Patent Publication Number: US-2009235957-A1

Title: Method for controlling dishwasher

Description:
This application claims the benefit of the Korean Patent Application No. 10-2007-0127524, filed on Dec. 10 2007, which is hereby incorporated by reference as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for controlling a dishwasher, and more particularly, to a control method of a dishwasher, which can achieve an improvement in washing performance via supply of steam. 
     2. Discussion of the Related Art 
     Generally, a dishwasher is an apparatus in which high-pressure wash washer is sprayed to dishes received in the dishwasher, so as to wash the dishes contaminated by, for example, food waste attached to surfaces of the dishes. 
     Such a dishwasher includes a water supply device to which wash water from an external source is supplied, and a drain device from which the used wash water is discharged to the outside. The dishwasher contains a tub defining a space in which dishes are received and washed. The tub is provided with spray nozzles capable of spraying wash water at a high pressure. 
     To collect the sprayed wash water, a sump is provided underneath the tub. The sump contains elements to filter and crush waste that is mixed with the wash water during washing of dishes. Then, under operation of a drive unit, the wash water is pumped upward so as to again be sprayed into the tub via a spray arm. In this way, the dishes received in the tub are washed via washing circulation of wash water. 
     Recently, a variety of measures have been proposed to improve washing performance of the above-described dishwasher. In particular, operations of the dishwasher may be classified into a preliminary washing operation and a main washing operation. In the preliminary washing operation, wash water is sprayed to dishes, to hydrate food waste attached to the dishes for easy washing of the dishes. The preliminary washing operation is followed by the main washing operation in which the dishes are washed in earnest. 
     To further improve the washing performance of the dishwasher, detergent is mixed into the wash water, so as to facilitate effective separation of food waste from the dishes. Some conventional dishwashers may also have a function to irradiate ultraviolet light to the washed dishes, for sterilization of the dishes. 
     However, conventional dishwashers have a problem in that, when used dishes are left for a long time and thus, food waste is dried and firmly adheres to the dishes, it is difficult to remove the food waste from the dishes via only spraying of high-pressure wash water. 
     To solve the above-described problem, although one might consider implementing the above-described preliminary washing operation having a hydrating function, it has been found that the preliminary washing operation cannot completely solve the above-described problem. Furthermore, implementation of the hydrating function requires a great time and thus, disadvantageously lengthens the entire washing operation. 
     Furthermore, even if the propagation of bacteria occurs as used dishes are left for a long time, conventional dishwashers have a difficulty to remove the bacteria. Although some conventional dishwashers have an ultraviolet sterilizing function, they exert sterilizing effects only on a local area to which ultraviolet light is irradiated, and cannot completely solve any problems, such as the propagation of bacteria, etc. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a method for controlling a dishwasher that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a method for controlling a dishwasher comprises: a preliminary washing operation for spraying wash water required to hydrate waste adhered to dishes; a main washing operation implemented plural times for washing the dishes contaminated by the waste; a rinsing operation implemented after the preliminary washing operation and also, after the main washing operation for rinsing the dishes from which the waste is removed; and a drying operation for drying the dishes after completion of the washing and rinsing operations, wherein the method further comprises a steam supply operation implemented at least once before, after, or during the main washing operation for supplying steam to the dishes. 
     The steam supply operation may be implemented to supply steam via operation of a heater having an adjustable heating capacity depending on the amount of wash water. 
     A hot-water supply operation for heating and supplying wash water may be selectively implemented during the main washing operation upon stoppage of steam supply. 
     A heating value of the heater in the steam supply operation may be controlled to be higher than a heating value of the heater in the hot-water supply operation. 
     A level of wash water in the steam supply operation may be controlled to be lower than a level of wash water in the main washing operation. 
     A lowest level of wash water in the steam supply operation may be controlled to be higher than an installation position of the heater. 
     The steam supply operation may be implemented to supply steam by adjusting the amount of wash water when a heater having a constant heating value is used. 
     A hot-water supply operation for heating and supplying wash water may be selectively implemented during the main washing operation upon stoppage of steam supply. 
     A level of wash water in the steam supply operation may be controlled to be lower than a level of wash water in the hot-water supply operation. 
     A lowest level of wash water in the steam supply operation may be controlled to be higher than an installation position of the heater. 
     The steam supply operation may be controlled to discharge water residue and supply new water, prior to implementing the steam supply operation. 
     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
         FIG. 1  is a schematic view illustrating the configuration of a dishwasher in accordance with an embodiment of the present invention; 
         FIG. 2  is an exploded perspective view illustrating components of a drive unit shown in  FIG. 1 ; 
         FIG. 3  is a sectional view of the drive unit shown in  FIG. 1 ; 
         FIG. 4  is a control block diagram of a control unit; and 
         FIG. 5  is a flow chart schematically illustrating a method for controlling the dishwasher shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Reference will now be made in detail to preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     Hereinafter, a preferred first embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic view illustrating the configuration of a dishwasher in accordance with an embodiment of the present invention. 
     As shown in  FIG. 1 , the dishwasher includes a cabinet  1  defining an external appearance of the dishwasher, a tub  10  defining a space in which dishes are washed, and a drive unit  20  to pump wash water, used to wash the dishes, into the tub  10 . 
     The tub  10  is provided therein with at least one rack  11  in which dishes are received, and at least one spray arm  12  to spray pumped wash water to the dishes. In this case, the spray arm  12  has a plurality of spray holes for spraying wash water. Preferably, the spray arm  12  is rotatably installed at a position corresponding to the rack  11 , to uniformly spray wash water to all the dishes. 
     The drive unit  20  is coupled to a water supply device  55 , through which wash water from an external tap water source is supplied. In addition, a drain device  50  is coupled to a sump  30 , and is used to discharge wash water received in the sump  30 . 
     The drive unit  20  is installed underneath the tub  10  and is configured such that wash water, sprayed and dropped from the spray arm  12 , is received in the drive unit  20 . Preferably, to pump the wash water upward from the drive unit  20 , a connecting pipe  13  is further coupled to the drive unit  20  while being connected to the spray arm  12 . 
       FIG. 2  is an exploded perspective view illustrating components of the drive unit shown in  FIG. 1 , and  FIG. 3  is a sectional view of the drive unit shown in  FIG. 1 . 
     As shown in  FIGS. 2 and 3 , the drive unit  20  includes the sump  30  in which wash water is received, the water supply device  50  to supply wash water into the sump  30 , the drain device  50  coupled to the sump  30 , from which wash water is drained, a washing pump received in the sump  30  and used to pump the wash water received in the sump  30 , and a filtering device to filter the pumped wash water remaining in the sump  30  after guiding some of the pumped wash water into the spray arm  12 . 
     The sump  30  defines a space in which wash water is received and is provided, at a portion thereof, with a drain chamber  32  that communicates with the drain device  50 . Preferably, a flow-path control device  35  is provided at the exterior of the sump  30  and in turn, a flow-path control valve  36  is axially coupled to the flow-path control device  35  via a shaft. 
     The water supply device  55  is connected with an external tap water source and serves to supply wash water into the tub  30 . Preferably, the water supply device  55  is connected to a specific position of the sump  30 . Of course, it will be appreciated that this installation of the water supply device  55  is provided only by way of example and wash water may be directly supplied into the tub  10 . In the present embodiment, the water supply device  55  consists of a water supply pipe  56  to supply wash water, and a water supply valve  57  to selectively open or close the water supply pipe  56 . 
     The drain device  50  may include a drain pump  53  provided in the drain chamber  32 . The drain pump  53  consists of a drain motor  51  and an impeller  52 . 
     The washing pump consists of a washing motor  41  provided underneath the sump  30  and used to generate a drive force, and an impeller  42  coupled to the filtering device and used to pump wash water. In addition, a disposer  43  is axially coupled to a shaft of the washing pump and functions to crush food waste via rotation thereof. Preferably, a screen  44  having a predetermined mesh size sufficient to filter a relatively large size of food waste is disposed above the disposer  43 . 
     The filtering device includes a pump housing  60  defining a space for installation of the impeller  42 , a filter housing  70  disposed to cover the top of the pump housing  60 , and a cover  80  coupled to the top of the filter housing  70  and sump  30 . Preferably, the pump housing  60  is disposed at a lower surface of the filter housing  70 , and the cover  80  is disposed at an upper surface of the filter housing  70 . 
     The filter housing  70  may contain, for example, a waste collecting chamber  71 , and the waste collecting chamber  71  may be coupled to a drain pipe  72  that communicates with the drain chamber  32 . For this, preferably, the drain pipe  72  is configured to protrude downward from the lower surface of the filter housing  70  by a predetermined length. 
     The cover  80  is preferably provided with a filter  81  such that the filter  81  corresponds to the waste collecting chamber  71  of the filter housing  70 . More preferably, the cover  80  is provided with a plurality of recovery holes  82  arranged around the filter  81 , and the recovery holes  82  communicate with the sump  30 . 
     The filter housing  70  is further provided with at least one main flow-path connected to the spray arm  12 , and is also provided with a sampling flow-path penetrating through the waste collecting chamber  71 . The flow-path control valve  36 , which is axially coupled to the flow-path control device  35 , is disposed in the filter housing  70  and is used to open or close the flow path(s) inside the filter housing  70 . 
     Preferably, regardless of any one main flow-path being opened or closed by the flow-path control valve  36 , some wash water is always introduced into the sampling flow-path. This serves to assure continuous filtering of wash water containing waste. 
     Once the wash water is introduced into the waste collecting chamber  71  through the sampling flow-path, the wash water flows through the filter  81  which is located above the waste collecting chamber  71 . Preferably, the filter  81  is used to filter waste contained in the wash water. 
     As the wash water, which is filtered via the above-described overflow process, as well as the wash water, which is sprayed from the spray arm  12  to thereby drop into the cover  80 , are introduced into the sump  30  through the recovery holes  82 , washing circulation of wash water is accomplished. 
     Hereinafter, a heater  100  provided at the drive unit  20  will be described in detail with reference to  FIGS. 2 and 3 . 
     In the present embodiment, the heater  100  is preferably provided, to supply steam into the tub  10 . 
     Steam is supplied because simply spraying high-pressure wash water to dishes may often fail to completely remove food waste adhered to surfaces of the dishes. Although one might consider providing a washing operation with a function for hydrating food waste adhered to surfaces of dishes, this still has a difficulty to completely remove dried food waste firmly adhered to dishes. Therefore, there is a risk of requiring an excessively long time to sufficiently hydrate food waste adhered to surfaces of dishes. 
     As will be appreciated, since steam has a considerably higher temperature than wash water and is able to be easily adsorbed into food waste adhered to dishes, the use of steam can accomplish sufficient hydration of food waste within a short time. Accordingly, when steam is supplied into the tub  10  as proposed in the present embodiment, food waste adhered to dishes may be easily removed by wash water, resulting in a remarkable improvement in washing performance of dishes. 
     Steam further has sterilizing effects since steam is a gas having a high-temperature of up to 100° C. and thus, provides the same effect as boiling of dishes. As compared to a conventional sterilizing device using ultraviolet light that is very expensive and exerts sterilizing effects only a local area to which ultraviolet light is irradiated, according to the present embodiment, steam is supplied to the entire inner surface of the tub  10  and surfaces of dishes, enabling sterilization of the entire dishwasher at low cost. 
     Preferably, steam is supplied before or after a main washing operation. This is advantageous to hydrate food waste adhered to dishes before the main washing operation, or to finally sterilize the dishes and tub after completion of the main washing operation. Of course, it will be appreciated that this supply of steam is given only by way of example and in the case where the main washing operation is implemented several times, steam may be supplied between the repeatedly implemented main washing operations, or may be supplied during the main washing operation. 
     Preferably, the heater  100  is installed to heat wash water received in the sump  30  for supply of steam. Of course, one might consider generating steam using water additionally supplied from the water supply device  55 , rather than using the wash water received in the sump  30 . However, when generating steam using the additionally supplied water, for example, a flow-path and valve connected to the water supply device  55 , a steam generator in which a receiving space for storing and heating water is defined, and a nozzle to supply steam into the tub are additionally required, entailing a risk of causing complexity in configuration. 
     Therefore, in the present embodiment, the heater  100  is installed in a bottom region of the sump  30  and is used to heat the wash water received in the sump  30  so as to generate steam. In this case, the steam is supplied into the tub  10  through a water collecting path, through which the wash water used to wash the dishes is collected. 
     Specifically, the wash water, sprayed into the tub  10 , drops downward, thereby being collected in the sump  30 . For this, most dishwashers have a water collecting path, which is connected to the top of the sump  30  while communicating with the tub  10  (here, the water collecting path is not an additional line, but a movement path along which the wash water drops). Steam rises because it is less dense than air and therefore, tends to be supplied into the tub  10  through the water collecting path. Accordingly, in the present embodiment, steam is supplied into the tub  10  through, for example, the recovery holes  82  perforated in the cover  80 . 
     Supplying steam through the recovery holes  82  of the cover  80  eliminates a need for an additional nozzle used to supply steam. When using the steam supply nozzle, there is a risk of steam being condensed at a surface of the nozzle. As compared to the use of the steam supply nozzle, the present embodiment can minimize condensation of steam during a steam supply operation, allowing a greater amount of steam to be supplied into the tub  10 . 
     Some of the steam generated by the heater  100  may be supplied by use of a path, through which wash water is supplied from the sump  30  into the tub  10 . Specifically, steam generated in the sump  30  has characteristics of diffusion and therefore, may be introduced into the filter housing  70  through an opening perforated in the bottom of the pump housing  60 . Thereafter, the steam may be supplied into the tub  10  via operation of the spray arm  12  after having passed through the main flow-path of the filter housing  70 . This has an advantage of sterilizing a flow path, along which the wash water is pumped from the sump  30  into the tub  10 . 
     Meanwhile, during washing circulation of the wash water used to wash the dishes, the heater  100  may heat the wash water to a high temperature sufficient to enable washing of dishes using high-temperature wash water. The higher the temperature of wash water, the easier it is to hydrate and remove food waste adhered to dishes and a further improved washing performance of the dishwasher can be accomplished upon washing circulation of the high-temperature wash water. 
     For this reason, in the present embodiment, the heater  100  heats the wash water received in the sump  30  according to operating modes of the dishwasher, so as to supply steam into the tub  10 , or to supply high-temperature wash water during the main washing operation. 
     For example, the heater  100  may heat wash water to more than 100° C., so as to generate steam. Otherwise, during the main washing operation, the heater  100  may heat wash water to less than 100° C., so as to generate high-temperature wash water. 
     In this case, to adjust the temperature of wash water heated by the heater  100 , a variety of methods may be adopted. 
     As the background of the above steam generation, the heater  100  may be a variable heater in which heating capacity is adjustable to adjust the temperature of wash water. In this case, when it is desired to supply steam into the tub  10 , it is preferable that the heater  100  heat wash water with an increased heating value sufficient to generate steam. 
     In addition, during washing circulation of wash water, it is preferable that the heater  100  heat wash water with a lower heating value than that required for the supply of steam, so as to supply a predetermined temperature of hot water. 
     If the heater  100  has no function of adjusting a heating value thereof, the temperature of wash water may be adjusted by adjusting the amount of wash water heated by the heater  100 . This is because, assuming that the heater has a constant heating value, the smaller the amount of wash water, the higher the temperature of wash water. 
     Accordingly, it is preferable that the amount of wash water during the supply of steam be kept at a lower value than the amount of wash water during washing circulation, to allow the wash water to be heated to a high temperature sufficient to generate steam. 
     In the present embodiment, in addition to the heater  100  in which heating capacity is adjustable according to an operating mode of the dishwasher, a control unit  110  is preferably provided, to adjust the amount of wash water received in the sump  30 . 
     As described above, it is preferable that the heater  100  heat the wash water during the supply of steam with a higher heating value than a heating value required during washing circulation of wash water. 
     In addition, the control unit  110  may control the amount of wash water such that a water level during the supply of steam is higher than a water level during washing circulation of wash water. This is because a steam generating time can be reduced according to the amount of wash water even though the heater  100  having an adjustable heating value is used. 
       FIG. 4  is a control block diagram of the control unit. 
     As shown in  FIG. 4 , in the present embodiment, the water supply device  55  and drain device  50  are preferably controlled, respectively, by the control unit  110 . Accordingly, the control unit  110  is able to control the water supply device  55  so as to effectively supply wash water, or to control the drain device  50  so as to adjust the amount of wash water to be received in the sump  30 . 
     In the present embodiment, a sensor unit  120  may be further provided, to measure a level of wash water received in the sump  30 . In this case, preferably, the control unit  110  acts to adjust the amount of wash water received in the sump  30  in response to a signal corresponding to a sensed level of wash water from the sensor unit  120 . The sensor unit  120  may consist of a high water level sensor  121  and a low water level sensor  122  (these sensors will be described hereinafter). 
     For example, if the sensor unit  120  senses, during an operation for washing dishes using wash water, that a level of wash water is lowered, thus making it impossible for the drive unit  20  to pump the wash water, the control unit  110  controls the water supply device  55 , so as to command supply of wash water. 
     Otherwise, if the sensor unit  110  senses, during an operation for supplying steam, that the amount of wash water is excessively increased to make it difficult to generate steam with a heating value of the heater  100 , the control unit  110  controls the drain device  50 , so as to command discharge of a predetermined amount of wash water. 
     The sensor  120  may include the high water level sensor  121 , to maintain an appropriate amount of wash water during generation of steam. Here, the high water level sensor  121  is preferably installed at a height corresponding to a position of the bottom opening of the pump housing  60 . 
     Preferably, the control unit  110  maintains a higher level of wash water received in the sump  30  than an installation position of the heater  100  during operation of the heater  100 . 
     Generally, the heater  100  used in the dishwasher is adapted to heat wash water by transmitting heat, generated by a heating element inside the heater  100 , to wash water in contact with an outer surface of the heater  100 . Therefore, when a level of wash water is not maintained to be higher than the installation position of the heater  100 , there is a risk of damage to the surface of the heater  100  due to overheating. In addition, this may cause remarkable deterioration in heating efficiency of the heater  100 . 
     For this reason, in the present embodiment as shown in the drawing, the heater  100  is disposed in the bottom region of the sump  30  and under control operation of the control unit  110 , a level of wash water is always maintained to be higher than the installation position of the heater  100 . In this case, the control unit  110  may sense a level of wash water using the sensor unit  120 . In addition, the sensor unit  120  may further include the low water level sensor  122  provided at the installation position of the heater  100  to sense a lowest level of wash water. 
     The control unit  110  is able to control the supply of wash water in such a manner that the wash water remaining in the sump  30  is discharged and new wash water is supplied, prior to generating steam via heating operation of the heater  100 . 
     In the case where the wash water contains waste separated from dishes during washing, heating the wash water containing waste entails a risk of generating serious foul odors. In particular, if the resulting steam is supplied for sterilization of dishes in a final step, such foul odors may cause a user discomfort. 
     Therefore, in the present embodiment, the control unit  110  preferably controls the water supply device  55  and drain device  50 , to selectively discharge the wash water remaining in the sump  30  and supply steam using newly supplied wash water, prior to generating steam. 
       FIG. 5  is a flow chart schematically illustrating a method for controlling the dishwasher shown in  FIG. 1 . 
     Hereinafter, a preferred control method of the dishwasher in accordance with the present invention will be described with reference to  FIG. 5 . 
     The dishwasher may be controlled such that washing of dishes is implemented using high-temperature wash water heated by the heater  100 . In addition, the dishwasher may be controlled to supply steam into the tub  10  via heating of wash water. That is, the dishwasher may be controlled to supply high-temperature wash water., or to supply steam into the tub  10  at a predetermined time when no washing of dishes is implemented, under operation of the single heater  100 . 
     As shown in  FIG. 5 , the dishwasher according to the present embodiment is basically controlled by the control unit  110  to implement a preliminary washing operation, a main washing operation, a rinsing operation, and a drying operation in this sequence. Additionally, before, after, or during the main washing operation, a steam supply operation is implemented at least once. 
     During the main washing operation, a hot-water supply operation for heating and supplying wash water may be selectively implemented when it is unnecessary to supply steam. Advantageously, prior to implementing the steam supply operation, the wash water remaining in the sump  30  is discharged and new wash water is supplied into the sump  30 . 
     The preliminary washing operation serves to preliminarily spray wash water to dishes, so as to hydrate, for example, food waste adhered to the dishes. The preliminary washing operation is followed by the rinsing operation and main washing operation in sequence. 
     The main washing operation is controlled in such a manner that wash water is sprayed plural times to wash the dishes and is followed by the rinsing operation. The washing and rinsing operations are selectively repeated, 
     The steam supply operation may be selectively implemented at least once before or during the main washing operation under control operation of the control unit  110 . 
     In the steam supply operation, preferably, the amount of wash water received in the sump  30  is adjusted before the wash water is heated by the heater  100  so as to generate steam. In this case, more preferably, a level of wash water received in the sump  30  during the steam supply operation is maintained to be lower than a level of wash water received in the sump  30  during the main washing operation or the hot-water supply operation. 
     The above washing operation requires more than a predetermined level of wash water required to pump the wash water using the drive unit  20 . Furthermore, in view of an improvement in washing efficiency, it is necessary to maintain more than a predetermined amount of wash water. 
     On the other hand, the steam supply operation has no need to pump the wash water using the drive unit  20 . This is because, assuming the heater  100  having a constant heating value, the smaller the amount of wash water, it is easier to generate steam via heating of the wash water. 
     Of course, under the assumption that the heater  100  is a variable heater in which heating capacity is adjustable, the steam supply operation may be controlled to generate steam with an increased heating value of the heater  100 . 
     That is, a heating value of the heater  100  in the steam supply operation is preferably higher than a heating value in the hot-water supply operation. In this case, note that heating a reduced amount of wash water is advantageous in view of a reduction in steam generating time. 
     Accordingly, in the use of a variable heater, a level of wash water received in the sump  30  during the steam supply operation is preferably maintained to be lower than a level of wash water during the hot-water supply operation. 
     That is, regardless of whether the heater  100  has a constant heating value or a variable heating value, it is preferable that a level of wash water received in the sump  30  be maintained at a lowest level for the purpose of an improvement in steam generation efficiency. 
     In the steam supply operation, after the amount of wash water is adjusted, the wash water is heated by the heater  100  until the wash water reaches a boiling point thereof, so as to generate and supply steam into the tub  10 . In this case, the supply of steam is continued for a predetermined time, so as to hydrate waste adhered to dishes for easy removal of the waste, or to sterilize the tub  10  and dishes. 
     Preferably, during implementation of the steam supply operation, a level of wash water is maintained to be higher than the installation position of the heater  100 , under control operation. 
     In the case where a steam supply time is excessively long, the wash water received in the sump  30  may evaporate, lowering a level of wash water to a level close to the bottom of the heater  100 . This may damage the surface of the heater  100  due to overheating and also, may cause remarkable deterioration in the heating efficiency of the heater  100 . For this reason, during implementation of the steam supply operation, it is preferable to sense a level of wash water received in the sump  30 , so as to keep the level of wash water above the installation position of the heater  100 . 
     In the present embodiment, preferably, a lowest level of wash water is set to be slightly higher than the installation position of the heater  100 . When the level of wash water received in the sump  30  reaches the lowest level in the steam supply operation, the steam supply operation can be controlled such that a predetermined amount of wash water is newly supplied into the sump  30 . 
     In this case, the newly supplied amount of wash water is preferably set in consideration of the capacity and heating efficiency of the heater  100 . Note that a level of wash water received in the sump  30  during the steam supply operation is preferably maintained to be lower than a level of wash water during the main washing operation. This is to assure effective generation of steam as described above. 
     In the case where the steam supply operation is followed by the main washing operation, it is preferable to additionally supply wash water prior to pumping the wash water received in the sump  30  for washing circulation of wash water. This serves to supplement wash water as much as a reduced amount of wash water received in the sump  30  during the steam supply operation, for the purpose of effective washing circulation of wash water. 
     In the case where the main washing operation is followed by the steam supply operation, it is preferable to implement an operation for discharging the wash water remaining in the sump  30  prior to implementing the steam supply operation. 
     This is because the wash water may contain waste separated from the dishes at the end of the main washing operation. Heating the wash water containing waste to generate steam may have a risk of causing, for example, serious foul odors. 
     Accordingly, when the steam supply operation is finally implemented for the purpose of sterilization, etc., it is preferable to discharge the wash water remaining in the sump  30  and supply new wash water, prior to implementing the steam supply operation. 
     In addition, even in the case where the steam supply operation is implemented during the main washing operation, it is preferable to discharge the wash water remaining in the sump  30  and supply new wash water, prior to implementing the steam supply operation. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.