Patent Publication Number: US-2004042173-A1

Title: Electronic apparatus having circulating path through which liquid coolant cooling heat generating component flows

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001] This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-255542, filed Aug. 30, 2002, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to an electronic apparatus of the liquid-cooled type, in which a circuit component such as a CPU (Central Processing Unit) is cooled with liquid coolant.  
       [0004] 2. Description of the Related Art  
       [0005] A CPU is incorporated in, for example, notebook-type portable computers. The heat that the CPU generates while operating is increasing as its data-processing speed rises, and it performs more and more functions. The higher the temperature of the CPU, the less efficiently it operates. To cool the CPU, so-called cooling systems of the liquid-cooling type have been developed in recent years. A liquid-cooling system uses a liquid coolant that has a far higher specific heat than air.  
       [0006] Japanese Patent Application KOKAI Publication No. 7-142886 discloses a cooling system of the liquid-cooling type, configured for use in a portable computers. The cooling system comprises a heat-receiving header, a heat-radiating header, and a tube for circulating the liquid coolant. The heat-receiving header is accommodated in a housing of the portable computer and thermally connected to the CPU. The heat-radiating header is accommodated in a display unit supported by the housing. The tube is arranged to extend between the housing and the display unit to connect the heat-receiving header and the heat-radiating header together.  
       [0007] With this cooling system, the liquid coolant absorbs heat from the CPU in the heat-receiving header. The liquid coolant thus heated is transferred to the heat-radiating header through the tube. While passing through the heat-radiating header, the liquid coolant releases heat from the CPU. The liquid coolant cooled by the heat-radiating header returns to the heat-receiving header through the tube. Then, the liquid coolant absorbs heat from the CPU again. This circulation of the liquid coolant efficiently transfers heat from the CPU to the heat-radiating header. This serves to improve cooling performance for the CPU compared to common conventional cooling systems of the air-cooling type.  
       [0008] The display unit of the portable computer accommodates a liquid crystal display panel. The liquid crystal display panel is adjacent to the heat-radiating header inside the display unit. Thus, when heat from the CPU is released from the surface of the heat-radiating header, the liquid crystal display panel is unavoidably thermally affected by the heat-radiating header. As is well known, when heated to high temperature, the liquid crystal display panel fails to control the orientation of liquid crystal molecules, resulting in degraded display quality. Thus, the surface temperature of the heat-radiating header, which thermally affects the liquid crystal display panel, must not be thoughtlessly increased. As a result, considering thermal effects on the liquid crystal display panel, the allowable amount of heat released from the heat-radiating header is estimated to be at most between 10 and 20W.  
       [0009] On the other hand, for example, the portable computer may be connected to an external display device having a large screen. In such usage, the liquid crystal display panel of the portable computer stops its display operation. Accordingly, even if the liquid crystal display panel is thermally affected by the heat-radiating header, its display quality is not degraded.  
       [0010] However, the amount of heat released from the heat-radiating header is kept small in order to reduce the thermal effects on the liquid crystal display panel as described above. Thus, the amount of heat released from the heat-radiating header is insufficient both while the liquid crystal display panel is operating and while the display operation remains stopped. Consequently, cooling performance for the CPU may be degraded, which would mean that it cannot sufficiently deal with an increase in the amount of heat generated by the CPU.  
       BRIEF SUMMARY OF THE INVENTION  
       [0011] According to an embodiment of the present invention, an electronic apparatus includes a main unit having a heat generating component; a heat receiving portion thermally connected to the heat generating component; a display unit supported by the main unit, which is movable between a closed position and an open position; a heat radiating portion accommodated in the display unit to release the heat from the heat generating component; a circulating path through which a liquid coolant circulates between the heat receiving portion and the heat radiating portion, the circulating path being used to transfer the heat from the heat generating component conducted to the heat receiving portion, to the heat radiating portion via the liquid coolant; and a control device which increases the amount of heat transferred from the heat receiving portion to the heat radiating portion when the display unit is moved to the closed position. 
     
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
     [0012] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
     [0013]FIG. 1 is a perspective view of a portable computer according to a first embodiment of the present invention provided with a cooling unit of the liquid cooling type;  
     [0014]FIG. 2 is a perspective view of the portable computer according to the first embodiment of the present invention, showing that a display unit has been rotationally moved to an open position;  
     [0015]FIG. 3 is a side view schematically showing that an external display device is connected to the portable computer according to the first embodiment of the present invention;  
     [0016]FIG. 4 is a sectional view of the portable computer according to the first embodiment of the present invention wherein the cooling unit of the liquid cooling type is mounted in the portable computer;  
     [0017]FIG. 5 is a sectional view of the portable computer according to the first embodiment of the present invention, showing the positional relationship between a CPU and a heat receiving portion;  
     [0018]FIG. 6 is a sectional view of the heat receiving portion according to the first embodiment of the present invention;  
     [0019]FIG. 7 is a sectional view of the portable computer according to the first embodiment of the present invention, showing the positional relationship between an electric fan and a heat radiating portion and a liquid crystal display unit;  
     [0020]FIG. 8 is a sectional view of the heat radiating portion according to the first embodiment of the present invention;  
     [0021]FIG. 9 is a block diagram showing a pump control system according to the first embodiment of the present invention;  
     [0022]FIG. 10 is a flow chart showing what control is provided from the detection of temperature of the CPU until the flow rate of a liquid coolant is increased according to the first embodiment of the present invention; and  
     [0023]FIG. 11 is a flow chart showing what control is provided from the detection of temperature of the CPU until the flow rate of a liquid coolant is increased according to a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0024] A first embodiment of the present invention will be described below with reference to FIGS.  1  to  10  wherein it is applied to a portable computer.  
     [0025]FIGS. 1 and 2 disclose a portable computer  1  as an electronic apparatus. The portable computer  1  comprises a main unit  2  and a display unit  3 . The main unit  2  has a housing  4  shaped like a flat box. The housing  4  supports a keyboard  5 .  
     [0026] The housing  4  is equipped with a printed circuit board  6 , a CD-ROM drive  7 , and batteries  8  as a power supply. The printed circuit board  6  and the CD-ROM drive  7  are electrically connected to the batteries  8 .  
     [0027] The display unit  3  comprises a liquid crystal display panel  10  and a display housing  11  that accommodates the liquid crystal display panel  10 . The liquid crystal display panel  10  has a screen  10   a  that displays images. The screen  10   a  is exposed from the display housing  11  through an opening  11   a  formed in the front surface of the display housing  11 . The display housing  11  is supported at the rear end of the housing  4  via hinges (not shown). Thus, the display unit  3  can be rotationally moved between a closed position and an open position. In the closed position, the display unit  3  lies on top of the housing  4 , thus covering the keyboard  5  from above. In the open position, the display unit  3  stands up to expose the keyboard  5  and the screen  10   a.    
     [0028] The liquid crystal display panel  10  is electrically connected to a liquid crystal driving circuit. The liquid crystal driving circuit is powered off when the display unit  3  is rotationally moved to the closed position. This causes the liquid crystal display panel  10  to stop its display operation. Furthermore, a temperature sensor  35 , shown in FIG. 1, is attached to the liquid crystal display panel  10 . The temperature sensor  35  detects the temperature of the liquid crystal display panel  10  and outputs a signal for this temperature.  
     [0029] As shown in FIG. 3, an interface connector  12  is mounted at the rear end of the printed circuit board  6 . The interface connector  12  is used to connect, for example, an external display device  13  to the portable computer  1 . The external display device  13  has a screen (not shown) which is larger than the screen  10   a  of the display unit  3  and which provides high image quality. If the external display device  13  is connected to the portable computer  1 , an operator operates a switch to make a choice as to whether to use the display unit  3  of the portable computer  1  or the external display device  13 . If the external display device  13  is chosen, the liquid crystal driving circuit of the liquid crystal display panel  10  is turned off to cause the liquid crystal display panel  10  to stop its display operation.  
     [0030] As shown in FIG. 7, the main unit  2  comprises a position sensor  14 . The position sensor  14  electrically detects whether or not the display unit  3  is in the closed position. The position sensor  14  is supported by the top wall  4 b of the housing  4 . When the display unit  3  is rotationally moved to the closed position, the position sensor  14  detects the presence of the display unit  3  on the basis of the contact between the position sensor  14  and the display housing  11 . At the same time, the position sensor  14  outputs a signal indicating that the display unit  3  has been rotationally moved to the closed position.  
     [0031] As shown in FIG. 5, a CPU  15  as a heat generating component is mounted on the top surface of the printed circuit board  6 . The CPU  15  has a base substrate  16  and an IC chip  17  mounted on the center of the base substrate  16 . The IC chip  17  generates a large amount of heat during operation owing to its increased processing speed and the increased number of its functions. The IC chip  17  needs to be cooled to keep operating in stable conditions. The CPU  15  contains a temperature sensor  18  (shown in FIG. 9). The temperature sensor  18  detects the temperature of the IC chip  17  and outputs a signal for this temperature.  
     [0032] As shown in FIGS. 1 and 3, the portable computer  1  is provided with a cooling unit  20  of the liquid cooling type that cools the CPU  15 . The cooling unit  20  comprises a heat receiving portion  21 , a heat radiating portion  22 , a circulating path  23 , and a pump  24 .  
     [0033] The heat receiving portion  21  is fixed to the printed circuit board  6 . As shown in FIG. 5, the heat receiving portion  21  is shaped like a flat box that is a size larger than the CPU  15 . The bottom surface of the heat receiving portion  21  constitutes a flat heat receiving surface  25 . The heat receiving surface  25  is thermally connected to the IC chip  17  via thermally conductive grease or a thermally conductive sheet (not shown).  
     [0034] The heat receiving portion  21  has a coolant channel  26 , an inlet port  27 , and an outlet port  28 . The coolant channel  26  is formed inside the heat receiving portion  21  and thermally connected to the IC chip  17  via the heat receiving surface  25 . The inlet port  27  is located at the upstream end of the coolant channel  26 . The outlet port  28  is located at the downstream end of the coolant channel  26 .  
     [0035] As shown in FIG. 7, the heat radiating portion  22  is accommodated in the display housing  11  of the display unit  3 . The heat radiating portion  22  is shaped like a rectangular plate substantially as large as the liquid crystal display panel  10 . The heat radiating portion  22  is arranged between the liquid crystal panel  10  and the rear surface of the display housing  11 . Thus, the heat radiating portion  22  is adjacent to the liquid crystal display panel  10  inside the display housing  11 .  
     [0036] As shown in FIG. 8, the heat radiating portion  22  comprises a first radiator plate  29  and a second radiator plate  30 . The first and second radiator plates  29  and  30  are each made of metal and are superimposed on each other. The first radiator plate  29  has a bulging portion  31  extending away from the second radiator plate  30 . The bulging portion  31  meanders substantially all over the surface of the first radiator plate  29 . It also opens toward the second radiator plate  30 . An opening end of the bulging portion  31  is closed by the second radiator plate  30 . The bulging portion  31  of the first radiator plate  29  constitutes a coolant channel  32  between itself and the second radiator plate  30 .  
     [0037] The heat radiating portion  22  has an inlet port  33  and an outlet port  34 . The inlet port  33  is located at the upstream end of the coolant channel  32 . The outlet port  34  is located at the downstream end of the coolant channel  32 . The inlet port  33  and the outlet port  34  are spaced from each other in the width direction of the display housing  8 .  
     [0038] As shown in FIGS. 1 and 4, the circulating path  23  comprises two pipes  36  and  37 . The first pipe  36  extends between the housing  4  and the display housing  11  so as to connect the outlet port  28  of the heat receiving portion  21  and the inlet port  33  of the heat radiating portion  22  together. The second pipe  37  extends between the housing  4  and the display housing  11  so as to connect the outlet port  34  of the heat radiating portion  22  and the inlet port  27  of the heat receiving portion  21  together. Thus, the coolant channel  26  in the heat receiving portion  21  and the coolant channel  32  in the heat radiating portion  22  are connected together via the circulating path  23 . The circulating path  23  and the coolant channels  26  and  32  are filled with a liquid coolant.  
     [0039] The pump  24  is installed in the middle of the second pipe  37 . The pump  24  is used to forcibly circulate the liquid coolant between the heat receiving portion  21  and the heat radiating portion  22 . In the present embodiment, the pump  24  is accommodated in the housing  4 . The pump  24  has an impeller  38  driven by a motor (not shown). The impeller  38  starts to be driven, for example, when the portable computer  1  is powered on or when the temperature of the CPU  15  reaches a predetermined value. The motor for the pump  24  is electrically connected to the batteries  8 , accommodated in the housing  4 . The rotation speed of the impeller  38  is varied by varying a voltage supplied to the motor.  
     [0040] When the impeller  38  of the pump  24  is rotated, the liquid coolant is delivered from the pump  24  to the heat receiving portion  21 . The liquid coolant then flows along the circulating path  23 . More specifically, the liquid coolant filled into the coolant channel  26  of the heat receiving portion  21  absorbs heat from the CPU  15  while flowing through the coolant channel  26 . The liquid coolant thus heated is delivered to the heat radiating portion  22  through the first pipe  36 . It then flows through the coolant channel  32 . While the liquid coolant is flowing through the channel  32 , the heat from the CPU  15  absorbed by the liquid coolant is diffused to the first and second radiator plates  29  and  30 . The heat is then released from the surfaces of the radiator plates  29  and  30 .  
     [0041] The liquid coolant is cooled by heat exchange in the heat radiating portion  22 . It then returns to the coolant channel  26  of the heat receiving portion  21  via the second pipe  37 . The liquid coolant absorbs heat from the CPU  15  again while flowing through the coolant channel  26 . It is then delivered to the heat radiating portion  22 . The repetition of such a cycle allows heat from the CPU  15  to be transferred to the heat radiating portion  22 . The heat is released from the heat radiating portion  22  to the exterior of the portable computer  1  through the display housing  11 .  
     [0042] As shown in FIGS. 1, 4, and  7 , an electric fan  40  is accommodated in the display housing  11  of the display unit  3 . The electric fan  40  is used to blow a cooling air against the heat radiating portion  22 . It is located at the lower end of left side of the heat radiating portion  22 .  
     [0043] The electric fan  40  comprises a centrifugal impeller  41  and a fan casing  42  in which the impeller  41  is accommodated. The impeller  41  starts to be driven by a motor  43 , for example, when the portable computer  1  is powered on.  
     [0044] The fan casing  42  has first and second suction ports  44   a  and  44   b  and an ejection port  45 . The first suction port  44   a  lies opposite first intake holes  46   a  opened in the front surface of the display housing  11 . The second suction port  44   b  lies opposite second intake holes  46   b  opened in the rear surface of the display housing  11 . The ejection port  45  opens toward the heat radiating portion  22 .  
     [0045] When the impeller  41  of the electric fan  40  is rotated, the air present outside the display unit  3  is sucked into the first and second suction ports  44   a  and  44   b  of the fan casing  42  through the first and second intake holes  46   a  and  46   b , respectively. The sucked air is ejected from the ejection port  45  toward the heat radiating portion  22 .  
     [0046] As a result, the flow of a cooling air is formed inside the display housing  11 . This cooling air forcibly cools the heat radiating portion  22 . Heat from the CPU  15  conducted to the heat radiating portion  22  is carried away by the flow of the cooling air. The cooling air heated by the heat exchange between itself and the heat radiating portion  22  is discharged to the outside of the display unit  3  from vents  47  opened at the upper end of the display housing  11 .  
     [0047] In the present embodiment, the electric fan  40  continues its operation even after the display unit  3  is rotationally moved to the closed position, unless the portable computer  1  is powered off. Further, when the display unit  3  is rotationally moved from the closed position to the open position, the electric fan  40  is controlled to increase the rotation speed of the impeller  41 .  
     [0048] The portable computer  1  configured as described above incorporates a program that serves to increase the amount of heat released from the heat radiating portion  22  when the display unit  3  is rotationally moved to the closed position.  
     [0049]FIG. 9 is a block diagram showing operation control of the pump  24 . As shown in FIG. 9, a controller  48  receives a signal for the temperature of the CPU  15 , which is outputted by the temperature sensor  18 , a signal for the temperature of the liquid crystal display panel  10  which is outputted by the temperature sensor  35 , and a signal indicating whether or not the display unit  3  is in the closed position, the signal being outputted by the position sensor  14 . The controller  48  is controlled by the CPU  15 . The controller  48  determines the current operational status of the portable computer  1  on the basis of various inputted signals. The controller  48  thus controls the operation of the pump  24 .  
     [0050]FIG. 10 is a flow chart showing a procedure of increasing the flow rate of the liquid coolant flowing through the circulating path  23  on the basis of operational status of the portable computer  1 .  
     [0051] While the portable computer  1  is in operation, the temperature of the CPU  15  is first detected via the temperature sensor  18  in the first step S 1 . The signal for the temperature of the CPU  15  is inputted to the controller  48 . The controller  48  determines whether or not the temperature of the CPU  15  is within a predetermined specified range.  
     [0052] If it is determined in step S 1  that the temperature of the CPU  15  exceeds the specified range, the procedure proceeds to step S 2 . In step S 2 , a process is executed to set the clock frequency of the CPU  15  to be lower than a normal value to reduce the amount of heat generated by the CPU  15 . In the next step S 3 , the temperature of the CPU  15  is monitored after the clock frequency has decreased. The temperature of the CPU  15  is then compared with a predetermined upper limit value. If the temperature of the CPU  15  is lower than the upper limit value, the procedure returns to step S 1  to repeat a process of detecting the temperature of the CPU  15 . In step S 3 , if the temperature of the CPU  15  exceeds the upper limit value, the procedure proceeds to step S 4 . In step S 4 , the controller  48  executes a process of shutting down the portable computer  1 .  
     [0053] On the other hand, if it is determined in step S 1  that the temperature of the CPU  15  is lower than the specified value, the procedure proceeds to step S 5 . In step  5 , the temperature sensor  35  detects the temperature of the liquid crystal display panel  10 . The signal for the temperature of the liquid crystal display  10  is inputted to the controller  48 . The controller  48  determines whether or not the temperature of the liquid crystal display panel  10  is within the standard range of temperatures at which the display panel  10  should be stored.  
     [0054] If the temperature of the liquid crystal display panel  10  exceeds the standard for the storage temperature, the controller  48  determines that the liquid crystal display panel  10  is significantly thermally affected by the heat radiating portion  22 . Then, the procedure shifts to the above step S 2 . The CPU  15  is thus prevented from generating heat, reducing the amount of heat transferred from the heat receiving portion  21  to the heat radiating portion  22 . This suppresses an increase in the temperature of the heat radiating portion  22 .  
     [0055] If the temperature of the liquid crystal display panel  10  is lower than the standard for the storage temperature, the procedure proceeds to step S 6 . In step S 6 , it is determined whether or not the display unit  3  is in the closed position. If it is determined in step S 6  that the display unit  3  is in the open position, then at the next step S 7 , a process is executed to detect the temperature of the liquid crystal display panel  10 .  
     [0056] In step S 7 , if it is determined that the temperature of the liquid crystal display panel  10  exceeds the operational standard, the controller  48  determines that the liquid crystal display panel  10  is significantly thermally affected by the heat radiating portion  22 . The procedure then shifts to the above step S 2 . The CPU  15  is thus prevented from generating heat, reducing the amount of heat transferred from the heat receiving portion  21  to the heat radiating portion  22 . This suppresses an increase in the temperature of the heat radiating portion  22 .  
     [0057] If it is determined in step S 7  that the temperature of the liquid crystal display panel  10  is lower than the operational standard, the procedure proceeds to step S 8 . In step S 8 , the controller  48  executes a process of increasing the clock frequency of the CPU  15  or a process of maintaining the clock frequency if it has already reached its limit.  
     [0058] If it is determined in step S 6  that the display unit  3  is in the closed position, the procedure proceeds to step S 9 . In step S 9 , a process is executed to increase the flow rate of the liquid coolant flowing through the circulating path  23 . Specifically, the controller  48  controls the operation of the pump  24  to increase a voltage supplied to the motor for the pump  24 . This increases the rotation speed of the impeller  38  and thus the amount of liquid coolant ejected per unit time. Therefore, the amount of liquid coolant delivered from the pump  24  to the heat receiving portion  21  increases, thus increasing the amount of heat transferred from the heat receiving portion  21  to the heat radiating portion  22 .  
     [0059] According to the first embodiment of the present invention, when the display unit  3  is rotationally moved to the closed position, the liquid crystal display panel  10  stops its display operation. Further, the rotation speed of impeller  38  of the pump  42  increases. This increases the flow rate of liquid coolant flowing from the heat receiving portion  21  to the heat radiating portion  22  and thus the amount of heat transferred from the heat generating portion  21  to the heat radiating portion  22 . As a result, the surface temperatures of the first and second radiator plates  29  and  30  increase to enhance the radiating performance of the heat radiating portion  22 .  
     [0060] In other words, the radiating performance of the heat radiating portion  22  is improved only while the liquid crystal display panel  10  stops its display operation. Thus, the display quality of the liquid crystal display panel  10  is not degraded even if the display panel  10  is significantly thermally affected by the heat radiating portion  22 . Consequently, even though the heat radiating portion  22  and the liquid crystal display panel  10  are adjacent to each other, it is possible to increase the amount of heat released from the heat radiating portion  22  to enhance the cooling performance for the CPU  15 .  
     [0061] According to the above configuration, the electric fan  40  is accommodated in the display housing  11  of the display unit  3 . The electric fan  40  blows the cooling air against the heat radiating portion  22  to positively cool the heat radiating portion  22  by the cooling air. As a result, the surface temperature of the heat radiating portion  22  decreases to reduce the thermal effects of the heat radiating portion  22  on the liquid crystal display panel  10 .  
     [0062] In particular, in the present embodiment, when the display unit  3  is rotationally moved from the closed position to the open position while the power supply to the portable computer  1  is on, the rotation speed of the impeller  41  increases. This increases the amount of cooling air blown against the heat radiating portion  22 . It is thus possible to positively cool the heat radiating portion  22  before the liquid crystal display panel  10  resumes its display operation. Consequently, the atmospheric temperature of the liquid crystal display panel  10  can be reduced in a short time to properly maintain the display quality of the liquid crystal display panel  10 .  
     [0063] In the above first embodiment, when the display unit  3  is rotationally moved to the closed position, the amount of heat released from the heat radiating portion  22  is increased. However, the present invention is not limited to this aspect. FIG. 11 shows a second embodiment of the present invention.  
     [0064] The second embodiment differs from the first embodiment in that the amount of heat released from the heat radiating portion  22  is increased while the power supply to the liquid crystal driving circuit is off. The other basic arrangements of the portable computer  1  are similar to those of the first embodiment.  
     [0065]FIG. 11 is a flow chart showing a procedure of increasing the flow rate of the liquid coolant flowing through the circulating path  23  on the basis of usage of the portable computer  1 . In FIG. 11, the processing executed between steps S 1  to  5 S is similar to that in the first embodiment. The processing after step  5 S differs from that in the first embodiment.  
     [0066] In the second embodiment, if the temperature of the liquid crystal display panel  10  detected in step S 5  is lower than the standard for the storage temperature, the procedure proceeds to step S 6  to detect whether or not the power supply to the liquid crystal driving circuit of the liquid crystal display panel  10  is off.  
     [0067] Specifically, after the external display device  13  has been connected to the portable computer  1 , when the operator chooses the use of the external display device  13 , the liquid crystal driving circuit is powered off. Then, the liquid crystal display panel  10  stops its display operation. Thus, in step S 6 , the condition of the power supply to the liquid crystal driving circuit is detected. If it is determined that the power supply is off, the procedure shifts to step S 7  to increase the flow rate of the liquid coolant flowing through the circulating path  23 . Control executed to increase the flow rate of the liquid coolant is similar to that in the first embodiment.  
     [0068] If it is determined in step S 6  that the power supply to the liquid crystal driving circuit is on, the procedure returns to step S 5  to repeat a process of detecting the temperature of the liquid crystal display panel  10 .  
     [0069] Also in the second embodiment, the radiating performance of the heat radiating portion  22  can be enhanced only while the liquid crystal display panel  10  stops its display operation. Accordingly, the cooling performance for the CPU  15  can be enhanced without degrading the display quality of the liquid crystal display panel  10 .  
     [0070] In the above first embodiment, the pump is accommodated in the housing of the main unit. However, the present invention is not limited to this aspect. The pump may be accommodated in the display housing of the display unit.  
     [0071] Furthermore, the electric fan is not an essential component and may thus be omitted.  
     [0072] Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.