By keeping the water in the refrigeration causes freeze-ups, corrosion, and acid formation which can damage compressor windings.

Water can come from many sources, such as trapped air from improper evacuation, system leaks, and motor windings, to name a few.

Another source is due to improper handling of polyester (POE) lubricants, which are hygroscopic; that is, they readily absorb moisture. Examples of this is when the oil cap is not tightly closed or left open.

POEs can pick up more moisture from their surroundings and hold it much tighter than the previously used mineral oils. This water can cause freeze-ups and corrosion of metallic components.

Water in the system can also cause a reaction with POEs called hydrolysis, forming organic acids.

To prevent the formation of these acids, the water within the system must be minimized. This is accomplished by the use of desiccants within the filter-drier. The three most commonly used desiccants are molecular sieve, activated alumina, and silica gel.