Sizing Compressors/Fans

While compressors are not directly sized by ANS, they are able to be indirectly sized by including an estimation of their cost. Their actual behavior is a result of the system. The appropriate sizing of a compressor requires a Cost Library and understanding of standard Compressor/Fan Sizing.

At the most basic, compressor sizing is simply selecting a compressor that is capable of meeting pressure and flow requirements. Too often this involves selecting a compressor that is larger than required, under the assumption that a bigger compressor will provide for operational margin. Unfortunately, over-sizing a compressor has negative impacts on reliability and efficiency, both of which incur higher cost. Over-sizing could also require the installation of ultimately unnecessary valves or restrictions to bring the flow back down to where it should be.

The ability to appropriately size compressors is built into AFT Arrow. By using the Compressor Sizing option, AFT Arrow will indicate the amount of pressure required to deliver the specified flow. It is up to the user to find a compressor that operates at its BEP at the calculated flow and head.

The best size of compressor to use when the pipe sizes are allowed to change as well is harder to handle, and requires iteration. See Sizing Pipes and Compressors Concurrently.

Note: Certain compressor configurations require special attention in ANS. See Special Concerns For Sizing Compressors.

Sizing Control Valves

Control Valve sizing is similar - AFT Arrow already has all of the tools needed to size a Control Valve for a specific set of pipes. AFT Arrow will report the Cv or K Factor necessary for a Control Valve to attain a given setpoint. From this value, an appropriate valve could be selected for minimizing cost. It would be typical to select the smallest Control Valve possible while still remaining in a good control range - typically 20-80% open.

Like compressors, selecting the best Control Valve while pipe sizes are changing is more complex. See Sizing Pipes and Control Valves Concurrently.

Sizing Other Junctions

Any other junction can be included in the sizing process as long as cost data exists.

For example, a Bend can have costs associated with it via a Table of Costs. From this table, ANS can determine the cost for a given diameter. ANS will assume that the Bend is of a diameter equal to the upstream pipe inside diameter, and include the corresponding cost in the Objective. If certain components are very expensive compared to piping, this could affect the selection of pipe sizes.

Sizing Ducts

Duct sizing has special considerations discussed in Sizing Duct Systems.