Abstract:
A refrigerator includes a freezing compartment and a door for selectively opening or closing at least a part of the freezing compartment The door includes a foam insulating layer and a sidewall contacting the foam insulating layer. A heater is installed correlative with the sidewall to supply heat to the sidewall.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a refrigerator, especially to a household or commercial refrigerator. 
       BACKGROUND ART 
       [0002]    The freezing compartment of a refrigerator can usually reach a temperature of lower than minus ten ° C. or more. The refrigerator body and the door of the refrigerator are provided with heat insulating layers to avoid losing of cold energy caused by heat exchange between the cold air within the refrigerator and the surrounding environment. However, as the increase in the refrigerator&#39;s volume and in the number of the components of the refrigerator door (for example, a dispenser for dispensing water or ice), some portions of the refrigerator door which are exposed to the atmosphere may have relatively low temperature under the influence of the storage compartment. When the difference between the temperature of the surface exposed to the atmosphere and the atmosphere temperature reaches dew point temperature, condensation will appear on the surface. 
         [0003]    The sidewall of the refrigerator door remote from the rotation axis of the door will be influenced by the freezing compartment during the closing of the door. Thus, it is possible to generate condensate drops on this side wall. This is especially obvious when the refrigerator door is provided with an ice dispenser. 
       SUMMARY OF THE INVENTION 
       [0004]    An object of the invention is to overcome at least one of the above technical problems existed in the prior art, and to provide a refrigerator for greatly reducing the possibility of presence of condensate drops on the freezing compartment door. 
         [0005]    An aspect of the invention relates to a refrigerator which comprises a freezing compartment and a door for selectively opening or closing at least a part of the freezing compartment, wherein the door comprises a foam insulating layer and a sidewall contacting the foam insulating layer, and the refrigerator is characterized by further comprising a heater which is installed correlative with the sidewall to supply the sidewall with heat. 
         [0006]    When the heater is turned on, the temperature of the sidewall can be increased, so as to greatly reduce the possibility that condensation appears on the sidewall of the freezing compartment door, and thereby occurrence of the dewing phenomenon on the freezing compartment door can be reduced. 
         [0007]    Other features which are disclosed individually or in combination as features of the invention are defined in attached claims. 
         [0008]    According to a preferred embodiment of the invention, the heater is located on the inner side of the sidewall. 
         [0009]    According to a preferred embodiment of the invention, the refrigerator includes adhesive means for attaching the heater to the inner side of the sidewall. 
         [0010]    According to a preferred embodiment of the invention, the refrigerator includes a heat conducting element for transmitting heat generated by the heater to the sidewall, the heat conducting element being located between the inner surface of the sidewall and the heater. Thereby, the heat generated by the heater can rapidly spread out so as to avoid overheating of any partial region of the door. 
         [0011]    According to a preferred embodiment of the invention, the door is rotatable around a rotation axis parallel to a vertical axis, and the sidewall is parallel to the rotation axis. According to a particularly preferred embodiment of the invention, the sidewall is located distant from the rotation axis. 
         [0012]    According to a preferred embodiment of the invention, the refrigerator further comprises a dispenser located on the door and configured to dispense ice. 
         [0013]    According to a preferred embodiment of the invention, the heater is, in a transverse direction, at least partially overlapped with the dispenser, so as to greatly reduce the possibility of generating condensate drops on a part of the sidewall which transversely overlaps the dispenser and is otherwise easy to generate condensate drops. Such a configuration can effectively reduce the possibility of generating condensate drops on the whole sidewall of the door. 
         [0014]    According to a preferred embodiment of the invention, the refrigerator further comprises a heating unit for supplying heat to the dispenser. 
         [0015]    According to a preferred embodiment of the invention, the refrigerator further comprises a control unit and a sensing unit for detecting at least one parameter, and the heater is automatically controlled by the control unit based on the parameter detected by the sensing unit. 
         [0016]    According to a preferred embodiment of the invention, the parameter comprises ambient temperature and/or ambient relative humidity and/or the temperature of the sidewall. 
         [0017]    The structure and other objects and advantages of the invention will be apparent from the description to the preferred embodiments with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention will be further understood by reading the following detailed description with reference to the drawings which are incorporated herein as a part of the description and illustrate the invention and in which: 
           [0019]      FIG. 1  is a schematic perspective view of a refrigerator according to a preferred embodiment of the invention. 
           [0020]      FIG. 2  is a schematic partial sectional view taken along a direction indicated by line I-I of  FIG. 1 . 
           [0021]      FIG. 3  is a schematic view of a partly assembled door of the according to the preferred embodiment of the invention. 
           [0022]      FIG. 4  is a schematic layout of a heating unit of a dispenser casing according to a preferred embodiment of the invention. 
           [0023]      FIG. 5  is a schematic block diagram of the refrigerator according to a preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0024]    Please refer to the drawings, in particular  FIGS. 1 and 2 . A refrigerator  1  comprises a refrigerator body  2  and two doors  3  connected to the refrigerator body  2 , as shown in  FIGS. 1 and 2 . 
         [0025]    The refrigerator body  2  comprises an outer shell  11 , an inner shell  12  and a heat insulating layer  6  disposed between the outer shell  11  and the inner shell  12 . In this embodiment, the heat insulating layer  6  is a foam-based insulating layer and is formed by foaming a heat insulating foam material. The refrigerator body  2  defines at least one storage space for storing food. In this embodiment, the storage space comprises a freezing compartment  7  and a refrigerating compartment (not shown) which are juxtaposed with each other. 
         [0026]    The doors  3  are pivotably connected to the refrigerator body  2  by hinges  4  respectively, and are rotatable about their corresponding rotation axes which are parallel to a vertical axis. As shown in  FIG. 2 , it is also provided with a foam-based insulating layer  6  inside each of the doors  3 . The doors  3  are usually closed to avoid escape of cold air from the freezing compartment  7  and the refrigerating compartment. When desired, the user may open the corresponding door  3  to perform an operation, such as taking out food from the freezing compartment or refrigerating compartment, or putting food into a corresponding storage compartment. The user can open or close the doors  3  by means of handles  5 . 
         [0027]    In this embodiment, each door  3  is configured to completely open or completely close a corresponding storage compartment. It can be understood that the invention is not limited thereto, and other embodiments are also possible. For example, in an alternative embodiment, one of the storage compartments may be opened or closed by two doors  3 . That is to say, each door  3  may only open or close a part of such a storage compartment. 
         [0028]    The door  3  which is corresponded to the freezing compartment  7  (hereinafter referred to as freezing compartment door) is equipped with a dispenser  8  to allow a user to take out ice and/or beverage (for example water), such as ice stored in the freezing compartment and water stored in a water tank arranged in the refrigerating compartment, without opening the door  3 . Although in this embodiment the dispenser  8  is arranged in the door  3  which is corresponded to the freezing compartment, it shall be appreciated that it is also possible to arrange the dispenser  8  in a suitable way in the door  3  which is corresponded to the refrigerating compartment. 
         [0029]    As shown in  FIGS. 2 and 3 , the freezing compartment door  3  comprises a door panel  13  forming its front surface and an inner lining  23  facing towards the freezing compartment  7  when the freezing compartment door  3  is in its closed position. In this embodiment, the door panel  13  is made of a sheet metal material, and both sides of the door panel  13  are bent backwardly and extend to form into first and second longitudinal sidewalls  48  and  49  respectively. The heat insulating layer  6  is in tight contact with the door panel  13  and the first and second longitudinal sidewalls  48  and  49 . 
         [0030]    The door panel  13  has an opening  9  corresponding to the dispenser  8 , which opening  9  having a substantially square or rectangular shape. The dispenser  8  comprises a dispenser casing  10  received between the door panel  13  and the inner lining  23 . The dispenser casing  10  forms a cavity  14  which is inwardly recessed and has a front open end. The shape and dimension of the front open end of the inner cavity  14  correspond to that of the opening  9  substantially. The inner lining  23  protrudes toward the freezing compartment  7  at the location corresponded to the dispenser casing  10 , with a predetermined distance between the protruding portion of the inner lining  23  and the dispenser casing  10  for disposing the heat insulating layer  6 . 
         [0031]    The dispenser  8  comprises a partition plate  15  within the inner cavity  14 . The partition plate  15  is parallel to a horizontal plane and separates the inner cavity  14  into upper and lower portions. The portion of the inner cavity  14  located below the partition plate  15  forms into a dispensing cavity  16  whose front end is kept open. The dispensing cavity  16  is configured to accept at least a part of an external container such as a cup. In this embodiment, the dispensing cavity  16  is recessed backwardly from the front surface of the door  3  with a certain curvature to a predetermined depth. 
         [0032]    The dispensing cavity  16  has a substantially flat support wall  17  for stably putting the external container thereon. The support wall  17  has a plurality of thin through holes (not shown), through which any liquid that is splashed out or overflows accidentally during an ice or water dispensing process flows into a water gathering slot  19  arranged below the support wall  17 . 
         [0033]    The refrigerator  1  comprises a control panel  20  arranged on the freezing compartment door  3 , and the control panel  20  comprises a display screen  21  and a plurality of buttons or a touch area  22  for controlling the refrigerator  1 . The display screen  21  can display the state of the refrigerator  1  and/or selectable parameters, etc. 
         [0034]    In this embodiment, the control panel  20  is arranged along the upper end of the opening  9 , closely adjacent to the dispensing cavity  16 . The portion of the opening  9  located above the partition plate  15  is adapted to be conformed to the out profile of the control panel  20 , such that the control panel  20  can be engaged by the corresponding edge of the opening  9 . The portion of the inner cavity  14  located above the partition plate  15  is shielded by the control panel  20 . 
         [0035]    The dispenser casing  10  comprises a cavity wall delimiting the inner cavity  14 . The cavity wall comprises a first portion  24  located below the partition plate  15 . The first portion  24  comprises a longitudinal wall  30  for forming a longitudinal boundary of the dispensing cavity  16 . The longitudinal wall  30  is perpendicular to the horizontal plane and has a substantially arc-shaped cross-section. The longitudinal wall  30  has a rear surface which is closely adjacent to the heat insulating layer  6  and an outer surface which is exposed to the atmosphere. 
         [0036]    The first portion  24  further comprises a bottom wall  31  which is connected to the lower end of the longitudinal wall  30  and extends forwardly. The bottom wall  31  is located below the support wall  17  and spaced from the support wall  17  by a certain distance so as to form the above-mentioned water gathering slot  19 . 
         [0037]    The cavity wall of the dispenser casing  10  further comprises a second portion  25  which is connected to the upper end of the first portion  24  and is located above the partition plate  15 . The second portion  25  comprises an inclined wall  26  which extends from the longitudinal wall  30  and is inclined forwardly. The inclined wall  26  comprises a through hole  27  which allows ice to pass therethrough. The through hole  27  is configured as a part of an ice transfer passage  29 . The ice transfer passage  29  is used for transferring ice from an ice storage unit  28  located within the freezing compartment  7  to the dispensing cavity  16 . The second portion  25  further comprises a top wall  32  which forms the upper boundary of the inner cavity  14 . The second portion  25  has a hole  33  through which a water supply pipe (not shown) passes, which water supply pipe transmitting drinkable water to the dispensing cavity  16 . 
         [0038]    An ice discharge pipe  34  forming a major part of the ice transfer passage  29  is embedded in the freezing compartment door  3 . One end of the ice discharge pipe  34  is connected to the second portion  25  and is in communication with the through hole  27 . The other end of the ice discharge pipe is oriented towards a discharge outlet of the ice storage unit  28  within the freezing compartment  7  when the freezing compartment door  3  is closed. Thereby, the ice discharged from the ice storage unit  28  enters into the ice discharge pipe  34 , and then is guided to the dispensing cavity  16  by means of an ice outlet  18  provided in the partition plate  15 . 
         [0039]    As shown in  FIG. 2 , the portion of the inner cavity  14  which lies above the partition plate  15  is shielded by the control panel  20 ; however, the second portion  25  of the dispenser casing  10  still communicates with the atmosphere, that is, the second portion  25  is still exposed to the atmosphere, because the partition plate  15  is provided with the ice outlet  18  which is in communication with the portion of the inner cavity  14  which lies above the partition plate  15 . 
         [0040]    To prevent air within the freezing compartment  7  from escaping from the freezing compartment  7  through the ice transfer passage  29  or prevent outside air from entering into the freezing compartment  7  through the ice transfer passage  29 , the dispenser  8  is equipped with a closure element  36  for opening or closing the ice transfer passage  29 . Usually, the ice transfer passage  29  is closed by the closure element  36 . When there is a need for dispensing ice, the ice transfer passage  29  is opened by means of the closure element  36  to allow the transfer of ice. The shape and dimension of the closure element  36  are substantially corresponded to that of the through hole  27 , such that in the closed position the closure element closes the through hole  27  and thus closes the ice transfer passage  29 . In this embodiment, the closure element  36  is connected to the second portion  25  of the dispenser casing  10  and is received in the inner cavity  14 . 
         [0041]    Under the influence of the freezing compartment  7 , the temperature of the dispenser casing  10  is usually lower than room temperature/ambient temperature. When the difference between ambient temperature and the temperature of the dispenser casing  10  reaches dew point temperature, condensate drops will be generated on the dispenser casing  10 . The condensation possibility is relatively high due to the fact that the second portion  25  of the dispenser casing  10  is close to the ice discharge pipe  34  and forms a part of the ice transfer passage  29 . For this end, the refrigerator  1  is provided with a heating unit  37  for increasing the surface temperature of the dispenser casing  10 . As shown in  FIG. 2 , the heating unit  37  is arranged between the dispenser casing  10  and the heat insulating layer  6 . 
         [0042]      FIG. 4  is a schematic diagram of the heating unit  37  according to a preferred embodiment of the invention. As shown in  FIG. 4 , the heating unit  37  comprises a first heater  38  and a second heater  39  adjacent to the first heater  38 , for supplying heat to the dispenser casing  10 . The first heater  38  and the second heater  39  are preferably resistance heaters, i.e. performing heating by resistors. 
         [0043]    In order to evenly transmit the heat generated by the first heater  38  and the second heater  39  to the dispenser casing  10 , the heating unit  37  further comprises a first heat conducting element  40  for transmitting the heat generated by the first heater  38  and the second heater  39  to the dispenser casing  10 . In this embodiment, the first heat conducting element  40  is an aluminum foil having a high heat conductivity. 
         [0044]    The first heat conducting element  40  has a hole (not shown) which is corresponded to the through hole  27 . The first heater  38  and the second heater  39  can be arranged according to the distribution characteristics of condensate drops on the dispenser casing  10 . In this embodiment, the first heater  38  comprises a plurality of arc-shaped heating segments  35  arranged around the hole. The second heater  39  is arranged close to the first heater  38  and preferably comprises a portion located between heating segments  35  of the first heater  38 . Preferably, this portion has a shape that corresponds to the heating segment  35 . 
         [0045]    After the first heater  38  and the second heater  39  are arranged in a predetermined pattern on one side of the first heat conducting element  40 , the other side of the first heat conducting element  40  is closely attached to the inner side of the dispenser casing  10 . 
         [0046]    The heating unit  37  is adhered to the inner side of the dispenser casing  10  by means of adhesive means (not shown), with the hole of the first heat conducting element  40  being aligned with the through hole  27 . The first heat conducting element  40 , the first heater  38  and the second heater  39  all are flexible and deformable, such that the portion of the heating unit  37  located between line A and line B is arranged on the inclined wall  26 , the portion thereof located above line A is bent and then is adhered to the top wall  32  of the dispenser casing, and the portion thereof located below line B is bent and then is connected to the upper end of the longitudinal wall  30 . Thereby, in this embodiment, the first heater  38  is mainly arranged on the inclined wall  26  and the top wall  32  of the dispenser casing  10 . The lower end portion of the first heater  38  extends to the upper end of the longitudinal wall  30 . The heating segment  35  most close to the through hole  27  is arranged around the through hole  27 . The major portion of the second heater  39  is arranged on the inclined wall  26 . The portion located below line B of the second heater extends to the upper end of the longitudinal wall  30  together with that of the first heater  38 . 
         [0047]    In this embodiment, the first heater  38  and the second heater  39  are disposed on a first region  51  and a second region  52  of the dispenser casing  10  respectively. The first region  51  is adjacent to the second region  52 , but they do not overlap each other. The first region  51  comprises the majorities of the inclined wall  26  and the top wall  32  as well as the upper end portion of the longitudinal wall  30  which is close to the inclined wall  26 . The second region  52  has an area smaller than the first region  51  and is surrounded by the first region  51 . 
         [0048]    Preferably, the power of the second heater  39  is lower than that of the first heater  38 . Preferably, the power density of the second heater  39  is configured in such a way that the dispenser casing  10  is not subjected to overheating even if the second heater  39  is turned on for a long time or always turned on. 
         [0049]    According to a preferred embodiment of the invention, the side of the longitudinal wall  30  which faces the heat insulating layer  6  is provided with a second heat conducting element  50 , the upper end of which is connected to the first heater  38  and the second heater  39  or connected to the first heat conducting element  40 . Thereby, the first and second heaters  38  and  39  and/or the first heat conducting element  40  serve as a heat source for the second heat conducting element  50 . 
         [0050]    Since the second heat conducting element  50  is of a high heat conductivity, the heat generated by the first and second heaters  38  and  39  is also transmitted to other portions of the longitudinal wall  30  that are not equipped with any heating element, such that the temperature of the whole longitudinal wall  30  can be increased so as to avoid the presence of condensate drops. Since the longitudinal wall  30  is located relatively distant from the ice transfer passage  29 , such a configuration allows to avoid the presence of condensate drops on the longitudinal wall  30  without arranging any heater on the longitudinal wall  30  or merely by arranging a heater on the marginal region of the longitudinal wall  30  where is not easy to be touched by the user. Thus, energy consumption can be lowered. In addition, the situation that the user touches the high temperature region of the longitudinal wall  30  can be avoided. 
         [0051]    Preferably, the second heat conducting element  50  comprises a metal foil of a high heat conductivity, such as aluminum foil. In a particularly preferable embodiment, the second heat conducting element  50  covers at least substantially most of the longitudinal wall  30 . For example, the longitudinal wall  30  is entirely covered by the second heat conducting element  50 . The second heat conducting element  50  is preferably adhered to the inner side of the longitudinal wall  30 . 
         [0052]    The first longitudinal sidewall  48  of the freezing compartment door  3  is located adjacent to the rotation axis of the freezing compartment door  3 , so that the second longitudinal sidewall  49  opposite to the first longitudinal sidewall  48  is located distant from the rotation axis of the freezing compartment door  3  and close to the door of the refrigerating compartment. According to a preferred embodiment of the invention, the freezing compartment door  3  is provided with a third heater  47  for supplying heat to the second longitudinal sidewall  49 , so as to avoid the presence of condensate drops on the second longitudinal sidewall  49  due to the difference between surface temperature and the atmosphere temperature. In this embodiment, the third heater  47  is attached to the inner side of the second longitudinal sidewall  49 . The third heater  47  is preferably a resistance heater, i.e. performing heating by resistors. 
         [0053]    The second longitudinal sidewall  49  is provided with a third heat conducting element  54  attached to the inner side thereof. The third heat conducting element  54  is located between the third heater  47  and the inner surface of the second longitudinal sidewall  49  to evenly transmit the heat generated by the third heater  47  to the second longitudinal sidewall  49 . Preferably, the third heat conducting element  54  is attached to the inner surface of the second longitudinal sidewall  49  by adhesive means (such as an adhesive tape). 
         [0054]    It is most preferably to arrange the third heater  47  and/or the third heat conducting element  54  on a region of the second longitudinal sidewall  49  which is corresponded to the dispenser  8  in the longitudinal direction. Preferably, the third heater  47  at least partially overlaps the dispenser  8  in a transverse direction. 
         [0055]      FIG. 5  shows a structural schematic diagram of the refrigerator according to a preferred embodiment of the invention. Now a control method of the first heater  38  and the second heater  39  will be described with reference to  FIG. 5 . 
         [0056]    The refrigerator  1  comprises a control unit  41 , and an input unit  43  and a display unit  44  coupled to the control unit  41  respectively, wherein the input unit  43  comprises the buttons or touch area  22  located on the control panel  20 , and the display unit  44  comprises the display screen  21  located on the control panel  20 . The control unit  41  comprises a microprocessor and a memory unit, such that some components of the refrigerator  1  such as the first heater  38  can be automatically controlled by means of a program stored in the memory unit. 
         [0057]    The refrigerator  1  further comprises a sensing unit  42  for detecting at least one environmental parameter. The sensing unit  42  is coupled to the control unit  41  and feeds back the detected parameter to the control unit  41 . In this embodiment, the sensing unit  42  comprises a temperature sensor for detecting ambient temperature. The sensing unit  42  controls the operation of the first heater  38 , including turning on and turning off the first heater  38 , based on the detected ambient temperature. 
         [0058]    In a preferred embodiment, when the detected ambient temperature is lower than zero ° C., the first heater  38  is turned off. When the detected ambient temperature is between 0° C. and 10° C., the first heater  38  operates at a first output power and/or operates at a duty cycle of lower than 0.3. When the detected ambient temperature is between 10° C. and 15° C., the first heater  38  is turned on at a second output power, or the first heater  38  is turned on and off in an alternative manner at a second duty cycle (for example, 0.4). When the detected ambient temperature is between 15° C. and 25° C., the first heater  38  is turned on at a third output power and/or operates at a predetermined third duty cycle (for example, 0.5). 
         [0059]    In an alternative embodiment, the sensing unit  42  further comprises a humidity sensor for detecting ambient relative humidity. The control unit  41  controls the operations of the first heater  38  based on the detected ambient temperature, ambient relative humidity and other factors. 
         [0060]    The second heater  39  is controlled independently of the first heater  38 . According to the invention, the second heater  39  is turned on only in an auxiliary heating mode, which is only manually initiated by the user. Thus, the user can, according to the dewing phenomenon on the refrigerator  1 , make an active decision as to whether the second heater  39  should be actuated to increase heat for removing or preventing dewing. 
         [0061]    In a preferred embodiment, the auxiliary heating mode is actuated by means of switching means  45  arranged on the freezing compartment door  3 . The switching means  45  is preferably arranged on the dispenser  8  or near the dispenser  8 . Particularly preferably, the switching means  45  is arranged on the partition plate  15 . 
         [0062]    In an embodiment, the switching means  45  is electrically connected to the second heater  39 , and the turning on and off states of the second heater  39  is determined by the switching on and off states of the switching means  45 . Preferably, when the switching means  45  is in the switching off state and the refrigerator  1  operates in a normal mode, the first heater  38  is turned on or off based on an instruction from the control unit  41 , and the second heater  39  is turned off. When the user operates the switching means  45  to switch on it, the refrigerator  1  actuates the auxiliary heating mode, the second heater  39  is turned on to supply extra heat to the dispenser casing  10 , and at the same time the first heater  38  is turned on or off based on an instruction from the control unit  41 . 
         [0063]    The switching means  45  can be provided independently of the control unit  41 . For example, there is no coupling between the switching means  45  and the microprocessor of the control unit  41 . In an alternative embodiment, the switching means  45  is connected to the control unit  41 . For example, the display unit  44  can display whether the refrigerator  1  is under the normal heating mode or the auxiliary heating mode, or the user can select the parameters displayed on the display unit  44  by means of the switching means  45  in order to initiate the auxiliary heating mode. 
         [0064]    The second heater  39  can be turned off by manually switching off the switching means  45 , so that the auxiliary heating mode is ended. In an alternative embodiment, the second heater  39  can also be automatically turned off. For example, the control unit  41  is configured in such a manner of automatically turning off the second heater  39  after the second heater  39  has been turned on for a predetermined time, such as 15 minutes. This can be achieved by virtue of timing means connected to the control unit  41 . The timing means is configured in such a way that it generates a signal when the second heater  39  has been turned on for a predetermined time, and then the second heater  39  is turned off based on this signal. Under the condition that the switching means  45  is not coupled with the microprocessor of the control unit  41 , this can be achieved by timing means connected to the switching means  45  or timing means embedded the switching means  45 . 
         [0065]    In the embodiment shown in  FIG. 5 , the control manner of the third heater  47  is the same as that of the first heater  38 , that is, being automatically controlled by the control unit  41  based on detected parameters. In a preferred embodiment, the parameter comprises ambient temperature, ambient relative humidity and/or the temperature of the sidewall  49 , such that the control unit  41  can control the third heater  47  based on the ambient temperature, the ambient relative humidity and/or the temperature of the sidewall  49 , so as to for example determine whether or not the third heater  47  should be turned on, or determine the frequency of turning on and off or the duty cycle of the third heater  47 .