Reducing system pressure to the minimum that is absolutely necessary is frequently the most cost-effective and quickest payback opportunity for energy savings in a CA system. It should be your first step in system optimization.
The goal of a CA system is to deliver air at the lowest appropriate pressure for system needs while supporting spikes in demand with stored CA. However, it is not unusual to find CA systems operating at higher pressures than necessary. This can happen for a variety of reasons, such as inadequate information about end-use requirements, changes to production demands over time, or suboptimal air-storage capacity. Operating your CA system above the minimum necessary pressure is wasteful for three reasons. First, the higher the air pressure needs to be, the more compressor energy it takes to reach appropriate levels. One rule of thumb is that for systems operating at about 100 pounds per square inch (psi), every increase of 2 psi raises input power to the compressor by 1 percent at full flow. The opposite is also true—one DOE case study found that a pressure drop of 2 psi resulted in a 1 percent reduction in power. Second, for unregulated CA end uses, the volume of air consumed depends on the air pressure—as pressure gets higher, more air is consumed, so CA requirements and energy costs can often be reduced significantly without affecting performance by simply reducing system pressure. Similarly, the higher the system pressure, the more air is driven through the leaks that are common to CA systems. For example, at 80 psi, about 21.4 cubic feet per minute (cfm) of air will flow through a one-eighth-inch diameter leak. At 100 psi, that flow would increase by over 20 percent to 26 cfm, wasting thousands of dollars annually.
Reducing pressure without affecting production processes requires that you be aware of the minimum pressure at which each CA end use can operate. If you find that none of the CA end uses in your plant require the pressure being delivered, you can save energy at almost no cost by dialing back compressor discharge pressure in small increments to the minimum that maintains satisfactory equipment performance.
Sometimes, elevated pressures are maintained to compensate for unacceptable pressure drops that would otherwise occur due to large, intermittent CA consumers on the same distribution system. In such cases, adding secondary storage capacity at or near the point of use is an inexpensive solution to smooth out systemwide pressure fluctuations. Section 5.F of the DOE’s Sourcebook describes how to calculate the volume of secondary storage needed for a particular application.
Another common reason CA systems operate at unnecessarily high pressure is that one or more end uses require it. In such cases, it can often be profitable to install either a booster compressor with local storage or a separate compressor and air distribution system dedicated to these high-pressure end uses. Doing so allows the rest of the plant to operate at lower pressure and can result in dramatic energy savings.
Finally, excessive pressure drop through the components of the air treatment and distribution system can necessitate higher compressor discharge pressure to ensure that the pressure will be adequate by the time it gets to the end use. In a well-designed and -maintained CA system, the pressure at the end use should be at least 90 percent of the initial compressor discharge pressure. Virtually every component of the CA system downstream of the compressor can be a source of pressure drop, such as dryers and filters on the supply side and undersized distribution piping, equipment hoses, disconnect couplings, filters, regulators, or lubricators on the demand side. If you find pressure at the end use to be significantly below 90 percent of compressor discharge, work upstream one component at a time to identify where the major pressure drops are occurring. When specifying or replacing this equipment, always ask manufacturers to provide information on pressure drop at the maximum anticipated flow rate and select equipment that minimizes it. Also, be sure to clean or replace filter elements regularly.
Finding the optimum pressure can also be an iterative process. Some of the other suggestions provided here, such as eliminating leaks, can result in higher air pressures at the end use. It’s important to measure and record air pressure at each end use, both before and after you make any improvements to the system, because you may be able to gain even greater energy savings by further reducing system pressure.