Industrial Utility Efficiency    

Compressor Controls

It was early summer, the air compressors were above the production floor on a mezzanine, and temperatures were heating up both outdoors and indoors. The compressed air system was comprised of three 500-horsepower centrifugal air compressors, and one 350-horsepower variable speed drive oil-free rotary screw air compressor.
The Technical Director of a large facility in the Mid-West (producing valves and meters) hired us to assist with a Site Energy & Utility Systems assessment. The compressed air system quickly became one of the main issues identified. The plant was built in the 1960’s and had expanded over the years. Recent reduction-in-force programs (to reduce costs) had impacted the maintenance department and the plant air system was high among the systems that were the most affected. Over the next several months, as management became attuned to all of the air system issues affecting utility costs and process quality concerns, resources were provided and significant improvements occurred.
TIGG Corporation, a manufacturer of activated carbon adsorption vessels, custom air receivers and other steel tanks and pressure vessels, substantially reduced its energy costs after implementing equipment, labor consolidation and procedural changes resulting from a compressed air energy audit. The audit was performed at TIGG's 155,000 square feet manufacturing facility in Heber Springs, Arkansas to determine the efficiency of the existing compressed air system and to set a baseline for TIGG's participation in Entergy Arkansas’ Large C&I Custom Incentive Program.
Not long ago most air compressors were controlled with mechanical pressures switches, relays and gauges. The setup of these units, especially when attempting to coordinate multiple compressors could be a frustrating and fruitless experience because often, no sooner than the controls were correctly adjusted, some sort of mechanical gremlin would throw something out of adjustment again.
It is common to see energy assessment specialists treat centrifugal compressors like positive displacement compressors when attempting to reduce compressed air system energy consumption. Unfortunately, centrifugal compressors are normally much larger, and miscalculations can easily represent hundreds of thousands of dollars in overestimated energy savings. These errors are not malicious; they result from oversimplified best practices perpetuated by individuals with limited centrifugal compressor knowledge. This type of knowledge is not readily available and most energy assessment specialists do not have access to engineering teams responsible for the technical development and design of centrifugal compressors.
This article reviews portions of an audit report commissioned to survey the condition of a compressed air system in a factory located in the U.S. The objective of this study is to determine the current operating conditions and make recommendations for improvement based upon application of industry recognized best practices. Due to article space limitations, this article will focus on portions of the over-all audit report provided to the factory.
Energy conservation measures (ECMs) associated with compressed air have received a significant amount of attention over the years, mostly due to a reasonably short financial return compared with other energy-consuming equipment. Over time, many of the recommended corrective actions to reduce compressed air energy consumption were simplified so much that they did not lead to positive results. One of the most common compressed air ECMs is reducing system pressure, and it leverages the best practice calculation —.5 percent power per psi — outlined in the Department of Energy’s Compressed Air Challenge. This article highlights more common issues associated with estimating energy conservation resulting from changing system pressure.
The facility generates compressed air with six (6) air compressors of various sizes located in three different compressor rooms. In recent years, the entire system converted to desiccant air dryers. Available equipment from other facilities has been utilized. The following is the layout of the facility showing the locations of the compressor rooms.
This plastic extrusion factory spent an estimated $180,711 annually on energy to operate the compressed air system at their Midwestern facility. Based on the air system operating 8,760 hours per year, the group of projects recommended below could reduce these energy costs by an estimated $116,520 or 67% of current use. Estimated costs for completing the recommended projects total $20,100. This figure represents a simple payback period of 2 months.
This facility operates one of the largest compressed air installations in industry, with 21 4000 hp process air and five 750 hp instrument air centrifugal compressors to support large-scale aerobic fermentation and related processes that require large volumes of oxygen.  
Energy management requires accurate and repeatable measurement of critical data, which is easily monitored and analyzed as required to stimulate required action. When a compressed air system assessment is implemented, the basic minimum measurement protocol to establish the baseline (pre‐measurement) and qualify and quantify the results (post‐measurement)