|Compressed Air System Upgrade Generates Significant Energy Savings at a Steel Mill|
|Written by USDOE Office of Industrial Technologies|
|Wednesday, 09 June 2010 10:45|
COMPRESSED AIR SYSTEM UPGRADE GENERATES SIGNIFICANT ENERGY SAVINGS AT A STEEL MILL
The main compressed air system is vital for the plant’s production process since it serves the Basic Oxygen Plant (BOP), two blast furnaces and the powerhouse. The principal applications of compressed air are pneumatic actuators and pistons that actuate large cylinders. Prior to the system over- haul, the plant’s compressed air system was served by six aging 400-hp air and oil-cooled, rotary screw compressors that were spread out over the plant. These compressors leaked oil, broke down frequently and could no longer provide air at the pressure level for which they were rated.
The Edgar Thomson plant engineers reviewed their compressed air system and realized that simply replacing the old compressors would not solve the system’s problems. Based on their experience, they knew that a system-level strategy was necessary to address the problem effectively. As a starting point the engineers examined the six compressors and determined that they were in poor condition and very costly to repair. The compressors had been in- stalled in the early 1980s. By 1996, they operated so poorly that they could not produce the airflow and pressures they were rated for, causing the plant to borrow compressed air from other systems in the plant.
In addition to being in poor operating condition, the oil that the compressors were leaking had several impacts on the system. First, it imposed excessive maintenance and lubricant costs on the plant because the oil had to be frequently replaced. Second, the oil was carrying over into the end use equipment, leading to unreliable product quality. Third, the oil had contaminated the air dryers and filters to the point that they were no longer performing effectively. Lastly, the oil’s presence in the dryers and filters severely obstructed the airflow, which led to a severe pressure drop across the system. Although they operated the compressors at discharge pressures of 90 psig or more, the end use applications were barely receiving air at their minimum required pressure of 60 psig because the oil had so congested the filtration equipment.
Finally, the plant identified some large leaks and inappropriate uses of com- pressed air that were contributing to excess air demand and were wasting energy.
The plant took the rotary screw compressors offline and installed two 600-hp centrifugal compressors in a central location that required minimal engineer- ing and equipment to connect the compressors. A few of the rotary screw compressors were kept for back-up use and during extreme hot or cold weather. Because of their poor condition, these rotary screw compressors are seldom used due to the risk of introducing oil and dirt into the system.
Next, the plant installed two new dryers, one for each of the new compressors and new filters in the air/lubricant separators. At the same time, the plant performed a leak detection/repair campaign and eliminated some inappropriate uses such as blow off and spot cooling applications.
Once all of the equipment was in place and the leak repair was complete, the plant began operating the new compressors at a discharge pressure of 90 psig. The plant engineers found that the end use applications were receiving air at 80 psig. Since they knew that the end use applications could operate at 60 psig, they began to gradually lower the compressor discharge pressure to 70 psig.
The overhaul of the plant’s compressed air system resulted in substantial energy savings. Prior to the project, the plant operated six compressors totaling 2400-hp at full capacity without being able to meet the end use applications’ air demand. In addition, the compressors leaked a lot of oil and had to be taken offline for frequent oil changes, leading to unreliable production. With the new system in place, the plant is able to operate more effectively with less total horsepower. Furthermore, the reduction in lubricant carryover and the compressor downtime it caused has improved the reliability of plant’s production and eliminated its dependence on other systems.
The plant is saving $140,000 (4,501,516 kWh) in energy costs annually. This is largely attributable to the lower, more stable pressure level at which the compressors are now required to operate and represents about 18% of the plant’s annual energy costs for compressed air. The plant is also saving $126,000 per year in compressor lubricating oil and $191,000 per year in repair and maintenance costs for a total annual project savings of $457,000. Since the cost of the project was $521,000, the simple payback for the project was 13.5 months.
The upgrading of a compressed air system’s components is most beneficial when performed within the context of a system level strategy to improve a compressed air system. As the compressors at the Edgar Thomson Plant reached the end of their useful lives, the plant decided to overhaul and reconfigure their entire system in order to fully benefit from the measures they were taking to improve it. Had the air treatment equipment not been replaced, the lubricant in the dryers and filters would have continued to obstruct the airflow and to maintain the pressure drop. Had the pressure not been lowered, the compressors would have continued to supply air at discharge pressures of 90 psig. This would have allowed artificial demand to prevent energy savings from being realized. By using a systems approach towards equipment replacement and adjusting the pressure to the lowest level that served the plant’s requirements, the Edgar Thomson Plant was able to increase the efficiency of its compressed air system. This has led to significant energy and maintenance savings.
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