| Compressed air
is your most expensive utility. |
| This is a fact that has been documented time
and time again. It takes 7 to 8 hp of electricity to produce 1 hp worth
of air force. Yet, this high energy cost quite often is overlooked. Here
are some questions you should ask yourself. |
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Do you know your compressed air costs?
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Do you know how much compressed air is really required for your plant?
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Do you select compressed air equipment with energy costs in mind?
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Do you monitor the use of compressed air like your other utilities?
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| It is the purpose of this reference material
to give you the information you need to answer these questions, save energy
and improve your compressed air system operation. |
| Why evaluate
energy costs? |
| Depending on plant location and local power
costs, the annual cost of electrical power can be equal to-or as much as
two times greater than-the initial cost of the air compressor. Over a 10
year operating period, a 100 hp compressed air system that you bought for
$40,000 will accumulate up to $800,000 in electrical power costs. Following
a few simple steps can significantly reduce energy costs by as much as
35%. |
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Chart
1
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| Identifying the
electrical cost of compressed air. |
| To judge the magnitude of the opportunities
that exist to save electrical power costs in your compressed air system,
it is important to identify the electrical cost of compressed air. Chart
1 shows the relationship between compressor hp and energy cost. In
addition, consider the following: |
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Every 10 psig increase of pressure in a plant
system requires about 5% more power to produce. For example:
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A 520 cfm compressor, delivering air at 110
psig, requires about 100 hp. However, at 100 psig, only 95 hp is required.
Potential power cost savings (at 10 cents per kWh; 8,760* hrs.) is $3,750/year.
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Indirect cost of pressure:
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System pressure affects air consumption on
the use or demand side. The air system will automatically use more air
at higher pressures. If there is no resulting increase in productivity,
air is wasted. Increased air consumption caused by higher than needed pressure
is called artificial demand.
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A system using 520 cfm at 110 psig inlet pressure
will consume only 400 cfm at 80 psig. The potential power cost savings
(520 efin - 400 cfm = 120 cfm = 24 hp, at 10 cents/ kWh; 8,760* hrs.) is
$18,000/year.
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Also remember that the leakage rate is significantly
reduced at lower pressures, further reducing power costs.
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* 8,760 hours is based on operating 24 hours/day, 7 days/week, 52
weeks/year.
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The cost of wasted air volume.
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Each cfm of air volume wasted can be translated
into extra compressor hp and is an identifiable cost. As shown by Chart
1, if this waste is recovered, the result will be $750/hp per year
in lower energy costs.
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Select the most efficient demand side.
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The magnitude of the above is solely dependent
on the ability of the compressor control to translate reduced air flow
into lower electrical power consumption.
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Chart
2
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Chart 2 shows the relationship between
the full load power required for a compressor at various air demands and
common control types. It becomes apparent that the on line-off line control
(dual control) is superior to other controls in translating savings in
air consumption into real power savings. Looking at our example of reducing
air consumption from 520 cfm to 400 cfm (77%), the compressor operating
on dual control requires 83% of full load power. That is 12% less energy
than when operated on modulation control. If the air consumption drops
to 50%, the difference (dual vs. modulation) in energy consumption is increased
even further, to 24%.
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Cost-justifying More
Efficient Compressors / Waste Heat Recovery
and the Importance of Maintenance
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© Copyright
2000 Maintenance Resources, Inc.
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Phone: 812.877.7119
- Fax: 812.877.7116 - E-Mail: info@maintenanceresources.com
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Address: 1983 North Hunt
Street - Terre Haute, IN 47805
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