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Compressed Air Systems 101




I.  Compressed air is the phantom utility.  Industry has to have it, but it can’t buy it. Industry can buy electricity and water.  They have to make their own compressed air.


II.  Why compress air?  Compressed air is used as a source of energy, much like electricity.  Compressed air is stored energy.  How is it used in industry?

            A.  Pneumatic motors

            B.  Pneumatic cylinders

            C.  Operate or position valves and instruments

            D.  Operate control devices

            E.  Convey materials

                        1. Push product through a pipe, i.e. cement and plastic pellets

                        2.  Convey materials along a tray or belt

                        3.  Align, rotate or flip materials, i.e. push empty Coke cans to one side of a conveying belt to allow their proper entry into the next machine

                        4.  Remove a bad product, i.e. an electronic eye determines that a can has fallen and removes the can with a blast of compressed air

            F.  Dry, heat or cool materials or finished products

            G.  Operate pneumatic tools

            H.  Spray abrasive material under pressure to remove coating

            I.  Painting or coating application

            J.  Inflate tires or other components

            K.  The list for the uses of compressed air is endless


III.  Why use compressed air in lieu of electricity?

            A.  Compressed air is safer

                        1. Non-sparking

                        2. Not explosive

            B.  Compressed air is typically non-damaging to the component. You can stall a pneumatic motor and cause no damage


IV.  What makes up the air that we breathe, the air that we compress?

            A. 78% Nitrogen

            B. 21% Oxygen

            C. 1% Argon and other rare gases


V.  Qualities of ambient air:

            A.  Humidity, we complain about the amount of gaseous water held by                         the air

            B.  Particulate matter, dust, pollen and other matter

            C.  Hydrocarbons

                        1.  Oil vapors

                        2.  Engine exhaust waste

                        3.  Industrial waste, pollutants

            D.  One cubic foot of air weighs 0.07 pounds.

            E.  Ambient air at sea level is 0 PSI (0 PSIG for gauge pressure) or 14.7 PSIA for absolute pressure.


VI.  Characteristics of compressed air:

            A.  An air compressor takes ambient air and compresses it an average of eight-fold.  117 PSI is eight times one atmosphere (14.7).  Most air compressors operate between 100 PSI and 125 PSI.  Air compressors basically take eight coffee cans of ambient air and shove it into one coffee can.

            B.  We now have eight times the contaminants:

                        1.  Eight times the water.

                                    a.  A 100 HP air compressor operating at full load for a twenty-four hour day at ambient conditions of 95 F and 40% average humidity takes in fifty-five gallons (a drum) of water a day! 

                                    b.  This water must be removed at each drain.  Poor removal of water at any drain in this system will overload the downstream equipment!

                                    c.  The ability to remove water directly relates to the operating temperature of the equipment.  It is critical to keep the coolers of the air compressor and air dryer clean.  The cooling air supply to these coolers must be ambient temperature or lower!

                        2.  Eight times the particulate matter.  Automobile manufacturers filter ambient air before it is compressed for use in the internal combustion engine.  We are eight times more contaminated.  This damaging matter can easily be removed via a particulate filer.  This is usually accomplished after the air compressor and before the air dryer.

                        3.  Eight times the hydrocarbons.  An industrial ambient atmosphere is typically measured at 15 PPM hydrocarbons.  After we compress the air we now have 120 PPM of damaging oils in the airstream.  Instruments and other components operating on the compressed air will fail with this level of oil in the compressed air.  A coalescer filter is used to coalesce or remove the hydrocarbons.  To coalesce means to bring together the aerosols molecules and drain them away.  While this is not really a filter, we name the component a coalescer filter.

                        4.  The air compressor discharge is a contaminant.  An oil flood rotary screw air compressor typically has an oil carry-over which adds more hydrocarbons and metal fines (particulates) to the airstream.

            C.  Definitions of Useable Compressed Air

                        1.  Plant Air

                                    a.  90 – 125 PSI

                                    b.  38 F pressure dewpoint (refrigerated air dryer)

                                    c.  Minimal particulate and hydrocarbon removal

                        2.  Instrument Air

                                    a.  40 – 90 PSI

                                    b.  0 F pressure dewpoint

                                    c.  99.997% removal of particulate and hydrocarbon removal

                        3.  Specialty Air

                                    a.  Medical Air

                                    b.  Pharmaceutical Air

                                    c.  Breathing Air

                                    d.  High Pressure Air


VII.  What makes up a typical industrial compressed air system?

            A.  Air compressor

                        1.  Rotary screw 5 – 500 HP, 15 CFM – 2,000 CFM

                        2.  Reciprocating (piston style like automobile engine) ˝ HP – 30 HP

                        3.  Double acting reciprocating 50 HP – 350 HP

                        4.  Centrifugal 200 HP – 1,100 HP

            B.  Rotary screw air compressor components:

                        1.  Intake air filter.  Critical component for lubricant and compressor air end life

                        2.  Controls

                        3.  Air end.  This is the rotating device that compresses the air.  Rotary screws compressor are preferred due to the continuous compression across the screw rotors.  Compressor lubricant is injected as the ambient air is compressed.  Thus this style of compression is referred to as oil flooded.

                        4.  Air oil separator

                        5.  Coolant thermal regulation valve

                        6.  Oil filter

                        7.  Aftercooler

                                    a.  Air cooled or water cooled

                                    b.  Integral on rotary screw air compressors

                                    c.  Required to cool the compressed air

                                    d.  Moisture separator follows to remove liquid water and hydrocarbons.  This drain point is typically responsible for 45% of the water removal.

            C.  Particulate filter

                        1.  Delta pressure gauge

                        2.  Combination strainer ball valve

                        3.  Electronic drain valve.  This drain must operate to prevent liquid slugging of the air dryer!

            D.  Air Receiver

                        1.  ASME coded and U-stamped for safe working pressure

                        2.  Pressure gauge

                                    a.  0-200 PSI

                                    b.  Most pressure gauges are accurate to plus or minus 2% and accurate only on the middle one third of the gauge face

                        3.  Pressure relief valve

                        4.  Condensate drain valve

            E.  Refrigerated air dryer

                        1.  Air dryer refrigeration system much like a small window unit air conditioner.  The difference is the media being cooled; the compressed air is pressurized and remains enclosed in copper tubes or similar material.

                        2.  Compressed air heats up in the compression cycle.  Charles law of physics sates that if we reduce the volume (compress) of a gas we produce heat as a byproduct.  As we increase the temperature of air we increase the water holding capacity.  When we utilize the compressed air we expand back to atmosphere (back to eight coffee cans).  Charles law states that the temperature will drop as we release the energy (decompress).  The water holding capacity of the air drops and liquid water appears at the discharge of the pneumatic device. 


Let’s use the example of blasting a metal part to near white metal condition.  We use compressed air to convey an abrasive material to collide with the metal object removing paint and oil.  The metal is now exposed and without oxidation protection.  The compressed air energy is released, the pressure drops and the water holding capacity of the air is reduced.  We end up with liquid water draining from the metal part.  Thirty minutes after we cleaned the part rusting has begun. 


A refrigeration air dryer is necessary to lower the temperature of the compressed air and remove the water from the airstream.  Compressed air is a closed system.  Once the water is properly removed via air drying, water can no longer create problems on the manufacturing floor.

                        3.  When the compressed air enters a refrigerant air dryer its temperature is reduced by the cold compressed air leaving the dryer.  This serves a dual purpose.  First this air to air temperature exchange lowers the temperature of the entering compressed air.  Secondly this heat exchange increases the temperature of the dry compressed air restoring the full pneumatic power; hot compressed air has more energy than cold compressed air.  This is why you will see some of our air receiver installations after the refrigerant dryer, painted an absorbing color and exposed to the sun.

Next the refrigerant is used to chill or lower the compressed air temperature to approximately 38 F.  This level of moisture removal is referred to as 38 F pressure dewpoint.  This is equivalent to 95% removal of water.

                        4.  Older refrigerants are considered ozone depleting.  Customers utilizing R-12 in older air dryers should be encouraged to upgrade the refrigerant air dryer to modern refrigerants.

                        5.  Refrigerant systems can only be repaired by certified refrigerant technicians.  Refrigerant must not be release to the atmosphere.  Air dryer repairs are commonly performed by qualified refrigerant subcontractors.

                        6.  Refrigerant air dryers are only as good as the condensate drain following the dryer.  This drain is critical to the removal of water form the airstream.  An electronic drain valve with a combination strainer ball valve preceding the valve is highly recommended.

                        7.  Absorption Dryers as alternate.  For applications that require superior moisture removal, absorption dryers utilize desiccant in an absorbing (versus adsorption) method for water removal.  This style of dryer is commonly referred to as regeneration or pressure swing.  Sophisticated instrumentation, pharmaceuticals, neutraceuticals, aerospace and electronics are industries that must use this type of moisture removal.

            F.  Coalescer filter. This filter should be installed after a refrigerant dryer or before a desiccant absorbent dryer.  Its job is to remove hydrocarbons (oil), but it often removes water.  When this filter is installed after a refrigerant dryer, a manual petcock is used to evaluate the amount of oil carryover from the air compressor.

            G.  Alternate location for air receiver.  In an effort to reduce the power cost associated with the production of compressed air it is critically important to create ‘true compressed air storage’.  An air receiver will not necessarily create compressed air storage.  We recommend that the air receiver be installed after the moisture has been removed and all filtration is accomplished.  With the air receiver installed last, we can create a large buffer between the supply of compressed air (the compressed air system) and the demand for compressed air (the plant).

Storage of compressed air is accomplished by creating a controlled pressure drop across the air receiver.  In layman’s terminology, we must put a high pressure in the air receiver and take the air out at a lower pressure.  This creates storage, reduces the consumption pressure in the plant and lowers the operational cost significantly.  A properly designed and selected pressure reducing device and a very accurate pressure gauge are required.

            H.  Condensate/waste oil separation

                        1.  Considered a hazardous waste by the EPA

                        2.  Generally easy to separate back to harmless water and compressor waste oil

            I.  Distribution air piping system

                        1.  Loop header system

                        2.  Properly designed compressed air drops

                        3.  Avoid quick connectors

                        4.  Pipe, valve and fitting selection

                        5.  Materials

                        6.  Subcontract installations


The Writer of 'Compressed Air Systems 101' can be reached at 727-847-4900.



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Revised: July 09, 2008
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