Chemical Engineering January 2013 - 41

Conventional air compression and drying system
Instrument air
Air intake filter and silencer
Drive 1:
Steam turbine
ST
Package boundary
Compressor
(1st stage)
Air intake filter and silencer
Drive 2: electric
motor
M
Stand-by
compressor
(1st stage)
Steam in
Condensate
out
Cooling
water supply
Stand-by
compressor
(2nd stage)
Cooling
water return
Air compression and supply
Air storage and drying
FIGURE 1. Shown here are the main components of a compressed air system
prepared that contains data, including
number and type of consumers,
minimum and maximum air-pressure
requirements of each user, air flow required
by each user, utilization factor
and so on.
Compressed air has a number of
industrial uses based on its service. A
major application of compressed air,
when used as instrument air, is valve
actuator control. Other common applications
of instrument air include use in
laboratories, rotating equipment seals,
paint spraying and powder coating,
climate control and so on. Industrial
workshops have consumer tools, such
as pneumatic hammers, drills, grinders
and such. Utility stations are often installed
in a plant for general purposes
and require plant air. Breathing air
stations are provided in most chemical
plants. Food, pharmaceutical and
electronic industries require mostly
process air. All of these users must be
carefully identified and listed.
Quality. The air quality depends on
the levels of contaminants that the
end users can tolerate without affecting
the smooth function of process
(Table 1). Typical contaminants commonly
encountered in compressed
air systems include solids (dirt, dust,
pipe scales, and particles from compressor
wear), liquids (water and oil)
and gases (water vapor, oil, chemical
vapors). Based on the services catered
to, the quality of compressed air
ranges from plant air (least critical),
process air and instrument air (critical)
to breathing air (most critical).
The cost of producing compressed
air goes up with each quality level.
Each increased quality level requires
installing additional purification
equipment and leads to a higher initial
capital investment. The future operating
cost will also rise due to increased
energy consumption and maintenance.
Therefore, the air quality level should
be determined as the first step.
The quality class of compressed air
can be assigned as listed in detail in
the international standard ISO 85731,
which bases the classes on particle
size, moisture and oil content in the air.
For example, the air quality specification
for instrument air is written as
ISO 8573-1 Class 2.2.1, which means
1 micron particulate filtration, -40°F
(-40°C) dew point and 0.08 ppm w/w
(0.1 mg/m3) oil filtration. The air class
may also change from client-to-client
based on the purity requirement of air
for the particular service.
The most stringent quality class
in this regard is Class 0. It does not
mean that the contaminant level will
be zero, but rather that the levels of
particulate matter, dew point and oil
content of the air supplied will be as
per any values (typically lowest) specified
by the user. Based on its equipment
capabilities, the manufacturer
must agree in writing that it can provide
air of such a class.
Some points to be considered when
talking about air quality are given
below:
1. Minimizing or eliminating sources
of contamination. Contaminants can
enter the system at the compressor intake
or could be introduced in the air
stream by the system itself. Though
equipment, such as separators,
filters,
dryers and condensate drains are
used to improve the air quality, we can
still try to reduce the load and thus
the quality level expected from them
by eliminating or minimizing sources
of contamination. This can be done in
a number of ways.
For example, locate the compressor's
air-intake filters in a safe nonhazardous
area in open air outside
the plant building away from sources
of dirt; dust; moisture; toxic, corrosive
and flammable gases; and also at sufficient
height (about 3 to 5 m) from
ground level to avoid dust, debris,
insects and so on. As the air intake
is subject to extreme conditions with
various contaminants causing fouling,
corrosion and other problems, the
material of intake filters should be
selected with great care. Typically, the
air intake filter and piping is made of
stainless steel.
Also, one should avoid using lubricated
air compressors in applications
where high quality is desired.
2. Grouping of consumers. Consumers
with similar air quality and pressure
level can be grouped along with
air-treatment equipment in close
proximity. If different air quality requirements
exist in the same plant
then the plant can be divided into difCHEMICAL
ENGINEERING WWW.CHE.COM JANUARY 2013 41
Air distribution
Moisture
separator
Inter-cooler
Inter-cooler
After-cooler
Flow orifice
After-cooler
Primary air
reciever
Automatic drain
trap
Automatic drain
trap
Shutoff valve
Flow orifice
Compressor
(2nd stage)
Moisture
separator
Pre-filter
After-filter
Plant air
Package
boundary
Dryer
Inst. air low
press. switch
PSL
Secondary air
reciever
P/F controller
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Chemical Engineering January 2013

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