Definition of “standard air” in compressor problems Standard air refers to air at which reference state and relative humidity?

Introduction / Context:
To compare compressors and air machinery consistently, engineers adopt a reference state called “standard air.” Using this common baseline allows fair comparison of volumetric flow rates, power, and performance across different locations and ambient conditions.

Given Data / Assumptions:

• Standard air is used as a conventional reference in many textbooks and problems.
• Humidity is included because the presence of water vapour affects density and thermodynamic properties.

Concept / Approach:
In many engineering curricula and design handbooks, “standard air” for compressor problems is taken as air at 1 bar absolute pressure and 20° C with 36% relative humidity. This selection provides a convenient, repeatable baseline representative of typical temperate ambient conditions.

Step-by-Step Solution:

Verification / Alternative check:
While some industries use other standards (e.g., 1.01325 bar and 15° C with specified humidity, or “standard day” in aerospace at 15° C and 0% RH), the compressor problem convention in many exam syllabi is 1 bar, 20° C, 36% RH.

Why Other Options Are Wrong:

0.1 bar options imply near-vacuum conditions, not a standard atmosphere.40° C options reflect hot ambient conditions, not the typical standard used in classroom compressor calculations.1 bar and 15° C with 0% RH is an alternative standard in some fields, but it is not the stated convention in the present context.

Common Pitfalls:
Assuming “standard” always means dry air at 15° C. Standards vary by field; read the problem statement or institutional convention carefully.

Final Answer:

1 bar and 20° C with 36% relative humidity