Costs and Energy Efficiency

Compressed air is one of the most expensive utilities in industrial facilities. Understanding costs is essential for optimization.

The Energy Reality

Efficiency Loss

Critical Fact

8 electrical HP = 1 pneumatic HP

Approximately 80% of compression energy is lost as heat.

$$\eta = \frac{E_{useful}}{E_{input}} \approx 20\%$$

Why Use Pneumatics Then?

Despite efficiency losses, pneumatics offers:

• Safety in hazardous environments
• High power-to-weight ratio
• Clean operation
• Simple and reliable equipment
• Easy distribution

Operating Cost Calculation

Basic Cost Formula

$$\text{Annual Cost} = \frac{HP \times 0.746 \times \$/\text{kWh} \times \text{hours} \times \text{days}}{\eta_{motor}}$$

Example: 50 HP Compressor

• 24 hours/day, 365 days/year
• $0.08/kWh
• 91% motor efficiency

$$\text{Cost} = \frac{50 \times 0.746 \times 0.08 \times 24 \times 365}{0.91} = \$28{,}725 \text{ USD/year}$$

Cost per CFM

For the above example at 250 CFM output:

$$\text{Cost per CFM} = \frac{\$28{,}725}{250} = \$114.90 \text{ per CFM/year}$$

Cost per PSI of Pressure Drop

$$\text{Cost per PSI} = \frac{\text{Annual Cost}}{P_{operating}} = \frac{\$28{,}725}{100} = \$287.25/\text{year}$$

Main Cost Factors

1. Leaks

Leaks are the #1 source of compressed air waste.

Leak Size	Air Loss @ 100 PSI	Annual Cost*
1/32"	1.5 CFM	$172
1/16"	6.5 CFM	$747
1/8"	26 CFM	$2,990
1/4"	104 CFM	$11,960

*At $0.08/kWh

Alert

A typical plant has 20-30% leaks!

2. Artificial Demand

Higher system pressure = higher consumption:

$$\Delta E \approx 0.5\% \text{ per PSI increase}$$

Every 2 PSI reduction in system pressure = ~1% energy savings

3. Pressure Drop

Component	Typical ΔP	Annual Cost (1000 CFM)
Dirty filter	5 PSI	$1,435
Dryer	5-15 PSI	$1,435-$4,305
Undersized piping	5-10 PSI	$1,435-$2,870

4. Inappropriate Uses

Some compressed air uses are extremely wasteful:

Application	Efficiency	Alternative
Cooling/blowing	4%	Fans, blowers
Vacuum generation	10%	Dedicated vacuum pumps
Agitation/mixing	5%	Mechanical mixers
Personal cooling	3%	Fans

Energy Optimization Strategies

Quick Wins

1. Repair leaks - Implement detection and repair program
2. Reduce pressure - Lower system pressure to minimum required
3. Turn off when not in use - Install solenoid shut-off valves
4. Replace filters - Monitor and maintain ΔP

Medium-Term Improvements

1. Regulate at point of use - Don't run entire plant at highest pressure
2. Upgrade to efficient dryers - Cycling refrigerated or blower desiccant
3. Heat recovery - Use compressor heat for space/water heating
4. Control optimization - Match compressor output to demand

Long-Term Investments

1. Variable speed drives (VSD) - Match compressor speed to demand
2. System redesign - Optimize piping, storage, zoning
3. Compressor upgrade - Modern compressors are more efficient

Heat Recovery

Since 80% of input energy becomes heat, recovering it makes sense:

$$Q_{recoverable} \approx 0.8 \times P_{input} = 0.8 \times 37.3 \text{ kW} \approx 30 \text{ kW} \approx 102{,}000 \text{ BTU/h}$$

Applications:

• Space heating
• Water heating (up to 140°F)
• Process heating
• Boiler water preheating

Heat recovery can reduce overall facility energy costs by 50-90% of compressor operating cost.

Key Metrics to Monitor

Metric	Formula	Target
Specific Power	kW / 100 CFM	18-22 @ 100 PSI
Leak Rate	Unloaded CFM / Total CFM	< 10%
Pressure Differential	$P_{supply} - P_{use}$	< 10 PSI
Load Factor	$t_{loaded} / t_{total}$	70-90%

Summary: The Real Cost of Compressed Air

Cost Component	% of Total Lifetime Cost
Energy	76%
Equipment	12%
Maintenance	12%

Energy dominates lifetime costs - even small efficiency improvements pay dividends.

Remember

The cheapest compressed air is the air you don't use. Reduce demand first, then optimize supply.