Patent Publication Number: US-7905119-B2

Title: Fabric treatment appliance with steam generator having a variable thermal output

Description:
BACKGROUND OF THE INVENTION 
     Some fabric treatment appliances, such as a washing machine, a clothes dryer, and a fabric refreshing or revitalizing machine, use steam generators for various reasons. The steam from the steam generator may be used to, for example, heat water, heat a load of fabric items and any water absorbed by the fabric items, dewrinkle fabric items, remove odors from fabric items, sanitize the fabric items, and sanitize components of the fabric treatment appliance. 
     Water from a water supply coupled with the steam generator typically provides water to the steam generator for conversion to steam. The water supply fills a steam generation chamber of the steam generator with water, and a heating element of the steam generator is activated to heat the water present in the steam generation chamber to generate steam. Steam generated in the steam generation chamber commonly flows from the steam generation chamber to a fabric treatment chamber via a steam supply conduit attached to the steam generator. 
     One problem associated with steam generators, especially in-line or flow-through steam generators, is that the heating element distributes heat in an inefficient manner. The heating element wraps around the steam generator in a manner providing, by conduction through the steam generator, substantially uniform thermal output into the steam generation chamber. For example, a standard in-line steam generator has a heating element formed from a resistive wire that is wrapped around the steam generation chamber. The steam generation chamber is often filled with an operating volume of water less than the total capacity of the steam generation chamber to provide for faster steam generation times and to provide room for expansion and boiling water. The operation volume of water results in an operational water level within the steam generation chamber. Air fills the steam generation chamber above the operational water level. However, the heating element is wrapped around the portion of the steam chamber containing both water and air. As the air is not a good conductor of heat, the portion of the heating element below the water level will more efficiently conduct heat into the water than the portion of the heating element above the water level. 
     In addition, inefficient heating of the steam generator can increase the buildup of scale inside the steam generation chamber. The temperature of the water in the steam generation chamber is limited, as it will eventually change phase to steam when it receives enough thermal output. The temperature of the steam, air, and vapor, however, is not limited. The upper portion of the steam generation chamber, therefore, has a tendency to reach higher temperatures. Higher temperatures convert soft calcium deposits in the steam generation chamber to hard calcium, which is not easily removed by the movement of water therein. If flow out of the steam generator or flow through the steam supply conduit becomes impaired due to the buildup of scale, the steam generator will malfunction and possibly damage the fabric treatment appliance. 
     SUMMARY OF THE INVENTION 
     A steam generator according to one embodiment of the invention includes a steam generation chamber for converting the water to steam, and a heating element thermally coupled with the steam generation chamber and having a first portion below an operational water level of the steam generation chamber and a second portion above the operational water level, with the first portion having a greater thermal output than the second portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a perspective view of an exemplary fabric treatment appliance in the form of a washing machine. 
         FIG. 2  is a schematic view of the fabric treatment appliance of  FIG. 1 . 
         FIG. 3  is a schematic view of an exemplary control system of the fabric treatment appliance of  FIG. 1 . 
         FIG. 4  is a perspective view of a steam generator, reservoir, and steam conduit from the fabric treatment appliance of  FIG. 1 , with the steam generator partially broken away to illustrate a heating element. 
         FIG. 5  is a schematic view of a first embodiment of a steam generator having a variable thermal output heating element according to the invention. 
         FIG. 6  is a schematic view of a second embodiment of a steam generator with a variable thermal output heating element according to the invention. 
         FIG. 7  is a schematic view of a third embodiment of a steam generator with a variable thermal output heating element according to the invention. 
         FIG. 8  is a schematic view of a fourth embodiment of a steam generator with a variable thermal output heating element according to the invention. 
         FIG. 9  is a schematic view of a fifth embodiment of a steam generator with a variable thermal output heating element according to the invention. 
         FIG. 10  is a schematic view of a sixth embodiment of a steam generator with a variable thermal output heating element according to the invention. 
         FIG. 11  is a schematic view of the steam generator of  FIG. 11  taken along line  11 - 11  in  FIG. 10 . 
         FIG. 12  is a schematic view of a seventh embodiment of a steam generator with a variable thermal output heating element according to the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     Referring now to the figures,  FIG. 1  is a schematic view of an exemplary fabric treatment appliance in the form of a washing machine  10  according to one embodiment of the invention. The fabric treatment appliance may be any machine that treats fabrics, and examples of the fabric treatment appliance may include, but are not limited to, a washing machine, including top-loading, front-loading, vertical axis, and horizontal axis washing machines; a dryer, such as a tumble dryer or a stationary dryer, including top-loading dryers and front-loading dryers; a combination washing machine and dryer; a tumbling or stationary refreshing/revitalizing machine; an extractor; a non-aqueous washing apparatus; and a revitalizing machine. For illustrative purposes, the invention will be described with respect to a washing machine with the fabric being a clothes load, with it being understood that the invention may be adapted for use with any type of fabric treatment appliance for treating fabric and to other appliances, such as dishwashers, irons, and cooking appliances, including ovens, food steamers, and microwave ovens, employing a steam generator. 
       FIG. 2  provides a schematic view of the fabric treatment appliance of  FIG. 1 . The washing machine  10  of the illustrated embodiment may include a cabinet  12  that houses a stationary tub  14 , which defines an interior chamber  15 . A rotatable drum  16  mounted within the interior chamber  15  of the tub  14  may include a plurality of perforations  18 , and liquid may flow between the tub  14  and the drum  16  through the perforations  18 . The drum  16  may further include a plurality of baffles  20  disposed on an inner surface of the drum  16  to lift fabric items contained in the drum  16  while the drum  16  rotates, as is well known in the washing machine art. A motor  22  coupled to the drum  16  through a belt  24  and a drive shaft  25  may rotate the drum  16 . Alternately, the motor  22  may be directly coupled with the drive shaft  25  as is known in the art. Both the tub  14  and the drum  16  may be selectively closed by a door  26 . A bellows  27  couples an open face of the tub  14  with the cabinet  12 , and the door  26  seals against the bellows  27  when the door  26  closes the tub  14 . The drum  16  may define a cleaning chamber  28  for receiving fabric items to be cleaned. 
     The tub  14  and/or the drum  16  may be considered a receptacle, and the receptacle may define a treatment chamber for receiving fabric items to be treated. While the illustrated washing machine  10  includes both the tub  14  and the drum  16 , it is within the scope of the invention for the fabric treatment appliance to include only one receptacle, with the receptacle defining the treatment chamber for receiving the fabric items to be treated. 
     Washing machines are typically categorized as either a vertical axis washing machine or a horizontal axis washing machine. As used herein, the “vertical axis” washing machine refers to a washing machine having a rotatable drum that rotates about a generally vertical axis relative to a surface that supports the washing machine. Typically, the drum is perforate or imperforate and holds fabric items and a fabric moving element, such as an agitator, impeller, nutator, and the like, that induces movement of the fabric items to impart mechanical energy to the fabric articles for cleaning action. However, the rotational axis need not be vertical. The drum can rotate about an axis inclined relative to the vertical axis. As used herein, the “horizontal axis” washing machine refers to a washing machine having a rotatable drum that rotates about a generally horizontal axis relative to a surface that supports the washing machine. The drum may be perforated or imperforate, holds fabric items, and typically washes the fabric items by the fabric items rubbing against one another and/or hitting the surface of the drum as the drum rotates. In horizontal axis washing machines, the clothes are lifted by the rotating drum and then fall in response to gravity to form a tumbling action that imparts the mechanical energy to the fabric articles. In some horizontal axis washing machines, the drum rotates about a horizontal axis generally parallel to a surface that supports the washing machine. However, the rotational axis need not be horizontal. The drum can rotate about an axis inclined relative to the horizontal axis, with fifteen degrees of inclination being one example of inclination. 
     Vertical axis and horizontal axis machines are best differentiated by the manner in which they impart mechanical energy to the fabric articles. In vertical axis machines, the fabric moving element moves within a drum to impart mechanical energy directly to the clothes or indirectly through wash liquid in the drum. The clothes mover is typically moved in a reciprocating rotational movement. In horizontal axis machines mechanical energy is imparted to the clothes by the tumbling action formed by the repeated lifting and dropping of the clothes, which is typically implemented by the rotating drum. The illustrated exemplary washing machine of  FIGS. 1 and 2  is a horizontal axis washing machine. 
     With continued reference to  FIG. 2 , the motor  22  may rotate the drum  16  at various speeds in opposite rotational directions. In particular, the motor  22  may rotate the drum  16  at tumbling speeds wherein the fabric items in the drum  16  rotate with the drum  16  from a lowest location of the drum  16  towards a highest location of the drum  16 , but fall back to the lowest location of the drum  16  before reaching the highest location of the drum  16 . The rotation of the fabric items with the drum  16  may be facilitated by the baffles  20 . Typically, the radial force applied to the fabric items at the tumbling speeds may be less than about 1 G. Alternatively, the motor  22  may rotate the drum  16  at spin speeds wherein the fabric items rotate with the drum  16  without falling. In the washing machine art, the spin speeds may also be referred to as satellizing speeds or sticking speeds. Typically, the force applied to the fabric items at the spin speeds may be greater than or about equal to 1 G. As used herein, “tumbling” of the drum  16  refers to rotating the drum at a tumble speed, “spinning” the drum  16  refers to rotating the drum  16  at a spin speed, and “rotating” of the drum  16  refers to rotating the drum  16  at any speed. 
     The washing machine  10  of  FIG. 2  may further include a liquid supply and recirculation system. Liquid, such as water, may be supplied to the washing machine  10  from a water supply  29 , such as a household water supply. A first supply conduit  30  may fluidly couple the water supply  29  to a detergent dispenser  32 . An inlet valve  34  may control flow of the liquid from the water supply  29  and through the first supply conduit  30  to the detergent dispenser  32 . The inlet valve  34  may be positioned in any suitable location between the water supply  29  and the detergent dispenser  32 . A liquid conduit  36  may fluidly couple the detergent dispenser  32  with the tub  14 . The liquid conduit  36  may couple with the tub  14  at any suitable location on the tub  14  and is shown as being coupled to a front wall of the tub  14  in  FIG. 1  for exemplary purposes. The liquid that flows from the detergent dispenser  32  through the liquid conduit  36  to the tub  14  typically enters a space between the tub  14  and the drum  16  and may flow by gravity to a sump  38  formed in part by a lower portion  40  of the tub  14 . The sump  38  may also be formed by a sump conduit  42  that may fluidly couple the lower portion  40  of the tub  14  to a pump  44 . The pump  44  may direct fluid to a drain conduit  46 , which may drain the liquid from the washing machine  10 , or to a recirculation conduit  48 , which may terminate at a recirculation inlet  50 . The recirculation inlet  50  may direct the liquid from the recirculation conduit  48  into the drum  16 . The recirculation inlet  50  may introduce the liquid into the drum  16  in any suitable manner, such as by spraying, dripping, or providing a steady flow of the liquid. 
     The exemplary washing machine  10  may further include a steam generation system. The steam generation system may include a steam generator  60  that may receive liquid from the water supply  29  through a second supply conduit  62 , optionally via a reservoir  64 . The inlet valve  34  may control flow of the liquid from the water supply  29  and through the second supply conduit  62  and the reservoir  64  to the steam generator  60 . The inlet valve  34  may be positioned in any suitable location between the water supply  29  and the steam generator  60 . A steam conduit  66  may fluidly couple the steam generator  60  to a steam inlet  68 , which may introduce steam into the tub  14 . The steam inlet  68  may couple with the tub  14  at any suitable location on the tub  14  and is shown as being coupled to a rear wall of the tub  14  in  FIG. 2  for exemplary purposes. The steam that enters the tub  14  through the steam inlet  68  may subsequently enter the drum  16  through the perforations  18 . Alternatively, the steam inlet  68  may be configured to introduce the steam directly into the drum  16 . The steam inlet  68  may introduce the steam into the tub  14  in any suitable manner. 
     An optional sump heater  52  may be located in the sump  38 . The sump heater  52  may be any type of heater and is illustrated as a resistive heating element for exemplary purposes. The sump heater  52  may be used alone or in combination with the steam generator  60  to add heat to the chamber  15 . Typically, the sump heater  52  adds heat to the chamber  15  by heating water in the sump  38 . The tub  14  may further include a temperature sensor  54 , which may be located in the sump  38  or in another suitable location in the tub  14 . The temperature sensor  54  may sense the temperature of water in the sump  38 , if the sump  38  contains water, or a general temperature of the tub  14  or interior of the tub  14 . The tub  14  may alternatively or additionally have a temperature sensor  56  located outside the sump  38  to sense a general temperature of the tub or interior of the tub  14 . The temperature sensors  54 ,  56  may be any type of temperature sensors, which are well-known to one skilled in the art. Exemplary temperature sensors for use as the temperature sensors  54 ,  56  include thermistors, such as a negative temperature coefficient (NTC) thermistor. 
     The washing machine  10  may further include an exhaust conduit (not shown) that may direct steam that leaves the tub  14  externally of the washing machine  10 . The exhaust conduit may be configured to exhaust the steam directly to the exterior of the washing machine  10 . Alternatively, the exhaust conduit may be configured to direct the steam through a condenser prior to leaving the washing machine  10 . Examples of exhaust systems are disclosed in the following patent applications, which are incorporated herein by reference in their entirety: U.S. patent application Ser. No. 11/464,506, titled “Fabric Treating Appliance Utilizing Steam,” U.S. patent application Ser. No. 11/464,501, titled “A Steam Fabric Treatment Appliance with Exhaust,” U.S. patent application Ser. No. 11/464,521, titled “Steam Fabric Treatment Appliance with Anti-Siphoning,” and U.S. patent application Ser. No. 11/464,520, titled “Determining Fabric Temperature in a Fabric Treating Appliance,” all filed Aug. 15, 2006. 
     The steam generator  60  may be any type of device that converts the liquid to steam. For example, the steam generator  60  may be a tank-type steam generator that stores a volume of liquid and heats the volume of liquid to convert the liquid to steam. Alternatively, the steam generator  60  may be an in-line steam generator that converts the liquid to steam as the liquid flows through the steam generator  60 . As another alternative, the steam generator  60  may utilize the sump heater  52  or other heating device located in the sump  38  to heat liquid in the sump  38 . The steam generator  60  may produce pressurized or non-pressurized steam. 
     Exemplary steam generators are disclosed in U.S. patent application Ser. No. 11/464,528, titled “Removal of Scale and Sludge in a Steam Generator of a Fabric Treatment Appliance,” U.S. patent application Ser. No. 11/450,836, titled “Prevention of Scale and Sludge in a Steam Generator of a Fabric Treatment Appliance,” and U.S. patent application Ser. No. 11/450,714, titled “Draining Liquid From a Steam Generator of a Fabric Treatment Appliance,” all filed Jun. 9, 2006, in addition to U.S. patent application Ser. No. 11/464,509, titled “Water Supply Control for a Steam Generator of a Fabric Treatment Appliance,” U.S. patent application Ser. No. 11/464,514, titled “Water Supply Control for a Steam Generator of a Fabric Treatment Appliance Using a Weight Sensor,” and U.S. patent application Ser. No. 11/464,513, titled “Water Supply Control for a Steam Generator of a Fabric Treatment Appliance Using a Temperature Sensor,” all filed Aug. 15, 2006, which are incorporated herein by reference in their entirety. 
     In addition to producing steam, the steam generator  60 , whether an in-line steam generator, a tank-type steam generator, or any other type of steam generator, may heat water to a temperature below a steam transformation temperature, whereby the steam generator  60  produces heated water. The heated water may be delivered to the tub  14  and/or drum  16  from the steam generator  60 . The heated water may be used alone or may optionally mix with cold or warm water in the tub  14  and/or drum  16 . Using the steam generator  60  to produce heated water may be useful when the steam generator  60  couples only with a cold water source of the water supply  29 . Optionally, the steam generator  60  may be employed to simultaneously supply steam and heated water to the tub  14  and/or drum  16 . 
     The liquid supply and recirculation system and the steam generation system may differ from the configuration shown in  FIG. 2 , such as by inclusion of other valves, conduits, wash aid dispensers, and the like, to control the flow of liquid and steam through the washing machine  10  and for the introduction of more than one type of detergent/wash aid. For example, a valve may be located in the liquid conduit  36 , in the recirculation conduit  48 , and in the steam conduit  66 . Furthermore, an additional conduit may be included to couple the water supply  29  directly to the tub  14  or the drum  16  so that the liquid provided to the tub  14  or the drum  16  does not have to pass through the detergent dispenser  32 . Alternatively, the liquid may be provided to the tub  14  or the drum  16  through the steam generator  60  rather than through the detergent dispenser  32  or the additional conduit. As another example, the liquid conduit  36  may be configured to supply liquid directly into the drum  16 , and the recirculation conduit  48  may be coupled to the liquid conduit  36  so that the recirculated liquid enters the tub  14  or the drum  16  at the same location where the liquid from the detergent dispenser  32  enters the tub  14  or the drum  16 . 
     Other alternatives for the liquid supply and recirculation system are disclosed in U.S. patent application Ser. No. 11/450,636, titled “Method of Operating a Washing Machine Using Steam;” U.S. patent application Ser. No. 11/450,529, titled “Steam Washing Machine Operation Method Having Dual Speed Spin Pre-Wash;” and U.S. patent application Ser. No. 11/450,620, titled “Steam Washing Machine Operation Method Having Dry Spin Pre-Wash,” all filed Jun. 9, 2006, which are incorporated herein by reference in their entirety. 
     Referring now to  FIG. 3 , which is a schematic view of an exemplary control system of the washing machine  10 , the washing machine  10  may further include a controller  70  coupled to various working components of the washing machine  10 , such as the pump  44 , the motor  22 , the inlet valve  34 , the detergent dispenser  32 , and the steam generator  60 , to control the operation of the washing machine  10 . If the optional sump heater  52  is used, the controller may also control the operation of the sump heater  52 . The controller  70  may receive data from one or more of the working components or sensors, such as the temperature sensors  54 ,  56 , and may provide commands, which can be based on the received data, to one or more of the working components to execute a desired operation of the washing machine  10 . The commands may be data and/or an electrical signal without data. A control panel  80  may be coupled to the controller  70  and may provide for input/output to/from the controller  70 . In other words, the control panel  80  may perform a user interface function through which a user may enter input related to the operation of the washing machine  10 , such as selection and/or modification of an operation cycle of the washing machine  10 , and receive output related to the operation of the washing machine  10 . 
     Many known types of controllers may be used for the controller  70 . The specific type of controller is not germane to the invention. It is contemplated that the controller is a microprocessor-based controller that implements control software and sends/receives one or more electrical signals to/from each of the various components (inlet valve  34 , detergent dispenser  32 , steam generator  60 , pump  44 , motor  22 , control panel  80 , and temperature sensors  54 ,  56 ) to effect the control software. As an example, proportional control (P), proportional integral control (PI), and proportional derivative control (PD), or a combination thereof, a proportional integral derivative control (PID control), may be used to control the various components. 
       FIG. 4  provides a perspective view of the reservoir  64 , the steam generator  60 , and the steam conduit  66 . In general, the reservoir  64  may be configured to receive water from the water supply  29 , store a volume of water, and supply water to the steam generator  60 . In the exemplary embodiment, the reservoir  64  may include an open-top tank  90  and a lid  92  removably closing the open top of the tank  90 . The reservoir  64  may include a water supply conduit  94  for supplying water from the water supply  29  to the tank  90 . In the illustrated embodiment, the water supply conduit  94  may extend through the lid  92  and include a water supply inlet connector  96  and a siphon break connector  98 . The water supply inlet connector  96  may be coupled to the second water supply conduit  62  ( FIG. 2 ) to receive water from the water supply  29  and provide the water to the water supply conduit  94 . The siphon break connector  98  may be coupled to a siphon break conduit  100  ( FIG. 2 ) to form a siphon break device. The siphon break conduit  100  may be coupled to atmosphere external to the washing machine  10 . The water supply inlet connector  96 , the siphon break connector  98 , and the water supply conduit  94  may be in fluid communication with one another. The reservoir  64  may further include a steam generator connector  102  for coupling the tank  90  to the steam generator  60  and supplying water from the tank  90  to the steam generator  60 . In the illustrated embodiment, the steam generator connector  102  may project laterally from the tank  90 . 
     The steam generator  160  comprises a tube  130  about a portion of which is wrapped a heating element  146 , which is illustrated as an electrically resistive heating element that conducts heat to the tube  130 . A cover  148  encloses most of the heating element  146 . In the illustrated embodiment, the tube has a circular cross-section. Alternatively, the tube  130  may have a cross-section of a different shape, such as triangular, square, or polygonal, for example. 
       FIG. 5  illustrates a schematic view of the steam tube  130  and the heating element  146  of the steam generator  60  with the cover  148  removed for clarity. The heating element  146  comprises a variable-pitch, coiled wire  150 , which is shown encapsulated in a protective coating in  FIG. 4 , but which has been removed for clarity in  FIG. 5 . The wire  150  wraps around the steam generation chamber  136  in a generally central location relative to first and second ends  132 ,  134 . Each 360° portion of the wire  150  extending radially from the bottom of the steam tube  130  to the top of the steam tube and back to the bottom again forms a winding. The wire  150  has at least one winding and may have any number of additional windings. The variable pitch heating element  150  includes a first portion  152  below an operational water level L of the steam generation chamber  136  and a second portion  154  above the operational water level L. The wire coils in the first portion  152  of the variable pitch heating element  150  may have a smaller pitch, which is the axial spacing between adjacent coils of the wire, than the second portion  154  of the variable pitch heating element  150 . The cross-sectional area of all of the coils of the variable pitch heating element  150  may be the same. In the illustrated embodiment, the coils all have a circular cross-section having the same diameter. Alternatively, the coils may have a cross-section of a different shape, such as triangular, square, or polygonal. 
     Due to the change in pitch between the first portion  152  and the second portion  154  of the variable pitch heating element  150 , a greater total length of the wire forming the variable pitch heating element  150  may be located below the operational water level L in the first portion  152  than the total length of wire above the operational water level L. As the heat outputted by the heating element is the same for a given lineal portion of the wire, the greater the length of wire below the operational water level L results in the heating element  146  having a greater thermal output below the operation water level than above the water level L. Therefore, a greater portion of the total thermal output of the heating element  146  is directed to the portion of the steam generation chamber  136  below the water level L. 
     A numerical example may be helpful. Assuming the heating element is a 1000 watt heater when operating at design conditions, if 25% of the wire lies above the operational water level L and 75% of the wire lies below the operation water level L, then 250 watts of thermal output is directed into the tube  130  above the operational water level L and 750 watts of thermal output is directed into the tube below the operation water level L. 
     The variable pitch heating element  150  may be formed by winding a wire around a shaped former, such as a rod. The pitch may be changed by winding the wire with an increased spacing between adjacent coils along portions corresponding to the second portion  154  of the variable pitch heating element  150 . Alternatively, the variable pitch heating element  150  may be formed by winding a wire around a shaped former to form a coil of uniform pitch and then slightly stretching the coiled wire along portions corresponding to the second portion  154  of the variable pitch heating element  150 . 
       FIG. 6  illustrates a second embodiment of the steam generator  60  according to the invention and having the standard heating element  146  replaced by a stretched heating element  160 . All of the other parts of the steam generator  60  are identical to those previously described. The stretched heating element  160  may be a coiled wire wrapped around the steam generation chamber  136  in a generally central location relative to the first and second ends  132 ,  134 . The stretched heating element  160  includes a first portion  162  below an operational water level L of the steam generation chamber  136  and a second portion  164  above the operational water level L. The first portion  162  of the stretched heating element  160  may have a greater number of coils than the second portion of the stretched heating element  160 . In the illustrated embodiment, the coils all have a generally circular cross-section. Alternatively, the coils could have a different shape, such as triangular, square, or polygonal. 
     The stretched heating element  160  may be formed by beginning with a coiled wire having generally similar coils with the same pitch. A portion of the coils are then pulled or stretched along a longitudinal axis to form a stretched portion, which becomes the second portion above the operation water level L. The longitudinal axis may be a central axis extending through the centers of the coils. In the illustrated embodiment, the longitudinal axis wraps around the tube  130 . More specifically, the stretched heating element  160  may be formed by winding the wire around a shaped former, such as a rod. The wire may be wound so as to have a uniform pitch, and the portions of the coiled wire corresponding to the second portion  164  may then be axially over-stretched so as to reduce the number of coils in the second portion. 
     The stretched coils tend to have a smaller effective diameter and a much greater pitch than the non-stretched coils, resulting in fewer coils per unit length along the longitudinal axis of the heating element  160 , which can also be characterized as less wire per unit length along the longitudinal axis. The reduction in coils and/or wire in the second portion as compared to the first portion results in the second portion having less thermal output than the first portion. Therefore a greater portion of the thermal output is located below the operational water level than above the operational water level. 
       FIG. 7  illustrates a third embodiment of the steam generator  60  according to the invention having the standard heating element  146  replaced by a variable coil size heating element  170 . All of the other parts of the steam generator  60  are identical to those previously described. The variable coil size heating element  170  may be a coiled wire wrapped around the steam generation chamber  136  in a generally central location relative to the first and second ends  132 ,  134 . The variable coil size heating element  170  includes a first portion  172  below an operational water level L of the steam generation chamber  136  and a second portion  174  above the operational water level L. The variable coil size heating element  170  may have a uniform pitch. The cross-sectional area of the coils in the first portion  172  of the variable size area heating element  170  may be larger than the cross-sectional area of the coils in the second portion of the variable coil size heating element  170 . In the illustrated embodiment, the coils of the variable coil size heating element  170  all have a generally circular cross-section. Alternatively, the coils could have a different shape, such as triangular, square, or polygonal. 
     Due to the change in the cross-sectional area between the coils in the first portion  172  and the coils in the second portion  174 , a greater total length of the wire forming the variable coil size heating element  170  is located below the operational water level L in the first portion  172 . Therefore a greater portion of the thermal output is located below the operational water level than above the operational water level. 
     The variable cross-sectional area heating element  170  may be formed by winding a portion of the wire corresponding to the first portion  172  around a first shaped former, such as a rod, having a first cross-sectional area. A remaining portion of the wire corresponding to the second portion  174  may then be wound around a second shaped former, such as a rod, having a second cross-sectional area smaller than the first cross-sectional area. Alternatively, a single shaped former having a plurality of sections corresponding to each of the first portion  172  and the second portion  174  with different cross-sectional areas may be used to form the variable coil size heating element  170 . 
       FIG. 8  illustrates a fourth embodiment of the steam generator  60  according to the invention having the standard heating element  146  replaced by a partially coiled heating element  180 . All of the other parts of the steam generator  60  are identical to those previously described. The partially coiled heating element  180  is a coiled wire coiled around the steam generation chamber  136  in a generally central location relative to the first and second ends  132 ,  134 . The partially coiled heating element  180  includes a first portion  182  below an operational water level L of the steam generation chamber  136  and a second portion  184  above the operational water level L. The first portion  182  of the partially coiled heating element  180  may be coiled while the second portion  184  of the partially coiled heating element  180  may be substantially straight. In the illustrated embodiment, the coils all have a generally circular cross-section. Alternatively, the coils could have a different shape, such as triangular, square, or polygonal. Due to the coils in the first portion  182 , a greater total length of the wire forming the partially coiled heating element  180  is located below the operational water level L in the first portion  182 . Therefore a greater portion of the thermal output is located below the operational water level than above the operational water level. 
     The partially coiled heating element  180  may be formed by winding a portion of the wire corresponding to the first portion  182  around a shaped former of a constant cross-sectional area, such as a rod, so that the coiled wire has a uniform pitch. The remaining wire corresponding to the second portion  184  is not coiled. 
       FIG. 9  illustrates a fifth embodiment of the steam generator  60  according to the invention having the standard heating element  146  replaced by a variable wire size heating element  190 . All of the other parts of the steam generator  60  are identical to those previously described. The variable wire size heating element  190  is a substantially straight wire coiled around the steam generation chamber  136  in a generally central location relative to the first and second ends  132 ,  134 . The variable wire size heating element  190  includes a first portion  192  below an operational water level L of the steam generation chamber  136  and a second portion  194  above the operational water level L. The first portion  192  of the variable wire size heating element  190  may be formed of a wire having a larger cross-sectional area than cross-sectional area of the wire forming the second portion  194 . In the illustrated embodiment, the wire has a generally circular cross-section. Alternatively, the wire could have a different shape, such as triangular, square, or polygonal. Due to the larger cross-sectional area of the wire in the first portion  192  of the variable size heating element  190 , a greater total portion of the variable wire size heating element  190  is located below the operational water level L in the first portion  192 . Therefore a greater portion of the thermal output is located below the operational water level than above the operational water level. 
     The variable wire size heating element  190  may be formed by stretching or rolling a wire of a constant cross-sectional area along portions of the wire that correspond to the second portion  194  of the variable wire size heating element  190 . Stretching or rolling the sections of the wire corresponding to the second portion  194  will decrease the cross-sectional area of the wire in the second portion  194  as compared to the cross-sectional area of the wire in the first portion  192 . 
       FIG. 11  illustrates a sixth embodiment of the steam generator  60  according to the invention having the standard heating element  146  replaced by a serpentine heating element  200 . All of the other parts of the steam generator  60  are identical to those previously described. The serpentine heating element  200  may be serpentine in shape and curves around a portion of the steam generation chamber  136  in a generally central location relative to the first and second ends  132 ,  134 . The serpentine heating element  200  includes a first portion  202  below an operational water level L of the steam generation chamber  136  and a second portion  204  above the operational water level L. In the illustrated embodiment, the wire has a generally circular cross-section. Alternatively, the wire could have a different shape, such as triangular, square, or polygonal. Due to the configuration of the serpentine heating element  200 , a greater total length of the wire forming the serpentine heating element is located below the operational water level L. 
     The serpentine heating element  200  may be formed by bending a wire so as to form a serpentine shape that curves around a portion of the steam generation chamber  136 , as is illustrated in  FIG. 12 . The serpentine heating element  200  may curve primarily around a portion below the operational water level L of the steam generation chamber  136 , with only a small portion of the serpentine heating element  200  extending above the operational water level L. 
     The different approaches of the previously described embodiments can be combined to form a heating element where a greater portion of the thermal output is located below the operational water level than above the operational water level. For example, any of the embodiments of  FIGS. 5-8  could incorporate the different cross-sectional areas for the wire forming the coils as disclosed in the non-coiled wire of  FIG. 9 . The non-coiled wire of  FIG. 9  could be used with a coiled or partially coiled wire as disclosed in any of  FIGS. 5-8 . The smaller diameter coils of  FIG. 7  and the different pitch coils of  FIG. 5  could be stretched as in  FIG. 6 . The different pitch coils of  FIG. 6  could be used with the smaller diameter coils of  FIG. 8 . These examples are merely illustrative and not limiting. The different approaches can be used alone or together to create a heating element that is discretely or continuously variable in its thermal output. 
     While the variable thermal output heating element has been described up to this point as varying the output relative to the top and bottom of the steam generator, it can also be applied to vary the thermal output from end-to-end. For example, it may be beneficial to vary the thermal output from the inlet end to the outlet end. One such approach is illustrated in  FIG. 12 , which illustrates the heating element of  FIG. 8  with the coils of the first portion  182  varying in pitch for each winding. The first three windings, when viewed in  FIG. 12  from left to right, have more windings per unit length than the last two windings. This places more of the thermal output at the inlet, which more quickly heats the entering water. 
     Although the heating elements of the various embodiments described above are illustrated as being coiled around an exterior of the tube  130 , the heating elements may alternatively be coiled within the steam generation chamber  136  along an interior of the tube  130 . 
     The steam generator  60  may be employed for steam generation during operation of the washing machine  10 , such as during a wash operation cycle, which may include prewash, wash, rinse, and spin steps, during a washing machine cleaning operation cycle to remove biofilm and other undesirable pests from the washing machine, during a refresh or dewrinkle operation cycle, or during any other type of operation cycle. The steam generator  60  may also be employed to clean the steam generator  60  itself. An exemplary operation of the steam generator  60  is provided below. 
     To operate the steam generator  60 , water from the water supply  29  may be provided to the steam generator  60  via the valve  34 , the second supply conduit  62 , the water supply conduit  104 , and the tank  90 . Water that enters the tank  90  from the water supply conduit  104  fills the volume of the tank  90  between the steam generator inlet and the tank bottom  92  to thereby form the water plug. Once the water reaches the steam generator inlet at the first end  132  of the steam generator tube  130 , the water flows into the steam generator tube  130  and begins to fill the steam generation chamber  136  and, depending on the configuration of the steam generator  60  and the steam conduit  66 , possibly a portion of the steam conduit  66 . In the exemplary embodiment, the water that initially enters the steam generation chamber  136  fills the steam generation chamber  136  and the steam conduit  66  to a level corresponding to the water plug without a coincident rise in the water level in the tank  90 . Once the water fills the steam generation chamber  136  to the level corresponding to the water plug, further supply of water from the water supply conduit  104  causes the water levels in the tank  90  and the steam generation chamber  136  to rise together as a single water level. If the steam generation chamber  136  becomes completely filled with water, further supply of water from the water supply conduit  104  causes the water level in the tank  90  to further rise. Due to the pull of gravity, the water supplied to the steam generation chamber  136  will fill the steam generation chamber  136  from the bottom up. 
     Water may preferably be supplied to the operational water level L, which is typically less than a maximum water level corresponding to filling a total volume of the steam generation chamber  136 . The operational water level L may correspond to a level of water present in the steam generation chamber  136  when the steam generation chamber is filled to a volume optimal for steam generation. Although the operational water level L is illustrated as a single level, the actual level of water present in the steam generation chamber  136  during operation of the steam generator  60  may vary. For example, the water is normally supplied to the steam generator based on time or to a sensed level. Steam is then created which lowers the water level. At some point the water level may drop low enough that water is re-supplied to prevent the steam generator from running out of water. Alternatively, the water may be re-supplied continuously or at discrete times to keep the water level within a desired range. In some in-line or flow through steam generators, the operational water level may vary from 5% to 50% of the total volume. In tank-type steam generators, the percentage may be much higher and very close to 100%. Moreover, when steam is being generated, the creating of bubbles in the water makes the water very turbulent and the water level may change quickly. Thus, the operational water level L may be thought of more as an expected, target, or effective water level and typically is machine and process dependent. 
     At any desired time, the heat source  138  may be activated to generate heat to convert the water in the steam generation chamber  136  to steam. For example, the heat source  138  may be activated prior to, during, or after the supply of water. Because a greater total portion of the heating element  150 ,  160 ,  170 ,  180 ,  190 ,  200  according to the invention is present in a first portion  152 ,  162 ,  172 ,  182 ,  192 ,  202  of the heating element positioned below the operational water level L, thermal output from the heating element is concentrated on the water present in the steam generation chamber  136 . This is because the thermal output is uniform along the length of the wire, so allocating a greater total length of wire to the first portion  152 ,  162 ,  172 ,  182 ,  192 ,  202  provides greater thermal output to the first portion. Water may be converted to steam by the addition of heat, but steam will only increase in temperature by the addition of heat. By concentrating the thermal output to areas of the steam generator  60  that have the greatest effect on creating steam, namely the area below the operational water level L, steam is generated more efficiently, and less heat is lost to the areas surrounding the steam generator  60 . 
     Additionally, the steam generator  60  is less likely to malfunction due to a buildup of scale or calcification by implementing the inventive heating element. When the thermal output from the heating element is concentrated towards the area below the operational water level L, steam, air, and vapor present in the steam generation chamber  136  above the operational water level L is cooler. Because higher temperatures convert soft calcium to hard calcium, which is more difficult to remove than soft calcium, the asymmetric thermal output provided by the inventive heating output reduces the amount of hard calcium buildup. 
     Steam generated in the steam generation chamber  136  flows from the steam generator tube  130  and through the steam conduit  66  to the treatment chamber. In some circumstances, such as, for example, excessive scale formation or formation of other blockage in the steam generator  60  or the steam conduit  66 , the steam may attempt to flow upstream to the water supply  29  rather than to the treatment chamber. However, the water plug between the steam generator inlet and the outlet of the water supply conduit  104  blocks steam from flowing from the steam generation chamber  136  backwards into the water supply conduit  104  and to the water supply  29 . 
     During the operation of the washing machine  10 , the siphon break device may prevent water or other liquids from the tub  14  and/or the drum  16  from undesirably flowing to the water supply  29  via the steam generator  60 . Any siphoned liquids may flow through the steam generator  60 , into the reservoir  64 , through the water supply conduit  104 , and through the siphon break conduit  116  ( FIG. 2 ) to the atmosphere external to the washing machine  10  or other suitable location. The siphoned liquids may flow through the siphon break conduit  116  rather than through the second supply conduit  62  to the water supply  29 . This type of siphon break device is commonly known as an air-gap siphon break, but it is within the scope of the invention for any type of siphon break device to be coupled with the reservoir  64 . Further, it is also within the scope of the invention for the siphon break device to be separate from the reservoir  64  or for the reservoir  64  to be employed without the siphon break device. 
     While the invention has been specifically described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be construed as broadly as the prior art will permit.