Managing Compressed Air Energy

Part I: Demand Side Issues

By R. Scot Foss, Plant Air Technology

Data from more than 250 plants show how compressed air energy may be distributed among key usage categories.  Use this information to help decide where energy management solutions should be applied first.

Although compressed air systems generally are the third highest energy user in an industrial plant, they represent the number one opportunity for both energy and operating cost reductions.

Compressed air systems convert electrical work energy to pneumatic work energy at the point of use.  All elements of this process need to be managed efficiently.  The optimum process would produce one unit of work energy in the form of expanded mass at the point of use for every 8.5 units of compressor input energy.  In industrial plant air systems, which represent more than 7.5 percent of the energy used in U. S. industry, there seems to be little understanding or effort made to achieve any level of efficiency other than the occasional attempt to buy the promise of efficiency with new equipment.

The manner in which compressed air is consumed offers a major opportunity for reduced energy and operating costs.  Typically, less than 60 percent of the total compressed air consumed contributes directly to the goods and services for which production was intended. Of this 60 percent, more than a third of it is poorly applied.

The net result is that less than 40 percent of the total consumption of compressed air in industrial plants is essential to process results.  The balance negatively influences the cost and quality of goods and services produced.  The combination of process efficiency and usage of compressed air makes plant compressed air systems one of the most significant economic opportunities in the industrial sector.  Despite this reality, compressed air energy has been increasing while the use of all other forms of energy in industry is diminishing.

Audit Results:

In the past five years, Plant Air Technology has thoroughly audited plant and process compressed air systems at 551 plants and cumulatively analyzed the audit results of 250 systems.  The percentage of total energy used for compressed air in these plants ranged from 6-29 percent, with an average of 9.5 percent.  This article will report the findings. It is particularly interesting to note that while most plant managers were aware of potential inefficiencies, the questions of how the system was specifically set up and adjusted and why it was operated the way it was went unasked and unanswered.

Most of the operating personnel in these plants did not know how much compressed air volume they used or needed.  They did not know the costs of operating the compressed air system. Only two of these plants monitored both input power and compressed air consumed.  There were no standards or operating procedures for the use or supply of compressed air other than maintaining a minimum acceptable result.  Generally, success in system operation was determined by the lack of complaints.

The majority of operating personnel acknowledged that their education regarding compressed air systems and their operation was lacking.  Most of the audited facilities did not know how their equipment was specifically adjusted and admitted that outside sources maintained the equipment and established equipment operating parameters.  In all cases, neither the owner nor the service agency had any records of how or why the equipment was adjusted. The utility costs ranged from a blended rate including demand charges of 0.035 cent-0.117 cent/kW of electricity consumed.

Low load or no load tests were performed at all audit locations in advance of the final audit. All operating conditions were investigated.  All parts of the system including supply, storage, distribution, and demand were measured. Problems in the system were evaluated and quantified.  Operating costs of the audited systems were determined including all ancillary equipment, maintenance, water, operator costs, and depreciation. Proposed solutions were detailed and costed.  Operating cost of the proposed system was determined to establish a return on investment.

Demand Side Energy:

The basics of demand side energy will be covered here.  Future articles will discuss usage factors that affect demand and supply side energy issues.

Most systems are evaluated based on perceived supply requirements.  If the pressure anywhere in the system is below what is believed to be the minimum, the diagnosis is insufficient supply. Little more is done to determine what is going on in the system. In existing systems, demand is determined by adding up the rated capacity of the compressors that are on regardless of power.  An "on" compressor is only an indication of cost, not an