Compression Ratio

The compression ratio indicates how many times we reduce the air volume.

What is it?

When we compress air, we reduce its volume to increase its pressure. The compression ratio tells us by how much.

Formula

$$CR = \frac{P_{abs,final}}{P_{abs,initial}}$$

Or in practical terms:

$$CR = \frac{P_{gauge} + P_{atmospheric}}{P_{atmospheric}}$$

Practical Example

Data:

• Suction pressure: 14.7 PSIA (atmospheric at sea level)
• Discharge pressure: 100 PSIG

Calculation:

$$P_{abs,discharge} = 100 + 14.7 = 114.7 \text{ PSIA}$$ $$CR = \frac{114.7}{14.7} = 7.8$$

The air is compressed 7.8 times.

Why does it matter?

1. Efficiency

CR	Efficiency	Temperature
Low (3-5)	High	Moderate
Medium (6-9)	Moderate	High
High (10+)	Low	Very high

2. Compression Stages

Total CR	Recommended stages
Up to 4	1 stage
4 - 16	2 stages
16+	3+ stages

3. Contaminant Concentration

If atmospheric air has 1 mg/m³ of particles and $CR = 10$:

$$C_{final} = C_{initial} \times CR = 1 \times 10 = 10 \text{ mg/m}^3$$

Contaminants multiply

Everything in the inlet air (dust, water, oil) concentrates by the compression factor.

Typical Compression Ratios

Application	Pressure	Approximate CR
Blowers	0.5 bar	1.5
Low pressure	3 bar	4
Standard industrial	7 bar	8
High pressure	40 bar	40
PET bottles	40 bar	40
Breathing	200 bar	200