Compressed Air Filters
Why purification of compressed air is required
Types of filters
Filtration Theory
Installation and maintenance recommendations
Filter sizing
Cost of filter pressure drop
Purification of compressed air is needed because the air we breathe carries contaminants. Airborne particles, water, microbes, and chemical gases enter compressors. At a compressed state these contaminants become concentrated and more destructive.
In the compressed air system, hard particles assault equipment and piping. The result is damage to the system and more particles generated. Examples of particles found in a compressed air system include desiccant dust, rust, pipe scale, metal oxides, and dirt. Particles in a compressed air system can:
- Plug orifices of sensitive pneumatic instrumentation
- Wear out seals
- Erode system components
- Decrease air dryer capacity
- Foul heat transfer surfaces
- Reduce air tool efficiency
- Damage finished products
Particulate filtration is used to combat the destructive effects of solid contaminants in a compressed air system. Particulate filters down to 0.003µm protect air systems from contaminants. The smallest particle the human eye can see is 40µm.
An additional effect of compressing air is aerosol and vapor concentration. Compressed air is saturated with water. Since the compressed air is heated during the compression process, an aftercooler is then used to remove the heat of compression. During this stage a significant quantity of aerosols and vapors are condensed into liquid at temperature. For example, a 25 hp (18 kW) compressor delivering 100 scfm (2.8 m3/hr) at 100 psig (7 bar) and 100ºF (38ºC) can produce 18 gallons (68 liters) of water a day. These liquids, if not removed, will cause corrosion, erosion, damage to pneumatic equipment and instruments.
Chemical gases, either alone or in combination with other contaminants, may cause additional damage or create a potential hazard to the process or personnel. Chemical gases found in compressed air systems include:
- Freon
- Chlorine
- Sulphur
- Carbon dioxide
- Hydrocarbons
- Carbon monoxide
Molds, fungus, and bacteria can all grow and spread through a compressed air system. Microbes require moisture to grow. In addition to being a health threat to workers, microbes produce acidic waste. This waste corrodes the air system.
Compressed air contaminants combine inside the air system forming sludge. Sludge clogs pipes and valves causing valves to jam (stiction). Filtering and drying processes remove particles, moisture, microbes, and chemicals from compressed air. Clean, dry air protects the air system, reduces maintenance costs and increases finished product yields.
Compressor Intake Filter
The first line of defense is the intake air filter, which reduces the bulk contaminant load protecting the compressor from dirt and solids. Generally, these are cartridge type filters with a greater than 98% air efficiency rating at 10µm. There has been a recent move to 3µm filtration. Over 80% of the contaminants challenging an intake filter are less than 2 um and therefore the majority of these contaminants enter the compressed air system with the intake air. Other compressed air filters are available to remove particulate contaminants down to 0.3 um and to remove chemical contaminants.
Compressor Air/Oil Separator
Oil injected rotary screw compressors have an air/oil separator. Generally, these separators are 3µm cartridge style compressed air filters. The air/oil separator allows compressed air to continue downstream and recycles the compressor lubricant to the compressor's oil sump. Some compressor lubricant does continue downstream; however, improvements in compressor design and air/oil separation has minimized lubricant carryover on newer models.
Coarse Coalescer Filter
A coarse coalescing (fig. F1-1) filter separates large water and oil droplets from the compressed air stream before the coalescing filter. The coarse coalescer also removes large solids. Typically, the coarse coalescer is a 3µm coalescer. By using staged filtration you protect the more expensive coalescer filter, making it last longer online. The idea is to change the less expensive coarse coalescing filter more often to reduce overall filtration cost. A coarse coalescer filter is optional equipment. Not all air systems utilize it.
Coalescing Filter
A high efficiency coalescer (fig. F1-1) filter removes water and oil aerosols and solid particles to a specified level. Many compressed air coalescers are rated at a liquid removal efficiency of 0.001 ppm by weight and by liquid aerosols down to 0.01µm. A coalescer stops particulate contamination through direct interception. Its main function is to remove water and oil aerosols by coalescing the aerosols into droplets. This happens partially because of torturous path and membrane characteristics and partially due to pressure drop. Coalescers remove both water and oil aerosols from the air stream.
It is important to realize that a coalescer is excellent at removing aerosols and liquids but not vapors. Vapors are organics like hydrocarbons and odors. A vapor filter removes vapors with an adsorbent.
Particulate Filter
Located downstream of a desiccant dryer, a particulate compressed air filter typically 1 to 3µm stops desiccant fines from migrating downstream in the air system. The particulate filter protects piping and pneumatic equipment from particulate damage by removing rust, pipe scale, metal oxides, and desiccant particles.
High Temperature Particulate Filter
Temperature spikes are common downstream of a heated desiccant dryer. Spikes happen when a regenerated tower comes back online. The heat generated during regeneration is picked up by the compressed air. Often these heat spikes are considerably higher than the maximum operating temperature of a particulate compressed air filter and housing. The particulate filter can catch fire and damage your air system and pneumatic devices.
A high temperature particulate filter protects against this risk of fire. A particulate compressed air filter operates at 100°F (38°C) and has a maximum temperature of 150°F (66°C). A high temperature particulate filter can operate daily at 350°F (177°C) and has a maximum operating temperature of 450°F (232°C). The level of protection with a high temperature particulate filter and housing is significant.
Vapor Filter (Charcoal Filter)
A vapor filter removes organics from the air stream. Organics, like tastes and odors need to be removed from breathing air systems. In general, industrial applications of vapor compressed air filters remove hydrocarbons and other organic chemical vapors from the air system. Depending on the airflow, vapor filters need to be replaced every few months because the effectiveness of the activated carbon degrades as it adsorbs.
Final Filter
Pharmaceutical and microelectronics are examples of industries that require ultra-clean, compressed air. Compressed air contaminants could destroy product batches worth millions of dollars in either industry. To protect these critical processes from contamination, absolute rated validated final compressed air filters are used. The filter is located as close to the pneumatic application as practical to minimize pipe scale and the possibility of contamination occurring downstream of the final compressed air filter.
A validated filter has a serial number that allows the tracking of components used to manufacture the filter. Additionally, the manufacturer has to add quality steps to the manufacturing process to ensure a consistent high quality product is produced. Both the microelectronics and pharmaceutical industries require it.
A 0.2µm to 0.003µm absolute rated cartridge filter is used as a final filter for compressed air. These filters have double o-rings at the bottom and a bomb fin top with locking tabs to eliminate the risk of bypass. This is commonly called a sanitary style filter assembly. A hydrophobic non-volatile membrane is used as the filter media.
Inertial impaction
Particles traveling in a fluid have a mass and velocity. The fluid will follow the path of least resistance. Some particles will impact onto the filter medium and be caught due to their inertia driving them into the filter. This is not a primary mechanism for particle retention. It is more common in gas streams.
Diffusional interception
Extremely small particles will bounce around randomly in a gas stream as they strike liquid or gas molecules. This motion is called Brownian Motion and is more pronounced in gases. The random path of the particle increases the chance it will strike the compressed air filter and be captured.
Direct Interception
If the particle is larger than the pore of the filter, it is retained. For example, the screen on your screen door is mesh. It allows air to pass but keeps insects and anything larger than the mesh out. Compressed air filters work in the same way; however, the flow path is not necessarily straight. The pores can be infinitely smaller, and there can be layer after layer of media for the liquid/gas to pass through. Direct interception is the most common form of retention in both gas and liquid service. Most filters maximize their direct interception with torturous flow paths, which increase the filter's retention capability.
Compressed air filters should be installed in a level pipeline, mounted vertically, the bowl downward with one element length clearance for element removal. The filter should be installed at the highest pressure point available, and as close to the equipment being protected as possible and have a drip leg immediately upstream.
Pressure drop is used to determine when to change a filter. Contaminants, pressure, temperature and other variables affect the service-life of a filter. Generally in a compressed air system, when the pressure drop across the filter reaches 8 to 10 psid, a filter should be changed. It is critical to drain the filter housing daily to release condensate that gathers in the bottom of the filter bowl. Automatic drain valves simplify this process and protect your system from water re-entrainment.
Inlet Flow and Inlet/Outlet Piping
The compressed air filter has to be rated for the inlet flow. Inlet flow is generally measured in scfm. Locate the filter at the highest pressure point available that offers an acceptable temperature. Match the inlet and outlet pipe size of the filter assembly to avoid generating excessive pressure drop across the compressed air filter.
Temperature
Filters have a maximum operating temperature. General use compressed air filters are designed for use at 100°F (38°C). It is a best practice to locate compressed air filters where the temperature is the lowest possible. There are filters designed to operate at high temperatures. For example, high temperature particulate filters. High temperature operation increases the rate of corrosion and can reduce the life of compressed air filters and other system components.
Pressure Drop
The filter housing and filter itself create some resistance to the flow of the compressed air. There is a pressure drop between the upstream and downstream sides of a filter. This is called Delta P. When you size a new compressed air filter and housing, you want a Delta P that is as low as possible. This is because the life of your filter is related to its Delta P. For 80% of a typical filters service-life pressure drop will stay approximately the same. Over the last 20% of the filter's
service-life the delta P rises. There is a point of diminishing returns. The filter plugs to the point where air pressure loss is too great. If a compressed air filter is not changed pressure will continue to build until a terminal Delta P is reached and the filter collapses. Delta P monitors are standard on most compressed air filters. Delta P monitors allow you to watch the filter's pressure. Some have alarms when change-out Delta P is reached.
Size your filters with the smallest pressure drop practical. The cost of pressure drop often exceeds the initial savings of a smaller filter housing. This annual cost of pressure drop chart (fig. F1-3) illustrates the cost of pressure drop.
Filter Pressure Drop Microprocessor Controls
Manufacturers have started to incorporate microprocessor controls into compressed air filter design. Traditional gauges indicate when to change a filter by a pressure or by a color indicator. Microprocessor controls monitor the filter's condition considering the hours of operation, cost of compressed air, cost of the filter, and filter pressure to determine the most cost effective point to change the filter. Microprocessor controls reduce compressed air utilities costs by reducing how long a compressed air filter stays online creating excessive pressure drop.
Several types of microprocessor filter controls are available. Some models calculate the most economical time to change a filter based on the cost of compressed air versus the cost of the filter element. Other microprocessor control features include:
- Time monitoring
- Pressure differential monitoring
- Filter performance monitoring
In addition, remote monitoring capabilities are available. These units use the latest, state-of-the-art technology to provide quality air with substantial cost savings.
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