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Study of Reciprocating Air compressor An air compressor is a machine to compress the air and to raise its pressure. The air compressor sucks air from the atmosphere, compresses it and then delivers the same under a high pressure to a storage vessel. From the storage vessel, it may be conveyed by the pipeline to a place where the supply of compressed air is required. Since the compression of air requires some work to be done on it, therefore a compressor must be driven by some prime mover. The compressed air is used for many purposes such as for operating pneumatic drills, riveters, road drills, paint spraying, in starting and supercharging of internal combustion engines, in gas turbine plants, jet engines and air motors etc. It is also utilized in the operation of lifts, rams, pumps etc. Single Stage Reciprocating Air Compressor: A single stage compressor consists of a cylinder, piston, inlet and discharge valves, as shown in figure. When the piston moves downwards (suction stroke), the pressure inside the cylinder falls below the atmospheric pressure. Due to this pressure difference, the inlet valve (I.V.) gets opened and air is sucked into the cylinder. When the piston moves upwards (delivery stroke), the pressure inside the cylinder goes on increasing till it reaches the discharge pressure. At this stage, the discharge valve (D.V.) gets opened and air is delivered to the container. At the end of delivery stroke, a small quantity of air, at high pressure, is left in the clearance space. As the piston starts its suction stroke, the air contained in the clearance space expands till its pressure falls below the atmospheric pressure. At this stage, the inlet valve gets opened as a, result of which fresh air is sucked into the cylinder, and the cycle is repeated.
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Working of Single Stage Reciprocating Air Compressor The compressor body is provided with fins to dissipate the heat generated during compression. In a double acting reciprocating compressor, the suction, compression and delivery of air takes place on both sides of the piston. It is thus obvious, that such a compressor will supply double the volume of air than a single acting reciprocating compressor (neglecting volume of piston rod). Multistage Reciprocating Air Compressors: In a single stage air compressor if the compression ratio is increased, the final temperature increases and the volumetric efficiency decreases. In such conditions leakage past the piston starts and high compression requires robust cylinder construction. A high compression temperature also affects the operation of the delivery valves, diminishes the lubricating properties of the oil and increases the risk of ignition in pipe line. For these reasons if a higher compression ratio (above 6 to 8) is needed, the overall compression ratio is subdivided into two or more stages with lower compression ratios. Usually the maximum compression ratio for small single stage compressors is 8 and for large machines it is 5. In practice, delivery pressure up to 5.6 bar in single stage, 5.6 to 35 bar in two stage and from 35 to 84 bar in three stages are used. An intercooler between the two stages cools down compressed air from the first stage before it enters the second stage. The temperature of the cooled air is nearly equal to initial temperature of first stage. By cooling the air in between stages in this way, the compression is made to approach isothermal. Thus the final temperature is appreciably lowered and the work required for compression is reduced. Two-stage Reciprocating Air Compressor with Intercooler: A schematic arrangement for a two-stage reciprocating air compressor with water cooled intercooler is shown in figure. In its operation, the fresh air is sucked from the atmosphere in the low pressure (L.P.) cylinder during its suction stroke at intake pressure P1 and temperature Tl. The air, after compression in the L.P. cylinder (i.e. first stage) from 1 to 2, is delivered to the intercooler at pressure P2 and temperature T2. The air is cooled in the intercooler from 2 to 3 at constant pressure P2 and from temperature. T2 to T3. After that, the air is sucked in the high pressure (H.P.) cylinder during its suction stroke. Finally, the air, after further compression in the H.P. cylinder (i.e. second stage) from 3 to 4, is delivered by the compressor at pressure P3 and temperature T4.
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Two stage Reciprocating Air Compressor Advantages of Multistage Compression: 1. Less power: Less power is required to run a multistage compressor as compared to single stage compressor for the same delivery pressure and the same quantity of free air due to intercooling. For a larger pressure range, the power reduced for drive is about 20 %. 2. Better Mechanical Strength: Due to phasing of operation in two or more stages, a better mechanical balance for machine is attained. 3. Increased Volumetric Efficiency: Volumetric efficiency increases due to lower temperature and lower pressure range. 4. Less Leakage Loss: Due to lower pressure range, there is less leakage loss past the piston. 5. Better Lubrication: Better lubrication is possible due to lower working temperature. 6. Lighter Cylinder: Due to lower pressure range a lighter cylinder serves the purpose. Energy Conservation in Air Compressor: 1. Leaks. Routinely check your system for leaks.
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2. Compressor pressure. If pressure loss in compressor system is greater than 10%, evaluate the distribution system and identify areas causing excessive pressure drops 3. Identify artificial demands. Artificial demand is created when an end use is supplied air pressure higher than required for the application. If an application requires 3.5 bar but is supplied 6 bar, excess compressed air is used. Use pressure regulators at the end use to minimize artificial demand. 4. Inappropriate use of compressed air. Look for inappropriate uses of compressed air at your facility. Instead of using compressed air, use air conditioning or fans to cool electrical cabinets; use blowers to agitate, aspirate, cool, mix, and inflate packaging; and use low-pressure air for blow guns and air lances. Disconnect the compressed air source from unused equipment. 5. Heat recovery. As much as 80 to 90% of the electrical energy used by an air compressor is converted to heat. A properly designed heat recovery unit can recover 50 to 90% of this heat for heating air or water. 6. Inlet air filters. Maintain inlet air filters to prevent dirt from causing pressure drops by restricting the flow of air to the compressor. Retrofit the compressor with large-area air intake filters to help reduce pressure drop.
