Relatively low productivity is a common problem for conventional tubular solar stills when running at atmospheric pressure. An improved tubular solar still is proposed to enhance the productivity

which can operate under conditions of vacuum pressure and feeding water continuously. An experimental prototype is tested under both constant heating temperature and constant heating power conditions. Its productivity and systematic temperature under vacuum condition is obtained

proving that it has better performance than a common distillation method running at atmospheric pressure. At operating pressure 40 kPa and fixed heat power of 200 W

the temperature difference between distilling water and condensation shell decreases 40%

and the daily accumulated freshwater yield increases 22.5%

with a value of 1.9 kg. An modified correlation to calculate the mass transfer inside the cavity under vacuum condition is obtained against the experimental data in steady state

based on that a model is built to predict the freshwater yield of the tubular solar still under vacuum condition. The model has deviations for present prototype of about 2.1% for daily accumulated productivity

and 4% for maximum freshwater yield rate.