Don't err with Air Compressors

Understand what's involved in getting a suitable supply of compressed air

By by David M. McCulloh, Mac Consulting Services

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Figure 1. Reduced capacities in centrifugal compressors can lead to a reversal of flow.

Centrifugal compressors have a characteristic pressure curve, where pressure increases as capacity decreases. This is an important consideration in capacity control, which is further complicated because pressurehead- making ability varies inversely with absolute inlet temperature and resultant mass. This means that, when designed for a given inlet capacity in actual cubic feet per minute (acfm), the mass flow increases inversely with absolute inlet temperature and the pressure generated also increases because of the denser inlet air. So, for a given discharge pressure, the compressor will have “grown.” See Figure 1, which presumes the same inlet temperature to each stage. Cooling water temperatures can further affect the curve. Figure 1 has a control pressure of 125 psig. It also shows a “surge” line, to the left of which, at reduced capacities, a flow reversal can occur and should be avoided.

Generally, plants want to maintain a relatively constant pressure to the system; controls will reduce capacity if the pressure tends to rise above the set point due to decreasing demand. With centrifugal compressors, a common means of capacity reduction is by progressively closing an inlet valve or guide vanes. This causes a pressure drop at the compressor inlet, reducing the mass flow in proportion to the absolute inlet pressure. Because inlet guide vanes impart rotation in a similar direction to the impeller, this provides energy savings compared with a simple butterfly valve. However, as stated, flow reduction must be limited to avoid surge and generally flow shouldn’t go below 80% or 70% of capacity.

The biggest potential energy problem is when further capacity reduction requires opening a blow-off valve, discharging already compressed air to atmosphere. This should be avoided. Some centrifugal compressors can be unloaded at this point to keep from wasting compressed air. Some units also can be run in a load/unload mode. Unloading involves closing the inlet valve and discharging the substantially reduced minimum flow to atmosphere, resulting in an unloaded power of approximately 15%. The bearing designs must be capable of running in this markedly reduced mass-flow condition; any desired change in capacity control should be discussed with the manufacturer to avoid potential problems.

Advantages of centrifugal compressors include:

  • oil-free air delivery
  • generally well packaged and no need for special foundations
  • relatively smooth air delivery
  • relative first cost per cfm or hp improves with size.

Disadvantages include:

  • limited constant discharge pressure capacity control range
  • need for specialized bearings for high rotational speeds and monitoring of vibrations and internal clearances.

At full capacity and 100-psig discharge pressure, operating cost typically runs 16–20 kW/100 cfm.

  • Rotary screw. Many chemical processes require oil-free air, so oil-free compressors are popular. Oil-injected compressors sometimes are used with additional clean-up equipment including specialized filtration — but this adds pressure drop, mandating a higher compressor discharge pressure than otherwise would be needed.

The oil-free rotary screw compressor generally requires two stages of compression, with inter-cooling, for pressures in the 100–150 psig range. Capacity reduction on constant speed compressors by inlet valve throttling is limited to about 80% of capacity, due to increasing discharge temperature. Below this it’s necessary to unload the compressor. Ample storage capacity then is required. Compressors with variable speed drive (VSD) provide more flexibility in capacity reduction.

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