Some people call compressed air the fourth utility (after electricity, water and gas) because it uses so much energy. In most compressed air systems, well over 70% of the electricity powering the system ends up as waste heat. Despite their inefficiency, compressed air systems are important in many manufacturing, healthcare and other environments where they (and steam systems) deliver force without the risk of igniting a fire.
Below is a diagram of a typical compressed air system. Most compressors take about 7 ft3 and compress it into 1 ft3, resulting in very high humidity. Most equipment downstream is used to dry out or filter the compressed air before it is used. Considering the amount of piping and tanks, leaks inevitably happen, requiring the compressor to work harder.
Users may not respect the cost of delivering compressed air. In many factories, I have seen people use compressed air to sweep the floor and have spitball fights. Educate your employees to use compressed air wisely.
Figure 1. Typical Compressed Air System. Source: Certified Energy Manager Training Workbook
Below are three steps to improve the efficiency of compressed air systems pressurized to 100 psi or greater.
Step 1: Reduce Leaks
I once did a compressed air audit at a factory that resulted in a $1.6 million energy savings per year. When I presented it, the plant manager looked at me and said, “Even if you're half wrong, this is still a great savings and the costs will be returned in less than a year.”
But don't take my word for it – just Google this term: “Industrial Assessment Center Database,” which is a Department of Energy (DOE) program that has been doing audits for the past 41 years at 25 universities around the country. Within the IAC database (searchable or downloadable as an Excel spreadsheet), you will find that the top recommendation is to fix compressed air leaks, which requires a relatively inexpensive investment with a quick payback. Among the 15,000 audited facilities in the database, it had the highest implementation rate. Information on how to estimate air leaks is available at the DOE website (https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/compressed_air3.pdf).
The table below provides rough estimates of the losses due to leaks of various sizes. Most people find that paybacks on repairing leaks is less than a year.
Figure 2. Rough Estimates for Energy Loss from a Compressed Air Leak. Source: Certified Energy Manager Training Workbook
Step 2: Reduce the Pressure
The higher the pressure, the more energy required — and the relationship is not linear. An analogy is climbing Mount Everest. The last 2,000 feet is much harder than the first.
Avoiding unnecessarily high pressure also saves energy. In many factories, I have seen compressors set to make 100 psi air when only 40 psi was needed at the application. Of course, some additional pressure at the compressor is necessary to move the air to the destination, but generally your compressor’s exit pressure should be only 10% higher than at a main distribution line near the final destination. Reduce the setpoint of the compressor a little every day until you get a complaint, then bump the pressure back up a few percent. Another rule of thumb is that for every 2 psi you can reduce, you will save 1% of energy used by a compressor operating in the 100 psi range.
Lower air pressures also reduce the flow rate at any leaks. Of course, if you raise the pressure, you magnify the leaks. One of my clients had a mile-long building with a compressed air station in the middle that piped air all the way to the end. When a new process needed roughly 20 psi more than what had previously been supplied, the pressure was increased throughout the plant by 35 psi. We determined that installing a small, dedicated compressor for the new process would return its cost in six months.
Step 3: Optimize Controls and Maintenance
A compressed air system has many parts. Keeping them optimized and maintained are important tasks. For example, we once investigated a system that was discharging compressed air to the sky because a control system had a safety valve set too low. There are many opportunities here that you can research, including sequencing, modulation and other techniques that improve efficiency.
Depending on your location, you may be able to make your compressor work less hard by reducing the temperature of the compressor’s intake air. In places like North Dakota and Colorado where it's routinely cold and dry, you can use outside air for your compressor intake. If you can reduce the temperature by 20 degrees Fahrenheit, you will save about 3.8% based on a ~70-degree inlet air temperature. The table below provides some estimates of the possibilities.
In addition, if you can recover the waste heat off an air compressor, you can save about 2,500 BTU per hour per horsepower. Even if you only recovered 1500 BTU per horsepower, at 100 HP you have 150,000 BTU per hour!
Figure 3. Relationship of Intake Air Temperature and Power.
Eric A. Woodroof, Ph.D., is the Chairman of the Board for the Certified Carbon Reduction Manager (CRM) program and he has been a board member of the Certified Energy Manager (CEM) Program since 1999. His clients include government agencies, airports, utilities, cities, universities and foreign governments. Private clients include IBM, Pepsi, GM, Verizon, Hertz, Visteon, JP Morgan-Chase, and Lockheed Martin. In August 2014, he was named to the Association of Energy Engineers (AEE) Energy Managers Hall of Fame.