Air Treatment

Air Saturation

The air we breathe from our atmosphere contains moisture in the form of water vapour; people often refer the amount of moisture in the air as levels of ‘humidity’. Humidity levels are a percentage of the saturation of air with water.
The maximum amount of water saturation in air varies greatly with temperature; hotter air can hold much more moisture than cold air. Due to the change in air pressure when compressing air its ability to hold moisture is seriously reduced, so some of the moisture content that was previously in the air will simply condense out of it when the air is compressed. This is why drainage systems and dryers are an important part of compressor systems.
If the water was allowed to travel out of the compressor into the pneumatic components that it was powering then this could cause big problems, for example ceasing valves and actuators.

Relative Humidity = Mass of water vapour present X 100
                          Mass of water needed to saturate it

Stages of air treatment

Filters
Compressed air requires filtration and drying before it leaves for pneumatic components, atmospheric air carries many particles of debris and moisture that unless filtered out can block valves or cause increased wear. Compressors usually filter air before its compressed, this is mainly to remove relatively large particles which could damage the compressor. A fine grade filter used after it is compressed helps filter the small particles which could clog or damage the pneumatic components that are used in pneumatic system set ups.

Air dryers
Before air can be used the excess moisture has to be removed, where well dried air isn’t a requirement all that may be needed is a basic intercooler followed by a separator unit; this is where the condensed water collects and is drained off.

Refrigeration units
The dew point can be lowered even further by cooling the air with a refrigeration unit which cools the air even further to around 0°C helping more moisture collect in the separator tank. This cold air is then routed through a separate section of the initial heat exchanger to help cool the warm air which is flowing through from the compressor.
This system of drying is suitable for most systems as air is dried out well.

Deliquescent dryer
Where completely dry air is a necessity then a chemical dryer should be used, the chemicals can remove moisture in two ways.
1) A chemical agent called a desiccant is used, this chemical agent collects water vapour and holds it while it eventually turns itself into a liquid once so much vapour has been collected and runs to the bottom of the unit where it is drained off. This chemical agent needs to be replaced quite regularly.

Absorption dryer
2) Within an absorption dryer a material called silicone dioxide or copper sulphate is used, this process works by attracting moisture to the sharp edges of these granular materials. Once these materials become saturated they are dried by passing heat through them, this process cant be carried out while it is still drying incoming air so they are usually set up with 2 of these moisture collectors which can be swapped over instantly, this allows one collector to collect the moisture while the remaining collector is left to dry itself out.