With energy cost increases averaging 7.5% per annum during the past few years, most facility managers and maintenance professionals seek projects that not only offer the best return on investment, but help drive operating costs in the opposite direction. Compressed air is an area where such improvements are readily available. But compared to traditional utilities such as lighting, chilled water, steam and HVAC, there’s limited knowledge of energy conservation in this venue.
Facilities generally operate with a “keep it running” mind-set and maintenance supports it at almost any cost. Fear provokes this approach. Fear flourishes in the absence of knowledge. Most production departments have no accountability for the cost of air or the consequences of their actions. At 7 cents per kilowatt hour, producing 100 scfm at 90 psig during 8,500 hours of air usage per year costs more than $13,750 annually in electricity alone. Adding more compressed air users should be considered a business decision.
Despite this fact, most plants freely allow anyone in production to add to compressed air consumption. Production equipment is evaluated on the basis of initial cost and workability with little to no interest in utility requirements or life cycle operating cost. Most air systems are configured to handle the peak usage with no consideration given to diversity or transient event management.
Waste in most air systems can be greater than 30% of the total usage. Even in a relatively small system, typical leakage can be 200 scfm, but because it’s a no-see-um, the waste goes unnoticed. Note that 200 scfm is equivalent to 1,496 gpm. If it were water leaking out, you couldn’t ignore it (Figure 1).
Fixing leaks can provide a nonlinear result if you don't control demand.
Do the detective work
Without quantifying the constituents of demand, you’re going to miss between 35% and 50% of any reduction opportunity. Considering this information, an audit that focuses on the air compressor won’t provide the best results and can miss the root causes of most of the energy waste and operating cost issues. Hundreds of companies offer air system audits, but most auditors don’t evaluate the constituents of demand or determine the root cause of any system problems uncovered.
When picking an audit team, focus on capabilities. You want the entire system analyzed, including demand constituents and critical pressures. Look for auditors who have examined a number of systems similar to yours, designed the implementation, demonstrated a positive return on investment and delivered the desired results. The deliverables should include a thorough financial evaluation, not just a look at electricity trends. Your auditors should understand air and how it’s used on the demand side of your system. Review a few of the company’s audit reports and call some of its clients to get their opinions of the auditor team.
Be up on the basics
To understand how a system works is to understand the opportunities. A compressed air system has five functions. It must:
- Compress the air as efficiently as possible.
- Convert a portion of the kinetic energy to potential energy to flat line the usage and control the rate of change in the system.
- Distribute the compressed air efficiently.
- Expand the compressed air to take advantage of the energy to operate the users.
- Have air users that are applied, installed, and operated correctly.
The audit team must evaluate these five system functions to achieve a reasonable solution and return on investment. Even so, most auditors don’t examine signal locations, the impact of differentials, supply system operation, set points or control configuration. These issues determine how well the equipment operates and what the units actually are displacing and why (Figure 2).
When a 200 scfm negative rate of change event (green) begins, the pressure drops. At a lower pressure, you can add either 200 (orange) or 400 (blue) scfm units. The preferred approach is 100 scfm (magenta), which controls the change. The longer it takes to recover, the less energy the system requires.
A compressor train has each compressor configured with its own filtration and drying equipment piped into a manifold in parallel with others. Compressor trains must be evaluated carefully because it’s so difficult to get the equipment to work together. Trains offer particularly great savings opportunities because, in most cases, the compressors are only partly loaded, even when pressure settings can’t be satisfied.
Flow meters used to determine air usage totals can provide inaccurate information and, by themselves, don’t determine what the compressors are doing. Air systems are in a constant state of change. In a positive rate of change, supply exceeds demand. In a negative rate of change, demand exceeds supply. In a neutral rate of change, supply and demand are stable. Evaluating usage based on a positive rate of change indicates the way that the compressors are reacting to demand. The larger the compressors, the higher the flow that will be measured (Figure 3).
The only true indication of demand is when the system is in a negative rate of change, when the pressure is falling. Measure pressure and flow simultaneously to determine the rate of change. Remember, only a portion of the data is useful for determining demand. This is why evaluating signals, differentials and set points is so important, even when you’re using flow meters to audit.