Compressed air is generally accepted as being the most expensive utility used in industrial plants. It takes approximately 8hp of electrical input to create 1hp of compressed air.

Following leakage, wasteful usage is probably the second largest waste of compressed air.

The Office of Industrial Technologies, part of the US Department of Energy has released a list of compressed air applications that can be replaced with more efficient solutions.

*Equipment that is temporarily not-in-use.
**Equipment that is no longer used.

Information courtesy of BestPractices, a program of the U.S. Department of Energy’s (DOE) Office of Industrial Technologies (OIT).

An air knife creates a curtain of air using the coanda effect. A small amount of compressed air is passed through a precise, slotted orifice. As the air exits the orifice, the surrounding air is pulled along with the compressed air. The compressed air and surrounding air combine to create a laminar sheet of airflow across the entire length of the air knife. The velocity of the air exiting the air knife is adjusted by regulating the input air pressure.

Air knives offer many advantages over open-pipe compressed air blow off. The cost of running an air knife is substantially lower since the air knife employs an amplification ratio of up to 40:1. This is the ratio of the compressed air input to the air knife output. Savings of between 40%-90% can be expected.

OSHA Considerations

The sound level of an air knife is only about 69 dBA, even when an input pressure of 80psi is used. This is well below the level that OSHA states is acceptable for workers to be exposed to for a full shift.

Static Elimination

For applications that require static elimination, a static dissipating air knife is available. At the discharge of the air knife, an ionizing bar fills the air stream with positive and negatively charged ions.

Air Knife Applications

• Air Knife Cleaning Systems
• Conveyor belt
• Web
• Dust blow-off before painting
• Air Knife Drying Systems
• Bottles and cans
• Produce
• Air Knife Cooling Systems
• Molded components
• Castings
• Other
• Bag opening (in fill applications)

The primary function of a check valve is to allow air to flow in one direction while restricting flow in the opposite direction. It is essentially a 2-way, 2-position, normally closed valve.

A ball-check valve consists of a spring-loaded ball that sits on a seat and covers the flow passage. If air pressure is applied above the ball, the pressure aids the spring in keeping the ball firmly against the seat. When air is applied to the underside of the ball, the pressure acts to push the ball off the seat and allows the air to flow through the valve.

Another design functions similarly to the ball-check but uses a poppet instead of a ball. A third design uses a flapper or swing-valve that opens up with flow in one direction, and closes tightly when flow is reversed.

Check valves are also used in hydraulics, process, and plumbing systems. They are sometimes referred to as backflow preventers.

Refrigerated air dryers reduce the temperature of compressed air to a point where the moisture contained within the air will condense into a liquid.

Pressure dew points as low as 35°F can be achieved with refrigerated air dryers.

Tube-In-Tube Direct Expansion Refrigerated Air Dryers

Moist compressed air flows through the inner tube of a tube-in-tube system while refrigerant flows through the outer tube. As the moist, compressed air is cooled, moisture within the air condenses and is collected and drained from the dryer.

In single-stage refrigerated air dryers, the exiting dry air is so cold that condensation forms on the outside of pipes downstream from the dryer.

A two-stage refrigerated air dryer adds a second heat exchanger to reduce the condensation problem considerably. The additional stage also uses a tube-in-tube system, with cold dry air flowing through the inner tube and moist, warm inlet air flowing through the outer tube. The extra stage offers two benefits: it raises the temperature of the outgoing air, and pre-cools the incoming air, which reduces the load on the dryer and allows it to operate more efficiently.

Chilled-Mass Refrigerated Air Dryers

A chilled-mass dryer uses refrigerant to cool an intermediate substance such as glycol or aluminum granules. As the compressed air passes through the midst of the secondary substance its temperature is lowered, and the moisture within it is condensed, collected and drained.

Water-Chiller Refrigerated Air Dryers

A water-chiller dryer is very similar to the chilled-mass dryer, but instead uses water as the secondary mass.

A refrigerant cycle cools the water that in turn cools the compressed air. A water pump is required to maintain water flow through the heat exchanger.

Deliquescent dryers consist of a tank containing compressed tablets of sodium chloride or calcium chloride that are consumed by the drying process. Moist air is introduced into the bottom of the tank and exits through the tank output at the top. As the moist air flows over and around the tablets, they absorb the moisture within the air creating a liquid solution of water and deliquescent chemical that must be disposed of periodically.

Deliquescent dryers reduce the dewpoint or the incoming air by approximately 15 to 25°F. Improved desiccants can increase the drying capacity of the deliquescent dryer.

Desiccant dryers remove water from compressed air by adsorbing it onto the surface of the desiccant. Desiccants, usually silica gel, activated alumina, or molecular sieve do not chemically react with the water.

Activated alumina is highly porous aluminum oxide and has a very large surface area for its size (350 sq. meters/gram). Its smooth uniform ball-shape prevents channeling of the airflow, which maintains low bed velocities and increases air contact time for efficient moisture removal and minimal pressure drop.

Heatless Twin-Tower Regenerative Dryers

Heatless twin-tower regenerative dryers consist of two identical tanks (towers) filled with desiccant. The moist compressed air enters the bottom of the active tower and moisture within the compressed air adsorbs onto the surface of the desiccant. The second tower’s desiccant is re-activated (dried) by directing about 20% of the dry compressed air output back through the inactive tank. After a set period of time, the flow through the towers is reversed.

Heatless twin-tower heatless regenerative dryers usually produce a dew point of -40ºF and can optionally be as low as -100ºF.

Heatless regenerative dryers can be completely controlled with pneumatic signals, they can be used in hazardous areas.

Heated Twin-Tower Regenerative Dryers

Heated twin-tower regenerative dryers function the same as heatless regenerative dryers. The only difference is that once the active tower’s desiccant becomes saturated, the desiccant is heated to dry it.

The heat can be supplied by elements embedded in the tower, or a heated jacket wrapped around the tower.

Dry compressed air is still used to purge the drying tank, but only about 3-8% of the dryer output is used to carry away the moisture.

Heated twin-tower regenerative dryers usually produce a dew point of -40ºF, but can be set-up to operate with a dew point of -100ºF.

Blower Purge Twin-Tower Regenerative Dryers

Blower purge twin-tower regenerative dryers are similar in concept to the heatless regenerative dryers except no purge air is redirected into the drying tank.

Instead, a separate blower supplies heated air from the surrounding environment to the drying tank.

Cylinder speed control is accomplished by adjusting the flow of air into and out of the cylinder. For air cylinders controlling the flow is usually done through the use of flow control valves.

Containing a metering needle and a check valve, flow control valves restrict the flow in one direction only. Air entering the valve in one direction is blocked by the check valve and is forced to pass through the needle valve. The setting on the needle valve is adjusted to determine the rate of flow. When the air flow is in the opposite direction it passes unrestricted through the check valve. The direction of the controlled flow is stamped on the body of the flow control valve for ease in installation.

To determine which direction to place the restriction, the pneumatic industry follows a rule of thumb. The rule is:

If in doubt, meter-out

The term ‘meter-out’ indicates that the air leaving the cylinder is controlled. By restricting the flow of air that leaves the cylinder smooth control can be attained. Restricting the flow of air into the cylinder (meter-in) will lead to chattering.

Air amplifiers can save up to 80% compressed air usage when compared to open pipes in blow-off applications.

A 1/8” open pipe can consume up to 70 cfm. That is the entire output of a 10hp compressor. An air amplifier can accomplish the same output flow using only 10 CFM.

Air amplifiers use the coanda effect to maximize the effect of a small amountof compressed air. Air amplifiers are essentially a cylindrical chamber with a specific profile on the inner surface. Compressed air, introduced into the circumference of the cylinder is channeled at high velocity through an annular gap, into the interior, and towards the outlet. The inner profile is shaped similar to an airplane wing and as the compressed air passes over it a region of low pressure is created. The low pressure induces the surrounding air to flow into the amplifier. The combined flow of primary compressed air and surrounding air travel towards the output of the amplifier.

Depending on the design of the amplifier, the output flow of the amplifier can be 25-40 times the flow of the compressed air that powers the amplifier.

OSHA Considerations

Not only do air amplifiers produce a jet of air more efficiently than an open tube but they are also substantially quieter. An open tube at 80psi produces approximately 100dBA, whereas an air amplifier can produce the same blast of air at only 74dBA. OSHA standard #29CFR-1910.95(a) states that workers can only be exposed to noise levels of 100dB or more for 2 hours, whereas 74dB is well below the levels that are acceptable for a full shift of exposure.

An open tube can be also be dangerous if the outlet pressure is over 30psi. Compressed air can be lethal at those pressures if it comes into contact with a person’s skin. Air amplifiers are open at both ends so dead-end pressure is not an issue. OSHA standard #CFR-1910.242(b) regulates the dead-end pressure of compressed air openings.

Primary Air Reciever Tank

Primary air receivers in a compressed air system are located after the compressor and the aftercooler but before any filtration or drying equipment.

The rule of thumb to use for sizing air receivers is:
compressor capacity (CFM) = air receiver size (gallons)

The actual formula is:

The primary air receiver’s responsibility includes the following:

Contaminant Removal - Air receivers provide an additional separation point for contaminants and liquid within the compressed air.

Air generally enters from the compressor and leaves to go to the system piping at high speed. With a properly sized air receiver, the compressed air is allowed to slow down. The residence time of air within the receiver tank depends on many system variables. Solid and liquid particles that were entrained within the compressed air are allowed to settle-out as the air slows down in the air receiver tank.

Liquids not drained from the tank can cause rust. A drain should be installed at the lowest point of the tank to remove contaminants. Unless a manual drain will be opened on a regular basis, an automatic-timer drain should be used. In order to provide necessary drainage, a horizontal tank can be raised slightly on one end to promote flow of contaminants to the drain end of the receiver tank.

Pressure Spike Dampening - Positive displacement compressors create surges in the compressed air supply (some more than others).

The volume contained within a receiver tank helps to dampen the surges from the compressor leading to a more consistent pressure supply.

The receiver tank also helps to protect the compressor from fluctuations in air requirements. An air tank allows the compressor to build up a store of compressed air each time it turns on. With the store of air available to downstream processes, the compressor can sit idle for longer periods of time and will operate more efficiently.

Secondary Air Reciever Tank

A secondary air receiver tank has a slightly different purpose than the primary air tank. It is installed near a piece of equipment that operates intermittently.

The advantage of installing secondary tanks at locations where higher flow rates are required on an intermittent basis is that if an intermittent process can be averaged out (which an air tank will do), the instantaneous requirement of the compressed air system is reduced.

Air Receiver Code Requirements

Many regions have a code requirement for the construction of air receiver tanks.

The American Society of Mechanical Engineers (ASME) has established a code for air receiver tanks and many companies rate their tanks to that code.

Safety Relief Valves

All tanks that contain compressed air should have a safety relief valve.

A rule of thumb is to set the relief valve to 10% than the highest system pressure requirement. But the relief valve should never be set higher than the pressure rating of the tank it is connected to.

Condensate Drains

All tanks should have a condensate drain to remove liquid from the tank.

If the manual drain cannot be opened on a regular basis, an automatic-timer drain should be used.

Pressure Gauge

The tank should have a large pressure gauge mounted to it. By using a pressure gauge rated to double the operating pressure, the normal needle location will be pointing straight up. A gauge snubber should also be used to protect the gauge from pressure spikes.

Free air delivery (FAD) is a standardized measure of the capacity of an air compressor.

To calculate the free air delivery of a compressor firstly the pressure and temperature at the inlet of the compressor are needed. Then the pressure and temperature of the outlet air along with the volume discharged are also measured. The output volume of the air is referenced back to inlet conditions using the Ideal Gas Law.

The value of V1 is the free air delivery of the compressor.

The standard method for measuring free air delivery is given in the Standard ISO 1217, annex C. The CAGI-Pneurop PN2 CPTC 2 standard can also be applied.