Air Compressor System provides high pressure compressed air with subsequent reduction in volume for a range of industrial and domestic applications. They have become imperative equipment in almost every industry. Due to its high efficiency, feasible operational parameters and diverse range of applications, they have gained widespread popularity and no industry is complete without a compact compressor system.
The primary objective of this report is to understand the importance of air compressor systems. In addition, the report covers a wide spectrum of compressor applications and processes. The different types of compressors, along with their merits and demerits have been delineated. The important operational parameters have been defined along with their importance in the compression process.
"Air is free, whereas compressed high pressure air is not". True to the phrase, generating high pressure compressed air is not an economical and energy efficient process. High initial and maintenance costs can increase the overall compressor cost to a great extent. Proper and well maintained compressor systems can reduce energy consumption from 30-50%. Various steps to reduce energy consumption and increase overall efficiency have been detailed in the report.
For a practical application of compressor acumen, a case study has been conducted on an ELGI Compressor which is under operation in Arabian Cement Industry. With the data collected from the study, certain operational parameters were calculated and corresponding inferences were drawn. At the end of the study, we are able to conclude the current state of the compressor and detail certain steps to improve efficiency and compressor performance.
The future of air compressor systems has been a constant research topic among industrialists. The modern techniques of Heat Recovery Systems in air compressors and use of Variable Speed Drives have been briefly detailed to provide a view into a bright future of air compressor systems.
Introduction to Air Compressor Systems
In most industrial applications, compressed air is considered as the "fourth utility", preceded by the three essential utilities; water, electricity and natural gas. Typically, a simple air compressor is an industrial device that intakes ambient air and increases the pressurewith reduction in volume by doing work on the working fluid, namely air or gas.
The primary objective of employing a compressor is to provide high pressure air using least possible power consumption. The compression can occur in any one of the three ways; adiabatic, isothermal or polytropic. For an Adiabatic compression process, there is assumed to be no heat loss or gain from the system. When the temperature of air at inlet equals temperature of compressed air at outlet, the compression process is Isothermal. Normally, a typical compression process involves Polytropic Compression. The pressure, temperature and volume of air it inlet and outlet vary with the respective conditions. The least possible work of compression is achieved in an adiabatic compression process as the net work done by the system is zero.  The adiabatic efficiency is the maximum possible compressor efficiency and compressors are designed with compressor efficiencies such that, there is little or no variation among the two.
As mentioned earlier, compressed air is considered a utility because of their diverse applications in various industrial and metallurgical operations. The compressed air supply is clean, convenient, safe and flexible for use. Industrial uses can be attributed to when they are in connection with compressed air motors for functioning of pneumatic tools, raw material conveying through ducts, in connection with a reservoir for storage of compressed air for future applications.
1.2 Historical Background
The earliest documented use of the compressor dates back to around 4th century B.C. In those times, the compressor was termed as a "water organ". The water organ was invented by Ctesibius of Alexandria. The design comprised of a chamber filled with air and water, a water pump, collection of pipes of various dimensions and connecting tubes and valves. The mechanism of operation was relatively simple; by pumping water into the chamber, the air gets compressed.  With advancement of time and technology, various improvements and innovations were made to the water organ.
In 1808, the Multi Stage Axial Compressor, designed by John Dumball, came into industrial application. However, this design did not achieve wide-spread popularity because the compressor comprised of moving blades without stationary blades to transfer the air flow into the successive compression stage.
During the 19th century, the invention of the Roots Blower was a huge step leading to invention of modern air compressor systems. Philander and Francis Roots designed the Roots blower, while devising a suitable replacement for the water wheel at their woolen mill. The roots blower consisted of numerous impeller pairs rotating in opposite directions.
Dr. Franze Stolze designed the modern compressor in 1900; which comprised of a multi stage axial flow compressor with a combustion chamber, multi-stage axial turbine along with a regenerator to heat the discharged air utilizing exhaust waste gases.
1.3 Applications of Compressed Air
High pressure compressed air is widely used for industrial and domestic applications due to their high efficiency and facile operational nature.
The powering of pneumatic equipment is one of the major applications of compressed air. Pneumatic tools are lighter, deliver more power and not prone to overloading, unlike electric motor run equipment. Compressed air driven tools can achieve high overall torque and optimum speed in a short span of time. They are safer as compared to electric driven tools because of no spark conditions and less overheating. 
Compressed air is also used for raw material transport, combustion processes; such as oxidation, thermodynamic operations; such as refrigeration and cryogenics. Filtration, aeration and dehydration processes are also driven by compressed air.
Non-industrial applications of compressed air include transportation, mining and agriculture.
1.4 Components of Air Compressor System
Due to the diverse applications of compressed air, air compressor systems merit an intricate and complex design. Air compressor systems consist of a number of sub-systems and components, which include the air compressor, motor and drive unit, control systems, air inlet filters, air dryers, after-coolers and various other retrofitted components.
During the operation of an air compressor system, the ambient air is taken into the system by air inlet filters. The motor and gear unit powers the compressor system to increase the pressure of air. The primary function of an integrated control system is to regulate the amount of air intake and air discharge from the system.
Treatment equipment, such as an Air Dryer and Air Separator are used to remove contaminants and provide moisture and oil free pressurized air. The Outlet and Distribution systems play the pivotal role of transporting the compressed air to the various required sources. In typical industrial applications, the pressurized air is stored in an Air Reservoir or Storage Tank for future use.
The major components of an air compressor are detailed below:
Intake Air Filter: Ambient air enters the compressor via inlet air filters. Filtration elements in intake filters facilitate filtration of particulate matter present in air. This allows the entry of clean and dry air into the compressor.
Compressor Cooling: A substantial amount of heat is generated during compressor process. As a result, cooling and lubrication of the system is mandatory for reducing wear and damage to the compressor spares; thereby increasing compressor efficiency. Compressors are normally cooled using air, water or oil (lubricant). 
Inter-Stage Coolers: Inter coolers act as heat exchangers to lower the temperature of air before it enters into successive stages of compression. This reduction of temperature is paramount to reduce the overall compression work and increase compressor efficiency.
After-Coolers: After coolers are employed at the end of compression process to lower the discharge temperature of air. During this final stage of operation, as the temperature is lowered, moisture present in air condenses and separated from the discharge air.
Air Separators: Separators act as an enhanced after-cooler. Theyare installed either after an inter-cooler or after-cooler to desiccate discharge air.
Air-Dryers: Normally, after the compressed air has passed through all the above mentioned equipment, discharge air is passed into the air-dryer before storage in the reservoir. The air that reaches the dryer is saturated. If this saturated air enters the piping, it can lead to corrosion and damage to the transport equipment. The Relative Humidity (moisture content) in atmospheric air is relatively high. When this air is compressed at high pressure, it occupies a small volume and high temperature. At a specific temperature, all the moisture present in air condenses. This temperature is known as Dew Point. This process is cycled in an air-dryer to ensure that moisture-free air reaches the storage area.
Pressure Regulators/Flow Controller: PFC's are auxiliary equipmentthat minimizes pressure drops in the system by stabilizing system pressure.
Lubrication: Lubrication is a paramount need for effective compressed air operation. A lubricator is employed to lubricate and cool the moving parts and prevent overheating. In some systems, the lubricator is retrofitted with a compressed filter and pressure flow regulator to fashion a system known as Filter Regulator Lubricant (FRL).
Air Receivers: The high pressure compressed air from the system is discharged to a storage tank or an air-receiver according to the demand. The capacity and type of air receiver varies depending on the demand of compressed air within the system. For spasmodic demand of compressed air, a large capacity reservoir is employed for a relatively small capacity compressor. Moreover, a receiver functions depending on demand, thereby reducing overwork and load on the compressor.
Air Transport and Distribution System: The final stage in a compression process is the proper distribution of compressed air to the demand points. A common distribution system includes piping, valves and hoses. To minimize the pressure drop during distribution, it is imperative that the pipe length is kept minimum and diameter to maximum.
Figure 1 depicts a typical air compressor system and corresponding components.
Fig 1: Components of Air Compressor System