A new air gap membrane distillation module based on hollow fiber with energy recovery was developed for desalination using 70 g·L-1 NaCl solution as the feed. The effects of membrane module length and membrane pore size were investigated. For measuring directly the effects of operating conditions

membrane module properties

and temperature and concentration polarization on the mass transfer coefficient

the total mass transfer coefficient was introduced. The effects of membrane pore size

feeding temperature and feeding flow rate were studied. The results showed that the total mass transfer coefficient increased with the increase of membrane pore size and feeding temperature. The membrane flux and gained-out put ratio were increased effectively with increasing membrane pore size. Since increasing membrane module length would reduce the membrane flux while enhance the gained-out put ratio

the membrane flux and gained-out put ratio should be considered comprehensively for selection of the appropriate membrane module length in the application process.