Curve Shape

The role of stagger, in developing the curve shape, was just covered; however, the shape deserves a few comments on comparison with other compressors. Even though it is a dynamic compressor, the axial's inherently steep pressure-volume curve makes it more akin to the positive displacement compressor shown in Figure 1-3. The steep curve contrasts with the flat curve of the centrifugal compressor. The horsepower characteristic also contrasts the shape of the centrifugal horsepower curve. Note that while the centrifugal's required horsepower increases with volume, the axial compressor's required horsepower does just the opposite (see Figure 6-8). This unique characteristic should be kept in mind when starting an axial. The axial is unloaded by opening a discharge bypass or otherwise removing the downstream load restriction during startup. This means that the lowest gas load is away from surge, compared to the cen trifugal which may be unloaded by discharge throttling, which tends to bring the machine up to speed in surge.

The steep pressure-volume curve permits the axial compressor to operate very well in parallel with other axial compressors. The pressures do not have to match precisely to permit load sharing, as the steepness of the curve allows for adjustment without danger of going into surge or taking wild load swings as sometimes happens when attempts are made to operate centrifugal compressors in parallel.

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Inlet Volume

Figure 6-8. Typical brake horsepower vs. inlet volume curves for a centrifugal and an axial-flow compressor.

The axial compressor matches up well with a centrifugal compressor in a tandem-driven series connected arrangement, when higher process pressures are needed. The interstage matching is quite easy as the steep curve will provide sufficient changes in discharge volume to easily accommodate the requirement of the centrifugal. It can be thought of as stacking a constant ratio compressor on top of the axial's more vertical curve. With the axial compressor's high volume and speed attribute, the two machines will match in speed and volumes very neatly without the use of interstage gears.

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