Industrial Utility Efficiency    

Compressor Controls

Load-sharing is an important part of a multiple centrifugal-compressor master control system. It minimizes blow-off based on the available turn-down. In addition, remote start-stop saves more energy if load floats between different ranges. Finally, adding a screw compressor and implementing a hybrid control system might save the most energy and provide the best back-up. In any case, a well-instrumented system allows engineers and operators to assess, optimize and tune the system.
The introduction of rotary screw air compressors controlled by variable speed drives (VSDs) is one of the best energy efficiency innovations introduced to the industry in the past few years. This style of compressor control can significantly reduce the energy wasted by compressors running in the unloaded condition. But the type of VSD control offered by various manufacturers can differ, and some of these differences can affect the efficiency of the system. This article discusses some little known tweaks to VSD compressor control, including some using hidden features that can sometimes be implemented to enhance the savings gained by the installation of this type of compressors.
In this article, Chad Larrabee from Ingersoll Rand writes about today’s status quo in most air compressor rooms – a group of air compressors all running off their individual controllers with different control schemes attempting to coordinate them. Larrabee then describes the advantages of a smart system controller, which can direct " compressors to respond to one common signal … dynamically matching compressed air supply with demand.” He concludes by outlining the benefits of remote connectivity and automated alerts for maintenance staff.
Compressed Air Best Practices® Magazine recently caught up with Rick Stasyshan, the Compressed Air and Gas Institute’s (CAGI) Technical Consultant, and John Kassin of Cameron to discuss variable inlet guide vanes (IGV). The following interview describes how centrifugal compressor efficiency can be improved thanks to recent developments in IGV technology.
As readers of this publication know, there are many ways to save energy in industrial compressed air systems. One common supply side technology is the variable frequency drive (VFD) of the compressor. It is well-documented that positive-displacement compressors with VFDs provide cost-effective savings in comparison to inlet modulating, load-unload, and variable displacement control.
Quite a number of worst-case compressed air scenarios have been encountered over the years but none may compare to the conditions that existed in a metal foundry somewhere in North America. For reasons you are about to discover, we will not reveal the name of this factory or its location, in order to protect the innocent from embarrassment.
Production complains about frequent work stoppages due to air supply related problems. It wants a more reliable consistent source of compressed air. Maintenance says it will need to replace an older compressor with a new one to improve the reliability and stability of the system. Maybe purchase a bigger one than currently needed in anticipation of future increases in air demands. Management wants assurances a good return on the investment will be realized from the expenditure before making a financial commitment. For comparing and evaluating alternatives, a benchmark must be established to determine the cost to run the current system. An assessment must be performed to identify the saving’s opportunities and assign dollar values. Questions about the cost of the assessment and what is to be expected in return need to be answered.
EnergAir’s unrivalled expertise in compressed air management is helping to save in excess of $50,000 per year at Whirlpool Corporation’s Ottawa, Ohio production facility. Whirlpool is the largest global manufacturer of home appliances and employs almost 70,000 people in more than 60 production and technology centres around the world. The Whirlpool plant in Ottawa manufactures a market-leading range of trash compactors, chest freezers, upright freezers and refrigerators.    
This is a food processing plant where processes and standards are controlled by FDA to AIB standards. Annual plant electric costs for compressed air production, as operating today, are $116,765 per year. If the electric costs of $3,323 associated with operating ancillary equipment such as dryers are included, the total electric costs for operating the air system are $120,088 per year. These estimates are based upon a blended electric rate of $0.085/kWh.
Specifying a control valve for Pressure/Flow Control service should be a relatively straightforward process. The range of compressed air flow and pressures must be known along with the target delivered air pressure. With this information, the performance specifications published by the various suppliers can be referenced for selecting a valve package. Often, however, the valve selected is too large in size to ensure it will have sufficient capacity to satisfy current and future flow requirements. Valve manufacturers use different design criteria in rating their units and in how their product is presented. There are no standards.
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.