Compressed air straight from the compressor contains moisture from the atmosphere. Although some water is removed during the intercooler and aftercooler stages, there is still a lot of water content in the air.
For many pneumatic applications, the compressed air has to be dry. Compressed air containing moisture is particularly damaging in a manufacturing environment. In a cold manufacturing environment, this type of compressed air tends to produce water droplets from condensation. It would be disastrous to the manufacturing products, and equipment.
Moisture tends to cause corrosion in machine parts. It will also cause changes in the property of lubricants, and other chemicals, resulting in sticking of moving parts.
Compressed air can be dried using the air dryer. Some dryers make use of refrigeration to cool down the air to its dew point in order to allow water to condense out. The water collected is then removed by draining it from the system. The air when it comes back to room temperature is drier than before.
A better dryness can be obtained by using desiccants.
Desiccant types of air dryer make use chemicals that are hygroscope in nature. These types of chemicals are able to absorb moisture in the air. Examples of some of these chemicals are silica gel, and alumina. They can absorb moisture in the air until they become saturated and cannot absorb anymore. They are also able to revert back to the original state of absorption by heating or by passing dry air through them. The latter is termed as regeneration. Both heating and regenerative types of desiccant air dryers are in common use.
Components of a desiccant regenerative dryer
The regenerative type dryer consists of two identical chambers. The piping is arranged so that wet air is led into one chamber at a time for drying of the air.
At a certain interval of time, or by means of a moisture-sensing signal, the air is then diverted into the next chamber. The previous chamber is then regenerated. There are some sequences to be followed, all controlled automatically. For example, the pressures are equalized before the change over. This prevents fluctuations in the outgoing pressures.
If it is a heat regeneration type, an electric heating coil will be switched on until all the moisture, which has been absorbed, is driven out.
For the regenerative type, a small flow of dry air from the operating chamber is led into the regenerating chamber, usually in the reversed direction. This regenerative dry airflow is switched off when the regeneration is complete.
The cycle of operation and regeneration is repeated for the running chambers.
Filters Because of the chemical nature of the desiccant type air dryer, it is very important not to contaminate it with oil, dust and other chemicals. Prefilters are installed for the compressed air before the dryers for this purpose. For the refrigeration type the stringent requirement of prefiltering the air is not so critical. However for good efficiency and practical operation, it is advisable to have one too. The desiccant type air may also pick up some extra dust in the form of desiccant powder in the discharge end. For this reason, it is common to install afterfilters on the discharge side.