all about air | Thomas W. Kreher
A Filter rated 2 times 40 SCFM, 80 SCFM, would have ¼ the pressure drop or 1.25 PSID @ 40 SCFM. For filtration requirements finer than 5 micron consider a coalescing filter after the general-purpose filter. COALESCING, OIL REMOVAL Filters flow from inside the filter media, through it and out. By direct interception, inertial impact and Brownian motion the filter media intercepts and retain incredibly small particles of solid and liquid contaminants. Coalescing is a continuing process. The useful life of the filter media is determined by the build up of solid particles. A generalpurpose pre-filter rated 5-micron or less protects and extends the useful life of the more expensive coalescing filter. Coalescing filters are effective in removing condensed water or oil aerosol. Water and Oil coalesced simultaneously in the same unit may gel and obstruct flow. The general purpose pre-filter should remove condensate, especially water, before the coalescer. Coalescing filters are more restrictive than general-purpose units of the same size. A larger body size may be required to match the flow capacity of the prefilter. The three simultaneous dynamics that coalesce different size particles depend upon an adequate velocity without exceeding the velocity that forces the oil back into the flowing air. As a rule of thumb a size should be chosen between 20% and 80% of the rated flow. The gradual build up of particles small enough to pass the pre-filter increases the pressure drop. Automatic drains should be used. Delta-P indicators or switches are a wise addition to signal a required filter element replacement. The incredible air quality following a coalescing filter may have all particles removed down to .01 micron (ISO Class 1) and all oil down to .01 ppm (ISO Class 2). ABSORBING, OIL REMOVAL Filters pass the air through a granular bed of activated carbon that will absorb oil vapor including the odor of oil. Following a pre-filter and a coalescing filter the absorbing filter may take the remaining oil content down to .003 ppm (ISO Class 1). Unlike the preceding general purpose and coalescing mechanical separators that use a continuous dynamic process the absorbing filter has a finite capacity to absorb and hold oil and oil vapor. The service interval or total flow capacity before the activated carbon should be replaced may be in the catalog literature or the specification sheets. Of the three filters the absorbing filter is the most restrictive, with the shortest service interval. This suggests that a generously sized absorbing filter should be selected first then the coalescing filter and finally the pre-filter. The air prepared with a pre-filter, coalescer and absorbing filter, properly sized and maintained, when expanded to atmospheric pressure is significantly cleaner than the air you are breathing as you read this. Please Note: Noxious gases and fumes that may be ingested by the compressor are not removed with filters. We do not suggest that compressed air of any degree of cleanliness be used for breathing air without proper monitors and controls to detect gases that may be harmful. Also air that has had moisture removed at a higher pressure will be much dryer at atmospheric pressure and could cause serious dehydration.
Air Preparation Filters
e will touch on inline filters designed to remove condensate, water and oil, and particles from compressed air prior to use. Compressed air, like a genie in a bottle waiting to serve you, should have remedial attention. Someone said that modern machinery would not run (very long) on the air we breathe. The heat, moisture and air borne particles of dust, ash, pollen etc. are all intensified into a smaller area as we compress the air. The filters in this discussion are point of use units, a last resort for “shop air” past the aftercooler, dryer, receiver devices or from a small tank mounted unit to do our work. The point of use filter is commonly a family product with matching regulator and aerosol lubricator. Most units have a maintenance sheet inside the box they are shipped in. Take a bow if you keep a loose-leaf binder and save a current copy of these sheets. Unfortunately when it is time for service this information often cannot be found. GENERAL PURPOSE Filters are used alone or as a prefilter for multiple filters in applications that require ultra filtration. Centrifical flow throws condensate and larger particles against the inside of the bowl or reservoir where it spirals downward to the drain area. The air now free of a high percentage of liquid and chunks passes through a filter material designed to intercept particles down to the rated micron size. These functions are independent of each other. A filter without a particle strainer actually removes moisture marginally better because the velocity is higher without the element or strainer. The filter element, strainer, will pass liquid and still retain finite particles as a water filter does. An automatic drain to disgorge the condensate easily pays for itself. Without an automatic drain frequent attention is required to keep retained liquid below the level where it is picked up by air turbulence and forced down stream. The time and attention required soon exceed the cost of the automatic drain. A preventative maintenance program for these filters might replace the filter element at 12-month intervals or more frequently if the pressure drop indicates. Delta P indicators and electrical differential pressure switches are available to signal service requirements. Some newer catalogs show flow curves with the expected pressure drop at various flow rates. When you chose a product from this data use 2 PSID or less as the maximum flow. Other typical catalog information for these filters might rate the maximum recommended airflow at 100 PSIG with a 5 PSID pressure drop. The filter rated at 40 SCFM in this manner should never be selected for an application that is expected to require 40 SCFM. A unit sized for maximum flow and a 5 PSID pressure drop when new will start expensively due to the pressure drop and get worse as the filter element loads with particles.
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FPJ May/June 2006
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4/17/06 11:52:41 AM
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