Oil Removal Filters – Keeping Compressed Air Clean

Compressed air filters help to keep the air clean and condensate free to protect equipment from dust, dirt, pipe scale, oil and water. Even though the compressed air system will typically have a main dryer, additional treatment is often necessary. For this discussion, we will focus on the oil removal process and filter type.

After the compressed air has passed through a particulate filter, the dirt, dust and water droplets have been removed.  Oil that is present is much smaller in size, and mostly passes though the particulate filter.  The installation of a coalescing filter will provide for the removal of the majority of the fine oil aerosols that remain. The coalescing filter works differently than the particulate filters. The compressed air flows from inside to outside through the coalescing filter media. The term ‘coalesce’ means to ‘come together’ or ‘form one mass.’  The process of coalescing filtration is a continuous process where the small aerosols of oil come in contact with fibers of the filter media. As other aerosols are collected, they will join up and ‘come together’ and grow to become an oil droplet, on the downstream or outside surface of the media.  Gravity will then cause the droplet to drain away and fall off the filter element.

Example of a 0.03 Micron Coalescing Oil Removal Filter

Some important information to keep in mind –

  • Change the filter regularly, not just when the differential pressures exceeds recommended limits, typically 5 PSI
  • Coalescing filters will remove solids too, at a higher capture rate due to the fine level of filtration, using a pre-filter for solids will extend the life
  • Oil free compressors do not provide oil free air, as the atmospheric air drawn in for compression contains oil vapors that will cool and condense in the compressed air system.

If you would like to talk about oil removal filters or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Line Loss: What It Means To Your Compressed Air Supply Pipe, Tubing, And Hose

“Leave the gun. Take the canolli.”

“What we’ve got here is failure to communicate.”

“I’ll get you my pretty, and your little dog too!”

“This EXAIR 42 inch Super Air Knife has ¼ NPT ports, but the Installation and Operation Instructions recommend feeding it with, at a minimum, a ¾ inch pipe…”

If you’re a movie buff like me, you probably recognize 75% of those quotes from famous movies. The OTHER one, dear reader, is from a production that strikes at the heart of this blog, and we’ll watch it soon enough. But first…

It is indeed a common question, especially with our Air Knives: if they have 1/4 NPT ports, why is such a large infeed supply pipe needed?  It all comes down to friction, which slows the velocity of the fluid all by itself, and also causes turbulence, which further hampers the flow.  This means you won’t have as much pressure at the end of the line as you do at the start, and the longer the line, the greater this drop will be.

This is from the Installation & Operation Guide that ships with your Super Air Knife. It’s also available from our PDF Library (registration required.)

If you want to do the math, here’s the empirical formula.  Like all good scientific work, it’s in metric units, so you may have to use some unit conversions, which I’ve put below, in blue (you’re welcome):

dp = 7.57 q1.85 L 104 / (d5 p)


dp = pressure drop (kg/cm2) 1 kg/cm2=14.22psi

q = air volume flow at atmospheric conditions (FAD, or ‘free air delivery’) (m3/min) 1 m3/min = 35.31 CFM

L = length of pipe (m) 1m = 3.28ft

d = inside diameter of pipe (mm) 1mm = 0.039”

p = initial pressure – abs (kg/cm2) 1 kg/cm2=14.22psi

Let’s solve a problem:  What’s the pressure drop going to be from a header @80psig, through 10ft of 1″ pipe, feeding a Model 110084 84″ Aluminum Super Air Knife (243.6 SCFM compressed air consumption @80psig)…so…

q = 243.6 SCFM, or 6.9 m3/min

L = 10ft, or 3.0 m

d = 1″, or 25.6 mm

p = 80psig, or 94.7psia, or 6.7 kg/cm2

1.5 psi is a perfectly acceptable drop…but what if the pipe was actually 50 feet long?

Again, 1.5 psi isn’t bad at all.  8.2 psi, however, is going to be noticeable.  That’s why we’re going to recommend a 1-1/4″ pipe for this length (d=1.25″, or 32.1 mm):

I’m feeling much better now!  Oh, I said we were going to watch a movie earlier…here it is:

If you have questions about compressed air, we’re eager to hear them.   Call us.

Russ Bowman
Application Engineer
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About Sliding Vane Air Compressors

Over the last few months, my EXAIR colleagues have blogged about several different types of air compressor types including single and double acting reciprocating and rotary screw. (You can select the links above to check those out.) Today I will review the basics of the sliding vane type, specifically the oil/lubricant injected sliding vane compressor.

The lubricant injected sliding vane compressor falls under the positive displacement-type, the same as the other types previously discussed.  A positive displacement type operates under the premise that a given quantity of air is taken in, trapped in a compression chamber and the physical space of the chamber is mechanically reduced.  When a given amount of air occupies a smaller volume, the pressure of the air increases.

Each of the previous positive displacement type compressors use a different mechanism for the reduction in size of the compression chamber.  The single and double acting reciprocating use a piston that cycles up and down to reduce the compression chamber size. The rotary screw uses two inter-meshing rotors, where the compression chamber volume reduces as the air approaches the discharge end.  For the lubricant sliding vane type, the basic design is shown below.

Sliding Vane2
Air enters from the right, and as the compression chamber volume reduces due to counterclockwise rotation, the pressure increases until the air discharges to the left

The compressor consist of an external housing or stator, and the internal circular rotor, which is eccentrically offset.  The rotor has radially positioned (and occasionally offset) slots in which vanes reside.  As the rotor rotates, the centrifugal forces on the vanes cause them to move outwards and contact the inner surface of the stator bore.  This creates the compression areas, formed by the vanes, rotor surface and the stator bore.  Because the rotor is eccentrically offset, the volume of the compression area reduces as the distance between the rotor surface and the stator reduces.  As the rotor turns counterclockwise, the vanes are pushed back into the rotor slots, all the while in contact with the stator surface.  The shrinking of the compression area leads to the increase in air pressure.

Oil is injected into compression chamber to act as a lubricant, to assist is sealing, and to help to remove some of the heat of compression.

The advantages of the lubricant sliding vane compressor type is very similar to the lubricant injected rotary screw.  A few key advantages include:

  • Compact size
  • Relatively low initial cost
  • Vibration free operation- no special foundation needed
  • Routine maintenance includes basic lubricant and filter changes

A few of the disadvantages include:

  • Lubricant gets into the compressed air stream, requires an air/lubricant separation system
  • Requires periodic lubricant change and disposal
  • Less efficient than rotary screw type
  • Not as flexible as rotary screw in terms of capacity control in meeting changing demands

EXAIR recommends consulting with a reputable air compressor dealer in your area, to fully review all of the parameters associated with the selection and installation of a compressed air system.

If you would like to talk about compressed air or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Diagram:  used from Compressed Air Challenge Handbook

About Rotary Screw Air Compressors

Recently, EXAIR Application Engineers have written blogs about reciprocating type air compressors: Single Acting (by Lee Evans) and Dual Acting (by John Ball.) Today, I would like to introduce you, dear EXAIR blog reader, to another type: the Rotary Screw Air Compressor.

Like a reciprocating compressor, a rotary screw design uses a motor to turn a drive shaft. Where the reciprocating models use cams to move pistons back & forth to draw in air, compress it, and push it out under pressure, a rotary screw compressor’s drive shaft turns a screw (that looks an awful lot like a great big drill bit) whose threads are intermeshed with another counter-rotating screw. It draws air in at one end of the screw, and as it is forced through the decreasing spaces formed by the meshing threads, it’s compressed until it exits into the compressed air system.

Rotary Screw Air Compressor…how it works.

So…what are the pros & cons of rotary screw compressors?


*Efficiency.  With no “down-stroke,” all the energy of the shaft rotation is used to compress air.

*Quiet operation.  Obviously, a simple shaft rotating makes a lot less noise than pistons going up & down inside cylinders.

*Higher volume, lower energy cost.  Again, with no “down-stroke,” the moving parts are always compressing air instead of spending half their time returning to the position where they’re ready to compress more air

*Suitable for continuous operation.  The process of compression is one smooth, continuous motion.

*Availability of most efficient control of output via a variable frequency drive motor.

*They operate on the exact same principle as a supercharger on a high performance sports car (not a “pro” strictly speaking from an operation sense, but pretty cool nonetheless.)


*Purchase cost.  They tend to run a little more expensive than a similarly rated reciprocating compressor.  Or more than a little, depending on options that can lower operating costs.  Actually, this is only a “con” if you ignore the fact that, if you shop right, you do indeed get what you pay for.

*Not ideal for intermittent loads.  Stopping & starting a rotary screw compressor might be about the worst thing you can do to it.  Except for slacking on maintenance.  And speaking of which:

*Degree of maintenance.  Most maintenance on a reciprocating compressor is fairly straightforward (think “put the new part in the same way the old one came out.”)  Working on a rotary screw compressor often involves reassembly & alignment of internal parts to precision tolerances…something better suited to the professionals, and they don’t work cheap.

Like anything else, there are important factors to take under consideration when deciding which type of air compressor is most suitable for your needs.  At EXAIR, we always recommend consulting a reputable air compressor dealer in your area, helping them fully understand your needs, and selecting the one that fits your operation and budget.

Russ Bowman
Application Engineer
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Super Air Knife Plumbing Kit Allows Installation In Tight Quarters

I recently had the pleasure of helping a long-time user of our Super Air Knives with a challenging application. They already use quite a few of our Model 110012SS 12″ Stainless Steel Super Air Knives to clean & dry their nonwoven material as it’s being rolled for packaging. They like them because they’re quiet and efficient, but also because they’re durable…this particular product off-gasses a mildly corrosive vapor, which used to corrode other equipment in the area. Not only does the Stainless Steel Super Air Knife resist corrosion itself, the air flow keeps these vapors contained. Two birds, one stone.

They have a new product…same kind of material, but much wider…that needed to be blown off, and the identified the Model 110060SS 60″ Stainless Steel Super Air Knife as a “no-brainer” solution. Thing is, it had to be a pretty even air flow across the length, and a 60″ Super Air Knife has to get air to four ports across its length for optimal performance. And, they wanted to install it at a point where it would serve not only as a blow off, but as a vapor barrier, just like the 12″ Super Air Knives they’re already so fond of. The space was a little limited, though, so they opted for the Model 110060SSPKI 60″ Stainless Steel Super Air Knife with Plumbing Kit Installed, which allowed them to simply run an air supply line to both ends.

EXAIR SS Super Air Knives can be ordered with a Plumbing Kit installed, or you can easily install a Plumbing Kit on your existing Super Air Knife.

If you want to find out more about an engineered solution for your compressed air application – cleaning, drying, vapor barrier, or all of the above – give me a call.

Russ Bowman
Application Engineer
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Intelligent Compressed Air: How to Develop a Pressure Profile

An important part of operating and maintaining a compressed air system is taking accurate pressure measurements at various points in the compressed air distribution system, and establishing a baseline and monitoring with data logging.  A Pressure Profile is a useful tool to understand and analyze the compressed air system and how it is functioning.

Pressure Profile 1
Sample Pressure Profile

The profile is generated by taking pressure measurements at the various key locations in the system.  The graph begins with the compressor and its range of operating pressures, and continues through the system down to the regulated points of use, such as Air Knives or Safety Air Guns.  It is important to take the measurements simultaneously to get the most accurate data, and typically, the most valuable data is collected during peak usage periods.

By reviewing the Pressure Profile, the areas of greatest drop can be determined and the impact on any potential low pressure issues at the point of use.  As the above example shows, to get a reliable 75 PSIG supply pressure for a device or tool, 105-115 PSIG must be generated, (30-40 PSIG above the required point of use pressure.)  As a rule of thumb, for every 10 PSIG of compressed air generation increase the energy costs increase 5-7.5%

By developing a total understanding of the compressed air system, including the use of tools such as the Pressure Profile, steps to best maximize the performance while reducing costs can be performed.

If you have questions about getting the most from your compressed air system, or would like to talk about any EXAIR Intelligent Compressed Air® Product, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Proper Supply Lines are Key to Air Knife Performance

A few weeks back I chatted with a customer on an Air Knife application where they were using our 48″ aluminum Super Air Knife to remove leftover dough from a baking pan. The knife was working somewhat, but they were seeing some residual dough being left in certain areas on the pans due to what they perceived as “weak” airflow. After reading through our catalog and installation guide, they noticed that there were available shim sets that would allow them to increase the gap setting to get more force and flow out of the knife.

Available in lengths from 3″ to 108″ in aluminum, 303ss or 316ss construction

Our aluminum Super Air Knives are shipped from stock with a .002″ shim installed. The optional shim set includes a .001″, .003″ and .004″ shim that would allow you to decrease or increase the performance. By operating the Super Air Knife with the .003″ shim installed, this would increase the force and flow by 1.5 times and using the .004″ shim would double the performance. Sometimes achieving greater force and flow may be required but with the customer saying they were seeing weak airflow, it seemed there may be a restriction on the supply side.

Super Air Knife with Shim Set

I asked the customer how the knife was plumbed and what size supply lines he was using. He advised that they were plumbing air to all 3 inlets on the bottom of the knife but they were using 3/4″ hose with a run of about 30′. I advised the customer that plumbing air to all 3 inlets is required for a 48″ Super Air Knife but we actually recommend 3/4″ Schedule 40 Pipe up to 10′ or 1″ pipe up to 50′. If using hose, he would need to go up a size to maintain a large enough ID to carry the volume required for the unit. In his case, since the length of the supply is close to 30′, he would need to use 1-1/4″ ID hose.

Improper plumbing line size is a common issue we deal with here at EXAIR. Using undersized supply lines can cause excessive pressure drops because they aren’t able to carry the volume of air necessary to properly supply the compressed air device. In this particular application, if the customer were to install either the .003″ or .004″ shim, while keeping his current plumbing size, the performance would actually be worse as now the lines are even more undersized due to the increased air volume requirement from the larger Super Air Knife gap.

If you are looking to change the performance with one of our Air Knives or if you would like to discuss a particular application or product, please contact one of our application engineers for assistance at 800-903-9247.

Justin Nicholl
Application Engineer