For many finishers, compressed air is
a vital utility. Compressed air runs essential tools and equipment, provides power
to material handling systems and supplies clean air to processes. Choosing the
right compressor for these applications can be difficult given new technological
developments, added concerns about even and steady pressure, maintenance and,
that traditional nemesis, electrical power.
The cost and dependability
of compressed air can have a tremendous impact on production processes and costs.
Surprisingly, compressed air costs are most often considered in terms of equipment.
Yet, energy consumption can represent up to 70% of the total cost of producing
compressed air. With energy costs escalating nationwide, selecting an energy-efficient
air system has become critical. Plus, there are other significant factors to consider,
such as reliability, productivity, systems support, automated features and noise.
Making a proper evaluation
about air compressor selection is ultimately based on a study of the available
technologies that may be appropriate for your application as well as evaluating
existing equipment. In some instances, such as when there is a continuous demand
at full load, a fixed-speed rotary screw compressor may be the best solution.
In cases where the base load varies with an additional load, it might be best
to consider supporting the base load with fixed flow compressors and adding a
unit with a variable speed drive (VSD) as a "trim" device to carry the
variation in the load.
Many compressed air users
are improving air system energy efficiency, reducing maintenance costs and lowering
noise levels with rotary screw compressors incorporating VSDs. Since many applications
do not have a consistent demand for air, a VSD system can meet the changing demand
on the fly.
Rotary screw compressors
with fixed-speed drives are limited in the number of times they can be stopped
and started in a given time frame. In applications with a variable compressed
air demand, drives may run in idle for long periods to avoid overheating caused
by frequent restarts. Although not producing compressed air while in idle mode,
a fixed-speed compressor running with modulation control still consumes about
70% of full load electrical power, which translates into substantial electrical
costs with no benefit. A fixed-speed compressor operating under dual control (stop/start
or online/offline) will use only 25% of full load electrical power and offer some
energy savings. However, compressors equipped with VSDs are much better able to
match their demand requirements, virtually eliminating the need for the compressor
motor to rest in the energy-consuming idle mode. Rotary screw compressors with
VSD technology save users 20-35% on electricity in situations where there are
variable loads.
|
Electric motors equipped
with VSDs have been around for some time. Traditional VSD applications include
fluid pumps, HVAC, conveyor systems and positive-displacement rotary-lobe blowers.
Only recently have they been applied to rotary screw air compressors.
The principal of variable
frequency control is accurately measuring the actual air main pressure with a
pressure transducer so that the volume of compressed air generated varies, achieving
a preset final pressure. Highly accurate sensors provide operational data. Combined
with a responsive drive system, pressure can be controlled to +/- 1.5 psig.
Some manufacturers are
retrofitting VSDs to their existing compressors rather than purchasing a system
that combines both features, but this may not always be the best approach. In
rotary screw compressors, the efficiency is based in part on airend speed. The
efficiency range can be plotted in a bell curve. In an efficient range there is
a flat top to the bell curve, but when you get out on the edge the efficiency
falls off very rapidly. In other words, as you go too slow or too fast, you use
more electricity and produce less air. By adding a VSD only onto an existing compressor
design, you don't know whether the compressor design is in the middle of the bell
curve or on the edge of the bell curve. When a compressor is on the edge of the
bell curve, if you slow it down at all, it becomes very inefficient. Plus, the
existing motor may not be designed to handle the conversion from fixed speed to
variable speed.
Therefore, it's necessary
to design the compressor airend to operate in the flat part (top) of the bell
curve and keep the whole speed range in the top of the bell curve so the user
can maintain maximum efficiency in terms of kilowatts in and compressed air out.
Compressors with large airends produce a much flatter curve.
The torque required by
the airend determines the drive size. When you have a small airend with higher
speeds, you can use a smaller, less expensive drive. But, if you need the efficiency
and operating range of a big airend with the higher torque, you also have to have
a bigger drive.
An inherent advantage of
a VSD-equipped compressor is the ability to start and stop the compressor as often
as desired. Unlike fixed-drive systems, VSD systems are "soft starting,"
incurring the lowest inrush current requirement. This enables unlimited starts
and stops of the motor. With a 100-hp fixed-drive system, for example, the user
would be limited to two or three starts and stops per hour because the inrush
current required to start it would heat up the motor windings. The motor has to
run for 20 min in order to cool the windings down before you can turn it off and
then turn it back on. Plus, the user may be penalized by the power company for
even one spike on the demand chart from high inrush motor starts. Also, newer
drive systems help stabilize plant air pressure, enhancing quality in the plant.
With fixed-drive rotary
screw compressors there is a 10-15 psig swing built into the controls. VSD compressors
have only a 1-2 psig swing, which is a significant advantage and can make a substantial
difference in product quality.
One of the issues with
retrofitting a compressor with a VSD is the danger of harmonics backing into the
plant's electrical system, which could disrupt or even destroy some of the other
equipment in the plant. However, with a completely integrated system, all feedback
should be isolated or eliminated so that no harmonic distortion goes into the
electrical grid of the plant.
Some VSD-equipped compressors
fail to keep the power factor near unity. When unloading the electric motor, the
power factor gets worse and worse. The power company may penalize the user based
on how far off unity (1.0) the power factor is. The user gets a power factor correction
penalty every month on its electric bill. However, some VSDs can maintain a power
factor close to 1.0 throughout the entire speed range. So, even as the motor is
unloaded, the power factor does not go down, eliminating any penalty from the
power company.
A factor often overlooked
when evaluating plant air systems is noise. Noise is partially due to compressor
speed and the packaging of the equipment. If a VSD is added to a standard compressor,
a lot of audible electrical noise will be generated. It makes a high-pitched "chirping"
sound in the motor. However, the noise can be reduced with large motors, large
airends, low speeds, a radial fan and an enclosed package.
Choosing the right air
compressor can be quite complicated. But, the right choice can save you lots of
money and help you produce better parts.
