Abstract:
An air conditioner includes a refrigerant circuit, a switching valve, an outdoor fan and a controller. The refrigerant circuit sequentially circulates refrigerant through a compressor, an indoor heat exchanger, a decompression mechanism and an outdoor heat exchanger during a heating operation. The switching valve is connected to the refrigerant circuit to switch a flow direction of the refrigerant discharged from the compressor. The controller executes a defrosting operation control in which the outdoor fan is deactivated and the switching valve directs refrigerant discharged from the compressor towards the outdoor heat exchanger during a defrosting operation. The controller further maintains the switching valve so refrigerant discharged from the compressor is directed towards the outdoor heat exchanger and executes a fan defrosting operation control in which the outdoor fan is rotated for a predetermined period of time after completion of the defrosting operation when a predetermined condition is satisfied.

Description:
CROSS-REFERENCE TO RELATED APPLICATONS 
     This U.S. National stage application claims priority under 35 U.S.C. §119 (a) to Japanese Patent Application No. 2008-293141, filed in Japan on Nov. 17, 2008, the entire contents of which are hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an air conditioner using a vapor compression refrigeration cycle. 
     BACKGROUND ART 
     The outdoor heat exchangers for the air conditioners function as evaporators for refrigerant during a heating operation. Therefore, the moisture contained in outdoor air is condensed as dew on the surfaces of the outdoor heat exchangers. Especially when outdoor temperature is roughly 0 degrees Celsius, frost markedly attaches to the outdoor heat exchangers. Frost attaches not only to the outdoor heat exchangers but also to the main bodies of the outdoor fans and their peripheral members such as bell mouths and fan guards. In the air conditioners such as one disclosed in Patent Literature 1 (Japan Laid-open Patent Application Publication No. JP-A-H04-366341), hot gas is configured to flow towards the outdoor heat exchanger during a defrosting operation for melting frost covering the surfaces of the outdoor heat exchangers. 
     In the air conditioners such as one disclosed in Patent Literature 1, it is possible to melt the frost attaching to the outdoor heat exchangers. However, it has been difficult to even melt frost attaching to the main bodies of the outdoor fans and their peripheral members such as the bell mouths and the fan guards. 
     SUMMARY 
     Technical Problem 
     It is an object of the present invention to provide an air conditioner for even removing frost attaching to devices and members positioned in the downstream of the airflow exchanging heat with an outdoor heat exchanger. 
     Solution to Problem 
     An air conditioner according to a first aspect of the present invention includes a refrigerant circuit, a switching valve, an outdoor fan and a controller. The refrigerant circuit sequentially circulates a refrigerant through a compressor, an indoor heart exchanger, a decompression mechanism and an outdoor heat exchanger during a heating operation. The switching valve is connected to the refrigerant circuit for switching a flow direction of the refrigerant discharged from the compressor. The outdoor fan blows the air towards the outdoor heat exchanger. The controller is configured to execute a defrosting operation control of deactivating the outdoor fan and causing the switching valve to direct the refrigerant discharged from the compressor towards the outdoor heat exchanger during a defrosting operation. Further, the controller is configured to keep the operation of directing the refrigerant discharged from the compressor towards the outdoor heat exchanger and execute a fan defrosting operation control of rotating the outdoor fan for a predetermined period of time after completion of the defrosting operation when a predetermined condition is satisfied. 
     Under predetermined conditions, frost attaching to the main body of the outdoor fan and its peripheral members (e.g., a bell mouth and a fan guard) does not melt even after completion of the defrosting operation. According to the air conditioner of the first aspect of the present invention, however, air elevates its temperature when passing through the outdoor heat exchanger and the warm air hits the main body of the outdoor fan and its peripheral members by means of rotations of the outdoor fan. Therefore, frost attaching thereto melts. 
     An air conditioner according to a second aspect of the present invention is the air conditioner according to the first aspect of the present invention. The air conditioner further includes an outdoor temperature sensor measuring an outdoor temperature. The controller is configured to execute the fan defrosting operation control when the outdoor temperature detected through the outdoor temperature sensor falls in a predetermined temperature range. According to the air conditioner of the second aspect of the present invention, the controller is configured to determine whether or not the fan defrosting operation control is executed depending on outdoor temperature. Therefore, the fan defrosting operation is prevented from being executed uselessly. 
     An air conditioner according to a third aspect of the present invention is the air conditioner according to the first aspect of the present invention. In the air conditioner, the controller is configured to activate the compressor during the fan defrosting operation control. According to the air conditioner of the third aspect of the present invention, the refrigerant flowing into the outdoor heat exchanger keeps its temperature high by means of activation of the compressor during the fan defrosting operation control. Therefore, reduction in temperature is inhibited for the warm air flowing towards the main body of the outdoor fan and its peripheral members. Consequently, a performance of defrosting the main body of the outdoor fan and its peripheral members is enhanced. 
     An air conditioner according to a fourth aspect of the present invention is the air conditioner according to the first aspect of the present invention. In the air conditioner, the controller is configured to activate the compressor during the fan defrosting operation control at a specific operating frequency lower than an operating frequency during the defrosting operation. According to the air conditioner of the fourth aspect of the present invention, a low operating frequency is preferably set for the compressor during the fan defrosting operation, for instance, when pressure is equalized within the refrigerant circuit after the fan defrosting operation. Therefore, actions after the fan defrosting operation will be smoothly executed by setting a specific operating frequency for the compressor in preparation for the actions after the fan defrosting operation. 
     An air conditioner according to a fifth aspect of the present invention is the air conditioner according to the first aspect of the present invention. In the air conditioner, the predetermined period of time is allowed to be selected from options at least in an initial setting at an installation site of the air conditioner. According to the air condition of the fifth aspect of the present invention, the period of time for executing the fan defrosting operation control is set to be suitable for a climate condition of the installation site of the air conditioner. Therefore, such a situation is avoided that frost remains on the main body of the outdoor fan and its peripheral members after the fan defrosting operation control. 
     An air conditioner according to a sixth aspect of the present invention is the air conditioner according to one of the third and fourth aspects of the present invention. In the air conditioner, the controller is configured to deactivate the compressor after completion of the fan defrosting operation control and before switching to the heating operation. According to the air conditioner of the sixth aspect of the present invention, the compressor is deactivated before starting of the heating operation. Therefore, pressure is equalized within the refrigerant circuit and switching to the heating operation is safely executed. 
     An air conditioner according to a seventh aspect of the present invention is the air conditioner according to one of the third and fourth aspects of the present invention. The air conditioner further includes a refrigerant heating device configured to heat the refrigerant flowing through the refrigerant circuit. In the air conditioner, the controller is configured to activate the refrigerant heating device during the fan defrosting operation control. 
     According to the air conditioner of the seventh aspect of the present invention, the refrigerant flowing into the outdoor heat exchanger keeps its temperature high by means of activation of the refrigerant heating device during the fan defrosting operation control. Therefore, reduction in temperature is inhibited for the warm air flowing towards the main body of the outdoor fan and its peripheral members. Consequently, a performance of defrosting the main body of the outdoor fan and its peripheral members is enhanced. 
     An air conditioner according to an eighth aspect of the present invention is the air conditioner according to the seventh aspect of the present invention. In the air conditioner, the refrigerant heating device is an electromagnetic induction heater. According to the air conditioner of the eighth aspect of the present invention, the pipes are directly heated. Therefore, the refrigerant increases its temperature elevating speed. 
     Advantageous Effects of Invention 
     According to the air conditioner of the first aspect of the present invention, air elevates its temperature when passing through the outdoor heat exchanger and the warm air hits the main body of the outdoor fan and its peripheral members by means of rotations of the outdoor fan. Therefore, frost attaching thereto melts. 
     According to the air conditioner of the second aspect of the present invention, the controller is configured to determine whether or not the fan defrosting operation control is executed depending on outdoor temperature. Therefore, the fan defrosting operation is prevented from being executed uselessly. 
     According to the air conditioner of the third aspect of the present invention, the refrigerant flowing into the outdoor heat exchanger keeps its temperature high by means of activation of the compressor during the fan defrosting operation control. Therefore, reduction in temperature is inhibited for the warm air flowing towards the main body of the outdoor fan and its peripheral members. Consequently, a performance of defrosting the main body of the outdoor fan and its peripheral members is enhanced. 
     According to the air conditioner of the fourth aspect of the present invention, a specific operating frequency is set for the compressor in preparation for the actions after the fan defrosting operation. Therefore, the actions after the fan defrosting operation will be smoothly executed. 
     According to the air conditioner of the fifth aspect of the present invention, the period of time for executing the fan defrosting operation control is set to be suitable for a climate condition of the installation site of the air conditioner. Therefore, such a situation is avoided that frost remains on the main body of the outdoor fan and its peripheral members after the fan defrosting operation control. 
     According to the air conditioner of the sixth aspect of the present invention, the compressor is configured to be activated during the fan defrosting operation control but is configured to be deactivated before starting of the heating operation. Therefore, pressure is equalized within the refrigerant circuit and switching to the heating operation is safely executed. 
     According to the air conditioner of the seventh aspect of the present invention, the performance of defrosting the main body of the outdoor fan and its peripheral members is enhanced. 
     According to the air conditioner of the eighth aspect of the present invention, the pipes are directly heated. Therefore, the refrigerant increases its temperature elevating speed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a refrigeration circuit diagram of an air conditioner according to an exemplary embodiment of the present invention. 
         FIG. 2  is an external perspective view of an outdoor unit seen from the front side thereof. 
         FIG. 3  is a perspective view of the outdoor unit that a front panel, a right side panel and a rear panel are removed therefrom. 
         FIG. 4  is a perspective view of the outdoor unit that members are removed therefrom excluding a bottom plate, an outdoor heat exchanger and outdoor fans. 
         FIG. 5  is a plan view of the outdoor unit that members are removed therefrom excluding the bottom plate and a machine room. 
         FIG. 6  is a cross-sectional view of an electromagnetic induction heating unit. 
         FIG. 7  is a time chart of a fan defrosting operation and its preceding and succeeding operations for the air conditioner. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An exemplary embodiment of the present invention will be explained with reference to figures. It is noted that the following embodiment is an illustrative embodiment of the present invention and is not intended to limit the technical scope of the present invention. 
     &lt;Air Conditioner&gt; 
       FIG. 1  is a configuration diagram of an air conditioner according to an exemplary embodiment of the present invention. In the air conditioner  1  of  FIG. 1 , an outdoor unit  2  as a heat source side device and an indoor unit  4  as a user side device are connected through a refrigerant piping and a refrigerant circuit  10  is thereby formed for executing a vapor compression refrigeration cycle. 
     The outdoor unit  2  accommodates a compressor  21 , a four-way switching valve  22 , an outdoor heat exchanger  23 , an expansion valve  24 , an accumulator  25 , outdoor fans  26 , a hot gas bypass valve  27 , a capillary tube  28  and an electromagnetic induction heating unit  6 . The indoor unit  4  accommodates an indoor heat exchanger  41  and an indoor fan  42 . 
     The refrigerant circuit  10  includes a discharge pipe  10   a , a gas pipe  10   b , a liquid pipe  10   c , an outdoor liquid pipe  10   d , an outdoor gas pipe  10   e , an accumulation pipe  10   f , a suction pipe  10   g  and a hot gas bypass  10   h.    
     The discharge pipe  10   a  connects the compressor  21  and the four-way switching valve  22 . The gas pipe  10   b  connects the four-way switching valve  22  and the indoor heat exchanger  41 . The liquid pipe  10   c  connects the indoor heat exchanger  41  and the expansion valve  24 . The outdoor liquid pipe  10   d  connects the expansion valve  24  and the outdoor heat exchanger  23 . The outdoor gas pipe  10   e  connects the outdoor heat exchanger  23  and the four-way switching valve  22 . 
     The accumulation pipe  10   f  connects the four-way switching valve  22  and the accumulator  25 . The electromagnetic induction heating unit  6  is attached to a part of the accumulation pipe  10   f . The accumulation pipe  10   f  is a copper pipe and at least a heated portion thereof, covered with the electromagnetic induction heating unit  6 , is enclosed by a stainless steel pipe. Excluding the stainless steel pipe, the other pipes forming the refrigerant circuit  10  are copper pipes. 
     The suction pipe  10   g  connects the accumulator  25  and the suction side of the compressor  21 . The hot gas bypass  10   h  connects a branch point A 1  disposed in an intermediate portion of the discharge pipe  10   a  and a branch point D 1  disposed in an intermediate portion of the outdoor liquid pipe  10   d.    
     The hot gas bypass valve  27  is disposed in an intermediate portion of the hot gas bypass  10   h . A controller  11  is configured to open and close the hot gas bypass valve  27  for switching the hot gas bypass  10   h  between a refrigerant circulation permission state and a refrigerant circulation prohibition state. Further, the capillary tube  28  is disposed in the downstream of the hot gas bypass valve  27  in order to reduce the cross-sectional area of the circulation path of the refrigerant. During a defrosting operation, the refrigerant ratio is thereby kept constant between the refrigerant circulating the outdoor heat exchanger  23  and the refrigerant circulating the hot gas bypass  10   h.    
     The four-way switching valve  22  is allowed to switch between a cooling operation cycle and a heating operation cycle.  FIG. 1  depicts a connected state for executing the heating operation with a solid line and depicts a connected state for executing the cooling operation with a dotted line. During the heating operation, the indoor heat exchanger  41  functions as a condenser whereas the outdoor heat exchanger  23  functions as an evaporator. During the cooling operation, the outdoor heat exchanger  23  functions as a condenser whereas the indoor heat exchanger  41  functions as an evaporator. 
     The outdoor fans  26  are disposed in the vicinity of the outdoor heat exchanger  23  in order to supply outdoor air to the outdoor heat exchanger  23 . The indoor fan  42  is disposed in the vicinity of the indoor heat exchanger  41  in order to supply indoor air to the indoor heat exchanger  41 . 
     The controller  11  includes an outdoor control unit  11   a  and an indoor control unit  11   b . The outdoor and indoor control units  11   a  and  11   b  are connected through a communication line  11   c . Further, the outdoor control unit  11   a  is configured to control devices disposed within the outdoor unit  2  whereas the indoor control unit  11   b  is configured to control devices disposed within the indoor unit  4 . 
     (External Appearance of Outdoor Unit) 
       FIG. 2  is an external perspective view of the outdoor unit seen from its front side. 
     In  FIG. 2 , the outer shell of the outdoor unit  2  is formed in a generally rectangular cuboid shape by a top plate  2   a , a bottom plate (not visible in the figure) opposed to the top plate  2   a , a front panel  2   c , fan guards  2   k , a right side panel  2   f , a left side panel (not visible in the figure) opposed to the right side panel  2   f  and a rear panel (not visible in the figure) opposed to the front panel  2   c  and the fan guards  2   k.    
     (Inside of Outdoor Unit) 
       FIG. 3  is a perspective view of the outdoor unit that the front panel, the right side panel and the rear panel are removed therefrom. In  FIG. 3 , the outdoor unit  2  is segmented into a fan room and a machine room through a partition plate  2   h . The fan room accommodates the outdoor heat exchanger  23  and outdoor fans (not illustrated in the figure) whereas the machine room accommodates the electromagnetic induction heating unit  6 , the compressor  21  and the accumulator  25 . 
       FIG. 4  is a perspective view of the outdoor unit that members are removed therefrom excluding the bottom plate, the outdoor heat exchanger and the outdoor fans. In  FIG. 4 , the outdoor heat exchanger  23  is a fin and tube heat exchanger molded in an L shape. Two sets of the outdoor fans  26  are disposed vertically adjacent to each other through a support base while being disposed between the fan guards  2   k  (see  FIG. 3 ) and the outdoor heat exchanger  23 . When the outdoor fans  26  rotate, the outdoor air is sucked through the air holes of the left side panel and the rear panel, passes through the fins of the outdoor heat exchanger  23 , and is blown out of the fan guards  2   k.    
     (Structures of Bottom Plate and its Periphery in Outdoor Unit) 
       FIG. 5  is a plan view of the outdoor unit that members are removed therefrom excluding the bottom plate and the machine room. It should be noted that  FIG. 5  depicts the outdoor heat exchanger  23  with a two-dotted dashed line for easy understanding of the position of the outdoor heat exchanger  23 . The hot gas bypass  10   h  is disposed on the bottom plate  2   b . The hot gas bypass  10   h  is extended to the fan room from the machine room where the compressor  21  is positioned, then circulates the bottom of the fan room, and returns to the machine room. Roughly half the entire length of the hot gas bypass  10   h  is positioned under the outdoor heat exchanger  23 . Further, a part of the bottom plate  2   b , positioned under the outdoor heat exchanger  23 , includes drainage ports  86   a  to  86   e  penetrating the bottom plate  2   b  along the thickness direction of the bottom plate  2   b.    
     (Electromagnetic Induction Heating Unit) 
       FIG. 6  is a cross-sectional view of the electromagnetic induction heating unit. In  FIG. 6 , the electromagnetic induction heating unit  6  is disposed for covering the radial outside of the heated portion of the accumulation pipe  10   f . The electromagnetic induction heating unit  6  is configured to heat the heated portion by means of electromagnetic induction heating. The heated portion of the accumulation pipe  10   f  has a double pipe structure of an inner copper pipe and an outer stainless steel pipe  100   f . Either ferrite stainless containing chromium of 16 to 18% or precipitation hardening stainless containing nickel of 3 to 5%, chromium of 15 to 17.5% and copper of 3 to 5% is selected as the stainless material used for the stainless steel pipe  100   f.    
     First, the electromagnetic induction heating unit  6  is appropriately positioned with respect to the accumulation pipe  10   f . Next, the top peripheral part of the electromagnetic induction heating unit  6  is fixed to the accumulation pipe  10   f  by means of a first hexagonal nut  61 . Finally, the bottom peripheral part of the electromagnetic induction heating unit  6  is fixed to the accumulation pipe  10   f  by means of a second hexagonal nut  66 . 
     A coil  68  is helically wrapped about the outer periphery of a bobbin body  65 . The coil  68  is accommodated in the inside of a ferrite case  71 . The ferrite case  71  further accommodates first ferrite parts  69  and a second ferrite part  70 . 
     The first ferrite parts  69  are formed by molding ferrite with a high magnetic permeability. When the coil  68  is electrified, the first ferrite parts  69  form a path for magnetic fluxes together with the stainless steel pipe  100   f . The first ferrite parts  69  are disposed on the both axial ends of the ferrite case  71 . 
     The position and shape of the second ferrite part  70  are different from those of the first ferrite parts  69 . However, the function of the second ferrite part  70  is roughly the same as that of the first ferrite parts  69 . The second ferrite part  70  is disposed in the vicinity of the outer periphery of the bobbin body  65  within the accommodation part of the ferrite case  71 . 
     &lt;Actions of Air Conditioner&gt; 
     The air conditioner  1  is allowed to switch back and forth between a cooling operation and a heating operation using the four-way switching valve  22 . 
     (Cooling Operation) 
     During the cooling operation, the four-way switching valve  22  is set to be in a state depicted with the dotted line in  FIG. 1 . When the compressor  21  is operated under the condition, a vapor compression refrigeration cycle is executed in the refrigerant circuit  10  where the outdoor heat exchanger  23  functions as a condenser and the indoor heat exchanger  41  functions as an evaporator. 
     The high pressure refrigerant, discharged from the compressor  21 , exchanges heat with the outdoor air in the outdoor heat exchanger  23 , and is thereby condensed. After passing through the outdoor heat exchanger  23 , the refrigerant is decompressed while passing through the expansion valve  24 . The decompressed refrigerant subsequently exchanges heat with the indoor air in the indoor heat exchanger  41 , and is thereby evaporated. The indoor air lowers its temperature through the heat exchange with the refrigerant, and is blown out to an air conditioning target space. After passing through the indoor heat exchanger  41 , the refrigerant is sucked into the compressor  21  and is therein compressed. 
     (Heating Operation) 
     During the heating operation, the four-way switching valve  22  is set to be in a state depicted with the solid line in  FIG. 1 . When the compressor  21  is operated under the condition, a vapor compression refrigeration cycle is executed in the refrigerant circuit  10  where the outdoor heat exchanger  23  functions as an evaporator and the indoor heat exchanger  41  functions as a condenser. 
     The high pressure refrigerant, discharged from the compressor  21 , exchanges heat with the indoor air in the indoor heat exchanger  41 , and is therein condensed. The indoor air elevates its temperature through the heat exchange with the refrigerant, and is blown out to the air conditioning target space. The condensed refrigerant is decompressed while passing through the expansion valve  24 . The decompressed refrigerant subsequently exchanges heat with the outdoor air in the outdoor heat exchanger  23 , and is therein evaporated. After passing through the outdoor heat exchanger  23 , the refrigerant is sucked into the compressor  21  and is therein compressed. 
     In the activation of the heating operation, especially when the compressor  21  is not sufficiently warmed up, the compressor  21  can compress the refrigerant in a heated state by heating the accumulation pipe  10   f  using the electromagnetic induction heating unit  6 . Consequently, the gas refrigerant to be discharged from the compressor  21  elevates its temperature, and the lack of heating performance is thereby compensated in the activation of the heating operation. 
     (Defrosting Operation) 
     When the heating operation is executed, moisture contained in the air is condensed as dew on the surface of the outdoor heat exchanger  23 . The condensed dew is changed into frost or ice and covers the surface of the outside heat exchanger. The heat exchange performance of the heat exchanger is thereby reduced. The defrosting operation is therefore executed for melting the frost or ice attaching to the outdoor heat exchanger  23 . The defrosting operation is configured to be executed in the same cycle as that of the cooling operation. 
     The high pressure refrigerant, discharged from the compressor  21 , exchanges heat with the outdoor air in the outdoor heat exchanger  23 , and is thereby condensed. The heat released from the refrigerant melts the frost or ice covering the outdoor heat exchanger  23 . The refrigerant, condensed as a result of the heat release, is decompressed while passing through the expansion valve  24 . The decompressed refrigerant subsequently exchanges heat with the indoor air in the indoor heat exchanger  41 , and is thereby evaporated. The indoor fan  42  is herein kept deactivated. This is because comfortableness is deteriorated by cooled air to be brown out to the air conditioning target space when the indoor fan  42  is activated. After passing through the indoor heat exchanger  41 , the refrigerant is sucked into the compressor  21  and is therein compressed. 
     Further, during the defrosting operation, the compressor  21  can compress the refrigerant in a heated state by heating the accumulation pipe  10   f  using the electromagnetic induction heating unit  6 . Consequently, the gas refrigerant to be discharged from the compressor  21  elevates its temperature, and the defrosting performance is thereby enhanced. 
     Yet further, during the defrosting operation, the high pressure refrigerant, discharged from the compressor  21 , also flows through the hot gas bypass  10   h . Even when growing on the bottom plate  2   b  of the outdoor unit  2 , frost or ice melts by means of the heat released from the refrigerant passing through the hot gas bypass  10   h . Water herein produced is discharged through the drainage ports  86   a  to  86   e . Further, the drainage ports  86   a  to  86   e  are also heated by the hot gas bypass  10   h . Therefore, the drainage ports  86   a  to  86   e  are prevented from being clogged by the frozen moisture. 
     &lt;Other Actions of Air Conditioner&gt; 
     (Fan Defrosting Operation) 
     A fan defrosting operation refers to an operation of causing the outdoor fans  26  to rotate for a predetermined period of time after completion of the defrosting operation in order to melt the frost attaching to the main bodies of the outdoor fans  26  and their peripheral members by means of the air having passed through the outdoor heat exchanger  23 . The fan defrosting operation will be hereinafter explained with reference to the figures. 
       FIG. 7  is a time chart of the fan defrosting operation and its preceding and succeeding operations for the air conditioner. In  FIG. 7 , the fan defrosting operation is configured to be executed for a predetermined period of time by maintaining the refrigerant cycle of the defrosting operation and setting the compressor  21  to have a specific operating frequency lower than the operating frequency during the defrosting operation. The predetermined period of time is set to be suitable for the climate condition of the installation site of the air conditioner. Specifically, three stages of 60, 80 and 100 seconds are available for the settings of the predetermined period of time. Any of the stages is set as the predetermined period of time by operating a setting button in the installation of the air conditioner  1 . Consequently, a situation is avoided that frost remains on the main bodies of the outdoor fans  26  and their peripheral members after the fan defrosting operation control. In the installation of the air conditioner  1 , however, such a setting is also available that prevents the air conditioner  1  from executing the fan defrosting operation. Alternatively, the predetermined period of time can be set anytime excluding in the installation of the air conditioner  1 . Further, execution/non-execution of the fan defrosting operation also can be set anytime excluding in the installation of the air conditioner  1 . 
     During the fan defrosting operation, the outdoor fans  26  rotate at a relatively low rotation speed. The rotation speed of the outdoor fans  26  can be switched in a range of steps 1 to 8 (excluding deactivation). The third lowest step 3 is selected during the fan defrosting operation. It should be noted the outdoor fans  26  are deactivated during the defrosting operation to be executed before the fan defrosting operation. 
     The fan defrosting operation is not always executed but is executed only when a predetermined condition is satisfied immediately before the start of the defrosting operation. The defrosting operation is normally executed under the condition that a predetermined period of time elapses after the previous defrosting operation and both of the outdoor temperature and the outdoor heat exchanger temperature are lower than or equal to a preliminarily set temperature. On the other hand, the fan defrosting operation is executed after completion of the defrosting operation when the outdoor temperature immediately before the start of the defrosting operation falls in a range of −5 to 5 degrees Celsius. It should be noted that the outdoor temperature is measured through an outdoor temperature sensor  102  attached to the outdoor unit  2 . 
     For example, frost attaches not only to the outdoor heat exchanger  23  but also to the fan guards  2   k  when the heating operation is executed under a high-humidity and low-temperature (roughly 0 degrees Celsius) circumstance. In the present exemplary embodiment, the outdoor fans  26  are propeller fans. When each outdoor fan  26  is of a type including a bell mouth in the surrounding of the propeller fan, frost also attaches to the bell mouth. Alternatively when each outdoor fan  26  is a turbo fan, frost also attaches to a fan blade. Even when the defrosting operation is completed under the condition, this results in only melting of the frost attaching to the outdoor heat exchanger  23  and does not result in melting of the frost attaching to, for instance, the fan guards  2   k  disposed in the surrounding of the outdoor fans  26 . According to the present exemplary embodiment, however, the outdoor fans  26  are activated by the fan defrosting operation. The air warmed by the outdoor heat exchanger  23  is supplied to the main bodies of the outdoor fans  26  and the peripheral members of the main bodies of the outdoor fans  26  such as the fan guards  2   k . Therefore, the frost attaching to the fan guards  2   k  and the like is also warmed and thereby melts. 
     Further, the compressor  21  is herein activated. Therefore, the refrigerant flowing into the outdoor heat exchanger  23  keeps its temperature high and this enhances the defrosting performance. Yet further, the compressor  21  can compress the refrigerant in a warmed state by heating the accumulation pipe  10   f  using the electromagnetic induction heating unit  6 . Therefore, the gas refrigerant discharged from the compressor  21  elevates its temperature and the refrigerant flowing into the outdoor heat exchanger  23  further elevates its temperature. This further enhances the defrosting performance. Consequently, a required time to melt the frost is reduced. 
     (Pressure Equalization Operation) 
     After completion of the fan defrosting operation, a pressure equalization operation is executed by deactivating the compressor  21  but activating the outdoor fans  26 . It should be noted that the pressure equalization operation is executed after completion of the defrosting operation when the fan defrosting operation is not executed. 
     The rotation speed of the step 6, greater than the rotation speed during the fan defrosting operation, is selected as the rotation speed of the outdoor fans  26  during the pressure equalization operation. An object of the pressure equalization operation is to eliminate pressure difference within the refrigerant circuit  10  or reduce the pressure difference to be equal to or less than a predetermined value. In the present exemplary embodiment, the pressure equalization operation is executed until 80 seconds elapses or the pressure difference within the refrigerant circuit  10  is equal to or less than 0.49 MPa after completion of the fan defrosting operation. Suppose the refrigerant cycle is switched into the heating operation without executing the pressure equalization operation, the devices such as the four-way switching valve  22  are subjected to negative effects due to the impact of the pressure difference within the refrigerant circuit  10 . 
     Prior to the pressure equalization operation, the compressor  21  preferably has a low operating frequency for quickly reducing the pressure difference within the refrigerant circuit  10  to be less than or equal to a predetermined value (0.49 MPa). In consideration of this, during the fan defrosting operation preceding the pressure equalization operation, the compressor  21  is set to have a specific operating frequency lower than the operating frequency during the defrosting operation. 
     &lt;Features&gt; 
     (1) 
     In the air conditioner  1 , the controller  11  is configured to execute the fan defrosting operation control of activating the outdoor fans  26  for a preliminarily set period of time after completion of the defrosting operation when the outdoor temperature falls in a range of −5 to 5 degrees Celsius immediately before the start of the heating operation. Consequently, this results in melting of the frost attaching to the main bodies of the outdoor fans  26  and their peripheral members (e.g., bell mouths and fan guards). 
     (2) 
     During the fan defrosting operation, the controller  11  activates the compressor  21  at a specific operating frequency lower than the operating frequency during the defrosting operation. Consequently, the refrigerant flowing into the outdoor heat exchanger  23  keeps its temperature high. This inhibits reduction in temperature of the warmed air flowing towards the main bodies of the outdoor fans  26  and their peripheral members. 
     (3) 
     The controller  11  is configured to deactivate the compressor  21  after completion of the fan defrosting operation control and immediately before switching of the refrigeration cycle into the heating operation in order to execute the pressure equalization operation of reducing the pressure difference within the refrigerant circuit  10 . Consequently, switching of the refrigeration cycle into the heating operation is safely executed. 
     INDUSTRIAL APPLICABILITY 
     The present invention is useful for the air conditioners intended to a cold and high humidity region.