Sweating astronauts

An astronaut consumes about 2.7 litres of water per day through eating and drinking. Most of this water leaves the body again – either in liquid state (in form of urine or sweat) or as vapour (through the pores or through breathing). If the water vapour leaving the bodies were not removed from the air, the Station would quickly feel like a sauna and the astronauts would have difficulties breathing.

The ISS life-support system has several tasks. It keeps the cabin air clean (filter the air for particulates and micro-organisms), provides the right level of gases, a preferable air pressure and the right temperature. As described in the previous paragraph, the humidity is also controlled – if the level is too high, the ISS life-support system makes sure that the surplus water vapour in the air is collected.

Imagine

Water droplets form due to condensation

a cold day and a person with glasses entering a warm humid room – the glasses steam up immediately. This “steam” is a layer of tiny water droplets on the glasses. The principle of water recovery on board the ISS is very similar: the warm humid air is blown across a cold surface where tiny water droplets will form (condensation). But on board the ISS there are weightless conditions, which means that the water droplets are not heavier than the air and will not run down a surface to be collected at the bottom. A solution to this is to spin the surface. The spinning will lead the droplets to the outside of the surface where they can be collected. Surfaces with hydrophilic coatings together with slurpers (tiny holes with suction tubes on the back) can also be used. The hydrophilic coating is used to make the water stick to the surface, the slurpers suck the water from the surface.