Compressed air: a more precious resource than it may seem

Compressed air has two main characteristics: pressure (e.g., 100 psi, or 690 kPa) and dew point (e.g., -40°C).

For example, let's take the typical compressed air found in most industrial facilities. A compressor will consume between 0.12 and 0.20 kW to produce one standard cubic foot per minute (SCFM) of air. This may not seem like much.

However, imagine a leak the size of a grain of rice on a line; it could consume up to 5 SCFM. If we take a system that’s under constant pressure (24 hours a day), this same leak would consume up to 8,760 kWh a year! If we add the air handling fee (a dryer can consume up to 15% of the air used to lower the dew point), we get almost 10,000 kWh.

Now, let's look at two scenarios. Take a grid-connected plant, where the average kilowatt-hour is $0.05, and another plant in a northern environment that generates its own electricity at an average cost of $0.30 per kilowatt-hour.

Our tiny leak will cost the first plant $500 a year and the second $3,525! When BBA team members performed audits using ultrasonic equipment, they detected leaks totalling hundreds of SCFMs at our client sites in the last year alone. We're talking about one or two days of work that led to tens of thousands of dollars in savings!

In addition to the money saved, remember that every kilowatt-hour that isn’t consumed helps lower greenhouse gas (GHG) emissions at client sites, where electrical energy is produced using generators. The leak at the second plant alone could save nearly 2,500 litres of diesel consumption.

Leaks aren’t the only aspect that could be harmful in compressed air systems. Unfortunately, production and handling equipment are often poorly sized or poorly operated. Sizing a new plant in design mode can sometimes be difficult. That’s why it’s essential to review equipment modulation and operation when the plant is running at steady state.

For example, during an audit, we found that a compressor hardly ever produced while still being in “unload” mode. It totalled nearly 25,000 hours in “running” mode and only 1,375 hours in production. It should be noted that the “unload” mode of a compressor consumes less, but still nearly 20% of its rated power. This means that the compressor has been running almost 95% of the time for nothing. For a 50 hp compressor, we're talking about 186,500 kWh wasted or, in this case, $50,000.

Even if compressed air is produced, processed and conveyed in the right way, using it can sometimes be problematic. As mentioned above, this type of energy isn’t free. Only 10% to 20% of the energy used to compress air can be used afterward by expanding it. The rest is just heat being released.

A common example of misuse is liquid agitation. Homemade agitators, with air-permeable pipes to let air through, are highly efficient for agitating liquids but are extremely energy-intensive. One such device used by one of our clients was replaced by a mechanical agitator with an electric motor, saving nearly $400,000 in energy costs.

Furthermore, with the race to lower GHGs, it’ll be that much more important to monitor our energy consumption, which includes compressed air. The phrase “Close the door. We’re not heating the neighbourhood!” holds more truth than ever.

BBA has an extensive project portfolio and a wealth of field experience in designing and optimizing compressed air systems. For more information, contact our team.