Selection
If the coalescing filter equipment is sized in standard cubic feet per minute (SCFM), select a filter for a particular application on the basis of airflow and system pressure rather than pipe size. The “real” capacity of a coalescing filter can be elusive since excessive air flow through the filter element raises pressure drop and, by increasing throughput velocity, increases the chance of contaminant reentrainment. Select a filter large enough to ensure that the air exits the filter at low velocity and does not carry over coalesced liquid. Economic factors are also important in the selection of a coalescing filter. As discussed earlier, a balance between performance and economic considerations will be required in the selection process. Filter service life, which has a major impact on the cost of a coalescing filter system, is discussed under “Maintenance” below.
Installation
Proper installation of a coalescing filter system can have a considerable impact on the cost and effectiveness of the system. If all air within your operation does not require purification to the same degree, install coalescing filters providing the desired efficiency and performance at points of use. Air flow through point-of-use filters will be lower than through a central filter system, allowing the use of a smaller filter and extending filter service life. What’s more, each point of use will receive air meeting specific cleanliness specifications. In addition to point-of-use filters, branch point filters can be installed at junctions in the air system where air purity needs change significantly. Where required by especially contaminated air, and/or the need for higher coalescing filter efficiency, install conventional 3 mm prefilters upstream from each coalescing filter to remove large quantities of contaminants. This will lengthen the coalescer’s service life. When installing coalescing filters at work stations, the additional 2-6 psid pressure drop contributed by the filter at rated flow through its life should be factored into equipment selection and distribution system sizing. When installing any filter, position it in a visible place where it will not be over-looked by maintenance personnel. Affix a label or tag to each filter detailing the installation date, the recommended filter element replacement date and the replacement element number. Inspection intervals should also be formalized and confirmation of inspections indicated on the label or tag. Finally, make sure coalescing filters are plumbed so that air flows from inside to outside and coalesced liquid will drip from the outside of the element tube to the wet zone of the filter housing. If plumbed outside to inside, liquid will not drain properly. This will result in reentrainment of oil, and a dramatic reduction in coalescing performance.
Maintenance
Filter element replacement cycles are an important factor in the design of a coalescing filter system. Generally speaking, more efficient filters will need to be replaced more often because they will entrap more contaminants, hastening clogging. The amount of solid contaminants in the air stream determines filter life. While liquids agglomerate and subsequently drain to the filter sump, solid contaminants become entrapped in the filter element, restricting air flow and increasing pressure drop. Use of a built-in prefilter to remove solid particulate matter can reduce dirt loading in the coalescer and prolong filter life.
Pressure Drop Signals Filter Replacement
Pressure drop is the “early warning” indicator of coalescing filter failure. Increasing pressure drop is a sign that the filter is becoming clogged and its efficiency is becoming impaired. Flow rate, operating pressure, and the amount and type of contaminants in the air stream all affect the rate of pressure loss across a coalescing filter. When a pressure drop of 8-10 psid is indicated, the filter should be replaced. Figure 4 shows a typical pressure drop curve for a coalescing filter system designed to provide optimum cost/efficiency. The initial low reading remains nearly constant for the first 75% of the filter’s life. Theoretically, no reentrainment of aerosols should occur in the first 80% of the filter’s life. Ignoring pressure drop can be costly, both in terms of reduced air quality due to contaminant reentrainment and the power costs associated with forcing compressed air through an obstructed filter. Figure 5 illustrates the point: To assure continuing clean air and lowest power costs, develop a service life profile for each coalescing filter. Establish an ongoing filter element replacement cycle that will assure that elements are replaced regularly at or before pressure drop reaches 8 psid. For optimum continuous air quality, element should be replaced when pressure drop is 75% of the 8 psid level. Between replacements, drain the filter bowl daily to remove coalesced liquid and prevent reentrainment, or install an automatic drain.
Figure 4. COALESCING FILTER LOADING CURVE.
Loading curve for a standard graded-porosity coalescing filter shows the classic relationship between pressure drop and life. Dirt loads into the filter element fairly constantly over the first 75% of a filter’s life; thus, pressure drop remains fairly constant over this period. Over the last part of a filter’s life, pressure drop increases drastically and oil reentrainment begins. Typically, a filter should be replaced when pressure drop reaches 8 to 10 psig, or earlier.
THE DIRECT COST OF PRESSURE DROP
At an electrical rate
of 9 cents per KW hour. . . . . .
A filter element is allowed to generate an extra 5 psid of pressure
drop operating in a 1000 SCFM system. . . . . .
The cost of generating that lost pressure is $28.80 every day. Use the
following calculation to identify pressure drop cost in your
application:
Calculation:
9¢/KW HR = .03¢/ft 3 @ 100 psig
.03¢/ft 3 @ 100 psig = 30¢/1000 ft 3 @ 100 psig
30¢/1000 ft 3 @ 100 psi = 1.5¢/1000 ft 3 @ 5 psid
Add: .5¢ for fixed charges, repair, operating cost.
2¢/min. @ 1000 SCFM, while dissipating 5 psid.
i.e. 2¢/min. = $1.20/HR = $9.60/shift = $28.80/day.