Patent Publication Number: US-2016238298-A1

Title: Hvac systems and methods with improved stabilization

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to heating, ventilating, and air conditioning (HVAC) systems, and more particularly, to HVAC systems and methods with improved stabilization. 
     BACKGROUND 
     Heating, ventilating, and air conditioning (HVAC) systems can be used to regulate the environment within an enclosed space. Typically, an air blower is used to pull air (i.e., return air) from the enclosed space into the HVAC system through ducts and push the air into the enclosed space through additional ducts after conditioning the air (e.g., heating, cooling, or dehumidifying the air). 
     The cooling aspect of an HVAC system may utilize an evaporator that cools return air from the enclosed space. An expansion valve meters refrigerant to the evaporator while receiving the refrigerant from a condenser. The expansion valve, the evaporator, and the condenser form part of a closed-conduit refrigeration circuit of the HVAC system. There are, at times, issues with stable operation of the expansion valve that could benefit from improvements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the present disclosure are described in detail below with reference to the attached drawing figures, which are incorporated by reference herein. 
         FIG. 1A  is a schematic diagram of a heating, ventilating, and air conditioning (HVAC) system having a heat flow modulator for improving stabilization of an expansion valve, according to an illustrative embodiment; 
         FIG. 1B  is a schematic diagram, with a portion shown in cross-section, of the expansion valve of  FIG. 1A , according to an illustrative embodiment; 
         FIG. 2A  is a schematic perspective view of a portion of a heating, ventilating, and air conditioning (HVAC) system having a heat-flow modulator for improved stability, according to an illustrative embodiment; 
         FIG. 2B  is a rear elevation view of the portion of the heating, ventilating, and air conditioning (HVAC) system shown in  FIG. 2A ; 
         FIG. 2C  is a schematic perspective view of a body of the heat-flow modulator of  FIG. 2A , according to an illustrative embodiment; 
         FIG. 2D  is a rear elevation view of the body shown in  FIG. 2C ; 
         FIG. 3A  is a perspective view of a body of a heat-flow modulator having a rectangular cross-section, according to an illustrative embodiment; and 
         FIG. 3B  is a rear elevation view of the body is shown in  FIG. 3A . 
     
    
    
     The figures described above are only exemplary and their illustration is not intended to assert or imply any limitation with regard to the environment, architecture, design, configuration, method, or process in which different embodiments may be implemented. 
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     Heating, ventilating, and air-conditioning (HVAC) systems commonly incorporate an expansion valve to regulate a flow of refrigerant from a condenser to an evaporator. The expansion valve, the condenser, and the evaporator are components of a closed-conduit refrigerant circuit, which also includes a compressor. The closed-conduit refrigerant circuit is operable to circulate refrigerant among its components, thus enabling the evaporator to produce a cooled airflow from unconditioned air. 
     To regulate the flow of refrigerant between the condenser and the evaporator, the expansion valve incorporates a movable pin that selectively occludes an internal flow orifice. The movable pin displaces along a pin stroke, positions of which, determine a degree of occlusion. An actuator is operable to displace the movable pin in response to a refrigerant temperature, which is sensed at an output of the evaporator. Such displacement typically occurs against a biasing member, such as a spring. The movable pin ceases its displacement when forces applied by the actuator and the biasing member balance. 
     As the refrigerant temperature changes, a force applied by the actuator changes and the movable pin adjusts to a new equilibrium point. This adjustment occurs dynamically as refrigerant traverses the expansion valve to flow from the condenser to the evaporator and the refrigerant temperature changes. However, under certain operating conditions, e.g., the HVAC system operating under reduced loads, the movable pin may oscillate around or excessively “hunting” the equilibrium point, causing unstable operation of the expansion valve. This unstable operation creates fluctuations in the flow of refrigerant, especially with regards to refrigerant temperature and pressure. These fluctuations negatively impact the HVAC system, reducing its efficiency and potentially damaging components of the closed-conduit refrigeration circuit. 
     The embodiments described herein relate to systems and methods for improving stabilization of a heating, ventilating, and air conditioning (HVAC) system. More specifically, the systems and methods include a heat-flow modulator for regulating an exchange of thermal energy between a flow of refrigerant and a sensory bulb. The exchange of thermal energy allows an expansion valve to respond to a refrigerant temperature using an actuator, which is coupled to the sensory bulb. The heat-flow modulator is formed of a body that includes a first contact surface and a second contact surface. The first contact surface is thermally-coupled to a suction line of the HVAC system, which conveys the flow of refrigerant. The second contact surface is thermally-coupled to the sensory bulb. The heat-flow modulator is operable to affect a flow of heat between the suction line and the sensory bulb, which includes providing a thermal resistance, a thermal capacitance, or both to the flow of heat. Such control regulates a response of the expansion valve to the refrigerant temperature and allows a pin within the expansion valve to reliably achieve an equilibrium point. Other systems and methods are presented. 
     Referring now to the drawings and primarily to  FIG. 1A , a schematic diagram is presented of a heating, ventilating, and air conditioning (HVAC) system  100  having a heat flow modulator  102  for improving stabilization of an expansion valve  104 , according to an illustrative embodiment. The expansion valve  104  is operable to regulate a flow of refrigerant within the HVAC system  100 . The HVAC system  100  includes a closed-conduit refrigeration circuit  106 . The closed-conduit refrigeration circuit  106  is shown in  FIG. 1A  by tubing that represents fluid coupling between components of the closed-conduit refrigeration circuit  106 . Sections of tubing  122 ,  126 ,  138 ,  140 ,  144  correspond to individual conduits of refrigerant and arrows  116 ,  118 ,  124 ,  136  indicate corresponding flows of refrigerant therein (i.e., when refrigerant is present in the HVAC system  100 ). 
     The closed-conduit refrigeration circuit  106  includes an evaporator  108  for enabling a cooling capacity of the HVAC system  100 . The evaporator  108  typically includes at least one evaporator fan  110  to circulate a return air  112  across one or more heat-exchange surfaces of the evaporator  108 . The evaporator  108  is configured to transfer heat from the return air  112  to refrigerant therein. The return air  112  is drawn in from a conditioned space and exits the evaporator  108  as a cooled airflow  114 . Concomitantly, a low-pressure liquid refrigerant  117  enters the evaporator  108  and leaves as a low-pressure gas refrigerant  119 . 
     The closed-conduit refrigeration circuit  106  also includes a compressor  120  fluidly-coupled to the evaporator  108  via a suction line  122 , or tubing. The suction line  122  is operable to convey the low-pressure gas refrigerant  119  from the evaporator  108  to the compressor  120 . During operation, the compressor  120  performs work on the low-pressure gas refrigerant  119 , thereby generating a high-pressure gas refrigerant  125 . The high-pressure gas refrigerant  125  exits the compressor  120  through a discharge line  126 , or tubing. In some embodiments, the compressor  120  includes a plurality of compressors that form a tandem configuration within the closed-conduit refrigeration circuit  106 . In such embodiments, the plurality of compressors may be fluidly-coupled to the suction line  122  through a common suction manifold and fluidly-coupled to the discharge line  126  through a common discharge manifold. Other types of fluid couplings are possible. 
     The closed-conduit refrigeration circuit  106  also includes a condenser  128  that is fluidly-coupled to the compressor  120  via the discharge line  126 . The condenser  128  typically includes at least one condenser fan  130  to circulate a non-conditioned air  132  across one or more heat exchange surfaces of the condenser  128 . The condenser  128  is configured to transfer heat from refrigerant therein to the non-conditioned air  132 . The non-conditioned air  132  exits the condenser  128  as a warmed airflow  134 . Concomitantly, the high-pressure gas refrigerant  125  enters the condenser  128  and leaves as a high-pressure liquid refrigerant  137 . In some embodiments, the condenser  128  includes a microchannel condenser. 
     The closed-conduit refrigeration circuit  106  includes a liquid line  138 , or tubing, and a refrigerant line  140 , or tubing. The liquid line  138  fluidly-couples the condenser  128  to the expansion valve  104  and is operable to convey the high-pressure liquid refrigerant  137  from the condenser  128  to the expansion valve  104 . The refrigerant line  140  fluidly-couples the expansion valve  104  to the evaporator  108  and is operable to convey the low-pressure liquid refrigerant  117  from the expansion valve  104  to the evaporator  108 . In some embodiments, a distributor  142  splits the refrigerant line  140  into a plurality of branches  144 . These branches  144  transition into a plurality of short heat-transfer circuits (not explicitly shown) upon entry into the evaporator  108 . In such embodiments, the plurality of short heat transfer circuits may prevent large drops in pressure that might otherwise occur if a single, long circuit were used. 
     The expansion valve  104  serves to regulate the flow of refrigerant through the HVAC system  100  and to control a conversion of high-pressure liquid refrigerant  137  into low-pressure liquid refrigerant  117 . Such regulation is assisted by a sensory bulb  146 , which is fluidly-coupled to the expansion valve  104  and operates cooperatively with the heat-flow modulator  102 . The heat-flow modulator  102  includes a first contact surface  148  that is thermally-coupled to the suction line  122  and a second contact surface  150  that is thermally-coupled to the sensory bulb  146 . Such thermal coupling enables the heat-flow modulator  102  to regulate an amount of thermal energy exchanged between the suction line  122  and the sensory bulb  146 . This regulation improves stability of the expansion valve  104  during operation. Aspects of the heat-flow modulator  102  will be described further in relation to  FIGS. 2A-2D  and  FIGS. 3A-3B . 
     Referring now primarily to  FIG. 1B , a schematic diagram is presented, with a portion shown in cross-section, of an expansion valve suitable for use as the expansion valve  104  of  FIG. 1A , according to an illustrative embodiment. It should be understood that the depiction of  FIG. 1B  is not intended as limiting and is presented for purposes of illustration only. Numerous types of expansion valves are suitable for use in the HVAC system  100  and might be incorporated therein in place of the one illustrated in  FIG. 1B . Some features of the expansion valve  104  are shown in both  FIGS. 1A and 1B  (e.g., the sensory bulb  146 ). The expansion valve  104  includes a body  152  formed with a flow orifice  154 . The flow orifice  154  is operable to convey the flow of refrigerant from an inlet port  156  to an outlet port  158 . The inlet port  156  is configured to fluidly-couple the expansion valve  104  to the liquid line  138  of the closed conduit refrigeration circuit  106 . The outlet port  158  is configured to fluidly-couple the expansion valve  104  to the refrigerant line  140  of the closed-conduit refrigeration circuit  106 . 
     The expansion valve  104  also includes a pin  160  having a longitudinal axis  162 . The pin  160  is operable to control a primary flow of refrigerant through the flow orifice  154 , which includes varying an occlusion of the flow orifice  154 . The pin  160  is operatively movable along the longitudinal axis  162  between a closed position and an open position. The closed position and the open position define terminal points of a stroke of the pin  160 , or pin stroke. In the closed position, the pin  160  occludes the flow orifice  154 . Such occlusion may involve the pin  160  sealingly engaging the body  152  along one or more surfaces that define the flow orifice  154 . In the open position, the pin  160  substantially unoccludes the flow orifice  154 . Motion of the pin  160  within the pin stroke alters the occlusion of the flow orifice  154 . As the pin  160  moves from the closed position to the open position, the occlusion progressively decreases. As the pin  160  moves from the open position to the closed position, the occlusion progressively increases. In  FIG. 1B , the pin  160  is depicted at a point along the pin stroke between the closed position and the open position. 
     In some embodiments, the expansion valve  104  includes a spring  164  arranged within the expansion valve  104  so as to bias the pin  160  in the closed position. In such embodiments, a spring guide  166  is typically operable to center the spring  164  along the longitudinal axis  162  of the pin  160 . In some embodiments, the pin  160  is disposed through the flow orifice  154 , as shown in  FIG. 1B . This depiction, however, is not intended as limiting. For example, and without limitation, the pin  160  could be configured to sealingly engage the body  152  proximate the flow orifice  154 , but not extend therethrough. Other configurations are possible. 
     The expansion valve  104  includes an actuator  168  coupled to the pin  160  and configured to move the pin  160  in response to a refrigerant temperature. The refrigerant temperature is sensed adjacent an output of the evaporator  108  via the sensory bulb  146 . In some embodiments, the actuator  168  includes a chamber  170  having a diaphragm  172  coupled to the pin  160 . This coupling may involve other elements, such as a flexible plate  174 . The diaphragm  172  partitions the chamber  170  into a first compartment  176 , which is at or near a minimum in  FIG. 1B , and a second compartment  178 . In such embodiments, the actuator  168  also includes a tube  180  coupling the chamber  170  to the sensory bulb  146 . The tube  180 , commonly a capillary transmission tube, enables fluid communication between the first compartment  176  of the chamber  170  and the sensory bulb  146 . 
     A fluid is disposed within a volume defined by the first compartment  176 , the sensory bulb  146 , and the tube  180 . The fluid is typically the same as a refrigerant used in the HVAC system  100 , although other fluids are possible. The fluid is operable to displace the diaphragm  172  in response to thermal energy entering or exiting the sensory bulb  146 . Such displacement adjusts a position of the pin  160 , thereby altering the flow of refrigerant through the flow orifice  154 . The expansion valve  104  is therefore able to regulate the flow of refrigerant through the HVAC system  100  in response to the refrigerant temperature of the low-pressure gas refrigerant  119  exiting the evaporator  108 . 
     In some embodiments, the expansion valve  104  includes a pressure equalizer port  182  fluidly-coupled to the suction line  122  of the closed-conduit refrigeration circuit  106 . In such embodiments, the pressure equalization port  182  enables the expansion valve  104  to sense a refrigerant pressure of the low-pressure gas refrigerant  119  exiting the evaporator  108 . The sensed refrigerant pressure is utilized by the expansion valve  104  to adjust the position of the pin  160 , thereby altering the flow of refrigerant through the flow orifice  154 . This alteration aids in regulating the flow of refrigerant through the HVAC system  100 . In some embodiments, the pressure equalizer port  182  is fluidly-coupled to the suction line  122  via a pressure equalization line  184 , such as that shown in  FIG. 1A . In these embodiments, the pressure equalization line  184  forms a junction  186  with the suction line  122  in close proximity to the output of the evaporator  108 . The pressure equalizer port  182  is configured to receive refrigerant from the suction line  122  and convey such refrigerant into the second compartment  178  and against the diaphragm  172  (or flexible plate  174 ). The diaphragm  172  (or flexible plate  174 ) may displace when contacted by such refrigerant, i.e., displace in response to the refrigerant pressure, thereby adjusting the position of the pin  160 . 
     It will be appreciated that the expansion valve  104 , when including the pressure equalization port  182 , uses the refrigerant temperature and the refrigerant pressure in combination to regulate the flow of refrigerant in the HVAC system  100 . For embodiments that incorporate both the sensory bulb  146  and the pressure equalization line  184 —such as that depicted in  FIG. 1A —the junction  186  is typically adjacent, but downstream a portion  188  of the HVAC system  100  that contains the sensory bulb  146 . More specifically, the portion  188  contains a segment of the suction line  122  thermally-coupled to the sensory bulb  146  via the heat-flow modulator  102 . However, other locations of the junction  186  are possible. 
     Now referring again primarily to  FIG. 1A , the HVAC system  100  includes a refrigerant disposed therein (e.g., see arrows  116 ,  118 ,  124 ,  136 ). The closed-conduit refrigeration circuit  106  serves to convey refrigerant between components of the HVAC system  100  (e.g., the expansion valve  104 , the evaporator  108 , the compressor  120 , the condenser  128 , etc.). Individual components of the closed-conduit refrigeration circuit  106  then manipulate the refrigerant to generate the cooled airflow  114 . 
     In operation, the evaporator  108  receives the low-pressure liquid refrigerant  117  as a cold fluid from the expansion valve  104  via the refrigerant line  140  and, if present, the distributor  142  and associated plurality of branches  144 . The cold, low-pressure liquid refrigerant  117  flows through the evaporator  108  and, while therein, absorbs heat from the return air  112 . Such heat absorption maybe aided by the at least one evaporator fan  110  and the one or more heat-exchange surfaces of the evaporator  108 . The at least one evaporator fan  110  enables a forced convection of return air  112  across the one or more heat-exchange surfaces of the evaporator  108 . Absorption of heat by the cold, low-pressure liquid refrigerant  117  induces a conversion from liquid to gas (i.e., boiling) of refrigerant within the evaporator  108 . The cold, low-pressure liquid refrigerant  117  therefore leaves the evaporator  108  as a warm, low-pressure gas refrigerant  119 . Concomitantly, the return air  112  exits the evaporator  108  as the cooled airflow  114 . 
     Conversion of the cold, low-pressure liquid refrigerant  117  into the warm, low-pressure gas refrigerant  119  often produces a superheated refrigerant whose temperature exceeds a saturated boiling point. Superheated refrigerant is generated when warm, low-pressure gas refrigerant  119  continues to absorb heat after changing from liquid to gas. Such absorption occurs predominantly within the evaporator  108 , but may also occur within the suction line  122 . A degree of superheat is typically measured in terms of temperature (e.g., ° F., ° C., K) and refers to a difference in temperature between the superheated refrigerant and its saturated boiling point. 
     After leaving the evaporator  108 , the warm, low-pressure gas refrigerant  119  traverses the suction line  122  of the closed-circuit refrigeration circuit  106  and enters the compressor  120 . The compressor  120  performs work on the warm, low-pressure gas refrigerant  119 , producing a hot, high-pressure gas refrigerant  125 . The hot, high-pressure gas refrigerant  125  exits the compressor  120  via the discharge line  126  and travels to the condenser  128 . The hot, high-pressure gas refrigerant  125  flows through the condenser  128 , and while therein, transfers heat to the non-conditioned air  132 . Such heat transfer may be assisted by the at least one condenser fan  130  and the one or more heat-exchange surfaces of the condenser  128 . The at least one condenser fan  130  enables a forced convection of non-conditioned air  132  across the one or more heat-exchange surfaces of the condenser  128 . Loss of heat from the hot, high-pressure gas refrigerant  125  induces a conversion from gas to liquid (i.e., condensing) within the condenser  128 . The hot, high-pressure gas refrigerant  125  therefore leaves the condenser  128  as a warm, high-pressure liquid refrigerant  137 . Concomitantly, the non-conditioned air  132  exits the condenser  128  as the warmed airflow  134 . 
     Conversion of the hot, high-pressure gas refrigerant  125  into the warm, high-pressure liquid refrigerant  137  often produces a subcooled refrigerant whose temperature is below a saturated condensation point. Subcooled refrigerant is generated when warm, high-pressure liquid refrigerant  137  continues to lose heat after changing from gas to liquid. Such loss occurs predominantly within the condenser  128 , but may also occur within the liquid line  138 . A degree of subcooling is typically measured in terms of temperature (e.g., ° F., ° C., K) and refers to a difference in temperature between the subcooled refrigerant and its saturated condensing point. 
     After leaving the condenser  128 , the warm, high-pressure liquid refrigerant  137  flows through the liquid line  138  to reach the expansion valve  104 . As explained more below, passage of the warm, high-pressure liquid refrigerant  137  through the flow orifice  154  induces a lowering of pressure and temperature that generates the cold, low-pressure liquid refrigerant  117 . The position of the pin  160  relative the flow orifice  154  serves to regulate flow through the expansion valve  104 , and hence, generation of the cold, low-pressure liquid refrigerant  117 . The cold, low-pressure liquid refrigerant  117  is then conveyed to the evaporator  108  by the refrigerant line  140  (and, if present, the distributor  142  and associated plurality of branches  144 ). 
     It will be appreciated that the closed-conduit refrigeration circuit  106  circulates the refrigerant to allow repeated processing by the evaporator  108 , the compressor  120 , the condenser  128 , and the expansion valve  104 . Repeated processing, or cycles, enables the HVAC system  100  to continuously produce the cooled airflow  114  during operation. During such cycling, the expansion valve  104  regulates the flow of refrigerant through the HVAC system  100 , which includes receiving the warm, high-pressure liquid refrigerant  137  from the condenser  128  and metering the cold, low-pressure liquid refrigerant  117  to the evaporator  108 . The former flow influences the degree of subcooling and the latter flow influences the degree of superheat. Higher degrees of superheat reduce a risk that the warm, low-pressure gas refrigerant  119  will enter the compressor  120  with a non-zero liquid fraction. Higher degrees of subcooling reduce a risk that the warm, high-pressure liquid refrigerant  137  will enter the expansion valve  104  with a non-zero gas fraction. 
     Now referring again primarily to  FIG. 1B , the expansion valve  104  regulates refrigerant flowing through the HVAC system  100  by receiving refrigerant through the inlet port  156  (see arrow  136 ). This received refrigerant traverses the body  152  and exits the outlet port  158  (see arrow  116 ). A presence of refrigerant within the body  152  enables the pin  160  to fluidly-couple to the flow orifice  154 . Such fluid coupling includes impeding refrigerant flowing through the flow orifice  154  (i.e., with the pin  160 ). When the pin  160  is in the open position, the flow of refrigerant exhibits a maximum magnitude. When the pin  160  is in the closed position, the flow of refrigerant substantially ceases. Between the open position and the closed position, i.e., along the pin stroke, the flow of refrigerant varies in magnitude between the maximum magnitude and substantially zero, respectively. 
     When the pin  160  is in the open position, the expansion valve  104  operates at “full load”. The expansion valve  104 , however, can transition into “part load” operation if the pin  160  moves along the pin stroke towards the closed position. “Part load” operation corresponds to that portion of the pin stroke where the flow of refrigerant exhibits a reduced, non-zero magnitude relative to the maximum magnitude. For example, and without limitation, “part load” operation may correspond to that portion of the pin stroke where the flow of refrigerant is  50 % or below that of the maximum magnitude. If the pin  160  moves into the closed position, the expansion valve  104  transitions into “no load” operation. In “no load” operation, the flow of refrigerant substantially ceases. 
     During operation, a plurality of forces acts on the pin  160  to determine the position of the pin  160  within the pin stroke. Refrigerant flowing from the inlet port  156  through the flow orifice  154  impinges on the pin  160 , biasing the pin  160  towards the open position and contributing to an opening force. The actuator  168  also contributes to the opening force depending on the refrigerant temperature, which is typically sensed proximate the output of the evaporator. For embodiments where the actuator  168  incorporates the diaphragm  172 , such as that illustrated in  FIG. 1B , the diaphragm  172  flexes in response to thermal energy transferring into or out of the fluid. Such transfer typically occurs at the sensory bulb  146 , which is thermally-coupled to the suction line  122  through the heat-flow modulator  102 . Because the fluid is sealed in the volume defined by the first compartment  176 , the sensory bulb  146 , and the tube  180 , thermal energy entering the fluid causes an increase in pressure that displaces the diaphragm  172  towards the body  152 . Conversely, thermal energy leaving the fluid causes a decrease in pressure that allows the diaphragm to relax away from the body  152 . By virtue of its coupling to the pin  160 , the diaphragm  172  contributes to the opening force when thermal energy enters the fluid. Such contribution decreases in magnitude when thermal energy leaves the fluid. 
     The spring  164  biases the pin  160  towards the closed position and contributes to a closing force. A strength of such bias increases as the pin  160  moves towards the open position, i.e., the spring  164  becomes increasingly compressed. An initial spring bias is typically determined by selecting an initial compression of the spring  164 . The pressure equalizer port  182 , if present, may also contribute to the closing force depending on the refrigerant pressure, which is typically sensed proximate the output of the evaporator (e.g., at the junction  186 ). The pressure equalizer port  182  is fluidly-coupled to the diaphragm  172  via the second compartment  178 . Such fluid-coupling allows the refrigerant pressure to be conveyed from the pressure equalizer port  182 , through the second compartment  178 , and against the diaphragm  172 . The refrigerant pressure displaces the diaphragm  172  away from the body  152  which, by virtue of its coupling to the pin  160 , contributes to the closing force. This contribution increases or decreases as the refrigerant pressure, respectively, increases or decreases. 
     As refrigerant flows through the expansion valve  104 , the pin  160  translates along the pin stroke until an equilibrium point is reached where the opening force balances the closing force. The equilibrium point changes dynamically in response to the refrigerant temperature and, in some embodiments, the refrigerant pressure. When integrated into the HVAC system  100 , it will be appreciated that the expansion valve  104  translates the pin  160  to meter refrigerant to the evaporator and to maintain a substantially constant degree of superheat therein. Thus, the expansion valve  104  sustains HVAC operating efficiencies while transitioning between “full load”, “part load”, and “no load” (i.e., as cooling demands on the evaporator  108  change). The expansion valve  104  also influences the degree of subcooling in the condenser. Translation of the pin  160  along the pin stroke alters refrigerant flow through the inlet port  156 , which due to fluid-coupling with the condenser  128 , varies a residence time of refrigerant flowing therein. 
     During certain operating conditions, without more, the expansion valve  104  may become susceptible to unstable regulation of the flow of refrigerant. For example, and without limitation, the HVAC system  100  may experience a reduced cooling demand that forces the expansion valve  104  into “part load” operation. If this “part load” operation corresponds to a small fraction of “full load” operation, e.g., less than 30% of the maximum magnitude, the pin  160  may experience difficulty finding the equilibrium point. 
     More specifically, the actuator  168  may contribute to the opening force in such a manner as to cause the pin  160  to oscillate around or excessively “hunt” the equilibrium point. Such oscillation or “hunting” may cause undesirable fluctuations the flow of refrigerant (e.g., unstable suction pressure in refrigerant flowing through the suction line  122 ). 
     To mitigate this behavior, the heat-flow modulator  102  provides a thermal resistance, a thermal capacitance, or both to a flow of heat caused by changes in refrigerant exiting the evaporator  108  (e.g., changes in the refrigerant temperature). These thermal characteristics aid the heat-flow modulator  102  in smoothing or delaying the flow of heat between the suction line  122  and the sensory bulb  146 . The actuator  168  is therefore allowed sufficient time to apply a steady force onto the pin  160 , allowing the pin  160  to reliably find the equilibrium point. Thus, the heat-flow modulator  102 , in cooperation with expansion valve  104 , ensures a controlled flow of refrigerant in the closed-conduit refrigeration circuit  106  and an improved stability of the HVAC system  100 . 
     Now referring primarily to  FIG. 2A , a perspective view is presented of a portion  200  of a heating, ventilating, and air conditioning (HVAC) system having a heat-flow modulator  202  for improved stability, according to an illustrative embodiment.  FIG. 2B  presents a rear elevation view of the portion  200 . The portion  200  of  FIG. 2A  is analogous to the portion  188  of  FIG. 1A . The portion  200  includes a section  204  of a suction line immediately downstream of an evaporator (see  108  in  FIG. 1A ). The portion  200  also includes a sensory bulb  206  thermally-coupled to the section  204  via the heat-flow modulator  202 . The sensory bulb  206  incorporates a tube  208  for fluidly-coupling to an actuator of an expansion valve. For purposes of illustration, the tube  208  is presented only partially in  FIG. 2A . 
     The heat-flow modulator  202  includes a first contact surface  210  and a second contact surface  212  (see  FIG. 2B ). The first contact surface  210  is thermally-coupled to the section  204  of the suction line and the second contact surface  212  is thermally-coupled to the sensory bulb  206 . This thermal coupling typically involves direct contact. However, in some embodiments, a thermal interface material (TIM) may be disposed between the first contact surface  210  and the section  204 , the second contact surface  212  and the sensory bulb  206 , or both. In such embodiments, the thermal interface material influences heat flow between neighboring components and may also secure one component to another. Non-limiting examples of thermal interface materials include thermal paints, frits, solders, pastes, epoxies, tapes, and glues. In some embodiments, the portion  200  of the HVAC system includes at least one bracket member  214  for urging the section  204  of the suction line (or portion thereof) towards the first contact surface  210 . In these embodiments, the at least one bracket member  214  also urges the sensory bulb  206  (or portion thereof) towards the second contact surface  212 . The at least one bracket member  214  may be configured to influence heat flow between the section  204  and the sensory bulb  206  (e.g., increase heat transfer between the section  204  and the sensory bulb  206 ). 
     The heat-flow modulator  202  typically includes a body  216 , or modulator body, that incorporates the first contact surface  210  and the second contact surface  212 . The body  216  may be deformable. The body  216  is configured to provide a thermal resistance, a thermal capacitance, or both to heat flowing between the first contact surface  210  and the second contact surface  212 .  FIG. 2C  shows a perspective view of the body  216  of the heat-flow modulator  202 . A rear elevation view of the body  216  is shown in  FIG. 2D . The body  216 , in cooperation with the first contact surface  210  and the second contact surface  212 , is operable to direct heat flow between the sensory bulb  206  and the section  204  of the suction line. Dimensions of the body  216  (e.g., length, width, height, etc.) and its shape may be selected to direct heat along a predetermined thermal flow path. In some embodiments, the first contact surface  210  is formed on the body  216  as a first concave portion and the second contact surface  212  is formed as a second concave portion that faces away from the first concave portion. In other embodiments, the body  216  may have a rectilinear cross-section such as a square or rectangular cross-section. Other dimensions and shapes, however, are possible. 
     In some embodiments, the body  216  is formed from a thermally-insulating material. The thermally-insulating material may have a thermal conductivity less than 1 W/(m·K). In other embodiments, the body  216  is formed from a thermally-conducting material. The thermally-conducting material may have a thermal conductivity greater than 10 W/(m·K). It will be appreciated that the body  216  is not limited to a single material, but in certain embodiments, may be formed from a plurality of materials (e.g., a textured composite). In these embodiments, the plurality of materials may be selected to impart predetermined thermal resistances and thermal capacitances to the body  216 . The plurality of materials may also be structured within the body  216  to constrain heat substantially along the predetermined thermal flow path. 
     In operation, the section  204  of the suction line conveys refrigerant from the evaporator towards a compressor and thereby attains an operating temperature that reflects a refrigerant temperature of refrigerant exiting the evaporator. If the operating temperature is greater than a temperature of the sensory bulb  206 , heat will traverse the heat-flow modulator  202  to flow from the section  204  to the sensory bulb  206 . If the operating temperature is less than the temperature of the sensory bulb  206 , heat will flow in an opposite direction, traversing the heat-flow modulator  202  to flow from the sensory bulb  206  to the section  204 . 
     Heat enters and leaves the body  216  substantially through the first contact surface  210  and the second contact surface  212 . Moreover, after entering the body  216 , heat flows substantially along the predetermined thermal flow path and experiences a resistance. This resistance stems from the thermal resistance, which retards a rate of heat transfer between the first contact surface  210  and the second contact surface  212 . This resistance may also be influenced by the thermal capacitance, which smooths the rate of heat transfer between the first contact surface  220  and the second contact surface  212 . Thus, the body  216 , by providing the thermal resistance, the thermal capacitance, or both, enables the heat-flow modulator  202  to regulate heat exchanged between the section  204  of the suction line and the sensory bulb  206 . 
     Although the heat flow modulator  202  is depicted in  FIGS. 2A-2D  as having a body  216  with concave portions, this depiction is not intended as limiting. The body  216  may have other portions and still remain with the scope of this disclosure. For example, and without limitation,  FIG. 3A  presents a perspective view of a body  316  of a heat-flow modulator  302  having a rectangular cross-section. The body  316  has a first contact surface  310  and a second contact surface  312  defined by planar, parallel portions. The rectangular cross-section is highlighted in  FIG. 3B , which shows a rear view of the body  316 . Features analogous to both  FIGS. 3A-3B  and  FIGS. 2A-2C  are related via coordinated numerals that differ in increment by a hundred. 
     According to an illustrative embodiment, a heat-flow modulator includes an insulating body that is operatively disposed between a portion of a suction line and a sensory bulb of an expansion valve. The insulating body has a first contact surface and a second contact surface. The first contact surface is thermally-coupled to the portion of the suction line and the second contact surface is thermally-coupled to the sensory bulb. The heat-flow modulator also includes a mounting bracket. The mounting bracket is operable to urge the portion of the suction line toward the first contact surface and to urge the sensory bulb (or a portion thereof) towards the second contact surface. During operation, refrigerant flows through the portion of the suction line. In response, heat is exchanged substantially through the mounting bracket between the portion of the suction line and the sensory bulb. A temperature of the portion is therefore communicated to the sensory bulb solely through the mounting bracket. The temperature of the portion may represent a temperature of refrigerant flowing therein. 
     According to an illustrative embodiment, a heat-flow modulator includes a body having an arc-shaped cross-section. The body is typically formed of an insulating material and has a length that varies between 0.25″ to 0.75″. The arc-shaped cross-section extends entirely through the length of the body and thereby forms a concave surface and a convex surface. The arc-shaped cross-section has a width of approximately 0.5″ and a thickness of approximately 0.0625″ (i.e., when viewed from an end perspective). The concave surface defines a first contact surface for thermally-coupling to a portion of a suction line. The convex surface of the body defines a second contact surface for thermally-coupling to a sensory bulb of an expansion valve. The specific dimension are not intended to be limiting but to offer one illustrative embodiment. 
     According to an illustrative embodiment, a method for stabilizing suction pressure within a heating, ventilating, and air conditioning (HVAC) system includes the step of using a heat-flow modulator to exchange heat between refrigerant in a suction line and a sensory bulb. The HVAC system has a closed-conduit refrigeration system. The method also includes the step of fluidly-coupling the sensory bulb to an expansion valve and the step of altering the flow of refrigerant through the expansion valve in response to heat exchanged between the sensory bulb and the heat-flow modulator. The expansion valve is configured to regulate refrigerant flow within the closed-conduit refrigeration circuit is also configured to meter the flow of refrigerant (i.e., flowing through the expansion valve) to the evaporator. 
     In some embodiments, the heat-flow modulator comprises a modulator body having a first contact surface and a second contact surface. In such embodiments, the first contact surface is thermally-coupled to the suction line and the second contact surface is thermally-coupled to the sensory bulb. 
     In some embodiments, the step of using a heat-flow modulator to exchange heat includes the step of directing heat along a predetermined thermal flow path of the heat-flow modulator. In some embodiments, the method further includes the step of conveying refrigerant from the evaporator to a compressor via a suction line. 
     In some embodiments, the step of using the heat-flow modulator to exchange heat includes flowing heat from the suction line through a first contact surface of the heat-flow modulator. In some embodiments, the step of using the heat-flow modulator to exchange heat includes flowing heat from the suction line through a first contact surface of the heat-flow modulator and through a second contact surface into the sensory bulb. 
     In some embodiments, the step of using the heat-flow modulator to exchange heat includes the step of flowing heat from the suction line through the first contact surface of the heat-flow modulator and through the second contact surface into the sensory bulb. In such embodiments, the method also includes the step of flowing heat from the suction line through at least one bracket into the sensory bulb. 
     In some embodiments, the step of altering the flow of refrigerant through the expansion valve includes the step of decreasing an occlusion of a fluid-flow orifice in the expansion valve in response to heat entering the sensory bulb. In these embodiments, the method also includes the step of increasing the occlusion of the fluid-flow orifice in the expansion valve in response to heat leaving the sensory bulb. 
     In some embodiments, the heat-flow modulator includes a body having a first concave portion and a second concave portion. In some embodiments, the heat-flow modulator includes a thermally-insulating material. In other embodiments, the heat-flow modulator includes thermally-conducting material. 
     While not limited to any certain theory, in some embodiments, it may be useful to consider that the heat flow modulator can modulate heat between the suction line and bulb using the following formula=kA(T_bulb−T_suction)/t, where k=thermal cond, A—contact area, t—thickness of modulator. Any one of the components (k, A, or t) can be changed to modulate. If an effective A and effective t were imagined, then either of these can be adjusted to modulate heat transfer rate. Change in material is one of these choices. It should be appreciated that the shape and size can also be changed to optimize the modulator. 
     Although the present invention and its advantages have been disclosed in the context of certain illustrative, non-limiting embodiments, it should be understood that various changes, substitutions, permutations, and alterations can be made without departing from the scope of the invention as defined by the appended claims. It will be appreciated that any feature that is described in connection to any one embodiment may also be applicable to any other embodiment. 
     It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. It will further be understood that reference to “an” item refers to one or more of those items. 
     The steps of the methods described herein may be carried out in any suitable order or simultaneous where appropriate. Where appropriate, aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples having comparable or different properties and addressing the same or different problems. 
     It will be understood that the above description of the embodiments is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples, and data provide a complete description of the structure and use of exemplary embodiments of the invention. Although various embodiments of the invention have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of the claims. 
     In the detailed description of the illustrative embodiments, reference is made to the accompanying drawings that form a part hereof. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is understood that other embodiments may be utilized and that logical structural, mechanical, electrical, and chemical changes may be made without departing from the scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the embodiments described herein, the description may omit certain information known to those skilled in the art. The detailed description above is, therefore, not to be taken in a limiting sense, and the scope of the illustrative embodiments is defined only by the appended claims. 
     In the drawings and description herein, like parts are typically marked throughout the specification and drawings with the same reference numerals or coordinated numerals. The drawing figures are not necessarily to scale. Certain features of the illustrative embodiments may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. 
     Unless otherwise specified, any use of any form of the terms “connect,” “engage,” “couple,” “attach,” or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the elements and may also include indirect interaction between the elements described. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to”. Unless otherwise indicated, as used throughout this document, “or” does not require mutual exclusivity. 
     As used herein, the phrases “fluidly coupled,” “fluidly connected,” and “in fluid communication” refer to a form of coupling, connection, or communication related to fluids, and the corresponding flows or pressures associated with these fluids. In some embodiments, a fluid coupling, connection, or communication between two components may also describe components that are associated in such a way that a fluid can flow between or among the components. Such fluid coupling, connection, or communication between two components may also describe components that are associated in such a way that fluid pressure is transmitted between or among the components. 
     As used herein, the terms “hot,” “warm,” “cool,” and “cold” refer to thermal states, on a relative basis, of refrigerant within a closed-conduit refrigeration circuit. Temperatures associated with these thermal states decrease sequentially from “hot” to “warm” to “cool” to “cold”. Actual temperatures, however, that correspond to these thermal states depend on a design of the closed-conduit refrigeration circuit and may vary during operation.