Maintenance Technology July 2017 - 23

ENERGY EFFICIENCY

Opposite page:
Far left. Leaks waste between 20% and 30% of all compressed
air produced in industrial plants.

Rethink Your Purchasing Approach

Left. Jacking up pressure can make users happy, but cause
problems in compressor rooms.

When purchasing compressors, these questions are at the top
of everyone's list. The "obvious" answer is not always correct:

in leakage that is repaired. This would be a system with a
good central compressor controller with multiple load/unload
compressors, or a system with a properly sized and coordinated
variable-speed-drive compressor.
"Some compressor control modes," Marshall cautioned, "have
less than optimum power-turndown capabilities, such as those
running in blow-off or inefficient modulation mode, where
the inlet flow to the compressor is choked off to control the
output." These compressors might only save 3% power for every
10% reduction in flow. Furthermore, this control mode causes
an increase in pressure as the flow reduces, which causes the
remaining compressed-air demand to consume more.

■ Can our plant save money by purchasing one large
air compressor instead of two or three smaller
ones? Not really. This approach may save on the initial
purchase and ongoing maintenance costs, but these
costs are typically only about 20% to 30% of the total
life-cycle costs of running an air compressor. Over-sizing
an air compressor will likely cost much more on the
largest component of the lifetime operating costs: electricity input. For example, a single 200-hp load/unload
compressor running full time at half capacity with a
small receiver tank might consume $130,000/yr. of
energy at $0.10/kWh. A well-designed system using two
100-hp units at the same average load would consume
about $74,500/yr. in energy. Over 10 years the savings
for choosing the two-compressor system would be worth
more than half a million dollars.

Misconception: Large pipe adds storage volume.
Adding storage volume to a compressed-air system can make
it run more efficiently. Rather than clearing space for a large
receiver tank in the compressor room, some people think they
can install extra storage by upsizing their compressed-air lines.
As Marshall noted, best-practice storage volume for load/
unload compressors is between 5 and 10 gal./cfm of your trim
compressor (the one that takes partial load). As an example, the
recommended system storage for a100-hp compressor that puts
out about 400 cfm would be between 2,000 and 4,000 gal.
Perhaps you want to upsize from 2-in. pipe to 3 in. to provide
storage capacity. You should be aware that 3-in. pipe contains
a volume of 38.7 gal./100 ft. To install the equivalent of a
2,000-gal. storage tank, you would need about 5,200 ft. of 3-in.
pipe-about a mile of piping. So, should you upsize your piping?
Marshall says yes, "but not to act as system storage." Larger sized
piping, he explained, reduces pressure loss in your system and

JULY 2017

■ Since all compressors are the same, can't we just go
with the least expensive? Not necessarily. The energy
characteristics of air compressors vary with make and
model. It's wise to check out these numbers before
purchasing a new compressor because the energy
savings from choosing an efficient compressor can
quickly offset any higher initial cost. Most large manufacturers have data sheets that show the power-versusflow characteristics using a data format developed by
the Compressed Air and Gas Institute (cagi.org). Look
up the numbers before you buy any new compressor
to ensure you are purchasing the most efficient unit. All
CAGI members are required to publish their numbers.
The effort could save you a bundle.

MAINTENANCETECHNOLOGY.COM | 23

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Table of Contents for the Digital Edition of Maintenance Technology July 2017