A simple reciprocating compressor consists of a Crankcase, crank, piton rod, cylinder, piston and valves. The input power to the reciprocating compressor is in the form of rotary motion. Earlier reciprocating compressors were single acting i.e. gas or air was handled on one side of piston/cylinder. Now double acting is commonly used due to inherent advantage and higher efficiency.
Functioning of a simple reciprocating compressor is explained below with the help of figure and p-v diagram. Ideally speaking one revolution of crank involves four cycles i.e. suction, compression, discharge, expansion
Suction:
When the piston moves towards BDC (bottom dead centre), the pressure within cylinder drops below suction pressure thereby forcing suction valve to open and allows gas in the cylinder. This movement of the piston is plotted as 4 and 1 on the p-v diagram.
Compression:
The movement of the piston from BDC towards TDC (top dead center) start increasing pressure thereby closing the open inlet valve and compressing the trapped gas. This movement of the piston is plotted as 1 to 2 on the p-v diagram.
Discharge:
As the piston approaches TDC, the discharge valve opens due to higher pressure in the cylinder than discharge pipeline, thereby discharging the trapped gas. This piston movement is marked as 2 to 3.
Expansion:
To avoid contact of the piston with cylinder end cover, small gap is maintained between the two. This results in small quantity of gas getting trapped and not discharged. This trapped gas expands in short duration during piston’s reverse movement from TDC to BDC. This expansion of gas is plotted as 3 to 4 on the p-v diagram.
MULTISTAGING WITH INTERCOOLING:
It is not always desirable or possible to achieve the required rise in pressure in a single compression stage. In multistaging gas discharge from the first stage is cooled in the inter cooler up to suction temperature of the first stage before going to the second stage. This is called as perfect intercooling.
Advantages of multistaging:
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 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.
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