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Metalcasting facility saves energy with compressed air system retrofit.

Techni-Cast, Southgate, Calif., is a metalcasting facility that specializes in centrifugal casting, producing cast components such as landing gear for aircraft, ship shaft liners, automotive gaskets, cylinder liners, pump parts and balls for ball valves. In addition to centrifugal casting, the firm makes its own alloys and secondary metal castings. The firm's compressed air applications require between 200-400 standard [ft..sup.3]/min (scfm) of compressed air at 90 lbs. [in..sup.2] gauge (psig) to operate reliably.

In 2002, Techni-Cast implemented a project to retrofit its compressed air system with more appropriately sized compressors while upgrading the controls and taking other measures to increase the compressed air system's efficiency. By implementing this project in conjunction with Accurate Air Engineering, Bakersfield, Calif., (a U.S. Dept. of Energy Allied Partner) the firm reduced compressor capacity by 50%, which greatly reduced its compressed air energy and maintenance costs.

The annual energy and maintenance savings from the project's implementation are 242,000 kilowatt-hrs (kWh) and $24,200. The project cost $38,000, but the firm received a $10,000 incentive payment from the California Public Utilities Commission, effectively reducing the total project cost to $28,000 and yielding a 14-month simple payback.

Before the project, the facility was served by two rotary screw compressors, one 100-hp unit and one 50-hp unit. Together, they generated 350-500 scfm. The larger compressor operated 16 hrs./ day, 6 days/wk., while the 50-hp unit came online for 5 hrs./day to support the facility's peak demand of 442 scfm. It was necessary for both compressors to generate air at a discharge pressure of 120 psig so that the end-use applications would receive air at no less than 90 psig.

Because of pressure fluctuations, both compressors ran simultaneously to prevent the header pressure from falling below 90 psig during periods of peak demand. In response to high energy costs and potential incentives, Techni-Cast reviewed the compressed air system for potential energy-efficiency opportunities and worked with Accurate Air Engineering to perform a system-level review.

The first problem the review identified was the compressor control scheme. Both the 100- and 50-hp units were operated at part load at the same time. When this happened, neither compressor operated at maximum efficiency. The 50-hp unit was controlled manually and started only during peak demand when pressure fell below 90 psig. It continued to run at part load even after peak demand ended, causing more than 30 hp of compressor capacity to operate unnecessarily.

The evaluation also identified the causes of the severe pressure drop, which included components, such as point-of-use filters, regulators and lubricators (FRLs) and additional air-treatment equipment. The FRLs caused a pressure drop of 10-20 psig because they were improperly configured. To fix the problem, regulators were adjusted for maximum pressure, causing a pressure gradient from the end use application back to the point of supply, which necessitated higher compressor discharge pressure to overcome the gradient.

The other air treatment equipment increased pressure drop in two ways. The coalescing filter on the dryer was undersized for the flow rate, causing a pressure drop. In addition, condensate drains were manually operated, and operators were not able to open the valves often enough. This resulted in accumulation of condensate, which further exacerbated the pressure drop.

The evaluation also found a large air leak and some inefficient uses of compressed air. The leak was in the grit blaster, which is one of the facility's most critical compressed air applications. The firm also used 10 diaphragm pumps. Accurate Air Engineering showed that the task the pumps were performing could be accomplished with electric pumps that use less energy.

The installations followed the recommendations made in Accurate Air's audit report, which showed that a 50-hp compressor could satisfy, the facility's normal demand of 200 scfm. The firm installed a new, 50-hp rotary-screw compressor as the base compressor. Automatic start-stop controls were added to the existing 50-hp unit, which allow it to come offline when it is no longer required.

Also, operators specified proper pressure levels for all end-use applications, and repaired the leak in the grit blaster. The dryer's coalescing filter was professionally cleaned and equipped with new elements. In addition, the existing condensate drains were replaced with zero loss condensate drains. Once these measures were implemented, the compressor discharge pressure was reduced from 120 psig to 100 psig, lowering the system pressure to 95 +/- 5 psig.

The system meets the plant's normal air demand during off-peak production, despite a 50% reduction in aggregate horsepower. The plant's normal air demand is 200 scfm, and briefly reaches 400 scfm during peak demand.

Select No. 002 at www.moderncasting.com/info
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Title Annotation:Case History
Publication:Modern Casting
Geographic Code:1USA
Date:Sep 1, 2004
Words:775
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