Abstract:
Check valves for use in fluid conducting structures such as thermostatic mixing valves are provided. Illustratively, the fluid conducting structure provides a passageway defining an axis, and the check valve comprises a cap coupled to the structure, a seat disposed in the passageway and spaced apart from the cap, a valve member axially movable between a sealed position engaging the seat and an opened position spaced apart from the seat, and a spring urging the valve member to engage the seat, the spring being coupled to the valve member and the cap.

Description:
RELATED APPLICATION 
     The present application is a divisional application of U.S. patent application Ser. No. 09/633,728 titled, “THERMOSTATIC MIXING VALVE” filed Aug. 7, 2000, now U.S. Pat. No. 6,315,210 which is a continuation of then U.S. patent application Ser. No. 09/165,880 titled “THERMOSTATIC MIXING VALVE,” filed Oct. 2, 1998 now abandoned, which applications are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a thermostatic mixing valve. 
     BACKGROUND OF THE INVENTION 
     Thermostatic mixing valves are known for the producing of a mixed fluid by combining the supplies of a first (relatively hot) fluid and of a second (relatively cold) fluid. Known arrangements for thermostatic mixing valves generally include a first fluid inlet, a second fluid inlet, a mixed fluid outlet, a mixing chamber, and a thermostatic control device. Known the thermostatic mixing valves generally vary the flow rate of at least the first fluid and often also of the second fluid, the temperatures, pressures, and flow rates of both of which are typically not known and may vary randomly during operation, to produce a mixed fluid of a substantially constant temperature. 
     It would be advantageous to provide for a thermostatic mixing valve to allow relatively high flow rates of first, second, and mixed fluids while incurring only relatively moderate pressure drops within the thermostatic mixing valve. It would also be advantageous for a thermostatic mixing valve to automatically shut off flow of at least a hot fluid upon failure of the thermostatic control device. It would further be advantageous to provide for a thermostatic mixing valve which allows for relatively high flow rates with only moderate pressure drops and which shuts off flow of at least the hot fluid. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a thermostatic mixing valve configured to produce a mixed fluid substantially of a particular temperature from the mixing of a first fluid of a temperature higher than or equal to the particular temperature and of a second fluid of a temperature lower than or equal to the particular temperature. The thermostatic mixing valve includes a valve body having a first fluid inlet, a second fluid inlet, and a mixed fluid outlet. The thermostatic mixing valve also includes a valve member configured to control the rate of flow of at least the first fluid. The valve member includes a thermostatic control device in communication with the mixed fluid and a shuttle coupled to the thermostatic control device, configured for movement within a liner, and oriented to adjustably engage the flow of at least the first fluid through at least one opening within a wall of the liner, the direction of movement of the shuttle with respect to the liner defining the major longitudinal axis of the thermostatic mixing valve, the direction of flow of the first fluid being at least partially transverse with respect to the major longitudinal axis of the valve. 
     The present invention also relates to a thermostatic mixing valve configured to produce a mixed fluid substantially of a particular temperature from a first fluid of a temperature higher than or equal to the particular temperature and a second fluid of a temperature lower than or equal to the particular temperature. The thermostatic mixing valve includes a valve body having a first fluid inlet, a second fluid inlet, and a mixed fluid outlet, and a valve member configured to control the rate of flow of the first fluid and the rate of flow of the second fluid. The valve member includes a thermostatic control device in communication with the mixed fluid and a shuttle coupled to the thermostatic control device, configured for movement within a liner, and oriented to adjustably engage in opposing relationship the flow of the first fluid and the flow of the second fluid, the direction of movement of the shuttle with respect to the liner defining the major longitudinal axis of the thermostatic mixing valve, the directions of flow of the first fluid and the second fluid being at least partially transverse with respect to the major longitudinal axis of the thermostatic mixing valve. 
     The present invention further relates to a mixing valve configured to produce a mixed fluid from the mixing of a first fluid and at least a second fluid. The mixing valve includes a valve body having a first fluid inlet, at least a second fluid inlet, and a fluid outlet, and at least one fluid inlet including a check valve configured to prevent fluid from flowing out of the valve through the at least one inlet. The check valve includes a first check valve member which is stationary within and with respect to the valve body, a second check valve member which is movable within the valve body in a defined path of motion and engageable with the first check valve member, and a biasing device for urging the second check valve member into engagement with the first check valve member and for defining the path of motion of the second check valve member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a thermostatic mixing valve according to a preferred embodiment of the present invention. 
     FIG. 2 is an exploded perspective view of the thermostatic mixing valve of FIG.  1 . 
     FIG. 3 is a front sectional elevation view of the thermostatic mixing valve of FIG.  1 . 
     FIG. 3A is a fragmentary elevation view of the thermostatic mixing valve of FIG.  3 . 
     FIG. 4A is a front sectional elevation view of the thermostatic mixing valve of FIG. 1 showing full cold fluid flow and partial hot fluid flow. 
     FIG. 4B is a front sectional elevation view of the thermostatic mixing valve of FIG. 1 showing cold fluid flow. 
     FIG. 4C is a front sectional elevation view of the thermostatic mixing valve of FIG. 1 showing full flow of both hot fluid and cold fluid. 
     FIG. 4D is front sectional elevation view of the thermostatic mixing valve of FIG. 1 showing the thermostat having failed and flow of only cold fluid. 
     FIG. 5 is a front elevation view of the thermostatic mixing valve according to an alternative embodiment. 
     FIG. 6 is a left side elevation view of the thermostatic mixing valve of FIG.  5 . 
     FIG. 7 is a front sectional elevation view of the thermostatic mixing valve of FIG. 5 
     FIG. 7A is a fragmentary elevation view of the thermostatic mixing valve of FIG.  7 . 
     FIG. 8A is a front sectional elevation view of the thermostatic mixing valve of FIG. 5 showing flow of both hot fluid and cold fluid. 
     FIG. 8B is a front sectional elevation view of the thermostatic mixing valve of FIG. 5 showing flow of only cold fluid. 
     FIG. 8C is a front sectional elevation view of the thermostatic mixing valve of FIG. 5 showing flow of only hot fluid. 
     FIG. 8D is a front sectional elevation view of the thermostatic mixing valve of FIG. 5 showing the thermostat having failed and no fluid flow. 
     FIG. 9 is an exploded perspective view of the thermostatic mixing valve of FIG.  5 . 
     FIG. 10 is an exploded perspective view of a valve member of the thermostatic mixing valve of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 through 4 show a thermostatic mixing valve according to a preferred embodiment for producing from a first fluid and a second fluid a mixed fluid substantially of a particular temperature which is intermediate the temperatures of the first fluid and the second fluid. The first fluid is higher in temperature than is the second fluid. For ease of understanding, the first fluid is sometimes referred to herein as a hot fluid and the second fluid as a cold fluid (though both may be “hot” or “cold” in terms of human sensory perception and they may be separated by only a relatively small temperature difference). 
     FIG. 1 shows a thermostatic mixing valve  102  having a valve body  104 , a cold fluid inlet port  110  associated with a cold fluid inlet designated by the reference letter “C”, a hot fluid inlet port  112  associated with a hot fluid inlet designated by the reference letter “H”, and a mixed fluid outlet port  114  associated with a mixed fluid outlet designated by the reference letter “M”. Thermostatic mixing valve  102  also includes a bonnet  116 , a cap  134 , and a cover screw  142  for limiting access to an adjusting screw  140  (shown in FIG.  2 ). Thermostatic mixing valve  102  further includes a first check valve  274  associated with hot fluid inlet H and a second check valve  274  associated with cold fluid inlet C, each check valve  274  including a check valve cap  276  in which is threadedly engaged a stem  286 . 
     FIG. 2 shows valve body  104  including cold fluid inlet port  110 , hot fluid inlet port  112 , and mixed fluid outlet port  114 . Ports  110 ,  112 , and  114  are configured for the connecting and sealing of appropriate fluid conduits (e.g., using pipe threads) to valve body  104 . A check valve  274  is assembled to valve body  104  in association with each inlet port  110  and  112 . Check valve  274  includes a seat  284 , a plug  282 , a check valve cap  276 , a stem  286 , a cylindrical filter screen  279 , and a biasing spring  280 . Check valve cap  276  is provided with threads  294  for engagement with threaded aperture  296  within valve body  104 , and is sealed to valve body  104  with an annular seal  278 . Stem  286  is provided with threads  290  for engagement with a threaded aperture  292  centrally located within check valve cap  276 , and is sealed to check valve cap  276  by an annular seal  285 . 
     Valve body  104  further includes a cavity  106  for the receiving of a valve member  144 . Valve body  104 , valve cap  134 , adjusting screw  140  and cover screw  142  may be made of various materials. According to any particularly preferred embodiment, valve body  104  and valve cap  134  are cast of brass, gray iron, or ductile iron, and adjusting screw  140  and cover screw  142  are machined of brass, bronze, or stainless steel. 
     A liner  146  is configured generally as a hollow cylinder having a side wall  152  and a lower end closed by a bottom wall  150  (shown in FIG.  3 ). Liner  146  further includes at least one transversely oriented upper opening  154  and at least one transversely oriented lower opening  156  for flow of cold and hot fluids, respectively, through side wall  152 . A circumferential groove  158  within the outer surface of side wall  152  is provided for a seal  254 . A seat  170  is secured to the inner surface of bottom wall  150  of liner  146  by a screw  172 , for seating of a lower edge  180  of a side wall  178  of a shuttle  174  and of a biasing spring  188 . 
     The position of shuttle  174  is adjustable within liner  146 . The orientation of sliding movement of shuttle  174  within liner  146  of valve member  144  defines the major longitudinal axis of valve member  144 , and hence of thermostatic mixing valve  102 . The upper end of biasing spring  188  is transversely restrained (or piloted) by a lower end  198  of a spring pilot  190  having a generally cylindrical shape, and is longitudinally restrained by a flange  192  circumscribing the outer surface of spring pilot  190 . Flange  192  is shown in a hexagonal configuration to provide wrench flats  200  for threaded assembly to a shuttle  174  and to a relief spring holder  204  shown in FIGS. 4A to  4 D. An upper end of spring pilot  190  includes a cavity  194  for the receiving and remaining of the lower end of a relief spring  202 . An upper end of relief spring  202 , and a disc  212  for spreading the axial load of relief spring  202  upon a lower end of a thermostat  214 , is received and retained within a cavity  206  oriented within a lower end of relief spring holder  204 . 
     In assembly of valve member  144 , a first valve member subassembly  240  is made by inserting disc  212  into cavity  206  within the bottom of relief spring holder  204 , inserting a first end of relief spring  202  into cavity  206  and upon disc  212  placing shuttle  174  upon the bottom of relief spring holder  204  so that a second end of relief spring  202  projects through an opening  186  within the upper surface of shuttle  174 , inserting the second end of relief spring  202  into cavity  194  of spring pilot  190 , and using wrench flats  200  of spring pilot  190  to fully engage threads  196  of spring pilot  190  with mating threads  208  within cavity  206  of relief spring holder  204 . This secures relief spring holder  204 , disc  212 , relief spring  202 , shuttle  174 , and spring pilot  190  together, with the top surface of shuttle  174  and relief spring  202  being clamped between a top surface of cavity  206  of relief spring holder  204  and a bottom surface of cavity  194  of spring pilot  190  to form, first valve member subassembly  240 . 
     An insert  242  is provided with a seal  246  which is seated within a peripheral groove located near a lower end of insert  242 . As shown in FIGS. 3 and 4A through  4 D, insert  242  is inserted into an upper end of liner  146  during assembly of valve member  144 , and seal  246  separates cold fluid from hot fluid within valve member  144 . Insert  242  includes at least one opening  264  for passage of cold fluid, as shown in FIGS. 3 and 4A through  4 D Insert  242  is held in position within a lower portion of bonnet  116  by liner  146 , which clamps insert  242  when liner threads  160  are engaged with mating threads within an opening  128  of bonnet  116 . 
     Referring again to FIG. 2, seat  170 , screw  172 , seal  254 , insert  242 , and seal  246  are preassembled to liner  146 , after which biasing spring  188  and first subassembly  240  are placed within the open end of liner  146 . A stem  248  is loosely received within a bellows  222  (shown in FIGS. 4A through 4D) of thermostat  214 , whereupon thermostat  214  with stem  248  is inserted through an opening  210  in a top surface of relief spring holder  204  to bear upon disc  212  (contained within first subassembly  240 ). 
     Valve member  144  is installed to opening  128  in a lower end of bonnet  116  using mating threads  160  and  162 . A seal  270  seals stem  248  to an aperture  249  within valve cap  134 . A second valve member subassembly  250  is then formed by further assembling to bonnet  116  a seal  130  and a seal  132 , valve cap  134  with a seal  136  using threads  138 , adjusting screw  140 , and cover screw  142 . Assembly of the thermostatic mixing valve is then completed by installing second valve member subassembly  250  to cavity  106  of valve body  104  by engaging threads  118  of bonnet  116  with threads  126  within the opening to cavity  106  of valve body  104 . 
     FIG. 3 shows a plurality of chambers formed within valve body  104  and valve member  144  of the thermostatic mixing valve. A hot fluid chamber  230  is in communication with hot fluid inlet port  112 , and a cold fluid chamber  232  is in communication with cold fluid inlet port  110 . Both hot fluid chamber  230  and cold fluid chamber  232  are open to valve member  144 . An inner passage  120  of bonnet  116  includes a preliminary mixing chamber  236 , which is in communication with a main mixing chamber  238 , which is in turn in communication with a mixed fluid outlet chamber  234 , itself in communication with mixed fluid outlet port  114 . Inner passage  120  and an outer passage  122  of bonnet  116  are separated by an annular inner bonnet wall  266  (which is coupled at a fixed distance from an annular outer bonnet wall  268  by at least two webs  124  (three, or four, are included in any particularly preferred embodiment for structural rigidity) oriented radially within outer passage  122 , having a thickness sufficient to structurally couple inner bonnet wall  266  to outer bonnet wall  268 ). Webs  124  are configured with a streamlined cross section having its greater dimension oriented vertically, in order to minimize obstruction of flow of mixed fluid. 
     FIGS. 4A-D show a thermostatic control device shown as thermostat  214  having a thermostat housing  216  is installed within both preliminary mixing chamber  236  and main mixing chamber  238 , which provides a large heat flow area for thermal convection to, and thermal conduction through, the walls of thermostat housing  216 . According to a particularly preferred embodiment, thermostat housing  216  includes at least one thin wall made of a material having a high coefficient of thermal conductivity (e.g., a copper alloy) in order to provide a low thermal impedance to a thermally responsive material  226  contained within thermostat housing  216 . Thermally responsive material  226  has a large coefficient of thermal expansion, and therefore expands substantially upon increasing in temperature and contracts substantially upon decreasing in temperature. Expansion upon increase in temperature increases a force exerted upon bellows  222  located within thermostat housing  216 . 
     Various substances are known to those skilled in the art for use as thermally responsive material  226 . According to an embodiment particularly preferred for economy of manufacture, an acetone is used for a thermally responsive material. According to an alternative embodiment particularly preferred for high performance when economy is a less important factor, a halogenated fluorocarbon such as MS-782 Vertrel XF manufactured and distributed by Miller-Stephenson Chemical of Danbury, Conn. is used for a thermally responsive material. 
     Bellows  222  is constructed in a manner (e.g., using circumferentially corrugated metal) which causes it to be radially stiff but longitudinally flexible. Bellows  222  has a closed end  224  located within thermostat housing  216 , and an open end  220  which is secured to an open end  218  of thermostat housing  216 . The periphery of the opening in open end  220  of bellows  222  may be sealed to the open end of thermostat housing  216  to prevent loss of thermally responsive material  226 . 
     Stem  248 , of generally cylindrical shape and a diameter which is slightly smaller than is the minimum inside diameter of bellows  222 , is placed within bellows  222  through open end  220 . An increase in temperature of thermostat  214 , caused by an increase in temperature of the mixed fluid surrounding thermostat  214 , causes an expansion of thermally responsive material  226  filling the space between the inner surfaces of thermostat housing  216  and the outer surfaces of bellows  222 , increasing a longitudinally oriented control force exerted upon closed end  224  of bellows  222  and thereby upon stem  248 , in a direction which tends to extend stem  248  out of thermostat  214 , and to thereby increase the combined lengths of thermostat  214  and stem  248 . 
     Upwardly oriented movement of stem  248  is prevented by adjusting screw  140  within valve cap  134 , so that any motion which occurs will be of thermostat  214  pressing against either relief spring  202  through disk  212  within first subassembly  240  or of thermostat  214  and first assembly  240  pressing against biasing spring  188 . Relief spring  202  is stiffer (i.e., has a higher spring rate) than is biasing spring  188 , so extension of stem  248  out of thermostat  214  results in a displacement of thermostat  214  vertically downward and an increase in compression of biasing spring  188 , the compressive force of biasing spring  188  balancing the force caused by the expansion of thermally responsive material  226  within thermostat  214 . Shuttle  174  is thereby displaced downwardly within liner  146 , decreasing open area associated with a hot fluid metering gap  258  of lower opening  156  and consequently flow rate of the hot fluid. 
     The setpoint temperature to which thermostat  214  controls is primarily a function or properties of thermally responsive material  226  and force of biasing spring  188 , which is influenced by the position of adjusting screw  140 . In any particularly preferred embodiment, such design parameters of the valve are selected by the valve designer and manufacturer so that, in normal operation of the valve using hot and cold fluid sources of typical pressures and temperatures, a desired mixed fluid outlet temperature can be obtained with adjusting screw  140  at or near the center of its range of screw thread travel. When adjusting screw  140  is rotated in a clockwise direction (assuming a right hand thread) to a position farther within valve cap  134 , it decreases the setpoint temperature by reducing the open area of lower openings  156  and thereby the flow rate of the hot fluid. Conversely, rotating adjusting screw  140  is an opposite direction to a position nearer the top of valve cap  134  similarly increases the setpoint temperature. Unauthorized tampering with adjusting screw  140  is discouraged by concealing adjusting screw  140  beneath a cover screw  142 . 
     Shuttle  174  and liner  146  thus cooperate to function as a hot fluid metering valve element. Because of the large diameter of the liner, wherein are located flow control openings  156 , relative to diameters of flow control openings of the poppet, plug, or globe types of valve element used in thermostatic control valves prior to the present invention, the cumulative open area of lower openings  156  is larger than is the open area of a comparably normally sized metering valve of the poppet, plug, or globe types, allowing a greater amount of flow at any given pressure drop trough thermostatic mixing valve  102 . A small change in position of shuttle  174  with respect to liner  146  in a preferred embodiment correspondingly results in a comparably greater change in flow rate of hot fluid than does a similar change in position of a hot fluid flow metering element in a thermostatic mixing valve of the poppet, plug, or globe type. 
     According to a particularly preferred embodiment (by way of example and not of limitation), of a thermostatic mixing valve, ports  110  and  112  are of 1 inch nominal pipe size and  114  is of 1¼ inch normal pipe size. Liner  146  is of approximately 1.491/1.492 inch inside diameter. Two lower openings  156  within the wall of liner  146  are spaced approximately 0.48 inch from two upper openings  154 . Each opening  154 ,  156  is configured as a slot cut through the wall of liner  146 , subtends an angle of approximately 145 degrees, and is approximately 0.13 inch in height, for hot and cold fluid flow areas at liner  146  of approximately 0.49 square inch, respectively. Testing of the thermostatic mixing valve using hot tap water of approximately 160 degrees Fahrenheit (F) and cold tap water of approximately 55 degrees F. produced the results shown in TABLE 1 below with a valve shuttle and stem stroked manually and controllably. The term “C v ” is a measure of valve flow capacity at a given pressure drop across a valve and is often from the relationship Q=C v *(Δp) ½ , wherein “Q” designates flow rate in U.S. gallons per minute (gpm) and “Δp” designates pressure drop in pounds per square inch (psi). 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Valve 
               
               
                 Shuttle And 
                 Hot Water 
                   
                 Cold Water 
                 Cold 
                 Flow 
               
               
                 Stem Stroke 
                 Flow Rate 
                 Hot Water 
                 Flow Rate 
                 Water 
                 Capacity 
               
               
                 (inches) 
                 (gpm) 
                 Δp (psi) 
                 (gpm) 
                 Δp (psi) 
                 (Total C v ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 4.9 
                 55 
                 51.0 
                 20 
                 12.0 
               
               
                 0.0093 
                 11.1 
                 55 
                 48.4 
                 20 
                 12.3 
               
               
                 0.0186 
                 16.6 
                 52 
                 47.1 
                 22 
                 12.3 
               
               
                 0.0279 
                 22.7 
                 42 
                 44.5 
                 24 
                 12.6 
               
               
                 0.0372 
                 27.1 
                 35 
                 42.1 
                 26 
                 12.7 
               
               
                 0.0465 
                 28.2 
                 30 
                 40.6 
                 28 
                 12.8 
               
               
                 0.0558 
                 30.5 
                 25 
                 38.6 
                 30 
                 13.1 
               
               
                 0.0651 
                 31.7 
                 21 
                 34.9 
                 33 
                 13.0 
               
               
                 0.0744 
                 23.9 
                 20 
                 31.1 
                 35 
                 12.8 
               
               
                 0.0837 
                 35.9 
                 16 
                 29.3 
                 40 
                 12.7 
               
               
                 0.0930 
                 36.2 
                 14 
                 19.8 
                 45 
                 12.6 
               
               
                 0.1023 
                 36.5 
                 11 
                 13.3 
                 50 
                 12.8 
               
               
                   
               
             
          
         
       
     
     The direction of movement of shuttle  174  within liner  146  is perpendicular to that of the fluid being metered, the fluid therefore not exerting a stagnation or velocity pressure against the face of shuttle  174  as it does against the flow control element of a poppet, plug, or globe valve. This enables control of higher flow rates at higher velocities and pressures using a smaller thermostat than is possible with thermostatic valve of the previously used poppet, plug, or globe types. Liner  146  is closed at its bottom end by a bottom wall  150  but has an opening  148  at its upper end, allowing the hot fluid to flow upwardly through the interior of shuttle  174  and passages  182  of shuttle  174 . Passages  182  are formed by a displacement of a top portion  184  of shuttle  174  from side wall  178  of shuttle  174 , top portion  184  being held in fixed relationship to side wall  178  by a web  176  of shuttle  174 . 
     FIG. 3 shows check valve  274  in an installed and operating condition (see FIG. 2 for exploded view). Spring  280  holds plug  282  against seat  284  in an absence of flow of mixed fluid from mixed fluid outlet M. fluid pressures being equal on both sides of plug  282  when there is no flow. When mixed fluid M is desired and flow is allowed from mixed fluid outlet M, back pressure drops on the downstream side of plug  282  and inlet supply pressure forces plug  282  upward, compressing spring  280  by a distance corresponding to the pressure difference across plug  282 . Spring  280  is configured to have a high lateral stiffness, so that it may not only serve to urge plug  282  against seat  284  but may also guide plug  282  in its path of motion between the opened and closed states of check valve  274 . 
     FIG. 3A is a detail of a portion of check valve  274  shown in FIGS. 2 and 3. Stem  286  is provided a tip  286   a  of a particular size and shape, and plug  282  is provided a recess  288  which coats with tip  286   a . These are included to maintain the position of plug  282  centrally located within check valve  274  during conditions of high flow rate and correspondingly high fluid velocity, when plug  282  is forced fully upward and plug  282 , with the associated end of spring  280 , may otherwise be dragged toward the center of thermostatic mixing valve  102  by drag of the high-velocity fluid. (Check valve  274  may also include other associated seals (such as annular seal  283 ) and washers.) For configuring of check valves  274  for operation of thermostatic mixing valve  102 , the position of threaded stem  286  within check valve cap  276  is adjusted upwardly as shown to provide plug  282  room to move upward. For service or maintenance of thermostatic mixing valve  102 , stem  286  may be turned to advance it downwardly and thereby force plug  282  against seat  284  and close off the associated inlet of thermostatic mixing valve  102 . 
     FIGS. 4A,  4 B,  4 C, and  4 D illustrate the operation of thermostatic mixing valve  102  in various conditions of operation. 
     FIG. 4A shows thermostatic mixing valve  102  in normal operation, with shuttle  174  intermediately oriented within liner  146 . Cold fluid from cold fluid inlet port  110  flows, through upper opening  154  of liner  146  and into preliminary mixing chamber  236 , and hot fluid from hot fluid inlet port  112  flows through lower opening  156  of liner  146  and through an at least one passage  182  of shuttle  174  into preliminary mixing chamber  236 . Mixing of the hot and cold fluids begins prior to flowing into preliminary mixing chamber  236 , continues in preliminary mixing chamber  236 , and is completed within main mixing chamber  238 . Thermostat  214  is immersed in the mixed fluid at a particular temperature within main mixing chamber  238 , and thermally responsive material  226  is at substantially the same temperature due to the effects of heat transfer (thermal conduction and convection) at the wall of thermostat housing  216 . Thermally responsive material  226  within thermostat housing  216 , and therefore bellows  222 , are neither fully contracted nor fully expanded, nor is biasing spring  188  fully extended or fully contracted. 
     In normal operation, the temperature of the mixed fluid is controlled by the longitudinal position of shuttle  174  within and with respect to liner  146 , which is in turn controlled by the corresponding specific volume of thermally responsive material  226  at that temperature and by the opposing force of biasing spring  188 , the latter corresponding to the position of adjusting screw  140 . The open area of a hot fluid metering gap  258  at lower openings  156 , and thereby the rate of flow through them, is metered by the longitudinal position of shuttle  174  and thereby by the amount that the side wall  178  of shuttle  174  overlaps and covers lower openings  156 . The flow of hot fluid continues in an upwardly oriented direction into preliminary mixing chamber  236 . Hot fluid is kept separated from cold fluid before leaving upper openings  154  and lower opening  156  of liner  146  by a shuttle seal  168  oriented within a peripherally oriented groove within side wall  178  of shuttle  174 . 
     Cold fluid similarly enters valve body  104  through cold fluid inlet port  110  and fills cold fluid inlet chamber  232 . Cold fluid then flows through transversely oriented openings, shown as upper openings  154 , which penetrate the wall of liner  146 , and immediately thereafter through similarly oriented transverse openings  264  penetrating a wall of insert  242 . Cold fluid then flows upwardly, meeting and mixing with hot fluid. The at least partially mixed fluid proceeds upwardly through preliminary mixing chamber  236  within bonnet inner passage  120  into main mixing chamber  238 , flowing over the surface of thermostat housing  216  of thermostat  214  as it does so and thereby maintaining thermally responsive material  226  within thermostat housing  216  at a temperature substantially equal to that of the mixed fluid. Mixed fluid then flows downwardly through an outer bonnet passage  122  into a mixed fluid outlet chamber  234 , from which it exits the thermostatic mixing valve through mixed fluid outlet port  114 . 
     FIG. 4B shows a condition of operation in which the mixed fluid has become too hot (e.g., caused by a large increase in temperature or supply pressure of the hot fluid) and thermally responsive material  226  has therefore expanded. This has forced thermostat  214 , and thereby lower edge  180  of side wall  178  of shuttle  174 , downward onto seat  170 , completely covering lower openings  156  to decrease the hot fluid metering gap to substantially zero and substantially stopping flow of hot fluid. Because lower edge  180  is now abutting seat  170 , biasing spring  188  can be compressed no farther. To prevent thermostat housing  216  and/or bellows  222  from rupturing due to excessive expansion of thermally responsive material  226  caused by excessively high temperature of the mixed fluid, relief spring  202  allows additional extension of stem  248  from thermostat  214  by compressing in response to the expansion of thermally responsive material  226 , thus relieving excessive force otherwise exerted by thermally responsive material  226 . 
     FIG. 4C shows a condition of operation in which the temperature of the mixed fluid has become too cold (e.g., caused by a large reduction in temperature and/or supply pressure of the hot fluid). Thermally responsive material  226  has cooled in response to the reduced temperature of the mixed fluid surrounding thermostat  214 , and has contracted and has reduced the force it exerts upon biasing spring  188  through thermostat  214  and first assembly  240 . This allows biasing spring  188  to lift first subassembly  240  and thermostat  214 , maintaining the abutting relationship between stem  248  and adjusting screw  140 . Shuttle  174  is a member of first subassembly  240 , and is therefore lifted with it, increasing the hot fluid metering gap of lower openings  156  fully. Hot fluid flow rate thereby increases and relieves the excessively cold condition of the mixed fluid, bringing valve member  144  back into equilibrium. 
     FIG. 4D shows an abnormal condition of operation which is encountered when thermostat  214  fails to function, in the illustrated instance due to leakage of thermally responsive material  226  through a rupture in bellows  222 . Since thermostat  214  is now unable to retain thermally responsive material  226  within housing  216 , spring  188  forces thermostat  214  and first subassembly  240  upward until stopped by abutting of a top surface of top portion  184  of shuttle  174  upon a lower surface, or an auxiliary seat  260 , of insert  242 . Although this fully opens lower openings  156  for maximum flow rate of hot fluid, the abutting of shuttle  174  top portion  184  upon auxiliary seat  260  constitutes closure of a backup shutoff valve  272  and prevents hot fluid from flowing beyond shuttle  174  into preliminary mixing chamber  236 . Cold fluid, however, continues to flow unimpended and unabated. Therefore, a failure of thermostat  214  results in a condition of an emergency shower bath remaining available (with cold fluid only) in spite of a failure of thermostat  214 . 
     FIGS. 5 through 10 show an alternative embodiment of the thermostatic mixing valve for the producing of a mixed fluid of a particular temperature from a cold fluid and a hot fluid, wherein all flow (i.e., flow of the cold fluid, the hot fluid, and mixed fluid) is stopped upon failure of the thermostatic control device (e.g., shown as a device which changes in length upon a change in temperature of a fluid in which the device is at least partially immersed). 
     FIGS. 5 and 6 show the alternative embodiment of a thermostatic mixing valve  302  including a valve body  304  having a cold fluid inlet port  310  and a hot fluid inlet port  312  (given reference letters C and H, respectively) and a single mixed fluid outlet port  314  (given a reference letter M). Ports  312 ,  310 , and  314  are configured for sealably connecting fluid conduits (e.g., using pipe threads). A valve cap  334  is mounted upon the top of valve body  304 , and holds an adjusting screw  340  and a cover screw  342 , both shown in FIG.  7 . Thermostatic mixing valve  302  further includes a first check valve  474  associated with hot fluid inlet H and a second check valve  474  associated with cold fluid inlet C, each check valve  474  including a check valve cap  476  in which is threadedly engaged a stem  486 . 
     Valve body  304 , valve cap  334 , adjusting screw  340 , and cover screw  342  may be made of various materials. According to any preferred embodiment, valve body  304  and valve cap  334  are cast of brass, gray iron, or ductile iron, and adjusting screw  340  and cover screw  342  are machined of brass, bronze, or stainless steel. 
     FIG. 7 shows valve body  304 , valve cap  334 , a thermostat  414 , thermostat adjusting screw  340  and cover screw  342 , a cold fluid inlet chamber  432  and a hot fluid inlet chamber  430 , a main mixing chamber  438 , and a fluid flow control element shown as a valve member  344 . Hot fluid inlet port  312  and cold fluid inlet port  310  are oriented near the right and left sides of the valve respectively, and mixed fluid outlet port  314  is located at the bottom of valve body  304  and is open to a mixed fluid chamber  434 . Valve body  304  further includes a cavity  306 , open at its top for the receiving of a valve member  344 . 
     A check valve (shown as check valve  474 ) is assembled to valve body  304  in association with each inlet port  310  and  312 . Check valve  474  includes a seal  484 , a plug  482 , check valve cap  476 , a stem  486 , a cylindrical filter screen  479  (with a centering taper), and a biasing spring  480 . Check valve cap  476  is provided with threads  494  for engagement with a threaded aperture  496  within valve body  304 , and is sealed to valve body  304  with an annular seal  478 . Stem  486  is provided with threads  490  for engagement with a threaded aperture  492  centrally located within check valve cap  476 , and is sealed to check valve cap  476  by an angular seal  485 . Spring  480  holds plug  482  against seat  484  in an absence of flow of mixed fluid from mixed fluid outlet M, fluid pressures being equal on both sides of plug  482  (which may have a tapering shape and may be provided with one or more annular seals) when there is no flow. When mixed fluid M is desired and flow is allowed from mixed fluid outlet M, back pressure drops on the downstream side of plug  482  and inlet supply pressure forces plug  482  downward, compressing spring  480  by a distance corresponding to the pressure a difference across plug  482 . Spring  480  is configured to have a high lateral stiffness, so that it may not only serve to urge plug  482  against seat  484  but may also guide plug  482  in its path of motion between the opened and closed states of check valve  474 . 
     FIG. 7A is a detail of a portion of check valve  474  shown in FIG.  7 . Biasing spring  480  is a compression coil spring, and is engaged with check valve cap  476  by a special thread  481  upon check valve cap  476  having a thread form, pitch, and pitch diameter matching the configuration of biasing spring  480 . Biasing spring  480  is similarly engaged with plug  482  by a similar thread  471 . For configuring of check valves  474  for operation of thermostatic mixing valve  302 , the position of threaded stem  486  within check valve cap  476  is adjusted downwardly as shown to provide plug  482  room to move downward. For service or maintenance of thermostatic mixing valve  302 , stem  486  may be turned to advance it upwardly and thereby force plug  482  against seat  484  and close off the associated inlet of thermostatic mixing valve  302 . 
     Valve body  304  is divided into various chambers including a main mixing chamber  438  (of an annular shape, oriented below valve cap  334 ), a cold fluid chamber  432  (of an annular shape, and in communication with cold fluid inlet port  310 ), a hot fluid chamber  430  (of an annular shape, and in communication with hot fluid inlet port  312 ), and mixed fluid outlet chamber  434  in communication with mixed fluid outlet port  314 . Valve member  344  is installed within cavity  306  of valve body  304  and is secured within valve body  304  by engagement of a screw thread  360  upon valve member  344  with a screw thread  308  within cavity  306 . A preliminary mixing chamber  436  (also shown in FIG. 8) is contained within valve member  344 , as is a shuttle  374  for modulating flows of hot and cold fluid (shown in FIGS.  8  and  10 ). 
     Referring to FIG. 9, which is a partially exploded view of thermostatic mixing valve  302  valve body  304  is shown with valve member  344  and valve cap  334 . Valve member  344  is generally cylindrical in shape and is installed with generally cylindrical valve body cavity  306  inside of valve body  304 . A threaded portion  360  of a liner  346  of valve member  344  is engaged with a lower threaded bore  308  within cavity  306  to secure valve member  344  within valve body  304 . An upper liner seal  452  and a lower liner seal  454  prevent leakage. Valve cap  334  has a headed portion  338  that is threaded into an upper threaded bore  326  of valve body  304  to secure valve cap  334  to valve body  304  and to close valve body cavity  306 . Valve cap  334  holds adjusting screw  340 , the position of which is secured against tampering by cover screw  342 . Adjusting screw  340  and cover screw  342  are engaged with screw threads located within an upper area of an aperture  427  extending through valve cap  334 , and an upper portion of thermostat  414  is installed with a lower portion of aperture  427  so that it bears upon the bottom of adjusting screw  340 . A seal  428  seals thermostat  414  to aperture  427  within valve cap  334 , while a seal  336  seals valve cap  334  to valve body  304 . 
     Valve member  344  includes cylindrical liner  346  and thermostat  414  having a cylindrical thermostat housing  416  that is at least partially received within the interior of valve cap  334  when valve cap  334  is threaded onto valve body  304 . Valve member  344  further includes a top flange  364  which includes a hub  362  (shown with a hexagonal shape to facilitate installation with a wrench) having a central circular opening  348  within which thermostat housing  416  freely slides. Cylindrical liner  346  of valve member  344  includes two sets of circumferentially oriented openings (shown as upper openings  354  and lower openings  356 ) which form passages though a side wall  352  of liner  346 . 
     Valve member  344  is shown in an exploded view of FIG. 10 so that the relationship of its elements may be more clearly described. 
     Thermostat  4 , having a thermostat housing  416 , is installed within both preliminary mixing chamber  436  and main mixing chamber  438 . According to a particularly preferred embodiment, thermostat housing  416  includes at least one thin wall made of a material having a high coefficient of thermal conductivity (e.g., a copper alloy) in order to provide a low thermal impedance from the mixed fluid to a thermally responsive material  226  (e.g. acetone) contained within thermostat housing  416  and thereby shorten response time of thermostatic mixing valve  302 . Thermally responsive material  226  has a large coefficient of thermal expansion, and therefore expands substantially upon increasing in temperature and contracts substantially upon decreasing in temperature. Expansion of thermally responsive material  226  within thermostat housing  416  upon an increase in temperature increases a force exerted upon bellows  422  located within thermostat housing  416 . 
     Bellows  422  is constructed in a manner (e.g., using circumferentially corrugated metal) which causes it to be radially stiff but longitudinally flexible. Bellows  422  is hollow and has a first end  424  which is closed and located within thermostat housing  416 , and a second end  420  which is open and secured to an open end  418  of thermostat housing  416 . Bellows  422  is installed to an open end  418  of housing  416  and is sealed thereto by a seal  462 . A valve stem  448  (e.g., a cylindrical rod) extends through an opening in a second end  420  and into bellows  422  so that the upper end of stem  448  bears upon the inner surface of the first end  424  of bellows  422 , and is maintained in this bearing relationship by a compressive coil biasing spring  388  pressing upon the lower end of stem  448  through a transversely oriented web  376  of shuttle  374 , a relief spring  402 , and a disc  412 . Shuttle  374 , having a cylindrical shape, is slidably received within liner  346  and is provided a seal  368  for sealing cold fluid from hot fluid. The orientation of sliding movement of shuttle  374  and of stem  448  defines the major longitudinal axis of valve  4  member  344 , and hence of thermostatic mixing valve  302 . Shuttle  374  includes a side wall  378  and a spring pilot portion  390 . Side wall  378  is joined to spring pilot portion  390  by a transversely oriented and ring-shaped web  376  having at least one passage  382  through which fluid flows in an axial direction. Spring pilot portion  390  of shuttle  374  has a closed bottom  398  and an open top with a threaded bore (visible in FIG. 8) which is used to assemble a top portion  384  of shuttle  374 , a relief spring  402  being retained within a relief spring holder  404 , configured as a cavity within spring pilot  390 , by top portion  384  of shuttle  374 . As shown in FIGS. 8 and 10, an annular space  391  exists between an outer surface of spring pilot portion  390  and an inner surface of side wall  378  of shuttle  374 . 
     Thermally responsive material  226 , expanding or contracting within thermostat housing  416  generally in correspondence to an increase or decrease respectively in temperature of the mixed fluid surrounding thermostat housing  416 , causes bellows  422  to contract and expand correspondingly and respectively, in opposition to biasing spring  388 . Stem  448 , in contact with bellows  422 , is thereby moved to correspondingly adjust (longitudinal position of shuttle  374 , which is coupled to stem  448 , within liner  346  and to thereby proportionally regulate the sectional flow areas of a cold fluid metering gap  456  and a hot fluid metering gap  458 , and thereby the temperature of the mixed fluid. Adjusting screw  340  changes the force exerted by biasing spring  388  by shifting position of the group of parts including thermostat  414 , stem  448 , shuttle  374 , disc  412 , and relief spring  402 , thereby adjusting temperature of the mixed fluid within main mixing chamber  438  at which shuttle  374  reaches a particular position within liner  346 . 
     The setpoint temperature, or temperature to which thermostat  414  controls is primarily a function of properties of thermally responsive material  226  and force of biasing spring  388 , which is influenced by the position of adjusting screw  340 . In any preferred embodiment, such design parameters of the valve are selected by the valve designer and manufacturer so that, in normal operation of the valve using hot and cold fluid sources of typical pressures and temperatures, a desired mixed fluid outlet temperature can be obtained with adjusting, screw  340  at or near the center of its range of screw thread travel. When adjusting screw  340  is rotated in a clockwise direction (assuming a right-hand thread) to a position farther within valve cap  334 , it decreases the setpoint temperature by reducing the open area or lower openings  356  and thereby the flow rate of the hot fluid. Conversely, rotating adjusting screw  340  in an opposite direction to a position nearer the top of valve cap  334  similarly increases the setpoint temperature. Concealing adjusting screw  340  beneath a cover screw  342  discourages unauthorized tampering with adjusting screw  340 . 
     Shuttle  374  and liner  346  thus cooperate to function as a fluid metering valve element. Because of the large diameter of the liner, wherein are located flow control openings  356 , relative to diameters of flow control openings of the poppet, plug, or globe types of valve element, the cumulative open area of lower openings  356  is larger than is the open area of a comparably nominally sized metering valve of the poppet, plug, or globe types, allowing a greater amount of flow at any given pressure drop through thermostatic mixing valve  302 . A small change in position of shuttle  374  with respect to liner  346  in any preferred embodiment correspondingly results in a comparably greater change in flow rate of hot fluid than does a similar change in position of a hot fluid flow metering element in a thermostatic mixing valve of the poppet, plug, or globe type. 
     The direction of movement of shuttle  374  within liner  346  is perpendicular to that of the fluid being metered, the fluid thereby not exerting a stagnation or velocity pressure against the face of shuttle  374  as it does against the flow control element of a poppet, plug, or globe valve. This enables control of higher flow rates at higher velocities and pressures using a smaller thermostatic than is possible with thermostatic valve of the previously used poppet, plug, or globe types. 
     Valve member  344  includes a top shuttle portion  384  having a central circular opening  386 . Valve stem  448  is inserted at its lower end through opening  386  and abuts disc  412 , which provides an enlarged area upon which relief spring  402  bears. Disc  412  and relief spring  402  are installed within spring pilot portion  390  of shuttle  374 , and are secured therein by top portion  384  of shuttle  374  when it is installed to spring pilot  390  portion by, e.g., screw threads. The lower end of valve stem  448  extends slidably through the central circular opening  386  within top portion  384 , and is maintained in contact with disc  412  by biasing spring  388 . 
     Liner  346  is provided a bottom wall  350 , which is configured as a separate part although it may alternatively be made integral with liner  346 . As shown, bottom wall  350  is a threaded plug having a central interior recess  366  for searing of biasing spring  388 . Bottom wall  350  also includes a seat  370  for seating of a bottom edge  380  of outer wall  378  of shuttle  374 . Biasing spring  388  is seared at its upper end upon ring-shaped web  376  and around the perimeter of spring pilot  390  portion of shuttle  374 . 
     Operation of thermostatic mixing valve  302  is described below in reference to FIGS. 8A through 8D. 
     FIG. 8A shows thermostatic mixing valve  302  in normal operation, with to shuttle  374  intermediately oriented within liner  346 . Cold fluid from cold fluid inlet port  310  flows through upper openings  354  within side wall  352  of liner  346 , and hot fluid from hot fluid inlet port  312  flows through lower openings  356  within side wall  352  of liner  346  and through passages  382  of shuttle  374 . Mixing of the hot and cold fluids begins immediately, continues in preliminary mixing chamber  436 , and is completed as the at least partially mixed fluids enter main mixing chamber  438  Thermostat  414  is immersed in the mixed fluid at a particular temperature within main mixing chamber  438 , and thermally responsive material  226  is at substantially the same temperature due to thermal convection at the wall of housing  416  and thermal conduction through the wall of housing  416 . Thermally responsive material  226  within thermostat housing  416 , and therefore bellows  422 , are neither fully contracted nor fully expanded, nor is biasing spring  388  fully contracted or fully extended. In normal operation, the temperature of the mixed fluid is controlled by axial position of shuttle  374  within and with respect to liner  346 , which is in turn controlled by the corresponding specific volume of thermally responsive material  226  at that temperature and by the opposing force of biasing spring  388 , the latter corresponding to the position of adjusting screw  340 . 
     In FIG. 8B, the valve is shown compensating for a hot outlet fluid condition (with respect to the temperature setting). Shuttle  374  is oriented fully downward (at the end of its normal axial path of travel) within liner  346  because thermally responsive material  226  has expanded and bellows  422  has therefore contracted, thereby moving shuttle  374  downwardly. 
     Were the mixed fluid to be still hotter, thermally responsive material  226  would attempt to expand further and, if stem  448  were blocked against further movement downward, thermally responsive material  226  could expand to the point that damage could result to housing  416 , bellows  422 , or the junction of bellows  422  with housing  416 . To prevent this from happening, relief spring  402  provides for additional movement of stem  448  when shuttle  374  is blocked by seat  370  of bottom wall  350  against further movement, thereby relieving force otherwise caused by excessive expansion of thermally responsive material  226 . Lower openings  356  within side wall  352  of liner  346  are closed, blocked by side wall  378  of shuttle  374 . The bottom edge  380  of side wall  378  of shuttle  374  rests against the top of seat  370  of bottom wall  350 , and side wall  378  of shuttle  374  closes lower openings  356 , reducing hot fluid metering gap  458  to substantially zero which substantially prevents the flow of hot fluid into preliminary mixing chamber  436 . Cold fluid flows through upper openings  354  of liner  346  and into preliminary mixing chamber  436  (above shuttle  374 ). The temperature of the mixed fluid in main mixing chamber  438  thus decreases because the flow from cold fluid inlet chamber  432  is in greater proportion of the total flow than it had been. As the temperature of the mixed fluid decreases, causing thermally responsive material  226  to contract, bellows  422  expands, readjusting the position of shuttle  374  and bringing the temperature of the mixed fluid into an equilibrium condition with respect to the temperature setting of the valve. 
     In FIG. 8C, the valve is shown compensating for a cold fluid condition (with respect to the temperature setting of the valve). Shuttle  374  is oriented upwardly (at the end of its normal axial path of travel as constrained by valve stem  448  within bellows  422 ) within liner  346  because thermally responsive material  226  has contracted, allowing bellows  422  to expand and thereby allowing biasing spring  388  to expand (within a constrained axial path of travel defined by valve stem  448  within bellows  422  of thermostat  414 ). Upper openings  354  of liner  346  are closed, blocked by side wall  378  of shuttle  374 , which reduces cold fluid metering gap  456  to substantially zero and thereby substantially prevents the flow of cold fluid into preliminary mixing chamber  436 . Hot fluid flows through lower openings  356  of liner  346  and into preliminary mixing chamber  436  (through passages  382  within shuttle  374 ). The temperature of the mixed fluid in main in a chamber  438  thus increases because the flow from hot fluid chamber  430  is in greater proportion of the total flow than it had been. Bellows  422  thereafter contracts as the temperature of the mixed fluid, and of thermally responsive material  226 , increases, readjusting the position of shuttle  374  and thereby bringing the temperature of the mixed fluid into an equilibrium condition with respect to the temperature setting of the valve. 
     In FIG. 8D thermostatic mixing valve  302  is shown in a failure condition caused by rupture of bellows  422  within thermostat housing  416 . Biasing spring  388  has fully expanded (no longer constrained by bellows  422 , see FIG.  8 C), driving shuttle  374  upward and thereby forcing disc  412  into valve stem  448  and driving top portion  384  of shuttle  374  fully upward into an auxiliary seat  460 , effectively forming a backup shutoff valve  472  within thermostatic mixing valve  302 . While hot fluid flows through lower openings  356  of liner  346  and through at least one shuttle passage  382  up into preliminary mixing chamber  436 , it is prevented from flowing beyond preliminary mixing chamber  436  and into main mixing chamber  438  by the engagement of upper portion  384  with auxiliary seat  460 . Moreover, upper openings  354  of liner  346  are blocked by side wall  378  of shuttle  374  to shut off flow of cold fluid. The seating of top portion  384  upon auxiliary seat  460  blocks all flow from preliminary mixing chamber  436  to main mixing chamber  438  by biasing spring  388  Consequently, no fluid (hot, cold, or mixed) flows through outlet port  314 . 
     As shown in the embodiments of FIGS. 1-3 and  5 - 7 , valve body  104 ,  304  includes a third fluid inlet  510  positioned between the check valve seat  284 ,  484  and the mixed fluid outlet port  114 ,  314 . Illustratively, the third fluid inlet port  510  is between cold fluid inlet port  110 ,  310  and the mixed fluid outlet port  114 ,  314 . Further illustratively, in the embodiment of FIG. 3, third fluid inlet  510  is adjacent plug  282 . As shown in FIGS. 3 and 7, third fluid inlet  510  is illustratively between the mixing chamber  238 ,  438  and check valve seat  284 ,  484 . 
     Although only a few exemplary embodiments for the present invention have been described in detail, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. For example, valve caps may be secured to valve bodies by machine screws; bellows nay be brazed or soldered to thermostat housing wails or bases to form substantially hermetic seals. Accordingly, all such modifications are intended to be included within the scope of the invention as defined in the following claims. In the claims, each means-plus-function clause is intended to cover the structures described herein as performing the recited function, and not only structural equivalents but also equivalent structures. Other substitutions, modifications, changes, and omissions may be made in the designs, operating conditions, and arrangements of the preferred embodiments without departing from the spirit of the invention as expressed in the appended claims.