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Air Compressors

Introduction to Air Compressors
Air Compressor Principals
Air Compressor Types (1.5 to 100 horsepower)
Positive Displacement Compressors
Reciprocating Compressors
Single Acting
Double Acting
Reciprocating Compressor Capacity Controls
Rotary Screw Compressors
Oil Lubricated Rotary Screw Compressor
Oil Lubricated Rotary Screw Capacity Controls
Rotary Compressor Lubricants
Oil-Free Rotary Screw Compressor
Oil-Free Rotary Screw Controls
Motors and Motor Control for Air Compressors
Air Compressor Installation and Maintenance
Compressed Air Purification

Introduction to Air Compressors

Air compressors are utilized to raise the pressure of a volume of air. Air compressors are available in many configurations and will operate over a very wide range of flow rates and pressures.

The first air compressor used by mankind was the human lung. Compressed air was expelled by primitive man to give glowing embers sufficient oxygen to allow them to flare up into a fire. Healthy human lungs are capable of processing 3.5 cubic feet of air per minute (0.1 m3/min.) and can compress this air up to 1.16 psig (0.08 bar). The human lung is also considered to have the highest reliability and lowest maintenance of all compressor types.

The middle of the Third Millennium saw the development of the first mechanical air compressor, a hand operated bellows. A current day hand bellows is shown in figure 1. Advances in metal working processes led to the development of the more efficient foot bellows compressor around 1500 B.C.

Hand and foot bellows compressors, along with a bellows design driven by a water wheel, provided compressed air for over two thousand years before the first "blowing cylinder"

Figure 1: Current day hand bellows

compressor was invented in 1762. "Blowing cylinder" compressors were capable of producing an air blast of approximately 14.5 psig (1 bar).

Construction of the Mt. Cenis tunnel in the Swiss Alps, started in 1857, was the first successful distribution of compressed air on a large scale. During the construction of the tunnel, compressors were installed at both ends of the tunnel. The compressed air was then piped to rock drills inside the tunnel. When completed, the compressed air distribution system included almost 23,000 feet (7010 m) of piping.

Air Compressor Principals

All air compressor designs utilize four (4) basic principals:

1. Staging

During the compression process, the temperature increases as the pressure increases (fig. ACP-1.1). This is known as polytopic compression. The amount of compression power also increases as the temperature increases.

Figure ACP-1.1 - Temperature Ratio for isentropic compression

Compressors are staged thereby reducing the temperature rise and improving the compression efficiency. The temperature of the air leaving each stage is cooled prior to entering the next stage. This cooling process is called intercooling.

Volumetric efficiency also increases with multi-stage compression since the pressure ratio over the first stage will be decreased.

2. Intercooling

A heat exchanger or intercooler is used to cool the compressed air between stages. The intercooler may be either air-cooled or liquid (water)-cooled. Perfect intercooling is achieved when the air temperature leaving the intercooler is identical to the air temperature entering the compressor. Minimum compressor power consumption can be achieved with perfect intercooling and when the pressure ratio across all stages is the same. Perfect intercooling is difficult to achieve in actual practice since the cost of either the heat exchanger(s) or the cooling water is prohibitive. Intercooler selection typically provides air at 10�F (-12�C) to 15�F (-9�C) above the ambient temperature.

Selection of the most efficient number of compressor stages is determined by the actual operating pressure and desired compressor efficiency.

3. Compressor displacement and volumetric efficiency

Theoretically, the capacity of the compressor is equal to the swept volume or amount of air drawn into the compression area. Actual compressor capacity will be reduced by pressure drop on the intake side, preheating of the intake air, internal and external air leakage and expansion of air in the clearance volume.

Volumetric efficiency is the ratio of compressor capacity to compressor displacement.

4. Specific energy consumption

The specific energy consumption of a compressor is defined as the shaft input power per unit of compressor capacity. Specific energy consumption is typically measured in bhp/100 cfm. A comparison of theoretical specific energy consumption for single and two stage isentropic compression is shown in figure ACP-4.1.

Figure ACP-4.1 Theoretical Specific Power Consumption

Air Compressor Types

Current day air compressor technology includes two (2) basic groups, positive displacement and dynamic. Positive displacement and dynamic compressors are further segmented into several compressor types as shown in figure ACT-1.1.

Figure ACT-1.1 Air Compressor Types

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Air Compressors

TABLE OF CONTENTS

Introduction to Air Compressors

Air Compressor Principals

Air Compressor Types