Nafion™ dryers absorb water on the internal surface of Nafion™ tubing, then perevaporate it into a surrounding dry purge gas (typically air). When the water is absorbed as water vapor then released as water vapor, there is no net change in phase and consequently no net energy is consumed. If water is present as a liquid rather than as a vapor within the tubing, it will still be absorbed as a liquid then released as a vapor, but now there is a change of phase from liquid to gas, requiring an input of energy.
If liquid water is permitted to enter the dryer, the transformation of the liquid into water vapor will draw heat from the dryer, cooling it. As the dryer cools, it condenses more water. As it absorbs this additional liquid water, it will cool more rapidly. Soon the dryer is cold and wet, and the dryer is then functioning as a condenser rather than as a permeable membrane. At this point the dryer has failed, and it must be removed from service and dried before it can again function properly. To prevent this failure, liquid water must not be permitted to enter the dryer during operation. The minimum operating temperature of the dryer is therefore limited by the sample dew point. As the gas sample passes through the dryer, water is removed and the sample dew point becomes progressively lower down the length of the dryer. The minimum operating temperature is the dew point of the sample at that point in the dryer, and is a temperature gradient, higher at the sample inlet and lower at the sample outlet.