LINE” WITH PNEUMATICS.
NORGREN, Littleton, Colorado
The cost of compressed air continues
to be a very elusive subject in industrial applications. The notion
that compressed air is free is a common misconception. Countless
attempts at making end users aware of the costs to produce compressed
air have largely been ignored. It seems that Operations Managers
are concerned with the power consumption of a couple of sixty-watt
light bulbs, but couldn’t care less about the power required
to run a 400 horsepower compressor! Let’s put this into perspective,
one horsepower equals 745.7 Watts. You can light 4,971 sixty-watt
light bulbs with the same power it takes to run a 400 horsepower
compressor! Indeed, the single largest electrical appliance in
a manufacturing plant may be the compressor motor.
This paper will provide some common sense approaches to determine
the cost of compressed air required in a typical pneumatic circuit.
It is the discovery of the approach and the requisite considerations
that we hope to explore. By clearing up several popular pneumatic
myths, we will be able to make better decisions to reduce the
cost of compressed air. Compressed air costs are typically hidden
in the operating overheads of most companies. Any overhead
cost reduction immediately falls to the bottom line in the form
of profit. The real test of this improved bottom line is determining
the additional sales a company must generate to produce a similar
profit.
Real Costs
Experts in the compressed air
field suggest the cost to produce compressed air varies from $.15
to $.40 per 1000 Standard Cubic Feet (SCF) (1), (2), depending
on geographical location. In spite of these estimates, a large
sector of the user public fails to complete the simple calculations
to determine what a machine will require in terms of Standard
Cubic Feet per Minute (SCFM) of compressed air, let alone for
an entire year or the life of the equipment. In order to improve
the bottom line with pneumatics, we must also expose three popular
pneumatic myths!
Myth #1: Compressed Air is Free
Myth #2: Pipe Size = Right Size
Myth #3: If a little bit’s good, a whole lot’s better
Myth #1: Compressed Air is Free!
In the past most people wouldn’t take the time to determine
the cost of the compressed air required by an actuator for a year.
It is ironic that hydraulic system designers have to do the calculations
in order to determine the size of the power unit required to operate
hydraulic cylinders. For far too long, little effort has been
made to recognize the similarities between hydraulic and pneumatic
systems. Typically, fluid power people have defended the differences
between hydraulic and pneumatic systems. By recognizing the similarities,
we can design and service the two mediums from similar perspectives,
including the need for safety, conservation, component sizing,
and cost justification.
There is a developing interest in determining the cost of compressed
air in a pneumatic system. We are beginning to see users specify
60 PSIG as the maximum pressure range for a pneumatic system.
Pneumatic component manufacturers are somewhat paranoid about
discussing the cost of compressed air in fear they might encourage
customers to apply a substitute for compressed air. Since compressed
air is readily available, affordable, clean, and has less force
hazards than hydraulic power, it seems reasonable that compressed
air will continue to be applied in the industrial sector.
Real Common Sense
To properly apply pneumatic components
in a system, the first component to be considered is the actuator.
We have seen a large number of cylinders grossly oversized resulting
in poor actuator performance, wasted compressed air, and high
initial component costs. Oversizing an actuator by one bore
size can result in a fifty percent increase in the cost of compressed
air required for the application. If the cylinder is sized
to move more than twice the load at the design pressure, the cylinder
speed will be adversely affected, and the cost of compressed air
will also increase. Using this simple observation can result in
significant savings
If care is taken at this step of the design process, every
component upstream of the actuator (valves, conductors, fittings,
filters, regulators, and lubricators) will have a better chance
of being correctly sized and applied. A good and correct start
in the process is essential to having an efficient operating system.
Real Calculations
A Cylinder Flow Calculation
is required for a number of reasons. It takes into consideration
the force required to move the load at the specified pressure,
the extend and retract stroke volumes of the cylinder, the cycles
per minute, the operating air pressure, and a conversion to Standard
Cubic Feet per Minute (SCFM). SCFM is the value used by most pneumatic
fluid power manufacturers to apply the correct components in a
system. SCFM is also linked to valve sizing using Coefficient
of Flow (Cv)
Once the cylinder flow calculation is completed, the designer
can determine the correct tubing, fittings, valves, and the Filter,
Regulator, and Lubricator (FRL) for the application. In spite
of the obvious benefits this information provides, we find few
designers attempting these critical calculations.
In a typical circuit (Figure 1.) comprised of a double-acting
pneumatic cylinder, a five-port, four-way valve, and two flow
controls, the typical approach to sizing exposes another popular
pneumatic myth.
Myth #2: “Pipe size = Right Size”.
Figure 1. Typical valve/cylinder/flow
control circuit
If the cylinder selected had a
½” pipe port, most installers would apply ½”
flow controls, ½” pipe and fittings, a ½”
valve, and quite possibly, a ½” FRL! This approach
leads to oversized, high priced components and higher long term
operating compressed air costs over the life of the equipment.
As a result, oversizing components occurs frequently and quickly
leads to another popular pneumatic myth.
Myth #3: If a little bit’s good, a whole lot’s
better!
Real Life Example?
Let’s consider the following
example. We have a cylinder load that requires 500 pounds of force
to move in the extend direction only. The retract stroke has no
effective load. We want to move the load twelve inches and be
able to do this at 30 cycles per minute, eight hours per day,
five days per week, 50 weeks per year. The cylinder will be operated
on a horizontal plane. Compressed air pressure is 80 PSIG. Of
course, we want this cylinder to move as fast as possible (AFAP)!
Specifications:
Load: 500 pounds
Operating Pressure: 80 PSIG
Stroke Length: 12 inches
Cycle Rate: 30 cycles per minute (CPM)
Time to Extend: 1 second
Time to retract: 1 second
Cylinder Velocity: AFAP
Real Solution?
There are five steps required
to accurately calculate a cylinder flow rate (in SCFM). The calculations
are necessary to achieve accurate results. After the calculations
are completed the system designer has the information needed to
make sound, cost effective decisions that include downsizing components
and conserving compressed air. (Calculations rounded to two decimal
places.)
Step #1: Size the Cylinder for Max Performance
This is the stage in sizing a cylinder where we find many designers
throwing in a little extra safety factor to cover breakaway forces
of a cylinder! (If a little bit’s good, a whole lot’s
better!)
Based on common practice and the orientation of the cylinder,
we have found the range of the force multiplier to be between
1.25 to 2.00 times the load (3) being moved, at the specified
pressure. This range of values will provide adequate force compensation
in the calculations and need not be exceeded.
To size the cylinder for maximum performance (quickest stroke
time) we will apply the X2 rule. Multiply the load by two and
apply the correct cylinder at the specified pressure. In this
case, 500 pounds X 2 = 1,000 pounds. Using the Formula, Force
is equal to Pressure times Area (F=PA) we solve for the cross-sectional
area we need for the cylinder bore. Written another way to solve
for the Area, the formula looks like A=F/P. 1000¸ 80=12.50
in.². 12.50 in.² is the cross-sectional area of the
bore needed to move our 500 pound load as fast as possible! This
is very close to a standard NFPA bore of 4″(12.57 in.²).
It is IMPORTANT to note here that any larger bore size will move
slower at 80 PSIG, and any smaller bore size will also move slower.
Use a 4″ bore cylinder.
Step #2: Calculate total volume per cycle
Total volume per cycle requires
some examination of the cylinder we will be applying. We need
to recognize that the extend stroke volume will be more than the
retract stroke volume on a typical double acting, single-rod,
cylinder, due to the volume displacement of the rod.
Since we’ve selected an NFPA
4″ bore cylinder we will apply the standard 1″ rod after
we’ve checked to avoid cylinder rod buckling! Extend Volume
is equal to the bore cross-sectional area times the stroke length.
12.57 in.² X 12 in. = 150.84 in.³
Retract Volume (compensating for the rod) calculation
is (12.57 in.² – .79 in.²) X 12 in. = 141.36 in.³
Total volume per cycle is:
150.84 in.³ + 141.36 in.³
= 292.20 in.³
For a 5″ bore the total volume
per cycle is:
235.68 in.³ + 226.20 in.³
= 461.88 in.³ or a 58% increase in the volume of the 4″
bore cylinder!
Step #3: Calculate total volume
per minute
Multiply the total volume per
cycle by the number of cycles per minute. 292.20 in.³/cycle
X 30 CPM = 8,766 in.³ per minute (CIM)
Step #4: Convert CIM to Cubic
Feet/Minute (CFM). (NOTE:
this calculation step is often overlooked ) 8,766 in.³ ÷
1728 in.³ = 5.07 CFM
Step #5: Convert CFM to SCFM
This conversion reduces the cylinder
flow calculation to the necessary and required terms.
To make this conversion we must
recognize the compression ratio of the compressed air being used
in the application. Compression ratio is the working pressure
expressed in absolute terms and converts compressed air to standard
conditions. (14.7 PSIA, 36% Relative Humidity, and 68º Fahrenheit
temperature). In most industrial applications, the ambient temperature
and relative humidity can be ignored since these variables have
little impact on the calculations.
Our Compression Ratio (CR)
calculation:
(80 + 14.7) ÷ 14.7 = 6.44
Multiplying the CFM by the CR
= SCFM,
5.07 CFM X 6.44 CR = 32.65
SCFM!
32.65 SCFM for this application
seems fairly harmless until you complete the compressed air cost
evaluation.
The cylinder flow calculation
provides the necessary information (SCFM) to more accurately determine
the correct valve (Cv) and the proper FRL for the system. Without
the cylinder flow calculation, sizing the rest of the components
in the system will be accomplished empirically, (trial and error
approach), or by using Myth #2.
With the cylinder flow calculation
complete, we can move on to the real cost of compressed air for
the application and sizing the rest of the components in the system.
Real Horsepower (HP) Requirements
Most compressor representatives
will use a few rules of thumb to determine the compressor capacity
required for an application. Depending on the type of compressor
used, compressors are typically rated to deliver four to five
SCFM per horsepower (rule of thumb). Most compressor representatives
will strongly recommend a duty cycle of 50% to 75% (rule of thumb),
again depending on the type of compressor. Duty Cycle is the percentage
of time the compressor motor is generally running under loaded
conditions. In our application, at 50% duty cycle, and at 4
SCFM/ HP, a 32.65 SCFM application will require an additional
compressor capacity of 16.32 HP! (32.65 SCFM÷4 SCFM/HP)÷50%
Duty Cycle = 16.32 HP
Real Compressed Air Costs!
If we carry our SCFM calculations
out to the number of SCF per year, per shift, pretty soon we are
talking about some serious compressed air usage! Consider how
many minutes there are in an eight-hour day, five days a week,
fifty weeks in a year? (60 minutes/hour X 8 hours/day X 5 days/week
X 50 weeks/year) =
120,000 minutes/year per eight
hour shift! (That’s
360,000 minutes/year for three shifts!)
32.65 SCFM X 120,000 Minutes
= 3,918,000 SCF/Year!
The 5″ bore cylinder would
require 51.6 SCFM, or 6,196,890 SCF/Year!
If your average cost per 1,000
SCF is only $.25, the cost of compressed air to operate this one
4″ bore cylinder for one shift, for the year is an incredible,
$979.50! [(3,918,000÷ 1,000) X .25]
Obviously, if your cost of compressed
air is $.50/1,000 SCF, your annual cost would be $1,959.00!
For the 5″ bore, air consumption
costs jump to $1,549.22 (at $.25/1,000 SCF) and $3,098.45 (at
$.50/1,000 SCF). Over a 50% increase for one bore size increase!
Real Concern
The cost of compressed air actually
used is a major concern for most manufacturing companies. If we
could reduce the compressed air consumption in our system by 30%,
most Chief Executive Officers, Chief Financial Officers, and plant
engineers would leap at the opportunity! Let’s consider another
approach to our application.
Since our 500 pound load is only
being moved in the extend direction, we could consider lowering
the air pressure to return the cylinder. For example, say we were
able to lower the return pressure from 80 PSIG to 20 PSIG. How
would that impact on the total system air consumption?
If you recall from Step #2 above,
the retract stroke volume was 141.36 in.³ per cycle,
or about 48.4% of the total cycle volume. Without taking you through
the additional math, the compressed air cost for the extend
stroke at $.25/1000SCF is (51.6% of $979.50) $505.42/year.
By changing the pressure on the
return stroke to 20 PSIG the compressed air consumption is reduced
from 15.8 SCFM to 5.8 SCFM. The impact on the compressed air bill
for the retract stroke is a reduction from $474.08/year to $174.03,
a reduction of $300.00 or 30%.
Even if the price of a ½”
regulator was $50 you would be able to expect a payback on the
cost of the regulator in about two months. That is, IF we use
Myth #2: Pipe Size = Right Size, or Myth #3: If a little bit’s
good, a whole lot’s better!
Please keep in mind this example
is for only one 4″ cylinder. How many cylinders are you applying?
What is your cost of compressed air?
When we examine the application
even closer, we are able to save even more in initial costs by
properly sizing the valve, the fittings, and tubing by the use
of Coefficient of Flow (Cv). It is sufficient to say the use of
Cv’s to size a system is reasonably accurate and provides
an element of cushion in most system calculations.
There are other approaches to
conserving compressed air in typical applications. If cylinder
speed is not important, using a force multiplier between 1.25
and 2.0 times the load will result in smaller cylinders and less
air consumption. Applying single-acting cylinders could significantly
reduce the long-term cost of compressed air
Real System Cv?
Using Cv’s we can evaluate
the typical circuit (Figure 1.) for potential bottlenecks! Each
component in the circuit has a Cv. With some effort it is possible
to determine the Cv of the cylinder port, Cv of the flow control
(in both the free flow direction as well as the WIDE OPEN controlled
flow direction), Cv of the piping/tubing and fittings, and the
Cv of the directional control valve. The System Cv is ALWAYS
going to be less than the component with the smallest Cv in the
system! A strong recommendation here is to make the most restrictive
component (“the weakest link”!) (4) in the system,
the most expensive component (usually the directional control
valve). This will minimize the initial cost of the components
in the system. It is fair to say that a ½” directional
valve will cost more than a ¼” valve. The difference
in pipe or tubing cost is marginally different.
Real Serious Savings?
With dual pressure savings, you
can see the fast payback on applying additional regulators. More
impressive savings can be realized by finding the leaks in the
compressed air system, and eliminating them! We have seen reports
suggesting compressed air losses due to system leaks and artificial
demand range from 20% to 45%! (5) (Perhaps the high loss
due to leaks is why compressor representatives suggest the 50-75%
duty cycle for compressors?) Compressed air leaks, left unattended,
will continue to grow in size and flow due to the abrasive effect
of the air-line contamination and particulate matter continuing
to attack the leak orifice. (6) The sooner leaks are discovered
and repaired, the less waste there is in power required to produce
the compressed air. Less wasted air reduces operating costs, and
can justify the expense of a maintenance patrol to quickly repair
air leaks.
As you can see in Figure 2., even
small diameter leaks can be very expensive.
Figure 2. Flow & Cost of
Leaks per Orifice Size
The values in Figure 2, are based
on twenty-four hours per day, seven days per week, fifty weeks
(8400 hours).
Real Maintenance
After all of this discussion to
reduce the operating costs associated with wasting compressed
air we must mention another, less obvious, source of waste. Users
should regularly check for excess pressure drop across air filters.
By applying pressure drop indicators, (also called service life
indicators, or delta P indicators) and changing filter elements
with greater frequency, you will avoid the escalating cost of
the pressure drop across the filter element. Electronic and mechanical
pressure drop indicators are commercially available to provide
reminders to service the filter elements on a regular basis. Using
pressure switches to monitor regulated pressure in the system
will avoid surpassing the X2 multiplier, ensure efficient use
of compressed air, and provide optimum performance of the system.
In hydraulic systems, a pressure
drop across a filter has serious consequences effecting the entire
hydraulic power unit adversely. Poor maintenance on hydraulic
filters results in catastrophic failures. In hydraulic systems,
leaks are quickly repaired due to the obvious hazards and cost
associated with hydraulic oil.
Rarely does poor maintenance on
a pneumatic filter result in catastrophic failure. However, excessive
pressure drop across a pneumatic filter is an ongoing operating
cost that is hidden from view. In pneumatic systems, leaks are
often ignored until they become so annoying (uncomfortably loud),
or they have caused such a significant system pressure drop, that
they MUST be repaired!
Summary
If you don’t care about the
cost of compressed air in your plant, don’t do the calculations,
and you will perpetuate the three pneumatic myths! Consider the
similarities between pneumatic and hydraulic systems, rather then
the differences.
If increased profits are of interest
to you, than you’ll find a hidden profit center in the cost-effective
use of compressed air. If you reduce your compressed air overhead
costs, avoid over -sizing components, and design your systems
to operate at an optimum pressure, you can improve system performance,
and you will improve the bottom line. The first step to recognizing
the potential savings available to you is completing these simple
calculations.
With a little time and
effort you can make a big improvement on your bottom line
with pneumatics.
References
Acknowledgements
A special thanks to my Norgren
colleagues, Jim Dietvorst, Brad Dittmer, Doug Kelly, Jerry Martin,
and Joseph Quinn, for their patience, support and technical expertise.
Cf.
- “Compressed Air Handbook
“, Electric Power Research Institute, Palo Alto, CA. 1994,
pp12. - Wagner, H.H., “Your Total
Compressor Cost May Be Too High” Plant Engineering, May
1, 1999. - Fleischer, H., “Stop Oversizing
Pneumatic Components” Machine Design, June 3, 1999, pp 101-106. - Fleischer, H., “Manual
of Pneumatic Systems Optimization” Chapter 2. Conductance
(Cv), 1st ed., McGraw-Hill, New York, 1995, pp 25-66 - Foss, R.S. “Improving Air
System Efficiency” Part 1, Hydraulics & Pneumatics,
April 1998, pp 41-68 - Foss, R.S. “Improving Air
System Efficiency” Part 7, Hydraulics & Pneumatics,
July 1999, pp 33-79