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

Intelligent Compressed Air: Things to Consider when Designing the Compressor Room


One common thing that can be easily overlooked is the importance of designing an efficient compressor room. After you’ve determined your overall requirements and selected the appropriate compressor, you can begin designing the layout of your compressor room. For starters, the compressor room should be located in a central location when possible, close to the point of use. This will help to minimize pressure drop as well as reduce installation costs as less piping will be required. If this isn’t possible, try to keep the compressor room close to the larger volume applications in your facility. Otherwise you will have to use larger diameter piping in order to ensure an adequate volume of air is available.

The diameter of the distribution piping should NOT be based on the connection size of the compressors, aftercoolers, or filters. According to the Compressed Air Challenge Best Practices for Compressed Air Systems handbook, piping should be sized so that the maximum velocity in the pipe is 30 ft/sec. When the distance between the compressor room and the point of use is lengthy, consider increasing the pipe diameter to minimize the pressure drop across the system.

Inside of your compressor room you’ll have a variety of different equipment, all dependent on the demand, quality, supply, storage, and distribution of your compressed air. Keeping all of the equipment in its own room will also provide some insulation from the noise associated with compressed air generation. It is crucial that the space selected as your compressor room is sufficiently large enough to accommodate everything without becoming cramped. As a general rule of thumb, keep about 3′ of space between equipment such as the compressor, receiver tanks, aftercooler, and dryer. This helps to prevent equipment from overheating as well as offers maintenance personnel adequate space with which to perform any regularly scheduled maintenance or repairs.

Once you’ve selected your equipment, piping, and determined the location, another thing to consider is ventilation. As compressed air is generated, the compressor gives off a good amount of heat. It is important that the exhaust air is not permitted to re-circulate throughout the compressor room. The exhaust needs to be ducted so that it the warm air is not drawn in at the air intake on the compressor. Some equipment, such as refrigerated dryers, requires a substantial amount of cooling air. In these situations, an exhaust fan can be used to provide that additional airflow.

To further enhance the efficiency of your facility, the heat generated from compression can be re-purposed instead of simply exhausting into the ambient environment. This process is commonly referred to as compressed air energy recovery. Some industries require a source of heat for many of their manufacturing processes. In these scenarios, the heat energy that is produced during compression can be reused rather than having to generate another source of heated air. If the heated air can’t be used for any of your manufacturing processes, the heat can be used as a means to heat your water supply or even to heat the facility itself. This can drastically reduce your electricity or gas requirements during cooler periods.

To reduce the amount of required maintenance and ensure that your compressor is operating as efficiently as possible, the compressed air intake must also be free from particulate and harmful gases. When dust and dirt is drawn into the compressor, it can cause wear on the internal components. If the ambient environment contains a lot of dust and particulate, a pre-filter can be used to prevent any future problems. In these instances, it is important to consider the pressure drop that will be caused when designing the system.

Keeping these tips in mind will serve to make your life much easier in the long run. Once you have everything installed and set up, visit the EXAIR website or give us a call to speak with an Application Engineer. EXAIR’s Intelligent Compressed Air Products  can help you reduce compressed air consumption and increase worker safety by adhering to both OSHA 1910.242(b) and 1910.95.

Tyler Daniel
Application Engineer
Twitter: @EXAIR_TD

Image Courtesy of  thomasjackson1345 Creative Commons.

Intelligent Compressed Air: What is an Air Compressor?

Example of the supply side of a compressed air system

One thing that is found in virtually every industrial environment is an air compressor. Some uses for the compressed air generated are: powering pneumatic tools, packaging, automation equipment, conveyors, controls systems, and various others. Pneumatic tools are favored because they tend to be smaller and more lightweight than electric tools, offer infinitely variable speed and torque, and can be safer than the hazards associated with electrical devices. In order to power these devices, compressed air must be generated.

types of compressors

There are two main categories of air compressors: positive-displacement and dynamic. In a positive-displacement type, a given quantity of air is trapped in a compression chamber. The volume of which it occupies is mechanically reduced (squished), causing a corresponding rise in pressure. In a dynamic compressor, velocity energy is imparted to continuously flowing air by a means of impellers rotating at a very high speed. The velocity energy is then converted into pressure energy.

Of the positive-displacement variety they are broken down further into two more categories: reciprocating and rotary. A reciprocating compressor works like a bicycle pump. A piston reduces the volume occupied by the air or gas, compressing it into a higher pressure. There are two types of reciprocating compressors, single or double-acting. Single-acting compressors are the most common and are available up to 30HP at 200 psig. Their small size and weight allow them to be installed near the point of use and avoid lengthy piping runs. These are the types of compressors that would be commonly found in your garage. The double-acting reciprocating compressor is much like its single-acting brethren, only it uses both sides of the piston and cylinder for air compression. This doubles the capacity of the compressor for a given cylinder size. They are much more efficient than single-acting compressors, but are more expensive and do require a more specialized installation and maintenance.

Rotary compressors are available in lubricant-injected or lubicrant-free varieties. These types of compressors use two inter-meshing rotors that have an inlet port at one end and a discharge port at the other. Air flows through the inlet port and is trapped between the lobes and the stator. As the rotation continues, the point intermeshing begins to move along the length of the rotors. This reduces the space that is occupied by the air, resulting in an increase in pressure. In the lubricant-injected compressors, the compression chamber is lubricated between the intermeshing rotors and bearings. This takes away the heat of compression and also acts as a seal. In the lubricant-free varieties, the intermeshing rotors have very tight tolerances and are not allowed to touch. Since there is no fluid to remove the heat of compression, they typically have two stages of compression with an intercooler between and an after cooler after the second stage. Lubricant-free compressors are beneficial as they supply clean, oil-free compressed air. They are, however, more expensive and less efficient to operate than the lubricant-injected variety.

On the other side of the coin, we have the dynamic compressors. These are comprised of two main categories: axial and centrifugal. These types of compressors raise the pressure of air or gas by imparting velocity energy and converting it to pressure energy. In a centrifugal air compressor, air continuously flows and is accelerated by an impeller. This impeller can rotate at speeds that exceed 50,000 rpm. Centrifugal air compressors are generally much larger and can accommodate flow ranges of 500-100,000 CFM. They also provide lubricant-free air.

Axial compressors are used for situations that require lower pressure but high flow rates. They do not change the direction of the gas, it enters and exits the compressor in an axial direction. It is accelerated and then diffused which creates the increase in pressure. A common application that would be served by this type of compressor is to compress the air intake of gas turbines. They have a relatively high peak efficiency, however their large overall size and weight as well as the high starting power requirements pose some disadvantages.

Just as you can find a wide variety of makes and models of automobiles, the same can be said for air compressors. The size, type, and features will be dictated by the types of applications that you’ll be needing the compressed air for in your facility. A quick chat with your local air compressor supplier will help you to determine which type is most suitable for you.

Of course, any of these types of compressors can be used to supply air to your engineered Intelligent Compressed Air Products. If you have an application in your facility that could benefit from an engineered solution, give us a call. An Application Engineer would be happy to discuss your options with you and see to it that you’re getting the most out of your compressed air!

Tyler Daniel
Application Engineer
Twitter: @EXAIR_TD


Images Courtesy of  the Compressed Air Challenge and thomasjackson1345 Creative Commons.

Intelligent Compressed Air: Refrigerant Dryers and How They Work

We’ve seen in recent blogs that Compressed Air Dryers are an important part of a compressed air system, to remove water and moisture to prevent condensation further downstream in the system.  Moisture laden compressed air can cause issues such as increased wear of moving parts due to lubrication removal, formation of rust in piping and equipment, quality defects in painting processes, and frozen pipes in colder climates.  The three main types of dryers are – Refrigerant, Desiccant, and Membrane. For this blog, we will review the basics of the Refrigerant type of dryer.

All atmospheric air that a compressed air system takes in contains water vapor, which is naturally present in the air.  At 75°F and 75% relative humidity, 20 gallons of water will enter a typical 25 hp compressor in a 24 hour period of operation.  When the the air is compressed, the water becomes concentrated and because the air is heated due to the compression, the water remains in vapor form.  Warmer air is able to hold more water vapor, and generally an increase in temperature of 20°F results in a doubling of amount of moisture the air can hold. The problem is that further downstream in the system, the air cools, and the vapor begins to condense into water droplets. To avoid this issue, a dryer is used.

Refrigerated Dryer
Fundamental Schematic of Refrigerant-Type Dryer

Refrigerant Type dryers cool the air to remove the condensed moisture and then the air is reheated and discharged.  When the air leaves the compressor aftercooler and moisture separator (which removes the initial condensed moisture) the air is typically saturated, meaning it cannot hold anymore water vapor.  Any further cooling of the air will cause the moisture to condense and drop out.  The Refrigerant drying process is to cool the air to 35-40°F and then remove the condensed moisture.  The air is then reheated via an air to air heat exchanger (which utilizes the heat of the incoming compressed air) and then discharged.  The dewpoint of the air is 35-40°F which is sufficient for most general industrial plant air applications.  As long as the compressed air stays above the 35-40°F temperature, no further condensation will occur.

The typical advantages of Refrigerated Dryers are-

  1.  – Low initial capital cost
  2.  – Relatively low operating cost
  3.  – Low maintenance costs

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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Twitter: @EXAIR_BB


Types Of Compressed Air System Dryers

Many times, when discussing product selection with a customer, we commonly reference supplying as clean and dry air as possible to promote peak performance. In iron piping systems for example, when moisture is present, rust can develop which can reduce the performance of end use compressed air operated devices like air tools or cause issues on the exhaust side as you could exhaust unwanted mist onto a surface, like in a painting operation.

Example of a desiccant dryer

Typically, an efficient and properly installed industrial compressed air system will include some type of dryer to remove any moisture that may be present in the supply.

Let’s take a look at the various types of dryers available.

Refrigerant and desiccant dryers are two of the more commonly used types of dryers.

Refrigerant based systems have several stages. The compressed air first passes through an air to air heat exchanger  which initially cools the air. The air is then delivered to an air to refrigerant exchanger where an external source of liquid refrigerant further cools the air and sends it to a separator, where the water vapors condensate and are removed through a drain trap. Now that the air is dry, it is then cycled back to the air to air exchanger where it is heated back to ambient temperature and exits the system.

Desiccant dryers typically incorporate 2 tanks containing a porous desiccant which causes the moisture to sort of “cling” to the surface. In these systems, compressed air flows through one tank, while, using it’s own regeneration cycle, heated or unheated air is blown through the desiccant in the other tank to remove the moisture and dry the air.

Membrane Dryers are typically used at the end use product. These types of systems utilize membranes to dissipate water vapor as it passes through the material, while allowing a small amount of the dry air to travel the length of the membrane to sort of “wipe” the condensate and remove it from the system.

Deliquescent Dryers use a drying agent which absorbs any moisture in the air. As the vapors react with the desiccant, like salt, the desiccant liquefies and is able to be drained at the bottom of a tank. These are the least expensive dryers to purchase and maintain because they have no moving parts and require no power to run.

When a dryer is being considered for a particular setup, there are 3 common reference points used when determining the dryers rating – an inlet air temperature of 100°F, supply pressure of 100 PSIG and an ambient air temperature of 100°F. Changes in supply pressure or temperature could change the performance of a particular dryer. You want to follow the manufacturer’s recommendations when dealing with variances as they will typically provide some type of conversion.

For help with this or any other topics relating to the efficient use of compressed air, please give us a call, we’d be happy to help.

Justin Nicholl
Application Engineer


Heated Desiccant Dryers image courtesy of Compressor1 via creative commons license

About Double Acting Reciprocating Air Compressors

My colleague, Lee Evans, wrote a blog “About Single Acting Reciprocating Compressors”, and I wanted to extend that conversation to a more efficient relative, the double acting reciprocating compressor.   As you see in the chart below, this type of compressor falls within the same family under the category of positive displacement compressors.

Compressor Types

Positive displacement compressors increase air pressure by reducing air volume within a confined space.  The reciprocating type of air compressor uses a motor that turns a crank which pushes a piston inside a cylinder; like the engine in your car.  In a basic cycle, an intake valve opens to allow the ambient air into the cylinder, the gas gets trapped, and once it is compressed by the piston, the exhaust valve opens to discharge the compressed volume into a tank.  This method of compression happens for both the single and double acting reciprocating compressors.  With a single acting compressor, the air is compressed only on the up-stroke of the piston inside the cylinder.  The double acting compressor compresses the air on both the up-stroke and the down-stroke of the piston, doubling the capacity of a given cylinder size.  This “double” compression cycle is what makes this type of air compressor very efficient.  A single acting compressor will have an operating efficiency between 22 – 24 kW/100 cfm of air while the double acting compressor has an operating efficiency between 15 – 16 kW/100 cfm.  Therefore, electricity cost is less with a double-acting reciprocating air compressor to make the same amount of compressed air.

To explore the internals a bit closer, the mechanical linkage used to move the piston is slightly different as well as the additional intake and exhaust valves.   Instead of the connecting rod being attached directly to the piston as seen inside a single acting compressor, a crosshead is added between the compression piston and the connecting rod (view picture below).  The rod that connects the crosshead to the compression piston can be sealed to keep the cylinder completely encapsulated.  For every rotation of the electric motor, the air is being compressed twice.  With the added heat of compression, the double acting compressors are generally water-cooled.  Also, with the added mechanism between the crank and the piston, the rotational speeds are typically less.  Because of the larger size, water jackets, and added parts, the initial cost is more expensive than the single acting compressor, but the efficiency is much higher.

Double Acting Reciprocating Air Compressor

Double acting compressors are generally designed for rugged 100% continuous operations.  Dubbed the work horse of the compressor family, they are also known for their long service life.  They are commonly used in high pressure services in multistage styles and can come in lubricated and non-lubricated configurations.   With the dual compression, slow speed and inter-cooling, it makes this type of air compressor very proficient in making compressed air.

John Ball
Application Engineer
Twitter: @EXAIR_jb


Photos:  used from Compressed Air Challenge Handbook