Actual P-V Diagram for Single-Stage Compressor

Theoretical p-V diagram  –  (1-2-3-4)

Theoretical inlet valve should open at point ‘4’ but in practice it is not opening at point ‘4’, due to the following two main reasons:

• Pressure difference is required across the inlet valve in order to move it.
• Inlet valve inertia
  • So negative pressure difference is required to open the valve and called as intake depression.
  • Similar situation occurs at 2, at the beginning of delivery stroke. Compressed air is delivered into receiver and hence external delivery pressure is also known as receiver pressure.
  • Gas inertia and turbulence also affecting the inlet and delivery.

Actual p-V Diagram of for Two Stage Compressor

• Inertia and friction effects, which results in valve flattering and increase the area of diagram slightly and hence their effects is to increase the total work of compression.
• LP and HP diagram overlap due to pressure drop in intercooler.
• Wavy lines during induction and delivery strokes are due to “Flatter” of valve’s disc.

Reciprocating Compressors

• Greater vibration problem.
• Mechanical efficiency is lower compare to rotary compressor because of several sliding and bearing members.
• Pressure ratio per state is 5 to 8.
• Delivery volume is small (250 – 300 m³/min) compared to rotary compressor (2000 – 3000 m³/min).
• Compression efficiency is high at compression ratio above 2.
• Reciprocating air compressors are having isothermal compression where as Rotary air compressors are having isentropic compression.

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