Compressed-air RFQ: state the flow basis and the pressure at use

Keep the reference conditions beside the flow

Imagine adding an air-operated station to a production line. One sheet gives L/min, another gives L/min(ANR), and a compressor offer gives FAD. The numbers cannot be aligned by changing the unit label alone. First establish what volume each number represents.

SMC’s PFM7 manual gives a concrete example: its reference-display selection distinguishes flow converted to 20°C and 1 atm from flow converted to 0°C and 1 atm.[1] This is a definition for that device, not a rule that every catalogue uses the same conditions. Keep the source, temperature, absolute reference pressure and humidity basis, where applicable, with each value.

Atlas Copco describes free air delivery as delivered air expressed at inlet/free-air conditions and stresses consistent conditions when comparing capacities.[2] Ask for the offered compressor’s test/reference basis and the delivery pressure at which capacity is stated. A free-air equivalent volume is not the actual compressed volume passing through a pipe at operating pressure. Request a checked conversion when the bases differ; do not guess a common meaning for “normal”.

Identify the pressure location and operating state

A pressure at the compressor outlet does not describe every downstream point. Mark the line inlet, treatment equipment, distribution and consuming station on a simple layout. State the pressure needed at the relevant point during use, with the flow and operating condition that go with it.

Label pressure as gauge or absolute. Atlas Copco distinguishes their atmospheric and vacuum reference points, and describes differential pressure between two locations.[3] Keep a pressure drop across equipment separate from either an inlet or outlet pressure. If a sheet only says “0.6 MPa”, ask which reference and location it means before using it in a comparison.

Describe when air is needed and what the air must meet

Give normal demand, short peaks and their duration, and which stations may operate together. Distinguish air used per cycle from flow per minute and record the cycle rate when converting the demand. Atlas Copco’s sizing discussion separates continuous and intermittent consumption.[2] A daily average alone does not describe a brief simultaneous peak.

For an existing line, share available pressure/flow records with the measurement location, instrument basis and observation period. For a proposed station, identify assumptions and equipment-maker data. Ask the supplier what additional information is needed; the total of nameplate maxima and an unsupported diversity factor are not interchangeable demand estimates.

State the intended use and required particle, moisture and oil characteristics, including the applicable measurement basis or customer requirement. Ask where those requirements must be met and what evidence is included. A compressor capacity figure does not establish the outlet quality of an entire dryer/filter/distribution arrangement.

Compare equipment on the same stated duty

Ask for the exact model and configuration, rated flow basis, pressure conditions, utilities and the work included. A compressor’s capacity, a filter’s processing flow and a station’s required flow have different roles; keep their definitions and applicable conditions visible when checking that they fit together.

Have the responsible supplier or system designer review distribution losses, transient demand and the treatment arrangement. This article does not select a receiver size, pipe size, pressure setting or air-quality class. Keep those design decisions and any additional tests with the offered configuration before accepting it.

Send HAM the equipment function, available data sheets, marked supply/use points, demand pattern, destination and questions still open. Agree which measurements or trials will confirm the order requirements. Keep unresolved conditions visible rather than treating an incomplete capacity comparison as an approved installation.