The purpose of an air dryer is to remove the moisture and contaminates from the compressed air before it reaches the supply reservoir or wet tank. The dryer is located between the compressor and the supply reservoir and is usually mounted on the frame rails behind the cab. There are two main types of desiccant air dryers. One type has the purge tank and desiccant cartridge contained inside the air dryer. The second type of air dryer is the one with an external purge tank and a spin on and off cartridge for easy maintenance of the air dryer. A large purge volume of air to desiccant produces the ideal filtering system and contamination protection for heavy-duty service today. Both types of air dryers must have regular maintenance performed on them. This means changing your air dryer desiccant at least once a year and checking the operation of your air dryer heater, purge valve and check valve. The desiccant in your air dryer does not absorb water but has the moisture in the air cling to the sides of the desiccant until the purge valve signals the dryer to purge. This blows the moisture off the desiccant and out the exhaust port.
So after a year of service you will find that there is oil and contamination on the desiccant that stops if from holding the moisture to it until the purge cycle. So the moisture and contaminates are able to travel through your air system causing all kinds of problems.
This is why I would recommend servicing your air dryer in the fall. This is when the dew point is the highest because the compressed air is warm and the ambient temperature is cool creating high levels of moisture. The spin on and off desiccant air dryer is becoming more popular because it can be maintained without taking the air dryer apart.
The heater in an air dryer is there to keep the purge valve from freezing up and is not involved in any form of drying the air. Without the purge valve the air dryer will not work. Some trucks have alcohol evaporators on them. If you are using an alcohol evaporator it must be plumbed in down stream of the air dryer. The alcohol will melt the desiccant in your air dryer if it is ahead or in front of the air dryer.
Air brake antifreeze is a better choice than alcohol for your air system. It has an additive in it that will lubricate air system components while removing moisture. This will prolong the life of your air system.
There is one other type of air dryer on the market today that is becoming very popular for highway use. It is made by CR and incorporates the spin on and off cartridge combined with the purge tank. This is the CR turbo 3000. This air dryer gives you the best of both worlds. Convenience and efficiency rolled into one. The spin on cartridge holds both desiccant and purge air and can be serviced in minutes. It is also turbo boost protected, so it can be used on for all engine application (fully compatible with Cummins, holset). An air dryer is an essential part of your air brake system. It will keep moisture and contamination out of you air brake system, if it is maintained properly.