There are many other uses requiring moisture removal to a low dew point. For example, railroad tank cars, which carry liquid chlorine, are padded (charged) with compressed air to enable pneumatic unloading. Chlorine will combine with water vapor to form hydrochloric acid; therefore, the compressed air must have minimum moisture content to prevent severe corrosion. Droplets of moisture in wind tunnel air at high- testing velocities may have the effect of machine gun bullets, tearing up the test models. Air used for low temperature processing (for example, liquefaction of nitrogen or oxygen) can form ice on cooling coils, thus requiring defrosting. The lower the moisture content of the air, the longer the periods between defrosting shutdowns.