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In this paper, a study of useful temperature range of heat pipes em-ploying ThermS 300, mercury, sulphur and sodium as the working fluid are presented.A ThermS 300 heat pipe is made from a SUS 304 stainless steel tube with 25mm diameter and 1.5mm thickness and is wickless type. The total length of the heat pipe is 1000mm, the length of heating zone is 180mm, adiabatic zone is 320mm and the remaing part is the condenser.Mercury heat pipe is manufactured from SUS 304 stainless steel tube with 12.7mm diameter and 1. 5mrn thickness. The capillary structure is two layers of 50 mesh stainless steel. The total length of heat pipe is 300mm, the length of heating zone is 60mm, adiabatic zone is 60mm and the remaining part is condenser. Sulphur heat pipe is made from STPA steel tube for boiler with 27.2mm diameter and 4. Omm thickness and is wickless type. The total length is 500mm. The length of heating zone is 100mm, adiabatic zone is 100mm and condenser zone is 300mm.Sodium heat pipe is manufactured from SUS 316 L stainless steel tube with 25mm diameter and 1. 5mm thickness and the capillary structure is two layers of 50 mesh stainless steel. The total length is 1000mm, in which heating zone is 265mm, adiabatic zone is 235 mm and condenser zone is 500mm.The experiment is carried out mainly by means of bottom heating, in which surface temperature fluctuation at evaporator and condenser zone and pressure fluctuation in heat pipe are measured at low pressures.As a result of the tests, the useful temperature range of these heat pipes is discussed.

In this paper, the results of the performance tests of the air to air heat exchanger and the heat pipe used in the unit are presented. The heat exchanger is consisted of 28water-copper grooved type heat pipes with 9.74mm diameter. The number of rows of heat pipes is 8 rows with plate Al fins. Face area is 0.01852m2.As the results of the tests, it can be concluded that(1) the experimental values of the over-all heat transfer coefficients are nearly equal to the values calculated by equation being based on over-all thermal resistance of the heat pipe.(2) the experimental values of outside heat transfer film coefficient including fin effi-ciency and contact resistance between fin and heat pipe are lower than the values obtained by using Vampola's equation by 10 percent.(3) the heat transfer rate of the heat pipe exchanger can be obtained diagrammatical on the curve of the evaporator zone surface temperature vs condenser zone surface temperature under a constant heat input.(4) the pressure drop across a heat pipe bunk is higher than the calculated values, but there is not a big difference between the experimental values and the calculated values.

Life tests of stainless steel heat pipes containing sodium working fluid were carried out at evaporator surface temperatures of 600 °C, 650 °C and 700 °C for 1, 200 hours in ambient enviroments and during the tests, the change of the thermal resistance of each heat pipe was examined.After finish of each the test, the containers were cutt off and corrosion at the each part of the containers and wicks was examined through X-ray analizer, metallurgical microscope etc.Heat pipes were fabricated from sus 316 L stainless steel with 50 mesh 2 layers screen wick of same material.The total length of the heat pipe was 1, 000 mm will 25 mm diam-eter and the length of heating zone by Ultra-gold furnace was 265 mm, adiabatic zone was 235 mm and the remainig part was the condensor.The following became clear after the examination:1) Cr crystal substance was deposited on the inner surfaces of the containers and the wick surfaces due to elution or precipitation of some Cr from the containers and mesh wicks.2) Selecting crystal boundary corrosion was observed at the wick and the nozzle on the heating side.However it is clear that such least amount of corrosion will not affect the life of heat pipes.