INTRODUCTION TO COMPRESSOR:
Compressors are work-absorbing devices that are used for increasing the pressure of the fluid ( Air, oil, Refrigerant ) at the expense of work done on fluid. The compressors used for compressing air are called air compressors. Compressors are invariably used for all applications requiring high-pressure air. Some of the popular applications of compressors are, for driving pneumatic tools and air operated equipments, spray painting, compressed air engine, supercharging in internal combustion engines, material handling (for transfer of material), surface cleaning, refrigeration and air conditioning, chemical industry, etc. Compressors are supplied with low-pressure air (or any fluid) at inlet which comes out as high-pressure air (or any fluid) at the outlet. Work required for increasing pressure of air is available from the prime mover driving the compressor. Generally, electric motor, internal combustion engine or steam engine, turbine, etc. are used as prime movers.
Reciprocating Compressor :
A reciprocating compressor is a positive-displacement machine that uses a piston to compress a gas and deliver it at high pressure. Various compressors are found in almost every industrial facility.
Reciprocating compressors have been the most widely used for industrial plant air systems. The two major types are single acting and double acting, both of which are available as one or two-stage compressors. The Single acting cylinder performs compression on one side of the piston during one direction of the power stroke. Two-stage compressions reach the final output pressure in two separate compression cycles, or stages, in series.
The double-acting compressor is configured to provide a compression stroke as the piston moves in either direction. This is accomplished by mounting a crosshead on the crank arm which is then connected to a double-acting piston by a piston rod. Distance pieces connect the cylinder to the crankcase. They are sealed to prevent the mixing of crankshaft lubricant with the air, but vented so as to prevent pressure built up.
Types of gases compressed include the following:
- Air for compressed tool and instrument air systems
- Hydrogen, oxygen, etc. for chemical processing
- Light hydrocarbon fractions in refining
- Various gases for storage or transmission
- Other applications
SINGLE-CYLINDER RECIPROCATING COMPRESSOR
Piston compressors are available as single or double-acting, oil-lubricated or oil-free with different numbers of cylinders in different configurations. With the exception of really small compressors with vertical cylinders, the V configuration is the most common for small compressors. On double-acting, large compressors the L type with vertical low-pressure cylinder and horizontal high-pressure cylinder, offer immense benefits and is why this the most common design. The construction and working of a piston-type reciprocating compressor is very much similar to that of an internal combustion engine.
Parts Of Reciprocating Compressors :
Piston type compressor consists of cylinder, cylinder head, and piston with piston rings, inlet and outlet spring-loaded valves, connecting rod, crank crankshaft and bearings.
Working of Reciprocating Compressor
Compression is accomplished by the reciprocating movement of a piston within a cylinder. This motion alternately fills the cylinder and then compresses the air. A connecting rod transforms the rotary motion of the crankshaft into the reciprocating motion of the piston in the cylinder. Depending on the application, the rotating crank (or eccentric) is driven at constant speed by a suitable prime mover (usually electric motor). Schematic diagram of the single-cylinder compressor is shown in Figure
Inlet stroke: –
Suction or inlet stroke begins with the piston at the top dead center (a position providing a minimum or clearance volume). During the downward stroke, piston motion reduces the pressure inside the cylinder below the atmospheric pressure. The inlet valve then opens against the pressures of its spring and allows air to flow into the cylinder. The air is drawn into the cylinder until the piston reaches a maximum volume position (bottom dead center). The discharge valve remains closed during this stroke
During compression stroke piston moves in the opposite direction (Bottom dead center to top dead center), decreasing the volume of the air. As the piston starts moving upwards, the inlet valve is closed and pressure starts to increase continuously until the pressure inside the cylinder is above the pressure of the delivery side which is connected to the receiver. Then the outlet valve opens and air is delivered during the remaining upward motion of the piston to the receiver.
Multistage Reciprocating Compressor :
It is not always desirable or possible to achieve the required rise in pressure in a single compression stage. In multistage gas discharge from the first stage is cooled in the intercooler up to a suction temperature of the first stage before going to the second stage. This is called as perfect intercooling.
Parts Of Multistage Reciprocating Compressor :
Fig shows various parts of three-stage (V type) reciprocating air compressor with the receiver (air tank). The pressure switch is connected to the electric motor. When the desired pressure in the air tank is reached it stops the motor and hence the compressor. The safety valve opens when the pressure in the air tank exceeds the set safe pressure.
Advantages of multi staging:
1. Good volumetric efficiency as compression is done in more than one stage and hence compression ratio is controlled.
2. Lower discharge temperature and hence selection of the material of construction for cylinder and its components and results in smaller size of subsequent stages.
3. Reduced work of compression, as due to intercooling, compression is closer to isothermal (gives rise to minimum work of compression). This results in to saving of power and smaller sizes of subsequent stages.
4. Limits pressure differential. This reduces excess strains in the frame.
The drain valve drains the condensate produced at the condenser and the receiver. Cylinders and intercoolers are either air-cooled (with fins) or water-cooled (with water jackets in the cylinder). The air-cooled compressor is used for low-pressure applications and water-cooled compressors are used for high
Range: Used of pressures up to 4-30 bar and low delivery volumes (< 10000 m3/h). For pressures exceeding 30 bar multi-stage compressors are required. The multi-stage compressors are available with pressure up to 250-350 bar.
Advantages of Reciprocating compressor
1. Piston type compressors are available in a wide range of capacity and pressure
2. Very high air pressure (250 bar) and air volume flow rate is possible with multi-staging.
3. Better mechanical balancing is possible by a multistage compressor by proper cylinder arrangement.
4. High overall efficiency compared to another compressor
Disadvantages of Reciprocating compressor
1. Reciprocating piston compressors generate inertia forces that shake the machine. Therefore, a rigid frame, fixed to a solid foundation is often required
2. Reciprocating piston machines deliver a pulsating flow of air. Properly sized pulsation damping chambers or receiver tanks are required.
3. They are suited for small volumes of air at high pressures.
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