Compressed air contains contaminants such as dirt, water and oil which must be removed before use. ISO8573.1 specifies air quality classes for these contaminants. Humidity is expressed in terms of Pressure Dew Point (PDP). PDP is the temperature at which air is fully saturated with moisture, when the air temperature falls below this point further condensation will occur.
Condensed water vapor within the compressed air system is by far the major contaminant, which if left untreated, leads to the malfunction of equipment, corrosion, product spoilage and bacterial growth. Freezing of condensate is of particular concern in outdoor and mobile applications during the winter months.
Desiccant air dryers are used for high purity applications where PDP’s of -40°C/F are required according to ISO8573.1 humidity class 2. In many applications however, there is a demand for the air to maintain a low Relative Humidity (RH) over a wide range of ambient temperatures e.g. -40°C to +40°C (-40°F to 104°F). Specifying %RH or Dew Point Suppression (DPS) enables optimised sizing of dryers for the application. This avoids oversizing and excessive purge air use, and provides protection over a wide range of seasonal temperature and humidity variations. For example, 20% RH closely corresponds to a DPS of 40°C (72°F).
Skroll Adsorbent Technology
Air Purification Skroll adsorption medium is revolutionary (patents pending). It consists of adsorbent crystals immobilised in a durable polymer support structure. It is produced in the form of a continuous sheet, typically 1 mm (0.04”) in thickness. When converted into a Skroll construction, flow paths are created between layers. This enables optimum performance to be achieved in applications such as dehydration of air and gases. It is tough and durable, recovers from misuse and has long service life.
When Skroll medium is incorporated into a compressed air dryer it provides a very robust long-life product. Skroll media’s durability ensures that even if flooded with water condensate or subjected to excessive air flow, it will, once normal conditions are resumed, fully recover its performance. Skroll’s polymeric binder ensures high chemical resistance (a known issue with semi-permeable membranes) and does not shed dust (a known issue with desiccant beads). This unique medium provides extremely stable dew point suppression, anywhere between that achieved by refrigerated and desiccant dryers. It can operate in a horizontal orientation and is not effected by pulsation from piston compressors.
Skroll medium is readily available in roll or sheet form to be fitted into new or existing PSA dryer designs. It can also be provided in cartridge form to suit customer requirements.
A close-up view of the Skroll adsorption medium.
Fast Kinetics-Lower Purge Air-Stable Dew Points
Skroll medium has a very open structure with fast kinetics. The SEM shows the surface of Skroll medium where each adsorption crystal is fully exposed on both sides to the adsorption/desorption process.
This means regeneration is very efficient requiring less energy. When the medium is exposed to excessive flow or higher temperatures, with the resultant increase in water vapor load, Skroll adsorption medium is not overwhelmed. DPS will reduce to a new and stable dew point but fully recover when normal conditions resume. In such conditions a beaded desiccant bed is likely to be ruined, since regeneration air would no longer be dry enough to carry out regeneration. This results in a downhill spiral leading to the loss of the beaded adsorbent bed through saturation. As the bed becomes saturated, the crush strength of the beads reduces significantly and the adsorbent beads will turn to dust. This may combine with condensate and spread downstream, causing malfunction or damage to pneumatic equipment.
An image of the adsorptive mediu.
Principles of Operation (PSA)
Heatless desiccant dryers are commonly used due to their simplicity. A heatless twin tower Pressure Swing Adsorption (PSA) dryer operates by removing moisture from the pressurised feed air by adsorption onto a desiccant bed (column A), typically at 7 bar (100 psig). A second column B (having previously been used in drying the air) is at atmospheric pressure and dry air from the outlet of column A is fed through a purge valve, expanded to near atmospheric pressure, and flowed in a contra flow direction through column B to affect its regeneration. When the first column (A) is saturated with water vapor (usually determined by a simple timer) the feed air (following repressurization) is switched back to column B and the cycle continues.
Beaded adsorbent designs may suffer from accidental misuse with excessive water loading if, for example, reduced pressure, or excessive flow, or excessive temperatures are encountered. Desiccant beads shed dust, especially when operating in challenging environments where, for example, shock and vibration are present. Operating in a horizontal orientation is a real challenge for adsorption beads.
APS Sizing Parameters
ISO 8573.1 humidity class 2 (-40°C/°F PDP) is often specified but, for many applications, this far exceeds the dryness required and therefore a dryer may be oversized and use excessive purge air (energy).
Skroll technology allows the user or designer to determine the dryness of the air to suit their application requirements by maintaining stable DPS or %RH over a wide range of conditions. Skroll technology is highly efficient, reliable and requires little or no maintenance. The medium has been developed for demanding applications in the purification of compressed air. It overcomes the disadvantages of adsorbent beads such as channelling, by-pass, bed fluidisation, orientation, dust generation, misuse (flooding), short service life and degradation due to high water loading.
Traditionally many beaded desiccant dryers operate on a 4-minute cycle (2 minutes drying and 2 minutes regeneration and re-pressurisation). At 7 bar (100 psig) bed volume equates to approximately 200 Nl/min/litre or 0.12 scfm/cubic inch of flow per volume of adsorbent bed. Pressure losses across beaded adsorption beds are typically up to 140 mbar (2 psi).
The Skroll medium operates at a 40°C (104°F) inlet temperature, 100% RH and 7 bar (100 psig) pressure for a DPS of 40°C (72°F) requires just 50% of the bed volume of beaded desiccant dryers with flow per unit volumes of 400 Nl/min/litre or 0.24 scfm/cubic inch. Contact time is lower and velocity increases but pressure losses remain very low, typically <35 mbar (0.5 psi).
Flow per unit of volume may be further increased (25%) where a 2-minute cycle (1-minute drying, 45 seconds purging and 15 seconds re-pressurisation) is used.
Example of Skroll medium being loaded into an aluminum tube. Internal diameter 62 mm (2.44”) bore x 218 mm (8.6”) long. Inlet flow capacity at 7 bar (100 psig) is 280 Nl/min (10 scfm) for a 40°C (72°F) dew point suppression.
Variable and Stable Dew Point Suppression (DPS)
The sizing described relates to a DPS of 40°C / 72°F. Skroll medium can operate at differing levels of DPS by varying the purge volume or air flow. This can be done by either varying the purge rate or the regeneration time. By making such adjustments to the purge volume, Skroll medium will produce stable dew points in a range of 20°C - 80°C (68°F - 176°F) DPS. Lower DPS means less purge air required e.g. 10%.
ISO8573.1 Quality Classes for Humidity
Humidity Class 2 may be specified without consideration being given to inlet conditions. ISO8573.1 simply states quality classes, it does not specify inlet conditions to achieve them. To achieve Class 2 for water (-40°C/F) a Skroll module may be sized to have increased DPS depending on the inlet conditions being specified. For a 20°C (68°F) inlet temperature, a 60°C (108°F) DPS will produce a ISO8573.1 humidity class 2 of -40°C/F.
Continental weather may vary from -40°C/F in the winter to +40°C (104°F) in the summer. Specifying compressed air dryness of 20% RH or 40°C (72°F) DPS prevents condensate forming in the system throughout the year. It prevents condensation in the summer which would wash out lubricants in pneumatic equipment (even more damaging if condensed water combines with oil and desiccant dust to form an abrasive sludge). In the winter, freezing of condensate will render equipment inoperative and is of particular concern during the overnight parking of vehicles.
NF F11-100, commonly used in rail, specifies a 40°C (72°F) DPS which recognizes seasonal variations.
Recovery from Misuse
There are many misuse scenarios including excessive volumetric flow. This increases the water loading resulting in a reduction in DPS. Crucially Skroll medium adjusts to a new and stable dew point which is unlikely to cause any short-term problems since condensation should not occur.
In extreme cases (where for example there was an interruption of power supply to the dryer) the normally open inlet valves would ‘fail open’ and maintain air supply. The Skroll medium would become saturated, however, once the power supply is resumed and the dryer starts to cycle, the Skroll medium will quickly recover its performance. The graph illustrates the loss of PDP over a period of several hours. Full recovery of performance is achieved within an hour of normal conditions being resumed. In such scenarios, traditional adsorption beads would, most likely, need to be replaced.
Pressure losses across some purification systems can be significant, made up from inlet filtration, dryer and outlet filter. Initial pressure losses may be as much as 0.7 bar (10 psi). This Differential Pressure (DP) may increase further as the pressure drop across the filters increases in use. Filters from the pneumatics industry are typically fitted with sintered elements of various micron ratings. They have high pressure losses especially when compared to developments in compressed air treatment. Such developments have resulted in highly efficient designs with very low and sustained pressure losses with no serviceable parts. This saves energy and results in significant through life cost savings.
Skroll in housing.
Many of the new compressed air systems use non- lubricated piston compressors and it is possible when using a Skroll dryer to dispense with the high efficiency coalescing inlet filters as well as the outlet dust filter.
A new Inlet Filter design developed for use with Skroll dryers produces air quality to ISO8573.1 quality class 3 for dirt and class 7 for water respectively (typically +99% removal of dirt particles down to 5 micron and water condensate). This new Inlet Filter operates using 2 powerful mechanisms of filtration: centrifugal separation and scrubbing action (removal of smaller particles). There are no service parts and pressure losses are very low and remain low in use, typically 50 mbar (0.7 psi).
For non-lubricated compressors, only one Inlet Filter is required. The overall pressure losses across the inlet filter and dryer using Skroll medium are in the order of 300 mbar (4 psi) representing a significant energy saving of up to 5% of compressor power.
For oil lubricated compressors, a second stage high efficiency coalescer is required to remove residual oil aerosols.
The use of piston compressors raises other issues, such as pulsation and aggressive condensate. Increased corrosion protection of the filters is necessary, while high efficiency filter elements must be of a construction to prevent rupture of the delicate material used in their construction. Generally, industrial filters are not designed to withstand pulsations and stop/start conditions.
Skroll medium is not a straight replacement for beaded technology since many desiccant dryers operate successfully in fixed locations where ambient temperatures, pressures and flow are controlled, and where servicing and supervision is readily available. Skroll medium has been developed specifically for use in arduous conditions where beaded PSA and membrane dryers are challenged beyond their normal operating parameters. Applications include where:
About the Author
Colin Billiet is the CEO of Air Purification Skroll Limited and the former CEO of the domnick hunter group PLC.
To read more about Compressed Air Purification Technology please visit, www.airbestpractices.com/technology/air-treatment.