Are you wasting compressed air?
Potentially Inappropriate Uses of Compressed Air
Compressed air is clean, readily available, and simple to use.
As a result, compressed air is often chosen for applications for
which other energy sources are more economical.
Inappropriate uses of compressed air include any
application that can be done more effectively or more
efficiently by a method other than compressed air.
Don't WASTE your Compressed Air. Check your facility for
wasteful and perhaps even un-safe uses of compressed air.
Examples of potentially inappropriate uses of compressed air include:
• Open blowing
• Sparging
• Aspirating
• Atomizing
• Padding
• Dilute-phase transport
• Dense-phase transport
• Vacuum generation
• Personnel cooling
• Open hand-held blowguns or lances
• Diaphragm pumps
• Cabinet cooling
• Vacuum venturis.
Here's an explaination of each potentially inappropriate use
and a suggested alternative:
Open Blowing
Open blowing is using compressed air applied
with an open, unregulated tube, hose, or pipe for one
of these applications:
• Cooling
• Bearing cooling
• Drying
• Clean-up
• Draining compressed air lines
• Clearing jams on conveyors.
The alternatives to open blowing are vast. Some are:
• Brushes
• Brooms
• Dust collection systems
• Non-air-loss auto drains
• Blowers
• Blowers with knives
• Electric fans
• Electric barrel pumps
• Mixers
• Nozzles.
Sparging
Sparging is aerating, agitating, oxygenating, or
percolating liquid with compressed air. This is a
particularly inappropriate application because liquid
can be wicked into a dry gas, increasing the dew point.
The lower the dew point of the compressed air, the
more severe the wicking effect. This can occur with
oil, caustics, water rinse materials, etc.
Alternatives to sparging include low-pressure blowers and mixers.
Aspirating
Aspirating is using compressed air to induce the
flow of another gas with compressed air such as flue
gas.
An alternative is a low-pressure blower.
Atomizing
Atomizing is the use of compressed air to disperse
or deliver a liquid to a process as an aerosol. An
example is atomizing fuel into a boiler. Fluctuating
pressure can affect combustion efficiency.
An alternative is a low-pressure blower.
Padding
Padding is using compressed air to transport
liquids and light solids. Air is dispensed over the
material to be moved. The expansion of the air moves
the material. The material is usually only moved short
distances. An example is unloading tanks or tank cars.
Molecular diffusion and wicking are typical problems with padding.
An alternative is low- to medium pressure blowers.
Dilute-Phase Transport
Dilute-phase transport is used in transporting
solids, such as powdery material, in a diluted format
with compressed air. Molecular diffusion and wicking
are typical problems with dilute phase transport.
An alternative is a low- or high-pressure blower or a lowpressure
air compressor designed for 35 psig. The
pressure required depends upon the moisture content
and size of the material being transported.
Dense-Phase Transport
Dense-phase transport is used to transport solids
in a batch format. This usually involves weighing a
batch in a transport vessel, padding the vessel with
compressed air, forcing the batch into a transport line,
and moving it in an initial plug with a boost of
compressed air at the beginning of the transport pipe.
Once the material is moving in a plug, the operation
may fluidize the material in a semi-dense or moderate
dilute-phase using fluidizers or booster nozzles along
the transport path. The material is typically transported
to a holding vessel that dispenses it on an as-needed
basis using pad air from the secondary transport vessel
to move it to the use location. A typical application
would be the dense-phase transport of carbon black.
There are typically four compressed air elements to
the transport. These elements are control air for the
equipment, pad air for the initial transporter, transport
air to move it in the piping, and fluidizers or booster
nozzles along the transport piping. Most dense-phase
manufacturers specify 80 to 90 psig with one single
line supporting the entire process. The control air and
booster nozzles typically use pressures in the 60 to
70 psig range. The actual article psig required for the
pad air and the transport air is typically 30 to 45 psig.
Because of the lack of storage in most of these
applications and the high-volume, short-cycle transport
times, the original equipment manufacturers request
80 to 90 psig and use the entire supply system as the
storage tank. As this usually has a negative impact on
the plant air system, separate compressors, filters, and
dryers are applied to this process at the elevated pressure.
Alternatives include supporting the control air,
pad air, and boosters with regulated plant air plus
metered storage, and using a two-stage, positivedisplacement
blower (28 psig) or single-stage compressor
(40 to 50 psig) for the transport air. Another alternative
is to use metered storage for both the pad air and
transport cycle. This necessitates providing the entire
requirement from storage and metered recovery per
cycle, with a metering adjustment to refill the vessel
just before the next transport cycle. The storage should
be sized to displace the required air first for the pad
and then for the transport cycles within an allowable
pressure drop to terminate the transport cycle pressure
at the required article pressure. This will flatten the
volumetric load on the system, eliminate any impact
on other users, and reduce the peak energy required to
support the process.
Vacuum Generation
The term vacuum generation describes applications
where compressed air is used in conjunction with a
venturi, eductor, or ejector to generate a negativepressure
mass flow. Typical applications are hold-downs
or 55-gallon, drum-mounted, compressed air vacuum
cleaners. This is by far the most inefficient application
in industry with less than 4 percent total efficiency,
although for very intermittent use (less than 30 percent
load factor), compressed air can be a reasonably
efficient solution.
An alternative is a vacuum pump. If a compressed-air-generated
vacuum is required, install a solenoid valve on the compressed
air supply line to shut this application off when it is not needed.
Vacuum generators are used throughout industry.
Some applications for vacuum generators include:
• Shop vacuums
• Drum pumps
• Palletizers
• Depalletizers
• Box makers
• Packaging equipment
• Automatic die-cutting equipment.
Vacuum generators are selected for safety, ease of
installation, physical size of the generator, the fact
that no electricity is required at the point-of-use, and
low first cost. Vacuum generators are usually less
economical to operate than central vacuum systems.
As a rule, in a base load situation, if the vacuum
generator is operating less than 30 percent of the time,
it will be more economical to operate than a central
vacuum system. Otherwise, vacuum generators are, in
general, less effective at pulling a vacuum and cost as
much as five times more to operate than a dedicated
vacuum pump. Using vacuum generators for shop
vacuums and drum pumps, which are typically peak
load applications, could cause another compressor to
turn on and stay on until it times out. Having to
operate a second compressor because of the added
demand associated with a vacuum generator eliminates
any apparent savings associated with a vacuum
generator, even if it operates only once a day for a
short period of time.
A dedicated vacuum pump, or the use of central
vacuum system will provide more suction force at a
fraction of the cost of vacuum produced by compressed
air. In this case, it is significantly more cost effective to
provide a system that is designed into the machine
from the beginning than to retrofit a piece of equipment.
This can be accomplished by being proactive at
the time the machine specifications are prepared and
the purchase orders issued. Vacuum generators must
be applied properly and only after taking life cycle
costs into consideration.
Vacuum venturis are a common form for vacuum
generation with compressed air systems. In a venturi
system, compressed air is forced through a conical
nozzle. Its velocity increases and a decrease in pressure
occurs. This principle, discovered by 18th century
physicist G. B. Venturi, can be used to generate
vacuum without a single moving part.
Multi-stage venturi devices provide a more
efficient ratio of vacuum flow to compressed air
consumed than single-stage venturi devices. Where
vacuum requirements vary significantly, or are cyclical
with a duty cycle of less than 30 percent, multi-stage,
venturi-type vacuum generators with pressure
regulators and automatic shut-off controls on the
compressed air supply may be more efficient than
continuously operating mechanical-vacuum pump
systems. These devices also can be equipped with a
vacuum switch that signals a solenoid valve to shut
off the air supply when a set vacuum level is attained,
thus reducing air consumption in non-porous
applications. They may also be suitable where it is
impractical to have a central vacuum system,
particularly where the uses may not be confined to
one area.
Personnel Cooling
Personnel cooling is when operators direct
compressed air onto themselves for ventilation. This is
dangerous because it can shoot particulates into the
skin. A 1/4-inch tube blowing air on an operator can
consume 15 to 25 brake horsepower (bhp) of compressed air.
An alternative is fractional horsepower fans of 1/4 bhp or less.
Open Hand-Held Blowguns or Lances
Unregulated hand-held blowing is not only a
violation of most health and safety codes, but is also
very dangerous. Hand-held blowguns that conform to
all occupational health and safety standards should be used.
There are different styles of blowguns that can
deliver various airflows, velocities, and concentrations.
The proper gun must be selected for each application.
Pipes installed in the end of hose and unregulated
non-approved guns must not be used. Blowguns must
have no more than 30 psig discharge nozzle pressure.
The nozzle should be constructed to relieve backpressure
if the nozzle is plugged or blocked. The blowgun must
also have a spring-operated throttle mechanism so it
shuts off automatically if it is dropped.
Diaphragm Pumps
A common error is to not size diaphragm pumps
for the maximum viscosity, highest pressure, and
highest volume required. The result is poor performance
and an increased supply pressure requirement.
Diaphragm pumps are commonly found installed
without regulators and speed control valves. Those
diaphragm pumps that are installed with regulators
are found with the regulators adjusted higher than
necessary. This is often because of undersized regulators
and supply piping or hose. The higher-than-necessary
setting of the regulator increases the demand on the
compressed air system and increases operating costs.
With a higher pressure setting, the amount of
compressed air admitted into the diaphragm chamber
is increased above that which is actually required to
move the product. The amount of product actually
transferred remains the same, but the amount of air
used increases with the increased pressure.
The regulator should be adjusted to equal the
maximum head that the pump is required to provide.
A flow control valve installed up stream of the regulator
will accomplish the required speed control. Operating
the diaphragm pump without speed control increases
the rate of compressed air consumption by increasing
the strokes per minute of the diaphragm pump. The
speed control should be adjusted to pump product in
the maximum allowable time. As a general rule, the
regulator and flow control valve are not included with
the standard pump package. Also, when the pump has
no liquid or slurry to pump, it will rapid cycle, wearing
out the diaphragm and wasting air. The pump controls
must be configured to turn the pump off when there
is nothing to pump.
Cabinet Cooling
Cabinet cooling should not be confused with
panel purging. The following are typical applications
where cabinet cooling is found.
• Programmable controllers
• Line control cabinets
• Motor control centers
• Relay panels
• Numerical control systems
• Modular control centers
• Computer cabinets.
When first cost is the driving factor, open tubes,
air bars (copper tube with holes drilled long the length
of the tube) and vortex tube coolers are often used to
cool cabinets. When life-cycle costs are taken into
consideration, these choices prove to be expensive. It
is not uncommon to find an open tube or air bar
consuming 7-1/2 horsepower (hp) of compressed air to
cool a cabinet. Vortex tube coolers can be an improvement
over open tubes and air bars because they are
often cycled with a thermostat control, which reduces
air consumption. However, air to air, air to water and
refrigerated cabinet coolers are available that only use
1/3 hp to accomplish the same task.
Other Potentially Inappropriate Uses
Other improper uses of compressed air are
unregulated end uses and those that supply air to
abandoned equipment, both of which are described
below.
Unregulated End Uses
A pressure regulator is used to limit maximum enduse
pressure and is placed in the distribution system
just prior to the end use. If an end use operates without
a regulator, it uses full system pressure. This results in
increased system air demand and energy use, since the
end use is using air at this higher pressure. High
pressure levels can also increase equipment wear,
resulting in higher maintenance costs and shorter end
use equipment life.
Abandoned Equipment
Many plants undergo numerous equipment
configuration changes over time. In some cases, plant
equipment is no longer used. Air flow to this unused
equipment should be stopped, preferably as far back in
the distribution system as possible without affecting
operating equipment.
Using Compressed Air
As a general rule, compressed air should only be
used if safety enhancements, significant productivity
gains, or labor reductions will result. Typical overall
efficiency is 10 to 15 percent. If compressed air is used
for an application, the amount of air used should be
the minimum necessary quantity and pressure and
should be used for the shortest possible duration.
Compressed air use should also be constantly monitored and re-evaluated.
