Air Compressors

Air compressors • Air compressors are utilized to raise the pressure of the given volume of air. • Air compressors are available in many configurations and will operate over a wide range of flow rates and pressures. • All air compressor designs are based on four basic principles: 1. Staging 2. Inter-cooling 3. Compressor displacement and volumetric efficiency 4. Specific energy consumption Staging • 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 inter cooling. • Volumetric efficiency also increases with multi-stage compression since the pressure ratio over the first stage will be decreased. Inter-cooling • 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 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 10ºF (-12ºC) to 15ºF (-9ºC) above the ambient temperature. • Selection of the most efficient number of compressor stages is determined by the actual operating pressure and desired compressor efficiency. 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. Specific energy consumption The specific energy consumption of a compressor is defined as the shaft input power per unit of compressor capacity.

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As shown in figure PD-1, positive displacement compressors increase the pressure of a given quantity of air by reducing the space occupied by the air at the original pressure. Atmospheric air (fig. PD-1.1) is drawn through the intake filter and intake valve into the one cubic foot cylinder by the movement of the piston towards the bottom of the cylinder. When the piston is at its lowest point in the cylinder (BDC), one cubic foot of air at atmospheric pressure (14.7 psia) is contained within the cylinder As the piston begins to move upwards (fig. PD-1.2) into the cylinder, the piston begins to increase the pressure of the air and closes the intake valve.

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The piston continues to move thereby reducing the volume of the cylinder. When the piston is at its' highest position top dead center (TDC), the volume of the cylinder has been reduced and the air pressure has been increased. Reciprocating Compressors • Reciprocating compressors were the first of the modern air compressor designs. Reciprocating compressors utilize a piston moving within a cylinder to compress low-pressure air to high pressure. They are available in single-acting and doubleacting configurations. Each of these configurations utilizes a variety of cylinder layouts as shown in figure RC-1.1.

Single Acting • This reciprocating air compressor utilizes automotive type pistons, connecting rods and crankshaft (fig. RC-2.1). • All compression takes place on the top of the piston. Single-acting air compressors are available in either single-stage or multi-stage compression and can either be air-cooled or liquid (water)-cooled. • Single-stage and two-stage are the most common reciprocating compressors utilized in (7 bar) plant air applications. Typical pressure, flow and horsepower ratings are shown below. • Single-Stage pressure, (bar)(1.7 to 8.6) • Flow rating, (m3/min) (0.11 to 0.57) • power, (kW) (0.75 to 3.73) • Two- Stage Discharge pressure, • (7 to 17.2bar) • Flow rating, (0.23 to 2.97)m3/min

• power, (kW)(1 to 22) Double Acting • This type of reciprocating air compressor utilizes a double-acting piston (compression takes place on both sides of the piston), piston rod, crosshead, connecting rod and crankshaft (fig. RC2.4). • Double-acting compressors are available in single and multi-cylinder and single and multi-stage configurations. Single-cylinder, single-stage and double-cylinder, two-stage configurations are the most common types found in industrial applications. • Most double-acting air compressors are available in either water-cooled or aircooled. Typical pressure, flow and horsepower ratings are shown . Rotary screw • Rotary Screw Compressors are the most widely applied industrial compressors in the 40 (30kW) to 500 hp (373 kW) range. • They are available in both lubricated and oil-free configurations. • The popularity of rotary compressors is due to the relatively simple design, ease of installation, low routine maintenance requirements, ease of maintenance, long operating life and affordable cost. •

Selection of the air compressor is only the first step in designing an efficient and reliable compressed air system. • The air exiting the compressor is saturated with moisture and will have compressor lubricants (lubricated compressors only). • Other chemicals that may have been drawn into the compressor intake may also be present. • This contamination is harmful to many processes, pneumatic tools, instruments and equipment. • Air purification equipment, filters, air dryers, breathing air purifiers, monitoring equipment, used alone or in combination will remove these contaminants. • Selection and purchase of the compressor and necessary purification equipment can be easily done based on Standards. FRLs are used in compressed air systems (pneumatic) in various industries for different applications Filter-Regulator-Lubricator's (FRL) are available in different configurations: • individual components • combination filter-regulator ("piggyback") • two-unit modular combinations • three-unit modular combinations • A porous device that removes solid contaminants, such as dirt or metal particles, from a liquid or gas (air); or that separates one liquid from another, or a liquid from a gas. The term filter describes the complete unit ... housing, filter element • An automatic or manual device designed to control pressure, flow, or temperature. • Regulators (fig. FRL1-4) govern line pressure changes and deliver a constant outlet pressure to the downstream air components. • Lubricators: Lubricators (fig. FRL1-5) provide a constant oil-air mixture to lubricate downstream air equipment for reduced friction and wear.