Replacement decision in brief
A useful air-consumption figure states the reference conditions, operating pressure, liquid duty and whether it includes the full transfer cycle. Require the same throughput and route assumptions as the existing pump. Avoid comparing an unspecified compressed-air volume with motor power: convert both options to a stated annual energy basis.
Separate actual and reference volumes
A cubic metre of gas at vessel pressure is not the same amount of gas as a cubic metre expressed at a reference pressure and temperature. Terms such as normal cubic metre, standard cubic metre and free-air delivery need their reference conditions stated. Never compare two consumption numbers solely because both use m³/h.
Account for the whole sequence
The gas needed to displace liquid is only part of the system use. Initial headspace pressurization, residual gas released during venting, valve actuators, leakage and any purge can add consumption. A preliminary ideal-gas estimate can support screening, but an actual utility guarantee should identify the cycle, product, operating pressure and measurement boundary.
Connect demand to the compressor
Ask for both average consumption and the short-term peak at the specified pressure. The plant must deliver the peak without falling below the skid requirement. If a local receiver is proposed, its usable pressure band and recharge behavior must be assessed. It does not create compressed air or remove the compressor energy cost.
Normalize a comparison
Use the same reference conditions, product throughput and operating hours for competing proposals. Express consumption per tonne or cubic metre of delivered product as well as per hour where useful. Include any separately supplied equipment that uses gas. During acceptance, meter at the agreed boundary and collect enough complete cycles to include pressurization and venting. Report the conditions with the result.
General engineering guidance. Numerical examples are illustrative; project specifications and acceptance criteria are agreed for the actual duty.