This invention relates to boiling and condensing heat transfer type cooling devices, and more particularly to such boiling type cooling devices for cooling power semiconductor switching elements of inverters and choppers, etc., of electric components mounted on railroad vehicles.
FIG. 1 is a sectional view of a conventional boiling and condensing heat transfer type cooling device (hereinafter referred to as a boiling type cooling device or simple as a boiling cooling device) for cooling thyristors, which is disclosed, in Mitsubishi Denki Giho (Mitsubish Electric Corporation's Techinical Journal), Vol. 48, No. 2, 1974, p. 231.
In FIG. 1, flat thyristor elements 1 coupled to fins 2 in pressured contact therewith are accomodated within an evaporator 3 containing a liquid fluorocarbon 4. A condenser 5 is disposed above the evaporator 3 such that the fluorocarbon vapor 4a generated within the evaporator 3 is introduced via a vapor conduit pipe 6 to the condenser 5. A liquid return conduit pipe 7 carries to the evaporator 3 the liquid fluorocarbon 4 condensed from the fluorocarbon vapor 4a within the condenser 5.
The operation of the device of FIG. 1 is as follows. When the thyristors 3 are in operation, the electrical power loss taking place therein results in the generation of an amount of heat as large as several hundred watts. The heat thus generated is transferred to the fluorocarbon 4 via the fins 2 in pressured contact with the cooled surfaces of the thyristors elements 1. The flux of heat flowing through the fins 2 reaches as high a value as 10.sup.5 W/m.sup.2. As a result, the fluorocarbon 4 boils, and the thyristors elements 1 are cooled by means of the so-called boiling fluorocarbon cooling. The fluorocarbon vapor 4a generated within the evaporator 3 by the boiling of the fluorocarbon proceeds via the vapor pipe 6 into the condenser 5 to be cooled and condensed therein by means of an exterior cooler fan (not shown), and then returns in the form of liquid fluorocarbon 4 to the evaporator 3 via the liquid return pipe 7. The fluorocarbon boils and condenses repeatedly as described above, such that the thyristors 1 are cooled efficiently.
FIG. 2 shows the principle of the conventional boiling type cooling device disclosed in the above mentioned journal (Mitsubishi Denki Giho vol. 148, No. 2). The heat generating element 1 such as thyristors is immersed in the pool of cooling medium, such as fluorocarbon R113 (trifluorotrichloroethane C.sub.2 Cl.sub.3 F.sub.3) contained in the evaporator 3. When the fluorocarbon boils or evaporates, the pressure within the evaporator 3 increases. Thus, the vapor generated within the evaporator 3 proceeds into the condenser 5 to be condenser therein. The latent heat of evaporation absorbed by the vapor within the evaporator 3 and carried by it through the vapor pipe 6 is discharged in the condenser 5 by the condensing vapor. The condensed cooling medium 4 returns to the evaporator 3 via the liquid return pipe 7. Thus, continuous heat transport by means of evaporation and condensing is established, and the thyristor elements 1 are cooled continuously.
FIG. 3a and 3b show another conventional boiling type cooler device disclosed, for example, pressed Japanese patent publication No. 59-41307. Semiconducts 1 are in into contact with the evaporator 3, and the heat generated in the semiconducts 1 is transmitted to the liquid cooling medium 4 via the cooling fins 3a disposed within the evaporator 3. The vapor 4a generated in the evaporator 3 is guided via the vapor pipe 6 to the condenser 5 having radiation pipes 5a provided with radiation fins 5b. The condensed liquid cooling medium 4 returns to the evaporator 3 via the liquid return pipe 7.
FIG. 4 shows a deaeration device for the cooling medium (e.g., fluorocarbon 113) of a conventional boiling type cooling device. Coupled to a deaeration bath 8 containing cooling medium 4 are a plurality of pipes 11, 13, and 16, having valves 12, 14, and 17, respectively a sealed container 15 accomodating a heat-generating electrical device such as thyristor elements is coupled to the outer end of the pipe 16. Further a branch pipe 18 having a valve 19 is coupled to the cooling medium sealing pipe 16. The bath 8 is provided with an agitator 9 comprising a motor 9a, a shaft 9b, and agitation wings or spoons 9c. Further, the bath 8 is surrounded by a cooling shell (cooling box) 10.
The deaeration is effected as follows. First, keeping valves 12 and 17 in the closed state, valve 14 is opened to exhaust the interior of the bath 8 by means of an exhaust pump (not shown) coupled to pipe 13. Next, valve 14 is closed and valve 12 is opened to introduce liquid fluorocarbon 4 into the bath 8 via the pipe 11. After the bath 8 is filled to a predetermined level with the liquid fluorocarbon 4, valve 12 is closed again. Then, the liquid fluorocarbon 4 is cooled by the cooling shell 10 and is agitated by the agitator 9. Thereafter, valve 14 is opened to remove the non-condensing gas such as air from the liquid fluorocarbon 4. When the deaeration is over, valve 14 is closed.
Next, valve 19 is opened to exhaust the interior of the electrical device container 15 via the branch pipe 18. Thereafter, valve 19 is closed and valve 17 opened such that the liquid fluorocarbon 4 is introduced into the electrical device container 15.
The above conventional boiling type cooling devices all utilize fluorocarbons as the cooling medium for boiling and condensing heat transfer. However, recent research has shown that the fluorocarbons discharged into the atmosphere reach the stratosphere and destruct the ozonosphere. Thus, there is an urgent need for a substitute for fluorocarbons. Water has good cooling characteristics when boiled and is an obvious candidate for a substitute. However, the operating temperature of chopper devices, etc., (typical electrical devices mounted on railroad vehicles) usually ranges from -20.degree. C. to 80.degree. C. Thus, water may freeze and cause failure in the cooling device if it is used as the cooling medium.
It is conceivable to add an antifreeze such as ethylene glycol to water. This, however, still leaves the following problems unsolved. Water boils and evaporates more easily than the glycol. Thus, the concentration of water decreases in the evaporator, while that in the condenser increases. As a result, the cooling efficiency is progressively reduced. In addition, the dilute condenser water may freeze in the condenser 5 or in the liquid return pipe 7 to cause a failure thereof at a low ambient temperature.