It’s safe, simple to use, easy to generate, and common to nearly all industrial plants. It’s also one of the most expensive sources of energy. We’re talking about compressed air. Consider its potential to be a sizeable electric load and the availability of tools and services to manage it. Are you getting the most out of your compressed air system?
The Cost of Compressed Air
When you look at the ½” or ¼” tubing leading to a process valve, it doesn’t seem like it would cost much to operate. However, the efficiency of compressed air can be as low as 10-15%. For example, imagine you need to pump the floor drain sump in a hazardous area of your plant. You choose an air operated sump pump since it’s simple and safe. If the pump is rated at 2 HP for a standard 100-PSI air supply, it will take 14-16 HP of electrical power to generate the air needed to operate the pump.
There’s a standard equation for determining the cost of running a traditional industrial compressor (see Figure 1). A general rule is that a compressed air system will cost the same amount to run the first year as it did to purchase the compressor and equipment. Thus, a $50,000 compressor and dryer may tally a similar $50,000 energy bill the first year.
Rather than immediately spending money on new equipment, consider older industrial compressors. The ones that are tucked away in a utilities room or stored on a mechanical mezzanine often deserve a second look. The same goes for the distribution system. Even down to the fittings and tubing serving the end devices.
Managing Your Compressed Air System
There are several strategies you can implement to save on the cost of operating your compressed air system, depending on your current set up.
Decrease the Pressure Set Point.
This simple strategy might make your production group raise an eyebrow, or even worse, scoff. Take courage; lowering the plant air pressure set point could result in a 1% reduction in system power consumption for every 2 PSI of set point drop (depending on system configuration). Make sure you get buy-in from production and make small adjustments over time. Sometimes the pressure set point is raised for a single component (e.g., an old air cylinder on your oldest machine that only runs in the summer and needs 110 PSI while the rest of the plant only needs 80 PSI). Sometimes replacing one needy component can lead to significant savings.
Eliminate or Isolate Unused Distribution Lines
Supply headers or unused branch lines that are still live are just opportunities for leaks and moisture entry. Valve them off or disconnect the unused lines.
Perform an Air Audit
Groups like Interstates Service Group have the tools and experience to conduct an air audit for your plant. Leaking connectors, piping, etc., can add up to an expensive drain on your system. Let experts use their equipment to find and tag issues – then either they can fix them or turn the list over to your maintenance group. Leaks the equivalent of 1/16” can cost as much as $500 a year.
It is surprising how many leaks turn up in an audit when the right tools are used. Here is an example from the US Department of Energy of the kind of findings and savings a plant-wide air audit can turn up.
A chemical plant undertook a leak prevention program following a compressed air audit at their facility. Leaks, approximately equivalent to different orifice sizes, were found as follows: 100 leaks of 1/32″ at 90 psig, 50 leaks of 1/16″ at 90 psig, and 10 leaks of 1/4″ at 100 psig. Calculate the annual cost savings if these leaks were eliminated. Assume 7000 annual operating hours, an aggregate electric rate of $0.05/kWh, and compressed air generation requirement of approximately 18 kW/100 cfm.
Cost savings = # of leaks x leakage rate (cfm) x kW/cfm x # of hours x $/kWh
Using values of the leakage rates from the above table and assuming sharp-edged orifices, cost savings include:
- 1/32″ leaks = 100 x 1.5 x 0.61 x 0.18 x 7000 x 0.05 = $5,765
- 1/16″ leaks = 50 x 5.9 x 0.61 x 0.18 x 7000 x 0.05 = $11,337
- 1/4″ leaks = 10 x 104 x 0.61 x 0.18 x 7000 x 0.05 = $39,967
Total cost savings from eliminating these leaks = $57,069
Note that the savings from the elimination of just 10 leaks of 1/4″ account for almost 70% of the overall savings. As leaks are identified, it is important to prioritize them and fix the largest ones first.
If your system has 2 or 3 large compressors, it’s possible that you continually run one of your compressors at partial load. That extra-large compressor running (if it doesn’t adjust power input to match your supply needs) is still consuming a full appetite of electrical power. By adding storage to the system in the form of a receiving tank, you can run that large compressor at full capacity for a longer period to fill the storage vessel. Then, let the compressor shut off while the air system is fed from the storage vessel.
Install a Variable Speed Swing Compressor
That same system with 2 or 3 large compressors can benefit from the addition of a small swing compressor. Usually these compressors have VFD control and use a control system to handle partial loads. The savings come in by keeping the large single speed compressors off until they can run at near-full capacity.
This information just scratches the surface on maximizing the value of compressed air. There are many resources available if you want to dig into the technical side of your compressed air system. Click here to learn how Interstates can help you.
Eric Oordt, Interstates Project Engineer