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There is no
justification for the wasted compressed air with techniques that
have become institutionalized.
In an article in
Motion Systems Design (MSD) magazine about air actuators on of the
panel of “Industry Experts” said: “Compressed air is not a free
utility. The energy to compress free air and move it through a plant
with attendant frictional loss is high.” At least some industry
experts are willing to accept the fact that we pay twice as much for
compressed air by our own doing and consider that normal. That is
what I mean when I say institutionalized. An
old ITT Pneumotive Air Data Book states: “Up to 300 feet of ½”
pipe (internally clean, no elbows) will handle 10 CFM at 100 PSI
with no appreciable pressure loss.” It is not necessary to
lose 50% of the air pressure and pay twice as much for compressed
air.
A small to medium
sized manufacturing plant might have a 100 HP compressor that can
provide 400 Standard Cubic Feet per minute of compressed air. Actual
usage is an average 200 SCFM at 100 psig at 50% duty cycle. Various
charts give the energy cost of compressing 1000 SCF to 100 PSIG as
$.50.
For this example
200SCFM = $.10 per minute, $6.00 per hour, $48 per 8 hour shift.
At $48 shift by 5 days a week x 52 weeks = $12,480 per year.
If the usage of air
is more or less than the example, in many cases half of that cost is
being wasted.
A cost savings goes
straight to the bottom line. If your enterprise is making 10% net,
saving $6,240 is like an increase in business of $62,000.
“What is the bull
about wasting half of our compressed air? Our air systems have less
leakage that average and everything is pretty much by the book.”
By the book and what everybody does IS the problem.
If your compressor is
set for 125 psig and your air pressure in the plant is 80 psig
(sound familiar?) you have lost 36% of the pressure just
transporting the compressed air from the source to the point of use.
“But 36% is a lot less than 50%.” A filter, regulator and
lubricator (FRL) at the point of use has been sized from catalog
data that suggests a maximum flow rate based on from 5 to 7 psid
pressure drop or a nominal 18 psi for three pieces. At half of that
or 9 psi from 80 psig we are down to 71 psig or a loss of 43%. If
you ever watched a gauge on a regulator dive 20 psi or more from the
static pressure setting each time a valve operates you are
witnessing this pressure drop phenomena.
Next is the valve and
cylinder with the valve sized for a 10 psi or greater pressure drop
to make the cylinder travel at the required velocity. At the end of
the pressure supply chain, the air cylinder is driven by 61 psig,
49% (loss of 51%) of the original 125 psig compressor output.
When the compressor
has performed a work stroke with 10 psid or more lost across the
valve the additional flow after the cylinder stops moving is a
total waste. Block the air when the cylinder stops or
replace the Valve and plumbing with larger components for a 2 psid
or less pressure loss. Consider a lower pressure or air spring to
return the cylinder after the work is done. Lower return pressure
also allows the forward or work stroke to develop speed rapidly.
Added to the pressure
drop in most systems, the loss from leaks which commonly waste 10%
or more of the compressed air may bring the efficiency of a
compressed air system down to 35% or a loss of 65%.
A drastic paradigm
shift is necessary in the way we think about, design, install and
maintain pneumatic systems and components. Womack earns an “Atta
Boy” for information in their FLUID POWER data book that shows they
get it. They give data about the amount of horse power required to
compress air and the loss of compressing to a higher pressure before
regulating the pressure back down. Also their data for air flow
through piping does not use a pressure drop of 5% or 10% per 100
feet. Many Mechanical Contractors, Facilities Engineers etc. work
from reference data that suggests a 5 or 10% pressure loss in 100
feet of pipe, 5 psid across a filter and other archaic practice..
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Most of us have been convinced that 14.7 is the
atmospheric pressure. When you change the word from atmospheric to
barometric it changes he perspective.
By the way what is atmospheric pressure? If we
had a cubic inch of air so that any one side was one square inch and
laid that on a scale in a prefect vacuum it would weigh .0000434 lbs
(.075 lb/cu ft / 1728 cu in/cu ft). How the heck can it have almost
15 pounds of pressure per square inch? The air around us is stacked
up approximately 200,000 feet. Let us imagine that you got on the
scale and weighed 200 Lbs. If a person who weighed 150 pounds sat on
your shoulders and a 100 pounder on theirs and a 50 pounder on top.
You would be the only one touching the scale but the scale would
show 500 pounds. Turbulent compressed air has increased resistance
caused by the turbulence. The pressure drop in slower air with
smooth laminar flow is primarily dependent upon the viscosity of
air. The pressure loss is significantly lower than the pressure loss
of viscosity and turbulent flow.
Remember, “Up to 300 ft. of ½” pipe
(internally clean, no elbows) will handle 10 CFM at 100 PSI with no
appreciable pressure loss.” No Appreciable Pressure Loss is
possible and should be the normal .
Double
the pipe diameter as a minimum for correct size from the old
charts that suggest 5 and 10% pressure drop per 100 feet. Go even
larger for less pressure drop and to allow for future growth. No
pipe is too large. A 6” pipe manifold that is 100 feet long would
equal a 150 gallon or 20 cubic foot reservoir with negligible
pressure drop under, at least, 2000 SCFM at 100 psig or a 500 Horse
Power compressor. See Figure 1. The pipe sizes shown in red should
be considered minimum for efficient pneumatic systems.
In addition to oversized plumbing cut the
pressure and centralize the air source. Compression, drying,
filtration, lubrication should be centralized to reduce the number
of point of use components and sized to cut the pressure drop they
cause.
For a pneumatic system that is efficient and
effective consider the following:
-
Double (or more) the size of your air system plumbing.
-
Reduce the compressor output pressure.
-
Centralize the source of clean, dry, lubricated compressed
air.
- Constantly monitor the air wasters and repair leaks on a PM
schedule.
- Centralize filtration and eliminate point of use or replace filter
elements with PM.
- Eliminate regulators or use non-relieving regulators.
- Centralize lubrication to eliminate point of use pressure drop.
- Use pressure switches, transducers and controls wisely
- Don’t add air to cylinders after they stop!
- Replace quick connectors with a larger size.
The initial cost for rework and installation of
larger plumbing and improvements can be amortized in energy saving
and man hours. After that the savings continue year after year.
See Table below |
