| Author: IMS - Revised 9/8/2009 |
| Air is EXPENSIVE!
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| The cost of compressed air
is far from free |
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| Too many people take the cost of compressed air for
granted, thinking "air is free!" Well they’re right; the company does not get
an air bill. But it gets a bill for electricity, a bill increased by the energy
needed to compress that air. |
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| We’ve talked to far too many people in far too many
shops who have said "I don’t care if I use more compressed air. We already have
a compressor, and my department is not charged extra for using it." |
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| Sure every plant already has a compressor, but every
time you use a compressed air device you make that compressor motor come on
more often. Every time it comes on, it costs you money. Eventually there could
be enough of a load on the compressor that you need to buy a bigger one... then
pay for it to cycle on with its bigger motor. You might even have to run an
expensive new electrical circuit to allow for the bigger load. That starts not
to sound like "free." |
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| Compressed air is actually one of the most expensive
items in a plant. According to the U.S. Department of Energy, compressed air is
about 10% of electricity consumed in most plants; some upwards of 30%. If
compressed air were 20% of your cost, imagine cutting that by 10%. You would
cut your total annual expense by 2% – showing up right there on the bottom
line. |
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| Some companies value air as high as 18 to 30 cents
per 1000 cubic feet of air. |
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| The U.S. governmenti has a formula to
approximate the cost of compressed air: |
| |
| Cost of Compressed Air
Formula |
| Cost = (BHP) x (.0746) x
(Hours) x (Cost per kWh) x (% of time) x (% work load) |
| Motor Efficiency |
|
| |
|
Where,
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Cost = the cost for compressed air
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BHP = brake horsepower (check the motor specifications)
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0.746 = the conversion constant from horsepower (HP) to kilowatts (kW)
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Hours = number of hours that unit is operating during a time period
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Cost per kWh = basic electricity cost (check with your electric supplier for
this value if it isn’t on your last bill)
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% of time = percentage of time that this motor operates at the work load
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% work load = BHP as percentage of horsepower at full load
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Motor Efficiency = efficiency at the current operating level
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|
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| EXAMPLE: 200hp motor
(which generates 215 BHP) operates 6800 hours per year. Loaded 85% of the time
in full load (motor efficiency .95) and 15% in standby mode 25% capacity (.90
efficiency) |
| (215bhp) x (.0746) x
(6800hours) x (.05 per kWh) x (.85) x (1.0) = $48,792 |
| .95 |
| (215bhp) x (.0746) x
(6800hours) x (.05 per kWh) x (.15) x (.25) = $2,272 |
| .90 |
| Annual Expense = $51,064 Cost |
|
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| Do not run out and turn your compressors off! It is
not time to panic, and not all is lost. With a little attention the cost can be
minimized. |
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| A little care with the "free" air in this plant can
make a major impact on the bottom line. The expense of maintenance and more
efficient equipment can easily be justified when the numbers are crunched. If
you don’t have time to apply the above formula, just remember that one
horsepower generates 4 cfm, which is equal to a 1/8" nozzle at 60 psi. That’s
not a whole lot of air. |
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| With prudent planning, these costs can be controlled.
IMS can show you annual savings without breaking the current budget. |
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| Here are some air-stealers to look for: |
|
|
Leaks – Wasted Air, Wasted Dollars
The biggest drain on a compressed air system is the easiest to control: leaks.
Leaks can account for hundreds of dollars in annual expense, please review
Graph #1 for lost air (in CFM) due to leaks of various diameters and pressures: |
| |
Pressure
(psig) |
Orifice
Diameter (inches) Graph #1 |
| 1/64 |
1/32 |
1/16 |
1/8 |
1/4 |
3/8 |
| 70 |
.29 |
1.1 |
4.6 |
18.62 |
74.4 |
167.8 |
| 80 |
.32 |
1.2 |
5.2 |
20.76 |
83.1 |
187.2 |
| 90 |
.36 |
1.4 |
5.7 |
23.1 |
92.0 |
206.6 |
| 100 |
.40 |
1.5 |
6.3 |
25.2 |
100.9 |
227.0 |
| 125 |
.48 |
1.9 |
7.6 |
30.6 |
122.2 |
275.5 |
|
| Granted it is unlikely that there will be a 3/8"
diameter leak, but what about an employee using compressed air for personal
cooling? As you can see several small leaks can quickly add to dollars in
annual operational costs. The two most common ways to find leaks are: |
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soapy water placed on all joints (any leaks cause bubbles), or
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an ultrasonic acoustic detector (IMS part# 138529)
|
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| Checking every joint with soapy water is a bit messy
and a potential slip hazard if soap gets on the shop floor. The Ultrasonic Leak
Detector requires an initial investment, but with the detection of one leak,
you can save the cost of enough wasted air to quickly recover the initial
expense. For example: |
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| Leak Cost Formula |
| Cost = (# leaks) x (cfm rate) x (kW/cfm) x (hours) x (Cost kWh) |
|
| |
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# Leaks= Number of leaks found of one size
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cfm rate= Refer to Chart #1
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kW/cfm= .18 (roughly 18 kW to generate 100 cfm)
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hours= Number of hours the system is pressurized
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Cost kWH= Cost per kWh
|
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| Example: 1/16" diameter
leak, 90psi system, 40 hour work week system is off when not in use. |
| 1 x (5.7) x .18 x 2000 x .05 = $102.60 |
|
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| The $102.60 a year is about one third the cost of a
detector, and that is a conservative estimate of one small leak with a system
that only operates 2000 hours a year. |
| |
 Air
Guns, Air Knives (Air Wipes)
Always use a well engineered orifice to administer compressed air. A well
designed air gun or air knife uses channels to profile the air stream for
maximum effect with minimum consumption (and minimum noise). Never use a
straight pipe for blowoff. That would be like washing your car with a hose
without a nozzle. You get wasteful full flow but much less effectiveness. |
| |
 We
have even seen people wastefully drill holes in a pipe instead of buying an air
knife. True the initial cost of an air knife (wipe) is much higher than pipe,
but that is where the savings stop; stop dead! Once all annual costs are
considered, the pipe that was cheaper to buy costs far more to operate. The
annual operating expenseii of a drilled pipe with twenty five 1/16" diameter
holes on a 60 psig system is $4,508 based on 8.3 cents per kWh and 2,080 hours
of annual operation (one shift of eight hours for five days per week for one
year – run more, and the cost is higher), but a super air knife under the same
circumstances is $1,417. The air knife (wipe) is engineered to amplify the
compressed air, for better performance with less air used. As you can see in
these annual costs, it takes less than two months to save enough in energy
costs to pay for the air knife (wipe). In addition to cutting energy expenses,
air knives (wipes) are easy to install and enhance the overall, professional
look of your plant. |
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| With the simple measures of checking and eliminating
compressed air leaks, and investment in equipment such as air guns and air
knives (wipes), you can control and minimize energy costs associated with using
air throughout your operation. The energy savings quickly outweigh your initial
expense and effort, giving you an attractive Return-on-Investment. |
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| i U.S. Department of Energy website:
compressed_air1.pdf |
| ii Information provided courtesy of Exair
Corporation. |
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| Products related to this article: |
Air Guns
|
|
Air Wipes and Knives
|