Efficient Plant February 2018 - 35

compressed air systems

Heed These
Signs of
Inefficiency
Ron Marshall

LOW-LOADED VS. RUN-TIME RATIO

STORAGE RECEIVER SIZE

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Some compressed air systems are more efficient than others in
serving end-use applications in plants.

WHILE COMPRESSED AIR is one of the most inefficient ways to power mechanical systems, it's also one of
the most widely used sources of such power in industrial
operations. For various reasons, some systems are simply
better than others at converting atmospheric air into
compressed air, and efficiently delivering it to end users.
Is yours one of them? The following seven signs may
point to inefficiency problems:

Ron Marshall has spent almost 25 years working with
compressed air systems, first as an industrial-systems officer with
Manitoba Hydro (hydro.mb.ca) and, since his retirement, as owner
of Marshall Compressed Air Consulting, based in Winnipeg,
Manitoba. For more information, email ronm@mts.net, or visit
compressedairaudit.com.

FEBRUARY 2018

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If your system runs in load/unload mode, i.e., wherein any compressor alternates between loaded and unloaded, record its loaded and run-time hours. If the
loaded hours are less than 70% of total run-time hours, it's likely a compressor
is running inefficiently. Are any of yours running in modulation mode? Modulation mode is the least-efficient way to run compressors in part load. If you don't
know the mode, ask your service provider.

How large is your storage receiver in relation to your trim compressor? The trim
compressor is the one that loads and unloads to take partial load when the plant
air demand only requires a fraction of a compressor. Sometimes, there could be
more than one trim. Take the number of gallons of storage capacity and divide it
by the CFM (cu-ft./min.) rating of the trim compressor. (Usually, CFM is about
4 times the horsepower rating). Is the ratio less than 5? For sites with lubricated
screw compressors, the effective storage should be larger than 5-gal. per trim
CFM. Best practice is about 10-gal. per CFM. For variable-speed compressors,
the storage volume would be 10 times the flow at minimum speed.

EXCESSIVE CYCLE FREQUENCY
When monitoring your load/unload operation, measure the number of seconds
the compressor is in the loaded condition and the time it's in the unloaded state.
An efficiently running system would have total cycle time (loaded plus unloaded)
above two minutes at around 50% loading (when load and unload are equal).

HIGH SYSTEM PRESSURE
If system pressure is running above 100 psi, the system may be inefficient. It
takes 1% more energy to produce compressed air for every two psi of higher
pressure.

HIGH LEVELS OF LEAKS
Visit your plant during non-production hours. Does the compressed air system
sound like a pit of angry vipers? Obvious audible leakage is indicative of
inefficiency.

LACK OF SYSTEM MONITORING
How much is the energy input to your compressed air system compared to the
flow produced? If you can't answer this question, your system is likely inefficient.

HIGH DRAINAGE LEVELS
Do you have condensate drains that have been cracked open manually or timer
units that are wasting expensive compressed air? These situations typically
reflect efficiency problems. EP
EFFICIENTPLANTMAG.COM |

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Table of Contents for the Digital Edition of Efficient Plant February 2018