Compressed Air (CA) is a major prime-mover for modern industry. Compressed Air is referred to as the fourth utility after electricity, gas and water. The article is published in two parts. This is first part of the article. Click here to read final part.
A properly managed Compressed Air system can:
- save energy
- reduce maintenance & decrease downtime
- increase throughput
- improve product quality depending on its end-use
Compressed Air Quality:
CA quality ranges from Plant air to Breathing air depending upon the end-use. Quality is determined by the dryness & the contaminant level. Higher the quality, higher the cost. The following figure (Fig. 1) gives the applications of compressed air with respect to quality.
Components of a Compressed Air system:
Compressed air systems consist of:
- a supply side which includes compressors and air treatment
- a demand side, which includes distribution and storage systems and end-use equipment.
A Compressed Air system broadly consists of:
- Prime mover
- Treatment equipment and accessories
- Distribution system.
Compressor Types (Fig. 2):
- The Air compressor is the heart of any CA system.
- There are two basic compressor types: positive-displacement and dynamic.
Reciprocating Compressor (Fig. 3):
This is a very versatile type of compressor and can be used for nearly all industrial applications. These are constant-capacity machines & deliver the air in pulses.
- High discharge pressures & relatively low to moderate volumetric flows (600-3000 cfm with a pressure range of 2000-5000 psig)
- Generally more maintenance intensive due to the many wearing parts
- Can be single-acting or double acting, single stage or multi stage, air cooled or water cooled and lubricated or non-lubricated.
- High efficiencies
- Occupy larger footprint
- Higher capital cost
- Control is usually by Load-unload with 3 or 5 step capacity control
- Multistage machines are used in place of single stage ones for following reasons:
- Single stage compression would generate excessive heat of compression
- MOC would have to be of high grade and hence expensive
- Power consumption of single machine would be higher
- Better efficiency
Rotary Compressor (Fig. 4):
The most common type of rotary compressor is the helical-twin, screw-type. Less common types include sliding-vane, liquid-ring, and lobe.
- Not usually suited for high discharge pressures & are most efficient for moderate air flows & low pressures (3000-6000 cfm with a pressure range of 300-400 psig)
- Low initial cost, compact size, low weight, and are easy to maintain.
- Dry or oil flooded type
- Lower efficiency
- VSDs provide good capacity control
Centrifugal Compressor (Fig. 5):
Centrifugal compressor develops pressure by increasing the velocity of the air going through the impeller & then recovering the velocity in a controlled manner to achieve the desired flow and pressure.
Best suited for continuous air flows in large quantities.
- Heat generated & power consumption is lower.
- The space requirement & maintenance is minimum.
- Inherently non-lubricated.
- Available for flows ranging from 300 to more than 100,000 cfm, but the common ones are 1200-5000 cfm with a pressure range upto 125 psig.
- Capacity control by inlet valve/guide vane throttling
- Surge/choke phenomena