Compressed Air: Central Filtration system VS Point of Use Filter
Central Filter and Point Of Use Filters are both crucial in maintaining an efficient, dry and clean compressed air distribution system. To understand how each play a role in maintaining an efficient compressed air system – it is helpful to learn how compressed air is produced.
Producing Compressed Air
When compressed air is produced it is hot and saturated with moisture. Therefore, compressed air systems usually consist of the following components:
The air receiver tank acts as a reservoir to store and cool the compressed air and helps make sure the system can cope with variations in demand. In the tank the compressed air further cools over time and water further condenses out but the compressed air can remain saturated with moisture.
The air cooler, main filter and dryer all treat the air at different points in the system. They remove impurities such as water, dirt and oil from the air taken in by the compressor, as well as those added by the compressor.
Compressed air may be fed to various uses on a site via a distribution system.
Unfiltered compressed air contains dust, oil, rust, moisture and other harmful substances, and therefore requires filtration. In the first main central stage of filtration, the compressed air is filtered using a large central air “separator” type of filter to clean the air of loose moisture and oil and dirt prior to entering the distribution system. Since water moisture is a part of atmospheric air, condensate in compressed air systems can’t be avoided. During the compression process, the air is heated. As it cools and nears the ambient temperature (approach), vapor condenses to liquid and can be removed by mechanical separation. As the air cools further, more condensate is present. Water moisture is not the only undesirable substance. There are also other pollutants in the compressed air: lubricating oil carried over from oil lubricated air compressors, atmospheric corrosive gases inhaled by the air compressor, aerosols and vapors, solid particles and rust from pipe system and pressure vessels and solid particles drawn in by the air compressor. It is very important to remove the most of these impurities from the compressed air. These filtering separators use centrifugal motion to force condensate out of compressed air. The spinning causes the condensate to join together on the centrifugal separators walls. Condensate is removed out of the system by the condensate drain.
A dryer placed after the air receiver can remove more moisture depending on the type of dryer used. So in fact, the compressed air produced centrally can be quite clean and dry prior to entering the compressed air distribution system.
However as the compressed air passes through the distribution system the system itself can still end up requiring further filtration at the point of use. Ordinary Industrial Compressed Air is still universally wet, dirty and contaminated with entrained oil vapors, some of which can get into the distribution system from leaks and other openings such as condensate traps and from the distribution system itself. All industrial compressed air put through a distribution system will still contain the following:
While some industrial operations continue to put up with these problems for years, they can be addressed at the point-of-use. If you ever hooked up a new air tool or air nozzle to a compressed air connection and had water spray out, that is the perfect example why point of use filtration is important. The water, dirt, and oil residue collects at low points in the air distribution system. Even if central filtration stops most of the contamination, some gets through and over time simply collects. For this reason alone point of use filtration is necessary. The net gain in efficiency from filtering the air at both the compressor site and the point-of-use more than pays for the initial investment because it:
There is a plethora of compressed air and gas filters offered in today’s market. They range from strainers such as wire mesh, sintered bronze or stainless steel elements, ribbon or string wood elements, fiber or cellulose discs to open pore sponge materials.
Filtration begins at the compressor site with Air Intake Filters, Aftercoolers, Refrigerated Dryers and even large central Coalescing Filters. The arrangement is more or less common in the industry today. However, contaminants remain such as 1) rust and scale 2) oil vapors, and 3) water vapors.
Different desiccants are available to do effectively one or all of the steps filtration requires. Coalescing filters, in general, will not remove oil vapors, water vapors or odors.
The presence of oil, oil vapors, and water at the point-of-use is ordinarily evidenced by the liquid in the bowl or trap having a milky appearance, which is an emulsion of oil and water. These contaminants are damaging to such sensitive equipment and instruments, pneumatic controls and hundreds of other industrial plant processes.
The following are typical applications where point-of-use filters are necessary:
|● Instrument Air
● Pneumatic Controls
● Process Air
● Air Operated Assembly Tools
● Injection Molding
● Filtering Gases
● Air Jet / nozzles
● Sorting Equipment
● Separating Pills
|● Spray Paint & Finishes
● Bottle & Container Cleaning
● Pneumatic Extruders
● Air for Packaging & Weighing
● Mixing & Blending
● Aeration & Agitating of Liquids
● Operating Clamps & Chucks
● Squeeze rolls
● Oxygen Analyzers
In many industries and processes the quality and extent of filtering is specified as in food and pharmaceutical processes. These air quality requirements (link to one of the air quality article) determine the extent of filtration required in any leg of the distribution system for the particular process.
There are two special separator type filters offered by Nex Flow Air Products that can be used in general manufacturing. These point of us system have the advantage of no cartridges to replace as is the case with other types of filters. The first is the Super Separator which can guarantee 99.99% removal of oil and water and dirt. It does not guarantee micron size but for any process where that is important additional membrane type filters can be placed after the Super Separator. Because of the high particulate and moisture removal of the Super Separator the time to replace cartridges in the membrane filters can be extended up to 4 times. For even better filtration there is the Expel Filter which removes 99.9999% oil and water and particulate and guarantees particulate removal to 1 micron. Another feature of these two special type of filters is the low pressure drop they have. Membrane type filters with cartridges that need replacement, are replaced after they build up so much collected dirt that the pressure drop across the system becomes very high. Any pressure drop across a filter, or for that matter, anything in a compressed air distribution system is an energy cost as this reduces the effective pressure downstream of the unit. So dirty filters need to be serviced regularly to keep the pressure drop within a small range.
These separator type filtration units, while they will not filter enough for more critical systems in food and pharmaceutical processes, they can reduce the load on filters used for much finer filtration and reduce the amount of maintenance and replacement needed on them, delaying pressure drop buildup, and reduce overall filter costs over time.
Both Central Filtration and Point Of Use Filtration is very important to maintain and efficient, and clean compressed air distribution system.