Compressor: Adiabatic and isentropic gas compression

A compressor can be idealized as internally reversible and adiabatic, resulting in an isentropic steady-state device with a 0-entropy change.

Isentropic efficiency of Compressors:

Isentropic compressor work / Actual compressor work

= Ws / Wa = h2s -h1/ h2a-h1 [ h is enthalpy, 1 and 2 are two points]

h2s and h1 are arrived enthalpies. h2a makes the difference

Therefore, the compressor's efficiency only depends on h2a. How much enthalpy eventually it can generate a net of frictional losses would decide its output work. Since there is no entropy loss, Q = 0, . U = - W. It is a challenge for actual users to what extent you can run your compressor internally reversibly. That is the key. It is the same as a Carnot heat engine in how much your machine is reversible.

Background

Two things are required for a compressor to be isentropic, with S = 0, ?[1] It must be an adiabatic process, and [2] it must be reversible. An isentropic process is defined in thermodynamics as an idealized thermodynamic process that is both adiabatic and reversible. This assumes that the system's work transfers are frictionless and that there is no net transfer of heat or matter. Such an idealized process can be used in engineering as a model for and comparison to real-world processes. This idealizes reversible processes that do not exist in reality. Thinking of a process as both adiabatic and reversible would mean that the initial and final entropies are the same, which is why it is referred to as isentropic (entropy does not change).

Why is the compressor thought to be an adiabatic process?

An adiabatic process is one that occurs without the transfer of heat or mass between the thermodynamic system and its environment. Simply put, an adiabatic process is one in which no heat is transferred to or from a system, resulting in Q = 0, and such a system is said to be adiabatically isolated.

In contrast to an isothermal process, an adiabatic process only transfers energy to its surroundings as work. The process of compression in compressors occurs too quickly for energy to enter or leave the system as heat, allowing a convenient "adiabatic approximation."

When it comes to compressors, it is assumed that the compression of a gas within a cylinder of an engine occurs so quickly that little of the system's energy can be transferred out as heat to the surroundings on the time scale of the compression process. Despite the fact that the cylinders are not insulated and are extremely conductive, the process is idealized to be adiabatic. The same can be said about such a system's expansion process.

If the system walls are adiabatic (Q = 0) but not rigid (W ≠ 0) [ that is what an adiabatic system is], and, in an assumed idealized process, energy is added to the system in the form of frictionless, non-viscous pressure-volume. Such a process is called an isentropic process and is said to be "reversible".

What makes it reversible?

If the process in a compressor were reversed, the energy could be recovered entirely as work done by the system. The main reason for this is that there is no heat Q transfer to the surroundings and all heat is contained within the system's four walls. Remember that you cannot reverse the energy in your surroundings. Once it's gone, it's gone for good. While energy can be reversed if it is contained within a system by reversing the process.

How entropy reduces, ΔS = 0

If the system contains a compressible gas and is reduced in volume, the uncertainty of the gas's position is reduced, which appears to reduce the ways the gas can organise itself [reduction in gas microstates] within the reduced volume, which is what the system's entropy is.