The Ac dryer is usually located between the condenser and the evaporator, and protects the AC against excess moisture. It absorbs the moisture particles by means of the chemical substances contained in it. The dryer must always be replaced with a new one (fitting a used one in doesn't make sense as it has already absorbed moisture). The dryer needs replacing not only because of a failure but also each time the AC system loses its tightness, the compressor, the evaporator or the condenser are replaced.