An experimental and numerical study is presented in which laminar air flow with low water-vapour concentration is cooled at temperatures well below 0 degrees C to remove the humidity contained in the carrier gas. The experiment uses a cold trap that consists of a vertical tube-bundle immersed in a cold shell. The analysis is carried out with the help of a model that simulates heat and mass transfer, as well as aerosol dynamics in the tube flow. It is found that an optimal temperature range exists for the efficiency of the process. The results are interpreted in terms of the competition between vapour diffusion to the wall and ice particle formation and growth.