There are several special concerns with sizing compressors. For a general overview of the process of sizing compressors see: Sizing Pipes and Compressors Concurrently.

End of Curve Flow Rate

Centrifugal compressors typically have an end of curve, or runout, flow rate. They cannot operate reliably past this point. This can be entered in Compressor Configuration, but this will not actually prevent the model from running the compressor with higher flows. Instead, the curve will be extrapolated past this point and a warning will be generated.

If it is important to prevent the sizing results from considering flows in this region as valid. A Design Requirement for maximum flow rate equal to the end of curve value should be placed on the compressor.

Partial Service Duration

Without other input, ANS will assume that any compressor included in the sizing operates constantly.

If this is not the case, then either an appropriate Service Duration or Multiplier should be defined in the Assign Cost Libraries panel.

Variable Speed Compressors with Partial Control Applicability

ANS can size compressors with a user defined fixed speed other than 100%, or a controlled compressor that modifies speed to meet a flow or pressure setpoint. However, pressure-type variable speed controlled compressors allow the user to optionally enforce Control only when the actual pressure is above/below a setpoint. This causes an issue with sizing.

If ANS allowed the compressor to change from a fixed 100% speed compressor (non-controlling pressure) to a variable speed compressor (when the setpoint is violated), it would be changing the characteristics of the system. This has a direct impact on the sizing process, so it cannot be allowed - ANS will always model this type of compressor as fixed 100% speed.

Either an Always Controlling compressor must be specified, or Design Requirements on pressure must be added to the appropriate pipes.

Unrealistic Sizing Results From Dependent Design Cases

If there are two design cases that have different compressor flow rate requirements, the ideal compressor curve may be unrealistic.

For example, one operating case may require 500 ft3/min, and another may require 750 ft3/min. It is desired to select the ideal compressor. After running ANS, it is determined that the compressor should generate 50 psid in the 500 ft3/min case and 70 psid in the 750 ft3/min case. To meet this requirement, we need a compressor curve that increases from 50 psid to 70 psid with a large increase in flow. This is an unrealistic demand.

To prevent this, a pressure rise Design Requirement can be added to maintain the high flow pressure at 70-90% of the low flow pressure. This will effectively force the sizing to consider only realistically shaped compressor curves.