What is Compression Efficiency ?

The formula for calculating compression efficiency includes:

Compression Efficiency (η_c) = theoretical power required / indicated power

Here is a breakdown of the terms:

1. Theoretical power

This is the power that would be required in a perfect, frictionless, thermodynamically ideal process. The two common theoretical benchmarks include:

• Isothermal power: The energy required if the air temperature remained constant throughout compression. It is the ultimate ideal, requiring the minimal possible energy but being practically unattainable.
• Adiabatic or isentropic power: This is the energy needed if there were no heat transfer to or from the air during compression. It is a more realistic ideal for fast-acting compressors but requires more power than isothermal. Adiabatic power is the most commonly used benchmark for real-world machines.

2. Indicated power

This power represents the actual power successfully transferred to the air, causing its pressure to rise. It is measured by analyzing the pressure-volume relation inside the compression chamber. This value accounts for internal losses such as fluid friction but excludes external mechanical friction and motor losses.

A single compressor power calculation does not exist. Rather, there is a chain of power calculations that describes the flow of energy from the power source to the compressed air. Each calculation measures the power at different stages and accounts for losses.

1. Theoretical power calculation

P_ad = (k/k-1) * P₁ * V̇₁ * [(P₂/P₁) (k-1)/k – 1]

Where:

• P_ad: Adiabatic power
• k: Specific air heat ratio
• P₁: Inlet absolute pressure
• P₂: Discharge absolute pressure
• V̇₁: Volumetric flow rate at the inlet

2. Indicated power calculation

Indicated power = adiabatic power / adiabatic efficiency

3. Shaft power calculation

Shaft power = indicated power + mechanical losses OR 

Shaft power = indicated power / mechanical efficiency

4. Motor power calculation

Motor power = shaft power / (motor efficiency * drive efficiency)

Real-world compression is not perfect. Losses occur due to internal friction within the air, turbulence as the air flows and internal leakage past screw tips and piston rings.

While they sound similar, these two terms measure different factors. Compression efficiency is a specific, internal metric that engineers use. It focuses on what happens inside the compression chamber and measures how effectively a compressor performs the compression work compared to a perfect theoretical ideal. Compression efficiency answers the question, “How well does the machine perform its task of compressing air?”

In contrast, air compressor efficiency focuses on the overall system’s efficiency. It measures the total amount of electrical energy the compressor consumes to deliver a specific volume of air at a target pressure. Air compressor efficiency accounts for all external and internal losses and answers the question, “How much compressed air do I get for the electricity I pay for?”

Each compressor design is engineered to excel in different applications. Reciprocating piston air compressors aim to efficiently achieve considerably high pressures. Their efficiency is linked to the precise operation of intake and discharge valves. Modern designs use advanced materials and engineering to ensure these valves operate with minimal loss, maximizing performance for high-pressure tasks.

Rotary screw air compressors are engineered for industrial reliability and continuous, 100% duty cycle operation. Because the clearances between the rotors can affect the unit’s efficiency, manufacturers design tighter clearances to maximize airflow for the energy used.