Efficient Plant April 2018 - 33

compressed air systems

WHEN TO GO BIG

MEASURING UP

Unlike the oversizing of air compressors, going 'bigger' really can be better for associated storage receivers and piping. Photo courtesy Marshall Compressed Air Consulting

Why Compressor
Size Matters

Ron Marshall

IN COMPRESSED AIR SYSTEMS, as in other areas, the size of equipment
does matter. For air compressors, it's crucial to ensure sufficient output capacity
to supply peak flows without causing pressure problems, as well as provide extra
capacity to cover unexpected flow caused by leakage, intermittent loads, and
future equipment additions.
Grossly oversizing compressors, however, can increase energy costs. For
example, with load/unload-controlled compressors, doubling the equipment size
from a 50-hp to a 100-hp unit could increase operating costs by about 70%, even
though the same amount of air is being produced. Consequently, in terms of air
compressors, bigger is not usually better.

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. To learn
more, email ronm@mts.net, or visit compressedairaudit.com.
APRIL 2018

Bigger can, in fact, be better for other components within a compressed air system. Specifically, if the storage
receiver and system piping are oversized, energy efficiency will improve. In short, if the size of your storage
receiver doesn't make visitors to your compressor room
do a double take, then the tank is probably too small.
Large storage tanks provide the greatest benefit for
lubricated screw compressors running in load/unload
mode. These units normally cycle from loaded to
unloaded at a specific frequency-and each of those cycles wastes energy. Thus, increasing the size of the storage receiver reduces the number of cycles and improves
efficiency. For example, installing a storage receiver
sized at 4,000 gal. for a 100-hp compressor would save
about 25% in operating costs, compared with one sized
at 400 gal. This is a savings of about $19,000/yr. at
10 cents/kWh for a compressor running full time at
50% average loading.
Installing piping that's too small causes
higher-than-normal compressor discharge pressures,
which increase the power consumed per unit output.
For every 2 psi of extra pressure that a compressor
must produce, power consumption increases by 1%. In
addition, for unregulated compressed air demands, each
psi of extra pressure increases the flow by about 1%,
further loading the equipment.
Right-sizing your compressor room and distribution
piping so there's less than 2% pressure loss across the
entire system (excluding air dryers and filters) ensures
that the equipment will run efficiently at the lowest
acceptable pressure. Of course, you should always
carefully monitor the pressure to ensure personnel
aren't unexpectedly increasing compressor settings for
some reason.
How does your plant's equipment measure up? To answer this question, sites should have their compressed
air systems monitored by professional service providers. They can check for compressor size and pressure
drop and make recommendations for improvements. EP
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Table of Contents for the Digital Edition of Efficient Plant April 2018