Receiver Driers and Accumulators and now Desiccant Bags all “Dry” and filter refrigerant in an A/C System.
There are both some obvious and not so obvious differences in the way they look and the function they perform. The first thing to be aware of is that Refrigerant is naturally hygroscopic which means that it absorbs moisture which then can react with the refrigerant and oil in an A/C system to form acids that in turn attack internal components that can lead to catastrophic system failure.
Receiver Driers apart from basic shapes to suit different mountings and applications have remained virtually unchanged from very early A/C systems. They “dry” the refrigerant by passing it through desiccant which is essentially the same material that you see in food bags and shoe boxes which absorbs moisture and stores it, locking it out of circulation in the system and preventing it from forming acids that occur in systems contaminated by moisture. Receiver Driers also filter the refrigerant by passing it through various types of gauze filters. The exit point for refrigerant in a RD is in a very strategic position, at the bottom, allowing any bubbles of refrigerant to rise and supplying a constant stream of Liquid refrigerant to the TX valve. Receiver Driers are also somewhere in line between the exit point of the Condenser and the TX Valve. They can in later cases quite often form part of the “Sub Cooling” or extra cooling process by also being in the airstream such as the Isuzu D-Max style below mounted in conjunction with the Condenser but behind the vehicle grille.
Accumulators also Dry and Filter the refrigerant but instead of having the outlet at the bottom and supplying a constant supply of liquid refrigerant like a Drier, an Accumulator has its outlet point at the top with the aim to ensure no liquid that exits the Evaporator without “boiling off” is allowed to return to the compressor which could be damaged due to its inability to compress a liquid. This is why the Accumulator is larger than a drier - to “accumulate” the refrigerant, in the low side of the system – to protect the compressor and has a large surface area – to pick up heat aiding complete “boiling off” of any remaining liquid. There is a small pick up point in the bottom of the Accumulator to allow any oil that gravitates to the bottom to be picked up and returned to the compressor to lubricate it.
Desiccant Bags are the internal section of a drier that is able to be changed independent of the external housing. These are normally built into the condenser and form a part of the sub cooling section of the condenser. Desiccant Bags along with Driers and Accumulators should be replaced every two years when a service is performed and always if a system is opened for any work.