Compressed air is too costly to use as a prime mover. Consider the fact that the price of 100-psig air is in the range of 18 cents to 32 cents per 1,000 standard cubic feet of free air. In the automobile industry, compressed air is a significant part of the energy cost, ranging from 10% in component plants to as much as 40% in stamping plants. In a typical Ford plant, this can represent anywhere from several hundred thousand dollars to well over a million dollars per year.
One way to reduce this cost is by applying best practices and a systems approach to improve compressed air system efficiency. Analyzing the case from only the supply side limits the opportunities for improvement. Focus on air user demands because that is what drives system requirements. Concentrating on proper end-use application, design, operation and maintenance ensures higher operating efficiency, lower cost and reduced production losses. Review these aspects of your current air system:
- Consider electro-technology conversion.
- Align supply side with demand side.
- Reduce system pressure.
- Improve maintenance.
- Eliminate inappropriate uses.
- Think in terms of life-cycle cost.
Electro technology conversion
The history of compressed air in the auto industry goes back to Henry Ford’s day. Then, it was a byproduct of electricity production: waste steam from the generator’s turbines powered the steam engine-driven compressors that produced compressed air. Electricity was in its infancy and couldn’t yet duplicate what could be accomplished with compressed air. But times have changed.
Electricity now produces compressed air, and it can take 8 hp of input power to deliver only 1 hp of work where compressed air is being used. At that rate, it’s obvious that it can be more economical to use electricity to drive mixers, dryers and blowers. Even direct-current nut runners are replacing air tools not just because of the energy efficiency but because of increased quality by being able to tie torque feedback to the line operation. The advances in electro-technology now offer many efficient options for replacing compressed air applications.
Aligning supply with demand
System demand drives the supply requirements in any compressed air system. You need to know the true air demands and how to fulfill them using proper compressor operation (number and total horsepower, duration, pressure and flow). Because the system is dynamic, it requires monitoring and controlling both the compressors and air users.
First, develop a pressure profile that quantifies system demand characteristics. Take pressure readings after the main supply components, at the beginning and end of the main distribution system and at several points of use. Spread your readings out over a period of time to establish the high, low and average system demand. The pressure variation you document indicates how the system and compressor react to the demands.
The adage, “If you can’t measure it, you can’t manage it,” applies to establishing your baseline. While temperature and dew point are useful air system measurements, the key metrics are pressure, rate of air flow and electrical consumption. This trio helps to determine the cost, monitor system operation and establish a baseline for evaluating future modifications.
Determine real-time air system efficiency using the flow rate (cfm free air) and power (kW). Let system size, component location and estimated air flow range determine the flow meter type and its location. Get your electrical consumption by calculating kW or from a kWh meter. For smaller systems, use voltage and current readings and apply the motor power factor to estimate power consumption. Convert your kW/cfm reading to cost by applying your electrical rate. Converting compressed air usage into dollars puts the system operation and improvements into terms that everyone can understand.
Apply controls to the compressors and other supply-side components as well as to air users that have the greatest effect on the system. The type of compressor control and operation depends on compressor type and system dynamics.
Control of an individual compressor requires consideration of demand variation and control of air users to minimize their effect on the system. Operate a minimum number of compressors necessary to base load (operate at full capacity), and use only one trim compressor to track the overall varying load. If you have multiple compressors of the same type, use sequencing controls to run all but one at full capacity. These sequencers not only control trim compressor turndown, but also will start and stop compressors according to system demand.
For systems with multiple compressor types, it may be beneficial to separate the control for each type. Sophisticated sequencing controllers and global systems now available can control more than one compressor type. When using these control schemes, don’t ignore compressor type. For example, rotary compressors with modulating, or load/unload capacity control should be run fully loaded, variable-speed rotary compressors should be used only for trim, and centrifugal units have relatively efficient but limited, reduced capacity modulation.
Primary and secondary storage also can help align supply with demand by minimizing the effects that air users have on the system. Air receivers are vessels that store air that’s needed to meet peak demand events with minimal effect on changes in pressure. Primary storage, located close to the compressors, reacts to any system event. Secondary storage, located close to an end use, minimizes the effect that a local high-volume, low-time duration event has on the upstream system.
In conjunction with storage, an application that requires a narrow pressure band can be equipped with a pressure/flow controller that monitors downstream pressure and reacts quickly to maintain line pressure stability.