Over 90% of manufacturing facilities worldwide use compressed air as part of their manufacturing process. This powerful utility is not without its problems, in the form of contamination. Therefore, air compressor filters are typically installed as part of a total purification system to ensure contaminants are reduced to acceptable levels and the compressed air system operates in a safe, efficient and cost-effective manner.
To ensure a continuous supply of high-quality compressed air is always available (and to keep energy costs low) filter elements must be replaced periodically. In this article, we address the myths regarding the best time to change filter elements and the role of differential pressure gauges in helping to ensure the effectiveness of air compressor filters.
Compressed air filters are typically installed to ensure the compressed air system operates in a safe, efficient and cost-effective manner.
The Foundation of Air Purification
The foundation of any purification system is its filtration and of the ten main contaminants found in a compressed air system, filtration is responsible for the treatment of nine of them. Coalescing filters are the most important piece of purification equipment as they reduce six of the ten contaminants and a look in any air compressor room will find a pair of coalescing filters (backed up with dry particulate and oil vapor removal filters).
For contamination reduction, compressed air filters use highly specialized filtration media in their filter elements and these elements function under extremely arduous conditions. Filtration media degrades over time and once the filtration media becomes damaged, the compressed air quality can no longer be maintained or guaranteed.
The components of coalescing and dry particulate filters are almost identical. They typically consist of a pressure envelope or housing (filter head & bowl), a filter element and a drain (the only major difference being a float drain on the coalescing variants and a manual drain on the dry particulate variants). Many filters are also supplied with differential pressure monitoring devices as standard or as optional extras.
The heart of the filter is the filter element. Filtration media is typically wrapped or pleated between support cylinders to form the element (elements are available in differing filtration grades). Endcaps are secured to the cylinders to form an “integral” seal and an outer drainage layer is added to prevent “re-entrainment” of coalesced liquids. These filter elements do not last forever and require periodic change, however there is often confusion surrounding why the filter element needs to be replaced (and when). This, despite the critical role it plays.
Even a tiny hole the size of a pinpoint can have disastrous consequences. Compressed air will always take the path of least resistance and once the filter media is damaged the compressed air will force through the media, tearing and rupturing the length of the element. This will result in all of the contamination being carried downstream.
Large pressure differentials or “pressure spikes” can also rupture filter elements in an instant, again, allowing contamination to be carried past the filter and into the compressed air system. When pressurising systems (or the filter after servicing), always ensure isolation valves are opened slowly to prevent damage.
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Filtration is about Air Quality
As air compressor filters are installed to provide contaminant reduction to a specific air quality requirement (usually in accordance with an ISO 8573-1 classification), the primary reason to change filter elements should always be to maintain the air quality selected.
Constant monitoring of compressed air quality for all the different contaminants present is typically not a viable option for compressed air users. Validation of air quality to ISO 8573-1 classifications requires multiple, off-line, laboratory based tests (excluding pressure dew point) and the digital, constant monitoring test equipment now becoming available to test for oil and particles in compressed are typically not accurate to the levels shown in ISO 8573-1 and therefore not endorsed by the ISO standards.
Sometimes filter elements are not changed because they look “clean” or just to save money, but commonly, it has been the practice to change air compressor filter elements based upon the “pressure drop” (also referred to as differential pressure or dP) measured across the filter, as this has a direct impact on energy costs.
Unfortunately, pressure drop is not an indicator of compressed air quality and can often hide a damaged filter element which is allowing contamination to pass downstream. One must remember that the reason for installing the filter in the first place is to reduce the contaminants found in the compressed air, and failure to maintain air quality can have a significantly higher financial impact compared to the energy consumed by a filter or the cost of replacing filter elements.
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Differential Pressure Gauges Often Misunderstood
Many filter housings are supplied fitted with differential pressure gauges, which are also called differential pressure indicators. They indicate differential pressure by means of a moving needle, pop-up indicator or digital display. Although common in the industry, the accuracy and purpose of these devices is often misunderstood.
- Accuracy: Generally, all of these devices, no matter how they display the change in differential pressure are only “indicators” and are not precise “gauges.” They typically have an accuracy of around plus or minus 25%. Calibration certificates will not be available for these devices.
- Display: Many of these devices mimic a real gauge, having graduated scales in mbar or psi; others simplify their display, dividing it either into two segments to indicate “Working within Parameters or Service Required” or three segments to include a “Needs Attention” warning. Segments can also be color coded “Green/Red” or “Green/Amber/Red.” The default for these devices is always “Green,” or “Good,” and it doesn’t indicate a problem with the filter element should the filtration media tear or rupture.
- Operation: Differential pressure monitoring devices show the difference in pressure created by the filter element as it blocks. While in principle, this is a straightforward measurement, there are several parameters that can affect the measurement.
- Airflow: Compressed air filters are sized to match the compressed air flow rate of the system. Often, they are slightly “oversized.” In addition, the system will very rarely operate at 100% maximum flow all of the time, therefore the compressed air flowing through the filter is constantly varying. Differential pressure is proportional to flow rate, therefore as the flow rate constantly varies, the differential pressure will also vary constantly. If a user looks at the differential pressure monitoring device at any time other than at maximum flow rate, they will not get a true indication from the device. This is especially true on compressed air systems with variable speed compressors installed.
- Element condition: If there is a hole, tear or rupture of the filter media, the indicator would not move as the compressed air will take the path of least resistance and pressure drop across the element would be almost non-existent.
The indicator of a differential pressure monitoring device will not move if the filter element is torn or ruptured.
If the differential pressure monitoring device is being used as a “service indicator,” the needle on the device (or pop-up indicator) would always indicate in the green area and the element would never be serviced until the user eventually detected contamination downstream. By this time, it is too late and once contamination is introduced into the storage and distribution system, it is very difficult to remove and may often require specialist cleaning of piping and pneumatic equipment before the compressed air system can be operated again.
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The Purpose of Differential Pressure Monitoring Devices
The differential pressure monitoring device fitted to a filter is seen by many as a service indicator. This is not helped by service manuals stating, “Change element when dP = xx psi,” or “Change element when needle is in the red.” Additionally, it is often thought that differential pressure is an indicator of air quality or total energy consumption, again this is not the case. In reality, the true purpose of a differential pressure monitoring device is to indicate premature blockage of a filter element and should not be used as a filter service indicator.
A differential pressure monitoring device is used to indicate premature blockage of a filter element. It should not be used as a filter service indicator.
Here are common myths about pressure drop and filters – and facts to help put these same myths to rest.
Myth: “dP is an indication of air quality therefore the ‘dP gauge’ is an air quality indicator.
Facts: Dry particulate filters are used to reduce atmospheric particulate, rust, pipe scale and microorganisms found in compressed air. In addition to these contaminants, coalescing filters also reduce oil aerosols and water aerosols to acceptable levels.
Differential pressure is the result of a filter element collecting contamination but in no way has a direct correlation to the delivered air purity. For example, high differential pressure and/or failure to change elements when advised can result in a tear/rupture of the filter media. This would allow all the contamination to pass downstream, while the dP indicator would indicate low dP.
Myth: My filter is fitted with a “dP Gauge” to indicate when to change my filter element.
Facts: The devices fitted to the top of many compressed air filters are not gauges; they are a monitor or indicator. These devices are not as accurate as a true differential pressure gauge would be. They typically have an accuracy of around plus or minus 25%. They are not calibrated, nor are they available with a calibrated certificate. Indicators, monitors or “dP gauges” should never be used as a service indicator, only as a premature blockage indicator. Always follow the manufacturers recommended instructions for element change.
Myth: Compressed air filter elements should only be changed when the differential pressure rises.
Facts: Differential pressure is a measurement of pressure loss in a compressed air system. To achieve a constant downstream pressure required to operate equipment and processes, an air compressor must often operate at a higher operating pressure and/or for longer periods to overcome the pressure losses.
Every 14.5 psi (1 bar) of differential pressure an air compressor (positive displacement type) must overcome is equal to approximately 7% increase in electrical consumption. So, while keeping differential pressure losses low is good practice, compressed air quality is the main reason for changing a filter element.
The Benefits of Regularly Changing Filter Elements
There are numerous benefits to regularly changing compressed air filter elements, including:
- High quality compressed air
- Protection of adsorption (desiccant) dryer beds
- Protection of downstream equipment, personnel and processes
- Reduced operational costs
- Increased productivity & profitability
- Continued peace of mind
It’s important to remember compressed air filters function under extremely arduous conditions, such as:
- Pressure variations
- Pulsing air demand
- Temperature variations – day/night and summer to winter
- High/low humidity – From 100% saturated air to extremely low dewpoints
- Chemical attack – from oily acidic condensate and lubricant additives
Coalescing filters are constantly soaked in oily, acidic condensate, while at the other end of the spectrum, dry particulate filters operate in very low humidity, which over time, weakens and degrades the strength of the filter media.
When the filter is operating, the filtration media in the filter element is bombarded by high velocity dirt particles. This constant bombardment will eventually lead to weakening and failure of the filter media. Once the filtration media becomes damaged, the required air quality can no longer be maintained or guaranteed. Filter elements therefore have a finite operating life and manufacturers will recommend an element change time to maintain delivered air quality, however after this time, air quality cannot be guaranteed. Not changing filter elements in accordance with the manufacturer’s recommendations can lead to:
- Inefficient production processes and damaged production equipment
- Spoiled, damaged or re-worked products
- Increased production downtime
- Increased manufacturing costs
- Contamination exhausting from valves and cylinders leading to unhealthy working environments
- Risk of personal injury, staff absences and personal injury claims
- Risk to consumer safety
A Balance Between Risk and Cost
Often a user’s emphasis is on the cost of maintenance and replacement parts when these costs are insignificant compared to those associated with product spoilage should a filter element fail. What seems like a cost savings in the short term can turn out to be a costly mistake. Therefore, filter elements should primarily be replaced based upon manufactures recommendations to maintain air quality.
Secondary consideration should also be given to system pressure losses; however, for almost all modern compressed air systems, this should not be the main reason to change filter elements. The exception to this may be more applicable to older, more heavily contaminated compressed air systems, when it may be more cost effective to change filter elements before the manufacturers recommendation as the cost of replacement elements will be significantly lower than the energy cost associated with operating with higher differential pressures.
Frequently Asked Questions – and Answers
Question: Can I wash filter elements?
Answer: No, it is not possible to wash contamination out of the filter media. Subjecting the filter media to hot soapy water or solvent will result in damage.
Question: My filter element doesn’t look too dirty, surely, I don’t need to change it?
Answer: Firstly, the part of the filter element you can see is the drainage layer (responsible for preventing coalesced liquids from being carried downstream). The main filtration media is below this and is not visible without dismantling the filter element. Secondly, coalescing and dry particulate filters capture aerosols and particles down to sub-micron sizes (1 micron = 1 millionth of a meter). The best a human eye can see is around 40-50 micron, therefore, it is not possible to see if a filter element is blocked or damaged.
Question: Are in-line activated carbon filters affected in the same way as coalescing and dry particulate filers?
Answer: In-line activated carbon filters often use the same housings as coalescing and dry particulate filters. These activated carbon filters however work differently. They utilize a bed of activated carbon to adsorb oil vapors (gaseous oil). Activated carbon has a fixed capacity to adsorb oil vapor and once this is used up, their elements or cartridges must be replaced. Smaller, in-line style activated carbon filters are typically matched to the system flow rate and pipe connections. The inlet concentration of oil vapor, the presence of liquid oil as well as the temperature, pressure and dew point of the air all affect the life of these elements and in-line air compressor elements require frequent changes throughout the year.
About the Author
Mark White is a Compressed Air Treatment Applications Manager at Parker Hannifin with 30 years of experience in the filtration and drying of compressed air.
About Parker Hannifin
Parker is the global leader in motion and control technologies, partnering with its customers to increase their productivity and profitability. Learn more at www.parker.com.
All photos courtesy of Parker Hannifin.
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