|  APPLICATION
Compressed Air Audit for reducing energy
consumption and proper sizing of compressors.
BACKGROUND
Compressed air is widely recognized
as industries fourth utility. It is the only utility the
end-user has the sole responsibility to produce and maintain
in-house. Compressed air is a very costly utility; it
is not “free”. Developing an understanding
of the dynamics of a compressed air system (CAS) is advantageous
to the end user for two main reasons.
First, a CAS is inherently inefficient.
Due to heat loss during the heat of compression, only
approximately 10% of the input energy is usable energy
at the point of use. Therefore, significant opportunities
exist to reduce the input energy consumed to meet the
compressed air demand, sometimes in the neighborhood of
35-40%.
Secondly, the CAS can be a source of
numerous problems that bring about added costs, product
rejection and unnecessary downtime due to the erratic
and inefficient performance of the CAS.
It is with these facts in mind that
industry owners, managers and engineers must recognize
that industrial compressed air systems can represent the
most insidious form of industrial waste in a plant, and
therefore, offer great opportunities for efficiency improvements
and increasing the bottom line.
REQUIREMENT
No two compressed air systems are identical.
A CAS is dynamic. The operation, response and performance
are different due to the constantly changing conditions,
equipment and demands. To better understand the dynamics
of a CAS, and create useful data needed in accomplishing
some goals discussed above, a thorough compressed air
audit must be performed.
During an air audit, several sensors
are installed at key locations to monitor the system.
The objective is to collect data that can be used to create
“profiles”. The three main profiles created
during an audit are flow, power and pressure.
Compressed air systems are split into
two main parts; the supply-side, most commonly referred
to as the compressor room or powerhouse, and the demand-side,
the main plant or production area. The bulk of the monitoring
during an audit is performed on the supply-side where
a system might consist of several air compressors, air
dryers and pieces of ancillary equipment. To properly
monitor a CAS, anywhere from 4 to 36 points or more must
be monitored simultaneously. The data then needs to be
logged or “stored” over time in an efficient
and reliable manner. This data is later downloaded to
a PC for post-audit data processing. In summary, to properly
monitor a CAS during a through audit can be quite a challenge
in data logging.
SOLUTION
The Logic Beach HyperLogger HL-1 data
logger has been configured and programmed into a data
acquisition system suitable for monitoring compressed
air systems. The versatility and expansion capabilities
of the Logic Beach HyperLogger HL-1 have made this an
excellent choice when working with the simplest or most
complex air systems.
As mentioned previously, there are no
two systems alike. Every audit performed is different
in regards to the equipment, layout, scope and the overall
objective. The sensors used in the data acquisition process
are; pressure transducers, current transformers, kW meters
and various types of flow meters, among others. Therefore,
the data acquisition system must be easily configurable
in the field to monitor and log the data from this list
of sensors.
The Logic Beach HyperLogger HL-1 data
logger, coupled with the HyperWare software, makes a great
package that allows the user to easily interface with
these various types and quantities of sensors while in
the field. The HyperWare software also allows the user
to view the data in real-time, as well as store for post-audit
download, processing and evaluation.
The Logic Beach HL-1 data logger is
a wonderful example of a versatile, rugged and dependable
data logger that has exceeded the compressed air auditor’s
needs.
SAMPLE AUDIT
The equipment used to collect data
during an audit, or any evaluation for that matter, is
very critical. The old adage applies here; garbage in
garbage out. To this point we have mentioned several times
the importance of a high quality data logger like the
Logic Beach HyperLogger HL-1. It is equally important
to use sensors that are of high accuracy and quality.
Sensors should be chosen that have an accuracy of ±1%
or better.
To perform a basic audit, it is necessary
to interface these high quality sensors; a pressure transducer,
current transformer, or power monitor, a mass flow meter
and a temperature sensor. This is easily accomplished
with the Logic Beach HL-1 data logger. The preferred sensors
are 4-20mA and can be excited from an isolated dc power
loop, or sensor excitation can be supplied from the logger
itself. Once the loop is created, the sensors are wired
to the logger Port of choice with the internal dip switches
configured for current input on the 20mADC scale. The
logger is then ready for the Program Net to be built utilizing
the HyperWare software. The following Program Net is simplified
NET utilizing a CASE Index develop by Southern California
Edison (explain later) and just a few of the powerful
programming Icons available in HyperWare from Logic Beach.
Figure 1
Figure 1 is a sample of a basic audit
Program Net as created in HyperWare the software used
for Logic Beach data logging instruments. HyperWare is
a graphical, icon-based, program designed for ease-of-use
and simplicity. See more here. The appropriate Input Icons
are first chosen that represent each sensor wired to the
logger Port, and the corresponding channels. The Sample
Rate Clock Icon is selected and should be set for a rate
of 6 to 15 second interval. This interval works well to
capture changes that happen rapidly within an air system,
large demands that occur within a small time frame.
The top portion of the Program monitor
and record air temperature and pressure. These are largely
used as a check if the Flow and Power move in a drastic
irregular manner. The Pressure (PSI) signal is converted
from a process signal to engineering units in the Math
Icon. ( 4th order polynomials are easily handles here).
Temperature from a thermocouple does not require signal
conversion and may be recording or processed directly
from the Input Icon.
The lower portion of the Program Net
represents the two 4-20mA signals from the Mass Flow Meter
and the Power (kW) sensor. After applying a Math Icon
to convert the process signals to engineering units each
program channel then flows in parallel to Averaging Icons
and then to a Math Icon where the CASE Index is calculated.
A Rate of Change Icon monitors the CASE Index fluctuation
over time and if the rate of change is greater than the
anticipated constant (Constant Icon) and alarm is triggered.
The data logger Memory Icon allows the
data to be stored within the internal memory of the logger.
This data can be accessed at anytime on a laptop computer
via a serial connection, and of course eventually downloaded
in full to a desktop PC for post-audit processing and
detailed evaluation. The Probe Point Icons allows the
user to view the data in real-time through the serial
connection. This is very convenient while the auditor
is still in the field and is certainly necessary to ensure
the sensors are set up, scaled and sensing properly.
POST-AUDIT
EVALUATION
The post-audit evaluations of the data collected and report
generation is equally important as the data collection
itself. There are several values that need to be extrapolated
to create a useful report. Basic information includes
the minimum, maximum and average flow, and power and pressure
values. These values can be calculated through the HyperWare
Program Net using the icons available, however, with different
shifts of operation and several other contributing factors,
it is best to extrapolate these values manually so the
auditor does not miss any event in the air system that
is of value to the end user. Knowing the exact time of
day, peak flow, duration and power consumed are just some
of the useful points of information the auditor needs
to extrapolate, examine and derive possible solutions
to improve the system.
Other useful values are the scfm/kW
ratio. An air system utilizing positive displacement rotary-screw
air compressors, operating at 100% rated full load and
100psig, should have a 100% dynamic efficiency close to
5 scfm/kW. A system that has room for improvement will
be operating in the 2 to 3 scfm/kW ratio range for instance.
Power consumed to produce the scfm required is critical
and is at the crux of most air system audits.
Southern California Edison (SCE) developed
a metric to measure compressed air system performance
and to compare its performance to other compressed air
system installations. The method involves monitoring and
recording flow, energy consumption of the system, temperature
and pressure of the air. A ratio results from this inputs
which is named CASE (Compressed Air Supply Efficiency)
Index and is the ratio of air supplied in standard cubic
feet per minute divided by the power in kWh. The ratio
may vary between 0 and 320 with a SCE recommended target
of 200 or above for averaged system period. Below 200
systems may be oversized, reservoirs undersized, and/or
individual machines operating ineffectively creating uneccessary
air loss and power consumption.
Most compressor manufactures, distributors
and independent auditors have created their own post-audit
evaluations programs. These programs take the raw data,
extrapolate the values desired, and allow the auditor
to then pour over the data to model potential solutions
to problems and help create the most efficient air system
possible. Needless to  say,
these programs are not made available to the public for
obvious reasons.
To date, the most widely accepted evaluations
program made available to the public is the AIRMaster+
software program. This is a software package developed
in part by the U.S. DOE to help the end user maximize
the efficiency and performance of their compressed air
system through improved operations and maintenance (O&M)
practices, usually the main goal of a compressed air audit.
AIRMaster+ provides comprehensive information on assessing
compressed air systems, including modeling and evaluating
energy efficiency. For more information on downloading
this program, and locating a qualified specialist to assist
you in identifying system improvement opportunities, please
visit the link provided below.
http://www1.eere.energy.gov/industry/bestpractices/software.html#air
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