If an air system is assumed to be so small that potential operating-cost reductions would not justify a detailed audit, the truth might be surprising. Even for many smaller systems, Compressed Air Consultants asserts, the advanced work is justifiable and profitable; often requires work in the plant and outside the compressor room; and, provides payback rivaling that for larger plants.

A cement import terminal, for example, had all the usual productivity problems attendant upon moisture in the air, including sticky valves, small boulder formation in the silos, and glazing in the baghouses.

SELECTING A STRATEGY

As a sister plant was pleased with results obtained from a recent Compressed Air Consultants audit, the terminal contacted CAC. Ensuing discussions revealed that the operation previously had run solely on a 75-hp compressor, and a 125-hp unit was used only when the facility was blending cement for bagging operations. Now, both compressors were required most of the time to prevent low air pressure.

Seizing an opportunity to increase productivity while potentially reducing operating costs, CAC offered two approaches: one focusing exclusively on the moisture issue; and, a second addressing air moisture content while evaluating the possibility of turning off one of the two compressors for significant periods of time.

Given estimated annual operating costs between $60,000 and $80,000, the facility arguably lacked the budget for a full audit. Nevertheless, in view of only two compressors running the system and the intent to turn off one unit, some demand side work would be required. With an eye to return on investment, therefore, identifying which demand side applications needed to be analyzed would be essential. The job's scope thus was tailored to accommodate the terminal's unique situation. The subsequent decision to pursue CAC's second option, a more detailed audit, produced some interesting and unpredictable results.

THE AUDIT

The terminal's three basic modes of operation included (1) standard ship unloading and truck loading, (2) bagging, and (3) shut down. Air moisture presented a problem during all operations, while the operating cost issue remained undetermined. The facility's original plan was to use only the 75-hp compressor during standard operation, while the 125-hp unit would come on during bagging to feed the blenders and dense-phase transporters — less than 5% of the week.

Defining and describing the system as well as implementing a data collection process constituted the first step in the audit. After pressure, temperature, and power data were collected for a week and a system profile emerged, several striking issues became evident.

1. The two air-compression systems comprised such a jumble of piping that auditors were hard pressed to track connections between them. Finally, they realized that the 125-hp compressor was feeding the 75-hp system upstream of the dryer, so that whenever air entered the latter unit from the larger one, it was wet.

2. Another root cause of elevated air moisture was found to be the temperature of the room housing the 125-hp compressor. Although the 125-hp was placed in a silo just like the 75-hp system, the smaller unit was situated in a far bigger silo with a greater transfer of air, allowing the 75-hp compressor and its dryer to operate at lower temperatures. Consequently, the 125-hp fed wetter and hotter air to the main system, since its aftercooler would be less effective. Additionally, air being fed to its own blender and dryers was so hot that the refrigerated air dryer failed to accomplish its function, providing air of less than adequate quality to the system.

3. During standard operation, one of the main system's internal processes was way out of tolerance: instead of consuming 25-40 cfm as designed, it consumed 150 cfm. Such faulty equipment operation was enough to require the use of the other compressor when intermittent events pushed the system to its limit, and the application fired off. Critical to eliminating the need for a second compressor was reducing the demand on this application, plus other infrastructure improvements related to piping, controls, and storage. If the auditor had stayed in the compressor room only, potential operating cost improvements would never have been realized.

4. Although blending and dense-phase transport applications were inefficient, their use was so limited that upgrading these systems at the time did not make economic sense. Until the application operates for a significant period, current practice would suffice.

Justified by return on investment, the audit addressed a moisture problem as it supplied the groundwork for improvements in supply, distribution and demand, enabling the facility to run on one compressor during standard operation. Benefits would include reduced operating costs in addition to increased longevity of the compressors and allowance for scheduled maintenance during normal working hours.

This article was adapted from a report by Paul Edwards of Charlotte, N.C.-based Compressed Air Consultants, 704-376-2600.

	Current	Future	Savings
Operating Costs	$70,791	$38,306	$32,485
Percent Reduction			46%
Project Cost	$59,750
Simple Payback	22 months
Moisture Problem	Solved as part of the project

CAC Air Audit Principles

1. Smaller systems can achieve returns on air system projects as impressive as those for larger systems.
2. Precise demand side work is often required to achieve significant results.
3. Prior to the audit, investigation is advised to determine if a project is likely that also facilitates the scope of work suitable to a specific operation.
4. Productivity upgrades can often be justified by operating cost reductions and long-term capital cost avoidance.