| Look beyond
the purchase price. |
| As indicated previously, over a 10
year period, the electrical power costs of a compressed air
system impact the bottom-line far beyond initial price and
maintenance. How can we evaluate the true impact each element of
the compressed air system has on the electrical power costs? |
| The air compressor package. |
The air compressor package has a very
significant impact on overall operating power costs. There are
other pieces of equipment in a compressed air system that
support the air compressor. These also impact on power costs,
both directly and indirectly. But the core of it all is the
air compressor package. Energy wasted at the air compressor
site can never be recovered.
|
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| What are the components of an
energy efficient compressor? |
- Compressor element (airend) - performance can vary
up to 20% depending on what airend size and style are used.
|
- Drive motor efficiency - there are high efficiency
motors available which can save up to 5% in power
consumption. Referring to the previous example, a 100 hp
compressor would save $3,750 annually.
|
- Compressor controls - are an important part of the
air compressor package, matching compressor supply to
demand. As outlined previously, the right control type (dual
control) is essential for efficient operation. Maximum
savings of 45% are possible. Any reduction in air usage in a
system accomplished by good demand side management (adequate
storage and flow controller) can be translated into real
power cost savings by the control system, and none does this
better than dual control.
|
 |
| State-of-the-art
control systems generate large savings. |
| Sequencing
compressors/controls. |
| A microprocessor-integrated sequencer
allows the system to maintain a stable system pressure and
ensures that only needed compressor units are operating at their
most efficient level. |
| User-friendly, PLC-based sequencers
can mix and match compressor supply to demand, including
automatically shutting off units not needed, and bringing on
backup units as required. |
| Sophisticated sequencers not only
sequence and select units as required, but ensure that no more
than one unit in a multiple-unit installation will be operating
at inefficient part load. All other units will be operating
efficiently at full load. |
| Electrical power savings result from
operating fewer compressors at a lower pressure than with
conventionally controlled compressors. |
Efficiency Rules of
Thumb
| • Most air
compressors deliver 4 to 5 cfm per hp at 100 psig
discharge pressure.
• Every 2
psig of pressure increases or decreases the power draw
of an air compressor 1%.
• Every
10 degrees Fahrenheit change in inlet air temperature
affects the efficiency about 1%. Colder temperature
increases and warmer temperature decreases efficiency.
• Power
cost for 1 hp for 3 shifts, 7 days a week (8,760 hours)
at 10 cents/kWh = about $750/year.
• A 50 hp
compressor rejects approximately 126,000 Btu per hour.
Approximately 119,000 Btu/hr of this is recoverable.
• Size
control air receiver located after compressor for about
1 gallon capacity per cfm of compressor capacity.
• Size
storage air receiver for about 2-4 gallon capacity per
cfm of compressor capacity. This results in an effective
demand side control management system.
• Total
pressure drop across all compressed air system
components, including piping, should not exceed 15 psi.
|
|
| Reduce
pressure drop in system components. |
| When buying or replacing equipment,
make sure it maintains low pressure drop over its entire service
life. Also, ensure that filters and dryer are sized and
maintained properly. The total pressure drop across all
compressed air system components, including piping, should not
exceed 15 psi. |
| Evaluating
Compressor Efficiency / Waste
Heat Recovery and the Importance of Maintenance |
