Limitations of a Single-Stage Air Compressor:
Refer to the enclosed diagram, the single stage air-compressor is compressing from pressure P1 to Pressure P2, completing the cycle 1234, where 3-4 is the clearance air expansion. Also V1-V4 is the effective swept volume or the effective volume where the fresh air from atmosphere is sucked in. The mass of air flowing through the compressor is controlled by this effective swept volume V1-V4.
If any restriction is placed on the delivery of the air compressor, for example: the discharge valve throttled, then the delivery pressure of the air compressor increases. From the diagram, let us say the new delivery pressure is P5. Then the operating cycle will be 1567, where 6-7 is the clearance expansion of air and the effective swept volume is V1-V7. Thus it is evident that the effective swept volume (V1-V4) is more than (V1-V7). Thus when the delivery pressure of the single-stage air compressor is increased, the effective swept volume is reduced.
If the delivery pressure is further increased (assuming the compressor is so strong to work), the delivery pressure reaches P8, and the compression follows the curve 1-8, where there will be no delivery of compressed air. Thus when the delivery pressure of a single-stage compressor is increased, the mass flow rate also increases.
Since the delivery pressure increases, the associated temperature also increases. Thus the temperature of the air after compression is so high as to cause mechanical problems and the amount of heat is actually the energy loss.
If a single-stage machine is required to deliver a high-pressure compressed air, then it requires
- Heavy moving/working components to compress air to such a high pressure,
- There might be some balancing problems due to heavy moving parts,
- The power requirement for such heavy parts movement is too high,
- There will high torque fluctuations,
- To compensate for the torque fluctuations, a heavy flywheel is required.
- Better cooling arrangements are required, and
- Lubricating oil which does not get vaporized at such high temperatures.