Inline Air Filters
Inline filters are installed within the compressed air piping system at various locations between the compressor and the point of air use. They are used to remove contaminants from the air flow.
Our research has found that many facilities are losing money because of misapplied inline filters. Here are some of the common mistakes involving inline air filters:
- Selecting filters based on low price rather than on lowest pressure drop.
- Installing a filter for the wrong contaminant or for a contaminant that does not exist at that point in the system.
- Trying to take out liquids without an understanding of the devices used for removing the different forms of liquid contamination.
- Filtering the air to a quality standard that is too high or too low for the application.
The key is to understand the potential contaminants in each section of your compressed air system. Then, pick the right product to remove any contaminant that will cause problems.
Four Guidelines for Inline Filter Selection
The need for inline filters is dependent upon the application. Here are some tips that will help you pick the right inline filter for most industrial situations. These tips do not apply to breathing air applications.
1) Match Filter Performance Levels to Air Quality Requirements
The starting point is to determine the appropriate cleanliness level of each application in your plant. You want to avoid the expense and problems that are caused by filtering to a standard that is below or above the necessary cleanliness level.
The International Organization for Standardization has established guidelines to help you with this subject. Specifically, ISO 8573-1:2010 is the standard dealing with compressed air contaminants and purity classes. You can learn more on their website at http://www.iso.org, and you can purchase 8573-1:2010 as a reference for your inline filtration requirements.
2) Avoid Wasting Energy
A pressure drop through an inline filter is wasting the energy that was used to produce the pressure.
Every pound of compressed air pressure requires a one half of one percent of electric power. You can reduce energy costs by using inline filters with a low pressure drop.
It is common to find inline filters with a high pressure drop in service because these products are often selected on the basis of low price. It may cost less to purchase but, it will cost more to operate when you consider the energy waste and more frequent element changes.
Here is a way to quantify the energy waste of pressure drop.
A plant operates a 200 horsepower compressor 8,000 hours a year. The motor has a .90 efficiency rating and the plant has a power rate of $.05 kWh. The annual power cost for this compressor is $66,311.
This is calculated by multiplying the horsepower of the compressor times .746 times the hours of operation times the power rate (HP x .746 x 8000 x $.05). Then, divide that number by the motor efficiency (.90).
Now, compare a filter with a 7 psig pressure drop to a filter with a 1 psig pressure drop. One pound of pressure is equal to one half of one percent of electric power. So, the extra 6 psig pressure drop will cost 3 percent in electric power.
This 3 percent amounts to $1,989 in wasted power, every year, for this one filter (3 percent of $66,311).
You can use this illustration to evaluate your existing inline filters. Gather your numbers and see how much money can be saved by upgrading inline filters to lower the pressure drop.
3) Know Your Contaminates
Inline filters are used to treat compressed air. The specific treatment will depend on the type of contaminant and the form it takes within the air system.
Solid contamination can be the dirt or other airborne solids that make it pass the inlet air filter. Another source is the rust and scale that can develop inside the air piping and air treatment equipment. It can also be a solid by product from the oil carryover that is associated with lubricated compressors.
Ambient air contains moisture which will enter the compressor with the inlet air as water vapor. Water can also enter a compressed air system through a leak in a heat exchanger.
Water can be present in a compressed air system in the form of liquid (free water), mist (aerosol) and vapor. A different type of product must be used to effectively remove each of these forms of water.
There are inline filters, which are called separators, that are used to remove free water. Another type of inline filters, which are called coalescing filters, are used to remove the mist form of water.
The vapor form of water contamination can not be filtered from the system. It must be removed by a compressed air dryer which is designed to handle only the vapor form of water.
The type of compressor has an impact on how oil contaminates a compressed air system.
a) Reciprocating Compressors
A lubricated reciprocating compressor, even with new piston rings and valves, will have oil carryover into the air piping system. This contamination can be found in the form of a solid, free liquid, mist and vapor.
The operating temperatures of the heads and valves can range from 350 to 400 degrees F. This will cause some oil to be burnt into flakes (solids). The rest of the oil carryover will be in the form of free liquid, mist and vapor.
b) Rotary Screw Compressors
The lubricated or oil flooded rotary screw (or rotary vane) compressor will inject oil directly into the air during the compression process. This oil and air mixture will pass through a filter (called the air oil separator) that will separate the oil from the compressed air.
The air oil separator is designed to pull the oil from the compressed air and return it to the lubrication system. The air that leaves the separator tank, under normal conditions, will have approximately 2 to 3 ppm of lubricant carryover.
The normal operating maximum temperatures this oil will see at the compressor discharge will range from 190 to 200 degrees F. This reduces the potential of having oil burnt into solid flakes and will usually lower the amount of oil vapor passed into the air system.
The oil carryover, from a rotary screw compressor, into the air piping will be found in the form of a mist and vapor when the air oil separator is functioning correctly. A failure or a malfunction of the air oil separator will result in a significant volume of lubricant passing into the air system in the free liquid form.
c) Non Lube Compressors
A compressor in this category can be either a reciprocating, rotary screw, centrifugal, scroll or lobe type. The risk of oil carryover is not usually a concern because oil is not present in the compression chamber. However, there is still a potential for oil contamination.
It is possible for the ambient air to have hydrocarbon content which will enter the compressor with the inlet air. This will become more concentrated as the air is compressed.
Another possibility is a malfunction within the compressor. Many non lube compressors have a lubricated drive train that is separated from the compression chamber by some type of sealing method. A failure in the sealing can allow lubricant into the compression chamber.
4) Match the Filter to the Contaminant
You have to use the appropriate type of filter for the contaminate you are trying to remove from the compressed air system. This requires an understanding of how the different filter types work so you can pick the right one for your application.
Some inline filters are promoted as being capable of handling multiple functions, such as filtering the air for particulates and for liquids. Our research has found that you will be better served by picking one filter for each purpose.
The following are the most common types of inline filters used in a compressed air system.
a) Particulate Filters
These filters are designed to remove solids from the air flow.
Particulate filters have a Micron Rating which is one way to compare performance levels. However, it is not always easy to make performance comparisons.
Micron ratings can be expressed as Absolute or as Nominal. This makes it possible for 2 filters to have the same numerical micron rating but different filtration characteristics.
An Absolute 3 micron rating means that the filter was designed to stop all particulates larger than 3 micron and an ever descending percentage of particulates smaller than 3 micron.
A Nominal 3 micron rating means that the filter will stop a majority of the particulates larger than 3 micron and an ever descending percentage of particulates smaller than 3 micron.
Particulate filters have a certain life which is partly based on the amount of surface area. This is why some people will oversize a particulate filter.
This idea will not improve filter efficiency but, it will extend the life of the replacement element and get a lower pressure drop. This is worth considering because particulate filter elements are changed by monitoring pressure drop across the filter.
b) Mechanical Filters
This type of filter is used to remove large volumes of liquids (water and oil) from a compressed air system.
The standard mechanical filter is the centrifugal type which is sometimes referred to as a separator. These work by directing the stream of wet air through a controlled centrifugal flow.
The centrifugal motion forces entrained liquids (and some solids) to the outer walls of the separator. There are breakers or baffle plates that prevent the separated liquids (and some solids) from returning to the air stream.
The separators are designed to allow the contaminants to gravity drain to the bottom of the filter. A drain is required to discharge the contaminants from the separator.
The centrifugal mechanical separators, when sized correctly and installed with proper drains, are limited to effective separation of particles 10 micron or larger. This is because particles smaller than 10 micron are less sensitive to the centrifugal separation.
The wire mesh coalescing separator is another type of mechanical filter that is used for removing large amounts of liquid contamination.
This is a two stage filter that is sometimes referred to as a coalescing separator or a mist eliminator. However, it should not be confused with the coalescing filters mentioned later in this report.
The first stage is a wire mesh that provides a coalescing action that increases the size of the liquid droplets as they travel through the mesh. The second stage is very similar to the standard centrifugal separator.
The wire mesh will convert the smaller droplets of liquid into drops that are large enough for the centrifugal separator to remove. The wire mesh coalescing separator, when sized and installed with proper drains, can remove 99 percent of the entrained liquids and particles that are 5 micron and larger in size.
c) Coalescing Filters
This type is used to remove water and oil when they are in the mist or aerosol forms.
The mist passes through several layers of filter media. In the process, the liquid will adhere to the fibers and form larger and larger droplets.
The fibers do not react with the liquid drops. They simply trap and coalesce them by utilizing the effects of surface tension. This continues until the drops of liquid have enough mass to gravity drain to a sump area in the bottom of the filter.
The liquid must not be allowed to accumulate in the bottom of the filter. This will saturate the element and dramatically lower the performance of the filter as the liquid wicks through. Also, coalescing filters may not be able to handle a slug of liquid on a continuing basis.
One version of coalescing filter is the loose pack, deep bed coalescing filters. These are sold by many vendors and are commonly referred to as mist eliminators.
The loose pack, deep bed coalescing filters will usually have 1 pound of pressure drop (or less) when new and are changed when they reach 3 pounds of pressure drop. This type may be more expensive than the standard coalescing filters but they will typically last many times longer (element design life is 5 to 10 years).
Coalescing filters operate based on velocity and are sized to handle the CFM flow and the pressure of each application. It is not a good idea to indiscriminately oversize the standard coalescing filter because you can lose the velocity required for proper coalescing. However, this is not true of the loose pack, deep bed type of coalescing filter.
Pressure drop is monitored to determine when the element must be replaced in a coalescing filter. The service life of this type of filter is reduced as oil and water residue or solid contaminants accumulate on the element.
Consider using a particulate filter before the coalescing filter in those situations where solid contamination is a real threat to the life of a coalescing filter. This move can be justified when you consider that a particulate filter is much less expensive than a comparable coalescing filter.
Filters and Drains
The mechanical and the coalescing filters are designed to remove liquids from the air flow. The liquid contamination is directed to a sump area in the bottom of the filter. This means that each filter must have a drain that can discharge the collected liquids.
The performance of the filter is dependent upon the reliable operation of the drain. The liquid contaminates will pass through the filter if the drain fails. Our research has found shortcomings with the many of the products being used to drain inline filters. The following are 3 requirements for an inline filter drain:
a) Automatically remove liquid when it appears at the drain.
b) Capable of handling the amount of liquid that is expected during the warmest and most humid times of the year.
c) Be easy to monitor and service.
Note: If you want help selecting a drain, send an email to firstname.lastname@example.org with a brief description of the drain location. We will reply with a recommendation and contact details for someone you can trust, who understands how to match a condensation drain to an application.
a) Avoid selecting inline filters that perform well beyond the required standards for your application. This will make the filters unnecessarily expensive to purchase and maintain.
b) You will need to know the CFM air flow of your system when sizing inline filters. This data is usually available on the compressor name plate or can be obtained from your compressor vendor.
c) A filter that is sized too small will cause additional maintenance attention including more frequent element replacement.
d) Once removed from your system, compressor condensate (and what it consists of), must be handled and disposed of according to the environmental regulations in your area.
Note: The information in this report does not apply to breathing air which requires different specifications and guidelines. If you want information on this topic, refer to ISO, CAGI, OSHA or whichever agency guidance you prefer.