Air Drying Units

Is your compressed air all wet? If you answered yes, you are not alone. Moisture, either liquid or vapour, is present in compressed air as it exits the compressor system. If this moisture is not properly removed, your compressed air system can lose efficiency and require dramatically increased maintenance, which can result in costly downtime.

To avoid these problems, compressed air system designers have a number of purification devices available to remove the remaining water vapor and other contaminants. The proper selection of these devices is critical as pneumatic applications and compressed air systems become increasingly sophisticated.

This article offers an overview and several strategies for selecting downstream purification equipment that effectively removes water and lubricant from compressed air systems

Where Does the Water Come from?
Ambient air, which includes atmospheric humidity (water vapour), is drawn into the compressed air system where it is compressed to a desired discharge pressure. Once the compressed air is discharged, its temperature is elevated and the moisture content is high. And, if a lubricated compressor is used, a small quantity of compressor lubricant, in both liquid and vapour form, is discharged with the compressed air.

Typical Example

Since the majority of pneumatic instruments and processes can not tolerate hot compressed air, compressors are normally supplied with aftercoolers and moisture separators.

Aftercoolers are heat exchangers that utilize either water or ambient air to cool the compressed air. As the water and lubricant vapors within the compressed air cool, a significant amount condenses into liquid. The amount of condensation is dependent upon the temperature of the air when it leaves the aftercooler.

Following this stage, the condensed water is collected and removed by the moisture separator, and discharged through a drain valve. However, it is important to remember that the compressed air is still saturated with water vapor at the discharge of the aftercooler/moisture separator. Additional condensation is generated downstream when the compressed air cools further.

Points to Review When Removing Water
Plant maintenance personnel and system designers must determine the air quality requirements for their specific compressed air applications.

Here are three areas that should be addressed:

	Review the air quality requirements of instrumentation, tools, and other air powered equipment, which is available from the manufacturer;
	Determine the air quality required for use in processes using compressed air, which can be obtained from the process designer; and
	Estimate the expected ambient conditions for all pneumatic equipment, processes and piping. For instance, outdoor locations during the winter months require compressed air to be dried to a lower dew point than indoor, heated locations.

An oversight within any of these issues can result in misapplied purification equipment, inefficient system operation, high operating and maintenance costs and even unnecessary capital expenditures.

Selecting Purification Systems
Condensed water, lubricant, water and lubricant vapours and other contaminants are removed with a variety of purification equipment used in combination.

Coalescing filters are the most common forms of compressed air purification. These filters remove liquid water and lubricants from compressed air and are installed downstream in a refrigerated air dryer system or upstream in a desiccant dryer system.

Most manufacturers claim a one psi "clean and dry" pressure drop, with the normal operating pressure drop between three and six psi. Manufacturers typically require filter changes when the pressure drop reaches 10 psi, which is approximately six to 12 months of operation. Coalescing filters will also remove particulate contamination; however, this will increase the pressure drop across the filter and shorten the filter element life. These filters are rated according to liquid particle retention size (micron) and efficiency, such as 0.50 micron and 99.99% efficient, or 0.01 micron and 99.9999% efficient.

High efficiency coalescing filters that feature low pressure drop (less than one psi) and long element life (five years minimum) should be specified when very low operating and maintenance costs are a requirement. These designs normally have a higher initial purchase price; yet, the resulting operating cost savings usually permit payback in less than one year.

When specifying a coalescing filter, be sure to confirm that the filter and element are compatible with the compressor lubricant. If these elements are not compatible, the filter system can fail and allow for contamination downstream.

Coalescing filters can only remove previously condensed liquids; they do not remove water or lubricant vapors from the compressed air. Any condensation produced from subsequent compressed air cooling will have to be eliminated.

When seeking to remove water and lubricant vapors from compressed air, specify air dryers. There are three styles of air dryers that are commonly specified: deliquescent, desiccant and refrigerated dryers.