Reciprocating Air Compressor - Case Study Example

 Reciprocating Air Compressor 1. Introduction: This technical report presents the product dissection of a compressor. Compressors could be classified under two categories, namely positive displacement type and dynamic type. According to this the common compressors are the reciprocating compressors, rotary screw compressors, rotary centrifugal compressors. This report intends to discuss the Reciprocating air Compressor in terms of product description, method of operation including the health and safety aspects. The report also gives a description of the components of the Reciprocating air Compressor along with the materials used for the design, design criteria and the methods of manufacturing. Finally a comparison of this Reciprocating air Compressor with respect to the other types of compressors in terms of Cost, Quality and Functionality is to be presented. 2. Description of product: The reciprocating air compressors are the predominant type of compressors used for compressing both air and refrigerant. These reciprocating find critical applications in production facilities like refineries, gas transmission systems, petrochemical plants and many other similar industries. These compressors are used in situations where the flow rate is less and high compression ratios are desired. These reciprocating compressors are found to be more expensive in terms of design, maintenance and operational factors. These are based on the mechanism of positive displacement, where the air pressure is increased by confining the air to a reduced volume. The main displacement element is the piston inside the cylinder . The following figure shows the common type of reciprocating air compressor. Fig. 1. Reciprocating Compressor. These reciprocating air compressors can be either single acting or double acting, according to the number of sides of the piston used for compression. If only one side of the piston is used for compression, then it is called as single acting. If both sides of the piston are used, then it is called as double acting. There are four configurations of reciprocating air compressors, namely vertical, horizontal, horizontal balance – opposed and tandem. According to the National Productivity Council (1993), for a capacity range of 50 to 150 cfm, the vertical reciprocating compressors are used and for a capacity range of 200 to 5000 cfm the horizontal balance opposed reciprocating compressors are used. 3. Method of Operation: The reciprocating air compressor works on the basis of increasing the pressure by reducing the volume of the air / gas. The compressor uses spring loaded valves that operate when the differential pressure across the valve is proper. The inlet valves allow the air inside when the pressure inside the cylinder is less than the intake pressure. The discharge valve lets the compressed air outside when the cylinder pressure is greater than the discharge pressure. The suction filter in the input side filters the dirt particles from the inlet air and provides clean air for compression. The suction filter needs to be of correct size. It should be fine so that it filters fine dust particles and at the same time it should be large enough so that it does not choke up frequently. The dirt particles are attracted towards the filter by a slight coating of lube oil. The main compressing element in a reciprocating air compressor is the piston. The piston has many grooves that serve the purpose of scraping and sealing. The number of grooves depend on the piston diameter and the compression pressure. The air compression is accomplished by the positive displacement of the piston, hence the reciprocating air compressors are often called as positive displacement compressors. Modern compressors are fitted with compression rings rather than groves in the piston. The cylinder head of the air compressor is generally heavy and capable enough to sustain the force of air and the excess heat generated during compression. The cylinder head has provisions for intake valves, delivery valves, intercooler and after cooler pipelines. The cylinder head is connected to the compressor body by proper nuts and studs. This allows proper intake and discharge of air. There may be coolers in an air compression system to reduce the heat generated during the air compression process. The various steps in the compression process include suction, compression, delivery and closure of valves. 3.1. Health and Safety Issues: Health and safety aspects in the reciprocating compressor need to concentrate on efficient cooling arrangements to maintain the temperatures during the compression process. Some of the critical issues that need prime attention are the air intake temperature, pressure drops within the air filter, leakage in the compressor, care for condense removal in filters and coolers, care for vibrations in the cylinder, speed of the crankshaft and maintenance of appropriate pressure levels. 4. Overview of components: The major components of a reciprocating air compressor are piston, suction filter, spring loaded valves ( suction and discharge), cylinder, connecting rod, crankshaft. The overview of these components are shown in the following Fig.2. Fig.2. Components of a reciprocating air compressor. 4.1. Materials: The materials used in the design of these major components of the reciprocating air compressor are critical with respect to the operation and maintenance of the compressor. Generally the suction filter is of impingement or mesh type. These suction filters are placed in a detachable casing. The material selection for casing is plastic or metals of less weight or sometimes aluminum. As the cylinder head is strong and heavy, it is usually made of cast iron so that it can accommodate the pressure and other process requirements. The studs that connect the cylinder head to the cylinder body are made of temperature resistant materials. The piston rod material selection seems crucial in the design of reciprocating compressor. The overall reliability of the compressor depends on the piston material and design. The wear of expensive components can be alleviated by the use of materials like polytetrafluoroethylene (PTFE). The piston rod seal meant to prevent leakage from the compressor are made with a coating of tungsten carbide (Almasi.A. , 2009). This coating improves the packing life of the piston rod seal. The materials of the crankshaft are made of high strength steel to withstand the torsional vibrations. The crankshaft is made of steel forgings. The connecting rod has been made of die forged steel. The bushings are characteristic of steel shell, leaded bronze, galvanic metal (tri – metallic). Steel material is used for the design of crossheads. 4.2. Design criteria: The design criteria for reciprocating compressors need to be based on the reliability, efficiency and maintainability of the compressors says Confederation of Indian Industries (http://greenbusinesscentre.com/documents/compressor.pdf ) . The major design aspects with respect to the design and manufacture of individual components of the reciprocating compressors has been described below. To seal the pressure and to minimize the wear of expensive components disposable piston rings, rider bands are to be used. The positioning of the piston must be such that there is free air movement. The cylinder valve type depends on the machine type. For high pressure – low speed machines, ring type valves are used and for high speed small machines, plate type valves are used. The choice of the valve size depends on the reliability and efficiency requirements of the compressor. Usage of liners inside the cylinders alleviates wearing and provide a renewable surface. Proper choice could be grey cast iron as suggested by Robin S. Wilson (2007). Use of inter stage and after coolers helps in maintaining proper temperature of the fluid being compressed. Drain traps in the form of steam traps to absorb moisture would enable efficient compression. Provisioning of spacers for tight tolerance above the bearings in the crankcases, provides stiffness for even distribution of strain and stress in the crankcase. The crankcases need to be provided with large openings so as to reduce cost of maintenance and time required for maintenance. Using a bi-metallic (Aluminum – Tin) main rather than tri-metallic materials does not need any initial adjustments and can handle higher load capacity. Lubrication at both the ends of the connecting rod can be achieved by an axial channel internal to the rod. Splitting the main bearing along the vertical plane relieves the surface discontinuities in the loaded areas. 4.3. Methods of manufacture: The manufacture of the individual components ( modular construction ) constitutes to the manufacture of the reciprocating compressor. The piston being made by aluminum could be manufactured by forging or die casting. Die casting leads to accurate dimensional tolerances, high strength casting and good surface finish. In general the manufacturing the reciprocating compressors with hyper ethylene would be capable enough of delivering pressures up to 3500 bar. The modern methods of design and manufacture aim at higher reliability, efficiency and safety. API 1 5. Product comparison: 5.1. Comparison with other similar products: The reciprocating compressors could be compared with other compressor in terms of capacity, efficiency, pressure in PSIG or pounds per square inch gauge, noise level in dB, oil carry over, vibration, the amount of air that a compressor can produce in CFM/ACFM/SCFM, or cubic feet per minute and maintenance(Sustainable Energy Development Office, 2002). The following Table 2.1. best describes the comparison of the reciprocating compressors with the rotary and centrifugal compressors. Table 2.1. Comparison of reciprocating compressor with other types. Parameter Reciprocating compressor Rotary compressor Centrifugal compressor Efficiency high Below 60% of full load Below 60% of full load Noise level noisy quiet quiet Size Large Compact Compact Oil carry over moderate Low low Capacity Low to high Low to medium Medium to high Vibration High Low Low Maintenance Wearing parts Not wearing parts Sensitive to air dust No load Efficiency. High Medium Medium Pressure 0.8 to 700 bar 0.8 to 24 bar 0.1 to 450 bar 5.2. Cost/Quality/Functionality comparison: The cost wise comparison of the various compressors shows that the reciprocating compressors (125 psi) are more costly compared to the centrifugal and rotary compressors of same specifications. Again comparing the cost of a 125 psi reciprocating compressor with 1000 psi reciprocating compressor, it can be seen that the 1000 psi reciprocating compressor costs much more than the 125 psi reciprocating compressors. Comparing the functionality, it can be seen from the above table that the efficiency and pressure levels are high for the reciprocating compressor than the other types. Comparing the other functionalities like vibration, maintenance and oil carry over, it can be seen that the reciprocating compressors are not so appreciable. 6. Conclusions: 6.1. Short review of report with final conclusions: The above report has dealt in detail the reciprocating compressors. The description of the product, the operating method and the health and safety aspects have been dealt. Also the major components of the compressor along with the design criteria, methods of manufacture have also been discussed. Finally a comparison of the reciprocating compressor with the other types has been done in terms of the capacity, pressure levels, efficiency, maintenance, noise level and other parameters. 6.2. General final observations: In general it can be seen that the reciprocating air compressor has characteristics that enables it’s use in many industrial environment. The efficiency of this reciprocating compressor has been found to be high compared to other types of compressors. Also the pressure range is found to be more than the other types. Thus these types of compressors could be widely used. References Almasi.A. (2009), “COMPRESSORS AND COMPRESSED AIR SYSTEMS” Electrical Energy Equipment: Compressors and Compressed Air Systems, Confederation of Indian Industries. Manual on Compressors and Compressed Air Systems. http://greenbusinesscentre.com/documents/compressor.pdf National Productivity Council, India. (1993) , Compressors. In: Technology Menu for Efficient Energy Use, Motor Drive Systems (NPC). Robin S. Wilson, (2007) "Reciprocating Compressor: Reliability Improvement Focusing on Compressor Valves", Piston and Sealing Technology, Compressor Optimization Conference, Sustainable Energy Development Office, (200) , Government of Western Australia. Compressed Air Systems. www1.sedo.energy.wa.gov.au/uploads Read More