Compressed air is prevalent in most manufacturing facilities. According
to the U.S. DOE OIT Best Practices, air compressors consume as much as
20% of all electricity usage in industrial plants. Compressed air is used
for pneumatic tools, mechanical drives, blowers and vacuum generators
among other applications.
Plant operators should consider compressed air as the fourth utility,
following electricity, gas and steam. Unlike traditional utilities, the
cost of compressed air is not typically measured or tracked. A typical
industrial facility can realize steam savings of 20% by improving their
steam system.
The presentation and workshop materials developed as part of the Illinois
IOF program are intended to motivate compressed air operators to scrutinize
their systems for energy saving opportunities and then provide the equations
and tools necessary to quantify the energy and cost savings potential.
Compressed Air System Characteristics
Compressed air is an inherently inefficient mechanism for transporting
energy, as it requires 8 horsepower of electrical energy to produce 1
mechanical horsepower of work. The remainder of that energy (84%) is converted
into heat during the compression of the air. In a typical system, other
losses, including leaks and pressure drops, reduce the effective work
of compressed air to 9% of the input energy as shown in Figure 1.
Figure 1. Compressed Air Energy Distribution
Like steam, the whole system must be considered for improvements
to best pursue reducing operating costs. However, improvements to the
distribution (leaks and pressure drop) and end users (inappropriate users)
can dramatically affect energy consumption at the compressor because of
the large ratio of input energy to work.
Steps to Improving System Efficiency
The Cycle
Compressed air systems are often overlooked and misunderstood areas of
a manufacturing facility. Many times in attempts to solve compressed air
problems in a plant, operators feed a vicious cycle of pressure and flow
problems. For example, if air pressure suffers on the plant floor, the
maintenance department typically increases the pressure output at the
compressor. While this might solve temporary or transient problems, the
compressor actually produces less air at higher pressures. Less airflow
often exacerbates problems on the plant floor. A new compressor is then
often purchased unnecessarily. Sometimes, a new compressor attached to
an existing distribution system creates turbulence and other pressure
drop problems that further hamper system performance.
Cost of Compressed Air
Instead of increasing pressure settings, compressed air operators should
address some basic energy savings options first. These include air leaks
and inappropriate air users. Then, operators should optimize the control
strategy of individual and groups of compressors in order to match the
compressor load to the plant air demand. Finally, the plant should employ
additional energy savings techniques to maximize system efficiency.
To assess the true cost of this utility and the potential energy savings
from each improvement, operators should determine the energy cost of compressed
air in terms of kW/scfm. A worksheet
developed for the Illinois IOF program details this calculation.
Calculating
the Cost of Compressed Air
Similar to the metric of $/thousand lbs for steam systems, this calculation
provides significant insight into compressed air operation. However, this
number can change dramatically as improvements are made to the system,
such as fixing large air leaks or turning of compressors. Operators should
recalculate kw/scfm after each major system change.
System Improvements
The workshop materials developed for the Illinois IOF program detail best
practices and potential savings for the following improvement strategies
-
Air Leaks: A poorly maintained system can lose 40% of air to leaks.
In contrast good systems have leak rates between 5% and 10%.
-
Heat Recovery: Up to 80% of compressor input electrical power can
be recovered for space heating, water heating or process applications.
For a 100 hp machine running at full load, energy savings total $6,000
(assuming $5/MMBtu natural gas costs)
-
Pressure Drop Problems: Most compressed air systems air piped improperly
within the utility room. Tee-junctions, short-radius elbows and small
pipe sizes all lead to unnecessary pressure drops that increase load
on the compressors and may seriously affect airflow and system control.
-
Storage: Most systems have inadequate air storage which forces compressors
to load frequently and allows pressure to fluctuate.
-
Compressor Controls: Often the most difficult to diagnose, using
the appropriate control strategy for a single or multiple compressors
is critical. Modulating controls are popular for regulating pressure,
but often waste energy.
-
Inappropriate Air Users: Compressed air is often used to blow off,
dry or cool parts in the manufacturing process. Traditional nozzles
such as open copper tubes or pipes with holes use significant amounts
of air and generate potentially damaging levels of noise. Engineering
nozzles are reduce noise and amplify air to maximize effectives and
minimize air usage. Higher volume applications should use high-pressure
blowers rather than compressed air.
