Carefully evaluating your compressed air system for leaks, piping problems, inadequate storage and other maintenance issues can help you optimize your system and avoid unnecessary equipment expenses.

Figure 1. A complete air system analysis can create a clear picture of a facility’s unique air usage profile. Dataloggers and precision sensors can be used to monitor and record information and provide vital information on temperatures, pressures and actual air demand.

A range of compressed air system equipment exists in today's market. Recent advances in air-demand analysis, variable-frequency drive technology and computer-based compressor controls have allowed processing facilities to apply state-of-the-art technology to improve air system reliability and overall productivity. Information on audits, compressed air production and the benefits of compressed air efficiency abound. However, equipment upgrades and system improvements are only as effective as the overall system design, and they require a solid compressed air delivery infrastructure to perform at peak efficiency and deliver maximum reliability.

Before purchasing a new compressor, companies should complete a total air system assessment to determine that new equipment is, in fact, required. Could system control adjustments, piping improvements, leak repairs or a better maintenance plan improve productivity? Surprisingly, many facilities overlook these factors and jump right into purchasing.

System Analysis. Any evaluation should begin with a thorough system analysis to determine pressure profiles and actual compressed air demand. Dataloggers and precision sensors can be used to monitor and record this information, creating a clear picture of a facility's unique air usage profile (figure 1). Using this baseline data, carefully review the air requirements for each piece of process equipment and its effect on the application. Consider your future needs in addition to your existing requirements, and also make sure that your facility has an adequate backup system in place to avoid unnecessary downtime.

Piping. Piping is frequently overlooked; however, especially in older facilities, piping might not be adequate to handle plant expansions that have occurred. Examine all pipes to ensure that they are the proper diameter and are in good condition. Cast iron piping will rust over time, releasing rust and scale into the compressed air and that can create buildups at various points in the system. This not only degrades the air quality but also significantly reduces the effective internal diameter of the pipe, creating unwanted pressure drops and velocity problems. Copper or aluminum piping appropriately sized for the required airflow provides good value and delivery characteristics with a minimum pressure drop. Modular piping products also can ease installation and provide good long-term performance.

Figure 2. This analysis shows the actual cost of air leakage based on 8,500 hours of operation and the cost of electricity at $0.08 per kWh.

Leaks and Artificial Demand. Identifying and repairing leaks is one of the first steps to improving air system efficiency -- and every facility has leaks. Studies indicate that approximately 35 percent of all compressed air produced is lost through leaks. That's an incredible amount of waste, especially when you consider the cost required to produce that air (figure 2). Ultrasonic leak detection devices and services are readily available and relatively inexpensive compared to the potential savings.

The second step to improve efficiency is to identify sources of artificial demand, such as the unnecessary and wasteful practice of using compressed air to blow debris off an area or work surface. Using compressed air over 30 psig for this purpose is a serious OSHA violation. Even small pieces of debris flying through the air at 110 psig can cause worker injury and perhaps permanent damage. Further, this random and unpredictable use of compressed air can divert air away from critical points in the plant's process and strain the system.

Many users also needlessly waste energy (and therefore money) by running at unnecessarily high pressures. Reduce this artificial demand by minimizing the system's operating pressure to meet but not exceed process requirements.

Storage. Appropriately sized air receiver tanks can add significantly to overall system efficiency. Through centrifugal force, these tanks provide a first stage of moisture separation to help maintain compressed air quality. However, their primary function is storing and delivering compressed air to help meet periods of peak demand and prevent excessive compressor cycling.

Receivers should be sized to minimize air velocity. A good rule of thumb is to allow for 5+ gal/cfm produced by the trim compressor (the compressor that kicks on to handle peak demand).

Clean Air Treatment. Your application requirements will determine the appropriate compressed air dewpoint and, in most cases, will dictate the type of compressed air dryer (refrigerated or desiccant air) and level of inline filtration (particulate and oil-removal filters, for example) needed.

Figure 3. Routinely changing the filter elements can help maintain compressed air quality and reduce pressure drop across the system.

The process of removing moisture and particulates such as dirt, dust and hydrocarbons is essential. Place coarser filters first to absorb the bulk liquid or particulate loading. Remember that the finer the filter, the less liquid and particulate loading the filter can endure. Develop a schedule for regularly checking and replacing filter elements by reviewing the manufacturer's recommendations and evaluating your specific usage patterns. This will help eliminate pressure drop across the system while maintaining effective filter performance (figure 3).

Drain traps are another crucial but frequently overlooked system consideration. Do not forget to include high quality automatic drain traps in the system. If the contaminants filtered and separated from tanks and filters are not removed, they will find their way back into the system. These same contaminants will clog machinery and process equipment and can lead to product rejects and quality control issues.

Air Main Charging Valve. If your existing clean air treatment equipment is not performing to expectations, consider installing an air main charging system before investing in new equipment. This economical device controls the air velocity during system pressurization so that dryers and filters are not overloaded when the compressor is started for the first time or after several days of sitting idle.

Pic: Mass flow meters should record the actual amount of compressed air being used at any given time -- not just an average flow.

Compare Equipment Options

If your inspection reveals that new compressors are necessary, developing a collaborative relationship with a supplier is the best way to ensure that you receive an efficient and reliable system. Consider whether the salesperson works to understand your facility's specific operational and application requirements and can clearly explain product features and relate their benefits to your needs. Beware of one-size-fits-all solutions, and compare compressor performance among manufacturers. While many reputable compressor manufacturers exist, there are marked differences in compressor performance and manufacturing techniques that can significantly affect the systems' day-to-day operations.

Cleveland-based Compressed Air and Gas Institute (CAGI), a nonprofit organization composed of compressed air and gas equipment manufacturers, provides a uniform reporting standard for member companies. These “CAGI data sheets,” which are readily available online from most major compressor manufacturers' websites, provide a straightforward, apples-to-apples performance comparison between the various brands. Training and educational materials are also available from CAGI as well as from the U.S. Department of Energy's Compressed Air Challenge.

Remember that value and return on investment are more important than initial price. Purchasing quality equipment now will save your plant time and money for years to come. Having a basic understanding of compressed air systems can make you a more effective consumer as well as a partner in the design and selection process. PCE

Sidebar: Simple Steps for Smooth Air Compressor Operation

Like water, gas and electricity, compressed air is often taken for granted -- until it's not available and plant operations cease. End users should have basic a understanding about compressor function and the steps that are required to keep the compressor in good working order. These include:

  • Place the compressor in a relatively clean, well-ventilated area.

  • Set the compressor pressure to meet but not exceed manufacturer's guidelines for the specific piece of process equipment. Do not overpressurize or increase the compressor's pressure setting in hopes of achieving more flow. This will exacerbate capacity deficits and increase the amount of air lost to leaks.

  • Adhere to the compressor manufacturer's recommendations for routine oil and filter changes.

  • Routinely change system's filters, drain manual condensate traps and responsibly dispose of compressor condensate.

  • Review your system periodically to ensure that it continues to meet your facility's needs.