Did you know that moisture is always present in atmospheric air, even if we can’t see it? When the air is compressed in a compressed air system and is cooled beyond its dew point, it condenses into liquid water. This creates a conundrum, as compressed air systems rely on dry air and cannot perform their functions properly if the air is wet. More specifically, the air must not contain any liquid moisture and should be at a relative humidity of less than 50% to prevent corrosion.
Moisture in Compressed Air is Damaging
Want to know exactly what troubles liquid water in air systems can cause? Check out the following:
It can block control air lines, prevent instruments from reading or actuating properly, and result in general instrumentation malfunctions;
It can damage air tools by preventing proper lubrication;
It can cause water hammer events that damage equipment and piping;
It can cause product spoilage and negate product integrity; and
Processes can be directly impacted when water enters from the air stream.
Removing moisture for corrosion protection is also important for the equipment using the air and the air system itself. Particulate created from rust and scale can foul lines and damage components of the air system. In the worst case, corrosion could lead to failure in the pipe work, creating leaks and preventing air from reaching the process where it is needed.
Most manufacturers use compressed air dryers as one of the methods for removing moisture. Air exiting a compressor is heated and 100 percent saturated with water. As the air cools, liquid water begins to condense. Because the air typically gets cooler the further it gets into the system (air systems also often pass through cold areas like the outdoors before reaching the process), it is much more effective to dry the air prior to putting it into the air net.
While filters and separators are able to remove liquid moisture droplets from a system, they are unable to remove water vapor. In order remove water vapor, you must use a dryer.
Removing Moisture from Compressed Air
There are five techniques for removing moisture from compressed air:
Cooling plus separation is exactly how it sounds. The hot, compressed air is cooled, which allows a large amount of water to condense. Once the water is condensed it can then be separated from the air. This is typically completed via an aftercooler, or a heat exchanger that cools the hot compressed air in order to precipitate the water that would otherwise condense.
Over-compression involves the compression of air to a higher pressure than the intended working pressure; after separation, the air is allowed to expand to the working pressure so it can be used in the intended process. Because this process is high-energy, over-compression is only suitable for very small air flow rates.
Membrane drying uses the process of selective permeation of the gas components in the air to separate the water vapor out. As the filtered, wet compressed air enters the cylinder, water vapor permeates the membrane coating and collects on the fibers; meanwhile, the dry air continues through the fibers.
Absorption drying is a chemical process in which water vapor is bound to an absorption material such as sodium chloride or sulfuric acid. The absorption material can be either a solid or a liquid and is much less common than other drying methods.
Adsorption drying involves the flow of moist air over a hygroscopic material or “desiccant” to be dried. Typical materials used are silica gel, molecular sieves and activated alumina, and include purge regenerated adsorption dryers, heated purge regenerated dryers, blower regenerated dryers, and heat of compression dryers.