Compressed air is found in almost any manufacturing environment. Often referred to as the “fourth utility,” it is widely used in control systems and for powering pneumatic equipment. In order for these systems to perform properly, the air must be dry.
“More specifically, the air must not contain liquid moisture and should be at a relative humidity of less than 50 percent to prevent corrosion,” says Mike Robinson, product marketing manager at Atlas Copco Compressors.
“Bulk liquid in compressed air systems can cause several issues: it can block control air lines and prevent instruments from reading or actuating properly; it can damage air tools by preventing proper lubrication; it can cause water hammer events that damage equipment and piping; and processes can be directly impacted when water enters from the air stream,” says Robinson.
He explains that removing moisture for corrosion protection is also important for the equipment using the air and the air system itself. Particulates 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.
The most common model of compressed air dryer is the refrigerated dryer, which cools the compressed air using a refrigerated heat exchanger to four to eight degrees above freezing. Refrigerated dryers use two heat exchangers, one for air-to-air and one for air-to-refrigeration. The cold outgoing air cools down the hot incoming air and reduces the size of compressor required. This temperature reduction causes the water in the air stream to condense; the condensed water is removed from the system with a separator and drain.
These dryers can come in cycling or non-cycling models. A cycling refrigerated air dryer will increase or decrease the refrigeration compressor load depending on the compressed air demand. Non-cycling dryers provide consistent dew points and typically have lower maintenance costs.
The second most common model is the desiccant dryer, according to Robinson. Desiccant dryers are also referred to as regenerative dryers and twin tower dryers. “In absorption,” says Robinson, “molecules adhere to the surface of a solid. A desiccant, a chemical product used as a drying agent due to its high surface area and affinity for water, absorbs the moisture in the compressed air.”
To remove this trapped moisture, the system goes through a regeneration process. During regeneration, the water is desorbed from the desiccant media using either expanded dry air from the system or heated ambient air. In order to make this process continuous, the dryer is designed with two drying chambers or towers so one side can function in the drying stage while the other regenerates.
Membrane dryer refers to the dehumidification membrane that removes water vapor from compressed air.
“This type of dryer is made of hollow fiber tubes that have small pores where water molecules pass through at a higher rate than gas molecules,” says Robinson. “Due to this type of preferential separation, the air that enters the membrane tubes exits at a lower moisture level.”
These dryers are used in applications such as pneumatic components, spray painting, air bearings, air spindles, medical equipment, air guns and pneumatic brakes for vehicles and trains.
Trends to Watch
“Technology advancements for compressed air dryers focus on improving the energy efficiency of the drying process,” says Robinson. “Changes in the system components can lead to improvements in the power required to operate the unit.”
Control strategies have a large impact on the amount of energy used in drying. By optimizing the dryer performance based on maintaining the proper relative humidity, a dryer can operate at a level that prevents water in a compressed air system while minimizing the power needed, he says.
Air losses due to purge and pressure drop are also key focuses for energy efficient design improvements because they often represent a higher portion of the dryer operating cost than direct power. “Anything that can be done to reduce those items while still maintaining the dryer performance will result in lower power costs for the user,” says Robinson.