellows 252 also starts moving from the first position to the second position, thus exerting an inward force on preloaded &#34;U&#34; springs 262 and thereby further increasing stresses in them. As a result, loops of preloaded &#34;U&#34; springs 262 which are proximate to each other, start moving away from one another as bellows 252 continues its expansion. As the forces in &#34;U&#34; springs 262 increase further, a vertical force component acting along the second prongs of &#34;U&#34; springs hooked at second end 260 of reciprocating member 254 also increases until it overcomes the frictional force present between faces 237 A and B and blades 240 A and B, respectively. &#34;U&#34; springs 262 rapidly relieve some of the stresses building up within them by snapping blades 237 A and B in a downward direction, and thereby putting orifices 236 A and B in a fluid communication with portals 237 A and B on faces 238 A and B, respectively. As a result first controller 226 opens and high pressure refrigerant from conduit 220 flows to conduit 232 such that pressure P1 and P2 are substantially equal. FIG. 2C (STATE 2) shows the aforementioned configuration. 
     High pressure in conduit 232 (P2) then causes second flow controller 228 to close. Particularly, the high pressure refrigerant exerts more force against check valve 228 than the low pressure refrigerant from conduit 230. Ball 244 is therefore forced into and held against seat 246, until P1 is at a higher pressure. 
     Referring to FIG. 2C (STATE 2), since orifices 236 A and B and portals 237 A and B are lined up, the pressure that had built up within bellows 252, falls at a rate equal to that of first evaporator 208. As a result, the force exerted by compression spring 266 exceeds the decreasing pressure provided by the high pressure refrigerant from conduit 220, and bellows 252 are compressed by the force of compression spring 266, which in turn causes preloaded &#34;U&#34; springs to push inwards toward one another and thereby exerting an upward force on reciprocating member 254 to snap it from the second position (STATE 2) to the first position (STATE 1). When the high pressure refrigerant discontinues flowing through first controller 226, second controller 228 then opens to allow the low pressure refrigerant from conduit 230 to flow to compressor unit 202. At this point device 218 is once again at the initial condition (STATE 1) and the process is repeated. The initial duration of each cycle of this alternating process is about 5 to about 6 seconds and as temperatures in evaporators 208 and 224 drop, the duration of each cycle extends to about 20 to about 60 seconds. 
     Refrigerant flow switching device 218 utilizes, in part, the pressure difference between the high and low pressure refrigerants or the pressure difference between the high pressure refrigerant and atmospheric pressure to control refrigerant flow. Device 218 is self-contained in that no outside energy source, e.g., electric power, is required to open and close the flow controllers. The preferred embodiment illustrated in FIGS. 2B-E therefore is particularly useful as the refrigerant flow control unit when it is desired to eliminate a need for any outside energy source to control refrigerant flow. 
     If energy efficiency and cost are primary concerns, it is contemplated that for system 200 of FIG. 2A having refrigerant flow switching device 218 of FIGS. 2B-C, compressor unit 202 is a single stage compressor. By utilizing a plurality of evaporators selected to operate at desired respective refrigeration temperatures, improved energy use results. Further, by utilizing a single-stage compressor rather than a plurality of compressors or a compressor having a plurality of stages, increased costs associated with an improved energy efficiency are minimized. 
     The refrigeration system 200 illustrated in FIG. 2A requires less energy than a single-evaporator, single-compressor circuit with the same cooling capacity. Some efficiency advantages come about due to the fact that the vapor leaving the higher temperature evaporator 208 is compressed from an intermediate pressure, rather than from the lower pressure of the vapor leaving the lower temperature evaporator 224. Since the vapor from phase separator 210 is at a higher pressure than the vapor from freezer evaporator 224, the pressure ratio is lower when vapor from phase separator 210 is compressed to a desired compressor outlet pressure than when the vapor from the freezer evaporator 224 is compressed. Thus, less compression work is required than if all the refrigerant was compressed from the freezer exit pressure. 
     FIG. 3 is a block diagram illustration of a household refrigerator 300 including an insulated wall 302 forming a fresh food compartment 304 and a freezer compartment 306. FIG. 3 is provided for illustrative purposes only, particularly to show one apparatus which has substantially separate compartments which require refrigeration at different temperatures. In the household refrigerator, fresh food compartment 304 and freezer compartment 306 are typically maintained at about +33° F. to +47° F. and -10° F. to +15° F., respectively. 
     In accordance with the present invention, a first evaporator 308 (high pressure evaporator) is shown disposed in the fresh food compartment 304 and a second evaporator 310 (low pressure evaporator) is shown disposed in freezer compartment 306. The present invention is not limited to the physical location of the evaporators and the location of the evaporators shown in FIG. 3, is only for illustrative purposes and to facilitate ease of understanding. It is contemplated that the evaporators 308 and 310 could be disposed anywhere in the household refrigerator, or even outside the refrigerator and the evaporator-cooled air from each respective evaporator is directed to the respective compartments via conduits, barriers, and the like. 
     First and second evaporators 308 and 310 are driven by a compressor unit 312 and a condenser 314 shown located in a compressor/condenser compartment 316. A control knob 318 is disposed in fresh food compartment 304 and a temperature sensor 320 is disposed in freezer compartment 306. Control know 318 adjusts via linking means, such as a flexible cable, the force provided by compression spring 266 of first flow controller 226 of refrigerant flow switching device 218 via pressure regulator 268, shown in FIGS. 2B and 2C. The temperature in compartment 304 may be controlled by the aforementioned adjustment of pressure because under a saturated condition (a two-phase refrigerant co-existing in a liquid and vapor state) typically existing in first evaporator 308 during its operation, a given pressure of the refrigerant is associated with a specific temperature of the refrigerant in first evaporator 308. Control knob 318 may be calibrated to read in gradations of temperature desired in fresh food compartment 304. Temperature sensor 320 sends a signal to compressor 312 to run or to stop according to the setting on it. First evaporator 308 is typically operated at about +15° F. to about + 32° F. and the second evaporator 310 is typically operated at about -30° F. to about 0° F. for maintaining fresh food compartment 304 at about +33° F. to +47° F. and freezer compartment 306 about -10° F. to +15° F., respectively. 
     In operation, and by way of example, control knob 318 of a typical household refrigerator of 19 cubic feet capacity is coupled to a refrigerant flow switching device of the present invention (not shown in FIG. 3). When control knob 318, for example is set at 38° F. in fresh food compartment 304, that setting corresponds to a refrigerant temperature of about 25° F. and pressure of about 45 psia in first evaporator 308 and first flow controller 226, shown in FIGS. 2B-C. When refrigerant pressure in bellows 229 exceeds 45 psia bellows 229 causes gate member 234 to switch from the closed position corresponding to STATE 1 to the open position corresponding to STATE 2 thereby conveying the high pressure refrigerant from conduit 220 to compressor unit 312. As compressor unit 312 evacuates first evaporator 308, part of the refrigerant present in evaporator 308 boils and thereby lowers the pressure and the temperature of the refrigerant present in first evaporator 308 to about 36 psia and to about 22° F., respectively. 
     At this point, compression spring 266 overcomes the force of the high pressure refrigerant in bellows 229 and causes it to move from the second position to the first position, thereby shutting off the flow of high pressure refrigerant to compressor unit 312. During a typical cycle of about 21 seconds, under the aforedescribed exemplary refrigerator conditions, the high pressure refrigerant from evaporator 308 is transported to compressor unit 312 by device 218 for about 5 seconds and the low pressure refrigerant form evaporator 310 is transported to compressor unit 312 by device 218 for about 16 seconds. It is understood that the allocation of conveying time between the high pressure and the low pressure refrigerant to compressor unit 312 is a function of the cooling capacity of first evaporator 308 and second evaporator 310. The capacity ratio between first evaporator 308 and second evaporator 310 for the aforedescribed refrigerator is about 3:1. A capacity ratio is defined as a ratio of the heat removing capacity in BTUs per hour of first evaporator 308 divided by that of second evaporator 310, i.e. in the aforementioned example first evaporator 308 removes heat at about three times the rate of second evaporator 310 from their respective compartments. Cycling of device 218 continues until the temperature set on thermostat 320 in freezer compartment 306 is reached, at that time, compressor unit 312 shuts down, until further demand signal from thermostat 320 is received. 
     Control knob 318 and sensor 320 are preferably user adjustable so that a system user selects a temperature, or temperature range, at which each respective evaporator is to be activated and/or inactivated. In this manner, operation of a refrigerant flow switching device is adjusted by the user. 
     As shown in FIG. 3, the illustrative refrigeration system includes two evaporators which are selected to operate at desired, respective, refrigeration temperatures. Reduced energy use is provided by utilizing a plurality of evaporators. Further, by utilizing, in one embodiment, a single-stage compressor as compressor unit 312 rather than a plurality of compressors or a compressor having a plurality of stages, increased costs associated with the improved energy efficiency are minimized. 
     FIG. 4 represents an illustrative refrigeration circuit having more than two evaporators in the system. 
     A conduit 404 conveys a high pressure refrigerant from a high pressure evaporator (not shown) into a first flow controller 414 of a first refrigerant flow switching device 410 of the present invention. An exemplary high pressure is about 60 psia. 
     A conduit 406 conveys a medium pressure refrigerant from a medium pressure evaporator (not shown) into a first flow controller 418 of the second refrigerant flow switching device 412 of the present invention. An exemplary medium pressure is about 40 psia. 
     A conduit 408 conveys a low pressure refrigerant into a check valve 420 of second switching device 412. An exemplary low pressure is about 20 psia. An output conduit 415 conveys either the low pressure refrigerant from conduit 419 or the medium pressure refrigerant from conduit 413 to check valve 416 of first flow switching device 410. An output conduit 402 conveys either the high pressure refrigerant from conduit 405 or an output from output conduit 403 to compression unit (not shown) of the refrigeration system. 
     In operation, since the temperature within a refrigeration system is progressively reduced during the initial phase, first flow switching device 410 is active and second flow switching device 412 is dormant because the high pressure in output conduit 403 prevents check valves 416 and 420 to open until the temperature and the pressure in the high pressure evaporator decreases sufficiently to allow check valve 416 to open. Thus during the initial stage, switching device 410 switches refrigerant flow between the high pressure refrigerant and the medium pressure refrigerant. As the temperature drops, the refrigerant flow pressure also drops. As a result, first switching device 410 becomes progressively less active and second switching device 412 becomes more active, i.e., refrigerant flow to conduit 402 is then alternated between the medium pressure refrigerant via conduit 415 and the low pressure refrigerant via conduit 419. 
     It is contemplated that in some refrigeration systems, all of the energy efficiencies and reduced costs provided by the present invention may not be strictly necessary. Thus, the invention as described herein may be modified or altered to vary efficiency and/or increased costs relative to the described embodiments. For example, a plurality of compressors or a compressor having a plurality of stages or any combination thereof, along with the refrigerant flow control means, may be utilized. Such modifications are possible, contemplated, and within the scope of the appended claims.