Patent Publication Number: US-2015076133-A1

Title: Heating-medium heating unit and vehicle air conditioner using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 13/499,994 filed on Apr. 3, 2012, which is a National Stage Application of PCT/JP2011/058417 filed on Apr. 1, 2011, which is based on and claims the benefit of priority from Japanese Patent Application No. 2010-093294, filed Apr. 14, 2010, which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a heating-medium heating unit that heats a heating medium using a positive temperature coefficient (PTC) heater, and to a vehicle air conditioner using the same. 
     BACKGROUND ART 
     One known heating-medium heating unit for heating a medium to be heated uses a PTC heater that uses a positive temperature coefficient thermistor device (PTC device) as a heating element. The PTC heater has a positive temperature thermistor coefficient and thus shows an increasing value of resistance as the temperature increases, which allows the current consumption to be controlled and the increase in temperature to be slowed, and thereafter, the current consumption and the temperature of the heat generating portion reach a saturation region and are stabilized; that is, the PTC heater has a self temperature control characteristic. 
     The PTC heater has the characteristic that the current consumption is reduced as the temperature of the heater increases, and thereafter when the temperature reaches a saturation region of a fixed temperature, the current consumption stabilizes at a low value. The use of this characteristic provides advantages in that current consumption can be reduced and an abnormal increase in the temperature of the heat generating portion can be prevented. 
     Therefore, PTC heaters are used in many technical fields. Also in the field of air conditioning, as disclosed in PTL 1, for example, in a hybrid-vehicle air conditioner, a heating-medium heating unit in which the PTC heater is applied to a heating unit for heating a heating medium (here, engine coolant) to be supplied to a radiator for heating air when the engine is stopped has been proposed. 
     In this heating-medium heating unit, two heating-medium circulation boxes are joined to each other via an O-ring in a liquidtight manner, and a flat PTC heater is closely interposed between the two heating-medium circulation boxes. The heating-medium circulation boxes are each configured such that a plurality of box components are joined via O-rings in a liquidtight manner, and the heating-medium circulation boxes each have therein a circulation path through which engine coolant, which is a heating medium, circulates. 
     The heating-medium circulation boxes each have a flat radiating surface in close contact with the PTC heater, and a grooved level-difference portion is formed between the flat surface and a joining surface formed on the outer periphery of each heating-medium circulation box (box component) (see FIG. 5 in PTL 1). 
     This is for the purpose of preventing the O-rings from being overheated by increasing the length of the heat transmission path from the PTC heater to the foregoing O-rings to prevent the O-rings interposed between the joining surfaces from deteriorating in quality due to high heat generated from the PTC heater, which would cause liquid leaks. The level-difference portion is provided with wiring members extending from the PTC heater. 
     CITATION LIST 
     Patent Literature 
     {PTL 1} Japanese Unexamined Patent Application, Publication No. 2008-56044 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, since the heating-medium heating unit described in PTL 1 has a configuration in which a flat PTC heater is closely interposed between a pair of heating-medium circulation boxes configured such that a plurality of box components are joined via O-rings in a liquidtight manner, as described above, a large number of O-rings are needed, thus increasing the number of components, and moreover, complicating the assembly work, and furthermore, needing the carving of fitting grooves in which the O-rings are fitted in the joining surfaces of the box components, which has caused an increase in the manufacturing cost of the heating-medium heating unit. 
     Furthermore, since level-difference portions, such as grooves, are formed between the radiating surfaces in close contact with the PTC heater and the joining surfaces formed on the outer peripheries of the heating-medium circulation boxes (box components), the number of man-hours for machining the box components is large, which causes an increase in the manufacturing cost of the heating-medium circulation boxes, and thus the entire heating-medium heating unit. 
     Furthermore, since wiring members extending from the PTC heater are disposed between the PTC heater and the outer peripheries (joining surfaces) of the heating-medium circulation boxes, the dimensions of the outer peripheries of the heating-medium circulation boxes are significantly larger than the area of the flat surface of the PTC heater, which also causes an increase in the manufacturing cost of the heating-medium heating unit. 
     The present invention has been made in consideration of such circumstances, and an object thereof is to provide a heating-medium heating unit which accommodates a PTC heater, in which the manufacturing cost of heating-medium circulation boxes through which a heating medium circulates is reduced, and in which leakage of the heating medium from the heating-medium circulation boxes is prevented so that the reliability can be enhanced, as well as a vehicle air conditioner using the same. 
     Solution to Problem 
     To achieve the above object, the present invention provides the following solutions. 
     A heating-medium heating unit according to a first aspect of the present invention includes a flat PTC heater; a first heating-medium circulation box in which a plurality of box components are stacked one on another, which is in close contact with one surface of the PTC heater, and in which a heating-medium circulation passage is formed in the interior; and a second heating-medium circulation box in which a plurality of box components are similarly stacked on one another, which is in close contact with the other surface of the PTC heater, in which a heating-medium circulation passage is formed in the interior, and which is joined to the first heating-medium circulation box in a liquidtight manner, wherein a heating medium that circulates through the heating-medium circulation passages in the first and second heating-medium circulation boxes is heated by heat radiated from both surfaces of the PTC heater, wherein at least one of a joining surface between the box components that constitute the first heating-medium circulation box and the second heating-medium circulation box and a joining surface between the first heating-medium circulation box and the second heating-medium circulation box is sealed with a liquid gasket; and wherein the heating-medium circulation passage of at least the first heating-medium circulation box or the second heating-medium circulation box is provided with a joining-surface cooling channel that is sealed with the liquid gasket and that cools the joining surface on which the heat from the PTC heater acts. 
     With the heating-medium heating unit, the heating-medium heating unit can be assembled by sealing spaces between the plurality of box components constituting the heating-medium heating unit using only liquid gaskets. The two heating-medium circulation boxes can also be assembled with the space therebetween being sealed using only a liquid gasket. This can therefore eliminate a large number of O-rings used in the related art, thus reducing the number of components and assembly man-hours, and moreover, can eliminate fitting grooves that are conventionally carved in the individual joining surfaces of the box components to fit the O-rings therein, thereby reducing the number of man-hours for machining the box components, thus allowing the manufacturing cost of the heating-medium circulation boxes to be reduced. 
     Moreover, since the liquid gaskets applied to the joining surfaces can be protected from the heat from the PTC heater by using the cooling channel provided in the heating-medium circulation passage of at least the first heating-medium circulation box or the second heating-medium circulation box, the durability of the liquid gasket is enhanced, and thus leakage of the heating medium from the joining surface can be prevented. 
     In the heating-medium heating unit according to the first aspect of the present invention, preferably, the joining-surface cooling channel is provided at a position closer to the joining surface sealed with the liquid gasket than to the PTC heater. This allows the liquid gasket applied to the joining surface to be protected from the heat from the PTC heater more reliably. 
     In the heating-medium heating unit according to the first aspect of the present invention, preferably, the joining surface is provided with an outside sealing section that seals a space between the heating-medium circulation passage and the outside and a board sealing section that seals a space between the heating-medium circulation passage and a portion communicating with a portion accommodating a board for controlling the PTC heater, in which the width of the board sealing section is larger than the width of the outside sealing section. This can reliably prevent coolant leakage to the control board while eliminating the O-ring so that the manufacturing cost can be reduced, thereby enhancing the reliability of the heating-medium heating unit. 
     In the heating-medium heating unit according to the first aspect of the present invention, preferably, a radiating surface of at least one of the first heating-medium circulation box and the second heating-medium circulation box, the radiating surface being in close contact with the PTC heater, and the joining surface between the first heating-medium circulation box and the second heating-medium circulation box are formed as a continuous flat surface without a level-difference. This can remarkably simplify machining of at least the first or second heating-medium circulation box, thus reducing the manufacturing cost of the heating-medium circulation box. 
     Furthermore, in the heating-medium heating unit according to the first aspect of the present invention, preferably, the PTC heater and the first and second heating-medium circulation boxes are formed in a rectangular shape, and a wiring member of the PTC heater extends from an end of the PTC heater in the longitudinal direction. This can eliminate the wiring members of the PTC heater interposed between the long side of the PTC heater and the long side of the heating-medium circulation box, and hence the outer peripheral dimensions of the heating-medium circulation box can be brought close to the planar outside dimensions of the PTC heater, and in addition, the heating-medium circulation box can be made compact, and thus the manufacturing cost can be reduced. 
     In the heating-medium heating unit, preferably, PTC devices that constitute the PTC heater are disposed in a plurality of rows along the channel direction of the heating-medium circulation passages, the plurality of PTC heaters have different widths, and ON/OFF states of the PTC devices can be individually controlled. With this configuration, the wiring members of the PTC heater can be easily provided together at one end of the PTC heater in the longitudinal direction, the amount of heat from the PTC heater can be controlled with a simple configuration, and thus a reduction in the manufacturing cost due to the size reduction of the heating-medium heating unit and enhanced reliability can be achieved. 
     Furthermore, a vehicle air conditioner according to a second aspect of the present invention includes a blower that circulates outside air or vehicle interior air, a cooler provided downstream of the blower, and a radiator provided downstream of the cooler, wherein a heating medium heated by the heating-medium heating unit according to the first aspect can circulate through the radiator. This can enhance the reliability while reducing the manufacturing cost of the heating-medium heating unit. 
     Advantageous Effects of Invention 
     Thus, with the heating-medium heating unit according to the present invention and the vehicle air conditioner using the same, it is possible to reduce the manufacturing cost of the heating-medium circulation box which accommodates a PTC heater and through which a heating medium circulates, and to prevent leakage of the heating medium from the heating-medium circulation box, and hence the reliability can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a vehicle air conditioner according to an embodiment of the present invention. 
         FIG. 2  is a perspective view of a heating-medium heating unit according to an embodiment of the present invention. 
         FIG. 3  is an exploded perspective view of the heating-medium heating unit according to the embodiment of the present invention. 
         FIG. 4  is a vertical cross-sectional view taken along line IV-IV in  FIG. 2 . 
         FIG. 5  is a vertical cross-sectional view taken along line V-V in  FIG. 2 . 
         FIG. 6  is a perspective view of a board-accommodating box shown in  FIG. 3 , turned upside down. 
         FIG. 7  is a bottom view of an upper heating-medium circulation box taken along arrows VII-VII in  FIG. 4 . 
         FIG. 8  is a top view of a lower heating-medium circulation box taken along arrows VIII-VIII in  FIG. 4 . 
         FIG. 9  is an enlarged diagram of part IX in  FIG. 4 . 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     An embodiment of the present invention will be described hereinbelow using  FIGS. 1 to 9 . 
       FIG. 1  shows a schematic configuration diagram of a vehicle air conditioner  1  according to this embodiment. The vehicle air conditioner  1  is, for example, a hybrid-vehicle air conditioner and is equipped with a casing  3  forming an air channel  2  through which outside air or vehicle interior air is taken in, controls the temperature thereof, and guides it into the vehicle interior. 
     The casing  3  accommodates, in sequence from the upstream side to the downstream side of the air channel  2 , a blower  4  that takes in outside air or vehicle interior air, increases the pressure thereof, and blows it downstream; a cooler  5  that cools the air blown by the blower  4 ; a radiator  6  that heats the air cooled by passing through the cooler  5 ; and an air mix damper  7  that adjusts the ratio of the amount of air passing through the radiator  6  to the amount of flowing air bypassing the radiator  6  to control the temperature of the air mixed downstream thereof. 
     The downstream side of the casing  3  is connected to a plurality of vents (not shown) through which the temperature-controlled air is blown out into the vehicle interior via a blowing-mode switching damper and a duct (not shown). The cooler  5  constitutes a refrigerant circuit together with a compressor, a condenser, and an expansion valve (not shown) and cools air passing therethrough by evaporating a refrigerant that is adiabatically expanded at the expansion valve. 
     The radiator  6  constitutes a heating-medium circulating circuit  11  together with a tank  8 , a pump  9 , an engine (not shown), and a heating-medium heating unit  10  according to the present invention. Engine coolant of a hybrid vehicle is used as a heating medium flowing through the heating-medium circulating circuit  11 . The heating-medium circulating circuit  11  heats the air passing through the radiator  6  in the casing  3  by heating the engine coolant with the heating-medium heating unit  10  when the temperature of the engine coolant, serving as the heating medium, does not significantly increase, such as during hybrid driving, and by circulating the heated engine coolant through the heating-medium circulating circuit  11  with the pump  9 . 
       FIG. 2  shows a perspective view of the heating-medium heating unit  10 ;  FIG. 3  shows an exploded perspective view of the heating-medium heating unit  10 ; and  FIGS. 4 and 5  show vertical cross-sectional views of the heating-medium heating unit  10 . 
     The heating-medium heating unit  10  is equipped with a first heating-medium circulation box A configured like a casing such that a plurality of box components  20 ,  21 , and  30  are stacked one on another; a second heating-medium circulation box B which is configured like a casing such that a plurality of box components  50  and  51  are similarly stacked one on another and which is joined to the lower surface of the first heating-medium circulation box A in a liquidtight manner; and a PTC heater  40  sandwiched between the first and second heating-medium circulation boxes A and B. 
     The first heating-medium circulation box A is formed such that the rectangular board-accommodating box  20 , to the upper surface of which the cap  21  is joined, and the upper heating-medium circulation box  30  having the same rectangular shape as the board-accommodating box  20  are joined together in a liquidtight manner. The second heating-medium circulation box B is formed of the lower heating-medium circulation box  50  having the same rectangular shape as the upper heating-medium circulation box  30  and the cap  51 , which is joined to the lower surface of the lower heating-medium circulation box  50  in a liquidtight manner. The first heating-medium circulation box A, the second heating-medium circulation box B, and the other box components  20 ,  21 ,  30 ,  50 , and  51  are tightened together with a plurality of bolts  58  to form a single unit, as shown in  FIG. 2  and  FIG. 4 . 
     The PTC heater  40  has a rectangular, flat shape smaller than those of the upper heating-medium circulation box  30  and the lower heating-medium circulation box  50 , the upper surface of the PTC heater  40  is in close contact with a flat radiating surface  38  formed at the lower surface of the upper heating-medium circulation box  30 , and the lower surface of the PTC heater  40  is in close contact with a flat radiating surface  56  formed on the upper surface of the lower heating-medium circulation box  50 , as will be described later in detail. 
     The board-accommodating box  20  is a rectangular half casing which is formed of a thermally conducting material, such as an aluminum alloy, whose upper surface is tightly sealed by the cap  21 , and whose interior serves as a board-accommodating space S, in which a control board  22  (see FIGS.  3  and  4 ) that controls the PTC heater  40  is accommodated. The control board  22  incorporates heat generating components and control circuits, such as field effect transistors (FETs)  23 , and is supplied with a high voltage of 300 V for driving the PTC heater  40  and a low voltage of 12 V for control. 
     The control board  22  is fixedly disposed on supporting portions  24  projecting from the bottom surface of the board-accommodating box  20  by being fastened with screws  25   a  at the four corners. The heat generating components, such as the FETs  23 , are disposed on the lower surface side of the control board  22  and are fastened and fixed with screws  25   b  to the upper surface of a cooling portion  26  provided on the bottom surface of the board-accommodating box  20  that is in contact therewith via an insulating layer (not shown) therebetween. The heat generating components, such as the FETs  23 , and the cooling portion  26  are disposed in the vicinity of the inlet of heating-medium circulation passages (circulation paths  33 ), described later, provided in the upper heating-medium circulation box  30 , to enhance the cooling effect on the heat generating components. 
     Wire insertion holes  27  are formed at one end face of the board-accommodating box  20  (see  FIGS. 3 and 6 ), through which wiring members  40   a  (see  FIG. 2 ) connected to the control board  22  are passed. Wire routing holes  28  (see  FIG. 6 ) through which a harness connecting the control board  22  and the PTC heater  40  passes are formed in the lower surface at one end of the board-accommodating box  20 . A harness insertion hole  29  (see  FIG. 3 ) is formed at the other end of the board-accommodating box  20 , through which an electrical harness  22   a  (see  FIG. 2 ) connecting to the control board  22  passes. 
       FIGS. 3 to 5  and  FIG. 7  illustrate the heating-medium circulation passages in the upper heating-medium circulation box  30 . The upper heating-medium circulation box  30  is a rectangular half casing which is formed of a thermally conducting material, such as an aluminum alloy, and whose upper surface is provided with a pair of inlet header  31  and outlet header  32  formed at both ends and parallel grooved circulation paths  33 , which are formed between the inlet header  31  and the outlet header  32  and are separated by a large number of fins  33   a . The upper surfaces of the inlet header  31 , the outlet header  32 , and the circulation paths  33  are sealed off by the bottom surface of the board-accommodating box  20  in a liquidtight manner (see  FIGS. 4 and 5 ). 
     Thus, an engine-coolant circulation passage through which the engine coolant flowing into the inlet header  31  is distributed to the large number of circulation paths  33  so as to flow simultaneously in parallel in the circulation paths  33  toward the outlet header  32  is formed between the board-accommodating box  20  and the upper heating-medium circulation box  30 . The engine coolant flowing in the circulation paths  33  does not flow directly into the outlet header  32  but flows into a circulation opening  35  (see  FIG. 7 ), described later, formed in the lower surface of the upper heating-medium circulation box  30 . The above-described cooling portion  26  formed on the bottom surface of the board-accommodating box  20  is cooled by the engine coolant circulating in the circulation paths  33 , described above, and thus constitutes a cooling structure for the control board  22 . 
     The inlet header  31  is provided with an engine-coolant inflow portion  34 , and the outlet header  32  is provided with the circulation opening  35  connecting to the lower heating-medium circulation box  50 , a circulation opening  36  through which the engine coolant flowing from the lower heating-medium circulation box  50  is made to flow outwards, as will be described later, and an engine-coolant outflow portion  37  communicating with the outside via the circulation opening  36 . The inflow portion  34  and the outflow portion  37  are provided with respective union members  34   a  and  37   a  (see  FIGS. 2 and 5 ) that allow hose members constituting the heating-medium circulating circuit  11  to be connected thereto. 
     Furthermore, the lower surface of the upper heating-medium circulation box  30  is provided with a wide depressed portion (see  FIGS. 4 ,  5  and  7 ) whose ceiling surface serves as the flat radiating surface  38  that is in close contact with the upper surface of the PTC heater  40 . This depressed portion faces the back surfaces of the circulation paths  33  through which the engine coolant circulates and is formed such that the PTC heater  40  is fitted therein. Wire insertion holes  39  (see  FIG. 3 ) are formed at the end of the upper surface of the upper heating-medium circulation box  30  opposite to the circulation openings  35  and  36 , and the wire insertion holes  39  match the wire routing holes  28  of the board-accommodating box  20 . 
       FIGS. 3 to 5  and  FIG. 8  illustrate heating-medium circulation passages in the lower heating-medium circulation box  50 . The lower heating-medium circulation box  50  is a rectangular half casing which is constituted by a thermally conducting material, such as an aluminum alloy, in which communication openings  52  and  53  (see  FIG. 8 ) are provided at one end thereof, and the communication openings  52  and  53  match the circulation openings  35  and  36  of the upper heating-medium circulation box  30 , respectively. 
     The lower surface of the lower heating-medium circulation box  50  is provided with parallel grooved circulation paths  54  that extend from the communication opening  52  toward the other end and that make a U-turn at the other end to return to the communication opening  53  and that are separated by a large number of fins  54   a  (see  FIG. 4 ). The supply channels and the return channels of the U-shaped circulation paths  54  are separated by a partition wall  54   b  (see  FIG. 4 ) higher than the fins  54   a . The lower surfaces of the circulation paths  54  are tightly sealed by the cap  51 , as described above, and the cap  51  has a U-shaped shallow depressed portion  55  (see  FIG. 3 ) that matches the shapes of the circulation paths  54  and the partition wall  54   b.    
     Thus, a heating-medium circulation passage through which the engine coolant flowing into the communication opening  52  is distributed from the communication opening  52  to the large number of circulation paths  54 , circulates in the individual circulation paths  54  simultaneously in parallel, and makes a U-turn at the other end to reach the communication opening  53  is formed between the lower heating-medium circulation box  50  and the cap  51 . 
     The communication opening  52  of the lower heating-medium circulation box  50  communicates with the circulation opening  35  provided in the outlet header  32  of the upper heating-medium circulation box  30  so that the engine coolant flowing in the circulation paths  33  of the upper heating-medium circulation box  30  flows therein. The communication opening  53  of the lower heating-medium circulation box  50  communicates with the circulation opening  36  provided in the outlet header  32  of the upper heating-medium circulation box  30  to constitute a passage through which the engine coolant flowing in the lower heating-medium circulation box  50  is made to flow outwards from the circulation opening  36  via the outflow portion  37 . 
     The upper surface of the lower heating-medium circulation box  50  serves as the radiating surface  56  (see  FIGS. 3 to 5  and  FIG. 8 ) and holds the PTC heater  40  with the flat radiating surface  38  at the lower surface of the upper heating-medium circulation box  30  therebetween like a sandwich, so that the radiating surfaces  38  and  56  are in pressure-contact with compressive heat conducting layers  44 , to be described later, bonded to both surfaces of the PTC heater  40 . 
       FIGS. 3 and 4  and  FIGS. 7 to 9  illustrate the configuration of the PTC heater  40 . The PTC heater  40  is rectangular in overall shape. The PTC heater  40  is constituted by PTC devices  41   a ,  41   b , and  41   c  serving as heat-generating elements, disposed in, for example, three rows, along the channel direction of the heating-medium circulation passages (circulation paths  33  and circulation paths  54 ). Of the three PTC devices  41   a ,  41   b , and  41   c , the PTC devices  41   a  and  41   c  at both ends are set to be, for example, twice as wide as the PTC device  41   b  at the center. 
     As shown in an enlarged cross-sectional view in  FIG. 9 , the PTC devices  41   a ,  41   b , and  41   c  each have a stacked structure in which electrode plates  42 , noncompressive insulating layers  43 , and the compressive heat conducting layers  44  are stacked in sequence. The PTC devices  41   a ,  41   b , and  41   c  are configured such that the ON/OFF states thereof can be individually controlled by control circuits incorporated in the control board  22 . 
     The electrode plates  42  are for supplying electric power to the PTC devices  41   a ,  41   b , and  41   c , are rectangular thin plates similar to the PTC devices  41   a ,  41   b , and  41   c , and have electrical conductivity and thermal conductivity. The noncompressive insulating layers  43  are rectangular thin plates, are each constituted by an insulating material, such as a polyamide film, and have thermal conductivity. The noncompressive insulating layers  43  are 0.1 mm or less in thickness. This is for the purpose of minimizing the thermal resistance between the PTC devices  41   a ,  41   b , and  41   c  and the electrode plates  42  and between the upper heating-medium circulation box  30  (radiating surface  56 ) and the lower heating-medium circulation box  50  (radiating surface  38 ) provided at the outside thereof and for providing sufficient electrical insulation. 
     Furthermore, the compressive heat conducting layers  44  are rectangular sheet members having compressibility, which are constituted by insulating sheets, such as silicone sheets and have thermal conductivity. The compressive heat conducting layers  44  are, if constituted by silicone sheets, set to be about 0.4 mm to 2.0 mm in thickness to reduce the thermal resistance between the PTC device  41  serving as a heat-generating element and the upper heating-medium circulation box  30  (radiating surface  38 ) and the lower heating-medium circulation box  50  (radiating surface  56 ). The thickness of at least 0.4 mm or more ensures a compressing function, allowing the upper heating-medium circulation box  30  and the lower heating-medium circulation box  50  to be reliably brought into close contact with the PTC heater  40  by using the compressibility when the PTC heater  40  is mounted between the upper heating-medium circulation box  30  and the lower heating-medium circulation box  50 , and allowing the mounting dimensional tolerance to be absorbed. 
     Thus, as shown in  FIGS. 4 and 5 , the PTC heater  40  can heat the engine coolant circulating in the upper heating-medium circulation box  30  and the lower heating-medium circulation box  50  provided in close contact with both sides thereof by radiating the heat from both sides. 
     The PTC heater  40  has wiring members  40   b  at one end thereof, and the wiring members  40   b  are bent upwards at right angles to the planar direction of the PTC heater  40  and are inserted into the wire insertion holes  39  of the upper heating-medium circulation box  30  and the wire insertion routing holes  28  of the board-accommodating box  20 . The wiring members  40   b  are guided to the control board  22 , so that the cable-like wiring members  40   a  (see  FIG. 2 ) are drawn outwards from the control board  22  through the wire insertion holes  27  of the board-accommodating box  20 , as described above. The wire insertion holes  27  are fitted with a waterproof, dustproof wire cap  40   c.    
     The heating-medium circulating circuit  11  is connected to the inflow portion  34  of the upper heating-medium circulation box  30 . Low-temperature engine coolant pumped from the pump  9  flows through the inflow portion  34  into the inlet header  31  and is distributed to the individual circulation paths  33  (see  FIG. 3 ). The engine coolant flowing through the circulation paths  33  toward the outlet header  32  is heated and increased in temperature by the PTC heater  40 , joins before the outlet header  32 , and flows into the communication opening  52  of the lower heating-medium circulation box  50  through the circulation opening  35 . 
     The engine coolant diverges at the communication opening  52  into the individual circulation paths  54 , flows as indicated by an imaginary line F in  FIG. 8  while being heated and increased in temperature again by the PTC heater  40 , makes a U-turn at the other end, passes through the communication opening  53  and then the circulation opening  36  of the upper heating-medium circulation box  30  and enters the outlet header  32 , passes through the outflow portion  37 , and flows back to the heating-medium circulating circuit  11 . Thus, the engine coolant passing through the interior of the heating-medium heating unit  10  flows along both surfaces of the PTC heater  40  and circulates in the heating-medium circulating circuit  11  while being heated by the heat from the PTC heater  40 , so that the temperature of the vehicle interior is controlled. 
     Since the PTC devices  41   a ,  41   b , and  41   c  that constitute the PTC heater  40  are configured such that the ON/OFF states can be individually controlled by the control circuits incorporated in the control board  22 , the individual PTC devices  41   a ,  41   b , and  41   c  are independently turned ON and OFF by the control board  22  according to the difference between the actual temperature of the engine coolant flowing into the heating-medium heating unit  10  and a necessary temperature (target temperature), and thus the heating capability is controlled. This allows the engine coolant to flow out while being heated and increased to a predetermined temperature. 
     Next, the relevant part of the present invention will be described. As shown in  FIG. 4 , the heating-medium heating unit  10  has a plurality of joining surfaces M 1  to M 4 . Joining surfaces between the first heating-medium circulation box A and the second heating-medium circulation box B, that is, a joining surface M 1  between the upper heating-medium circulation box  30  and the lower heating-medium circulation box  50 , joining surfaces M 2  and M 3  between the board-accommodating box  20  and the cap  21  and the upper heating-medium circulation box  30  that constitute the first heating-medium circulation box A, and a joining surface M 4  between the lower heating-medium circulation box  50  and the cap  51  that constitute the second heating-medium circulation box B, are configured to be sealed with liquid gaskets. Examples of the liquid gaskets include a waterproof, heat-resistant silicone sealant that becomes rubbery when it hardens. 
     Furthermore, the circulation paths  33  that serve as the heating-medium circulation passages of the first heating-medium circulation box A and the circulation paths  54  that serve as the heating-medium circulation passages of the second heating-medium circulation box B are provided with joining-surface cooling channels C 1  and C 2 , respectively. These joining-surface cooling channels C 1  and C 2  are provided to particularly cool, of the joining surfaces M 1  to M 4  sealed by the liquid gaskets, the vicinity of the joining surface M 1 , on which a considerable amount of the heat from the PTC heater  40  acts, thereby preventing the liquid gasket applied to the joining surface M 1  from being degraded due to the heat. 
     The joining-surface cooling channel C 1  constitutes, of the plurality of circulation paths  33 , one or two circulation paths close to the joining surface M 1 , and the joining-surface cooling channel C 2  constitutes, of the plurality of circulation paths  54 , one or two circulation paths close to the joining surface M 1 . These joining-surface cooling channels C 1  and C 2  are provided at positions closer to the joining surface M 1  than to the edge of the PTC heater  40 . Therefore, the heat from the PTC heater  40  is subjected to heat exchange by the engine coolant that flows through the joining-surface cooling channels C 1  and C 2  before being transmitted to the joining surface M 1 , which makes the heat difficult to be transmitted to the joining surface M 1 . Accordingly, this allows the liquid gasket that seals the joining surface M 1  to be protected from the heat, thus enhancing the durability, which prevents the heating medium from leaking through the joining surface M 1 . 
     Of the joining surfaces M 1  to M 4 , the joining surface M 3  between the lower surface of the board-accommodating box  20  and the upper surface of the upper heating-medium circulation box  30 , through which the wiring members  40   b  of the PTC heater  40  pass, includes an outside sealing section M 3   a  that seals a space between the heating-medium circulation passages (circulation paths  33 ) and the outside and a board sealing section M 3   b  that seals a space between the heating-medium circulation passages (circulation paths  33 ) and the wire routing holes  28 , which are portions communicating with the board-accommodating space S, as shown in  FIG. 6 , which illustrates the shape of the lower surface of the board-accommodating box  20 . The width W 2  of the board sealing section M 3   b  is set to be larger than the width W 1  of the outside sealing section M 3   a . For example, W 1  is set at 5 mm, and W 2  is set at 8 mm. 
     As shown in  FIG. 4 , either one of the upper and lower surfaces of the individual joining surfaces M 1  to M 4  are each provided with a level-difference portion Mc along the inner peripheral edge thereof. Forming the level-difference portions Mc allows the liquid gasket to be maintained at a predetermined thickness by the level-difference portion Mc and to be able to harden without being subjected to a pressing force. If both surfaces of the individual joining surfaces M 1  to M 4  were made flat without the flat level-difference portion Mc, the liquid gaskets applied therebetween would be completely pushed out from the range of the joining surfaces when a pressing force is applied thereto, thus posing a worry that sufficient sealability would not be maintained. The height of the level-difference portion Mc may be about 0.5 mm to 2.0 mm. 
     As shown in  FIGS. 3 ,  4 , and  8 , for example, in the lower heating-medium circulation box  50  constituting the second heating-medium circulation box B, the radiating surface  56  in close contact with the PTC heater  40  and the joining surface M 1  between it and the first heating-medium circulation box A are formed as a continuous flat surface without a level-difference. 
     The heating-medium heating unit  10  according to this embodiment is configured as described above. This heating-medium heating unit  10  provides the following advantages. 
     First, since the joining surfaces M 1  to M 4  between the box components  20 ,  21 ,  30 ,  50 , and  51  that constitute the first heating-medium circulation box A and the second heating-medium circulation box B are configured to be sealed with the liquid gaskets, the O-rings that are conventionally interposed between the joining surfaces M 1  to M 4  can be eliminated. This can reduce the number of components and the number of man-hours for assembling the heating-medium heating unit  10 , and moreover, can eliminate fitting grooves that are conventionally carved in the individual joining surfaces M 1  to M 4  to fit the O-rings therein, thereby reducing the number of man-hours for machining the box components  20 ,  21 ,  30 ,  50 , and  51 , thus allowing the manufacturing cost of the heating-medium circulation box  10  to be remarkably reduced. 
     Since the joining-surface cooling channels C 1  and C 2  are provided in the heating-medium circulation passages (circulation paths  33  and  54 ) of the first heating-medium circulation box A and the second heating-medium circulation box B, of the four joining surfaces M 1  to M 4  sealed with the liquid gaskets, the joining surface M 1  on which a considerable amount of the heat from the PTC heater  40  acts can be cooled favorably. This can therefore prevent the liquid gasket applied to the joining surface M 1  from being degraded due to heat, and realizes a sealing technique only with the liquid gasket without using an O-ring, thus significantly contributing to a reduction in the manufacturing cost of the heating-medium heating unit  10 . 
     Furthermore, since these joining-surface cooling channels C 1  and C 2  are provided at positions closer to the joining surface M 1  than to the edge of the PTC heater  40 , the liquid gasket applied to the joining surface M 1  can be more reliably protected from the heat from the PTC heater  40 . For the joining surfaces M 3  and M 4 , since the positions of the circulation paths  33  and  54  are closer to the joining surfaces M 3  and M 4  than to the PTC heater  40 , it is difficult for the joining surfaces M 3  and M 4  to be affected by the heat from the PTC heater  40 . 
     The shapes of the joining-surface cooling channels C 1  and C 2  are not limited to those of this embodiment; they may be other shapes. For example, in this embodiment, although the depths and widths of the joining-surface cooling channels C 1  and C 2  are equal to or smaller than those of the adjacent circulation paths  33  and  54 , the depths and widths may be larger than those of the circulation paths  33  and  54  so that much more engine coolant will flow through a portion closer to the joining surface M 1 , thereby further enhancing the cooling performance of the joining surface M 1 . 
     Furthermore, in this heating-medium heating unit  10 , of the joining surfaces M 1  to M 4 , the joining surface M 3  through which the wiring members  40   b  of the PTC heater  40  pass is configured such that the width W 2  of the board sealing section M 3   b  is set to be larger than the width W 1  of the outside sealing section Mia; therefore, while the O-ring on the joining surface M 3  is eliminated so that the manufacturing cost can be reduced, coolant leakage to the board-accommodating space S in which the control board  22  is accommodated can be reliably prevented, and hence the reliability of the heating-medium heating unit  10  can be enhanced. 
     Furthermore, in this heating-medium heating unit  10 , since the radiating surface  56  of the lower heating-medium circulation box  50  that constitutes the second heating-medium circulation box B and the joining surface M 1  to the first heating-medium circulation box A are formed as a continuous flat surface without a level-difference, the upper surface of the lower heating-medium circulation box  50  can be made completely flat, thereby remarkably facilitating processing of the lower heating-medium circulation box  50 , thus reducing the manufacturing cost of the heating-medium circulation box  10 . 
     Furthermore, in this heating-medium heating unit  10 , since the PTC heater  40 , the first heating-medium circulation box A, and the second heating-medium circulation box B are formed in a rectangular shape, and the wiring members  40   b  of the PTC heater  40  are extended together from the end of the PTC heater  40  in the longitudinal direction, the wiring members of the PTC heater  40  are not interposed between the long side of the PTC heater  40  and the long side of the heating-medium circulation box  10  as in the related art. Therefore, the outer peripheral dimensions of the heating-medium circulation box  10  can be brought close to the planar outside dimensions of the PTC heater  40 , and the width dimension of the heating-medium circulation box  10  can be reduced, and thus the manufacturing cost can be reduced. 
     Furthermore, in this heating-medium heating unit  10 , since the PTC devices  41   a ,  41   b , and  41   c  that constitute the PTC heater  40  are disposed in a plurality of rows along the channel direction of the heating-medium circulation passages (circulation paths  33  and  54 ), the plurality of PTC devices  41   a ,  41   b , and  41   c  have different widths, and the ON/OFF states of the PTC devices  41   a ,  41   b , and  41   c  can be individually controlled, the wiring members  40   b  can be easily provided together at one end of the PTC devices  41   a ,  41   b , and  41   c  in the longitudinal direction, the quantity of heat from the PTC heater  40  can be controlled with a simple configuration, and thus a reduction in the manufacturing cost due to the size reduction of the heating-medium heating unit  10  and enhanced reliability can be achieved. 
     Furthermore, since the vehicle air conditioner  1  according to the present invention is provided with the blower  4  that circulates outside air or vehicle interior air, the cooler  5  provided downstream of the blower  4 , and the radiator  6  provided downstream of the cooler  5  and is configured to circulate engine coolant heated by the heating-medium heating unit  10  according to the present invention through the radiator  6 , the reliability of the heating-medium heating unit  10  can be enhanced, and furthermore, the reliability of the entire vehicle air conditioner  1  can be enhanced while achieving miniaturization of the heating-medium heating unit  10  and a reduction in the manufacturing cost. 
     Although this embodiment has been described as applied to an example in which the heating-medium heating unit is used in a vehicle air conditioner, the heating-medium heating unit according to the present invention may be applied to air conditioners that are not designed for vehicles, heaters, refrigerators and so on. 
     REFERENCE SIGNS LIST 
     
         
           1  vehicle air conditioner 
           4  blower 
           5  cooler 
           6  radiator 
           10  heating-medium heating unit 
           20  board-accommodating box serving as box component 
           21  cap serving as box component 
           22  control board for controlling PTC heater 
           28  wire insertion hole serving as portion communicating with board-accommodating space 
           30  upper heating-medium circulation box serving as box component 
           33  circulation path serving as heating-medium circulation passage 
           38  radiating surface PTC heater 
           40   b  wiring member of PTC heater 
           41   a ,  41   b ,  41   c  PTC device 
           50  lower heating-medium circulation box serving as box component 
           51  cap serving as box component 
           54  circulation path serving as heating-medium circulation passage 
           56  radiating surface 
         A first heating-medium circulation box 
         B second heating-medium circulation box 
         C 1 , C 2  joining-surface cooling channel 
         M 1  to M 4  joining surface 
         M 3   a  outside sealing section 
         M 3   b  board sealing section 
         S board-accommodating space 
         W 1  width of outside sealing section 
         W 2  width of board sealing section