After discussing the operation and working of an air compressor and theoretical P-V diagram, it is necessary to understand the practical P-V diagram and the reasons for compressing air in multiple stages.
Practical P-V diagram:
In my last article, we have seen the P-V diagram and understood the operation of an air compressor. But in practice, the diagram is not so perfect. The figure shown here represents the actual practical P-V diagram of an air compressor. The points, 1234 represents the theoretical diagram. But there are some shaded portions above and below the work done area. It is necessary to give certain explanation for these additional areas which add up to the work done by the compressor.
Referring to the diagram, at point 4, when the clearance air has reduced to the atmospheric pressure, the inlet valve will not open immediately. The pressure drops lower than the atmospheric pressure and the inertia of the valves are overcome by the pressure difference. Thus the valve is forced open by the atmospheric air and it rushes into the cylinder chamber. There is a “valve bounce" and the pressure does not remain constant inside the cylinder. The pressure slightly increases and then decreases after which reaches somewhat steady intake of air. This negative pressure difference is called as the “Intake Depression".
The same occurs at the point 2, where the delivery valve delays to open. The compressed air pressure inside the cylinder of compressor reaches a pressure slightly more than the air receiver pressure. The delivery valve then opens causing a slight decrease in pressure but always above the air receiver pressure enabling the compressor to deliver air. Then it reaches a stable point after some “Valve Bounce" and then reaches point 3 where the delivery valve closes to continue its cycle. Thus in practice, the compressor requires more power to compensate for the additional work done due to the depressions in delayed opening of the valves.
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.
Thus it is clear that a single stage compressor cannot contribute to high delivery pressure demands. In my next article, let us discuss the effects and advantages of multi-staging of an air compressor.