Effect of the length to inner diameter ratio of vapor tube on the performance of a vacuum closed loop thermosyphon

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Sarawut Sirikasemsuk
Thanaphol Sukchana


The effect of steam pipe size on the performance of closed loop thermosyphon under vacuum condition was studied to determine the relationship of heat load to pipe size affecting the saturation temperature and overall performance of the boiler. A closed loop thermosyphon was made of cylindrical copper pipes with a capacity of 3000 cc. Distilled water was used as the working fluid at 10% (300 cc) of capacity volume. The water was boiled by electric heaters of 530, 1085, 1640, 2210, 2780 and 3350 W. The steam pipes for testing were made of type L copper pipes of 7 sizes in accordance with ASTM B88 STD with a test section length of 400 mm. Length to the inside diameter (L/d) of pipe was in the ratio of 82.82, 49.94, 36.63, 28.90, 23.65, 20.05 and 17.50. The boiler was under vacuum initial condition in the experiment, the water cooled condenser had a constant temperature of 25°C and constant flow rate of 1 kg/min. The results showed that the L/d value had an effect on the closed loop thermosyphon performance and when heated at a constant volume, the saturation temperature increased with the L/d and was greater than 20. The total thermal resistance was also decreased with a decrease in L/d. The lowest total thermal resistance of 0.0343°C/W was when the heat load content was 2780 W with L/d of 17.50.


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