Industry
Dryers and filters for industrial compressed air
The US Department of Energy compressed air sourcebook (third edition, 2016) describes the refrigerant dryer as the most common dryer in industry and twin-tower desiccant dryers as the most effective at removing moisture, typically rated at a pressure dew point of −40 °F, with particulate, coalescing or adsorbent filters chosen for the purity required.
Why compressed air is dried
Air leaving an aftercooler and moisture separator is typically saturated, and further cooling in the distribution piping causes condensation, with corrosion and contamination of point-of-use processes. The sourcebook notes that, in general, reducing the temperature of saturated compressed air by 20 °F reduces its moisture content by approximately 50 percent. It also says drying should only be performed to the degree needed for the equipment and the end use: drying beyond the required pressure dew point results in unnecessary energy and costs.
Dryer types
| Type | How it works, according to the DOE sourcebook (2016) |
|---|---|
| Refrigerant | Non-cycling dryers cool the air in a refrigerant-to-air heat exchanger, where the evaporation of a liquid refrigerant makes the moisture condense; it is drained by a separator and drain, and a hot gas bypass valve controls the air temperature leaving the evaporator. Cycling dryers cool the air indirectly through a thermal storage medium. The most common type, with relatively low initial and operating costs; not recommended in sub-freezing ambient temperatures. Pressure drop across the dryer is usually 3 to 5 psid. |
| Regenerative desiccant | A porous desiccant (silica gel, activated alumina or molecular sieve) adsorbs the moisture, normally in two towers: one dries while the other is regenerated at reduced pressure with previously dried purge air. Twin-tower desiccant dryers are the most effective at removing moisture and are typically rated at a pressure dew point of −40 °F. |
| Heat of compression | A regenerative desiccant dryer that uses the heat generated during compression to regenerate the desiccant; used almost exclusively with centrifugal or lubricant-free rotary screw compressors. |
| Membrane | Water vapour passes through the membrane pores faster than air; the dew point achieved is usually 40 °F, and lower dew points, to −40 °F, cost additional purge air. |
| Deliquescent | The drying medium absorbs the moisture, is used up and cannot be regenerated. |
Filters
Depending on the air purity required, the sourcebook describes particulate filters for solid particles, coalescing filters for lubricant and moisture, and adsorbent filters. It recommends a coalescing filter before a desiccant dryer, to prevent fouling of the desiccant bed, and a particulate filter after it, to remove desiccant fines. Filtering only to the level each application requires minimises pressure drop and energy use, and elements should be replaced as indicated by pressure differential.
Ultra Controlo air treatment
- ULTRASEC refrigeration dryer: aluminium heat exchanger for compressed air inlet temperatures up to 70 °C (100 °C for ULTRASEC-HT), hot gas bypass valve for a constant dew point, digital temperature indicator and electronically controlled condensate drain. The datasheet estimates that each extra bar on the compressor costs about 7% of the installed motor power, and links lower dryer and filter pressure drop to lower energy consumption.
- ULTRADRY adsorption dryer: desiccant in replaceable cartridges, coalescing filter at the inlet and dust filter at the outlet, pressure dew point range of −40 °C to −70 °C, pressure from 4 to 16 bar g and inlet temperature from +5 °C to +50 °C.
- ULTRAK activated carbon tower: separates oil vapours from compressed air by adsorption (dry separation); a coalescing filter is required upstream and a 1 µm dust filter is recommended downstream. Operating pressure 16 bar, ISO 8573-1 oil quality class 0/1, nominal residual oil vapour content below 0.003 mg/m³ and a pressure drop of 20 mbar when new and dry.
More questions
Refrigerant or desiccant dryer?
The DOE sourcebook (2016) says refrigerant dryers are the most common and are not recommended in sub-freezing ambient temperatures, while twin-tower desiccant dryers are the most effective at removing moisture; drying beyond the required dew point wastes energy.
Where do the filters go around a desiccant dryer?
The sourcebook recommends a coalescing filter before a desiccant dryer to prevent fouling of the bed, and a particulate filter after it to remove desiccant fines.
Sources
- US Department of Energy and Compressed Air Challenge, Improving Compressed Air System Performance: A Sourcebook for Industry, Third Edition (2016), Section 1 and Fact Sheet 7
- ULTRASEC datasheet, p. 2
- ULTRADRY datasheet, pp. 2 and 3
- ULTRAK datasheet, p. 2
Ultra Controlo figures come from its technical datasheets. Third-party facts were read at the source on the date shown.