*4.1. Operational Factors*

The need for compressed air is primarily defined by end-users' requirements in terms of:


The CAS performance and e fficiency do not rely exclusively on such primary factors. Yet, primary factors are strictly interconnected to several secondary ones: among these, we can find heat and thermal capacity, linked to the air temperature, power, work, but also volume, density and mass flow rate of air, directly connected to its pressure and flow rate.

#### *4.2. Economic and Energetic Factors*

*Pay-back time*. Pay-back time has been widely recognized as an easy ye<sup>t</sup> indicative factor supporting industrial decision-makers with limited resources [7,118].

*Initial expenditure*. Regardless of the type of investment [119], the initial expenditure is a crucial factor and may represent a major hurdle hindering EEMs adoption, especially among SMEs, due to their limited capital availability [120,121].

*Energy savings*. The amount of saved energy is a critical indicator of savings stemming from the adoption of an EEM [7] and it refers to monetary quantification of the physical energy source (either primary or secondary).
