| 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. |
- Do you know your compressed air costs?
|
- Do you know how much compressed air is really
required for your plant?
|
- Do you select compressed air equipment with energy
costs in mind?
|
- Do you monitor the use of compressed air like your
other utilities?
|
| 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%. |
 |
| Chart
1 |
| 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: |
|
|
Every 10 psig increase of pressure in
a plant system requires about 5% more power to produce. For
example:
|
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.
|
- Indirect cost of pressure:
|
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.
|
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.
|
Also remember that the leakage rate
is significantly reduced at lower pressures, further reducing
power costs.
|
* 8,760 hours is based on operating 24 hours/day, 7
days/week, 52 weeks/year.
|
- The cost of wasted air volume.
|
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.
|
- Select the most efficient demand side.
|
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.
|
 |
| Chart
2 |
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%.
|
| Cost-justifying
More Efficient Compressors / Waste
Heat Recovery and the Importance of Maintenance |
