Between every stage inter cooling of gas is done. Now the only way to make the compression more isothermal is, by dividing the process to a number of stages. Jacket cooling of compressor makes the compression polytropic. So it better resembles an adiabatic process. But compression is practically a fast process. Work of compression can be minimized by isothermal compression. ![]() Polytropic compression is neither isothermal nor adiabatic. Specific heat is defined as the heat energy required to raise the temperature of unit mass of substance by unit degree. It can be seen from the indicator card that, work of compression is maximum in adiabatic compression. All the thermodynamic process resembles adiabatic process. For a perfect adiabatic process, process must be very fast. Whatever heat produced during compression is kept inside the gas only, or heat transfer is zero in an adiabatic compression. From the indicator card, it is clear that, work of compression is minimum in isothermal compression. ![]() For a process to be isothermal, the process must be very slow, which is impractical. In other words, it is the compression, keeping temperature of the gas constant. Isothermal Compressionĭuring the process of compression, whatever heat produced is taken away by a cooling medium. The indicator card (PV diagram) above shows 3 types (or processes) of compression possible. This is the limiting factor for more number of stages. Work can be reduced further by increasing number of stages and inter cooling, but as the number of stages increases design becomes complex, constructional cost increases, maintenance cost also increase, which may nullify the effect of work saved during operation. ![]() Hence when comparing with a single stage compressor work can be reduced by inter cooling in a multistage compressor. Now we can see that the work of compression corresponding to the shaded area in the diagram is saved by incorporating an inter cooling between two stages. Here first stage compresses gas from atmospheric pressure to 3 bar and then gas is cooled isobarically (at constant pressure, refer diagram above). Gears used for driving the screws are called timing gears, since they are properly timed to maintain the close-clearance between the screws. One screw is called driving screw which is coupled with a drive, say an electric motor, while the other screw is called as the driven screw, since it is driven by the driving screw. In addition the flow from screw compressor is generally more uniform and has fewer pulsations than the flow from a reciprocating compressor.Ī twin screw compressor consist of two meshing helical rotors mounted on counter rotating parallel shafts that are enclosed within close-clearance casing. Unlike the reciprocating compressors which are also positive displacement machines, screw compressors does not typically require internal suction or discharge valves. ![]() Screw compressors (also called as helical lobe compressors) are positive displacement machines in which gas is being compressed is forced through the casing by two screws. RECIPROCATING AIR COMPRESSOR Screw Compressor
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