Patent Publication Number: US-7707844-B2

Title: Thermostatic expansion valve with bypass passage

Description:
FIELD OF THE INVENTION 
   The present invention relates to thermostatic expansion valves for controlling the flow of refrigerant to an evaporator in an air conditioning system. 
   BACKGROUND OF THE INVENTION 
   Thermostatic expansion valves are used to control or meter the flow of refrigerant to an evaporator in an air conditioning system, to provide a refrigerant flow rate into the evaporator that approximately matches the refrigerant flow rate exiting the evaporator. The refrigerant flowing through the thermostatic expansion valve experiences an expansion and a drop in pressure, which results in a refrigerant vapor being supplied to the evaporator. The vapor is then superheated in the evaporator before it enters the suction inlet to the compressor of the air conditioning system. 
   The typical thermostatic expansion valve operates via a working fluid having a “charge” pressure that changes in response to sensing the temperature of the refrigerant suction line to the compressor. The working fluid pressure acts against a diaphragm in the thermostatic expansion valve to effect opening and closing of a valve. By controlling the refrigerant flow to the evaporator, the thermostatic expansion valve maintains a predetermined amount of superheat in the evaporator to ensure that only vapor is leaving the evaporator. If there is insufficient refrigerant or superheat in the evaporator, un-evaporated liquid refrigerant leaving the evaporator could enter the suction inlet to the compressor. Liquid refrigerant entering the suction inlet to the compressor could cause overheating or damage to the compressor. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a thermostatic expansion valve that has a diaphragm for movably controlling a first valve element, which regulates fluid flow to the valve outlet. If a loss of charge pressure occurs due to a leak, for example, the loss of charge pressure against the diaphragm would cause the first valve element to move to a closed position and remain closed. An extended restriction of flow would lead to insufficient refrigerant in the evaporator and possible compressor damage. The present thermostatic expansion valve also has a bypass passage for allowing fluid flow to the outlet when the diaphragm moves in a direction to close the first valve element and continues to move beyond the point of closure by more than a predetermined distance, as would occur upon loss of charge pressure. In accordance with one aspect of the present invention, various embodiments of a thermostatic expansion valve are provided that comprise an inlet, an outlet, and a first valve element between the inlet and outlet, and a movable diaphragm. The diaphragm has a first side in communication with a pressurized fluid external to the thermostatic expansion valve, and a second side in communication with the pressurized fluid within the valve chamber outlet. In one embodiment, the diaphragm is movable relative to a neutral position in response to changes in pressure between the outlet fluid pressure and the external fluid pressure, wherein the movement of the diaphragm controls the first valve element to regulate the fluid flow rate through the valve. Various embodiments further comprise a second valve element in connection with the diaphragm, where the second valve element permits fluid flow from the inlet to the outlet through a bypass passageway when the diaphragm moves in a direction to close the first valve element and continues to move in the same direction more than a predetermined distance beyond the closure of the first valve. 
   In another aspect of the present invention, another embodiment of a thermostatic expansion valve is provided that comprises an inlet, an outlet, first and second flow paths through the valve, a first valve element in the first flow path between the inlet and outlet, and a movable diaphragm having a first side acted on by a first fluid pressure and a second side acted on by at least a second fluid pressure. The diaphragm is movable relative to a neutral position in response to changes in the pressures against the first and second sides of the diaphragm, wherein the diaphragm movement controls the position of the first valve element to regulate the fluid flow through the first flow path. This embodiment further comprises a spring for biasing the first valve element towards a closed position. The diaphragm is movable to permit increased fluid flow through the first flow path when the force against the first side is greater than that against the second side, and is movable to restrict fluid flow through the first flow path when the force against the first side is less than that against the second side. The thermostatic expansion valve further comprises a second valve element in connection with the diaphragm for permitting fluid flow through a second flow path, when the diaphragm moves to close the first valve element and continues to move more than a predetermined distance beyond the closure of the first valve element. 
   In yet another aspect of the invention, other embodiments of an expansion valve are provided that permit fluid flow through the valve when an opening in a slidable valve element is slideably moved into the flow path between the inlet and the outlet of the valve. In one exemplary embodiment, the valve comprises a slideable valve element in the flow path between the inlet and outlet, the slidable valve element having first and second openings therein, each of which may be slidably moved into the flow path to permit fluid flow through to the outlet. The valve further comprises a spring for providing a force for biasing the slidable valve element against a moveable diaphragm, which has a first side acted on by a first fluid pressure and a second side acted on by a second fluid pressure and the spring biasing force. The diaphragm is movable relative to a neutral position in response to changes in the pressures against the first and second sides of the diaphragm, wherein the diaphragm movement controls the position of the first opening in the slide valve element relative to the flow path to regulate the fluid flow through the first valve opening. The diaphragm is movable in a first direction to increase fluid flow through the first opening in the slideable valve element when the force against the first side is greater than that against the second side, and is movable in a second direction to restrict fluid flow through the first opening in the slideable valve element when the force against the first side is less than that against the second side. A second opening in the slide valve element permits fluid flow from the inlet to the outlet when the diaphragm allows the slide valve element to move in the second direction to move the first opening out of the flow path to a restricted flow position and the second opening into the flow path. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a cross-sectional side view of one embodiment of a thermostatic expansion valve in accordance to the principles of the present invention; 
       FIG. 2  is a cross-sectional side view of a second embodiment of a thermostatic expansion valve in accordance to the principles of the present invention; 
       FIG. 3  is a cross-sectional side view of a third embodiment of a thermostatic expansion valve in accordance to the principles of the present invention; 
       FIG. 4  is a side view of a slide valve element of the third embodiment of a thermostatic expansion valve according to the principles of the present invention; and 
       FIG. 5  is a side view of a fourth embodiment of a thermostatic expansion valve according to the principles of the present invention. 
   

   Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   The following description of the various embodiments are merely exemplary in nature and are in no way intended to limit the invention, its application, or uses. 
   One embodiment of a thermostatic expansion valve in accordance with the present invention is generally shown in  FIG. 1  at  120 . The thermostatic expansion valve  120  comprises an inlet  122 , an outlet  124 , a first valve element  126  between the inlet  122  and outlet  124 , and a movable diaphragm  128 . The movable diaphragm has a first side  128   a  in communication with a pressurized fluid  150  external to the thermostatic expansion valve, and a second side  128   b  in communication with the outlet  124 . In the first embodiment, the diaphragm  128  is movable relative to a neutral position in response to changes in pressure between the outlet  124  and the external fluid pressure at  150 , wherein the movement of the diaphragm  128  controls the first valve element  126  to regulate the fluid flow rate through the valve  120 . The first embodiment further comprises a second valve element  134  in connection with the diaphragm  128 , for permitting fluid flow to the outlet  124  through a second valve opening or bypass passageway  140  when the diaphragm  128  moves in a direction to close the first valve element  126  and continues to move in the same direction by more that a predetermined distance. In the first embodiment, the predetermined distance or stroke beyond the point of closure of the first valve which will open the second valve element is in the range of about 0.001 inches to about 0.010 inches. 
   Some embodiments of a thermostatic expansion valve further comprise an actuator member  144  for engaging the movable diaphragm  128  and the first valve element  126  to permit the movement of the diaphragm  128  to control the movement of the first valve element  126  to regulate the fluid flow through opening  138 . A spring  146  provides a biasing force against the first valve element  126  to move the valve element  126  towards a closed position. The spring  146  also applies a biasing force via actuating member  144  to the second side  128   b  of the diaphragm  128 , such that the spring biasing force and the force of the outlet fluid pressure both act against side  128   b  of the diaphragm. Thus, the diaphragm provides a balancing of the forces of the external fluid pressure acting against side  128   a , and the spring biasing force and outlet fluid pressure acting against side  128   b.    
   In the first embodiment, the diaphragm  128  is movable to displace a first valve element  126  to permit increased fluid flow through first valve opening  138  when the force against the first side  128   a  of the diaphragm  128  is greater than that against the second side  128   b . The first valve element  126  is preferably a tapered needle valve disposed within the first valve opening  138 , wherein movement of the tapered needle valve varies the cross-sectional area through the opening to allow for regulating fluid flow. Alternatively, the first valve element  126  may comprise a contoured poppet valve or other valve element disposed within the opening  138  that is suitable for varying or regulating the fluid flow. The first embodiment also includes a second valve element  134 , which comprises a pin  132  that is slidably disposed within a cavity  152  in a buffer plate. The pin  132  and second valve element  134  are biased by a second spring  148  against the second valve opening  140  of a bypass passageway  142  relative to the buffer plate  129 . Accordingly, when upward movement of the buffer plate  129  is not restricted or prevented by the diaphragm  128 , the biasing spring  148  expands such that the pin/second valve element  134  is no longer biased against the second valve opening  140 . The second valve element  134  (and pin  132 ) is biased in a closed position against the bypass opening  140  when the diaphragm downwardly displaces the first valve element  126  to permit fluid flow through first valve opening  138  to the exit  124 . Thus, there is no fluid flow through the bypass passageway  142  when the first valve element  126  permits fluid flow through the first valve opening  138  to the valve exit  124 . 
   The working fluid at  150  functions to apply an effective amount of pressure against side  128   a  of the diaphragm, so as to move the diaphragm  128  in a direction for opening the first valve element  126 . A force is applied against side  128   b  of the diaphragm by the pressure of the fluid internal to the valve  120 , which is in communication with the exit  124  of the thermostatic expansion valve  120 . A biasing force is also applied against side  128   b  of the diaphragm by the spring  146  when the first valve element  126  is in an open position relative to the first valve opening  138 . Thus, the force applied against side  128   a  by the working fluid must be greater than the force applied against side  128   b  by the internal fluid pressure and the spring  146  for the diaphragm to move the first valve element  126  to an open position. When the force against the first side  128   a  is greater than that against the second side  128   b , the diaphragm  128  is movable in a first direction to permit increased fluid flow through the first valve opening  138 . When the force against the first side  128   a  is less than that against the second side  128   b , the diaphragm  128  is movable in a second direction to cause the first valve element  126  to restrict fluid flow through the first valve opening  138 . Fluid flow is completely restricted when the diaphragm  128  moves in the second direction to allow the valve element  126  to completely close against the first valve opening  138 . The diaphragm may continue to move in the second direction beyond the closure point if the force against side  128   a  is less than the force of the internal fluid pressure acting against side  128   b . Such a situation could occur where a leak or loss of pressure in the external pressure source causes a loss of charge pressure. In this situation, pressure against side  128   b  may move the diaphragm  128  in the second direction beyond the point of closure of the first valve element  126 , to permit the second valve element  134  to open relative to a bypass or second valve opening  140 . Specifically, when the diaphragm  128  is displaced more than a predetermined distance beyond the position of closure of the first valve element  126 , the buffer plate  129  is no longer restricted by the diaphragm and moves upward by virtue of the second spring  148 . The second spring  148  accordingly expands and removes the spring force holding the second valve element  134  closed. The fluid pressure at  122  and opening  140  causes the second valve element  134  to move upward to an open position. Thus, the diaphragm  128  is movable to control a first valve element  126  for regulating fluid flow through a first valve opening  138  to the exit  124 , and is further movable upon closure of the first valve element  126  against opening  138  to open a second valve element  134  to permit fluid flow to the exit  124  though a bypass opening  142  when the diaphragm  128  is displaced more than a predetermined distance beyond the point of closure of the first valve element  126 . 
   The working fluid pressure  150  in communication with the first side  128   a  of the diaphragm  128  is provided by a pressurized fluid from an external device, such as a capillary tube having a working fluid pressure that is generally higher than the internal fluid pressure or pressure at the exit of the valve. The capillary tube or bulb may be positioned adjacent to the refrigerant suction line of a compressor in a typical air conditioning system, and provides a working pressure that is responsive to the temperature of the refrigerant suction line to the compressor. The bulb pressure varies with suction line temperature changes and acts against the diaphragm  128  to effect opening and closing of the valve element  126  against a spring bias and an equilibrium pressure against side  128   b  of the diaphragm. By sensing the suction line temperature and controlling the refrigerant flow through the valve exit  124  to the evaporator, the thermostatic expansion valve  120  maintains a predetermined amount of superheat in the evaporator of the air conditioning system. During normal operation of an air conditioning system, it is possible that a loss of working fluid pressure could occur, due to a leak in the capillary tube or a rupture in the diaphragm  128 . In such a situation, the loss of pressure against the diaphragm  128  leads to a force against side  128   a  that is less than the force against side  128   b  resulting in closure of the valve element  126  against the first valve opening  138 . This blocks the flow of refrigerant to the evaporator of the air conditioning system, which could lead to low suction pressure, and an inadequate superheat in the evaporator entering the suction inlet to the compressor. Such a situation could cause overheating or damage to the compressor, and is especially of concern for high efficiency scroll compressors. 
   In the event of a loss of working pressure, where fluid flow is completely restricted when the diaphragm  128  moves the valve element  126  to a closed position, the force against side  128   a  is less than the force of the internal fluid pressure against side  128   b . Thus, the diaphragm  128  moves in a direction for closing the first valve element  126  in the first flow path  138 A, and continues to move in the same direction to open a second valve element  134  when the diaphragm is displaced more than a predetermined distance beyond the position of closure of the first valve element  126 . The opening of the second valve element  134  relative to the bypass opening  140  in the second flow path  140 A permits a predetermined flow of refrigerant through a passageway  142  to the valve outlet  124  and to the evaporator, to enable the air conditioning system to operate at a nominal level in the event of a loss of working pressure. 
   The bypass opening  140  and the passageway  142  are sized to provide a predetermined nominal flow rate for nominal operating conditions of a typical air conditioning system. This first embodiment of a thermostatic expansion valve provides control of fluid flow relative to changes in a working fluid pressure to regulate the amount of superheat in the evaporator, and also provides a predetermined amount of superheat in the event of a loss of working pressure for a limp along mode of air conditioning operation. The thermostatic expansion valve  120  accordingly provides protection to a compressor by ensuring an adequate level of suction pressure to prevent overheating or damage to the compressor. 
   In a second embodiment, a thermostatic expansion valve  220  is provided that comprises an inlet  222 , an outlet  224 , a first flow path  238  and second flow path  240  through the valve, a first valve element  226  in the first flow path between the inlet  222  and outlet  224 , and a movable diaphragm  228  having a first side  228   a  acted on by a first fluid pressure and a second side  228   b  acted on by at least a second fluid pressure. The diaphragm  228  is movable relative to a neutral position in response to changes in the pressures against the first and second sides  228   a  and  228   b  of the diaphragm, wherein the diaphragm movement controls the position of the first valve element  226  to regulate the fluid flow through the first flow path  238 . The second embodiment further comprise a spring  246  for biasing the first valve element  226  towards a closed position. The diaphragm  228  is movable to permit increased fluid flow through the first flow path  238  when the force against the first side  228   a  is greater than that against the second side  228   b , and is movable to restrict fluid flow through the first flow path  238  when the force against the first side  228   a  is less than that against the second side  228   b . The second embodiment of a thermostatic expansion valve  220  further comprise a second valve element  234  in connection with the diaphragm  228  for permitting fluid flow from the inlet  222  to the outlet  224  through a second flow path  240  only when the first valve element  226  is in a closed position. The second valve element  234  is biased in a closed position against the bypass opening  240  whenever the diaphragm  228  has displaced the first valve element  226  to permit fluid flow through the first valve opening  238  to the exit  224 . Thus, there is no fluid flow through the bypass or second valve passageway  242  when the first valve element  226  permits fluid flow through the first valve opening  238  and the valve exit  224 . The diaphragm  228  may also continue to move beyond closure of the first valve element  226  if the force against side  228   a  is less than the force of the internal fluid pressure against side  228   b . In this situation, the diaphragm  228  moves in the direction to close the first valve element  226 , and continues to move in the same direction allowing the second valve element  234  to open when the diaphragm  228  is further displaced beyond the point of closure of the first valve element  226  by more than a predetermined distance. 
   In the second embodiment, the second fluid pressure is provided by a pressurized fluid from an external source, such as the outlet of the evaporator, to provide an externally equalized pressure rather than an internal fluid pressure to side  228   b  of the diaphragm. In this second embodiment, the diaphragm moves relative to changes between the working fluid pressure at  250  that is responsive to the suction line temperature, and changes in the pressure drop across the evaporator at inlet  260 . Thus, this embodiment provides diaphragm control of a first valve element to regulate refrigerant flow in response to relative pressure changes external to the valve, and also provides a predetermined amount of flow in the event of a loss of working pressure for a limp along mode of air conditioning operation. The thermostatic expansion valve  220  accordingly provides protection to a compressor by ensuring an adequate level of suction pressure to prevent overheating or damage to the compressor. 
   Referring to  FIG. 3 , a third embodiment of a thermostatic expansion valve is shown. The thermostatic expansion valve  320  comprises an inlet  322 , an outlet  324 , a slide valve element  326  between the inlet chamber  322  and outlet chamber  324 , and a movable diaphragm  328 . The movable diaphragm has a first side  328   a  in communication with a pressurized fluid  350  external to the thermostatic expansion valve, and a second side  328   b  in communication with the outlet  324  via passage  342 . The diaphragm  328  is movable relative to a neutral position as shown in  FIG. 3 , in response to changes in pressure between the outlet  324  and the external fluid pressure  350 . The movement of the diaphragm  328  regulates the fluid flow rate through the valve  320 , by controlling or positioning a first valve opening port  338  in the slide valve element  326  relative to the opening  330  in the inlet chamber. The slide valve element  326  further comprises a second valve opening or bypass port  340  that permits fluid flow from the inlet  322  to the outlet  324  when the diaphragm  328  moves against the fluid pressure acting on side  328   a  to the extent that the slide valve element  326  moves the first valve opening port  338  to a closed position and further moves in the same direction by more than a predetermined distance. 
   In the third embodiment, the slide valve element  326  also acts as an actuator member for engaging the movable diaphragm  328  and a spring  346 , which permits the balancing of the spring force and the force against the diaphragm  328  to control the movement of the first opening port  338  in the slide valve element  326  to regulate the fluid flow through opening  330 . The spring  346  provides a biasing force against the slide valve element  326  to move the first opening port  338  towards a closed position away from the opening  330  in the inlet chamber. The spring  346  also applies a biasing force via the slide valve element  326  to the second side  328   b  of the diaphragm  328 , such that the spring biasing force and the outlet fluid pressure both act against side  328   b  of the diaphragm. Thus, the diaphragm provides a balancing of the forces of the external fluid pressure acting against side  328   a , and the spring biasing force and outlet fluid pressure acting against side  328   b.    
   In the third embodiment, the diaphragm  328  is movable to move the first opening port  338  in the slide valve element  326  into the opening  330  to permit increased fluid flow through the opening  30  when the force against the first side  328   a  of the diaphragm  328  is greater than that against the second side  328   b . The slide valve element  326  is preferably a plate having first and second ports that are disposed within a slot  336 , wherein movement of the first port opening  338  into the opening  330  varies the cross-sectional area through the opening  330  to regulate the flow of fluid. For manufacturing convenience, the opening  330  in the inlet chamber and the opening port  338  in the slide plate  326  are preferably generally circular openings. Alternatively, the opening  330  in the inlet chamber and the first opening port  338  in the slide plate  326  may comprise a rectangular, oval, or tapered or contoured opening shape suitable for varying the cross-sectional area of an opening  330  to regulate the fluid flow through the valve  320 . The slide valve element  326  also includes a second opening or bypass port  340 , which may be a generally circular or rectangular opening. Alternatively, the second opening port  340  in the slide plate  326  may comprise a rectangular, oval, or tapered or contoured opening shape suitable for varying the cross-sectional area of an opening  340  to regulate the fluid flow through the valve  320 . The second opening port  340  is biased in a closed position when the force against the first side  328   a  of the diaphragm  328  is greater than that against the second side  328   b . Thus, there is no fluid flow through the second bypass opening port  340  when the first opening port  338  is within the opening  330  to permit fluid flow to the valve exit  324 . The second opening port  340  moves into the opening  330  of the inlet chamber when the force against the first side  328   a  of the diaphragm  328  is significantly less than the force against the second side  328   b , such that upward movement of the diaphragm  328  allows the slide valve element  326  to move the first opening port  338  to a closed position and to further move in the same direction by more than a predetermined distance. 
   The working fluid at  350  functions to apply an effective amount of pressure against side  328   a  of the diaphragm, so as to move the diaphragm  328  in a direction for moving the first opening  338  in the slide valve element  326  to an open position. A force is applied against side  328   b  of the diaphragm by the pressure of the fluid internal to the valve  320 , which is in communication with the exit  324  via passage  342 . A biasing force is also applied against side  328   b  of the diaphragm by a spring  346  via the slide valve element  326 . Thus, the force applied against side  328   a  by the working fluid must be greater than the force applied against side  328   b  by the internal fluid pressure and the spring  346  for the diaphragm to move the first port opening  338  in the slide valve element  326  to an open position relative to opening  330 . When the force against the first side  328   a  is greater than that against the second side  328   b , the diaphragm  328  is movable in a first direction to permit increased fluid flow through opening  330  via the first opening port  338 . When the force against the first side  328   a  is less than that against the second side  328   b , the diaphragm  328  is movable in a second direction to restrict fluid flow through opening  330 . Fluid flow is completely restricted when the diaphragm  328  moves the slide valve element  326  in the second direction to the extent that the first opening port  338  is moved out of the opening  330  to close the opening passage  330 . The diaphragm  328  may continue to move in the second direction beyond the point of port  338  moving to a closed position, if the force against side  328   a  is less than the spring force and force of the internal fluid pressure against side  328   b . In this situation, the diaphragm  328  may continue to move in the second direction to the extent that the second bypass opening port  340  in the slide valve element  326  moves into the opening  330  when the diaphragm is displaced more than a predetermined distance beyond the position of port  338  moving to a closed position. Thus, the diaphragm  328  is movable to control a first opening port  338  for regulating fluid flow through opening  330  to the exit  324 , and is further movable upon closure of the first opening port  338  to open a second bypass port  340  to permit fluid flow to the exit  324  when the diaphragm  328  is displaced more than a predetermined distance beyond the position of port  338  moving to a closed position. 
   The working fluid pressure  350  in communication with the first side  328   a  of the diaphragm  328  is provided by a pressurized fluid from an external device, such as a capillary tube having a working fluid pressure that is generally higher than the internal fluid pressure or pressure at the exit of the valve. The capillary tube or bulb may be positioned adjacent to the refrigerant suction line of a compressor in a typical air conditioning system, and provides a working pressure that is responsive to the temperature of the refrigerant suction line to the compressor. The bulb pressure varies with suction line temperature changes and acts against the diaphragm  328  to effect opening and closing of the first valve opening port  338 , against a spring bias and an equilibrium pressure against side  328   b  of the diaphragm. By sensing the suction line temperature and controlling the refrigerant flow through the valve exit  324  to the evaporator, the thermostatic expansion valve  320  maintains a predetermined amount of superheat in the evaporator of the air conditioning system. During normal operation of an air conditioning system, it is possible that a loss of working fluid pressure could occur, due to a leak in the capillary tube or a rupture in the diaphragm  328 . In such a situation, the loss of pressure against the diaphragm  328  leads to a force against side  328   a  that is less than the force against side  328   b  resulting in an upward movement of the diaphragm that moves the first opening port to a closed position. If the flow of refrigerant to the evaporator of the air conditioning system were closed off, this could lead to low suction pressure, and an inadequate superheat in the evaporator entering the suction inlet to the compressor. The situation of low suction pressure entering the suction inlet to the compressor could cause overheating or damage to the compressor, and is especially of concern for high efficiency scroll compressors. 
   In the above situation of a loss of working pressure, where the diaphragm  328  moves the first opening port  338  in the slide valve element  326  to a closed position, the force against side  328   a  is less than the force of the internal fluid pressure against side  328   b . Upon a loss of working fluid pressure at  350 , the upward displacement of the diaphragm  328  allows the slide valve element  326  to move in a direction for shifting the first opening port  338  upward to a closed position, and to further move in the same direction by more than a predetermined distance that is sufficient to cause the second bypass port  340  to move into the opening  330  to permit fail-safe fluid flow through the second opening. The movement of the second bypass opening port  340  relative to the opening  330  permits a predetermined flow of refrigerant through the second bypass port  340  to the valve outlet  324  and to the evaporator, to enable the air conditioning system to operate at a nominal level in the event of a loss of working pressure. 
   As shown in  FIG. 4 , the first bypass opening port  338  and the second bypass opening port  340  are spaced apart from each other such that the distance from the bottom of the first port  338  to the top of the second port is at least greater than the height of the opening  330  of the inlet chamber. More preferably, the distance between the first port  338  and second port  340  is at least that of the opening  330 , plus a predetermined distance the slide valve element  326  must move beyond the point of closure of the first opening port  338  to cause the second bypass port  340  to move into the inlet opening  330  for establishing a fail-safe open position. In the third embodiment, this predetermined distance is preferably in the range of about 0.001 to about 0.010 inches, but may alternatively comprise a greater predetermined distance in other valve embodiments. It should be noted that this predetermined distance may be scalable depending on the overall stroke distance of the valve. 
   The second bypass opening port  340  is sized to provide a predetermined nominal flow rate for nominal operating conditions of a typical air conditioning system. This third embodiment of a thermostatic expansion valve provides control of fluid flow relative to changes in a working fluid pressure to regulate the amount of superheat in the evaporator, and also provides a predetermined amount of superheat in the event of a loss of working pressure for a limp along mode of air conditioning operation. The thermostatic expansion valve  320  accordingly provides protection to a compressor by ensuring an adequate level of suction pressure to prevent overheating or damage to the compressor. 
   A fourth embodiment of a thermostatic expansion valve in accordance with the present invention is generally shown in  FIG. 5  at  420 . The thermostatic expansion valve  420  comprises an inlet  422 , an outlet  424 , a first valve element  426  between the inlet  422  and outlet  424 , and a movable diaphragm  428 . The movable diaphragm has a first side  428   a  in communication with a pressurized fluid  450  external to the thermostatic expansion valve, and a second side  428   b  in communication with the outlet  424 . In the fourth embodiment, the diaphragm  428  is movable relative to a neutral position in response to changes in pressure between the outlet  424  and the external fluid pressure at  450 , wherein the movement of the diaphragm  428  controls the first valve element  426  to regulate the fluid flow rate through the valve  420 . The fourth embodiment further comprises a second valve element  432 , for permitting fluid flow to the outlet  424  through a bypass passageway  440  when the diaphragm  428  moves upward in a direction to close the first valve element  426  and continues to move in the same direction by more than a predetermined distance, such as when a pressure loss occurs at  450 . 
   The working fluid at  450  functions to apply an effective amount of pressure against side  428   a  of the diaphragm, so as to move the diaphragm  428  in a direction for opening the first valve element  426 . The first valve element  426  generally comprises a tapered portion on the shaft  427  disposed within the first port opening  438 , wherein movement of the first valve element  426  varies the cross-sectional area through the opening  438  to allow for regulating fluid flow. A force is applied against side  428   b  of the diaphragm by the pressure of the fluid internal to the valve  420 , which is in communication with the exit  424  of the thermostatic expansion valve  420 . A biasing force is also applied against side  428   b  of the diaphragm by the spring  446  and shaft  427  when the first valve element  426  is in an open position relative to the opening  438 . Thus, the force applied against side  428   a  by the working fluid at  450  must be greater than the force applied against side  428   b  by the internal fluid pressure and the spring  446  for the diaphragm to move the first valve element  426  towards an open position. When the force against the first side  428   a  is greater than that against the second side  428   b , the diaphragm  428  is movable in a first direction to permit increased fluid flow through the valve opening  438 . When the force against the first side  428   a  is less than that against the second side  428   b , the diaphragm  428  is movable in a second direction to restrict fluid flow through the valve opening  438 . Fluid flow is completely restricted when the diaphragm  428  moves the valve element  426  in the second direction to close against the first valve port opening  438 . 
   In the fourth embodiment of a valve  420 , a buffer plate  436  is provided for distribution of force from shaft  427  against side  428   b  of the diaphragm  428 . Slidably coupled to buffer plate  436  is a second valve element  434 , having an opening  432  therein for receiving a spring  446  for biasing the second valve element  434  and towards the second valve port opening  440 . The second valve element  434  is slidably coupled to the buffer plate  436  by virtue of a lip  433  on the second valve element  434  that retains the second valve element over a tab  435  on the buffer plate  436 . The spring  448  urges the second valve element to extend away from the buffer plate  436 , and the tab  435  on buffer plate  436  engages lip  433  on the second valve element  434  to limit the amount of extension. Alternatively, in place of the lip  433 , the second valve element  434  may comprise a locking ring inserted into a groove, or other suitable means for engaging the tab or ring  435  on the buffer plate  436 . The slidably coupled valve element  434  and buffer plate  436  permits the diaphragm  428  and buffer plate  436  to move relative to the stationary second valve element  434  for displacing the shaft, while permitting the second valve element to fully extend such that the second valve element  434  may move away from the second valve opening  440 . 
   When the valve  420  is at or near equilibrium, the spring  446  biases the first valve element  426  on shaft  427  to a closed position against the first valve port opening  438 , and pressure against diaphragm side  428   a  maintains buffer plate  436  against the end of shaft  427 . In this position, a spring  448  biases the second valve element  434  towards a closed position against the second valve port opening  440 . Accordingly, the valve  420  may restrict flow from the inlet  422  to the outlet  424 . Increased pressure at  450  causes the diaphragm  428  to push down on buffer plate  436  and shaft  427  to open the first valve element  426  relative to port  438  as shown in  FIG. 5 . This permits regulation of fluid flow from the inlet  422  through the first valve opening  438  to the valve outlet  424 . The second valve element  434  is maintained in a closed position against the second valve opening  440  by biasing spring  448 . A seal  449  is also provided for sealing around the shaft  427  and the second valve element  434 . Thus, there is no fluid flow through the bypass passageway  440  when the first valve element  426  permits fluid flow through the first valve opening  438  to the valve exit  424 . 
   In the event of a loss of charge pressure at  450 , spring  446  biases the first valve element  426  on shaft  427  to move upward to a closed position against the first valve port opening  438 . However, with no pressure at  450 , the internal pressure against side  428   b  will cause the diaphragm  428  to continue to move in an upward direction. This allows spring  448  to extend until the tab  435  on buffer plate  436  engages lip  433  on the second valve element  434 . The pressure at inlet  422  and at the second valve port opening  440  acting against the second valve element  434  will cause the second valve element  434  to continue to move the diaphragm  428  in an upward direction (since there is no opposing pressure at  450 ). This permits fluid flow from the inlet  422  through the second bypass valve opening  440  to the valve outlet  424 . Thus, the diaphragm  428  moves in a direction to close the first valve element  426 , and continues to move in the same direction to open the second valve element  434  when the diaphragm  428  is further displaced beyond the point of closure of the first valve element  426  by more than a predetermined distance. The opening of the second valve element  434  relative to the second valve opening  440  permits a predetermined flow of refrigerant through the bypass passageway  440  to the valve outlet  424  and to the evaporator, to enable the air conditioning system to operate at a nominal level in the event of a loss of working pressure. 
   The bypass port opening  440  and passageway are sized to provide a predetermined nominal flow rate for nominal operating conditions of a typical air conditioning system. This fourth embodiment of a thermostatic expansion valve provides control of fluid flow relative to changes in a working fluid pressure to regulate the amount of superheat in the evaporator, and also provides a predetermined amount of superheat in the event of a loss of working pressure for a limp along mode of air conditioning operation. The thermostatic expansion valve  420  accordingly provides protection to a compressor by ensuring an adequate level of suction pressure to prevent overheating or damage to the compressor. 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.