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Air Compressor Energy-Saving Tips
Approximately
70 percent of all manufacturers have a
compressed air system. These systems power a variety
of equipment, including machine tools, material handling
and separation equipment, and spray painting equipment.
Energy audits conducted by the U.S. Department of Energy
(DOE) suggest that over 50 percent of compressed air
systems at small to medium sized industrial facilities
have low-cost energy conservation opportunities.
Significant air emissions are released
when electricity is produced. In Minnesota, one-fourth
of the energy-related emissions of carbon dioxide, sulfur
dioxide, lead and mercury are from generating electric
power. Industry uses over 50 percent of this electricity.
Reducing electricity used by compressed air systems
will help improve Minnesota’s air quality.
This fact sheet about air compressors
will help you calculate their operating cost, understand
your system and identify easy to implement energy conservation
strategies.
Compressed
Air Energy Cost
Compressed air is one of the most expensive uses of
energy in a manufacturing plant. About eight horsepower
of electricity is used to generate one horsepower of
compressed air. Calculating the cost of compressed air
can help you justify improvements for energy efficiency.
To find the annual cost of electricity used to power a compressed air system, calculate the cost for running the system under loaded and unloaded conditions. For each, multiply:
- horsepower (hp)
- conversion factor 0.746 kW/hp
- total operating hours per year (hr/yr)
- cost per kilowatt-hour ($/kWh)
- percent time fully-loaded or unloaded
- percent full-load hp, loaded or unloaded
Divide the product by the motor’s efficiency.
| Cost
per year = |
| (hp)(0.746 kW/hp)(hr/yr)($/kWh)(% time)(% full-load hp) |
 |
| motor
efficiency |
Calculate the cost of compressed air for specific end uses. This allows you to determine if compressed air should be used in specific applications (ie. as fans or blowers), or if other electric-motor operated equipment would be more efficient.
First calculate the volume of air produced annually for a specific operation by multiplying:
- horsepower (hp)
- cubic feet per minute per horsepower (cfm/hp)
- total operating hours per year (hr/yr)
- 60 minutes per hour (60 min/hr)
- percent time fully loaded
- percent full-load horsepower
Volume of air produced annually =
(hp)(cfm/hp)(hr/yr)(60 min/hr)(% time)(% full-load hp)
Then calculate the cost per 1,000 cubic feet (cf) by dividing the total energy cost to operate the air compressor by the volume of air produced annually, then multiply by 1,000.
| Cost per 1,000
cf = |
| energy cost |
x
1,000 cf |
|
| air produced |
Example Calculations
The following example represents
a typical small job-shop manufacturer.
A facility operates a 100 hp air compressor
4,160 hours annually. It runs fully loaded, at 94.5
percent efficiency, 85 percent of the time. It runs
unloaded—at 25 percent of full load—at 90
percent efficiency, 15 percent of the time. The electric
rate is $0.06 per kWh, including energy and demand costs.
The cost per year to power the air compressor will be
as follows.
Fully loaded = |
| (100 hp)(0.746 kW/hp)(4,160 hr)($0.06/kWh)(0.85)(1.0) |
|
| 0.945 |
| =
$16,748 |
|
Unloaded = |
| (100 hp)(0.746 kW/hp)(4,160 hr)($0.06/kWh)(0.15)(0.25) |
|
| 0.90 |
| =
$776 |
The total annual energy cost to operate
the air compressor is $17,524.
The following calculation shows how
much it will cost to use compressed air to operate a
specific end use. Assume 3.6 cfm per horsepower and
that this rate applies when the compressor is fulled
loaded.
Volume of air produced annually
=
(100 hp)(3.6 cfm/hp)(4,160 hr)(60 min/hr)(0.85)(1.0)
= 76,377,600 cf
| Cost per 1,000
cf = |
| $17,524 per year |
x
1,000 cf = $0.23 |
|
| 76,377,600 cfm |
Over the life of a compressor, energy
costs will be five to 10 times the compressor’s
purchase cost. Energy savings can rapidly recover the
extra capital required to purchase an energy-efficient
air compressor motor.
A 1.17 rated horsepower air operated
mixer uses 45 cfm at 80 pounds-per-square-inch (psi)
and operates 40 hours per week. The cost of the compressed
air to operate this motor over a year is $1,292. A comparably
sized electric motor of Energy Policy Act (EPACT) efficiency,
rated for hazardous locations, is around $350. The cost
to operate the EPACT motor under the same conditions
is less than $100 per year. Including installation,
payback is under one year.
Understand
Your System
Before implementing energy reduction strategies, be
familiar with all aspects of your compressed air system.
System supply. Analyze the
supply side of your compressed air system for the types
of compressors used and the type, suitability and settings
of capacity controls and other operating conditions.
Understand the basic capabilities of the system and
its various modes of operation.
Verify that air compressors are not
too big—oversized—for end uses. For example,
an air compressor is oversized if the end use only requires
air pressure that is 50 percent of the pressure that
the compressor is capable of producing.
Once the big picture is in view, supply
side operating conditions can be modified, within the
constraints of the compressed air unit, to better match
the demand side uses of compressed air.
System demand. Identify all
the uses of compressed air in the plant. Quantify the
volume of air used in each application and generate
a demand profile, quantity of air used as a function
of time, for the compressor. Equipment specifications
for operations that use air are good resources for obtaining
data on air volume use rates. The profile highlights
peak and low demand. A general assessment of compressed
air use will help identify inappropriate uses of air.
System diagram. Develop a sketch
of your compressed air system—including compressors,
air supply lines with dimensions, and compressed air
end uses—to provide an overall view of the entire
compressed air process.
Distribution system. Investigate
the distribution system for any problems related to
line size, pressure loss, air storage capacity, air
leaks and condensation drains. Verify that all condensation
drains are operating properly—inadequate drainage
can increase pressure drop across the distribution system.
Maintenance. Evaluate maintenance
procedures, records and training. Ensure that procedures
are in place for operating and maintaining the compressed
air system, and that employees are trained in these
procedures.
Conservation
Strategies
Identify easy to implement energy conservation opportunities
in your compressed air system by conducting a walk-through
assessment. Simple conservation opportunities can result
in savings up to 25 percent of the current cost to run
the compressed air system.
Leaks. Routinely check your
system for leaks. A distribution system under 100 pounds-per-square-inch
gauged (psig) of pressure, running 40 hours per week,
with the equivalent of a quarter-inch diameter leak
will lose compressed air at a rate of over 100 cfm—costing
over $2,800 per year. In noisy environments an ultrasonic
detector may be needed to locate leaks.
Compressor pressure. The compressor
must produce
air at a pressure high enough to overcome pressure
losses in the supply system and still meet the minimum
operating pressure of the end use equipment. Pressure
loss in a properly designed system will be less than
10 percent of the compressor’s discharge pressure—found
on a gage on the outlet of the compressor. If pressure
loss is greater than 10 percent, evaluate your distribution
system and identify areas causing excessive pressure
drops. Every two pounds-per-square-inch decrease in
compressor pressure will reduce your operating costs
1.5 percent.
Identify artificial demands.
Artificial demand is created when an end use is supplied
air pressure higher than required for the application.
If an application requires 50 psi but is supplied 90
psi, excess compressed air is used. Use pressure regulators
at the end use to minimize artificial demand.
Inappropriate use of compressed
air. Look for inappropriate uses of compressed air
at your facility. Instead of using compressed air, use
air conditioning or fans to cool electrical cabinets;
use blowers to agitate, aspirate, cool, mix, and inflate
packaging; and use low-pressure air for blow guns and
air lances. Disconnect the compressed air source from
unused equipment.
Heat recovery. As much as 80
to 90 percent of the
electrical energy used by an air compressor is converted
to heat. A properly designed heat recovery unit can
recover 50 to 90 percent of this heat for heating air
or water. Approximately 50,000 British thermal units
(Btus) per hour is available per 100 cfm of compressor
capacity when running at full load. For example, consider
a 100 hp compressor that generates 350 cfm at full load
for 75 percent of the year. If 50 percent of heat loss
is recovered to heat process water, the savings, at
$0.50 per therm, would be about $4,100 per year in natural
gas.
Inlet air filters. Maintain
inlet air filters to prevent dirt from causing pressure
drops by restricting the flow of air to the compressor.
Retrofit the compressor with large-area air intake filters
to help reduce pressure drop.
Compressor size. If your compressor
is oversized add a smaller compressor and sequence-controls
to make its operation more efficient when partially
loaded. Sequence-controls can regulate a number of compressors
to match compressed air needs, as they vary throughout
the day.
Air receiver/surge tank. If
your compressed air system does not have an air receiver
tank, add one to buffer short-term demand changes and
reduce on/off cycling of the compressor. The tank is
sized to the power of the compressor. For example, a
50 hp air compressor needs approximately a 50-gallon
air receiver tank.
Cooler intake air. When intaking
cooler air, which is more dense, compressors use less
energy to produce the required pressure. For example,
if 90° F intake air is tempered with cooler air
from another source to 70° F, the 20° F temperature
drop will lower operating costs by almost 3.5 percent.
V-belts. Routinely check the
compressor’s v-belts for proper tightness. Loose
belts slip more frequently which reduces compressor
efficiency.
For More Information
Additional resources are available online at the following
Web pages.
DOE
Office of Industrial Technology Compressed Air BestPractices
Airmaster+
Free software to help identify energy conservation opportunities
for compressed air systems.
Compressed
Air Challenge
MnTAP has a variety of technical assistance services available to help Minnesota businesses implement industry-tailored solutions that maximize resource efficiency, prevent pollution, increase energy efficiency, and reduce costs.Our information
resources are available online. Or, call MnTAP at 612.624.1300
or 800.247.0015 from greater Minnesota for personal
assistance.
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