For convenience, manufacturers usually base the performance of individual impellers on an air test. Figure200-14 represents a typical curve which characterizes a certain impeller design. The vertical axis is usually called the head coefficient m; and the horizontal axis is called the flow coefficient, f . (See Section212 for definitions of m and f). In this way, impeller performance data are concisely cataloged and stored for use by designers. When a compressor is originally sized, the designer translates the wheel curve data into ACFM, discharge pressure, and RPM in wheel-by-wheel calculations to select a set of wheels that satisfy the purchaser’s requirements.
Theoretically, an impeller should produce the same head, or feet of the fluid, regardless of the gas weight. However, in practice, a wheel will produce somewhat more head (than theoretical) with heavy gases, and less with lighter gases. Gas compressibility, specific heat ratio, aerodynamic losses, and several other factors are responsible for this deviation. Manufacturers should apply proprietary correction factors when the effect is significant. This effect contributes to variance from the wellknown fan laws or affinity laws. (See the next sub-section.)
Notice in Figure 200-14 that the heavier gas causes surge at a higher Q/N, that is, it reduces stability. The opposite is true of a lighter gas. Similar non-conformance can sometimes be observed when the wheel is run at tip speeds considerably higher or lower than an average design speed. The higher tip speed would surge at higher Q/N, and the lower tip speed would surge at a lower Q/N.
Figure 200-15 illustrates the effects of using movable inlet guide vanes. Notice that as the head or discharge pressure is reduced, the surge volume (defined by the dashed line) is also reduced. The effect is similar to that of speed reduction on a variable speed machine. Inlet throttling, although less efficient, will produce similar curves.
Centrifugal compressors recognize actual inlet cubic feet per minute (ACFM at inlet conditions, or ICFM). Performance curves are most commonly plotted using ACFM. This means that a curve is drawn for a specific set of suction conditions, and any change in these conditions will affect the validity of the curve.
Performance curves often plot discharge pressure on the vertical axis, and flow (ACFM) on the horizontal axis. To estimate performance for varying suction pressures, the curve should be converted to pressure ratio on the vertical axis. This can be done by dividing the discharge pressures on the vertical axis by the suction pressure on which the original curve was based. The effect of a small variation in suction temperature can be estimated by using a ratio of absolute temperatures with the original temperature in the denominator. This ratio is used to correct the inlet capacity on the X-axis by multiplying inlet capacities by the temperature ratio.
For a rough estimate for molecular weight changes of less than 10%, the pressure ratio on the curve can simply be multiplied by the ratio of the new molecular weight over the original. Unless there are gross changes in the gas composition causing large changes in specific heat ratio, this estimating method will only have an error of 1–2% for pressure ratios between 1.5 and 3. For more accurate estimates, a curve with polytropic head on the vertical axis must be obtained.
Remember that any change that increases the density of the gas at the inlet will increase the discharge pressure and the horsepower. Also, the unit will tend to surge at a slightly higher inlet volume.