Abstract:
A heat pipe which can prevent frost damage of the pipe body due to freezing of the typical working fluid in cold climates by encapsulating an aqueous solution containing about 0.5 to about 10 wt % glycols as a working fluid, and which has a working performance almost comparable to that of the heat pipe in which water is used as the working fluid.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a heat pipe, and especially to a heat pipe which is improved in solving the problem of frost damage in cold climates.  
         [0002]     The present invention relates to the subject matter contained in Japanese Patent Application No. 2003-173467, filed on Jun. 18, 2003, which is expressly incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0003]     In the prior art, a heat pipe, in which a working fluid such as water is encapsulated in a metal pipe excellent in heat conductivity such as a copper pipe, and which utilizes latent heat generated by a phase change of the working fluid in the system from a liquid phase to a gas phase, as well as from the gas phase to the liquid phase, is used for the purpose of removing the heat from equipments, or of heating. For example, those heat pipes are widely employed for heat exchange in electronic equipments such as a personal computer, and for local heating of train stations, roadways, points, cars and so on in cold climates.  
         [0004]     The liquid to be employed as a working fluid of the heat pipe are required to have: (1) excellent heat conductivity; (2) large critical heat transporting capacity; (3) favorable compatibility with a container and a wick so as not to generate gas due to corrosion; (4) innocuousness; (5) incombustibility, etc.  
         [0005]     The most applicable working fluid to meet those requirements is water. However, in a cold condition, water freezes into ice and its volume expands to cause breakage of the heat pipe. In case the heat pipe comprising water as the working fluid is used in cold climates, for example, the internal working fluid freezes and expands, and the expanding pressure may burst the pipe.  
         [0006]     The bursting of the pipe can be avoided if Hydrochlorofluorocarbon such as alcohol, hydroflorocarbon or hydrofluoroether is used in place of water; however, heat conductivity of the alternatives to chlorofluorocarbon is inferior to that of water.  
         [0007]     As a measure for cold climates, an antifreeze liquid is used as the working fluid of automobiles or the like. The antifreeze liquid is the liquid the freezing point of which is lowered so as not to freeze even below freezing temperature by adding water with ethylene glycol or propylene glycol. As specified in the description of Japan Industrial Standard K2234, the widely used antifreeze liquid is made by adding water with approximately 30 volume percent or 50 volume percent of ethylene glycol and/or propylene glycol, in order not to freeze even at minus 10 degrees C.  
         [0008]     However, if a large amount of ethylene glycol and/or propylene glycol is mixed into water, the boiling point rises and the viscosity of the working fluid increases to degrade its heat conductivity. As a result of this, the performance of the heat pipe is deteriorated.  
       SUMMARY OF THE INVENTION  
       [0009]     A main object of the present invention is to provide a heat pipe in which the frost damage of the pipe body due to freezing of the working fluid in cold climates is prevented.  
         [0010]     Another object of the present invention is to provide a heat pipe having a comparable working performance to that of the heat pipe using water as the working fluid.  
         [0011]     Still another object of the present invention is to provide a heat pipe excellent in heat conductivity.  
         [0012]     According to the present invention, therefore, there is provided a heat pipe wherein water containing about 0.5 to about 10 wt % glycols is used as the working fluid.  
         [0013]     According to the present invention, moreover, ethylene glycol and/or propylene glycol is/are preferable as the aforementioned glycol.  
         [0014]     According to the present invention, still moreover, distilled water or deionized water is preferable as the water. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]      FIG. 1  is a cross-sectional view showing one example of a heat pipe according to the invention.  
         [0016]      FIG. 2  is a table showing results of the tests for heat conductivity according to Examples 1 and 2, and Comparative Examples 1 to 3. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     Preferred embodiments of the present invention will be described hereinafter. In  FIG. 1 , there is shown one example of the heat pipe according to the invention. According to the heat pipe  1 , a working fluid  3  is encapsulated in a container  2  made of a metallic material such as copper, copper alloy, aluminum, stainless steel or the like. The container  2  comprises a heating portion  4  and a heat radiating portion  5 . A fin  6  or fins are formed on the heat radiating portion  5 . Moreover, inside of the pipe is kept depressurized. Although  FIG. 1  shows a wickless heat pipe, which does not have a wick, and in which gravity is used as a motive power, the present invention can be applied not only to a double-pipe type and a loop type wickless heat pipe, but also to a heat pipe having a wick.  
         [0018]     Basically, water having a large evaporation latent heat is used as the working fluid. The working fluid is brought to boil and evaporated at the heating portion  4  where a heat source such as a heater is arranged (heater not shown). At this time, the heat outside of the heat pipe is drawn. The generated vapor ascends in the heat pipe and liquefies at the heat radiating portion  5 . At this time, the heat is radiated. The working fluid in a liquid phase flows down again in the heat pipe by its own weight to the heating portion  4 .  
         [0019]     The heat pipe can be operated full-time by activating the heat source, but normally, in view of the efficiency of thermal energy, it is operated only when needed. As a result of this, in cold climates, there arises a problem in that the working fluid inside of the heat pipe freezes when the heat pipe is not under operation.  
         [0020]     According to the present invention, a liquid comprises the water, to which a certain amount of glycol is added, as the working fluid to be circulated inside of the heat pipe.  
         [0021]     In the present invention, glycols can be exemplified by a low-molecular weight, room-temperature and liquid organic compound which has hydroxyl groups on both its ends, and specifically by ethylene glycols such as ethylene glycol, diethylene glycol or triethylene glycol; propylene glycols such as propylene glycol or dipropylene glycol; and butanediol or the like. A mixture of those liquid organic compounds can also be applied to the present invention.  
         [0022]     The addition amount of glycols for 100 wt % of the working fluid should be in the range from about 0.5 to about 10 wt %, preferably from about 0.7 to about 5 wt %, and more preferably from about 1.0 to about 3 wt %. Even if the addition amount of glycols is within the above-mentioned range, it is impossible to prevent the working fluid from freezing. On the other hand, if the addition amount of glycols exceeds the above-mentioned ranges, the heat conductivity of the working fluid degrades so that the object of the present invention cannot be attained.  
         [0023]     In order to prevent glycols from deteriorating at high temperature, distilled water containing no metal ions or deionized water is preferable as the water component in the aqueous solution.  
         [0024]     Inventors of the present invention discovered that an aqueous solution containing glycols within the above ranges freezes into sherbet-like ice containing a solid-liquid mixture, and the strength of the frozen solution is lowered. Therefore, this does not burst the heat pipe. Moreover, the heat conductivity is also excellent and comparable to that of the water.  
         [0025]     The following provides a description of specific examples. However, although the invention will be explained below in more detail by reference to the following Examples, the invention should not be construed as being limited to the following Examples only. It is to be expressly understood, that the Examples and Figures are for purpose of illustration only and are not intended as a definition of the limits of the invention.  
       EXAMPLE 1  
     Test for Heat Conductivity  
       [0026]     A device as illustrated in  FIG. 1  was used for the tests of heat conductivity. A pipe having a diameter of 16 mm and a length of 1100 mm is provided with double-pipe condensers over 500 mm of an upper portion, and 500 mm of a lower portion is heated by a heater. As the working fluid, an aqueous solution containing 1% weight concentration of ethylene glycol was used and 51 cm 3  (i.e., 40% of capacity of the heating portion) thereof was encapsulated in the pipe. The air was adequately removed from the system.  FIG. 2  shows a table of overall heat transfer coefficients kw/(m 2 K) when a heat gauge, which is arranged on a pipe wall at 550 mm from the bottom of the pipe (i.e., a heat insulating portion), indicates 40 degrees C. and 80 degrees C. after 10 kw/m 2  and 50 kw/m 2  of heat is supplied to the heating portion  4  of the heat pipe.  
       EXAMPLE 2  
       [0027]     In example 1, an aqueous solution containing 2% weight concentration of ethylene glycol was used for the working fluid instead of the aqueous solution containing 1% weight concentration of ethylene glycol. No alteration was made in the rest of the conditions of example 1. Results are shown in  FIG. 2 .  
       COMPARATIVE EXAMPLES 1 to 3  
       [0028]     In example 1, aqueous solutions containing 0%, 20%, and 40% weight concentration of ethylene glycol were used for the working fluid instead of the aqueous solution containing 1% weight concentration of ethylene glycol. No alteration was made in the rest of the conditions of example 1. Results are shown in  FIG. 2 .  
       EXAMPLE 3  
     Test for Congelation and Solidification  
       [0029]     Glass test pipes having a diameter of 16 mm, a length of 150 mm and a thickness of 1 mm are used for the test for congelation and solidification. The aqueous solution containing 1% weight concentration of ethylene glycol was filled in each test pipe in the amount of 44 cm 3 , and stored for 20 hours in a freezer kept at minus 20 degree C. It was found that the aqueous solution of 1% weight concentration of ethylene glycol froze into sherbet-like ice, and the test pipe did not burst.  
       EXAMPLE 4  
       [0030]     In example 3, an aqueous solution containing 2% weight concentration of ethylene glycol was used for the working fluid instead of the aqueous solution containing 1% weight concentration of ethylene glycol. No other alteration was made in the rest of the conditions of example 3. As a result, the aqueous solution of 2% weight concentration of ethylene glycol froze into sherbet-like ice, and the test pipe did not burst.  
       COMPARATIVE EXAMPLE 4  
       [0031]     In example 3, an aqueous solution containing 0% weight concentration of ethylene glycol was used for the working fluid instead of the aqueous solution containing 1% weight concentration of ethylene glycol. No other alteration was made in the rest of the conditions of the example 3. As a result, in case of pure water, the test tube burst.  
       COMPARATIVE EXAMPLE 5  
       [0032]     In example 3, an aqueous solution containing 20% weight concentration of ethylene glycol was used for the working fluid instead of the aqueous solution containing 1% weight concentration of ethylene glycol. No other alteration was made in the rest of the conditions of example 3. As a result, the aqueous solution of 20% weight concentration of ethylene glycol froze into sherbet-like ice and the test pipe did not burst.  
       COMPARATIVE EXAMPLE 6  
       [0033]     In example 3, an aqueous solution containing 40% weight concentration of ethylene glycol was used for the working fluid instead of the aqueous solution containing 1% weight concentration of ethylene glycol. No other alteration was made in the rest of the conditions of example 3. In this case, the working fluid did not freeze into ice, and the test pipe did not burst.  
         [0034]     As thus far described, according to the heat pipe of the present invention, the working fluid does not freeze at a temperature below the freezing point at minus 20 degrees C., but instead turns into sherbet-like ice in which the solid-liquid is mixed. Therefore, the strength of the frozen solution is low enough so as not to burst the heat pipe. As a result of this, the pipe-damaging problem in cold climate is solved. Moreover, the heat conductivity of the heat pipe is also excellent, and it is almost comparable to that of water.  
         [0035]     Since the heat pipe of the present invention has the above-mentioned characteristics, this can be suitably applied to applications such as removing electrical heat from a personal computer; melting snow from a platform of a train station, a roadway, a fence or the like; freeze proofing of a point; and heating.