Compresors

3. COMPRESSED AIR SYSTEM
Syllabus
Compressed air system: Types of air compressors, Compressor efficiency, Efficient compressor operation, Compressed air system components, Capacity assessment, Leakage test,
Factors affecting the performance and efficiency

3.1

Introduction

Air compressors account for significant amount of electricity used in Indian industries. Air compressors are usedin 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.

3.2

Compressor TypesCompressors 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. Dynamiccompressors are basically centrifugal compressors and are further
classified as radial and axial flow types.
Bureau of Energy Efficiency

45

3. Compressed Air System

The flow and pressure requirements of a given application determine the suitability of a particulars type of compressor.

Positive Displacement Compressors
Reciprocating Compressors
Reciprocating compressors are the mostwidely 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, horizontalbalance-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.
Reciprocating compressors are also available in variety of types:

•
•

Lubricated and non-lubricated
Single ormultiple cylinder

Bureau of Energy Efficiency

46

3. Compressed Air System

•
•

Water or air-cooled.
Single or multi stage

In the case of lubricated machines, oil has to be separated from the discharge air. Non-lubricated compressors are especially useful for providing air for instrumentation and for processes
which require oil free discharge. However non-lubricated machines havehigher specific power
consumption (kW/cfm) as compared to lubricated types.
Single cylinder machines are generally air-cooled, while multi-cylinder machines are generally water cooled, although multi-stage air-cooled types are available for machines up to 100
kW. Water-cooled systems are more energy efficient than air-cooled systems.
Two stage machines are used for high pressures and arecharacterized by lower discharge temperature (140 to 160°C) compared to single-stage machines (205 to 240°C). In some cases, multi-stage
machines may have a lower specific power consumption compared to single stage machines operating
over the same total pressure differential. Multi-stage machines generally have higher investment costs,
particularly for applications with high discharge pressure (above 7bar) and low capacities (less than 25
cfm). Multi staging has other benefits, such as reduced pressure differential across cylinders, which
reduces the load and stress on compressor components such as valves and piston rings.
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...