The water vapor contained compressed air will condense out if the temperature of the air in plant piping falls below the compressed air dew point. If the pipes are below the freezing point of water, such as in an outdoor installation in winter, then ice forms. The ice will build up and block the flow of compressed air, causing severe pressure drop.
Even during cold winter days, when the ambient conditions in your compressor room are very dry, the compressed air produced by an air compressor will be at or near full saturation at room temperature. An air compressor raises the dew point of the air by compressing it to a smaller volume, making the air less able to hold water vapor. For this reason, some sort of air dryer must be used to remove enough water vapor to keep the pipes dry.
If your system uses refrigerated air dryers, you have a problem if you have pipes exposed to very low temperatures, these dryers produce compressed air with dewpoints of around 35° to 38° F (3° to 5° C). In order to prevent the lines from freezing, you will need secondary air drying. This application might use desiccant or membrane air dryers to produce compressed air with dew points of around minus 40°.
To save operating costs, it is best to dry only the air that goes outside to such low dew points—if you don’t need such dry air elsewhere in the plant. Air dried to low levels will cost between 15% and 30% more, just for dryer purge … so design should be done carefully. It is important to size the secondary dryer for worst case conditions, accounting for peak flows.
Already have desiccant dryers but still having your lines freeze? Try testing your air quality to determine the dew point. Often, some dryer malfunction or piping problem, such as a leaky bypass valve will cause unexpected and unacceptable dew points. If you find a problem, then investigate!