Electric motors account for approximately 70 percent of the energy used by industry in the United States. Compressed air equipment consumes a large portion of this energy in many industries, including the chemical industry. By reducing the energy-related compressed air costs, chemical companies can save a significant amount of money.
Many of today's plants have outdated compressed air systems with inefficient equipment, obsolete controls and undersized piping. Such plants could realize as much as 25 percent savings in energy costs through the installation of newer equipment, controls and system components ," potentially boosting facility profitability.
Although numerous energy-improvement programs currently are being touted, many of these programs involve expensive retrofits. These retrofits can achieve significant energy savings, but they often bring with them a payback in the five- to seven-year range. Because most chemical facilities require a payback of less than two years, these types of energy conservation projects might never be implemented.
Plants can achieve real energy savings, however ," and circumvent at least some of the high capital cost expenditures ," through implementation of a five-step approach that stresses the identification of energy-savings opportunities and works within payback restraints. It is very common for plants to save a significant amount of energy without buying new compressed air equipment. This article details each of these steps.
Identify savings opportunities
Compressed air systems are often less understood than other utilities such as electricity, steam and chilled water, so in many plants these systems have been ignored for years. The result is that the compressed air system might be operating at much less than optimum efficiency.
Fig. 1 illustrates the typical life-cycle costs of a compressed air system.
Figure 1. Life-Cycle Costs of a Compressor
* The cost to the customer resulting from lost production can exceed both the capital and energy costs, and can be significant.
Although compressed air is a "mystery" to many engineers, the savings opportunities are huge, with approximately 90 percent of available savings coming from three broad areas, including:
Demand or consumption reduction. This most often is accomplished by controlling leaks and putting an end to improper use.
Compressor operating-pressure reduction.
Continuous monitoring and load management of the system using master control systems.
The first step in identifying savings opportunities calls for a complete review of the entire compressed air system. To accomplish this, the plant first should put together a compressed air team made up of representatives from the facility's engineering and maintenance staff, assuming these plant personnel have sufficient experience with compressed air systems. If significant compressed air system knowledge is not available on-site, the facility should consider hiring a consultant with the necessary system experience.
This team then should implement a complete compressed air system study that includes not only the compressor room equipment, but also the distribution system. In addition, it should take a look at how the compressed air is used.
This study should include an aggressive leak detection/elimination program and investigate existing demand-side system components that could be improved. Even small leaks or inefficiencies in the system can waste a substantial amount of energy. See Fig. 2.
Figure 2. Energy-Savings Opportunities
Specifically, the team should:
Evaluate the demand and energy consumption of compressed air usage.
Check system fittings. Are they the best choice for minimizing leaks?
Check point-of-use pressure regulation and filtration for proper control and pressure drop.
Examine system traps. Are they of a high-enough quality to prevent air loss?
Analyze pressure drop. Are the air dryers and filters optimized? Are pipe restrictions present? Is the piping sized correctly to provide low pressure drop? (See http://air.inger soll-rand.com/AST/piping.htm for some piping tips.) Remember: Each 1 psig in pressure drop is equivalent to as much as a 1/2 percent change in the total energy required. For example, a 4,000-horsepower (hp) compressor system operating 10 psig higher than necessary as the result of excessive pressure drops could be consuming as much as 5 percent, or 200 hp, more than necessary. This means more than $65,000 in wasted energy at $0.05 per kilowatt-hour (kWh) of electric power, 8,500 hours per year operation and 95 percent motor efficiency.
Look at compressor operation and plant needs. Are the compressors operating at their lowest possible pressures? Are they being shut off when not needed?
Compressed air systems normally must operate within a fixed pressure range and deliver a varying air volume, based on demand. Control systems are necessary to monitor system pressure and to automatically decrease or increase compressor output when the pressure reaches a specific level.
By examining the plant's existing control strategy, the team will be able to determine if the approach is really the most suitable ," or if another alternative would make more sense. Therefore, the team should scrutinize all aspects of controller functions, including pressure control, surge/valve control and health monitoring, data transfer and storage, asking:
Is the compressed air demand constant like a process load, or does it vary widely?
Does the compressed air system have a master control system that looks at system demand? Does the control system include load-sharing and start/stop capabilities? Master control systems can offer significant savings with relatively low investment.