08-11-2012, 12:06 PM
COMPRESSED AIR SYSTEM
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Introduction
Air compressors account for significant amount of electricity used in Indian industries. Air compressors
are used in a variety of industries to supply process requirements, to operate pneumatic
tools and equipment, and to meet instrumentation needs. Only 10 – 30% of energy reaches the point
of end-use, and balance 70 – 90% of energy of the power of the prime mover being converted to
unusable heat energy and to a lesser extent lost in form of friction, misuse and noise.
Compressor Types
Compressors are broadly classified as: Positive displacement compressor and Dynamic
compressor.
Positive displacement compressors increase the pressure of the gas by reducing the volume.
Positive displacement compressors are further classified as reciprocating and rotary
compressors.
Dynamic compressors increase the air velocity, which is then converted to increased pressure
at the outlet. Dynamic compressors are basically centrifugal compressors and are further
classified as radial and axial flow types.
Positive Displacement Compressors
Reciprocating Compressors
Reciprocating compressors are the most
widely used type for air compression.
They are characterized by a flow output
that remains nearly constant over a range
of discharge pressures. Also, the compressor
capacity is directly proportional
to the speed. The output, however, is a
pulsating one.
Reciprocating compressors are
available in many configurations, the
four most widely used of which are
horizontal, vertical, horizontal balance-
opposed and tandem. Vertical
type reciprocating compressors are
used in the capacity range of 50 – 150
cfm. Horizontal balance opposed compressors are used in the capacity range of 200 – 5000 cfm
in multi-stage design and upto 10,000 cfm in single stage designs.
Rotary Compressors
Rotary compressors have rotors in place of pistons and give a continuous, pulsation free discharge air.
They are directly coupled to the prime mover and require lower starting torque as compared to reciprocating
machine. They operate at high speed and generally provide higher throughput than reciprocating
compressors. Also they require smaller foundations,
vibrate less, and have a lower number of parts - which means less
failure rate.
Among rotary compressor, the Roots blower (also
called as lobe compressor) and screw compressors are
among the most widely used. The roots blower is essentially
a low-pressure blower and is limited to a discharge
pressure of 1 bar in single-stage design and up to 2.2 bar
in two stage design.
The most common rotary air compressor is the single
stage helical or spiral lube oil flooded screw air compressor.
These compressors consist of two rotors, within a casing
where the rotors compress the air internally. There are no
valves. These units are basically oil cooled (with air cooled
or water cooled oil coolers) where the oil seals the internal clearances. Since the cooling takes
place right inside the compressor, the working parts never experience extreme operating temperatures.
The oil has to be separated from discharge air. Because of the simple design and few
wearing parts, rotary screw air compressors are easy to maintain, to operate and install.
Dynamic Compressors
Dynamic compressors are mainly centrifugal
compressors and operate on similar principles
to centrifugal pump. These compressors have
appreciably different characteristics as compared
to reciprocating machines. A small
change in compression ratio produces a
marked change in compressor output and efficiency.
Centrifugal machines are better suited
for applications requiring very high capacities,
typically above 12,000 cfm.
The centrifugal air compressor depends on
transfer of energy from a rotating impeller to
the air. The rotor accomplishes this by changing
the momentum and pressure of the air. This
momentum is converted to useful pressure by
slowing the air down in a stationary diffuser.
Dust Free Air Intake
Dust in the suction air causes excessive wear of moving parts and results in malfunctioning of
the valves due to abrasion. Suitable air filters should be provided at the suction side. Air filters
should have high dust separation capacity, low-pressure drops and robust design to avoid frequent
cleaning and replacement. See Table 3.3 for effect of pressure drop across air filter on
power consumption.
Air filters should be selected based on the compressor type and installed as close to the compressor
as possible. As a thumb rule "For every 250 mm WC pressure drop increase across at
the suction path due to choked filters etc, the compressor power consumption increases by
about 2 percent for the same output"
Hence, it is advisable to clean inlet air filters at regular intervals to minimize pressure drops.
Manometers or differential pressure gauges across filters may be provided for monitoring pressure
drops so as to plan filter-cleaning schedules.