Patent Publication Number: US-2007107255-A1

Title: Drying apparatus

Description:
TECHNICAL FIELD  
      The present invention relates to a drying apparatus used for drying clothing or bathroom, or used for dehumidify a room.  
     BACKGROUND OF THE INVENTION  
      As a conventional drying apparatus, there is a cloth drier in which a heat pump apparatus is used as a heat source and drying air is circulated (see patent document 1, for example).  FIG. 11  shows a structure of the conventional drying apparatus described in the patent document 1.  
      In the drying apparatus (cloth drier) shown in  FIG. 11 , a rotation drum  2  is used as a drying room. The rotation drum  2  is provided in a body  1  of the clothing dryer so as to rotate freely. The rotation drum  2  is driven by a motor  3  through a drum belt  4 . Further, a blower  22  is driven by the motor  3  through a fan belt  8 . Drying air is sent from the rotation drum  2  to a circulation duct  18  through a filter  29  and a rotation drum-side air intake  10  by the blower  22 .  
      Further, the heat pump apparatus is provided with: an evaporator  23  which evaporates a refrigerant to dehumidify drying air; a condenser  24  for condensing the refrigerant to heat the drying air; a compressor  25  for generating a pressure difference in the refrigerant; an expansion mechanism  26  such as a capillary tube for maintaining the pressure difference in the refrigerant; and a pipe  27  through which the refrigerant passes. A portion of the drying air heated by the condenser  24  is discharged outside from the body  1  through an exhaust port  28 . An allow B shows flow of the drying air.  
      Next, the operation of the cloth drier shown in  FIG. 11  will be explained. First, clothing  21  to be dried is placed in the rotation drum  2 . Then, when the motor  3  is rotated, the rotation drum  2  and the blower  22  rotate, whereby the flow B of the drying air is generated. The drying air absorbs water from the clothing  21  in the rotation drum  2  and takes up much moisture, and then, the drying air is sent to the evaporator  23  of the heat pump apparatus through the circulation duct  18  by the blower  22 . The drying air from which heat is absorbed by the evaporator  23  is dehumidified and sent to the condenser  24  to be heated therein, and the air is again circulated into the rotation drum  2 . A drain outlet  19  is provide in a middle portion of the circulation duct  18 , and a drain generated by dehumidifying the drying air in the evaporator  23  is discharged out through the drain outlet  19 . As a result, the clothing  21  is dried.  
      (Patent Document 1)  
      Japanese Patent Application Laid-open No. H7-178289  
      However, the cloth drier shown in  FIG. 11  cannot control a superheat value that changes in the drying process.  
      In this regard, a reason why the superheat value changes with the progress of the drying operation will be explained. Generally, in the case where a solid body is to be dried using warm air, content of water on a surface of the solid body to be dried is reduced with the progress of the drying operation, whereby the drying speed is reduced. In other words, with the progress of the drying operation, the amount of water included in the drying air after passing through a subject to be dried is reduced, and the absolute humidity of suction air in the evaporator  23  is reduced. This makes an endothermic value due to condensation of water in the evaporator  23  lower, whereby the superheat value is reduced. If the superheat value becomes zero, the refrigerant sucked in the compressor  25  becomes gas-liquid two-phase state. Therefore, in the case where the compressor  25  carries out compression of the liquid refrigerant, a risk that the compressor is damaged may occur.  
      Further, there is relation, as shown in  FIG. 9 , between a superheat value (SH) and coefficient of performance (COP) of the heat pump apparatus (that is, heating capability/compressor input), and an optimum superheat value exists therein. This principle is shown in  FIG. 10 . In the case where the superheat value becomes too large (SH Large), a workload of the compressor (that is, an enthalpy difference between those in suction and discharge conditions when the refrigerant is adiabatically compressed from a compressor suction condition) is increased as compared with the case in the optimum superheat value (Optimum SH), whereby the heat pump performance is deteriorated. On the other hand, in the case where the superheat value becomes too small (SH Small), compressor discharge temperature is lowered, and heating performance is deteriorated, whereby the heat pump performance is deteriorated. For this reason, if the superheat value can be controlled to the optimum value, it is possible to reduce power consumption required for the drying operation.  
      It is therefore an object of the present invention to provide a drying apparatus which can avoid liquid back to a compressor that has been a conventional problem by controlling a superheat value to a predetermined value.  
      Further, it is known that, as a general drying property, a drying layer between an evaporation surface and a surface of a subject to be dried becomes heat transfer resistance, and heat quantity from drying air to moisture that exists on the evaporation surface is lowered. For this reason, in the case where the heat pump apparatus operates so as to maintain the optimum superheat value as shown in  FIG. 9  even close to completion of drying, it tends to make the drying time become longer.  
      It is therefore another object of the present invention to provide a drying apparatus that can reduce drying time by changing a superheat value.  
     DISCLOSURE OF THE INVENTION  
      A first aspect of the present invention provides a drying apparatus including a heat pump apparatus composed by sequentially connecting in series: a compressor that compresses a refrigerant; a radiator that radiates the refrigerant discharged from the compressor; an expansion valve that expands the refrigerant radiated in the radiator; and an evaporator that evaporates the refrigerant expanded by the expansion valve, and an air channel in which drying air heated in the radiator is introduced to a subject to be dried. In this case, the drying air that absorbs moisture from the subject to be dried is dehumidified in the evaporator, and the dehumidified air is then heated in the radiator again to reuse the dehumidified air as the drying air. The drying apparatus includes:  
      a first temperature sensor for detecting the temperature of the refrigerant between the outlet of the evaporator and the inlet of the compressor; and  
      control means for controlling a superheat value by changing flow resistance of the expansion valve based on a detected value of the first temperature sensor.  
      According to the first aspect, it is possible to maintain an optimum superheat value by changing flow resistance of the expansion valve based on the detected value of the first temperature sensor.  
      A second aspect of the present invention is characterized that, in the drying apparatus of the first aspect, the drying apparatus further includes:  
      storage means for storing correlation data between time elapsing from start of operation of the heat pump apparatus and evaporation temperature of the refrigerant in the evaporator, and a target superheat value in advance;  
      a timer for detecting operation time of the heat pump apparatus; and  
      processing means which estimates the evaporation temperature of the refrigerant based on the operation time detected by the timer and the correlation data stored in the storage means, and then estimates a superheat value based on the estimated evaporation temperature and the detected value detected by the first temperature sensor,  
      wherein the control means controls the flow resistance of the expansion valve so that the superheat value estimated by the processing means becomes the target superheat value stored in the storage means.  
      According to the second aspect, it is possible to control the estimated superheat value so as to become the target superheat value in the drying process, and this makes it possible to reduce power consumption or time required for the drying process.  
      A third aspect of the present invention is characterized that, in the drying apparatus of the first aspect, the drying apparatus further includes:  
      storage means for storing a target superheat value in advance;  
      second temperature sensor for detecting the temperature of the refrigerant between the outlet of the expansion valve and the inlet of the evaporator; and  
      processing means which calculates a superheat value based on a detected value detected by the second temperature sensor and the detected value detected by the first temperature sensor,  
      wherein the control means controls the flow resistance of the expansion valve so that the superheat value calculated by the processing means becomes the target superheat value stored in the storage means.  
      According to the third aspect, it is possible to measure the superheat value in the drying process more precisely.  
      A fourth aspect of the present invention is characterized that, in the drying apparatus of the second aspect, the control means controls the flow resistance of the expansion valve so that the superheat value after the operation time of the heat pump apparatus elapses beyond predetermined time becomes larger than that before the predetermined time elapses.  
      According to the fourth aspect, by making the superheat value become larger after the operation time of the heat pump apparatus elapses beyond the predetermined time, it is possible to shorten the drying time.  
      A fifth aspect of the present invention is characterized that, in the drying apparatus of the third aspect, the drying apparatus further includes:  
      a timer for detecting operation time of the heat pump apparatus, wherein the control means controls the flow resistance of the expansion valve so that the superheat value after the operation time of the heat pump apparatus elapses beyond predetermined time becomes larger than that before the predetermined time elapses.  
      According to the fifth aspect, by making the superheat value become larger after the operation time of the heat pump apparatus elapses beyond the predetermined time, it is possible to shorten the drying time.  
      A sixth aspect of the present invention is characterized that, in the drying apparatus of the fourth or fifth aspect, the drying apparatus further includes:  
      selection means for selecting whether to apply the superheat value larger than that before the predetermined time elapses to that after predetermined time elapses or not.  
      According to the sixth aspect, it is possible to select either reduction of the power consumption or shortening of the drying time in response to intention of a user of the drying apparatus.  
      A seventh aspect of the present invention is characterized that, in the drying apparatus of the first aspect, the drying apparatus further includes:  
      a third temperature sensor for detecting the temperature of the refrigerant between the outlet of the compressor and the inlet of the expansion valve.  
      According to the seventh aspect, it is possible to measure the temperature of the refrigerant discharged from the compressor in addition to the superheat value.  
      An eighth aspect of the present invention is characterized that, in the drying apparatus of the sixth aspect, in the case where a detected value detected by the third temperature sensor becomes predetermined temperature or more, the control means controls the expansion valve so as to make the flow resistance of the expansion valve smaller.  
      According to the eighth aspect, it is possible to prevent any component of the compressor (for example, a seal member) or refrigerating machine oil from deteriorating due to abnormal rise in the temperature of the refrigerant in the drying process, and this makes it possible to enhance the reliability of the compressor.  
      A ninth aspect of the present invention is characterized that, in the drying apparatus of the first aspect, the drying apparatus further includes:  
      discharge pressure detecting means for detecting discharge pressure of the compressor.  
      According to the ninth aspect, it is possible to measure the pressure of the refrigerant discharged from the compressor in addition to the superheat value.  
      A tenth aspect of the present invention is characterized that, in the drying apparatus of the eighth aspect, in the case where a detected value detected by the discharge pressure detecting means becomes predetermined pressure or more, the control means controls the expansion valve so as to make the flow resistance of the expansion valve smaller.  
      According to the tenth aspect, since the refrigerant pressure does not exceed an upper limit value of withstanding pressure in the drying process, it is possible to enhance the reliability of the drying apparatus.  
     EFFECT OF THE INVENTION  
      According to the drying apparatus of the present invention, it is possible to control the superheat value to a target value in the drying process. Therefore, it is possible to avoid liquid back to the compressor that has been a conventional problem, and to shorten the drying time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows a structure of a drying apparatus of a first embodiment according to the present invention;  
       FIG. 2  shows a control flowchart of the drying apparatus of the first embodiment;  
       FIG. 3  shows a structure of a drying apparatus of a second embodiment according to the present invention;  
       FIG. 4  is a control flowchart of the drying apparatus of the second embodiment;  
       FIG. 5  shows a structure of a drying apparatus of a third embodiment according to the present invention;  
       FIG. 6  is a control flowchart of the drying apparatus of the third embodiment;  
       FIG. 7  shows a structure of a drying apparatus of a fourth embodiment according to the present invention;  
       FIG. 8  is a control flowchart of the drying apparatus of the fourth embodiment;  
       FIG. 9  shows a relation between the superheat and COP (coefficient of performance);  
       FIG. 10  shows a Mollier diagram showing behavior of a refrigeration cycle when the superheat is changed; and  
       FIG. 11  shows a structure of the conventional drying apparatus described in the patent document 1. 
    
    
     DESCRIPTION OF THE NUMERALS  
       11  storage means  
       12  operating time detecting means (Timer)  
       13  processing means  
       14  control means  
       31  compressor  
       32  radiator  
       33  expansion valve  
       34  evaporator  
       35  pipe  
       36  subject to be dried  
       37  blowing fan  
       38  first pipe temperature detecting means (first temperature sensor)  
       39  second pipe temperature detecting means (second temperature sensor)  
       40  third pipe temperature detecting means (third temperature sensor)  
       41  circulation duct (air channel)  
       42  discharge pressure detecting means  
     BEST MODE FOR CARRYING OUT THE INVENTION  
     First Embodiment  
      Preferred embodiments of the present invention will be explained with reference to the appending drawings.  FIG. 1  shows a structure of a drying apparatus of a first embodiment according to the present invention.  FIG. 2  shows a control flowchart of the drying apparatus of the first embodiment.  
      Referring to  FIG. 1 , a drying apparatus of the present embodiment includes a heat pump apparatus, and an air channel  41  in which the heat pump apparatus is used as a heat source for drying a subject to be dried and drying air is circulated and reused. The heat pump apparatus includes: a compressor  31  for compressing a refrigerant; a radiator  32  for condensing the refrigerant by heat radiation effect to heat the drying air; an expansion valve  33  for reducing the pressure of the refrigerant; and an evaporator  34  for evaporating the refrigerant by endothermic effect to dehumidify the drying air. These elements of the heat pump apparatus are connected in series to one another through a pipe  35  in this order. As the refrigerant used for this heat pump apparatus, a refrigerant which can be brought into a supercritical state on the radiation side (that is, a discharge portion of the compressor  31  to the radiator  32  to the inlet of the expansion valve  33 ), for example, carbon dioxide or the like is charged into the pipe  35 .  
      Further, in the air channel  41  of the drying apparatus, the radiator  32  and the evaporator  34  are disposed. The radiator  32  and the evaporator  34  respectively heat and dehumidify drying air which absorbs moisture from a subject to be dried  36  (for example, clothing, bathroom and the like). The drying air is circulated in the air channel  41  by a blowing fan  37 .  
      Moreover, in the present embodiment, the drying apparatus is provided with a first temperature sensor  38  for detecting refrigerant temperature (compressor suction refrigerant temperature) T 1  between the outlet of the evaporator  34  and the inlet of the compressor  31 . In this case, a method of detecting the refrigerant temperature by means of the first temperature sensor  38  includes a method of directly measuring the refrigerant temperature and a method of indirectly measuring the refrigerant temperature by detecting pipe temperature.  
      Furthermore, in the present embodiment, the drying apparatus further includes storage means  11 , a timer  12 , processing means  13 , and control means  14 . Correlation data between time elapsing from start of operation of the heat pump apparatus and evaporation temperature of the refrigerant in the evaporator  34 , and a target superheat value are stored in the storage means  11  in advance. The timer  12  detects operation time of the heat pump apparatus by detecting temperature and/or humidity in the air channel  41  in addition to detection by means of count-up. The processing means  13  estimates the evaporation temperature of the refrigerant based on the operation time detected by the timer  12  and the correlation data stored in the storage means  11 , and then estimates a superheat value based on the estimated evaporation temperature and the detected value detected by the first temperature sensor  38 . The control means  14  controls flow resistance of the expansion valve  33  so that the superheat value estimated by the processing means  13  becomes the target superheat value stored in the storage means  11 . If the pressure or transition of evaporation temperature of the evaporator  34  in accordance with the operation time of the heat pump apparatus is grasped in advance, it is possible to estimate the evaporation temperature at the time using detected values detected by the timer  12  and the first temperature sensor  38 . Thus, a superheat value can be obtained as the difference between the estimated evaporation temperature and the detected value by the first temperature sensor  38 . In this regard, in  FIG. 1 , solid arrows represent the flow of the refrigerant, while hollow arrows represent the flow of the drying air.  
      Next, the operation of the drying apparatus will be explained.  
      The refrigerant is compressed by the compressor  31  to be brought into a high temperature and high pressure state, and the refrigerant exchanges heat with drying air discharged from the evaporator  34  in the radiator  32  to heat the drying air. The refrigerant cooled in the radiator  32  is decompressed by the expansion valve  33  to become a low temperature and low pressure state. Then, the refrigerant decompressed by the expansion valve  33  exchanges heat with the drying air that has passed through the subject to be dried  36  in the evaporator  34  to cool the drying air. The refrigerant dehumidifies the drying air by condensing moisture included in the drying air, and is heated by the drying air to be sucked in the compressor  31  again. That is a principle of the operation of the heat pump.  
      Further, the drying air is dehumidified by the evaporator  34 , and then heated in the radiator  32  to become a high temperature and low humidity state. When the drying air is forcibly brought into contact with the subject to be dried  36  by the blowing fan  37 , the drying air absorbs moisture from the subject to be dried  36  to become a high humidity state, and is then dehumidified by the evaporator  34  again. That is a principle of the drying operation in which moisture is absorbed from the subject to be dried  36 .  
      In this regard, in the case where flow resistance of the expansion valve  33  is made to be larger, the temperature of the refrigerant sucked in the compressor  31  is raised. This is because the pressure of the suction side (from the outlet of the expansion valve  33  to the suction portion of the compressor  31  through the evaporator  34 ) becomes lower and the amount of the refrigerant in the evaporator  34  is decreased, whereby the refrigerant is easily evaporated and overheated, if the flow resistance of the expansion valve  33  is made to be larger. Therefore, if the flow resistance of the expansion valve  33  is made to be smaller, the temperature of the refrigerant sucked in the compressor  31  can be lowered.  
      Next, the control operation of the drying apparatus will be explained.  
      As shown in  FIG. 2 , operation time t of the heat pump apparatus is detected by the timer  12 , and evaporator pressure Pe (=evaporation temperature Te) are estimated using Table of the operation time t and the evaporator pressure Pe (=evaporation temperature Te) prepared in advance (Step  41 ). Suction temperature Ts of the compressor  31  is then detected by the first temperature sensor  38 , and a superheat value TSH (=Ts−Te) is estimated using the detected value Ts and the evaporation temperature Te estimated at Step  41  (Step  42 ). Next, the superheat value TSH estimated at Step  42  is compared with the target superheat value TC (Step  43 ). In the case where the superheat value TSH is larger than the target superheat value TC at Step  43 , the control means  14  controls flow resistance of the expansion valve  33  to become smaller (Step  44 B), and then, the procedure is returned to Step  41 . On the other hand, in the case where the superheat value TSH is smaller than the target superheat value TC at Step  43 , the control means  14  controls flow resistance of the expansion valve  33  to become larger (Step  44 A), and then, the procedure is returned to Step  41 .  
      In the present control operation, by using the detected values of the timer  12  and the first temperature sensor  38 , it is possible to control the superheat value to a value close to the optimum value at which the COP becomes maximal.  
      In the drying apparatus of the present embodiment, the superheat value can converge in the vicinity of the target value, and this makes it possible to avoid lowering the heat pump performance (COP). Namely, it is possible to reduce power consumption as compared with a conventional drying apparatus. In other words, since it is possible to avoid deteriorating the operating efficiency of the drying apparatus, it is possible to utilize CO 2  refrigerant which hardly has impact on the global warming.  
      The drying apparatus of the present embodiment uses a transition critical refrigeration cycle using CO 2  refrigerant. Therefore, as compared with a conventional subcritical refrigeration cycle using HFC refrigerant, heat exchanging efficiency between CO 2  refrigerant and the drying air in the radiator  32  can be enhanced, and the temperature of the drying air can be increased to high temperature. Thus, the ability for absorbing moisture from the subject to be dried  36  is increased, and this makes it possible to dry the subject to be dried  36  within a short time.  
      In this regard, in the present embodiment, CO 2  refrigerant which is brought into supercritical state on the radiation side is used, but the conventional HFC refrigerant may be used. Further, even in the case where HC refrigerant such as propane and isobutene is used, the same effect can be obtained.  
     Second Embodiment  
       FIG. 3  shows a structure of a drying apparatus of a second embodiment according to the present invention.  FIG. 4  is a control flowchart of the drying apparatus of the second embodiment. In this regard, in the following explanation for the second embodiment, the same structures as those of the first embodiment are designated with the same symbols, explanation thereof will be omitted, and the structures of the second embodiment which are different from those of the first embodiment will be explained.  
      The drying apparatus of the present embodiment includes a second temperature sensor  39  for detecting the refrigerant temperature between the outlet of an expansion valve  33  and the inlet of an evaporator  34  in addition to the structure of the drying apparatus of the first embodiment, and processing means calculates a superheat value based on the difference between the detected values of a first temperature sensor  38  and the second temperature sensor  39 . Further, a plurality of values as target superheat values and predetermined time values for applying each of the target superheat values to the heat pump apparatus are stored in the storage means  11 . In this case, if the second temperature sensor  39  is applied to a portion in which liquid refrigerant exists, the second temperature sensor  39  may be mounted on the body of the evaporator  34 .  
      Hereinafter, the control operation of the drying apparatus of the second embodiment will be explained.  
      As shown in  FIG. 4 , operation time t of the heat pump apparatus detected by a timer  12  is compared with predetermined time t 1  stored in the storage means  11  (Step  51 ). In the case where the operation time t is longer than the predetermined time t 1  at Step  51 , a superheat value TSH 1  obtained from the difference between the detected values of the first temperature sensor  38  and the second temperature sensor  39  is compared with a target superheat value TC 1  (Step  52 ). In the case where the superheat value TSH 1  is larger than the target superheat value TC 1  at Step  52 , control means  14  controls flow resistance of the expansion valve  33  to become smaller (Step  53 A), and then, the procedure is returned to Step  52 . On the other hand, in the case where the superheat value TSH 1  is smaller than the target superheat value TC 1  at Step  52 , the control means  14  controls flow resistance of the expansion valve  33  to become larger (Step  53 B), and then, the procedure is returned to Step  52 .  
      Further, in the case where the operation time t is shorter than the predetermined time t 1  at Step  51 , a superheat value TSH 2  obtained from the difference between the detected values of the first temperature sensor  38  and the second temperature sensor  39  is compared with a target superheat value TC 2  (Step  54 ). In the case where the superheat value TSH 2  is larger than the target superheat value TC 2  at Step  54 , the control means  14  controls flow resistance of the expansion valve  33  to become smaller (Step  55 A), and then, the procedure is returned to Step  51 . On the other hand, in the case where the superheat value TSH 2  is smaller than the target superheat value TC 2  at Step  54 , the control means  14  controls flow resistance of the expansion valve  33  to become larger (Step  55 B), and then, the procedure is returned to Step  51 . In this regard, the target superheat value TC 2  is a superheat value at which the COP becomes optimal, and the target superheat value TC 1  is set to a superheat value larger than the target superheat value TC 2 .  
      According to the control operation, since the superheat value is made to become larger after predetermined time elapses from the start of the drying process, the temperature of the drying air can be raised. Thus, by adding selection means (not shown in the drawings) for selecting whether the target superheat value TC 2  is applied to the heat pump apparatus or not to the drying apparatus of the present embodiment, it is possible to select either reduction of the power consumption or shortening of the drying time in response to intention of a user of the drying apparatus. In this case, although the case where the target superheat value is changed from the target superheat value TC 2  to the target superheat value TC 1  at the predetermined time t 1  has been explained in the present embodiment, the target superheat value may be raised with three steps or more, or may be raised consecutively. Moreover, in the first embodiment described above, the plurality of target superheat values may be set as the present embodiment. In the case of setting the plurality of target superheat values, it is preferable to add selecting means (not shown in the drawings) to the drying apparatus of the first embodiment.  
     Third Embodiment  
       FIG. 5  shows a structure of a drying apparatus of a third embodiment according to the present invention.  FIG. 6  is a control flowchart of the drying apparatus of the third embodiment. In this regard, in the following explanation for the third embodiment, the same structures as those of the second embodiment are designated with the same symbols, explanation thereof will be omitted, and the structures of the second embodiment which are different from those of the second embodiment will be explained.  
      The drying apparatus of the present embodiment includes a third temperature sensor  40  for detecting the temperature of the refrigerant between the outlet of a compressor  31  and the inlet of an expansion valve  33  in addition to the structure of the drying apparatus of the second embodiment. Control means  14  controls flow resistance of the expansion valve  33  using the difference (that is, a superheat value) between the detected values of a first temperature sensor  38  and a second temperature sensor  39  and the detected value from the third temperature sensor  40 . In this case, the drying apparatus of the third embodiment does not include a timer  12  for detecting operation time of the drying apparatus with which the drying apparatus of the second embodiment is provided.  
      Hereinafter, the control operation of the drying apparatus of the third embodiment will be explained.  
      As shown in  FIG. 6 , suction temperature Td detected by the third temperature sensor (suction temperature detecting means)  40  is compared with preset temperature Tm (for example, 100° C.) (Step  61 ). In the case where the suction temperature Td is higher than the preset temperature Tm at Step  61 , the control means  14  controls flow resistance of the expansion valve  33  to become smaller (Step  64 ), and then, the procedure is returned to Step  61 . In the case where the suction temperature Td is lower than the preset temperature Tm at Step  61 , a superheat value TSH obtained from the difference between the detected values of the first temperature sensor  38  and the second temperature sensor  39  is compared with a target superheat value Ta (for example, 10 deg) (Step  62 ). In the case where the superheat value TSH is larger than the target superheat value Ta at Step  62 , the control means  14  controls flow resistance of the expansion valve  33  to become smaller (Step  64 ), and then, the procedure is returned to Step  61 . On the other hand, in the case where the superheat value TSH is smaller than the target superheat value Ta at Step  62 , the control means  14  controls flow resistance of the expansion valve  33  to become larger (Step  63 ), and then, the procedure is returned to Step  61 .  
      Generally, in the case where the superheat value is increased, the compressor suction temperature is increased and the compressor discharge temperature is increased. However, in the drying apparatus of the third embodiment, by detecting the discharge temperature of the compressor  31  and the superheat value and controlling flow resistance of the expansion valve  33  based on the detected values, the superheat value can converge in the vicinity of the target value at which the COP becomes maximal in a state where the discharge temperature does not exceed a permissible range of the compressor  31 . Thus, it is possible to prevent employed materials of the compressor  31  (for example, a seal member) or refrigerating machine oil from deteriorating, and this makes it possible to exert the maximum heat pump performance while ensuring the reliability of the compressor  31  more surely. In other words, the heat pump apparatus can carry out the heat pump cycle operation with stability and high efficiency. In this regard, even in the present embodiment, the superheat value may be made to become larger after predetermined time elapses from the start of the drying process as the second embodiment, and the temperature of the drying air may be raised. Further, by adding determining means for determining whether the target superheat value Ta is applied to the heat pump apparatus or not, it is possible to select either reduction of the power consumption or shortening of the drying time in response to intention of a user of the drying apparatus. In addition, the target superheat value may be raised with three steps or more even in the present embodiment.  
     Fourth Embodiment  
       FIG. 7  shows a structure of a drying apparatus of a fourth embodiment according to the present invention.  FIG. 8  is a control flowchart of the drying apparatus of the fourth embodiment.  
      The drying apparatus of the present embodiment includes discharge pressure detecting means  42  for detecting the discharge pressure of a compressor  31  in addition to the structure of the drying apparatus of the second embodiment. Control means  14  controls flow resistance of an expansion valve  33  using the difference (that is, a superheat value) between the detected values of a first temperature sensor  38  and a second temperature sensor  39  and the detected value from the discharge pressure detecting means  42 . In this case, the drying apparatus of the fourth embodiment does not include a timer  12  for detecting operation time of the drying apparatus with which the drying apparatus of the second embodiment is provided.  
      Hereinafter, the control operation of the drying apparatus of the fourth embodiment will be explained.  
      As shown in  FIG. 8 , discharge pressure Pd detected by the discharge pressure detecting means  42  is compared with preset pressure Pm (for example, 12 MPa) (Step  71 ). In the case where the discharge pressure Pd is higher than the preset pressure Pm at Step  71 , the control means  14  controls flow resistance of the expansion valve  33  to become smaller (Step  74 ), and then, the procedure is returned to Step  71 . In the case where the discharge pressure Pd is lower than the preset pressure Pm at Step  71 , a superheat value TSH obtained from the difference between the detected values of the first temperature sensor  38  and the second temperature sensor  39  is compared with a target superheat value Tb (for example, 10 deg) (Step  72 ). In the case where the superheat value TSH is larger than the target superheat value Tb at Step  72 , the control means  14  controls flow resistance of the expansion valve  33  to become smaller (Step  74 ), and then, the procedure is returned to Step  71 . On the other hand, in the case where the superheat value TSH is smaller than the target superheat value Tb at Step  72 , the control means  14  controls flow resistance of the expansion valve  33  to become larger (Step  73 ), and then, the procedure is returned to Step  71 .  
      Generally, when the flow resistance of the expansion valve  33  is made to be larger in order to increase the superheat value, the compressor discharge pressure is increased. However, in the drying apparatus of the fourth embodiment, by detecting the discharge pressure of the compressor  31  and the superheat value and controlling flow resistance of the expansion valve  33  based on the detected values, the superheat value can converge in the vicinity of the target value at which the COP becomes maximal in a state where the discharge pressure does not exceed a permissible range of the compressor  31 . Thus, the heat pump apparatus can carry out the heat pump cycle operation below the withstanding pressure of a shell of the compressor  31 , and this makes it possible to exert the maximum heat pump performance while ensuring the reliability of the compressor  31  more surely. In other words, the heat pump apparatus can carry out the heat pump cycle operation with stability and high efficiency. In this regard, even in the present embodiment, the superheat value is made to become larger after predetermined time elapses from the start of the drying process as the second embodiment, and the temperature of the drying air may be raised. Further, by adding determining means for determining whether the target superheat value Ta is applied to the heat pump apparatus or not to the drying apparatus of the present embodiment, it is possible to select either reduction of the power consumption or shortening of the drying time in response to intention of a user of the drying apparatus. In addition, the target superheat value may be raised with three steps or more even in the present embodiment.  
     INDUSTRIAL APPLICABILITY  
      The drying apparatus of the present invention can suitably be used for drying clothing, bathroom and the like. Further, the drying apparatus can also be used for other application such as for drying tableware, garbage and the like.