Compressed Air Purity Levels and Application of Dryers and Filters

In industries using compressed air, there is a requirement to ensure that the air quality meets the standard for its intended use. Compressed air usage is generally broken down into two categories, being “power” and “process”.

  • “Power” applications include such things as air tools, air cylinders, air motors and devices that require compressed air to actuate a mechanical component.
  • “Process” applications include processes where air is being mixed with, or is in direct contact with a chemical, gas or product to agitate, fluidize, convey, clean, shape or in some manner, finish the final product.

Before a dryer can be selected, the air purity requirement must be established. The “International Organization for Standardization” (ISO) has created a table for air purity levels under document 8573.1. This document defines purity levels based on particulate levels, water vapor levels and oil aerosol levels. The first reality, will be to understand that the process of compressing air will almost always result in the formation of condensate after compression and cooling. Air taken into the compressor intake at normal room temperatures and relative humidity, will at some point after compression and temperature reduction, become saturated. Further reduction in temperature beyond that point, must result in condensate forming in the airline. Any particulate/impurities that were small enough to avoid being captured at the compressor inlet, will now become compressed into a much smaller space. If the system operating pressure is 100 PSIG, the space will be approximately 1/7 of the atmospheric space, so those particles/impurities must now reside in closer quarters.

The other consideration will be for the level of oil aerosols in the compressed air. When the system employs a lubricated air compressor, some compressor lubricating oil will be carried over into the compressed air. If an oil-less compressor is used, even though the compressor will not add oil to the compressed air, very small amounts of oil aerosols may be drawn in at the compressor intake. In either case, an oil coalescing filter is normally recommended to remove aerosols to acceptable levels. The first order of business in any system will be to determine what level of purity is required.

Normally, there is much less concern with “power” applications as the type of equipment using the air will usually only require that there be no free condensate in the air, and that it be reasonably particle free. If there are small amounts of oil aerosols in the line, they will not normally cause a problem, as most air tools require lubrication anyway. For most of these “power” applications then, a refrigerated air dryer can be selected if the airlines are all located indoors in heated areas. Provided the refrigerant dryer will maintain a pressure dew point lower than the lowest ambient temperature where the airlines are run, with a safety buffer of say 10-15 degrees F., there should be no condensate forming in the airlines. A coalescing prefilter before the dryer will normally remove particulate to a suitable level, and will protect the dryer’s heat exchangers from oil varnishing.

The problem may arise when air lines are exposed to temperatures in the 3-5 deg. C (35-40 deg. F) range or lower. Most refrigerant dryers will only provide dew points down to this range, so if the actual compressed air temperature falls to these levels, you will have no buffer, and there is a good chance that condensate may form in the airlines downstream of the dryer, and cause deterioration of tools due to rusting/corrosion, and perhaps cause them to malfunction due to condensate ingestion. If the airlines are exposed to temperatures below freezing, it is likely that condensate will turn to ice, causing airline blockage, and shutting the system down. In these cases, a dryer which yields sub-freezing dew points will be required. Either a desiccant dryer, or a membrane dryer will be necessary. Likewise, in most process applications, mixing air containing water vapor or other impurities with chemicals or other products (may be food, paint, plastic polymers, etc.) will be unacceptable. In many of those cases, air which is expanded quickly will see a rapid temperature drop which may result in condensing of vapor into water and/or frost.

The most popular dryers on the market are refrigerant, heatless desiccant, heated purge desiccant dryers, and membrane. Refrigerant dryers as discussed above, have performance limitations, and can only be used in applications requiring higher dew points. Alternatively, the heatless desiccant compressed air dryer is preferred by most industries to provide low dew points. Some of the reasons for heatless dryer popularity are as follows:

What Makes the Heatless Desiccant Compressed Air Dryers So Popular?

  • The initial capital cost and installation for heatless dryers is lower than heated purge type dryers.
  • These air dryers have the capability to deliver high performance, and require minimal maintenance which can be performed by most industrial maintenance mechanics.
  • The dryers do not use supplemental heating for regeneration, which results in consistent dew points throughout the time cycle.
  • The dryers require less than 1 amp on a 1/60/120V power supply.
  • Life expectancy of the desiccant is 5-6 years, as opposed to approximately 3 years for heated purge dryers.
  • Purge rates are approximately 15% of intake volume as opposed to 20-25% for membrane dryers.
  • Various options are available to customize the dryer to the user’s application. APPL’s “Purge Economizer Controls” option offers quick return on investment in purge air savings, while providing the user with extensive data/history for the dryer operation.
  • APPL dryers employ up-flow drying and down-flow purging. Down-flow purging and rapid blow down of the towers results in the most efficient regeneration, while preventing desiccant fluidizing during tower depressurization.
  • Standard APPL dryers use microprocessors, unlike many manufacturers using proprietary printed circuit boards with only one source for replacement.
  • APPL dryers do not use proprietary desiccant cartridges which can grossly increase desiccant replacement costs. Many desiccant cartridge dryers require earlier replacement based on time accumulation rather than performance deterioration.
  • Smaller APPL dryers (up to 75 CFM) do not use 4-way sliding spool valves on the inlet which are sensitive to desiccant dust. Channeling purge air back through spool valves usually results in premature valve failure. APPL small dryers use poppet-style valves only which are impervious to abrasive desiccant dust.
  • -40°C dew point is standard for APPL dryers (10 minute time cycle), with on-board controls capability of reducing dew points to -60 ºC to -70ºC range by changing to a 4 minute time cycle. Lower dew points to -80 °C can be achieved using molecular sieves.

Air Power Products Ltd. is a leading manufacturer of heatless twintower desiccant compressed air dryers, and has a vast experience in the field (since 1983). Many dryers built in the 1980’s are still in service, which is testament to our quality in design and product components.