Most compressed air systems at process plants offer significant opportunities for improvement. A rigorous evaluation of an existing or new system can establish sustainable best practices for maximizing reliability and performance and minimizing lifecycle costs. In addition, it can forestall product contamination and scrap as well as the possibility of environmental damage.
The same optimization strategies and techniques apply to new systems and improvement of existing ones. They reflect a key common denominator: energy costs alone generally exceed the purchase price of a compressor in its first year of operation. Often, addressing the inappropriate application of compressed air and the proper control of the compressed air system provide the greatest savings.
Efforts must begin with analysis of demand. Every new plant will have specific initial requirements while an existing plant has established consumers. However, both of these likely will change over time, so always consider anticipated growth.
HOW MUCH AIR IS NEEDED?
Start by creating a "demand profile" form and listing each steady demand, based on manufacturers' specifications, including the required pressure and operating flow rates at minimum, average and peak conditions. Identify and separately list all intermittent demands with compressed air "on/off" cycle times in seconds or minutes. Include the number of shifts and resulting variations and potential future additions. Indicate whether the air is for process or general plant use or serves as breathing air.
The type and size of compressors needed and the air quality necessary for the applications are very important factors. Consider the air quality requirement for the applications in selecting the compressors and the air treatment. While a specific process or plant standard may call for an oil-free air compressor, some sites have successfully installed lubricated compressors with proper filtration and excellent maintenance to provide oil-free air. In other cases, where a plant requires some oil-free air but has an almost equal demand in other applications that can tolerate a given lubricant level, two separate systems may make sense. International Organization for Standardization (ISO) standards classify the level of particulates, degree of dryness and amount of lubricant in the air.
Operating at the lowest possible system pressure allows for the most efficient operation. For example, in many compressors, reducing the compressor discharge pressure 10 psi can cut specific power by 5%. Many plants supply compressed air to the main distribution system at a pressure that is at least 5 psi above the required minimum, which is determined either by the most significant uses or one most sensitive to lower pressure.
Figure 1 shows components of demand and their typical levels. "Artificial Demand" relates to increased air consumption required by any unregulated uses due to system pressure in excess of what actually is necessary. "Inappropriate Uses" refers to applications where replacing compressed air with other options potentially can improve effectiveness or efficiency. "Leaks" covers unintended losses.
Leaks typically represent 10–20% of total demand. In the past, recommendations were to maintain the leakage rate below 10%. However, compressed air now is recognized as a very expensive utility and standards at many plants call for holding leaks to less than 5% of peak flow.