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Air Compressors - Sardar Vallabhbhai Patel Institute of PowerPoint Presentation

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On : Dec 09, 2014

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  • Slide 1 - ACTIVE LEARNING PROCESS Prepared By : Keval Doshi (035) -Tilak Patel (032) -Mohit Chavda (027) Guided By : Prof. Jugal M Panchal Branch: Electronic & Communication
  • Slide 2 - Air Compressors -
  • Slide 3 - What are air compressors ? The machine that takes in air or any other gas at low pressure and compresses it to high pressure are called compressors. They are power consuming machines in which mechanical work is converted into the pressure head of air or gas. A compressor which is used for increasing the pressure of air is called air compressor.
  • Slide 4 - Classification of compressor Method of compression Delivery pressure Principle of operation The number of stages The number of cylinder The pressure limit Volume of air delivered Fluid to be compressed
  • Slide 5 - Reciprocating Compressors
  • Slide 6 - The Construction of Reciprocating Compressors Reciprocating compressor compresses the vapor by moving piston in cylinder to change the volume of the compression chamber, as shown in Fig.10-2. The main elements of a reciprocating compressor include piston, cylinder, valves, connecting rod, crankshaft and casing.
  • Slide 7 - Small two cylinder reciprocating compressor.
  • Slide 8 - Principle of Operation The piston is driven by a crank shaft via a connecting rod. At the top of the cylinder are a suction valve and a discharge valve. A reciprocating compressor usually has two, three, four, or six cylinders in it.
  • Slide 9 - The states of the refrigerant in a reciprocating compressor can be expressed by four lines on a PV diagram as shown in Fig.10-4
  • Slide 10 - Line 4-1 The suction valve opens at point 4. As the piston travels toward the bottom dead center, the volume of the cylinder increases and the vapor flows into the cylinder. The pressure inside the cylinder is slightly less than suction line pressure. The pressure difference pushes the valve open on during the suction stroke.
  • Slide 11 - Line 1-2 The suction valve returns to close under the spring force as the vapor pressure across the valve is equalized when the piston has reached the bottom dead center. The piston then changes the moving direction at point 1. The cylinder volume decreases as the piston moves towards the top dead center, raising the pressure inside the cylinder. The shape of the compression line (Line 1-2) is determined by the properties of the refrigerant and the compression exponent
  • Slide 12 - Line 2-3 At point 2, the pressure inside the cylinder has become slightly greater than discharge line pressure. This causes the valve opening allowing the gas to flow out of the cylinder. The volume continues to decrease toward point 3, maintaining a sufficient pressure difference across the discharge valve to hold it open.
  • Slide 13 - Line 3-4 At point 3, the piston reaches the top dead center and reverses direction. At top dead center, as the piston comes to a complete stop prior to reversing direction, the pressure across the valve is equal. So, the discharge valve is closed. As the piston moves towards point 4, the volume increases and the pressure decreases in the cylinder. The gas trapped in the cylinder expands as the volume increases until to point 4. At point 4, the gas pressure inside the cylinder becomes less than the suction line pressure, so the suction valve opens again. The cycle then starts over again. The shape of the re-expansion line (Line 3-4) is dependent on the same compression exponent that determines the shape of the compression line.
  • Slide 14 - Clearance Space and Clearance Fraction In order to prevent the piston from striking the valve plate, a clearance volume must be allowed at the end of the piston compression stroke. Manufacturing design tolerances require this to allow for reasonable bearing wear, which would effectively lengthen the stroke. The space between the bottom and top of the valve assembly adds extra to the clearance volume.
  • Slide 15 - The clearance volume will cause the vapor not being completely discharged after compression. The remaining vapor trapped in the clearance volume will re-expend in the next suction stroke. As a result, the volume of the vapor sucked in by the compressor in each stroke is less than the volume the piston swept through. Here we only consider the actual and the clearance volumetric efficiencies. The actual volumetric efficiency is defined as
  • Slide 16 - Rotary Compressors In a rotary compressor, air or gas is compressed due to the rotation of impeller or blades inside a casing similar to rotary pump.
  • Slide 17 - Types of Rotary Compressors Rotodynamic Compressors -Centrifugal Compressors -Axial Flow Compressors 2. Positive Displacement -Roots Blower -Vane Type Blower Screw Compressor
  • Slide 18 - Comparison between Reciprocating and Rotodynamic compressors. Reciprocating compressor Rotodynamic compressor Maximum delivery pressure can be achieved as high as 1000 bar. Rotational speed is slow Air discharge is not continuous, so a receiver is needed. Maintenance cost is more. Running cost is more. Maximum delivery pressure is upto 10 bar. Rotational speed is very high (may be upto 30000 RPM) Air discharge is continuous so a receiver is not needed. Maintenance cost is less. Running cost is less.
  • Slide 19 - Thank You

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